J. Nig. Soc. Phys. Sci. 5 (2023) 527

Journal of the
Nigerian Society

of Physical
Sciences

Benchmark Studies on the Isomerization Enthalpies for
Interstellar Molecular Species

E. E. Etim∗

Department of Chemical Sciences, Federal University Wukari, Katsina-Ala Road, P.M.B. 1020 Wukari, Taraba State, Nigeria & Department of Chemistry, Indian
Institute of Technology Bombay, Powai, Mumbai 400 076, India.

Abstract

With the well-established correlation between the relative stabilities of isomers and their interstellar abundances coupled with the prevalence
of isomeric species among the interstellar molecular species, isomerization remains a plausible formation route for isomers in the interstellar
medium. The present work reports an extensive investigation of the isomerization energies of 246 molecular species from 65 isomeric groups using
the Gaussian-4 theory composite method with atoms ranging from 3 to 12. From the results, the high abundances of the most stable isomers
coupled with the energy sources in interstellar medium drive the isomerization process even for barriers as high as 67.4 kcal/mol. Specifically,
the cyanides and their corresponding isocyanides pairs appear to be effectively synthesized via this process. The following potential interstellar
molecules; CNC, NCCN, c-C5H, methylene ketene, methyl Ketene, CH3SCH3, C5O, 1,1-ethanediol, propanoic acid, propan-2-ol, and propanol
are identified and discussed. The study further reaffirms the importance of thermodynamics in interstellar formation processes on a larger scale
and accounts for the known isomeric species. In all the isomeric groups, isomerization appears to be an effective route for the formation of the
less stable isomers (which are probably less abundant) from the most stable ones that are perhaps more abundant.

DOI:10.46481/jnsps.2023.527

Keywords: Isomers, Energy barrier, Interstellar chemistry, Astrochemistry, Hydrogen Bonding.

Article History :
Received: 19 December 2021
Received in revised form: 28 January 2023
Accepted for publication: 12 February 2023
Published: 18 April 2023

© 2023 The Author(s). Published by the Nigerian Society of Physical Sciences under the terms of the Creative Commons Attribution 4.0 International license

(https://creativecommons.org/licenses/by/4.0). Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Communicated by: B. J. Falaye

1. Introduction

That the interstellar medium (ISM) is chemically rich is not
an argument with the discoveries of over 200 different molecu-
lar species in this thin space between the stars which was earlier
regarded as a vacuum dotted with stars, black holes, and other
celestial bodies [1-6]. These molecules are important in vari-
ous fields such as atmospheric chemistry, astrochemistry,
prebiotic chemistry, astrophysics, astronomy, astrobiology, etc,

∗Corresponding author tel. no: +2349079192231
Email address: emmaetim@gmail.com (E. E. Etim )

and in our understanding of the solar system, “the world around
us” with each successfully detected interstellar molecule telling
the story of the chemistry and physics of the environment from
where it was detected. They serve as the most important tools
for probing deep into the interior of the molecular clouds and
the molecular clouds are significant because it is from them that
stars and consequently new planets are formed.

The symmetric rotors serve as interstellar thermometers
while the metal-bearing species provide useful information re-
garding the depletion of these molecular species into the molec-
ular dust grains. Understanding how the simple molecules that
were present on the early earth may have given rise to the com-

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E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 2

plex systems and processes of contemporary biology is one
question the biologically related interstellar molecules can be
used to address. Molecules also provide the cooling mecha-
nism for the clouds through their emission [3, 7, 8].

A careful look at the list of known interstellar and circum-
stellar molecules reveals some interesting chemistries among
these molecular species. Tables 1 and 2 contain the list of the
currently (as of June 2021) known interstellar and circumstellar
species arranged according to the number of atoms making up
the molecules.

Some of the chemistries existing among the interstellar
molecular species include the dominance of the linear car-
bon chain species of the form Cn, H2Cn, HCnN, CH3(CC)nH,
CH3(C≡C)nCN and CnX (X=N, O, Si, S, H, P, N-, H-) which
account for more than 20% of all the known interstellar and
circumstellar molecular species; the presence of about 30 alkyl
group containing interstellar molecules mostly observed from
the same or similar molecular clouds; periodic trends in which
elements from the same group appear to form similar com-
pounds with similar properties as in the case of oxygen and sul-
phur (group 6/16 elements) in which of the over 25 S-containing
molecules observed in ISM, about 20 have the corresponding
O-containing molecules uniquely detected in ISM and the abun-
dance of S-compound relative to its O-analogue is approxi-
mately equal to the cosmic S/O ratio, 1/42 as seen in methyl
mercaptan, thioisocyanic acid, etc, except where the effect of
interstellar hydrogen bonding dominates [9, 10]; successive hy-
drogen addition where larger species are believed to be formed
from the smaller unsaturated species via successive hydrogen
addition in which both species could be shown to be chemically
and spatially related [11]. Isomerism is yet another prevailing
chemistry among the interstellar molecular species. Apart from
the diatomics and a few species which cannot form isomers,
about 40% of all the known interstellar molecules have isomeric
analogues ranging from the isomeric pairs to the isomeric tri-
ads which are believed to have a common precursor for their
formation process [12]. Table 3 lists some of the known iso-
meric species and their isomeric groups.

Because of the conditions (low temperature and pressure)
in the interstellar medium, there is hardly a consensus as to
how these molecules are formed but some of the chemistries
listed above and the effect of thermodynamics serve as clues as
to how these molecules could be formed in ISM. Focusing here
on isomerization, the prevalence of isomeric species among
the known astromolecules coupled with the energy sources
in ISM such as the shock waves (which could arise from the
interaction of the Earth’s magnetic field with the solar wind,
molecular outflows during star formation, supernova blasts and
galaxies colliding with each other) which provide energy for
both the formation and distribution of large interstellar species,
place isomerization as one of the most plausible routes for
the formation of interstellar molecules [13]. As observed in
some studies, the most stable isomers are found to be more
abundant than their less stable analogues except where other
factors dominate; thus, the isomerization of the most stable

isomer (which is probably the most abundant) to the less
stable isomers can be a very effective and efficient formation
mechanism in ISM. Also, apart from the energy sources in
ISM, the high abundance of the most stable isomer can drive
the isomerization process irrespective of the energy barrier.

According to the minimum energy principle [14], iso-
merization is the most important process in determining the
relative abundances of the isomers in ISM. Isomers of the
same generic formula are said to have a common intermediate
in their formation and destruction routes. After reaching the
generic formula, the equilibration process is said to occur.
This implies an internal isomerization with a low activation
barrier, assisted isomerization, or catalytic isomerization at the
grain/ice surface [14].

The present work aims at estimating accurate isomerization
enthalpies for 246 different molecular species from 65 isomeric
groups using the Gaussian 4 theory composite method [15].
The molecules range from the 3 atomic species to those with
12 atoms with at least one known interstellar molecule from
each isomeric group. The results account for the extent and ef-
fectiveness of isomerization as a plausible formation route in
ISM; and the rationale behind the successful observation of the
known species. Among other things, potential candidates for
astronomical searches are highlighted and discussed.

2. Computational Details

A concerted effort between theory and experiment is found
in the heart of some of the most successful scientific stud-
ies. Considering the large range of molecules examined in this
study, only very few of them have experimentally measured en-
thalpies of formation while others are so unstable that they can-
not be probed experimentally in the terrestrial laboratory but
all of these can be comfortably handled computationally. The
Gaussian 4 (G4) theory composite method is employed in esti-
mating accurate standard enthalpies of formation (∆fHO) for all
the molecular systems investigated in this work. The G4 com-
posite method is very accurate for several systems and bench-
mark studies; in its release note, it has an average absolute de-
viation of 0.8 kcal/mol from experimental values for the en-
thalpies of formation of 270 molecular species. The G4 method
is a modification of the G3 method which has an average abso-
lute deviation of 1.19 kcal/mol for the same number of molecu-
lar systems [15, 16]. In the G4 method, geometry optimization
and zero point energy (ZPE) are carried out at the B3LYP level
of theory using the 6-31G(2df,p) basis set. The ZPE is scaled by
0.9854. Single-point calculations and energy are done using the
MP4/6-31G(d) method modified by corrections from additional
calculations (with MP4 and other methods) while core correla-
tion is obtained via higher-level correction terms. The Gaussian
09 suite of programs is used for all the computational studies
reported here. Only stable geometries with no imaginary fre-
quencies are considered. The standard enthalpies of formation
are obtained from the atomization energies using the approach
described in previous studies [4-6, 17-23]. 	 	

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E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 3

Table 1: Interstellar molecules between 2 and 7 atoms

2 atoms 3 atoms 4 atoms 5 atoms 6 atoms 7 atoms
H2, CO, CSi, H2O, H2S, NH3, H2CO CH4, SiH4 CH3OH, CH3SH CH2CHOH
CP HCN, TiO2,

HNC, CO2, H2CS, C2H2 CH2NH, C2H4, HC4H c-C2H4O
CS, NO, NS, SO2 NH2CN
SO HNCS CH3CN, CH3NC HC(O)CH3

MgCN, CH2CO,
HCl, NaCl, H3O+, SiC3 HCOOH HCONH2, H3CCCH

NaCN , C3S, H2CN HCCCN, HC2C(O)H CH3NH2
FeCN, KCN,

AlCl, AlF, PN HCCNC
c-C3H, l- HC3NH+ CH2CHCN

AlOH,
SiN, SiO, SiS C3H c-C3H2, l-

H2Cl, HC4N HC4CN
NH, OH, C2 H2O+, HCCN, CH3 C3H2

C5N, C5H C6H
H2Cl+,

CH2CN,
CN, HF, FeO N2O, NH2, C2CN, C3O

H2COH+
H2C4, H2CCNH CH3NCO

OCS
LiH, CH, CH+ HCNH+,

C4Si
C5N- HC5O,

HOCO+
CH2, HCO,

CO+, SO+, SH,
C3 C5

C3N-, c-H2C3O HOCH2CN
C2H, C2O, HNCCC HNCHCN, C5S, HNCHCCH,

O2, N2, CF+
C2S , AlNC,

SiH3CN,
HSCN, C3N,

C4H
HNO HC4NC,

PO, HD PH3
z-HNCHCN,

SiCN, N2H+ C4H- c-C3HCCH,
SiH, AlO, HMgNC

SiNC, c- MgC4H
HC(O)CN,

ArH+ , OH+, SiC2 HCCO, CH3O,
CN-, SH+, NCCP, HNCNH, CH3CO+,
HCl+, TiO, MgCCH,

HCO+, H2NCO+, CH2CCH,
CrO, NS+, HOCN, NH3D+,

H2CCCS,
CH3Cl,

HCS+, H3+
VO, HeH+ CNCN

OCN-, HCP,
CCP, SiCSi , H2O2 , MgC3N,
S2H, HCS, NH2OH,
HSC, NCO, t-HONO HC3O+,
CaNC, HC3S+,
NCS, HO2 C4S,

t-HC(O)SH,
H2CCS,
NCCNH+

3. Results and Discussion

The standard enthalpies of formation (∆fHO) for all the 246
molecular species from 65 isomeric groups examined in this

study are contained in the supporting information. The relative
enthalpies for each isomeric group are presented and discussed
in this section. The different isomeric groups investigated are

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E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 4

Table 2: Interstellar molecules between 8 and >12 atoms

8 atoms 9 atoms 10 atoms 11 atoms 12 atoms > 12 atoms
CH3COOH (CH3)2O (CH3)CO HC9N C6H6 HC11N
HCOOCH3 CH3CH2CN HOC2H4OH CH3C6H C3H7CN C60
HOCH2CHO CH3CH2OH H3CCH2COH HCOOC2H5 C2H5OCH3 C60+
H3C3CN, CH3CH2SH CH3C4CN, CH3OCOCH3 branched- C70
C7H, CH3CHCHO, C3H7CN ,
(NH2)2CO CH3C4H CH3COCH2OH c-

CH3OCH2OH c-C5H5CN C6H5CN
H2C6 HC7N c-C5H6

C10H7CN
H(CC)3H C8H NH2CH2CH2OH

c-C9H8
H2CCHCHO CH3CONH2
CH2CCHCN C8H-
H2NCH2CN, CH3CHCH2,
CH3CHNH CH3NHCHO,
CH3SiH3, HC7O
(NH2)2CO HCCCHCHCN
HCCCH2CN, H2CCHC3N
HC5NH+, H2CCCHCCH
CH2CHCCH,

Table 3: Known interstellar isomeric pairs, triads and their isomeric groups

Isomeric Group Astronomically observed isomers
CHN HCN, HNC
CNMg MgCN, MgNC
CNSi SiCN, SiNC
CHO+ HOC+ , HCO+

CHSN HSCN, HNCS
HC3 c-C3H , l-C3H
H2C3 c-C3H2 , l-C3H2
CN2H2 NH2CN, HNCNH
C3H2O HCCCHO, c-H2C3O
C4H2 H(CC)2H, H2C4
C6H2 H2C6, H(CC)3H
C4H3N CH2CCHCN, CH3CCCN
C2H6O (CH3)2O, CH3CH2OH
C3H6O (CH3)2CO, CH3CH2C(O)H
C3H6O2 HC(O)OCH2CH3, CH3OC(O)CH3
C4H7N n-CH3CH2CH2CN , b-CH3CH2CH2CN
CHON HNCO, HCNO, HOCN
C2H4O2 CH3COOH, HC(O)OCH3, HOCH2C(O)H
C2H4O CH2CH(OH) c-C2H4O, HC(O)CH3
C2H3N CH3CN, CH3NC, H2CCNH
C3HN HC2CN, HC2NC, HNCCC

grouped according to the number of atoms beginning from 3 to
12.

3.1. Isomers with 3 atoms

In Table 4, the relative enthalpies or isomerization energies
of the 26 molecular species from 13 isomeric groups with 3

atoms are shown alongside the current astronomical status of
these species. At least one isomer from each of these isomeric
groups is a known interstellar molecule [24]-[43]. The isomer-
ization enthalpies range from 0.2 to 41.5 kcal/mol for groups
where both isomers have been detected. This range illustrates
the effectiveness of the isomerization mechanism as a plausible
route for the formation of these molecular species in ISM; it
also shows how far or the extent to which the energy sources
within the ISM can drive some chemical processes. For the
isomeric groups where only one isomer has been detected, the
observed range of the relative enthalpy of formation suggests
that species like NaNC, AlCN, ONC- and even HOC can be
formed from their most stable isomers that have been detected
already. The high abundances of the stable isomers coupled
with the energy sources in ISM imply the possibility of these
less stable isomers being formed from their most stable isomers
via the isomerization mechanism.

With the exception of C2N group where only the less stable
isomer is been detected, in all other cases where only one iso-
mer is detected, it is the most stable isomer which supports the
fact that the most stable isomer is probably the most abundant
and the most abundant species is easily detected compared to
the less stable isomer. Where both isomers have been detected,
the most stable isomer is found to be the most abundant ex-
cept where other processes dominate. CNC is more stable than
CCN but the less stable isomer has been detected while the most
stable isomer is yet to be astronomically observed. From litera-
ture perusal, there is no information regarding the spectroscopic
parameters of CNC that would have warranted it astronomical
searches. Thus, the detection of CNC awaits the availability of
accurate spectroscopic parameters.

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E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 5

Table 4: Isomerization enthalpies for isomers with 3 atoms

Isomeric
group

Isomers Relative
∆fHO

(kcal/mol)

Astronomical
status

Ref.

CNH HCN 0.0 Observed [24]
HNC 13.5 Observed [25]

CNNa NaCN 0.0 Observed [26]
NaNC 2.5 Not

observed
CNMg MgNC 0.0 Observed [27]

MgCN 0.7 Observed [28, 29, 30]
CNAl AlNC 0.0 Observed [31]

AlCN 7.3 Not
observed

CNSi SiNC 0.0 Observed [32]
SiCN 0.2 Observed [33]

CHO HCO 0.0 Observed [34]
HOC 38.2 Not

observed
CHP HCP 0.0 Observed [35]

HPC 75.7 Not
observed

C2N CNC 0.0 Not
observed

CCN 2.1 Observed [36]
C2P CCP 0.0 Observed [37]

CPC 85.3 Not
observed

CHO+ HCO+ 0.0 Observed [38]
HOC+ 37.3 Observed [39, 40]

CHS+ HCS+ 0.0 Observed [41]
HSC+ 94.1 Not

observed
CNO- OCN- 0.0 Observed [42]

ONC- 17.1 Not
observed

CHS HCS 0.0 Observed [43]
HSC 41.5 Observed [43]

Ions (both cations and anions) play important role in
the formation processes of interstellar molecules. Under the
conditions of the ISM, neutral atoms and molecules tend to be
unreactive toward molecular hydrogen but in the presence of
ions, most of the reactions become very efficient since the ions
can easily react without having to overcome the reaction barrier
[7, 44]. This can be seen in the case of the CHO+ group where
HOC+ with a relative enthalpy of formation of 37.3 kcal/mol
has been detected. Theoretical calculations and laboratory
experiments have shown that the isomerization process in the
CHO+ isomers has essentially no barrier [14]. Thus, with the
availability of accurate spectroscopic parameters, the HSC+

and ONC- ions (where their most stable isomers are already
detected) could be successfully detected.

Table 5: Isomerization enthalpies for isomers with 4 atoms

Isomeric
group

Isomers Relative
∆fHO

(kcal/mol)

Astronomical
status

Reference

CHON Isocyanic acid 0.00 Observed [45]
Cyanic acid 29.0 Observed [46,

47]
Fulminic acid 67.4 (0.0) Observed [48]
Isofulminic
acid

86.1
(18.7)

Not
observed

CHSN HNCS 0.0 Observed [49]
HSCN 11.2 Observed [50]
HCNS 40.6 (0.0) Not

observed
HSNC 43.3 (2.8) Not

observed
C3H c-C3H 0.0 Observed [51]

l-C3H 3.1 Observed [52]
C3N l-C3N 0.0 Observed [53,

54]
C2NC 22.7 Not

observed
c-C3N 28.8 Not

observed
C2NH HC2N 0.0 Observed [55]

HCNC 28.0 Not
observed

C3O l-C3O 0.0 Observed [56,
57]

c-C3O 21.6 Not
observed

C3S l-C3S 0.0 Observed [58,
59,
60]

c-C3S 26.3 Not
observed

SiC3 c-C3Si 0.0 Observed [61]
l-C3Si 49.8 Not

observed
C2NP NCCP 0.0 Observed [62]

CNCP 24.1 Not
observed

C2N2 NC2N 0.0 Not
observed

CNCN 24.7 Observed [63]

Isomerization appears to be a favourable route for the
cyanide/isocyanide pair. Table 4 contains 7 cyanide/isocyanide
pairs of which both cyanide and isocyanide have been detected
in three groups with isomerization energy ranging from 0.2
to 13.5 kcal/mol. Except for the ONC- ion with a relative
enthalpy of formation of 17.1kcal/mol; the remaining members
of the cyanide/isocyanide pairs have relative enthalpy of
formation in the range of 0.0 to 7.3 kcal/mol which is well
within the range of those already detected. Thus, NaNC, AlCN

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E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 6

Figure 1: Optimized structures of cyclic C5N isomers.

Figure 2: Optimized structures of cyclic C5H isomers.

Figure 3: Optimized structures of cyclic C5S isomers.

Figure 4: Optimized structures of cyclic SiCH3N isomers.

Figure 5: Optimized structures of cyclic C5O isomers.

and CNC are most likely to be formed from their corresponding
analogues and could be successfully detected.

3.2. Isomers with 4 atoms

For the isomers with 4 atoms, 25 molecular species from
10 isomeric groups are examined. Table 5 contains the
isomerization energies of these molecular species and their

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E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 7

Table 6: Isomerization enthalpies for isomers with 5 atoms

Isomeric
group

Isomers Relative
∆fHO

(kcal/mol)

Astronomical
status

Reference

C3HN HCCCN 0.0 Observed [65]
HCCNC 24.1 Observed [66]
HNCCC 45.4 Observed [67]
CC(H)CN 50.5 Not

observed
HCNCC 72.6 Not

observed
C2H2O Ketene 0.0 Observed [68]

Ethynol 38.8 Not
observed

Oxirene 81.9 Not
observed

C3H2 c-C3H2 0.0 Observed [69]
l-C3H2 13.7 Observed [70]

N2H2C NH2CN 0.0 Observed [71]
CH2NN 26.3 Not

observed
NH2NC 43.4 Not

observed
C2H2N CH2CN 0.0 Observed [72]

CH2NC 22.2 Not
observed

C2HNO CNCHO 0.0 Observed [73]
HCONC 12.9 Not

observed
c-C2NHO 30.2 Not

observed
HNCCO 69.2 Not

observed
HC2NO 83.3 Not

observed

current astronomical status. Fourteen of these molecular
species have been detected from different astronomical sources
[45]-[63]. In the CHON, CHSN, and C3H groups where more
than one isomer has been detected, the relative enthalpy of
formation ranges from 3.1 to 67.4 kcal/mol. For the remaining
7 isomeric groups with only one known molecular species
from each, the relative enthalpy of formation ranges from 21.6
to 49.8 kcal/mol. This range falls within that of the known
species, thus, pointing to the possibility of detecting the less
stable isomers of these groups that could be formed via the
isomerization process.

Among the isomers with 4 atoms examined here, there
are eight cyanide/isocyanide pairs. In two of these pairs,
both cyanide and isocyanide have been detected with a
relative enthalpy of formation in the range of 2.8 to 29.0
kcal/mol. For the other cyanide/isocyanide pairs where only
the cyanides are observed, the relative enthalpy of formation
ranges from 18.7 to 24.7 kcal/mol which is below the 29.0

Table 7: Isomerization enthalpies for isomers with 6 atoms

Isomeric
group

Isomers Relative
∆fHO

(kcal/mol)

Astronomical
status

Reference

C5N l-C5N 0.0 Observed [74]
c-C5N* 17.4 Not observed
C4NC 20.7 Not observed
c-C5N** 21.0 Not observed
c-C5N*** 100.1 Not observed

C5H l-C5H 0.0 Observed [75,
76,
77]

c-C5Ha 1.0 Not observed
c-C5Hb 34.7 Not observed
c-C5Hc 60.8 Not observed

C2H3N Methyl cyanide 0.0 Observed [78]
Methyl isocyanide 23.0 Observed [79]
Ketenimine 23.1 Observed [80]
Ethynamine 42.3 Not observed
2H-azirine 46.5 Not observed
1H-azirine 80.8 Not observed

SiCH3N SiH3CN 0.0 Observed [62]
SiH3NC 4.5 Not observed
c-SiCH3Nd 33.4 Not observed
H2SiCNH 46 Not observed
c-SiCH3Ne 50.2 Not observed
H2NCSiH 57.0 Not observed

C4HN HC4N 0.0 Observed [81]
HC3NC 17.4 Not observed

H2C3O Methylene ketene 0.0 Not observed
Propynal 7.3 Observed [82]
Cyclopropenone 10.6 observed [83]

H3CON Formamide 0.0 Observed [84]
Hydroxymethylimine 12.6 Not observed
Nitrosomethane 60.8 Not observed

C5S l-C5S 0.0 Observed [62,
85]

c-C5S# 26.9 Not observed
c-C5S## 41.8 Not observed
c-C5S### 84.1 Not observed
C4SC 114.2 Not observed

C5O l-C5O 0.0 Not observed
c-C5Oσ 28.6 Not observed
c-C5Oσσ 46.3 Not observed
c-C5Oσσσ 84.5 Not observed
C4OC 114.3 Not observed

*Figure 1a, **Figure 1b, ***Figure 1c; aFigure 2a, bFigure 2b,
cFigure 2c; #Figure 3a, ##Figure 3b,
###Figure 3c; dFigure 4a, eFigure 4b; σFigure 5a, σσFigure 5b, σσσFigure 5c.

kcal/mol relative enthalpy of formation calculated for the
known cyanide/isocyanide pairs. Thus, isomerization remains
a plausible mechanism for the formation of the less stable
isocyanides from their corresponding cyanides which are
probably more abundant.

The C2N2 isomeric group is the isoelectronic analogue of
the C2NP isomeric group. Just as there is interesting chemistry
between the O and S-containing interstellar molecular species;

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E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 8

such also exists for the N and P-containing species, though
it is not well explored as that of O and S. Over 80% of all
the known interstellar and circumstellar molecules have been
detected via their rotational spectral features because, at the
low temperature of the ISM, rotational excited states are easily
populated. NC2N (the most stable isomer of the C2N2 group)
is microwave inactive meaning that it cannot be astronomically
detected via radio astronomy. There are reasons to believe the
presence and detectability of NC2N; its protonated analogue,
NC2NH+ has been detected [64]. NCCP, the isoelectronic
analogue of NC2N is also known [62]. Both the protonated
analogues of NC2N and its isoelectronic analogue that have
been observed are likely to be less abundant than NC2N
because of the reactive nature of the ion and the less cosmic
abundance of P as compared to N. Thus, infrared astronomy
of NCCN or radio astronomy of its isotopologues (which are
microwave active) is likely to be successful.

The observed isomers in all the groups shown in Table 5 are
also the most stable isomers.

3.3. Isomers with 5 atoms

Isomerization energies for 20 molecular species from
6 isomeric groups are presented in Table 6 alongside their
current astronomical status. Nine of these species are known
astromolecules [65]-[73]. In the C3HN and C3H2 groups where
more than one isomer has been detected, the isomerization
energy ranges from 13.7 to 45.4 kcal/mol, and the isomeriza-
tion energies for some of the isomers with five atoms whose
most stable isomers have been detected fall within this range.
These include; ethynol, CH2NN, NH2NC, CH2NC, HCONC
and c-C2NHO. These species can as well be formed from their
most stable isomers via the isomerization mechanism.

Again, isomerization appears to be a favourable route
for the formation of isocyanides from their corresponding
cyanides. As shown in Table 6, an isomerization enthalpy as
high as 45.4 kcal/mol is noted for an isocyanide formation.
This strongly supports the formation of other isocyanides from
their corresponding cyanides. The barrier for other isocyanides
whose corresponding cyanides have been detected ranges from
12.9 to 43.4 kcal/mol which is within the limit of the one that
has been observed.

The fact that the most stable isomers are probably the most
abundant and are easily detected in ISM is well demonstrated
among these isomers. From Table 6, all the observed isomers
are the most stable ones in their respective isomeric groups as
compared to the ones that have not been detected.

3.4. Isomers with 6 atoms

The isomerization energies and the current astronomical
status of the different isomeric species with 6 atoms inves-
tigated in this study are presented in Table 7. Figures 1 to
5 display some of the cyclic isomers highlighted in Table
7. One-third of all the molecular species presented in Table

7 have all been detected from several astronomical sources
[74]-[85]. In the C2H3N and H2C3O groups where more than
one isomer has been detected, the isomerization enthalpies
range from 7.3 to 23.1 kcal/mol. Except for some of the
cyclic isomers which are highly unstable, most of the unknown
isomers (c-C5N*, C4NC, c-C5N**, c-C5Ha, SiH3NC, HC3NC,
methylene ketene, hydroxymethylimine) have isomerization
enthalpies in the range of the known isomers, suggesting their
possible astronomical observation if they could be formed via
isomerization as the ones that have been observed. Ketenimine
for instance is reported to be formed from methyl cyanide
via tautomerization (i.e., an isomerization pathway in which
the migration of hydrogen atom from the methyl group to the
nitrogen atom is accompanied by a rearrangement of bonding
electrons) and this process is said to be driven by shock waves
that provide the energy for both the formation and distribution
of large interstellar species [80].

The C5H isomers; l-C5H and c-C5Ha are almost iden-
tical in energy. If the spectroscopic parameters of c-C5Ha

(Figure 2a) are accurately probed, either experimentally or
theoretically; this species (c-C5H) could be detected in ISM.
The four cyanide/isocyanide pairs among the isomers with 6
atoms presented in Table 7 have isomerization enthalpies in
the range of 4.5 to 23.0 kcal/mol. Interestingly, the C2H3N
group where methyl cyanide and methyl isocyanide have been
detected has the highest relative enthalpy of formation of 23.0
kcal/mol which implies the possible detection of the other
isocyanides with lower barriers (4.5 to 20.7kcal/mol) assuming
isomerization as their plausible formation route.

Methylene ketene and methyl ketene have been proposed as
potential interstellar molecules for many reasons. The ketenes
are found to be more stable than their corresponding isomers
(Tables 6, 7, and 9); they are less affected by interstellar
hydrogen bonding assuming surface formation processes;
ketene and ketenyl radical (H2C2O and HC2O respectively) are
known interstellar molecules [64, 65, 66, 67, 68].

Just as every known O-containing interstellar molecule
points to the presence and the detectability of the S-analogue.
For every known S-species, the O-analogue is not only present
in detectable abundance, but it can also be said to have even
been overdue for astronomical detection. C5O is the O-
analogue of C5S that has been detected. Thus, it is an important
potential interstellar molecule.

3.5. Isomers with 7 atoms
Table 8 shows the isomerization energies and the current

astronomical status of different isomeric species with 7 atoms
investigated in this study. Of the 22 molecular species in
the Table, 7 have been astronomically observed [86]-[93].
In the H4C2O isomeric group where all the isomers have
been detected, the isomerization energies range from 12.2 to
27.8 kcal/mol. All the non-observed isomers in the H3C3N
and C3H4 isomeric groups including, cyanomethanol, imi-
noacetaldehyde, and methyl cyanate (from C2H3NO isomeric

8



E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 9

group) have isomerization enthalpies in the range calculated
for the H4C2O isomers which have all been detected. This
points to the possibility of detecting these molecules in ISM
with relative enthalpy of formation within those of the known
interstellar molecules. There is no cyanide/isocyanide pair
observed among these systems, however, the relative enthalpy
of formation in the range of 19.9 to 27.3 kcal/mol is within
the range of those noted for other observed cyanide/isocyanide
pairs.

As in the previous examples, all the observed isomers
are the most stable ones in their respective groups. In the
H4C2O isomeric group where all the stable isomers have
been observed, the most stable isomer; acetaldehyde was first
observed before the other isomers. As would be expected, the
most stable isomer (acetaldehyde) is present in high abundance
in the different astronomical sources where it has been detected
as compared to the abundances of the less stable isomers in the
same sources [94, 95, 96].

Table 8: Isomerization enthalpies for isomers with 7 atoms

Isomeric
group

Isomers Relative
∆fHO

(kcal/mol)

Astronomical
status

Reference

H4C2O Acetaldehyde 0.0 Observed [86,
87]

Vinyl alcohol (syn) 12.2 Observed [88]
Vinyl alcohol
(anti)

13.9 Observed [88]

Ethylene oxide 27.8 Observed [89]
H3C3N Acrylonitrile 0.0 Observed [90]

Isocyanoethene 19.9 Not observed
C3H4 CH3C2H 0.0 Observed [91]

H2CCCH2 7.8 Not observed
c-C3H4 23.6 Not observed

C2H3NO Methyl isocyanate 0.0 Observed [92,
93]

Cyanomethanol 13.6 Not observed
Iminoacetaldehyde 20.1 Not observed
Methyl cyanate 27.3 Not observed
2-Aziridinone 36.0 Not observed
2-Oxiranimine 40.8 Not observed
Methyl fulminic
acid

56.5 Not observed

2-Iminoethenol 60.9 Not observed
Nitrosoethene 69.3 Not observed
2H-1,2-Oxazete 79.8 Not observed
Methyl
isofulminic acid

84.8 Not observed

N-
Hydroxyacetylenamin

93.6 Not observed

(Aminooxy)acetylene 94.7 Not observed

3.6. Isomers with 8 atoms

Table 9 contains the isomerization energies for 33 isomeric
species from 7 groups containing eight atoms each. The astro-
nomical statuses of these species are also shown in the table.
Figure 6 highlights the cyclic isomers from the C2H5N and
CH4N2O groups. As shown in Table 9, 11 of these species have
been astronomically detected [97]-[109]. The isomerization

energies of these species range from 3.1 to 50.0 kcal/mol.
Except for 1,2-dioxetane, 1,3-dioxetane, and epoxyproprene,
the isomerization energies for all the unknown isomers with
8 atoms fall within the range (of 13.0 to 46.9 kcal/mol) of
the known isomers (3.1 to 50.0 kcal/mol). Thus, some of the
unknown isomers in Table 9 could be formed from their most
stable isomers (which are probably more abundant) via the
isomerization process.

The three cyanide/isocyanide pairs among these isomers
have isomerization enthalpies ranging from 19.0 to 25.6
kcal/mol. Though no cyanide/isocyanide pair has been de-
tected among these molecular species, the relative enthalpy of
formation is within the range of those where the pairs have
been detected. CH3CCNC and H2NCH2NC could be formed
from their corresponding cyanides that have been detected,
thus, they could also be detected provided their spectroscopic
parameters are accurately known. Methyl ketene, the most
stable isomer of the H4C3O group is yet to be astronomically
detected. The reasons for its presence and detectability in ISM
are the same as discussed for methylene ketene (among the
isomers with 6 atoms).

Table 9: Isomerization enthalpies for isomers with 8 atoms

Isomeric
group

Isomers Relative
∆fHO

(kcal/mol)

Astronomical
status

Reference

C4H3N CH3CCCN 0.0 Observed [97]
CH2CCHCN 3.1(0.0) Observed [98,

99]
HCCCH2CN 13.0 Not observed
CH3CCNC 25.6 Not observed
CH2CCHNC 25.7 (22.6) Not observed

C2H4N2 H2NCH2CN 0.0 Observed [100]
H2NCH2NC 19.0 Not observed

C2H4O2 Acetic acid 0.0 Observed [101]
Methylformate 17.7 Observed [102,

103]
Glycolaldehyde 33.2 Observed [104]
1,3-dioxetane 52.9 Not observed
1,2-dioxetane 103.0 Not observed

H4C3O Methyl ketene 0.0 Not observed
Propenal 2.3 Observed [105]
Cyclopropanone 17.3 Not observed
Propynol 30.8 Not observed
Propargyl alcohol 35.2 Not observed
Methoxy ethyne 42.0 Not observed
1-cyclopropenol 44.0 Not observed
2-cyclopropenol 45.5 Not observed
Epoxypropene 65.8 Not observed

H2C6 HC6H 0.0 Observed [106]
H2C6 50.0 Observed [107]

C2H5N CH3CHNH 0.0 Observed [108]
H2CCHNH2 2.8 Not observed
CH3NCH2 8.1 Not observed
c-C2H5Nm 19.8 Not observed

CH4N2O H2NCONH2 0.0 Observed [109]
H2NNHCHO 11.0 Not observed
HN2CH2OH 35.0 Not observed
c- CH4N2On 42.4 Not observed
CH3NHNO 43.4 Not observed
H2NCHNHO 46.9 Not observed

mFigure 5a; nFigure 5b.

9



E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 10

Figure 6: Optimized structures of cyclic C2H5N (A) and CH4N2O (B) isomers.

Table 10: Isomerization enthalpies for isomers with 9 atoms

Isomeric
group

Isomers Relative
∆fHO

(kcal/mol)

Astronomical
status

Reference

C2H6O Ethanol 0.0 Observed [110,
111]

Dimethyl ether 12.6 Observed [112]
C2H6S CH3CH2SH 0.0 Observed [113]

CH3SCH3 1.4 Not observed
C2H5ON Acetamide 0.0 Observed [114]

N-
methylformamide

9.7 Not observed

Nitrosoethane 64.7 Not observed
1-aziridnol 77.9 Not observed
Cyanoethoxyamide 134.4 Not observed

C3H5N Cyanoethane 0.0 Observed [115]
Isocyanoethane 20.8 Not observed
Propylenimine 22.0 Not observed
2-propen-1-imine 25.3 Not observed
N-methylene
ethenamine

26.0 Not observed

Azastene 32.2 Not observed
Cyclopropanimine 37.2 Not observed
Methylene
azaridine

43.8 Not observed

Propargylamine 45.1 Not observed
1-
azabicyclo(1.1.0)butane

53.5 Not observed

C5H4 CH3C4H 0.0 Observed [116]
H2C3HC2H 5.2 Not observed
H2C5H2 8.1 Not observed
c- C5H4 x 26.1 Not observed
c- C5H4 y 31.6 Not observed

C3H6 CH3CHCH2 0.0 Observed [117]
c- C3H6 z 8.2 Not observed

C7HN HC7N 0.0 Observed [118]-
[120]

HC6NC 27.1 Not observed
xFigure 6a; yFigure 6b; zFigure 6c.

Figure 7: Optimized structures of cyclic C5H4 (A and B) and C3H6 (C) isomers.

3.7. Isomers with 9 atoms
In Table 10, 28 isomeric species from 7 groups are

presented with their isomerization enthalpies and current
astronomical status. Eight of these species are known inter-
stellar molecular species [110]-[120]. Figure 7 shows some

Table 11: Isomerization enthalpies for isomers with 10 atoms

Isomeric
group

Isomers Relative
∆fHO

(kcal/mol)

Astronomical
status

Reference

C2H6O2 1,1-Ethanediol 0.0 Not observed
Ethylene glycol 6.1 Observed [123]
Methoxy methanol 15.9 Not observed
Ethyl
hydroperoxide

51.9 Not observed

Dimethane
peroxide

57.7 Not observed

C3H6O Propanone 0.0 Observed [124,
125]

Propanal 8.1 Observed [126]
Propen-2-ol 13.8 Not observed
1-propen-1-ol 18.6 Not observed
Methoxy ethene 25.6 Not observed
2-propene-1-ol 27.1 Not observed
1,2-epoxypropane 30.0 Not observed
Cyclopropanol 31.2 Not observed
Oxetane 33.2 Not observed

C6H3N CH3(CC)2CN 0.0 Observed [127]
CH3(CC)2NC 25.3 Not observed

Table 12: Isomerization enthalpies for isomers with 11 atoms

Isomeric
group

Isomers Relative
∆fHO

(kcal/mol)

Astronomical
status

Reference

C3H6O2 Propanoic acid 0.0 Not observed
Ethylformate 11.8 Observed [128]
Methyl acetate 14.3 Observed [129]
Lactaldehyde 28.1 Not observed
Dioxolane 36.1 Not observed
Glycidol 52.1 Not observed
Dimethyldioxirane 81.7 Not observed

C9HN HC9N 0.0 Observed [130]
HC8NC 27.2 Not observed

Figure 8: Optimized geometries of CNC and CCN at MP2(full)/6-311++G**
level.

of the cyclic molecules with 9 atoms. Among the species
shown in Table 10, only ethanol and dimethyl ether from the
C2H6O group have been observed in more than one isomeric
form with a relative enthalpy of formation of 12.6 kcal/mol.
CH3SCH3, the S-analogue of dimethyl ether remains a poten-
tial candidate for astronomical detection for many reasons; the
well-established chemistry of S and O-containing interstellar
molecules, the high abundance of CH3CH2SH (the most stable
isomer of the group which can isomerize to CH3SCH3) and the
low relative enthalpy of formation (1.4 kcal/mol) as compared
to that of dimethyl ether (12.6 kcal/mol). Assuming accurate
spectroscopic parameters are available, interstellar CH3SCH3

10



E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 11

Table 13: Isomerization enthalpies for isomers with 12 atoms

Isomeric
group

Isomers Relative ∆fHO

(kcal/mol)
Astronomical
status

Reference

C6H6 Benzene 0.0 Detected [137]
Fulvene 34.2 Not

detected
3,4-
dimethylenecyclopropene

62.0 Not
detected

1,5-hexadiene-3-yne 62.3 Not
detected

2,4-hexadiyne 65.2 Not
detected

1,2,3,4-hexateraene 71.7 Not
detected

1,3-hexadiyne 73.2 Not
detected

Trimethylenecyclopropane 79.8 Not
detected

1,4-hexadiyne 82.3 Not
detected

1,5-hexadiyne 85.8 Not
detected

C3H8O Propan-2-ol 0.0 Not
observed

Propanol 3.7 Not
observed

Ethyl methyl ether 8.2 Observed [139]
C4H7N Isopropyl cyanide 0.0 Observed [138]

Propyl cyanide 0.4 Observed [140]
2-isocyanopropane 18.2 Not

observed
2-aminobutadiene 18.2 Not

observed
3-pyrroline 20.2 Not

observed
2,2-dimethylethylenimine 20.5 Not

observed
But-1-en-1-imine 22.6 Not

observed
2,3-butadiene-1-amine 38.1 Not

observed
N-vinylazaridine 40.1 Not

observed
N-methyl-1-propyn-1-
amine

41.4 Not
observed

3-butyn-1-amine 43.2 Not
observed

N-methyl propargylamine 49.3 Not
observed

2-
azabicyclo(2.1.0)pentane

51.9 Not
observed

Table 14: Relative energies of CNC and CCN

Method
Relative energy (kcal/mol)

CNC CCN
CCSD/6-
311++G**

0.0 1.5

MP2(full)/6-
311++G**

0.0 6.2

B3LYP/6-
311++G**

0.0 2.0

G4 0.0 3.7

will soon become a reality.

Isomerization remains one of the plausible formation routes

Table 15: Relative energies of linear and cyclic stable isomers of C5H

Method
Relative energy (kcal/mol) Dipole moment (Debye)
c-C5H l-C5H c-C5H l-C5H

MP2(full)/6-
311++G**

0.0 18.7 3.4 4.2

MP2/aug-cc-
pVTZ

0.0 17.5 3.4 4.2

G4 10.9 0.0 3.7 4.6
CCSD/6-
311++G**

3.0 0.0 3.4 4.6

B3LYP/6-
311++G**

10.4 0.0 3.7 5.2

B3LYP/aug-cc-
pVTZ

10.3 0.0 3.7 5.2

for the formation of the less stable isomers of the different
groups here whose most stable isomers have been detected.
The isomerization enthalpies for the two cyanide/isocyanide
pairs range from 20.8 to 27.1 kcal/mol. Though none of these
pairs has been detected, the possibility of their formation
from their corresponding cyanides (via isomerization) which
could lead to their successful detection cannot be ruled out.
HC6NC is the highest member of the HC2nNC linear chains
with experimentally measured rotational transitions that can be
used for its astronomical search [121]. However, a recent study
using a combined experimental and theoretical approach has
provided accurate rotational constants for higher members of
the HC2nNC linear chains [6]. HNC remains the only member
of this series that has been detected in ISM [122].

3.8. Isomers with 10 atoms
Isomerization energies and astronomical statuses for 16

molecular species from three isomeric groups comprising 10
atoms each are presented in Table 11. Of these three groups,
only the C3H6O group contains isomers (propanone and
propanal) that have been detected in more than one isomeric
form with a relative enthalpy of formation of 8.1 kcal/mol
[123, 124, 125, 126, 127]. CH3(CC)2NC is the isocyanide ana-
logue of CH3(CC)2CN that has been detected. It has a relative
enthalpy of formation of 25.3 kcal/mol which is within the
range of the known cyanide/isocyanide pairs. However, there
is little or no information regarding the rotational spectrum
of CH3(CC)2NC that could warrant its astronomical search.
Thus, the availability of accurate spectroscopic parameters for
this species remains the starting point in the steps toward its
astronomical detection.

1,1-ethanediol; the most stable isomer of the C2H6O2 group
has not been detected whereas ethylene glycol, the next sta-
ble isomer of the group with an isomerization enthalpy of
6.1kcal/mol has been detected in good abundance. The delayed
astronomical detection of 1,1-ethanediol is directly linked to
a lack of spectroscopic parameters for this molecule. The
rotational spectrum of 1,1-ethanediol is yet to be probed
either experimentally or theoretically. Once this is done, this
molecule could be successfully observed.

11



E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 12

3.9. Isomers with 11 atoms
There are currently seven known interstellar molecules

(HC9N, CH3C6H, ethyl formate, methyl acetate, hydroxyace-
tone, cyclopentadiene and ethanolamine) containing 11 atoms
[128]-[134]. In Table 12, the 9 isomeric species from two
groups containing 11 atoms are shown with their isomeriza-
tion enthalpies and current astronomical status. For the known
isomers in the C3H6O2 group, the isomerization energy ranges
from 11.8 to 14.3 kcal/mol. The non-detection or delayed de-
tection of propanoic acid (the most stable isomer of the group)
has been traced to the effect of interstellar hydrogen bonding on
the surface of the dust grains which reduces the overall abun-
dance of this species in the gas phase, thus, influencing its suc-
cessful astronomical observation. HC9N is the second largest
member of the cyanopolyyne chain that has been detected in
ISM. As seen in other cyanide/isocyanide pairs, the isomeriza-
tion of HC9N to HC8NC with the relative enthalpy of forma-
tion of 27.2 is achievable. An accurate rotational constant for
HC8NC is now available from a recent study [6, 17]. Thus,
HC8NC could be astronomically searched.

3.10. Isomers with 12 atoms
Only very few cyclic molecules are known among the

interstellar molecular species [17, 20, 21]. From the different
isomeric groups considered in this study, it is clear that the
cyclic isomers are found to be among the less stable isomers
in their respective groups as compared to their corresponding
straight-chain analogues. This low stability could affect
their interstellar abundance and influences their astronom-
ical observation. The C6H6 isomeric group is one of the
few groups where the cyclic molecules are found to be the
most stable species. Apart from cyanocyclopentadiene and
2-cyanocyclopentadiene that have been recently observed
in the interstellar medium [135, 136], the other four known
interstellar molecules with 12 atoms, [137, 138, 139, 140]
their corresponding isomers, and isomerization enthalpies are
presented in Table 13.

The only known branched-chain interstellar molecule; iso-
propyl cyanide is almost equivalent in energy to its linear chain
analogue; propyl cyanide with a relative enthalpy of formation
of 0.4kcal/mol. Propan-2-ol and propanol (the two most sta-
ble isomers of the C3H6O group) are yet to be astronomically
observed. These species have also been shown to be strongly
affected by interstellar hydrogen bonding as compared to ethyl
methyl ether that has been detected [6, 18, 19, 20, 21]. The sta-
bility of ethyl methyl ether is also enhanced by the stabilizing
effect of the two alkyl substituents while propanol and propan-
2-ol have only one alkyl substituent each.

3.11. Potential Interstellar Molecules
Knowing the right candidates for astronomical searches

is vital in reducing the number of unsuccessful astronomical
searches considering the time, energy and resources involved in
these projects. From the present study, few molecular species
have been identified as potential interstellar molecules. These
are briefly summarized below:

Isocyanomethylidyne, CNC:
This is the isocyanide analogue of C2N that has been de-

tected [141]. CNC is found to be more stable than its cyanide
analogue. Table 14 contains the relative energies of CNC and
CCN from different quantum chemical calculation methods
while Figure 8 shows the optimized geometries of these iso-
mers at the MP2(full)/6-311++G** level with the bond angle
in degrees and bond distance in angstroms. As discussed under
the isomers with 3 atoms, all the observed species are the most
stable ones and these stable species are also found to be more
abundant. Where both species have been detected, the most
stable is reported to present in high abundance than the less
stable. For instance, HCN is found to be more abundant than
HNC in different molecular clouds and this is also the case for
the MgNC/MgCN abundance ratio measured in the asymptotic
giant branch (AGM) stars [142, 143, 144, 145]. CNC is more
stable than CCN which implies that CNC should be present in
high abundance in ISM than CCN that has been detected. Thus,
CNC remains a potential candidate for astronomical detection.
CNC is microwave inactive with a zero-dipole moment; thus,
infrared astronomy remains the best approach for its astronom-
ical observation.

NCCN:
The rationale for the choice of NCCN as a potential in-

terstellar molecule is well discussed under the isomers with 4
atoms.

c-C5H:
With the limited number of known cyclic interstellar

molecules, it is always exciting finding rings that are as stable
as their corresponding chains. c-C5H (Figure 2a) is the cyclic
analogue of C5H that has been detected. From Table 7, the rel-
ative enthalpy of formation between the linear chain and this
cyclic analogue is just 1 kcal/mol. Table 15 shows the relative
energies of these species at different quantum chemical calcula-
tion methods. While the MP2 method predicts the cyclic isomer
to be more stable than the linear, others methods predict the re-
verse but the magnitude of the difference in energy is much at
the MP2 level as compared to other methods. Figure 9 displays
the optimized structures of these species. As shown in Table
15, c-C5H is microwave active with a very good dipole moment
making its astronomical searches in the radio frequency possi-
ble. If the spectroscopic parameters of c-C5H can be accurately
probed, either experimentally or theoretically, the possibility of
its astronomical observation is very high.

Figure 9: Optimized structures of C5H most stable isomers.

Methylene and Methyl Ketenes:
As highlighted in the text, the ketenes are found to be

more stable than their corresponding isomers in all the isomeric
12



E. E. Etim / J. Nig. Soc. Phys. Sci. 5 (2023) 527 13

groups examined in this study (Tables 6, 7 and 9) and as such be
present in detectable amounts in ISM. Ketenes have also been
shown to be less affected by hydrogen bonding on the surface
of the interstellar dust grains as compared to their correspond-
ing isomers. Also, the fact that ketene (H2C2O), ketenyl radical
(HC2O) and isomers of ketenes (like propynal, cyclopropenone,
and propenal) are known interstellar molecular species further
supports the presence and detectability of methyl ketene and
methylene ketene in ISM. Thus, methyl ketene and methylene
kenetne remain potential interstellar molecules pending their
astronomical searches. No unsuccessful astronomical search
has been reported for any of these ketenes.

C5O:
This is the oxygen analogue of C5S which is a known

interstellar molecule. Without any exception, an interstellar
O-containing molecular species is more abundant than its S-
analogue. This could simply be traced to the cosmic abundance
of O and S. Thus, for every known S-species, the O-analogue is
not only present in detectable abundance, but it can also be said
to have even been overdue for astronomical detection because
for sure the O-species are more abundant than their S-analogue
and as such could be detected with less difficulty as compared to
its S-analogue. The column density of 2.14x 1012cm-2 reported
for C5S suggests the- high abundance of C5O in ISM [62, 85].
The microwave spectrum of C5O that could guide its successful
astronomical observation is available [146]. Interstellar C5O is
just a matter of time.

CH3SCH3:
This is the S-analogue of dimethyl ether; a known interstel-

lar molecule. The interstellar chemistry of S- and O-containing
species is well established. Every known O-containing inter-
stellar molecule points to the presence and detectability of the
S-analogue. As earlier mentioned, except where the effect of
interstellar hydrogen bonding dominates, the ratio of an inter-
stellar sulphur molecule to its oxygen analogue is close to the
cosmic S/O ratio. Assuming dimethyl ether is formed from
ethanol via isomerization with a barrier of 12.6 kcal/mol, then
the isomerization of C2H5SH to CH3SCH3 is a much more fea-
sible process with a relative enthalpy of formation of just 1.4
kcal/mol (nine times lower than that of CH3OCH3) compared
to that of the O-analogue. Thus, because of the unique chem-
istry of S- and O-containing interstellar molecules and the low
relative enthalpy of formation, CH3SCH3 is considered a poten-
tial candidate for astronomical observation assuming its spec-
troscopic parameters are accurately known.

1,1-Ethanediol, Propanoic Acid, Propan-2-ol, and Propanol:
These molecules are found to be the most stable isomers in

their respective groups (Tables 11, 12 and 13). Whereas their
less stable isomers have been astronomically detected; success-
ful detection of these most stable isomers is highly feasible.
The spectroscopic parameters for propanoic acid, propan-2-ol,
and propanol are known but those of 1,1-ethanediol are yet
to come by. Though the delayed astronomical observations
of some of these species have been linked to the effect of

interstellar hydrogen bonding; there are high chances for their
successful astronomical detection.

4. Conclusion

An extensive investigation of the isomerization energies
of 246 molecular species from 65 isomeric groups is reported
in this study. From the results, isomerization is found to be one
of the plausible mechanisms for the formation of molecules in
ISM. The high abundances of the most stable isomers coupled
with the energy sources in ISM drive the isomerization pro-
cess even for barriers as high as 67.4 kcal/mol. However, the
chemical reaction pathways can also influence the final abun-
dance of any species in the ISM. Specifically, the isomerization
process is found to be very effective in converting cyanides to
their corresponding isocyanides and vice versa. Thus, for every
cyanide or isocyanide, the corresponding isocyanide or cyanide
could be synthesized via isomerization. Also, from the results,
the following potential interstellar molecules; CNC, NCCN, c-
C5H, methylene ketene, methyl Ketene, CH3SCH3, C5O, 1,1-
ethanediol, propanoic acid, propan-2-ol, and propanol are high-
lighted and discussed.

Acknowledgement

EEE acknowledges a research fellowship from the Indian
Institute of Science, Bangalore, and fruitful discussions with
Prof. E. Arunan of the same Institute. The visiting position
from the Indian Institute of Technology Bombay where the final
part of this work was done is also acknowledged.

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