ACCEPTED MANUSCRIPT This is an early electronic version of an as-received manuscript that hasbeen accepted for publication in the Journal of the Serbian Chemical Society but has not yet been subjected to the editing process and publishing procedure applied by the JSCS Editorial Office. Please cite this article as R. V. Kapustin, I. I. Grinvald, A. V. Vorotyntsev, A. N. Petukhov, V. M. Vorotyntsev, S. S. Suvorov, A. V. Baryscheva, J. Serb. Chem. Soc. (2022) https://doi.org/10.2298/JSC211003087K This “raw” version of the manuscript is being provided to the authors and readers for their technical service. It must be stressed that the manuscript still has to be subjected to copyediting, typesetting, English grammar and syntax correc- tions, professional editing and authors’ review of the galley proof before it is published in its final form. Please note that during these publishing processes, many errors may emerge which could affect the final content of the manuscript and all legal disclaimers applied according to the policies of the Journal. https://doi.org/10.2298/JSC211003087K J. Serb. Chem. Soc.00(0)1-13 (2022) Original scientific paper JSCS–11415 Published DD MM, 2022 1 Formation of intermediate gas-liquid system in aromatics’ thin layers ROSTISLAV V. KAPUSTIN1, IOSIF I. GRINVALD1*, ANDREY V. VOROTYNTSEV2, ANTON N. PETUKHOV2,3, VLADIMIR M. VOROTYNTSEV1, SERGEY S. SUVOROV2 and ALEXANDRA V. BARYSCHEVA2 1Alekseev State Technical University of Nizhny Novgorod, Nizhny Novgorod, Russia; 2Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod, Russia and 3Dmitry Mendeleev University, Moscow, Russia (Received 3 October; Revised 25 November; Accepted 9 December 2022) Abstract: The present work discusses the IR spectroscopic experiments and quantum-chemical DFT study of structure and intermolecular binding in the intermediate gas-liquid systems of aromatics, namely, benzene, furane, pyridine and thiophene. These systems can be generated in thin layers near a solid surface by two different methods, depending on the physical properties of the sample. The first method includes evaporation with a subsequent compression of a sample in a variable thickness optical cell is applied to volatile components: benzene, furane, thiophene. For benzene and pyridine, the second method is used, which involves a heating-initiated evaporation into a closed inter-window space with an after- cooling of a sample. It was shown that the formed layer is not an adsorbate or a condensate. The IR data obtained by these two methods allow to conclude that the revealed systems of the considered aromatics manifest dual gas-liquid spectral properties which can pass into each other in case of varying external conditions. According to the DFT calculation results, the spatial arrangement in the aromatic thin layers can be described as a combination of π- and σ- bonded clusters, which simulate the gas and the liquid phase state properties. Keywords: fluid-like; intermediate phase; IR spectroscopy; DFT calculations INTRODUCTION The concept of thin layers in the near-surface area was mentioned in the works of Ananikov’s group, devoted to the mechanisms of heterogeneous catalysis. 1 This term primarily refers to catalytically active metal particles, and it can mean both a nanoscale layer and molecular clusters. However, later it was shown that the very concept of the thin layer has a much wider application. The thin-layer effect was observed in different phase states of organic liquids: in a solid-film-encapsulated *Corresponding author E-mail: grinwald@mts-nn.ru https://doi.org/10.2298/JSC211003087K Ac ce pt ed M an us cri pt mailto:grinwald@mts-nn.ru https://doi.org/10.2298/JSC211003087K 2 KAPUSTIN et al. form, in a thin-layer liquid on a solid surface, in dense vapours near a solid surface, and also in a low-temperature matrix. 2 Various theoretical and experimental methods for studying transient thin-layer systems have been suggested, including ab initio DFT calculations in the Gaussian software package (B3LYP 6–311++G [2d, 2p] basis set), which make it possible not only to optimize the geometry of molecular clusters, but also to compare theoretical IR vibrational spectra with experimental ones directly. 3 However, the available computing schemes does not allow to calculate the entire supramolecular system in all its complexity and heterogeneity, being limited only to isolated clusters. Therefore, it is only experimental in situ IR spectral methods for studying organic-compound thin layers that make it possible to reveal their unique phase properties and make assumptions about their spatial structure. 4 The structure and properties of the thin layers can vary and manifest themselves differently depending on the method of generation, the state of aggregation, and the class of the compound, as well as the types of intermolecular interaction prevailing in it. For example, in liquid chlorocarbons, chlorine acts as the main binding particle for gaseous clusters’ and condensed supramolecular structures. Accordingly, tetrachloromethane forms the most stable thin-layer gas- phase system among them. The unusual intermolecular properties of tetrachloromethane were predicted in the spectral study. 5 There was shown that gas-phase tetrachloromethane exists not only in the single molecular shape that has Td symmetry, but also in the transformed shape having C3V symmetry. This supposition was confirmed by the spectra of gaseous tetrachloromethane at various temperatures. It was resumed that the pyramidal structure relates to the cluster shape, where the chlorine atom provides the binding between molecules. The chlorine atom shift in the cluster can occur owing to the association of the molecules. It leads to the transformation of the molecular geometry to the almost planar D3h symmetry, in which A1 stretching band is forbidden in IR spectra. The above interpretation of the liquid chlorocarbons’ IR spectral data based on the symmetry point groups’ theory showed the appearance of two structural modifications – the pyramidal C3V and the biplanar D2h or D2V symmetry. The DFT calculations also predicted the transformation of the initial isomer into biplanar and pyramidal ones. Thus, it was concluded that the phase characteristics in chlorocarbons can be combined: in the gas phase, some interactions resembling liquid ones may retain, and vice versa. The most unusual and chemically resistant form of the studied thin layers are solid-phase chlorosilane films with an encapsulated liquid, generated in an argon atmosphere – they are able to selectively protect the internal component, preventing its air hydrolysis (which could be explosive), 6 but allowing slow oxidation over months, which was repeatedly confirmed by IR spectroscopy and electron microscopy data. 7 Ac ce pt ed M an us cri pt FORMATION OF INTERMEDIATE AROMATIC SYSTEMS 3 Systems that combine gaseous and liquid physicochemical properties are known as supercritical fluids (SCF). 8 They can dissolve large volumes of gases and mix indefinitely with each other. In addition, a slight adjustment of pressure and temperature within the limits of supercritical values makes it possible to influence the density of the SCF and its other parameters to such an extent that, in the phase diagram, the SCF is even conditionally divided into gas-like and liquid- like states. Although there is actually no clear boundary between these two states, many researchers conditionally draw it along the so-called Widom line, the transition through which is described by the term pseudo-boiling, which once again emphasizes the variability degree of the SCF physicochemical properties under changing external conditions. One of the most reliable methods for tracking such phase transformations is considered to be IR spectroscopy, since it allows one to unambiguously assign individual characteristic bands to the gas or liquid phase and thus clearly distinguish between the gas and liquid properties of the sample. 9 In the thin layers of chloromethanes and chloroethanes, it was possible to reproduce the main properties of SCF. Specifically, in the IR spectra, two separate phases – gas-like and liquid-like – were distinguished by their manifestation in the characteristic bands that change a shape depending on the phase state of the substance. Similar to the SCF, in the thin layers these states smoothly and continuously pass into each other with a slight adjustment of pressure and temperature. Depending on the boiling point of the component, under ambient conditions it may appear in the thin layers more as a gas-like or liquid-like one. 10 It is important to note that the terms ‘gas-like’ and ‘liquid-like’ used in the thin- layer system description are not accidental or intended to emphasize the similarity of a given state with a fluid. The states observed by IR spectroscopy are in fact neither a real gas nor a real liquid, but only exhibit some of their spectral properties, which is easy to prove: it is almost impossible to obtain the IR spectrum of a real gas at an equilibrium vapor pressure in the thin spectral cell (1–6 mm). As already mentioned, the properties of thin-layer gas-liquid systems are largely determined by the types of intermolecular bonding. For this reason, the compounds with aromatic properties, both proper hydrocarbons and heterocycles, are of particular interest for study since they are distinguished from the other previously studied organic compounds in terms of intermolecular interactions. Back in the 70s Shakhparonov, who predicted the existence of the specific interactions in the non-polar hydrogen-bonded liquids, suggested the formation of molecular π-complexes (molecular stacks) between aromatic rings. 11 Later, this concept was confirmed and developed by Abramovich’s group 12–14 . The single benzene molecule geometry is usually taken as a planar ring. This geometry corresponds to D6h symmetry point group; according to the selection rules, only one stretching C–H band (E-specie) should be active in its IR spectra. Nevertheless, in the real liquid benzene spectra there are three bands, and in the Ac ce pt ed M an us cri pt 4 KAPUSTIN et al. solid benzene there are even four bands in C–H stretching region. Three bands that have isotopic H/D shift close to the theoretical prediction also are observed in benzene-d6 spectrum. This spectral picture is assigned to the existence of two molecular forms in the liquid phase: a planar shape, in which one IR band is active (D6h symmetry), and a shape with two IR bands (A1- and E-species), corresponding to the C3V symmetry. Assumedly, the benzene molecule exists in the liquid state as a cluster system, where C–H bonds deviate from the ring plane to the neighbouring molecule, which leads to a distortion of the C–H stretching vibrations’ symmetry. 15 In solution of dichlorobenzene, there are strong neighbouring-molecule interactions between chlorine atom and carbon in the aromatic ring, and between carbon atoms as well. Since the arrangements of molecules in a crystal always correspond to the potential energy minima of the system, it can be used to verify the results of molecular light scattering experiments combined with modelling proce- dure and consequently reveal the structure and characteristics of the mutual arrange- ment in molecules as well as the intermolecular binding types in the liquid phase. 16 A structural element that determines the molecular arrangement in the benzene liquid phase, was earlier presented as a set of dimers that have several geometry configurations. Later, the trimers were considered as a formed element in the ‘stack model’ of the benzene liquid phase. The conducted DFT calculations with different transformations of the initial molecular geometry predict that the optimal configuration is the stack with a ‘chair’ shape of the central and two planar rings; the trimers are bonded in a spatial structure by the hydrogen bridges. 17 This concept explained the IR data outside the scope of traditional assignment. In the trimer spectrum, a C–H stretching band assigning to both planar rings should be observed while the C–H central-ring stretching exhibits two bands: the first one assigns to a pair of equivalent (C1–H1) and (C4–H4) bonds, and the second one – to a quartet: (Ci–Hi), i = 2.3, 5.6. The stretching bands of the hydrogen bridges shift in the middle IR region due to the mixing of the C–H and C–C stretching. Consequently, there is a pair of bands that corresponds to two bridging C–H bonds’ stretching in the stacks bound in two mutually perpendicular plains, as it was shown earlier. 18 Based on the data presented above, it can be stated that the formation mechanism of the thin-layer systems with dual gas-liquid phase characteristics is caused and provided by the intermolecular binding. Using the example of chlorocarbons and chlorosilanes, one can see how the properties of the thin layers change depending on the supramolecular geometry. The formation of certain stable structures with unusual phase properties indirectly confirms the presence of nonstandard intermolecular binding realized through such structures. Thus, the experimental studies of the thin aromatic layers considered in the present work are intended to find a new and more reliable confirmation of the above assumptions about the supramolecular geometry of aromatics. If the intermolecular binding is Ac ce pt ed M an us cri pt FORMATION OF INTERMEDIATE AROMATIC SYSTEMS 5 indeed the thin-layer systems’ basis, then the properties of such systems for aromatics should differ from chlorides to the same extent as their supramolecular characteristics do. Pure benzene, a common organic solvent that can be directly compared with liquid chlorocarbons, is studied as a typical representative of aromatics. Moreover, it is very promising to study heterocycles, which are known to have some aromatic properties as well. Below we consider the question of whether the heterocyclic intermolecular interactions, and hence the characteristics of their thin-layer systems, differ from those of benzene. In order to clearly distinguish between the proper aromatic properties, which manifest themselves to the same extent in various heterocycles, from the intermolecular interactions of the heteroatom itself, which change depending on the compound, we have selected three aromatic heterocycles with different heteroatoms: thiophene, furane, and pyridine. The main objective of the work is to study the four selected substances under conditions as close as possible to those in which the chlorides were considered, for the most accurate comparison. As a part of the presented work, we have concluded an extensive quantum-chemical study based on the DFT calculations is also expected to complement the earlier theoretical studies of chlorocarbons with the data on intermolecular binding in the aromatics and allow the quantum chemical model of the gas-liquid thin-layer system structure to be evaluated for the varied classes of compounds in terms of DFT study. EXPERIMENTAL In the present work, as mentioned above, four components were studied: benzene, thiophene, furane, and pyridine. The optimal parameters for thin-layer gas-liquid systems’ generation (evaporation time τ, cell type, heating temperature T and boiling point TB) depend on the individual properties of each component, primarily its boiling point, as shown in the Table I below. TABLE I. The optimal parameters for the selected compounds’ transient state formation Compound τ / min Cell type T / K TB / K Benzene 60 VTOC/MCOH 295/323 353 Thiophene 60 VTOC 295 357 Furane 25 VTOC 295 304 Pyridine 10 MCOH 363 389 For the selected components, whose intense evaporation do not require additional heating, a Perkin-Elmer variable-thickness optical cell was applied (here we use the abbreviation VTOC). This method was described in detail in the earlier paper.10 The formation of the dual gas-liquid thin-layer system occurred under the compression-extension procedure of the cell inter-window distance from 6 to 1 mm and in reverse. The optimal evaporation time required for the gas-liquid state formation depends on the boiling point of compound and varies from 25 to 60 minutes. The effect of the thin gas-liquid layers generation for benzene and low-volatile pyridine has been achieved by the method of evaporation of liquid sample placed between optical Ac ce pt ed M an us cri pt 6 KAPUSTIN et al. window in following way. The sample drops (Fig. 1, 1) on the edge of a KBr optical window (Fig. 1, 2) so that to prevent its falling into the IR optical beam (Fig. 1, 3) under a Teflon 1-mm- thickness gasket (Fig. 1, 4), covering them from above with another optical window (Fig. 1, 5). Then both windows with a gasket between them are placing and clamping in a manual collapsible optical holder (here we use the abbreviation MCOH) for subsequent heating up to optimal for this experiment’s temperature (Table I). Figure 1. The scheme of the thin-layer system heating-accelerated generation and detection For the spectral detection and study of the generated state, IR spectra recorded in the VTOC and the MCOH were compared with those obtained in a gas cell (for a gas phase) and for a liquid phase between optical windows at ambient conditions. The effectiveness and accuracy of the two methods used for the study of the thin-layer systems generation was previously demonstrated in our works.7,10 It was proved that the revealed layer is not an adsorbate or a condensate because the IR spectra of substances, evaporated into the VTOC at 6 mm thickness, disappeared after the cell thickness compression to 0.05 mm. Besides, the layer origin was tested by the heating method (see above). The layers have been formed by the heating-accelerated evaporation into the cell with 1 mm gasket while at the same procedure conducted with 0.2 mm gasket, the intermediate shape was not detected. IR-spectra were recorded by IR-Fourier spectrometers IR Affinity1 (Shimadzu Co. Inc.) and FSM 1202 (InfraSpec Co. Ltd.) in 500 – 4000 cm–1 range with 2 cm–1 resolution and 60 scans. The spectra fragments are given in the figures with the wavenumber axis expansion according to the original record. The VTOC used was manufactured by Perkin-Elmer Co. Ltd. The purity of the components was not less than 99.9 % as it was confirmed by chromato- mass spectrometry data obtained by GCMS–QP2010 Plus spectrometer (Shimadzu Co. Inc.). RESULTS AND DISCUSSION In this section, the IR results for selected aromatics, including benzene and heterocycles, are presented. We highlighted the C–H out-of-plane bending vibration (800 – 600 cm –1 ) and high frequencies’ (3000 – 2000 cm –1 ) regions. The Ac ce pt ed M an us cri pt FORMATION OF INTERMEDIATE AROMATIC SYSTEMS 7 main manifestations of the thin layer formation could be expected in these ranges, because the first of them is sensitive to the intermolecular “stack” binding and the second one can display the E‧‧‧H (E=C, O, S or N) intermolecular binding in “chains”. 17 The most valuable results for benzene were obtained by heating of the sample between optical windows and subsequent evaporation into VTOC (see section “Experimental”). In the experiments with furane and thiophene, the gas-liquid systems were generated in VTOC, while for pyridine, they were obtained by heating between optical windows. Gas-liquid system in the thin layers of benzene The band of C–H out-of-plane bending vibration with maximum at 673 cm –1 , resembling the band in a gas phase (Fig. 2, spectrum A) with poorly resolved P-, Q-, R-branches, was revealed at the sample evaporation procedure in VTOC by thickness compression from 6 to 1 mm (Fig. 2, spectrum C). At the minimal cell thickness of 0.05 mm, the C band disappears. At the thin-layer generation by heating of the sample between optical windows without a gasket, the C band of spectrum was not observed as well. Therefore, we can conclude that this band is not assigned to the adsorbed molecular layer. As the spectrum of the equilibrium gas phase cannot be observed at 1 mm optical cell thickness, the observed band can be attributed to the transient shape that appears in the thin layers and combines gas and liquid properties. The band of liquid benzene can be considered as the gas-phase band transformation through the transitional phase state band (Fig. 2, spectrum A). Figure 2. Fragment of benzene spectra in C–H out-of-plane bending vibration range. Spectra: A – liquid phase; B – gas phase; C – transient phase in the thin layer In the spectra obtained by evaporation of the sample, placed between optical windows, with the heating to 323 K, the new bands at 654, 685 and 730 cm –1 were revealed (Fig. 3, spectrum B). The first two of them locate near the P- and R-bran- Ac ce pt ed M an us cri pt 8 KAPUSTIN et al. ches of gaseous benzene, at 659 and 687 cm –1 , respectively (Fig. 3, spectrum A), while the third band is located closely to the position of the similar mono-substituted arenes’ band. The spectrum “B” after cooling transforms into the spectrum of the transitional shape, presented in Fig. 2 (spectrum C). These data can be interpreted as a manifestation of the benzene thin layer formation with the structure and binding that differ from the ones obtained in the method above by evaporation into VTOC. Figure 3. Fragment of benzene spectra in C–H out-of-plane bending vibration range. Spectra: A – gaseous benzene; B – benzene transient shape in the thin layer. Gas-liquid systems in the thin layers of thiophene and furane In the spectra of furane, obtained at the evaporation into VTOC and subsequent lowing of the cell thickness from 6 to 1 mm, the bands at 759, 749 and 727 cm –1 were detected (Fig. 4A, spectrum C). The position of the central band is very close to one in the spectrum of liquid (Fig. 4A, spectrum A) and gaseous furane (Fig. 4A, spectrum B). As the spectrum of a real gas phase cannot be recorded at 1 mm cell thickness, the spectrum C can be assigned to the transient gas-liquid system of furane forming in the thin layer, as well as in the benzene spectrum above. The new bands of the thin layer system are detected at 2598 cm –1 , unlike the benzene gas-liquid system (Fig. 4B, spectrum C). In the spectra of liquid and gaseous furane, these bands are absent (Fig. 4B, spectra A and B). Since this region (3600 – 3000 cm –1 ) is characteristic for the O–H stretching vibrations, the revealed band can be assigned to the intermolecular O‧‧‧H stretching that arises in the thin- layer gas-liquid shape. The similar spectral picture was observed for thiophene in the C–H out-of- plane bending and the high-frequency regions (Fig. 5A). The band at 2151 cm –1 can be assigned to the S‧‧‧H intermolecular stretching in the thin-layer gas-liquid system of thiophene (Fig. 5B). Ac ce pt ed M an us cri pt FORMATION OF INTERMEDIATE AROMATIC SYSTEMS 9 A B Figure 4. Furane spectra in C–H out-of-plane bending vibration (A) and high-frequency (B) ranges. Spectra: A – liquid phase; B – gas phase, C – transient shape in the thin layer A B Figure 5. Thiophene spectra in C–H out-of-plane bending vibration (A) and high-frequency (B) ranges. Spectra: A – liquid phase; B – gas phase; C – transient phase in the thin layer Gas-liquid system in the thin layers of pyridine For pyridine, the generation of the thin layer system can be realized by heating to 373 K in the inter-window space with a 1 mm cell gasket. The observed IR absorption (Fig. 6A, spectrum B) is presented in comparison with the liquid phase (Fig. 6A, spectrum A). The spectrum of pyridine equilibrium gas phase spectrum cannot be recorded due to low volatility of the substance. However, in this case, the transition shape with dual gas-liquid spectral properties was also observed. In the high-frequency region, a new band in the IR spectrum of the generated thin layer at 3420 cm –1 was observed (Fig. 6B, spectrum B). This band can be assigned to an intermolecular N‧‧‧H hydrogen bond forming within an intermediate shape. Ac ce pt ed M an us cri pt 10 KAPUSTIN et al. A B Figure 6. Pyridine spectra in C–H out-of-plane bending vibration (A) and high-frequency (B) ranges. Spectra: A – liquid phase; B – transient phase in the thin layer DFT modelling of intermolecular binding in the thin layers The description of the intermolecular binding in a condensed state can be considered in terms of DFT study only as a quantitative model. It is well-known that ab initio quantum chemical calculations simulate the condensed phase for organic molecules as a set of dimers, trimers and rarely as polymolecular clus- ters. 3,17,18 Therefore, our effort to evaluate the intermolecular binding in the gas- liquid system of the thin layers is based on the similar conception, including the calculation of the dimer structure parameters for the different molecular shapes. The aromatics’ system can exist in spatial arrangements of so called “stack” shape – with parallel and perpendicular oriented aromatic rings. 18 However, the IR data indicate the formation of the C‧‧‧H interstack σ-binding as well. 17,18 The results of DFT study, in terms of the similar concept, are presented in this Section. The calculations were caried out in terms of DFT technique with B3LYP and GD3BJ functionals 19,20 with 6–311++G (2d, 2p) basis set. For geometry optimization procedure we have used the algorithm 21 . The intermolecular distances calculated by the B3LYP functional are given on Figs. 7 to 10 in brackets. The additional criterium of a system geometry correct calculation is the absence of negative vibration frequencies in the final data set. Benzene system. Two types of clusters, which represent the benzene spatial arrangement, are given in Fig. 7. DFT calculations of the optimized geometry for structure A (“stack” shape) in the variant with the B3LYP functional gives a geometry with aromatic rings arranged at an angle, so the comparison of intermolecular bond lengths with the GD3BJ functional can be taken rather conditionally, hence the bond lengths obtained by the B3LYP functional are not given. The calculated spectral data for these structures generally agree with the experimental IR picture in CH stretching region, in which three bands are observed instead of one CH stretching band prognosticated for the planar isolated ring (this Ac ce pt ed M an us cri pt FORMATION OF INTERMEDIATE AROMATIC SYSTEMS 11 problem was discussed earlier). 18 The calculations predict two bands assigning to the shape with parallel (Fig. 7A) and one band for the “T” shape (Fig. 7B). The combination of these arrangement variants leads to the appearance of three bands in the spectra (Table II). Figure 7. Elements of spatial arrangement with optimized geometry parameters of the benzene clusters(the distances are given in Å) The calculated IR frequencies correctly predict the existence of three bands in the CH stretching region arising due to the bands’ combination of two molecular shapes – A and B. 17 TABLE II. The experimental and calculated frequencies data of CH stretching bands Wavenumber, cm-1 Experimental 3092, 3071, 3036 Calculated 3122, 3053 (shape A) 3179 (shape B) Furane and thiophene systems For the furan molecule, the geometry optimization procedure gives two variants of σ-bonded dimers – one with the at-an-angle aromatic rings’ arrange- ment and the intermolecular (C–C) distance of about 3.1 Å (Fig. 8A) and with the distance of about 2.7 Å (Fig. 8B). The thin-layer transient gas-liquid system, in terms of the similar model, can form under the O‧‧‧H intermolecular bonded clusters with two molecular arrangement types. The structure of thiophene gas-liquid system in the thin layer predicted in DFT calculation can be presented as a state with two different shapes π-bonded (“stack” shape) and σ-bonded S‧‧‧H shape (Fig. 9). In this case, DFT calculations with the B3LYP potential do not give an optimized geometry for the “stack” shape, so these values are not shown in Figure 9. Ac ce pt ed M an us cri pt 12 KAPUSTIN et al. Figure 8. Elements of spatial arrangement with optimized geometry parameters of the furan clusters (the distances are given in Å) Figure 9. Elements of spatial arrangement with optimized geometry parameters of the thiophene clusters (the distances are given in Å) The calculated IR frequencies in CH out-of-plane bending (Q-branch only) and CH stretching ranges, where the manifestation of intermediate phase shape formation was observed, in Table III are presented (for the GD3BJ functional only). The calculated and relative experimental intensities were not listed in Table III, because these values strongly depend on the experimental conditions and do not give any useful information in terms of the considered problem. TABLE III. The calculated and experimental IR frequencies of heterocyclic aromatics in ranges of CH out of plane bending – ρ(CH) and CH‧‧‧X stretching – q(CH) Frequencies Wavenumber, cm-1 C5H5O C5H5S C6H6N ρ(CH) q(CH‧‧‧O) ρ(CH) q(CH‧‧‧S) ρ(CH) q(CH‧‧‧N) Experimental 749 2598 709 2151 698 3420 Calculated Shape A 756 3259 723 3253 712 3156 Shape B 757 3259 723 3253 716 3174 Ac ce pt ed M an us cri pt FORMATION OF INTERMEDIATE AROMATIC SYSTEMS 13 Significant differences for the calculated and experimental frequencies for the CH‧‧‧X stretching vibrations are probably caused by the hydrogen atom shift to the heteroatoms in the real systems. 22 Pyridine system The cluster structure of pyridine system predicted in the DFT study with the GD3BJ functional can be presented as a state with combination of “stack” shape (Fig. 10A) and σ- one formed under intermolecular hydrogen N‧‧‧H bond (Fig. 10B). Figure 10. Elements of spatial arrangement with optimized geometry parameters of the pyridine clusters (distances are given in Å) CONCLUSION The presented experimental and computational data allow to make the following main conclusions. 1. The gas-liquid systems of aromatics including heterocyclic aromatic compounds can be generated in the thin layers by evaporation and subsequent compression of the substances in VTOC or by heating of the sample placed between optical windows and evaporation into the small inter-window space. 2. These systems manifest the dual gas-liquid spectral properties of the considered aromatics. In terms of the presented concept, their structure can be interpreted as a transient phase state arising at the liquid state transformation into the gas. 3. The spatial arrangement of the thin layers can be described as a combination of π- and σ- bonded clusters, which can simulate the gas and the liquid phase state properties. Therefore, the thin layers can be considered as an intermediate state of organic liquids. Acknowledgements: The study was supported by Ministry of Science and Higher Education of the Russian Federation as part of the scientific project FSSM-2021-0013 of the Laboratory of Smart Materials and Technology (LSMT). Ac ce pt ed M an us cri pt 14 KAPUSTIN et al. ИЗВОД ФОРМИРАЊЕ ИНТЕРМЕДИЈАРНОГ СИСТЕМА ГАС-ТЕЧНО У ТАНКИМ СЛОЈЕВИМА АРОМАТИЧНИХ ЈЕДИЊЕЊА ROSTISLAV V. KAPUSTIN1, IOSIF I. GRINVALD1, ANDREY V. VOROTYNTSEV2, ANTON N. PETUKHOV2,3, VLADIMIR M. VOROTYNTSEV1, SERGEY S. SUVOROV2 и ALEXANDRA V. BARYSCHEVA2 1Alekseev State Technical University of Nizhny Novgorod, Nizhny Novgorod, Russia; 2Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod, Russia и 3Dmitry Mendeleev University, Moscow, Russia У овом раду разматрани су подаци добијени ИЦ спектроскопским експериментима и кванто-хемијским DFT испитивањем структуре и интермолекулских веза у интермеди- јерним системима гас-течно ароматичних једињења бензена, фурана, пиридина и тиофена. Ови системи могу настати у танким слојевима у близини чврсте површине применом две различите методе, у зависности од физичких особина узорка. Прва метода која укључује исправање а затим компресију узрока у оптичкој ћелији променљиве дебљине је примењена на испарљива једињења: бензен, фуран и тиофен. За бензен и пиридин, коришћена је друга метода, која подразумева испаравање иницирано загревањем у затвореном простору између прозора са накнадним хлађењем узорка. Показано је да настали слој није адсорбат или кондензат. На основу ИЦ података добијених овим двема методама може се закључити да добијени системи кондензованих ароматичних једињења показују дуалне гас-течно спектралне особине које могу прелазити из једних у друге у случају варирања спољних услова. Резултати DFT прорачуна показаују да се просторно уређење у танким слојевима ароматичних једињења може описати као комбинација π- и σ- повезаних кластера, које симулирају особине гасне и течне фазе. (Примљено 5. октобра; ревидирано 25 новембра; прихваћено 9. децембра 2022.) REFERENCES 1. D. B. Eremin, V. P. Ananikov, Coord. Chem. Rev. 346 (2017) 2 (https://dx.doi.org/10.1016/j.ccr.2016.12.021) 2. I. I. Grinvald, I. Y. Kalagaev, A. N. Petukhov, A. I. Grushevskaya, R. V. Kapustin, I. V. Vorotyntsev, J. Struct. Chem. 59 (2018) 313 (https://dx.doi.org/10.1134/S0022476618020087) 3. M. A. Palafox, Phys. Sci. Rev. 3 (2018) (https://dx.doi.org/10.1515/psr-2017-0184) 4. B. C. 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