INTERPRETATION OF THE VIBRATIONAL SPECTRA OF TERT- BUTYL HYDROPEROXIDE AND DIMETHYLETHYNYLMETHYL HYDROPEROXIDE UDC

Transcription

1 basis of Raman Spectra," Article deposited in the All-Union Institute of Scientific and Technical Information of the Academy of Sciences of the USSR, Moscow (VlNITI), No. 984, February 5, D. A. Lee, Inorg. Chem., ~, No., (1964). 8. B. Keil (editor), Laboratory Techniques of Organic Chemistry [Russian translation], Moscow (1966). 9. L. Weissberger, E. Proskauer, J. Riddick, and E. Toops, Jr., Organic Solvents -- Physical Properties and Methods of Purification, Interscience (1955). i0. Ch. Vul'fson (ed), Preparative Organic Chemistry [in Russian], Moscow (1964), p ii. B. Basak~ Inorg. Chem. Acta, 45, No. i, (1980). 1. G. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules, D. Van Nostrand, New York (1946). 13. D. Paoli, M. Lucon, and M. Chabanel, Spectrochim. Acta, 34A, (1979). 14. D. Paoli, M. Lucon, and M. Chabane!, Spectrochim. Acta, 35A, (1979). INTERPRETATION OF THE VIBRATIONAL SPECTRA OF TERT- BUTYL HYDROPEROXIDE AND DIMETHYLETHYNYLMETHYL HYDROPEROXIDE I. P. Zyat'kov, Yu. A. Ol'dekop, A. P. Yuvchenko, N. M. Ksenofontova, G. A. Pitsevich, D. I. Sagaidak, V. I. Gogolinskii, and V. L. Antonovskii UDC The important role which is played by hydroperoxides in the aging and oxidation processes of petroleum products, fats~ polymersj and other materials, calls for the development of sensitive spectroscopic methods for their quantitative determination. However, the features of the vibrational spectra of these compounds, particularly of the Raman spectra, have heretofore scarcely been studied. The Raman spectra of some hydroperoxides are presented in [ID ]. A number of studies [3-5] have been devoted to the IR absorption spectra, but their interpretation is contradictory in a number of cases. In the present work we interpreted the vibrational spectra of tert-butyl hydroperoxide HOOC(CHs)s (I), dimethylethynylmethyl hydroperoxide HOOC(CHs)C~CH (II), and their alcoholic analogs HOC(CHs)s (IA) and HOC(CHs)=C~ 6~ (IIA). The IR absorption spectra were recorded on a Specord 75-IR spectrometer with a spectral slit width of 3 cm -~. The samples consisted of the pure substance in a thin layer between KBr windows. In the case of the hydroperoxides, which actively interact with KBr, NaCI windows were used to record the spectra in the cm -~ region, and the KBr windows were used only for recording in the 700,400-cm -x range. The gas-phase IR absorption spectra were recorded in a gas cuvette with a thickness of i0 cm at atmospheric pressure and room temperature. In the came of the alcohols~ the gas cuvette was heated. The Raman spectra were recorded on a Spex-Ramalogspectrometer with a spectral slit width equal to cm -~ under dc conditions. The excitation was effected by an argon laser operating at 19,435 cm -x with a power of 0.3 W. ~le values of the frequencies and the accuracy of the polarization measurements were determined relative to CC14. The samples were prepared in the form of the pure liquid in a glass capillary. A calculation of the spectrum of the normal modes of vibration of compound I was carried out with the use of the program described in [6]. The force field of molecule I was composed by a simulation method on the basis of the data from the calculations of the vibrational spectra of tert-butyl alcohol in [7] and of dimethyl peroxide in [8]. Quantum-chemical calculations of themolecules of the compounds investigated were carried out in the framework of the standard SCF--MO--LCAO method in the MNDO approximation [9] with complete optimization of all the parameters of the geometric structure. A stereochemical treatment of the molecule of peroxide I shows that it is characterized by a single stable conformation and the absence of molecular symmetry. Therefore, 4 vibrations should be displayed in its vibrational spectrum, and all the lines in the Raman spectrum should be polarized. Actually, less than 4 vibrations are displayed in the IR spectra and in the Raman spectra. Seven lines in the Raman spectrum 6f the hydroperoxide are de- Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 48, No. 4, pp , April, Original article submitted February 9, /88/ Plenum Publishing Corporation

2 polarized. It is known that in the case of pure deformation vibrations, the value of the degree of depolarization may not differ from 0.75, regardless of the symmetry of the vibration and the molecule. However, besides the pure deformation vibrations of the C--H bonds in the cm -I range, there are a number of depolarized lines in the low-frequency region of the spectrum ( cm-x). Here the vibrations generally contain a contribution from the stretching vibration of the skeleton of the molecule. There are also two depolarized lines in the region of the C--H stretching vibrations ( cm-1). This suggests that in the molecule of I there is a grouping which has a higher local symmetry. On the basis of the hypothesis that C3v symmetry is maintained for the OC(CHs)s fragment in the molecule of I, its vibrations may be represented according to the symmetry types in the following manner: Fc3~ = 8 A A -[-, 1 E. The lines caused by the vibration with A~ symmetry may be polarized in the Raman spectrum. According to a group-theoretical analysis, these eight vibrations include two ~CH vibrations, one VCO vibration, two ~HCH vibrations, one ~CCH vibration, one ~OCC vibration, and one vcc vibration. The O-(>-H grouping produces five more polarized lines in the Raman spectrum, which are associated with the following vibrations: one ~00, one ~OH, one ~CO0, one ~OOH, and one TOO. Thus, in the case of the realization of Cs~ symmetry for the OC(CHs)s fragment~ the number of polarized lines inthe Raman spectrum of compound I should not exceed 13, including no more than two polarized lines in the region of the C--H stretching vibrations. The total number of polarized lines in the Raman spectrum of compound I exceeds the number indicated, and in the region of the C--H stretching vibrations there are five polarized lines. Therefore, the tert-butyl fragment in the molecule of I has a lower symmetry than C3v, but a higher symmetry than C~. According to the data in [i0], the tetrahedral structure of the OC(CHs)s fragment in the molecule of tert-butyl peroxide in the crystalline phase is distorted as a result of the nonbonded interaction with an oxygen atom not found on the third-order axis (the d effect). The O-C--C angle formed by the C--C bond located in the trans position to the 0-O bond is decreased by 9 ~ and the other two O-C--C angles are increased by 3 ~. This results in lowering of the symmetry of the tert-butyl fragment to C s. Then, its vibrations may be represented according to their symmetry types in the following manner: Fc~ = ~17. According to a group-theoretical analysis, the vibrations of type include 5VCH, 56HC H, 36CC H, VCC, 6CC c, 16OC c, IVCO, 16CO o, lvo o, 1TCH 3, and the 17 vibrations of type include 4VCH, 46HC H, 36CC H, lvcc,!6occ, I~CH~, 1TCO, 16cc C, 1TCH 3- A comparison of the theoretical conclusions with the experimental results allow us to assert that the local C s syrmnetry of the OOC (CHs) s fragment in the molecule of I unequivocally describes both the number of polarized lines in the Raman spectrum and the appearance of several depolarized lines, including some of the region of the C--H stretching vibrations. The assignment of the stretching and deformation vibrations of the C--H and O--H bonds with consideration of the polarization of the lines in the Raman spectrum does not present any difficulties, since they are very characteristic and are displayed in narrow spectral ranges, which do not overlap with the regions where other vibrations are displayed. In the cm -I region of the spectrum there should be six rocking vibrations of the C--H bonds, one stretching vibration of the C-O bond~ one stretching vibration of the O-0 bond, and three stretching vibrations of the C--C bonds. The stretching vibrations of the C--O bond should be displayed in the cm -I region of the spectrum with a larger relative intensity in the IR spectrum than in the Raman spectrum. These specifications are satisfied by the line at 146 cm -~. The lines associated with the rocking vibrations of the C--H bonds were interpreted with the aid of the data in [7, ii], in which the IR spectra and the Raman spectra of isobutane HC(CHs)s, in whose molecule the Csv symmetry of the tert-butyl fragment is maintained, were interpreted. The symmetric rocking vibrations of the C--H bonds of hydroperoxide I are formed from one totally symmetric vibration of the C--H bonds of type A~ with a frequency of 1189 cm -~ in the isobutane molecule, as well as from two degenerate vibrations of type E, which appear at 961 and 1173 cm -I when the degeneracy is removed. The antisymmetric vibrations of these bonds are formed from the two ~ibration of type E Just indicated and one vibration of type A, whose wave number is equal to i000 cm -I. Taking into account 391

3 the spectral ranges indicated and the polarization of the lines in the Raman spectrum of compound I, the lines at 93, 103, and 154 cm -~ were assigned to rocking vibrations of type, and the lines at 101 and 1199 cm -: were assigned to the corresponding vibrations of type. The three C--C stretching vibrations include two ow and one of type. The lowfrequency symmetric ~C-C vibration is an analog of the totally symmetric ~C-C vibration Of type in the molecule of isobutane. According to [7, ii], it is displayed in the Raman spectrum in the form of an intense and strongly polarized line at 799 cm -~. The line at 747 cm -~ in the IR and Raman spectra of compound I is associated with this vibration, since it has similar spectral characteristics. The high-frequency symmetry and antisymmetric vc-c vibrations are formed as a result of the splitting of the doubly degenerate vibration of the C--C bonds of type E in the molecule of HC(CH3)s, which, according to [7, ii], is displayed at 900 cm -~. The symmetric ~C-C vibration in molecule I should clearly be displayed near 900 cm -~ in the form of an intense polarization line in the Raman spectrum. This condition is met by the line at 884 cm -~. Then the line at 917 cm -~ may be assigned to the antisymmetric vibrations of these bonds. We shall single out the lines in the IR spectrum and the Raman spectrum of compound I which are associated with vibrations of the peroxide group. A comparison of the IR spectra and the Raman spectra of compounds I and IA reveals that the spectrum of the latter does not contain lines at 884, 843, 69, and 161 cm-~. The line at 55 cm -~ has an analog in the Raman spectrum of compound IA at 48 cm-~; however, the large difference between the values of their frequencies and relative intensities must be explained. Since the tert-butyl fragments in the molecules of I and IAhave the same symmetry, their IR spectra and Raman spectra should be distinguished only by the absence of the three lines caused by the ~00, ~OOC, ~and ~00 vibrations in the spectra of compound IA. Since four, rather than three, lines are absent in the spectra of compound IA, these fragments clearly have different symmetries in the molecules of I and IA. The hydrogen atom probably does not have such a strong distorting influence on the geometry of the tert-butyl grouping as does the oxygen atom. This hypothesis is supported by the calculation performed in the MNDO approximation. In the case of hydroperoxide I, the distortions in the configuration of the tert-butyl fragment (the calculated values of the C--C--O angles for the trans and gauche positions relative to the 0-0 bond are equal to i00 and Iii ~ practically coincide with the experimental distortions for the molecule of tert-butyl peroxide [i0]. In the case of compound IA, they are considerably smaller (the calculated values of the C--C-O angles for the trans and gauche positions are and i09.9 ~ respective), i.e., the C3v symmetry is practically maintained. Therefore, the differences between the IR spectra and between the Raman spectra of compounds I and IA may be attributed not only to the presence of the peroxide linkage in I, but also to the different symmetries of the tert-butyl fragments in them. On the basis of the frequency ranges for the appearance of the torsion, deformation, and stretching vibrations of the peroxide group established in [8], the lines at 161, 69, and 843 cm -~ should be associated with them. The absence of the line at 884 cm -~ in the Raman spectrum of the ahcohol may be attributed either to the absence of the contribution from the stretching vibration of the peroxide group or to the weaker d effect of the hydrogen atom in comparison to the oxygen atom. In order to determine which of the mechanisms indicated is more important, we performed a calculation of the normal modes of vibration of molecule I with a geometry in which the C3v symmetry of the tert-butyl fragment was maintained and with a geometry in which the influence of the d effect was taken into account. The magnitude of the distortions was assigned on the basis of the experimental data for tert-butyl peroxide [i0]. The force field of the molecules was identical in both cases. An analysis of the results of the calculation made it possible to draw the following conclusions. First, all three bonds take part in the low-frequency symmetric vibration of the C--C bonds (the calculated value was 715 cm-~); howeverj the predominant contribution to the potential-energy distribution of this vibration and, therefore, to its high relative intensity in the Raman spectrum is provided by the C--C bonds located in the gauche positions relative to the 0-0 bond, as follows from the group-theoretical analysis. The potential-energy distribution of the highfrequency symmetric vibration of the C-C bonds (its calculated wave number is 909 cm -I) contains an insignificant contribution (less than 10%) from the peroxide group, and the main contribution is provided by the C--C bond located in the trans position relative to the peroxide group. However, while this contribution to the potential-energy distribution of the vibration indicated is the same regardless of whether the d effect in molecule I is taken into account or not, in the case of the C s symmetry of the tert-butyl grouping, there are also significant contributions to the potential-energy distribution from two other ~C-C vibrations. In the case with C3v symmetry, such a contribution is completely absent. This should result in lowering of the intensity of the manifestation of this vibration in the Raman spectrum. The calculation predicts a decrease in the difference between the frequen- 39

4 J TABLE i. Interpretation of the Vibrational Spectra of Molecule I Symmetry of Calcuthe vibration lated of the OOC- values (CH3) a fragment 100 A Potential-energy distribution 99 "Coo 86 "rcc 98 tcc 99 "rcc 66 Too, 15 6cc c 39 6oo c, 39 8oc c 35 6oc o 4 8ccc, Too 50 8cc c, 35 8oc c 68 8cc O 15 8oc c 5 8oc o 9 6cc c 38 6OO O 0 6OC O 10 6CO 69 VCC, 11 ~'CO, 7 6OO c 68 Voo, 13 Vcc, 8 Vco 41 Vcc, 3 8CCH, 5 VCC 63 Vcc, 33 8cc H 36 6cc H, 30 Vco, 1 Vcc 90 8CC H 8 8CC H 8 8cc H 47, 40 Vcc, 11 6CC C 51 8CCH, 8 'V'CC, 10 6cc c I Ra~an spectrum cm ~ l19g 155! IR spect_rum i [ O cm_ I I C sh ),5 },3 ),75 1,75 ),66 ),75 ),19 ),0 ),5 ),33 ),75 ),4 0,75 0,4 0,75 0, sh 8 6O 0 cies of the symmetric and antisymmetric vc_ C vibrations and some mixing of their modes in the latter case, in good agreement with the experimental data. The calculated value of the wave number of the stretching vibration of the peroxide bond is equal to 88 cm-*. The calculation confirms the previously given assignment, according to which the line at 843 cm-* is caused by the stretching vibration of the peroxide bond, and the line at 884 cm-* is caused by the symmetric stretching vibration of the C-C bonds. The deformation and rocking vibration of the C--C bonds are displayed below 600 cm -~. Since the frequency of the rocking vibrations must be greater than the frequency of the deformation vibrations, the intense line at 54 cm-* in the Raman spectrum of compound I, whose analog in the Raman spectrum of the alcohol is considerably less intense, should be associated to the symmetric rocking vibration of the C--C bonds. In the case of the totally symmetric O-C--C vibration, there is a significant contribution from the deformation vibration of the O-O-C angle, which clearly provides for the high intensity of the line at 55 cm-* in the Raman spectrum. The other rocking vibrations of type, in which there is no contribution from the deformation of the O-O-C angle, according to the calculation, is displayed at a frequency of 413 cm-*. The line at 465 cm -~ is assigned to the antisymmetric rocking vibration of the C--C bonds, and the lines at 363 and 348 cm-* are assigned to deformation vibrations of these bonds of types and, respectively (see Table i). The introduction of the P:-CEC group into the tert-butyl group in place of a methyl substitution in the case of hydroperoxide II is a more significant physical factor, which lowers the sy=metry of the latter to Cs, than is the influence of the d effect. Therefore, the tertiary fragments in themolecules of II and IIA should have the same C s local symmetry, and the spectra of these compounds should be distinguished only by the absence of the three Ifnes associated with the vibrations of the 0-O bond in the latter. While the internal rotation around the C--O bond in compound I could not result in the formation of conformers, the formation of three conformers is possible in the case of compound II. The dependence of the energy of a molecule on the angle of rotation of the tertiary grouping E = E(~) was calculated in 393

5 TABLE. Interpretation of the Vibrational Spectra of Molecule II Raman spectrum IR spectrum Assignv, cm- i I,o v, cm -I i ment ~ ,75 7 0,7 10 0,35 3 0,6 6 0,35 6 0,76 8 0,3 4 0,1 1 0,75 7 0,4 50 0, , ,01 1 0, , 8 0, ,3 O 0, , " 1 0,75 0,3 3 0, O ha O I TOO, "~CO "I:CH ~ 6coo 6occ 6occ 6occ 6c-cc ~CCC 6ccc 8C=CH 6CCC 6C~CH WCC VCC WOO WOO ~CC ~/CC 6ccH the MNDO approximation. The curve was calculated point by point with fixation of the value of the angle 9 and complete optimization of the geometric structure with respect to all the remaining parameters. The calculation showed that the realization of three different conformers is possible for hyperperoxide II. The trans configuration of the 0-43 and C--C bonds is energetically most favorable. In this case, the OOC(CH3)C~CH fragment and molecule of IIA have C s symmetry. The energy of the second conformer with a gauche configuration of the 0-0 and C-C bonds is only 3.1 kj/mole higher. The energy of the third conformer, in which a gauche configuration of these bonds is also realized, is 8.37 kj/mole higher than the energy of the first conformer due to the close approach of the hydrogen atom and the C~C--H group. Thus, at room temperature, the existence of two conformers with trans and gauche configurations of the 0-O and CEC--H bonds is possible for compound II. The local symmetry of the tert-amyl fragments in these conformers is Cs and C~, respectively. Since ghere are depolarized lines in the region of the C--H stretching vibrations in the Raman spectra of compounds II and IIA, it may be asserted that the conformer with a trans configuration of the O-O and CEC--H bonds is present in the sample. Acccording to theory, the total number of vibrations for one conformer is equal to 39. However, the number of lines in the IR spectra and in the Raman spectra of compound II exceeds this value, i.e., the presence of two conformers also follows from the experimental data. The 36 vibrations of the OOC(CH3)CECH grouping in the trans:conformer of molecule II may be divided according to symmetry types in the following manner: According to the group-theoretical analysis, they are caused by vibrations of the following natural coordinates : --. I~CH,, lvc~ c, Vcc, lvoo, lvco, 3VCH, 16 c CH' 16C CC' 16CCO' 6CC c, 36HCH, 6CC~, 16COO, H'cH,, 394

6 --lvcc, 3VcH, 16c~c H, 16c~cc, 16ccc, 36HC H, 6CC H, lvch, I~CO, 16OC c 9 In molecule II there are three more vibrations, which are associated with the hydroxyl group: one ~OH, one ~OOH, and one xo0. The lines at 84 and 86 cm -~ in the Raman spectrum of compound II do not have analogs in the Raman spectrum of eompobnd IIA, and they may be assigned to the stretching and deformation vibrations of the peroxide group. The line at 84 cm -~ has a low-frequency shoulder at 831 cm -~, whose intensity is approximately two ties smaller than the intensity of the principal line. IX the line at 84 cm -x belongs to the trans conformer, the line at 831 cm-* may be assinged to the stretching vibration of the O-O bond in the gauche conformer. Both lines have intense analogs in the IR absorption spectrum. The three stretching vibrations of the C.-C bonds for the trans conformer of molecule II include two of type and one of type. According to the data from the calculation of the normal modes of vibrations for hydroperoxide I, the high-frequency vibrations with the participation of the C--C bonds are delocalized; therefore, the gauche conformer may have its own set of vibrations for these bonds. Since the C s symmetry for the tert-amyl fragment in molecules II and IIA is achieved as a result of the appearance of the C~C--H substituent, rather than as a result of the d effect, the line associated with the high-frequency ~C-C vibration in the spectrum of the trans conformer of hydroperioxide II should have an intense analog in the Raman spectrum of compound IIA. In fact, the intense line at 889 cm -~ in the Raman spectrum of hydroperoxlde II, which should be assigned to the high-frequency 9C-C vibration, has an intense analog at 891 cm -x in the Raman spectrum of alcohol IIA. The low-frequency vibration of the C-C bonds of type in the trans conformer of compound II is displayed at a frequency of 705 cm -I. In the Raman spectrum of compound II there is also an intense line at 74 cm-*, which, according to its spectroscopic parameters (half-width and polarization), is similar to the line at 705 cm-*. Since, according to the data in [1, 13], it cannot be assigned to a vibration of the C--CECH substituent, it is associated to the stretching, vibration C-C bonds in the gauche conformer. The line at 934 cm-* is assigned to the antisymmetric vibration of the C-C bonds in the trans cq~fqrmer of compound II, and the lines at 945 and 973 cm -I are assigned to the stretching vibration of the C-C bonds of the gauche conformer. The band at 356 cm -~ is assigned to the symmetric deformation vibration of the C--C bonds, and the band at 410 cm -~ is assigned to the antisymmetric vibration. The intense absorption~band in the IR spectrum at ].33 cm -~ is assigned to, the stretching vibration of the C-O bond in compound II. According to the data from the calculation of the normal modes of vibration of molecule I, the localized rocking vibration of the C--H bonds should appear in the em -~ region of the spectrum. Their frequencies should be identical for the trans and gauche conformers. The lines at 1008 and 1045 cm -~ are assigned to the rocking vibrations of the C--H bonds of types and, respectively (the values of p for them are 0.3 and 0.75). The lines at 1156 and 1196 cm -~ are caused by a second pair of rocking vibration of types and A, which, as in the case of molecule I, are mixed with the stretching vibration of the C-C bonds. The assignments of the other bands and lines are presented in Table. Thus, it has been established that the stretching vibration of the 0-0 bond in the case of tertiary hydroperoxides is localized and appears at 840 cm-* with a moderate relative intensity in both the Raman and IR spectra. The contribution of ~00 to the potential-energy distribution reaches 70%. The absence of the line at 880 cm -z caused by the ~C-C vibration in the spectra of the alcoholic analogs of the hydroperoxides is evidently due to the influence of the d effect. It has been established that two conformers are realized for molecule II and that the low-frequency symmetric stretching vibration of the C--C bonds in the tertiary fragment is conformation-sensitive. i LITERATURE CITED A. J. Melverger, L. R. Anderson, C. T. Ratcliffe, and W. V. Fox, Appl. Spectr., ~, No. 3, (197). P. A. Budinger and S. R. Moon,y, Anal. Chem., 53, No. 6, (1981). H. R. Williams and S. R. Mosher, Anal. Chem., 7, No. 4, (1955). O. O. Shreve and M. R. Heether, Anal. Chem., 3, No., 8-85 (1951). H. A. Szymanski, Progr. Infr. Spectr., ~, No. I, (1966). V. A. ~ement'ev, V. I. Smirnov, and L. A. Gribov, "FORTRAN programs for calculating molecular vibrations," Article deposited in the All-Union Institute of Scientific and Technical Information of the Academy of Sciences of the USSR (VINITI), Moscow, No. 4069, October ii,

7 7. B. Schreder and J. Pacansky, J. Phys. Chem., 88, No. 1, (1984). 8. M. E. Butwill Bell and J. Loane, Spectrochim. Acta, 8A, No. ii, (197). 9. M. J. S. Dewar and W. Thiel, J. Am. Chem. Soc., 99, No. 15, (1977). i0. Yu. L. Slovokhotov,T. V. Timofeeva, M. Yu. Antipin, et al., J. Mol. Struct., i1, No. I, (1984). ii. J. S. Evans and J. H. Bernstain, Can, J. Chem., 34, No. i0, (1956). 1. P. N. Daykin and S. Sundaram, J. Chem. Phys., 37, No. 5, (196). 13. R. W. Bayer, S. Sundaram, and W. F. Edgell, J. Chem. Phys., 37, No. i0, (196). CALCULATION OF BAND INTENSITIES IN IR SPECTRA AND CONFORMATION OF AROMATIC ESTERS S. N. Abdullin and V. L. Furer UDC Calculation of the vibrational spectra of aromatic esters is the initial stage in our work on the study of the structure and properties of liquid-crystalline aromatic polyesters. At the present time synthesis of a number of aromatic polyesters capable of forming the mesophase has been accomplished and intensive work is being conducted on the study of the effect of liquid-crystalline order on the conformational and orientational characteristics of this type of polymers [i, ]. We hope that calculation of the vibrational spectra of these substances will permit solving a number of problems concerning conformational transformations in polymorphic transitions. Before embarking on a theoretical analysis of vibrational spectra of polymers it is necessary to perform additional work on calculation of force and electromagnetic fields, and also conformations of compounds which are fragments of mesogenic groups. As models were taken: methylbenzoate (I), ethylbenzoate (II), phenyl acetate (III), and phenyl benzoate (IV) --RICOOR, where R, = Ph, R= = CHs (I), R~ = Ph, R = CHs (II), R~ = CH3, R= = Ph (III) R, = Ph, R = Ph (IV). Such a choice of model compounds permits determining the force and electrooptical parameters for various types of aromatic esters. Besides, it is important to clarify how the interaction of the phenyl and ester groups reflects on the force and electrooptical fields used. There are rather many data on vibrational spectra of benzoates and their deutero derivatives in the literature, attribution of the bands is given, and the vibrational frequencies have been calculated [3-5]. Energetically more suitable for molecules I-II is the planar structure of the skeleton, but in Ref. 6, devoted to an investigation of the IR spec, tra of these substances in the crystallinestate, the assumption of the presence of rotational isomers on account of rotation of the benzene ring by the angle 9 = 90 ~ (Fig. i) is made, and also of the methyl group by the angle ~ = 180~ bands sensitive to conformational transformations are given. In Ref. 7 the dipole moments of the bonds of molecule III were determined and it was assumed that three rotational isomers of this molecule are present: ~ = 0, 60, and 90 ~. The IR spectra of solutions I-IV of different concentrations in CCI~ were measured on a UR=0 spectrophotometer, and the integrated intensities were calculated using a D3-8 computer, switched into the spectrometer. The values obtained are given in Table i. The vibrational problem for molecules I-IV was solved by themethod of fragment calculation [8]. As initial fragments the methyl acetate and benzene molecules were chosen. The force and electric fields of methyl acetate were determined by us previously and will be published, and the parameters of benzene were taken from [8]. The calculation was performed on an EC-I033 computer using a complex of programs [8]. The shape of the Bands was assumed to be Gaussian, and the half widths of the bands were taken equal to i0 cm -I. In the first stage the absorption curves in the IR spectrum of moleculeswith a planar structure of the skeleton were calculatedj since such a conformation is considered predominant. The results of such calculations indicated the need to change the parameters of the initial molecules. Attempts to obtain satisfactory agreement with experiment by variation of the parameters at the attach- Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 48, No. 4. pp , April, Original article submitted November 19, /88/ Plenum Publishing Corporation