J. CHEM. SOC. FARADAY TRANS., 1994, 90(5), 689-695 Structures and Vibrational Spectra of CH,OCH,CH,OH :The Hydrogen-bonded Conformers Francisco P. S. C. Gil,t R. Fausto, A. M. Amorim da Costa and J. J. C. Teixeira-Dias* Department of Chemistry, University of Coimbra ,Pi3049 Coirnbra, Portugal ~ ~~~ Ab initio calculations at the MP2/6-31G* and MP2/6-31G*//6-31G* levels have been carried out for the monomer of 2-methoxyethanol (CH,OCH,CH,OH). The MP2/6-31G* results indicate that the two more stable conformers (tGg’ and gGg‘) display intramolecular hydrogen bonds directed from the hydroxy H atom to one of the lone pairs of the ether 0 atom, and that the tGg’ conformer is 6.3 kJ mol-’ more stable than the gGg’ conformer. As the MP2/6-31G* and MP2/6-3lG*//6-31G* calculations do not yield results differing by more than a few tenths of a kJ mol-‘, it is concluded that the structure-sensitive and the dynamic correlation corrections are far from being additive.While the optimization of geometry for the correlated wavefunction generally leads to increase of bond lengths and reduction of bond angles, on the whole the geometrical parameters undergo similar changes in different conformers. Ab initio harmonic 6-31G* derived force fields were used to perform normal mode analyses for the more stable conformers. The calculated v(CH) frequencies are found to correlate linearly with some of the ab initio calculated CH bond lengths. An interpretation of the FTIR and Raman spectra for the liquid phase consonant with the structural and vibrational ab initio results is presented.Two spectral features observed both in Raman and in FTIR spectra and associated with v(0H) in monomeric species are ascribed to conformers, in accord with the theoretical and experimental results. On the whole, both the structural and the vibrational results presented point to a distinction between the hydrogen-bonded G-type conformers (tGg’ and gGg‘) and the higher energy T-type conformers (tTg and tTt). Compounds with the general formula Normal-coordinate analyses were performed’ using the ab C,H,, + l(OCH,CH,),OH, abbreviated C,E, ,display a wide initio derived force fields. While correlated wave functions at range of interesting molecular properties and aggregation the MP2/6-31G* level were used for the determination of patterns in solution ’-’ with important pharmaceutical and structures and energies, the less expensive and more tractable industrial applications.On the whole, these properties result 6-31G* basis set was used for the evaluation of the force from a subtle interplay between the conformational degrees fields at reference geometries obtained with the same basis of freedom, the possibility of intramolecular hydrogen set.21 bonding, and the relative importance of the polar and non- Local C, symmetry internal coordinates were used polar fragments in the extent of intermolecular interactions throughout. As the calculated frequencies vs. observed fre- especially of the hydrogen-bonding type. While C,E, com-quencies yielded a straight line with a high correlation coefi- pounds have been widely studied by different spectroscopic cient (0.998 87), the calculated frequencies of the most stable = 0.9123and thermodynamic techniques, an assessment of the relative conformer were appropriately scaled [v(~~~~~~) -importance of conformational effects and hydrogen-bond x v(,,~,)72.5).On the whole, 26 frequency values were interactions at the unimolecular level is still lacking, though it used. The mean error in this scaling was 0.5%, and the largest is of fundamental importance for the understanding of the error which occurred in some low-frequency modes did not properties of these compounds at the oligomer level and of exceed 17%. Modes which were doubtfully assigned or could their aggregation patterns in solution.not be observed were excluded from the linear regression. In this work, the structures and relevant conformations of The same frequency scaling was assumed for the less stable CIEl (CH,OCH,CH,OH) are determined by ab initio MO conformers. calculations at the MP2/6-3 lG* level. Ab initio harmonic Fig. 1 represents schematically the four more relevant con- 6-31G* derived force fields are used to perform normal mode formations of CIE,, numbers the atoms, and identifies the analyses for the more stable conformers. The structural and conformations. For the identification of the atoms in struc- vibrational ab initio results are discussed in the light of tural parameters and whenever ambiguity does not occur, the Raman and FTIR spectra for the liquid phase, and of concen- numbers of the atoms are omitted and a left-to-right order of istration and temperature variation spectroscopic studies for the underlined atomic symbols in ~(H,)~(H,)C(H,)OJ the v(0H) region.adopted. The optimized geometries of C,E, are identified by a three-letter acronym specifying the CO-CC (lower case), OC-CO (upper case) and CC-OH (lower case) axes as Computational and Experimental Methods trans (t, T), +gauche (9, G) or -gauche (g’, G’)arrangements. Ab initio calculations at the MP2/6-31G* and MP2/6-31G*// 2-Methoxyethanol was obtained from Aldrich. Samples of 6-31G* levels were carried out with the GAUSSIAN 92 the pure compound and dilute solutions in CC1, were pre- program system” adapted to VAX computers.The absolute pared. IR spectra were recorded on a Nicolet FTIR 740 spectro- errors in bond lengths and bond angles with respect to the meter, equipped for the 4000-400 cm-’ region with a germa- equilibrium geometrical parameters are less than 1 pm and nium on CsI beam splitter and with a DTGS detector with 0.1”, respectively, and the stopping criterion for the SCF iter- CsI windows. Temperature variation was carried out using a ative process required a density matrix convergence of less variable-temperature AgCl cell (accuracy to f1 K).than loe8. Raman spectra were recorded on a triple monochromator Jobin-Yvon T MOO0 Raman system (focal distance 0.640 m, t Department of Physics. aperturefl7.5). The pre-monochromator stage was used in the tTt Fig. 1 Numbering of atoms for the four more stable conformations of CIEl (schematic) subtractive mode.The system is equipped with holographic gratings corrected for aberration (1800 grooves mm- '), a thermoelectrically cooled photomultiplier and a non-intensified CCD, for optional mono- and multi-channel detec- tion, respectively. The 514.5 nm line of an argon laser (Coherent, model Innova 300-05) was used as excitation radi- ation. Under the experimental conditions used, the estimated frequency errors were approximately 1 em-'. Results and Discussion Energies and Geometries Table 1 presents the relative conformational energies and dipole moments, and the most relevant optimized structural parameters for the four more stable forms, calculated at the MP2/6-31G* level.The order of conformational energies is the following: tGg' < gGg' < tTg -c tTt. As can be seen, tGg' and gGg' are the two more stable conformers and tGg' is J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the most stable conformer, in agreement with previous calcu- lations.lo Consideration of the short non-bonded atom O...O(H) distances (Table 1) suggests the and O...H(O) occurrence of intramolecular hydrogen bonds in the tGg' and gGg' conformers, between the ether 0 atom and the hydroxy H atom. These intramolecular interactions seem to be deci- sive to explain the greater stability of these conformers, since the non-hydrogen bonded G-type conformers g'Gt, tGt, and tGg are less stable than the T-type conformers tTg and tTt.Model comparison of the tGg' and gGg' structures suggests that the repulsive interaction between one of the methyl H atoms and one of the H atoms in the methylene group bonded to the alcohol 0 atom should be mainly responsible for the higher energy of the gGg' conformer. One important consequence of this repulsive interaction is the opening of the OCC angle in the gGg' conformer by 4.3" with respect to its value in the most stable conformer (Table 1). The stabilization effect of correlation increases with the number of gauche arrangements along the series tTt < tTg < tGg' < gGg', being largest for gGg' (Table 1). This order seems to indicate that the effect of electrons avoid- ing each other is less important for trans arrangements which tend to keep the lone pairs of distinct 0 atoms and the H atoms bonded to different atoms further apart.Considering the various trans +gauche transitions involving the studied conformers, the stabilization energy (kJ mol- ') increases in the following order: CO-CC (tGg' +gGg' = 3.3) < OC-CO (tTg + tGg' = 5.3) < CC-OH (tTt -+ tTg = 8.7). These correlation corrections are of the same magnitude, or in some cases larger than, conformational energy differences and change appreciably with the conformational transition considered. This stresses the importance of including corre- lation whenever conformational energy differences in this type of molecules are evaluated.In addition, since these correlation corrections reach their largest value for the trans -+gauche transition in the CC-OH axis, it is concluded that correlation is also of great importance to account ade- quately for the energetics of hydrogen-bond formation. Comparison of the MP2/6-3 lG* and MP2/6-31G*//6-31G* results enables us to assess the extent of the structure- sensitive correlation correction, as the first calculation includes geometry optimization at the MP2 level and the second uses the 6-31G* optimized geometries. As far as con- formational energy differences are concerned, these two cal- Table l conformer CO-CC; OC-CO; CC-OH/degrees EIE,"AE~ AEcorrelation b,c P/Ddbond lengths/pm' c-00-c c-cc-0 O-H bond angles/degrees' 0-c-cc-c-0 C-0-H non-bonded atom distances/pm 0...O(-H) 0-*H(-O)* Relevant MP2/6-31G* results for the more stable conformers of C,E, tGg' gGg' tTg tTt -173;60; -50 82; 54; -42 -178; 180; 72 180; 180; 180 -268.700 41 1 2 -.268.698 024 1 --268.694 894 1 -268.694 856 9 0.0 6.3 14.5 14.6 0.0 -3.3 5.3 14.0 2.86 2.87 2.35 0.32 142.0 142.4 141.8 141.7 142.4 143.1 141.7 141.8 151.3 151.9 151.8 151.3 142.1 142.1 142.7 142.7 97.4 97.5 97.2 97.1 105.8 110.1 107.4 107.1 110.4 109.8 11 1.1 106.3 104.6 104.6 107.3 107.6 275.1 277.6 363.8 358.4 225.1 221.8 392.1 429.4 E, = 2625.5 kJ mol-'.Energies in kJ mol-'; AE = E(conformer)-E(tGg').For tGg', Eforrelation= -1966.2 kJ mol-'. D = 3.33564 x C m. 'For identification of the atoms, a left-to-right order in C(H,)OC(H,)C(H,)OH is followed. Ta le 2 Vibrational spectra and PED, for relevant conformations of CIEl" tGg' gGg' intensity intensity description R (liq) IR (liq) freq. n-sc. (sc.) Raman IR PED (%)b freq. n-sc. (sc.) Raman IR PED (%)b 1 v(0H) 3608" 360T 4097 (3665) 43 57 1 :1w 4090 (3659) 42 56 1 [lo01 2 v(CH, as.A) 2988 2980 3307 (2944) 89 46 2 391 3308 (2945) 93 43 2 [89] 3 vCC(9)H as.] 2952 -3278 (2918) 88 61 3 :76] + 6[22] 3269 (2910) 93 44 3 [57] + 6[29] + 5[12] 4 v(CH, as.A) 2935 2931 3232 (2876) 61 99 4 1991 3241 (2884) 55 79 4 [98] 5 v[C(6)H as.] 2897 2895 3221 (2866) 23 103 5 1731 + 6[21] 3286 (2925) 68 71 5 [64] + 8[21] + 3[ll] 6 vCC(9)H s.] 2884 2882 3199 (2846) 168 11 6 1521 + 3[21] + 5[20] 3182 (2830) 29 13 6 [49] + 7[26] + 3[22] 7 v(CH, s.) 2834 2828 3186 (2834) 127 84 7 :65] + 8[22] 3186 (2834) 42 60 7 [46] + 8[26] + 6[12] + 5[10] 8 v[C(6)H s.] 2780 -3172 (2821) 9 28 8 170) + 7[25] 3205 (2851) 177 89 8 [52] + 7[19] + 5[12] 9 "(6)H21 1477 1472 1674 (1455) 10 1 9 1601 + 13[18] + ll[12] 1652 (1435) 16 4 9 [64] + ll[18] + 13[ll] 10 W(9)H,I 1457 1456 1660 (1442) 14 2 10 1891 + ll[ll] 1670 (1451) 8 2 10 [78] + 13[12] 1 1 S(CH, as.A') --1654 (1436) 5 8 11 1662 (1444) 6 5 11 [59] + 13[12] + 10[12] 12 S(CH, as.A") 1413 1408 1645 (1428) 18 5 12 c1011 1648 (1431) 18 3 12 r86i 13 6(CH3 s.) 1376 1369 1634 (1418) 11 5 13 [77] + 9[15] 1630 (1415) 11 14 ~[c(~)H,I -1326 1596 (1384) 4 42 14 [45] + 15[29] + 25[15] 1587 (1375) 3 15 oCC(6)HJ 1287 -1551 (1342) 2 65 15 1542 (1334) 2 16 6(COH) 1243 I233 1515 (1310) 11 14 16 I 1505 (1301) 12 17 tCC(6)H21 I199 1194 1398 (1203) 9 27 17 I1593 1445 (1 246) 7 18 y(CH, A) 1160 I157 1383 (1189) 5 37 18 I1311 + 17[21] + 24[16] + 19[15] 1359 (1 167) <1 87 18 c52 I + 24[26] 19 tCCP)H,I I129 1124 1322 (1134) 4 101 19 I1281 + 16[16] + 21[16] + 18[16) + 24[12] 1332 (1143) 6 28 19 [42 I + 16[27] 20 y(CH3 A") 1095 -1300 (1113) 6 5 20 [351 1298 (1112) 4 8 20 [85 21 v[C(1)-0(2)] 1072 1066 1284 (1099) 7 93 21 I1461 + l8[2l] + 24[15] 1279 (1094) 4 56 21 [21 + 23[20] + 24[12] + 22[ll] 22 YCC(6)H,I 1048 -1242 (1061) 3 15 22 [:37] + 23[36] + 26[13] 1214 (1035) 4 4 22 [27 + 18[17] + 26[14] + 24[12] + 25[ll] 23 v[C(9)-0( 12)] 1022 1017 1200 (1022) 2 127 23 [:38] + 25[18] + 26[14] + 16[13] 1206 (1028) 4 153 23 [60 ++ 26[24]16[14] + 24[20]24 v[O(2)-C(6)] 97 1 963 1129 (957) 5 17 24 I:28] + 21[22] + 26[12] 1097 (928) 8 6 21 [33 25 v(C-C) 895 891 991 (832) 6 12 25 i1311 + 22[27] + 23[22] 969 (812) 7 11 25 [35 + 22[27] + 23[20] 26 YCC(~)H,I 839 833 925 (771) 9 25 26 I1391 + 24[28] + 25[13] 911 (759) 9 36 24 [33 + 26[24] + 25[18] + 21[12] 27 S(CC0) 545 539 590 (466) 2 10 27 I1381 + 30[22] + 22[ll] + 29[10] + 26[10] 571 (448) 1 17 30 [43 + 27[39] + 26[17] + 22[13] 28 z(CC0H) --448 (336) 3 166 28 I:961 439 (328) 2 130 28 [93 29 S(C0C) --395 (288) 1 10 29 I1451 + 27[32] 468 (354) tl 8 29 [65 30 S(0CC) --296 (1 98) <1 5 30 I:39] + 29[30] + 32[27] 320 (219) <1 12 27 [38 + 30[36] + 29[12] 31 z(HC0C) --239 (146) <1 7 31 I1761 + 30[10] 217 (125) 6 7 31 [45 + 32[23] + 33[16] 32 z(OCC0) --158 (72) <1 5 32 I1811 + 30[13] + 31[17] + 28[ll] 151 (65) <1 5 32 [66 + 31[37] + 30[21] + 28[ll] 33 z(C0CC) --99 (18) tl 2 33 I391 79 (73) <l 1 33 [ll '3 + 32[28] + 31[26] + 28[19] " Frequencies in cm-'; n-sc.= non-scaled; sc. = scaled; for the frequency scaling see 'Experimental and Computational Methods'; v, stretching; 6, bending;w, wagging;y, rocking;t, twisting;5, torsion; s, symmetric; as, asymmetric. PED values smaller than 10% are not shown. Values obtained in diluted solutions in CCl, . culations do not yield results differing by more than a few tenths of a kJ mol-'.This leads us to conclude that the correlation correction at the MP2/6-3 lG*//6-31G* level accounts also for the energy change which results from geometry optimization at the MP2/'6-31G* level, i.e. the structure-sensitive and the dynamic correlation corrections are far from being additive. In addition, from the small changes of the conformational energy differences obtained by these two types of calculations, it can be concluded that the geometry optimization carried out with the correlated wave function tends to stabilize the intramolecular hydrogen bond, since it increases slightly the energy separation between the hydrogen bonded G-type conformers (tGg' and gGg') and the T-type conformers (tTg and tTt). Considering now the geometrical parameters in both calcu- lations, it is concluded that the optimization of geometry for the correlated wave function leads, in general, to an increase of bond lengths and a reduction of bond angles, following a previously mentioned trend which improves general agree- ment of the absolute values with experimental data.22.23 However, on the whole, the geometrical parameters undergo similar changes in different conformers. This observation is of great practical importance, as it indicates that the more time- consuming and expensive geometry optimization carried out at the MP2 level is not strictly necessary to evaluate changes in the geometrical parameters which are conformationally induced.Based on this conclusion we will obtain geometrical parameters profiles along different dihedral coordinates and force fields for different conformers using, in both cases, 6-3 lG* optimized geometries.At room temperature, approximately 92% of the molecular population should have the tGg' conformation, leading us to expect that this is the most dominant form in the gaseous phase at sufficiently low pressures or in very dilute solutions in inert solvents, when both oligomer formation and aggre- gation in general are not likely to occur. By contrast, the single symmetric form among those studied herein, i.e. the all-trans conformer, besides having an energy much higher than that of the tGg' conformer (14.6 kJmol-'), presents also a very small dipole moment (0.32 D), in fact, the smallest dipole moment among the considered conformers (Table 1).Hence, dipole-dipole non-specific intermolecular interactions like those which are expected to occur in diluted solutions of polar aprotic solvents should not contribute much for the stabilization of the symmetric all-trans conformer in solution. In order to characterize the most important intramolecular interactions in the various conformers, the conformational dependence of some relevant structural parameters (Table 1) is now considered and discussed. Starting with the more stable form (tGg'), the most pro- nounced variations, dihedral angles and 0.* .O(-H) and 0...H(-O) distances apart, occur for the CCO angle, which opens by ca. 4,and for the COH angle, which closes by 3", with respect to the tTt form.These variations are consistent with the formation of an intramolecular hydrogen bond (0.h.H-0) in the tGg' conformer, as they point clearly towards a more linear O...H-O axis and a shorter 0-n.H distance (see Table 1). It is interesting to point out that similar changes are observed in the same structural param- eters for the gGg' conformer, in particular, the CCO angle increases by 3.6" and the COH angle diminishes by 3". However, if these two conformers (tGg' and gGg') are com- pared (Fig. 1) and the tGg'+gGg' transition is considered, the OCC angle opens by ca. 4" in the gGg' form in order to reduce the steric repulsive interaction in this conformer between one of the methyl H atoms and one of the H atoms in the methylene group bonded to the alcohol 0 atoms (see Fig. 1).This repulsive interaction in the gGg' conformer may J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 explain the greater stability of the tGg' conformer. The tGg' +gGg' transition is concomitant with an increase of 2.5 pm in the O...O(-H) distance and a reduction of 3.3 pm in the 0.. .H(-O) distance (Table 1). The latter variation seems to result from the fact that the ether 0 atom becomes more in line with the H-0 bond in the gGg' conformer than in the tGg' conformer, as the C-0 and 0-C bonds increase slightly in the gGg' conformer, Finally, for the tTg form, the most pronounced geometrical change with respect to the tTt conformer occurs for the CCO angle which is ca.5" larger in the tTg form in order to reduce a repulsive steric interaction between the hydroxy H atom and one of the H atoms in the methylene group bonded to the ether 0 atom (see Fig. 1). . In conclusion, the OCC, CCO, and COH bond angles are very sensitive to different conformational changes due to internal rotations about the CO-CC, CC-OH and OC-CO axes, respectively. Vibrational Frequencies In this section, the 6-31G* derived force field is used to calcu- late the frequencies, the potential-energy distribution (PED), and the IR and Raman intensities of the more relevant con- formers. Emphasis is given to the modes which exhibit large sensitivities, in frequency and/or composition, to conforma- tional changes. Table 2 presents the vibrational calculated results for the tGg' and gGg' conformers.The occurrence of two 'windows' at ca. 2700-1500 and 800-600 cm-', both in the calculated and experimental Raman and IR spectra of C,E,, immediately suggests the consideration of three spectral regions for the discussion of the vibrational spectra: 3700-2700 cm-', 1500-800 cm-', and below 600 cm-'. However, since the OH stretching is particularly sensitive to aggregation which, in turn, should not be involved in the discussion of the conformational degrees of freedom of the monomer for clarity of this text, the v(0H) region will be separately considered at the end of this section in a 'spectral region' of its own. For sufficiently diluted solutions in CCl, , the occurrence of two IR spectral features associated with v(0H) in monomeric species is now well e~tablished.'~~*~~~~*~~~-~~With the excep- tion of this spectral region, the number of intense (vs, s, and m) and well defined bands in the spectra of the liquid at room temperature does not exceed the total number of distinct fre- quencies expected for the fundamental vibrations of a single conformer (3N -6 = 33).This observation is consonant with the large population calculated for the most stable conformer at room temperature (92%). Hence, only the frequencies of the relatively intense Raman and IR bands of the pure liquid (Fig. 2) and the calculated frequencies for the most stable conformer were considered in the vexp vs. vcalc linear regres- sion.3200-2700 cm-'Region This region includes the v(CH,) and v(CH,) vibrations which correspond, for a single molecule, to seven CH oscillators and as many distinct frequencies since no degeneracies occur. The four highest frequencies should be ascribed to the anti-symmetric modes (two for the CH, group, one for each CH, group), the lowest three frequencies should correspond to the symmetric modes. This expected general pattern is observed for the tGg', gGg' and tTt conformers. The antisymmetric components of the CH, groups, in par- ticular of (0)CH2,change appreciably, both in frequency and composition, with conformation. Both v(CH, as A') and v(CH, as A") calculated non-scaled frequencies increase with the number of gauche arrangements along the series tTt < tTg < tGg' < gGg'.In addition, these frequencies (cm-') present reasonable linear correlations with methyl CH J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3200 2400 1600 800 I I I I - bond lengths (pm): v(A’) = -134.99r(C1H,)+ 17 537 (correlation coefficient R2 = 0.892); v(A”) = -100.54r(C1H4)+ 13 812 (R2= 0.973). Hence, it can be concluded that the methyl CH bond lengths, in particular r(C,H,) and r(C1H4), are appreciably sensitive to conformation. For isolated CH stretching vibrations of alkanes, good linear correlations of v(CH) against r(CH) have already been con~idered.~~ 1500-800 an-’Region This region includes the CH, and CH, bending vibrations, the CC and CO stretching modes, and the COH bending vibration, corresponding to a total of 18 fundamental vibra- tions in the monomer.In particular, 14 bending vibrations (5 involving the CH, group, 4 for each CH, group, 1 COH bending), and 4 stretching vibrations (1 CC and 3 CO) are considered in this region. The modes which exhibit the largest sensitivities, both in frequency and composition, to conformational changes are 6(COH), v(0C) and v(CC), as well as the wagging, twisting and rocking vibrations of the CH, groups. In the most stable conformer, an extensive mixture of coordinates occurs for tCH,, for the CH, group adjacent to OH. In addition, the calculated 6COH frequency exhibits a decrease of ca. 150 cm-’ on going from the conformations with a gauche arrangement in the CC-OH axis (tGg’, gGg’, tTg) to the all- t trans conformer.This large frequency shift is consonant with the calculated profile of the COH angle variation us. CC-OH, as discussed above. Region below 600cm-’ For a single molecule, this region includes a total of seven fundamental vibrations, namely, three skeletal bending vibra- tions (CCO, COC, OCC) and four torsional modes (COCC, OCCO, CCOH, HCOC). Among the conformers considered herein, only for the most stable conformer does S(0CC) mix with z(OCC0) and to an appreciable extent (27%).It is interesting to point out that, for n-butane, a similar vibrational coupling has been previously observed and discussed between S(CCC) and z(CCCC), also for the gauche c~nformation.~’ For z(COCC), z(OCC0) and z(HCOC),the frequency shifts due to conformational changes are larger whenever the con- formational coordinate corresponds to the predominant tor- sional mode involved.Generally speaking, z(CC0H) and z(C0CC) are pure modes in all the considered conformers, except for gGg’, where z(C0CC) mixes appreciably with the remaining torsional coordinates. For the tTt and tTg conformers, z(HC0C) does not change appreciably, either in frequency (ca. 150 cm-’)or in composition [over 86% of PED is from t(HC0C) and approximately 12% from z(0CCO)l. In gGg', z(HC0C) occurs at the lowest frequency value among all the studied conformers, 125 cm- ',and mixes appreciably with z(C0CC) and z(OCC0). For this all-gauche form, an appreciable steric interaction is expected to occur between H atoms of the methyl and of the C(9)-methylene groups.Finally, for tGg', z(HC0C) occurs at 146 cm-', with a significant 10% PED element from S(0CC). On the whole, z(HC0C) is not expected to be particularly sensitive to conformational changes, except through an indirect mechanism, whenever repulsive steric interactions involving at least one of the methyl H atoms occur. v(0H) Stretching Region This region displays various important spectral features, occurring, both in the FTIR spectra (Fig. 3) and in the Raman spectra (Fig. 4). Among these, two are ascribed to monomeric species and occur at ca. 3640 (shoulder) and 3608 cm-'(hereafter referred to as A and B, respectively).In addi- tion, a very wide and convoluted complex of bands, referred to as C, occurs in the range 3550-3200 cm- Band B is the most sharp feature which becomes progres- sively more prominent at lowering concentrations of C,E, in CCl,, both in the FTIR and Raman spectra. In paricular, for a 0.01 mole fraction, the IR peak absorbance of B is twice that of C (Fig. 3). It is interesting to mention that the same approximate ratio of intensities (B/C x 2) is observed, in the Raman spectra (Fig. 4), for a much less dilute solution (0.09 mole fraction in CCl,). In addition, for the same concentra- tion, the Raman spectrum of feature C clearly shows two maxima at ca. 3460 and 3310 cm-', probably ascribed to different degrees of aggregation, whereas the same feature C shows a single maximum in the FTIR spectrum.wavenu m ber/cm -' 1.o h cn w.-C P Y \-3 0.0 3600 3500 3400 3300 3200 wavenumber/cm-' Fig. 3 FTIR spectra of 2-methoxyethanol in CCl, solutions, after smoothing:(a)0.02 mole fraction solution spectra at 1,263; 2,283; 3, 303 and 4, 323 K; (b)room temperature spectra with mole fraction: 1,0.02; 2,O.Ol; 3,0.002 and 4,0.0002 J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 4 3600 3520 3440 3360 3280 3200 wavenumber/cm-Fig. 4 Raman spectra of 2-methoxyethanol in CC1, solutions at room temperature, after smoothing. Mole fraction: 1, pure liquid; 2, 0.50;3,0.17 and 4,O.m The absorbance ratio A/B (A is the high-frequency shoul- der of B) increases with temperature [Fig.3(u)], but is rela- tively insensitive to concentration variation [Fig. 3(6)], at least for the range of concentrations considered (mole frac- tions varying from 0.02 to 0.0002). By contrast, C is a very broad band whose width increases with concentration, its absorbance and peak frequency being dependent both on temperature and concentration. In particular, the absorbance of C increases and its peak frequency decreases with both reduction in temperature and increase in concentration (Fig. 3)-Previous papers on C,E, based their assign- 1*2*4*798~13-1 ments of IR bands A and B on assumptions of the more probable molecular structures, and estimated enthalpy differ- ences from the inverse temperature dependence of the logarithm of the ratio of intensities.To the best of our know-ledge, it is the first time that ab initio calculations at the levels considered here are used to provide a theoretical and quanti- tative assessment of the vibrational spectra and to shed light on the interpretation of the above-mentioned spectral fea- tures. The order of the a6 initio scaled v(0H) frequencies (cm- ') for the various conformers is gGg'(3659) < tGg'(3665) < tTg(3678) < tTt(3690). These results indicate an increase of v(0H) with the number of trans arrangements, a trend opposite to that observed for v(CH,,). In addition, the hydrogen-bonded conformers (tGg' and gGg') have close v(0H) scaled frequencies (Av = 6 an-'), a result which suggests the assignment of these conformers to the same observed vibrational band.As expected, the T-type conformers have higher v(0H) frequencies than the G-type conformers, since the OH oscillators of the latter conformers are intramolecularly hydrogen bonded. It is also interesting to notice that the v(0H) frequency of the all-trans conformer J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 is shifted ca. 30 cm-’ from the centre of the tGg’-gGg’ pair, to higher frequencies. The same approximate frequency differ- ence between bands A and B is observed both in the FTIR (Fig. 3) and Raman spectra (Fig. 4). The ab initio results clearly point to the tGg’ conformer as the best candidate for band B, since this conformer corre- sponds to the highest monomer population (92% at room temperature).A small contribution to band B from the gGg’ conformer is also expected, since the vibrational calculations yield close v(0H) frequencies for the tGg’ and gGg’ con- formers (Av = 6 cm-’), and the MP2/6-31G* calculations yield a significant population for the gGg’ conformer (ca. 7%). The proximity of the calculated v(0H) frequencies for the tGg’ and gGg’ conformers is not surprising, since the different CO-CC arrangements in these two conformers are not expected to affect the v(0H) frequency appreciably. For the all-trans conformer, the calculated v(0H) frequency is shifted by ca. 30 cm- to higher frequencies with respect to the centre of v(0H) for the G-type conformers (3662 cm-’), in accord with the observed frequency separations between the centres of bands A and B in the FTIR and Raman spectra (ca. 32 cm-’; Fig.3 and 4). This agreement points clearly to the tTt conformer as the best candidate for band A, though a negligible contribution from the t Tg conformer should not be totally excluded as this latter conformer is very close in energy to the all-trans conformer (Table 1)and the calculated v(0H) frequencies for the tTt and t Tg conformers fall within the observed width of band A. In addition, while the barrier for the tTt+tTg transition has not been evaluated, it is unlikely to exceed the value of thermal energy (RT = 2.5 kJ mol-’ at room temperature), hence providing a mechanism for interconversion between the tTt and t Tg conformers.The deconvolution of bands A and B is difficult and subject to large errors at least for the weak shoulder, A. However, the relative intensity of this shoulder, A/B, was found to be approximately insensitive to concentration varia- tion in a non-polar solvent (CCl,), in agreement with the very low dipole moment of the all-trans conformer (Table 1). In addition, from the temperature dependence of the intensities ratio ZJZB, it is possible to estimate a conformational energy difference which falls in the range 7-10 kJ mol-’, in accord with the energy difference between the gGg’ conformer and the T-type conformers. This observation gives further support to the above assignment of conformers to bands A and B. While the calculated intensities should not be considered totally reliable and refer to the isolated molecule as, in fact, all the calculated quantities, it is worth pointing out that the calculated v(0H) Raman intensity for the t Tt conformer approximately doubles the corresponding intensities for the remaining conformers studied, whereas all the conformers exhibit approximately the same calculated IR line intensities.Apparently, these theoretical results do not agree with experi- ment, since band A is observed both in the Raman and FTIR spectra. However, as the concentration of C,E, is progres- sively reduced, band A becomes clearly distinguishable in the Raman spectra for much less dilute solutions than those which enable observation of the same feature in the FTIR spectra.In fact, for band A to become evident in the FTIR spectra, a more dilute solution (roughly, one tenth of the mole fractions ratio) had to be prepared. These findings lead to the conclusion that the v(0H) band is a more sensitive probe to the presence of monomers in the Raman spectra than in the FTIR spectra. Since the theoretical results refer to the isolated molecule, care should be exercised in extrapolating the calculated quan- tities to the liquid-phase spectra, especially to the pure liquid or to concentrated solutions. In particular, the formation of intermolecular hydrogen-bonded oligomers is likely to affect both the geometries and energies of the monomers present in the oligomeric species, hence leading to changes in the observed IR and Raman spectra.Moreover, non-specific intermolecular interactions may also affect the structures and vibrational spectra of monomers. However, both the structur- al and vibrational results presented herein point to a clear distinction between the hydrogen bonded G-type conformers (tGg’ and gGg’) and the higher energy T-type conformers (tTg and tTt). 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