BIBECHANA Vol. 21, No. 1, April 2024, 23–36 ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher:Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University)Biratnagar An investigation of vibrational analysis, thermodynamics properties and electronic properties of Formaldehyde and its stretch by substituent acetone, acetyl chloride and methyl acetate using first principles analysis Susmita Limbu, Tulsi Ojha, Rishi Ram Ghimire, Krishna Bahadur Rai∗ Department of Physics, Patan Multiple Campus,Tribhuvan University, Nepal ∗Corresponding author. Email: krishna.rai@pmc.tu.edu.np Abstract This study finds the equilibrium configuration, vibrational analysis, thermodynamic properties, and electronic properties of formaldehyde and its derivatives, namely acetone, acetyl chloride, and methyl acetate, using First Principles Analysis. It emphasizes the impact of substituents on the carbonyl group and the need for this comprehensive analysis. The computational meth- ods employed in this work are Gaussian DFT, GaussSum and Moltran calculations. For formaldehyde, the optimization step starts from the energy of -114.51282 Hartree and gets optimized in the energy -114.5129634 Hartree. Similarly for Acetone, Acetyl chloride and Methyl acetate, optimization step start from -193.74375 Hartree, -613.26 Hartree, -266.805 Hartree and gets optimized in the energy -193.1744033 Hartree, -613.2941798 Hartree, - 266.8358066 Hartree respectively. Infrared (IR) spectroscopy is used to analyze vibrational frequencies. The C-H and C=O vibrations are highlighted, showing characteristic peaks for each compound. Heat capacity at constant volume, heat capacity at constant pressure, internal energy, enthalpy, entropy and Gibb’s free energy change with change in temperature. Among the derivative of Formaldehyde, Methyl Acetate has the highest energy gap (i.e. 7.4222 eV) and Acetyl Chloride has the small energy gap (i.e. 5.6137 eV). The Chemical parameters like ionization potential, electron affinity, chemical hardness, chemical potential, electronegativity, electrophilicity index, and chemical softness have been also calculated. Electrostatic Potential Surfaces, Molecular Electrostatic Potential, and Electron Density are visualized to understand charge distribution and reactivity regions. Density of State spectra illustrate the density of electron states per unit energy. Keywords Density Functional Theory, Electrostatic Potential, HOMO-LUMO, IR spectroscopy, Thermody- namic properties, Density of State. Article information Manuscript received: September 20, 2023; Revised: December 10, 2023; Accepted: December 16, 2023 DOI https://doi.org/10.3126/bibechana.v21i1.58684 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons. org/licenses/by-nc/4.0/ 23 http://nepjol.info/index.php/BIBECHANA krishna.rai@pmc.tu.edu.np https://doi.org/10.3126/bibechana.v21i1.58684 https://creativecommons.org/licenses/by-nc/4.0/ https://creativecommons.org/licenses/by-nc/4.0/ Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 24 1 Introduction Formaldehyde is the first of the series of aliphatic aldehyde. It was discovered by Butlerov in 1859 and has been manufactured since the beginning of the twentieth century. At ordinary temperature Formaldehyde is colorless gas with pungent odor. Because of its relatively low cost, high purity and variety of chemical reactions, Formaldehyde has be- come one of the world’s most important industrial and research chemical. Its biggest use is in the production of industrial resins, urea-formaldehyde, phenol formaldehyde and melamine form aldehyde [1–3]. Formaldehyde (HCHO) is a simple organic compound consisting of a carbon atom bonded to two hydrogen atoms with single bond and an oxy- gen atom with the carbon atom in the center, form- ing a carbonyl group (C=O). When the Formalde- hyde has its stretch by substituent acetone, acetyl chloride, and methyl acetate, the number of ef- fects on the vibrations, electronic and thermody- namic properties appear on the carbonyl group (C=O). Acetone (CH3COCH3) is colorless highly volatile and flammable liquid with pungent ordor. Acetyl Chloride (CH3COCl) is a reactive and cor- rosive. Methyl Acetate (CH3COOCH3) also known as acetic acid methyl ester or Methyl ethanoate. It is colorless, flammable liquid with a fruity or- dor. It is used in Manufacture of flavors, fragrances and pharmaceuticals [4–6]. The presence of the methyl (CH3) groups on both sides of the carbonyl group forms the Acetone. Acetyl chloride is an acyl chloride, which contains a carbonyl group at- tached to a chlorine atom. Methyl acetate is an ester, which contains a carbonyl group bonded to an oxygen atom. Gustav E. Lienhard and William P. Jencks investigate the kinetics and mechanisms of thiol addition to carbonyl compounds, form- ing hemithioacetals and hemithioketals [7]. Car- bonyl compounds are commonly found both out- doors and indoors. Their polar nature, stemming from the oxygen atom’s high electronegativity, en- ables a wide range of chemical reactions. Moreover, the presence of substituents and conjugated dou- ble bonds further shapes their physical and chem- ical characteristics [8]. The substituent additivity effects in adiabatic ionization energies imply that cation molecular orbitals are not directly related to those of the neutral molecule [9]. Laurence et al., in- vestigated the impact of substituents on the basicity of carbonyl compounds through various measure- ments including enthalpy changes in iodine complex formation, solvent sensitivity and carbonyl stretch- ing vibrations. The enthalpic basicity exhibited a strong correlation with protonation scales. The cu- mulative impact of substituents showed additive ef- fects for enthalpy. The study concluded that induc- tive effects predominantly influence the basicity of carbonyl groups towards iodine [10]. An investigation that has not been performed previously regarding the equilibrium configuration, vibrational analysis, thermodynamics properties, and electronic properties of formaldehyde and its stretch by substituents acetone, acetyl chloride, and methyl acetate using First Principles Analy- sis is now essential and important for further study. Therefore, this study aims to gain a deep under- standing of the molecular behavior and the effects of substituents on the geometry optimization, vi- brational analysis, thermodynamic properties [spe- cific heat capacity at constant volume (Cv), specific heat capacity at constant pressure (Cp), internal energy (U), enthalpy (H), entropy (S) and Gibb’s free energy (G) with change in temperature], high- est occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) analysis, Global reactivity descriptor, Electrostatic potential (ESP) surfaces, Molecular Electrostatic Potential (MEP), Electron Density (ED) and density of states (DOS) of formaldehyde and its derivatives, namely acetone, acetyl chloride, and methyl acetate using Density Functional Theory (DFT) with B3LYP/6- 311+G(d ′ ,p ′ ) basis set together with GaussSum 3.0.2 and Moltran. DFT method is used due to their ability to accurately describe molecular structures, vibrational spectra and other differ- ent specific characteristics and properties of title molecules. B3LYP is hybrid functional mostly used in DFT and has the exchange correlation energy function in DFT that incorporate a portion of exact exchange from Hartee-Fock theory with the rest of the exchange correlation energy from other sources. It provides accuracy for a wide range of molec- ular system. 6-311+G(d ′ ,p ′ ) is a basis set rela- tively large and includes polarization and diffuse function. It allows more accurate representation of electron density around atoms specially for the sys- tem with heavy atoms. It is the standard, split valence triple zeta basis set. Its function describes the core and valence orbitals. The core orbital is a contracted Gaussian type orbital made of six Gaus- sians and the valence orbital is described by two or- bitals, one contracted Gaussian type orbital made of three Gaussians and two Gaussian type orbitals. In this study, 6-311G(d ′ ,p ′ ) is 6-311G basis set with added ’d’ polarization functions on non-hydrogen atoms and ’p’ polrizations for hydrogen. Thus, it also describes a diffuse s and p functions for non- hydrogen atoms. Triple zeta has much finer result than that of double zeta basis sets and is more ac- curate. Sometimes, Convergence of ab initio results is disappointingly slow with respect to basis set for non-DFT methods. DFT is less dependent on basis set size than wave function based method. Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 25 2 Methodology – Computational details Geometric Optimization is an important in molec- ular mechanics. It was performed as a first task of the computational work for the formaldehyde and its substitute (Acetone, Acetyle chloride, Methyl Acetate). The optimized geometry, vibrational fre- quency, HOMO-LUMO analysis and Global reac- tivity descriptor of given molecules were computed employing the Density Functional Theory (DFT) method using Gaussian 09W software program [11] employing 6-311+G(d ′ ,p ′ ) basis set based on Becke’s three parameter (local, non-local, Hartree- Fock) hybrid exchange functional with Lee- Yang pair correlation functional B3LYP [12,13] at charge value zero. Gaussian calculation is the best pre- pared using Gauss View 6.0 interface. Gaussian 09W is a general-purpose ab initio electronic struc- ture package which can predict many properties of molecule and reaction pathways based on various uncorrelated and correlated wave function. After processing Gaussian, the .log file and .chk file ob- tained from Gaussian used to generate various sur- face representation and to observe various result [14]. The basis set used here is the 6-311+G(d ′ ,p ′ ) augmented by ‘d’ polarization functions on heavy atom and ‘p’ polarization functions on hydrogen atom as well as diffuse function for both hydrogen and heavy atoms [15]. Thermodynamic parame- ter such as heat capacity at constant volume (Cv), heat capacity at constant pressure (Cp), entropy (S), internal energy (U), enthalpy (H), entrophy (S) and Gibb’s free energy (G) were calculated using Moltran software. In addition to this, GaussSum 3.0.2 was used to calculate the density of states (DOS). 3 Results and Discussion 3.1 Geometry Optimization Geometry Optimization is a method to predict the three-dimensional arrangement of the atoms in a molecules with minimum energy [16]. It is the first task to do while performing the computational work for the given molecules. Figures 1(a) (b) (c) and (d) show the optimized structure, symbol and num- bering of atoms of Formaldehyde, Acetone, Acetyl Chloride, Methyl Acetate. Listed Figures 1(a) (b) (c) and (d) are from the DFT calculation and they have their respective total energy versus op- timization step number in optimization state. For formaldehyde, the optimization step starts from the energy of -114.51282 Hartree and gets optimized in the energy -114.5129634 Hartree. Similarly for Ace- tone, Acetyl chloride and Methyl acetate, optimiza- tion step start from -193.74375 Hartree, -613.26 Hartree, -266.805 Hartree and gets optimized in the energy -193.1744033 Hartree, -613.2941798 Hartree, -266.8358066 Hartree respectively [17]. 3.2 Vibrational analysis IR spectroscopy or vibrational spectroscopy is a tool for the measurement of interaction of infrared radiation with matter by absorption, emission or reflection. Basic principle of it states molecules ab- sorb specific frequencies that are characteristics of their structure. The interaction of the matter in in- frared radiation signifies the IR spectroscopy and its spectrum shows the relationship between frequency and wave length of the infrared light (transmittance or absorption) [18]. The greater the frequency of os- cillation of chemical bond, greater is the wavenum- ber. By using IR spectrum, it is distinguished that which chemical bond and possible functional group oscillates with how much of specific frequency that corresponds to certain amount of energy. C-H vibrations Most organic molecules contain alkane residues and their general appearance can be seen in the in- frared spectrum. Figure 2 is the IR spectrum of Formaldehyde and it reveals the presence of H-C- H in-plane scissoring stretching at 2910.1248 cm−1, C-H asymmetric stretching at 1257.5142 cm−1, C- H symmetric stretching at 1528.7213 cm−1, H-C- H out-of-plane wagging at 1190.7995 cm−1. Fig- ure 3 is the IR spectrum for Acetone having C- H asymmetric stretching at 3161.0988 cm−1, H-C- H assymetric stretching at 3105.5663 cm−1, C-H symmetric stretching at 3039.159 cm−1, C-H in- plane scissoring at 1382.2427 cm−1 and C-C-C-H in plane rocking at 1234.2899 cm−1. Figure 4 is the IR spectrum of Acetyl Chloride, having C-H asym- metric stretching at 3331.3231 cm−1 and 2201.8905 cm−1, C-C-H in-plane rocking at 282.2516 cm−1. Figure 5 is the IR spectrum of Methyl Acetate hav- ing C-H asymmetric stretching at 3179.0767 cm−1, 3122.3869 cm−1, 3041.8403 cm−1, H-C-H in-plane scissoring at 1393.9274 cm−1, H-C-H out-of-plane wagging at 999.1881 cm−1. C=O vibrations The carbon-oxygen double bond formed by π − π between carbon and oxygen, and the lone pair of electrons on oxygen also determines the nature of carbonyl group. The C=O stretching is a character- istic wavenumber of carboxylic acid. The carbonyl C=O stretching vibrational mode is expected to oc- cur in the region 1680–1715 cm−1. Figure 2 is the IR spectrum of Formaldehyde and it shows C=O stretching at 1816.9839 cm−1. Figure 3 is the IR spectrum for Acetone and it has C=O stretching at 1789.4345 cm−1. In figure 4, spectrum for C=O stretching at 1747.0313 cm−1 is obtained for Acetyl Chloride. Figure 5 is the IR spectrum of Methyl Ac- etate showing C=0 stretching at 1821.1828 cm−1. Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 26 C-O-C vibration C-C-O-C in-plane scissoring is observed at 788.341 cm−1 and C-O-C stretching at 1089.4461 cm−1 for Methyl Acetate is as shown in figure 5. C-Cl vibrations Figure 4 shows C-Cl stretching of Acetyl Chloride molecule at 483.3359 cm−1. Therefore, IR spectroscopy is used for the de- tailed discussion of structural changes in the given molecule. Structural changes affect their vibra- tional spectra and provide information about the vibrational modes, vibrational frequencies and in- tensities of these modes. In a molecule, atoms are connected by chemical bonds, and these bonds have associated vibrational modes. The vibrational fre- quencies depend on factors such as bond strength, bond length, and mass of the atoms involved. When the structure of the molecule changes, the vibra- tional modes are influenced due to bond stretch- ing, bending, or the introduction of new functional groups. By analyzing the vibrational spectra of a molecule, we can gain insights into its structure and chemical environment. Changes in vibrational fre- quencies due to structural modifications can impact the entropy term in the Gibbs free energy equation (∆G = ∆H - T∆S) and heat capacity of the sys- tem. Changes in bond strengths due to alterations in molecular structure affect the enthalpy of a re- action. The enthalpy change (∆H) is composed of contributions from bond energies and vibrational energies. Stronger or weaker bonds, as indicated by shifts in vibrational frequencies, influence the enthalpy aspects of a chemical reaction. Thermody- namic properties are also connected to the equilib- rium constants of reactions. Alterations in molec- ular structure, reflected in vibrational spectra, can affect the position of equilibrium by influencing the enthalpy and entropy changes. Figure 1: Optimized structure of (a) Formaldehyde (b) Acetone (c) Acetyl Chloride and (d) Methyl Acetate. Figure 2: IR spectroscopy for neutral Formaldehyde molecule by DFT method. Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 27 Figure 3: IR spectroscopy for neutral Acetone molecule by DFT method. Figure 4: IR spectroscopy for neutral Acetyl Chloride molecule by DFT method. Figure 5: IR spectroscopy for neutral Methyl Acetate molecule by DFT method. Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 28 3.3 Thermodynamic analysis The Formaldehyde, Acetone, Acetyl chloride and Methyl acetate molecules taken for this thermo- dynamic properties analysis are based on the out- put file of DFT calculation with B3LYP/6-311+G (d ′ ,p ′ ) basis set. Moltran software was em- ployed in computing thermodynamic parameters of Formaldehyde, Acetone, Acetyl chloride, Methyl ac- etate. On the basis of vibrational analysis, the thermodynamic parameter such as entropy (S), en- thalpy (H), Gibbs free energy (G), internal energy (U), heat capacity at constant volume (Cv), heat capacity at constant pressure (Cp) were obtained. While performing DFT calculations the molecules was considered to be at room temperature of 298.15 K and one atmospheric pressure. Figures 6, 7, 8 and 9 show the different values of thermodynamic pa- rameters for Formaldehyde, Acetone, Acetyl chlo- ride, Methyl Acetate respectively and these depict the correlation of heat capacity at constant volume (Cv), heat capacity at constant pressure (Cp), in- ternal energy (U), enthalpy (H), entropy (S) and Gibb’s free energy (G) with temperature. All the entropies (S) for all the given molecules sharply in- crease from 10 K to 50 K and decreases the in- crease in rate from 50 K to 500 K. Parameters such as Cv, Cp, U and H except G increase relentlessly with temperature going from 10 K to 500 K. These increase in thermodynamic parameters are due to enhancement of the molecular vibration intensities with increment in temperature. However, G in all the Figures 6, 7, 8, and 9 decrease with increase in temperature. It is because of the G depends upon the S and as the S increases G decreases [19]. S changed shows that the atom has greater adapt- ability of changing its own thermodynamic system regarding the temperature [20]. The G represents the energy that is free to do useful work for spon- taneous process. The S and H changes revealed that the tittle compound possess more flexibility of changing its own thermodynamic system with re- spect to the temperature. Figure 6: Correlation plot of the thermodynamic properties at different temperature of the Formalde- hyde. Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 29 Figure 7: Correlation plot of the thermodynamic properties at different temperature of the Acetone. Figure 8: Correlation plot of the thermodynamic properties at different temperature of Acetyl Chloride. Figure 9: Correlation plot of the thermodynamic properties at different temperature of the Methyl Acetate. Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 30 3.4 HOMO-LUMO analysis The electronic transition corresponds from HOMO (characterizes of electron giving) to the LUMO (characterizes of the electron accepting) orbitals. Both the frontier molecular orbitals (FMOS) (i.e. HOMO and LUMO) are the main orbitals taking part in the chemical reaction [21]. High value of HOMO energy is likely to indicate a tendency of the molecule to donate electrons to appropriate ac- ceptor molecule of low empty molecular orbital en- ergy. The lower value of the LUMO energy shows more probability to accept electrons. The energy of the HOMO is directly related to the ionization potential and the energy of the LUMO is directly related to the electron affinity. So, the energy gap difference between the HOMO and LUMO is an important stability index. A higher value of the energy difference is the stability of the molecu- lar system. Low gap value refers the higher elec- tron transition and vice versa. A large HOMO- LUMO gap implies high stability for the molecule in the sense of its lower reactivity in chemical re- action [22, 23]. The electronic absorption relates to the transition from the ground to the first ex- cited state and mainly described by one electron excitation from HOMO to LUMO [24]. Figures 10(a), (b), (c) and (d) show the three-dimensional plot of the HOMO–LUMO of Formaldehyde, Ace- tone, Acetyl Chloride and Methyl Acetate molecules using the DFT with B3LYP/6-311+G(d ′ ,p ′ ) basis set. In the Figure 10(a), it is observed that -7.6708 eV of energy is required to free an electron from formaldehyde molecule and -1.7265 eV amount of energy is released while attaching an electron to neutral atom of formaldehyde. The energy gap be- tween HOMO and LUMO for this Formaldehyde from Figure 10(a) is found to be -5.9443 eV. Fig- ure 10(b) shows that ionization potential of Ace- tone is -7.0183 eV and electron affinity for Acetone is -0.7371 eV. The calculated energy difference be- tween HOMO and LUMO for Acetone from Fig- ure 10(b) is found to be -6.2812 eV. Similarly, from Figure 10(c), it is observed that -7.0882 eV amount of energy is necessary to release an electron from the highest occupied orbital of Acetyl Chloride and -1.4745 eV energy must release to attach an elec- tron to the lowest occupied molecular orbital of Acetone. From this Figure 10(c), it shows the cal- culated energy gap of HOMO and LUMO for this Acetyl Chloride compound is -5.6137 eV. Also, Fig- ure 10(d) implies that -7.6379 eV amount of en- ergy is required to free an electron from HOMO of Methyl acetate and -0.2157 eV energy is released while attaching an electron to the LUMO of Methyl acetate. This Methyl Acetate from Figure 10(d) shows the HOMO-LUMO gap -7.4222 eV. In addi- tion to that, it is observed that Methyl acetate has the highest stability implying lower reactivity in the chemical reactions in the sense of its higher energy gap of 7.4222 eV as compared to that of our mother compound formaldehyde (5.9443 eV). On the other hand, Acetyl Chloride molecule is closely stable as Formaldehyde and Acetone is second highest stable compound among the given molecules. Kinetic sta- bility increases with the increase of HOMO-LUMO gap. As a result, removal of electrons from ground state HOMO to excited state LUMO requires more energy. From the above energy gap, it is clear that the molecule (Formaldehyde, Acetone, Acetyl chloride, Methyl Acetate) under investigation is very soft since it has a small HOMO–LUMO gap. Global reactivity descriptor This study introduces a range of chemical reactivity descriptors aimed at enhancing our comprehension of the properties of Formaldehyde, Acetone, Acetyl Chloride, and Methyl Acetate molecules. The ion- ization potential (IP) and electron affinity (EA) of these compounds were estimated utilizing the cal- culated HOMO and LUMO energies. Additionally, parameters such as chemical hardness (η), chemi- cal potential (µ), electronegativity (χ), global elec- trophilicity index (ω) and chemical softness (S) for these molecules have been computed and are pre- sented in Table 1. By using HOMO and LUMO energy values for a molecule, we can calculate the Global reactivity descriptor such as hardness (η), chemical potential (µ), electronegativity (χ), elec- trophilicity index (ω) and softness (S) using Koop- man’s theorem for closed-shell molecules [25–27]. The equations (1)–(5) provide the necessary for- mulas for computing the chemical hardness (η), chemical potential (µ), electronegativity (χ), global electrophilicity index (ω) and chemical softness (S), and they are expressed as follows: The hardness of the moelcule (η) can be expressed as η = (I −A)/2 (1) The chemical potential (µ) of the molecule is the negative of hardness µ = −(I +A)/2 (2) The electronegativity (χ) of the molecule is given by the equation χ = (I +A)/2 (3) The electrophilicity index ()ω of the molecule is ω = µ2/2η (4) The Softness of the molecule is the reciprocal of hardness S = 1/η (5) where I = - EHOMO called ionization potential and A = - ELUMO called electron affinity. The ion- ization potential (I) and an electron affinity (A) of Formaldeyde, Acetone, Acetyl chloride, Methyl Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 31 acetate are -7.6708 eV, -7.0183 eV, -7.0882 eV, - 7.6379 eV and -1.72656 eV, -0.73715 eV, -1.47458 eV, -0.215786 eV respectively. Usually, the com- pound having higher HOMO-LUMO gap are com- paratively harder [28]. The Acetone molecule shows the most hardness (3.140605 eV) in its neutral state. Similarly, it has the most chemical softness (0.318409 (eV)−1) in its neutral state. In Table 1, chemical potential is expressed in negative values. Chemical potential measures the potential energy of the substance. A more negative chemical potential suggests a molecule that is more prone to react or release energy. In this context, Acetone has the most negative chemical potential, implying a high reactivity. Methyl acetate has a less negative chemical potential, indicating lower reac- tivity. Electronegativity is a measure of the ability of an atom to attract electrons in a chemical bond. From Table 1, Formaldehyde has the highest elec- tronegativity value (4.698725 eV) indicating it has a strong tendency to attract electrons so it is reactive. For Methyl Acetate, it has the lowest electronega- tivity (0.489788 eV) suggesting it is less inclined to attract electrons. Normally, strong electrophiles have a value greater than 1.5 eV, moderate elec- trophiles have a value between 0.8 eV and 1.5 eV, and marginal electrophiles have a value less than 0.8 eV [29]. Table 1 shows that the Acetone molecule has strong electrophiles in its neutral state with val- ues of 1.570297 eV and the methyl acetate has the least electrophiles with the value 0.137800 eV in neutral state indicating marginal electrophiles. The moderate values are found in the Formaldehyde and Acetyl chloride in the neutral state with their val- ues 1.486082 and 1.403425 respectively. Chemical softness is the inverse of global hardness and rep- resents the ease with which a molecule can accept or donate electrons. A higher softness value indi- cates a molecule’s greater responsiveness to electron exchange. From Table 1, Methyl acetate has the highest chemical softness (3.649608 eV) indicating it can more readily engage in electron transfer pro- cesses, while Acetone has the lowest chemical soft- ness (0.318409 eV) suggesting it is less responsive to electron exchange. Reactivity descriptors and thermodynamic pa- rameters are fundamental concepts in understand- ing chemical reactions and it allows to predict and control reactions, optimize reaction conditions, and design new chemical processes. The global reactiv- ity descriptor associated with the electronic struc- ture principles is also used for analyzing structures, properties, reactivity, bonding, interactions, and dynamics in the context of various physicochemi- cal processes such as molecular vibrations, internal rotations, chemical reaction, stability of isomers, ion-atom collision, atom-filled interactions, solvent effect, etc. Also, global reactivity descriptors such as electronegativity, hardness has a correlation with change in dipole moment which is a measure of ionic character of bond. Figure 10: The atomic orbital compositions of the frontier molecular orbital (HOMO-LUMO) by DFT for (a) Formaldehyde (b) Acetone (c) Acetyl Chloride (d) Methyl Acetate. 3.5 Electrostatic potential (ESP) sur- faces, Molecular Electrostatic Poten- tial (MEP) and Electron Density (ED) ESP is used for predicting sites and relative reac- tivity towards electrophilic attack and for study- ing of biological recognition and hydrogen bond- ing interactions and MEP is used as a reactivity map displaying most probable regions for the elec- trophilic attack of charged point-like reagents on organic molecules [30]. It provides a visual method to understand the relative polarity of the molecule. One of the most important non-bonded interac- tions that can be studied with potential maps is the hydrogen bond. ED iso-surface mapped with ESP depicts the size, shape, charge density and site of chemical reactivity of the molecules. Fig- ure 11(a) shows the electrostatic potential (ESP) of the Formaldehyde that ranges from 9.963e-3 to 9.963e-3 a. u. while the Figure 11(b) for molecular electrostatic potential (MEP) i.e. mapped potential Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 32 ranges from -4.309e-2 to 4.309e-2 a. u. in its neu- tral state for the same molecule. Various colors on the surface correspond to distinct electrostatic po- tential values, which follow a progressive order of increase: from red to orange, then to yellow, green, and finally blue. The chemical reactivity of specific locations is dictated by the molecular electrostatic potential. In this context, the negative regions rep- resented by red and yellow, and are indicative of electrophilic reactivity, while the positive region de- picted in blue signifies nucleophilic reactivity [31]. Similarly, ESP of the Acetone ranges from 1.238e-2 to 1.238e-2 a. u. while the mapped MEP ranges from -5.283e-2 to 5.283e-2 a. u. as shown in figure 12(a) and (b). Figure 13(a) and (b) imply the po- tential of the acetyl chloride ranges from -1.367e-2 to 1.367e-2 and the mapped potential ranges from -9.480e-2 to 9.480e-2 a. u. in neutral state. Like- wise, Figure 14(a) and (b) are the electrostatic po- tential having the ranges from -1.603e-2 to 1.603e-2 and mapped potential with the range from -6.020e- 2 to 6.020e-2 a. u. of the Methyl acetate. On comparing both the ESP and MEP of each of the molecule, it is observed that oxygen atom is mostly tilting towards the negative potential and rest of the region is mostly feasible to the zone of positive potential. From all the Figures of 11(a) (b), 12(a) (b), 13(a) (b), and 14(a) (b), we can see that the highest electronegativity is located in the carboynl group. The MEP of each title molecule from Figure 11(b), 12(b), 13(b) and 14(b) clearly demonstrates the present of a hydrogen atom has the most posi- tive electrostatic potential which can be considered as possible sites for nucleophilic attack. For ESP of figure 11(a), 12(a), 13(a) and 14(a), the nega- tive potential is localized near Oxygen due to the more electrophilic attack (red and yellowish region), while the positive potential is localized on the rest surface indicating nucleophilic attack. Regarding the ED plot of the Formaldehyde, Acetone, Acetyl Chloride and Methyl Acetate from the Figure 11(c), 12(c), 13(c) and 14(c) using DFT method of calcu- lation, these show uniform charge distribution and the probability of an electron being present at the specific location. Table 1: Global Reactivity Descriptors of Formaldehyde, Acetone, Acetyl Chloride, and Methyl Acetate molecules in neutral state. Property Energy (eV) values of different molecules Formaldehyde Acetone Acetyl Chloride Methyl Acetate Global Hardness (η) 2.972165 3.140605 2.806855 0.274002 Chemical potential (µ) -2.972165 -3.14060 -2.806855 -0.274002 Electronegativity (χ) 4.698725 3.877755 4.281435 0.489788 Global Electrophilicity (ω) 1.486082 1.570297 1.403425 0.137800 Chemical Softness (S) 0.336455 0.318409 0.356270 3.649608 Figure 11: (a) ESP (b) MEP (c) ED by DFT for Formaldehyde. Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 33 Figure 12: (a) ESP (b) MEP (c) ED by DFT for Acetone. Figure 13: (a) ESP (b) MEP (c) ED by DFT for Acetyl Chloride. Figure 14: (a) ESP (b) MEP (c) ED by DFT for Methyl Acetate. 3.6 Density of state (DOS) The consideration of only HOMO and LUMO may not provide a realistic description of the frontier or- bitals, it is because of the neighboring orbitals may show quasi-degenerate energy levels in the bound- ary regions [31, 32]. For this reason DOS has been calculated using GaussSum 3.0.2 program. The most important application of the DOS plot is to demonstrate the MO composition and their contri- butions to the chemical bonding through positive and negative charges [33]. The plot reveals to deter- mine whether the interaction between two orbitals is of a bonding or anti-bonding nature. When the DOS exhibits a high intensity at specific energy lev- els, it signifies the presence of numerous available states for occupation. Conversely, a zero intensity indicates the absence of available states, while a negative intensity signifies an anti-bonding inter- action [34]. In addition to this, he energy values presented in the HOMO-LUMO analysis also align well with the energy gap depicted in the DOS spec- trum. Figure 15 represents the DOS plot for high- est and lowest energy gap of the Formaldehyde and its derivatives in the energy range from – 20 eV to 0 eV and with a full width at half maximum (FWHM) of 0.3 eV. In Figure 15(a), the energy gap from DOS spectrum (i.e. 5.9176 eV) is lower than 5.9443 eV for FMO of formaldehyde and it means that there are only (5.9176 eV) states available for Susmita Limbu et al./ BIBECHANA 21 (2024) 23-36 34 occupation. Figure 15(b) shows that Acetone has 6.2825 eV number of electron state per unit volume per unit energy which is very much close to energy gap (6.2812 eV) obtained from FMO’s of Acetone. Similarly, figure 15(c) is the DOS for Acetyl Chlo- ride having 5.6233 eV unit energy per unit volume which is a little higher than energy gap (5.6137 eV) obtained from FMO’s of Acetyl Chloride. In figure 15(d), it is found that DOS for Methyl Acetate hav- ing 7.1838 eV unit energy per unit volume which is less than the energy gap obtained (7.4222 eV) from FMO’s of Methyl Acetate. The energy values pre- sented in the HOMO-LUMO analysis are well coor- dinated with the energy gap depicted in the DOS spectrum. DOS spectrum for HOMO and LUMO in figure 15 for formaldehyde and its substitutions shows exact stability of given compounds and found that Methyl Acetate is most stable due to O–C=O group present in it. Figure 15: DOS spectrum of (a) Formaldehyde (b) Acetone (c) Acetyl Chloride (d) Methyl Acetate at B3LYP with basis set 6-311+G(d ′ ,p ′ ). 4 Conclusion This study conducted an in-depth analysis of the molecular properties of Formaldehyde, Acetone, Acetyl Chloride, and Methyl Acetate using vari- ous computational methods. Gaussian calculations were conducted using the DFT method with the 6-311+G(d ′ ,p ′ ) basis set in which the optimiza- tion process successfully minimized the energy for each molecule. Infrared (IR) spectroscopy ana- lyzed the vibrational modes of the molecules in which specific vibrational frequencies correspond- ing to C-H, C=O, C-O-C, and C-Cl bonds were identified. Thermodynamic parameters such as en- tropy, enthalpy, Gibbs free energy, internal energy, heat capacity at constant volume, and heat capac- ity at constant pressure were computed and the data showed how these parameters varied with tem- perature and provided insights into the thermody- namic behavior of the molecules. Frontier molec- ular orbitals (HOMO and LUMO) were analyzed to understand the electronic structure and reac- tivity of the molecules. The energy gap between HOMO and LUMO was used as an indicator of molecular stability. Methyl Acetate was found to be the most stable molecule due to its higher HOMO- LUMO energy gap. Chemical reactivity descrip- tors, including ionization potential, electron affin- ity, chemical hardness, chemical potential, elec- tronegativity, electrophilicity index, and chemical softness, were calculated to assess the reactivity of the molecules. Acetone exhibited strong elec- trophilic reactivity, while Methyl Acetate had the lowest reactivity. 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