Vibrational spectra and molecular conformations of alkoxy(alkylthio)methanes

wrnal of Molecular Structure, 96 (1983) 267-275 lsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

‘IBRATIONAL SPECTRA AND MOLECULAR IF ALKOXY (ALKYLTHIO)METHANES

CONFORMATIONS

IROATSU MATSUURA and HIROMU MURATA ‘epartment of Chemistry, Faculty aka-ku, Hiroshima 730 (Japan)

of Science,

Hiroshima

University,

Higashisenda-machi,

IASAAKI SAKAKIBARA acuity of General Education,

Tottori

University,

Koyama-cho,

Tottori

680 (Japan)

Zeceived 11 June 1982)

BSTRACT The Raman and infrared spectra of methoxy(methylthio)methane, ethoxy(methylrio)methane and methoxy(ethylthio)methane have been measured in the liquid, glassy id crystalline states. Normal coordinate treatment of these molecules has been perumed in order to analyse the spectra and determine the molecular conformations. ethoxy(methylthio)methane crystallizes in the gauche-gauche conformation and ethoxynethylthio)methane in the transgauche-gauche conformation about a series of axes ,H,-0-CH,-SCH,. It is shown that many conformers are present in the liquid state of le molecules studied. JTRODUCTION

In a previous communication [ 11, the Raman spectra of methoxy(methyl.io)methane CH30CH2SCH3 were reported in relation to the molecular n-iformation. In the present work, spectroscopic studies on alkoxy(alkyl.io)methanes, ROCH,SR’, which are structurally inttrmediate between DCH,OR’ and RSCH,SR’, were extended to ethoxy(methylthio)methane, IH50CH2SCH3, and methoxy(ethylthio)methane, CHJ OCH2SC2H5.Analysis i the Raman and IR spectra, based on a normal coordinate treatment, ,ovides information on the molecular conformations of these molecules. 3r CH30CH2SCH3, the IR spectra were obtained for the first time and the aman spectra were carefully remeasured. The results of the spectroscopic udies of these molecules are reported here. WERIMENTAL

Samples of ROCH,SR’ (R, R’ = CH3 and C,H,) were prepared from OCH$l and NaSR’. The Raman spectra were recorded on a JSOL JRS1ODspectrophotometer equipped with a CR-3 argon-ion laser. The spectra ere measured in the liquid state and in the solid state, obtained on cooling 122-2860/83/0000-0000/$03.00

0 1983 Klsevier Scientific Publishing Company

268

the liquid substance by liquid nitrogen. The IR spectra of the liquid and solid states were recorded on a Perkin-Elmer 621 spectrophotometer. The solid for IR measurement was obtained by depositing the vapourized substance onto a caesium iodide plate cooled with liquid nitrogen. The Raman spectra of CzH50CHzSCH3, CH30CH2SC2H5 and CH30CH,SCH3 are shown in Figs. 1-3 and the observed and calculated wavenumbers and vibrational assignments based on a normal coordinate treatment are listed in Tables l-3. NORMAL COORDINATE

CALCULATION

Normal coordinate analysis was carried out using a computer program MVIB [2] on a HITAC M-200H computing system at Hiroshima University. The force constants in the group coordinate force field [ 21 were initially transferred from alkyl ethers and alkyl sulphides [ 3,4] . The calculated results from these zero-order force constants satisfactorily reproduced the observed wavenumbers so that only the constants for the 0CH.S part were refined to give closer agreement between the observed and calculated wavenumbers. The structural parameters used were: r(C-C) = 1.539, r(C-0) = 1.410, r(C-S) = 1.816, r(C-H) = 1.100 A, L(C-C--O) = 108.1, L(C-0-C) = 111.8, L(C-C-S) = 113.6, L(C-S-C) = 98.6, L(O-C-S) = 114.1, L(C-C-H) =

(b)

dd 1500

1000 Wovenumber /-mu’

500

-

0

Fig. 1. Raman spectra of C,H,OCH,SCH,: (a) liquid state at room temperature; (b) glassy state at liquid nitrogen temperature; (c) crystalline state at liquid nitrogen temperature.

269

Wavenumber/cm

ig. 2. Raman spectra of CH,OCH,SC,H,: ate at liquid nitrogen temperature.

(a) liquid state at room temperature;

(b) glassy

10.4,L(O-C-H) = 109.8 and L(S-C-H) = 108.8”. Dihedral angles of 180, D and -6O”, as defined in refs. 2 and 3, were assumed for the skeletal conm-nations of tram, gauche+ and gauche-, respectively.

A full list of the force constants used in the calculation has been deposited ith the British Library at Boston Spa, Yorkshire, U.K., as Supplementary ublication No. SUP. 26230 (4 pages). 1

g. 3. Raman spectra of CH,OCH,SCH,: (a) liquid state at room temperature; (b) glassy ate at liquid nitrogen temperature; (c) crystalline state at liquid nitrogen temperature.

!70 FABLE

1

vTibrationa1 wavenumbers for C,H,OCH,SCHja xamanwavenumbersb

(cm“)

IR wavenumbersb~c (cm-‘)

Liquid

Glassy

Crystalline

Liquid

Glassy

Crystalline

1482 w 1458m

1484vw 1460 w

1482 w

1484 w 1460 w

1476m 1464m

1444 m

1444 w

1442m 1432 m

1440 m 1435 m

1428m

1424 w

1485 1463 1452 1444 1431 1422

1395 w

1395vw

1395vw

1393m

1367 w 1325 w 1302~ 1272~ 1262 w

1365~~ 1325 w 1305vw 1272 w.sh 1262 w

1367~~ 1327 w 1309 VW 1270~ 1261~

1365~~ 1326~ 1302m 1273 W.&I 1264m

1365~ 1326 m 1301 m 1270 m 1261 w,sh

1160~~ 1112w 1092 w 1086 w

1157vw 1115 w

1157vw 1116 w

1393m 1383 w,sh 1366 VW 1325 w 1302 s 1273 w,sh 1260m 1202 vw 1169m 1161 w.sh llllm.sh

1443 w 1438m 1425 m 1416 w 1394m

1080 w

1017 w

1085 vs

1083 vs

1015 w 1002 w

1065 vs 1014m 1002m

1017 m 1003 s

1065 w 1060~~ 1013 w 1017 vw 1000 vw,sh 1000 vw

w w w w w w

972vw

968nv

97ovw

908 VW 877 vw 839 m

908vw

908 vw

839 w 816 vw

839 w 822vw

’ 776~~ 731 s 708 w 693 s,sb 684 vs 512m 448 vw 37ow 330 w 320 w 271 w 238 w 219 m

737 s

684~s 520 w.sh 513 w

1731s 1725 vw.sh 694 w.sh 687 vs 514w

974w 961vw 908 m 871 VW 838 s

1483 1459 1452 1445 1443 1431

(C)CH,(O) scissor CH,(Cj ip asym. deform. CH,(C) op asym. deform. (S)CH, ip asym. deform. (O)CH,(S) scissor (S)CH, op asym. deform.

1392 (C)CH,(O) 1373 1321 1304 1274 1251

wag

CH,(C) wm. deform. (S)CH, sym. deform. (O)CH,(S) wag (O)CH,(S) twist (C)CH,(O) twist

1154m 1159 CH,(C) OP rock 1119 m 1129 CH,(C) ip rock 1094 m,sh 1073 (C)O-CH,(S) stretch, 1082~s (C) cH,--O(C) stretch 1017 s 1004 vs

1018 (S)CH, 999 (S)CH, (C)O--CH,(S)

ip rock ip rock. stretch

962w 909 w

966m 909 s

958 (S)CH, OP rock 928 (O)CH,(S) rock

836 s 825~~

842 (C)CH,-O(C) stretch 816 (C)CH,(O) rock

8Olvw 78Ovw 731 s

836 m 82Ovw 804vw 781 vw 732m

731 s 727 vw,sh

736 S-CH,

stretch

682~s

683 s

690 s

680 CH,-S

stretch

515vw 454vw 373w

51ovw

505 COC bend, OCS deform.

373w

382 CSC bend, OCS deform.

320~

320~

312 CC0 deform.

375w

380 w

512~~ 448 VW 369 vw

323 w 276 VW 243 w 225 w

328~

320 VW

234 w 207vw 154w 108 w 99w 92w 79w 58~ 39w

1171vw.sb 1159w 1115m

Calculated wavenumber (cm-‘) and mode for the TGG formd

254 CH,-C torsion 211 CSC bend

aWavenumbers greater than 1500 cm-’ are not included. bAbbreviations: vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder. %frared spectra below 300 cm+ were not recorded. dThe molecule assumes the TGG form in the crystalline state. Calculated wavenumbers and modes below 200 cm“ and those for other forms are not given. ip, in-plane; op, out-f-plane.

271 ,E 2 tional wavenumbers for CH,OCH,SC,H,a n wavenumbersb

1

IR wavenumbersb* c (cm-‘)

1

Glassy

Liquid

w,sh

1472 w

1466 m 1463 m

1467 m 1461 m

m m

1451 m 1441 m 1429 w 138Ovw

1449 m

1450 m

1423 1375 1325 1311 1278 1263

1422 m 1375 m

W VW

W W W VW VW VW VW W W W W VW W W W

m vw vw s S vs W

w,sh W VW W W VW

m

1319vw 1288 w 1266~~ 1253 VW 1237 vw 1185 vw 1156~~ 1080 w 1064 w,sh 1059 w 1048 w 1032 vw 983w 971w 948 w 897 m 788 vw 759vw 710s 670s 655 vs 474w 408 w,sh 388 w 348 w 323vw 301 w 263 vw 211m =50m

m m w,sh m m m

1183s 1153w 1083 vs 1057 w,sh

978 w,sh 970 w 943 m 897 s 782~ 757 m 711s 668 w 652 m 471 w 386~ 343 vw 321 VW

-

Glassy

Calculated wavenumber (cm-‘) and mode for the GGG formd 1467 1463 1462 1462 1454 1443 1431 1377

CH,(O) ip asym. deform. (C)CH, op asym. deform. CH,(O) sym. deform. (C)CH, ip asym. deform. CH,(O) op asym. deform. (O)CH,(S) scissor (S)CH,(C) scissor (C)CH, sym deform.

1315 m 1285 m 1263 m 1250 vw,sh 1236~~ 1183s 1154w 1078 vs

1309 1280 1273 1255

(O)CH,(S) wag (O)CH,(S) twist (S)CH,(C) wag (S)CH,(C) twist

1059 w,sh 1049 w,sh 103ovw 981 w 969m 944 m 896 s 785 w 756 m 710s 669 w 654 m 472 m

1051 (C)CH, ip rock

387 m 347 w 322vw

1190 CH,(O) ip rock 1157 CH,(O) op rock 1080 CH,+ stretch

1028 (C)CH, op rock 981 CH,--CH, stretch 934 O-CH, stretch 886 (O)CH,(S) rock 762 (S)CH,(C) rock 714 (O)CH,-S(C) stretch 648 (C)S-CH,(C) 462 COC bend

stretch

391 CSC bend 324 SCC deform. 253 C-CH, 201 CH,-G

torsion torsion

iee footnotes a, b and c, respectively, to Table 1. dCalculated wavenumbers and modes

r 200 cm-’ and those for other forms are not given. ip, in-plane; op, out-of-plane.

272 TABLE 3 Vibrational wavenumbers for CH,OCH,SCH,* Raman

wavenumber& (cm-‘)

IR wavenumbersbJ

(cm-‘)

Liquid

Glassy

CrYstalline

Liquid

Glassy

Crystalline

1465m

1469 m

1471 m

1466 w

1465m

1459 VW

1444 m

1453 w 1445 w

1448 m.sh

1448 m,sh

1448 m

1442 m

1440 m

1433 m

1424m 1327 w 1305 w 1280 w

1422 m 1330 w 1307w 1285 w

1431 m 1422 w 1320 w 1293 w 1280m

1183~~ 1154w 1083 w

1184-/w 1155 w 1079 w

1008 VW 96Ovw 938w 920m

1012vw 963vw

888 m 786 w 731 s

888m

921 m

731 s

1182~~ 1151 w 1075w 1018 w 1015w 1 963vw 918 m 893 w.sh i 889 m 134vs i 12lw

1448 w

1437vw

1428 w 1327~~ 1303m 1280~

1424m 1327 w 1302m 1282 m

1184 s 1155vw 1082 vs

1181 s 1150w 1076 vs

1421 w 1325~~ 1296m 1279m 1187 VW& 1181 s 1150w 1076~s

1008~ 962w 937 w,sh 921 s

1012 w 961w

1015m 963w

889 w 186 VW 733 m

881 m

921 s 890 m 887 m

734m

733 m

920 s

108 m 683~s 473 m 408 w

684~s 473 m

685~s 471m

685s 472~1

682 s 472 w

684m 470w

379 m

383 m

379 m

379vw

377 w

376 w

243 m

250m

241 VW

331 VW

239 m

195vw =115m

IJ 55m

235~ 205 VW 176~ 133m 107 m 91w 8lm 7ow 60m 42~ 32~

Calculated wavenumber (cm-‘) end mode for the GG formd 1467 CH,(O) deform. 1462 CH,(O)

ip asym. sym. deform.

1454 CH,(O) op asym. deform. 1445 (SjCH, ip asym. deform. 1443 (O)CH,(S) scissor 1432 (S)CH, op asym. deform 1321 (SjCH, sym. deform. 1309 (O)CH,(S) wag 1278 (O)CH,(S) twist 1190 CHJO) 1157 CH,(O) 1080 CH,-C

ip rock OP rock stretch

1007 (SW& ip rock 958 (S)CH, OP rock TG 939 O-CH, stret& 932 O-CH, stretch 883 (0)CHJ.S) rock TG 785 CH,-S stretch 736 S-CH, stretch TG 705 S-CH, stretch 679 CH,-S stretch 462 COC bend TG 413 CH& stretch, COC bend; TT 401 COC bend 392 CSC bend. OCS deform. GT 352 OCS deform.; TT 344 CSC bend, OCS deform. 235 CSC bend: TG 244 CSC bend 190 CH,-C

torsion

a*bSee footnotes a and b, respectively, to Table 1. CInfrared spectra below 200 cm-’ were not recorded. dThe molecule assumes the GG form in the crystalline state. Calculated wavenumbers and modes for other forms are given only when the bands are assigned preferentially to these forms. Calculated wavenumbers and modes below 190 cm-’ are not included. ip, in-plane; op, out-of-plane.

273 lESULTS AND DISCUSSION

?thoxy(me

thylthio)methane,

C,H50CH,SCH,

The unannealed solid of CLH,OCH,SCH, for both Raman and IR measurenents was found to be in the non-crystalline glassy state, which, on annealng, changed into the crystalline state. Most of the Raman and IR bands n the liquid state persist in the glassy and crystalline states. Comparison of he wavenumbers of the crystalline-state bands with the results of the normal oordinate treatment indicates clearly that the observed wavenumbers are onsistent only with those calculated for the molecular conformation of runs-gauche-gauche (TGG) for a series of axes C2H5-O-CH2-SCH3 (see ‘able 1). The spectra in the glassy state are similar to those in the crystalline tate indicating that the molecule assumes predominantly the same conormation as in the crystalline state. In the liquid state, additional bands to those in the crystalline state are ssigned to conformers other than the TGG form. The C-S stretching waveumbers are well known to be sensitive to the molecular conformation 5-81. Accordingly, the observed bands in the C-S stretching region (80050 cm-‘) were examined on the basis of a normal coordinate treatment. ‘he calculated results show that the Raman band at 731 cm-’ is assignable 3 the XGY and XTT forms (X and Y being either T or G), the band at 08 cm-’ to the XGT forms, the band at 693 cm-’ to the XTG forms, the and at 684 cm-’ to the XGG forms and the very weak band at 776 cm-’ 1 the XTY forms, for a series of axes CzH5-O-CHz-SCH3. Therefore, le XGG, XTG and XGT forms are definitely present in the liquid state. [owever, no positive evidence for the existence of the XTT form is found. The molecular conformations in each of the aggregation states are sumiarized in Table 4. rethoxy(ethyZthio)methane,

CH30CHzSC2H5

Cooling liquid CH30CH2SC2H, gave the non-crystalline glassy state, which id not change, unlike C2H50CH2SCH3, into the crystalline state on repeated lnealing in both Raman and IR measurements. Figure 2 indicates that ABLE 4 olecular conformations

of ROCH,SR’a

ggregation state

CH,OCH,SCH,

C,H,OCH,SCH,

CH,OCH,SC,H,

rystalline lassy quid

GG GG GG, TG, GT

TGG Predominantly TGG XGG, XTG, XGT

GXY and others GXY and others

: and Y are either T (tram) or G (gauche).

274

almost all the bands in the liquid state persist in the glassy state, although several differences in spectral intensities are noted between the two states. In the region between 800 and 650 cm-‘, five bands are observed in the Raman and IR spectra. Normal coordinate calculations show that the bands at 654 and 668 cm-‘, which are stronger than the others in the Raman scattering but not necessarily strong in the IR absorption, are due to the S-&H5 stretching mode. A closer examination reveals that the 654 cm-’ band is associated with the XGG forms (X being either T or G) for a series of axes CH@-CH2-S+2HS and the 668 cm-’ band may be assigned to the XTY and XGT forms (Y being either T or G) for the same axes. The band at 710 cm-‘, which is the strongest in the IR spectra between 800 and 650 cm-‘, is assignable to the OCH2-S stretching mode of the GXY forms. The remaining two bands at 784 and 758 cm-‘, which exhibit very weak Raman intensities, are assignable to the OCHz-S stretching mode of the TXY forms in addition to the (S)CH2 rocking mode of all the possible conformers. Accordingly, these bands are not suitable for discussing the molecular conformation. However, it may be rationalized that the TXY forms exist in very small quantities, if at all, as suggested by the very weak Raman intensities, since the C-S stretching mode generally gives rise to strong Raman intensities. In the skeletal deformation region, the band at 474 cm-l is assigned only to the GXY forms. The above results lead to the following conclusion on the molecular conformation of CHJOCH2SC2HS.In the liquid and glassy states, the presence of the GGG, GGT, GTT, GTG and GTG’ forms, i.e. all possible forms of GXY, is likely, whereas the presence of other forms, i.e. TXY, is less likely. Comparison of the liquid- and glassy-state spectra shows that the spectral features do not differ much between the two states, although the temperature of the spectral measurement is considerably different. This observation implies that the energy differences between the various conformers are small and that none of the conformers is predominant in conferring conformational stability. The experimental fact that this molecule barely crystallizes may be a consequence of this phenomenon. Methoxy(methylthio)methane,

CH30CH2SCH3

The observed wavenumbers for CH30CH2SCH3 have been reported previously only for the Raman spectra [l] . Table 3 lists the Raman wavenumbers of the remeasured spectra as well as the IR wavenumbers obtained for the first time in the present work. The previous study [l] showed that the molecule assumed the GG conformation in the crystalline and glassy states for a series of axes CH30--CH2-SCH3 and the GG and TG conformations in the liquid state, the presence of the GT and TT conformers being undetermined. The spectral measurements of this study further clarify that, in the liquid state, either the GT or TT form, or both the GT and TT forms

275

re present as evidenced by the observation of Raman bands at 337 cm-’ assignableto the GT and TT forms) and 408 cm-’ (TG and TT forms). Condering that the GG conformation of this molecule is the most stable in the quid state [l] , the presence of, at least, the GT form is probable. onformational

stability

According to the results obtained in the present study, the GG conformaon is energetically most stable about the bond axes (C)O-CH,-S(C). lthough no quantitative estimation of the energy differences among the >nformers was made, this conformational stability is in agreement with rat for the (C)O-CH,-O(C) and (C)S-CH,-S(C) groups. The enthalpy lfference between the GG and TG forms of CH30CH20CH3 in the liquid ate was determined by Raman spectroscopy to be 1.2 kcal mol-‘, the GG um being the more stable [ 91. It is now established that the gauche conumation is more stable than the trans conformation about the (O)C-O(C), ;)C-O(C), (S)C-S(C) and (O)C-S(C) axes. This conformational stability mtrasts with that about the (C)C-O(C) and (C)C-S(C) axes; namely the ans conformation about the former axis is much more stable than the mche [lo] , and the gauche conformation about the latter axis is only ightly more stable than the tram [ 11, 121. These findings indicate that le oxygen or sulphur atom adjoining the C-O or C-S bond lowers the iergy of the gauche conformation about this bond axis relative to that of Letrans conformation. CKNOWLEDGEMENTS

We thank Mr. Takayuki Shuin for his assistance with the experiments. EFERENCES H. Matsuura, K. Kimura and H. Murata, J. Mol. Struct., 64 (1980) 281. H. Matsuura and M. Tasumi, in J. R. Durig (Ed.), Vibrational Spectra and Structure, Elsevier, Amsterdam, 1982, Vol. 12, Chap. 2. T. Shimanouchi, H. Matsuura, Y. Ogawa and I. Harada, J. Phys. Chem. Ref. Data, 7 (1978) 1323. H. Matsuura and H. Murata, Bull. Chem. Sot. Jpn., 55 (1982) 2835. N. Nogami, H. Sugeta and T. Miyazawa, Bull. Chem. Sot. Jpn., 48 (1975) 2417. M. Ohta, Y. Ogawa, H. Matsuura, I. Harada and T. Shimanouchi, Bull. Chem. Sot. Jpn., 50 (1977) 380. M. Sakakibara, I. Harada, H. Matsuura and T, Shimanouchi, J. Mol. Struct., 49 (1978) 29. H. Matsuura, J. Matsumoto and H. Murata, Spectrochim. Acta, Part A, 36 (1980) 291. M. Sakakibara, Y. Yonemura, H. Matsuura and H. Murata, J. Mol. Struct., 66 (1980) 333. T. Kitagawa, K. Kusaki and T. Miyazawa, Bull. Chem. Sot. Jpn., 46 (1973) 3685. N. Nogami, H. Sugeta and T. Miyazawa, Bull. Chem. Sot. Jpn., 48 (1975) 3573. M. Sakakibara, H. Matsuura, I. Harada and T. Shimanouchi, Bull. Chem. Sot. Jpn., 50 (1977) 111.