Specrrochimica Acta, Vol. 42A, No. 8, pp. 913-918, 1986.

Printed in Great Britain. 0584-8539/86 S3.00 + 0.00

PW8Ut,OII Journals Ltd.

Vibrational spectroscopic studies on 1,3-dithiacyclohexane and some 2-substituted 1,3-dithia-2-boracyclohexanes

GEORGE DAVIDSON and KENNETH P. EWER

Department of Chemistry, University of Nottingham, Nottingham, NG7 2RD, U.K.

(Received 7 November 1985; inPnalform 10 February 1986; accepted 12 February 1986)

Abstract-Infrared and Raman spectra are reported for 1,3dithiacyclohexane and 2-X-1,3dithia-2- boracyclohexanes, where X = Cl, Br or Ph. Assignments are made in each case assuming a molecular symmetry of C,. The assignments of ring modes for the dithiaboracyclohexanes are almost all very close to those for the corresponding dithiaboracyclopentanes.

INTRODUCTION

We have earlier reported vibrational spectroscopic studies of five-membered heterocyclic complexes con- taining boron [ 1,2], phosphorus or arsenic [3]. There are, however, very few data available on the vibrational characteristics of derivatives containing six-membered rings with sulphur at the l- and 3-positions. Almost the only published report is that of FINCH and PEARN [4] who obtained i.r. spectra (1600-400 cm- 1 only) for the compounds I, with X = Cl or Ph.

c i>-X

I

As an extension to our work on related five- membered ring compounds, and to obtain reliable vibrational data for 2-substituted 1,3-dithia-2-bora- cyclohexanes, we now report the i.r. and Raman data for I, where X = Cl, Br or Ph, subsequently abbrevi- ated as 2-X-DTBCH, together with as full an assign- ment as possible. Because of the paucity of published data on such ring systems, and in order to assist in the vibrational assignments for the boron-containing compounds, we also report the i.r. and Raman spectra of 1,3-dithiacyclohexane (m-dithiane, subsequently ab- breviated to DTCH). A few i.r. bands have previously been assigned for this [S], but much more complete studies have been made on l&dithiacyclohexane (p- dithiane) [6,7] and 1,3,%ithiacyclohexane [8].

EXPERIMENTAL

The compounds 2-X-DTBCH (X = Cl, Br or Ph) were prepared by literature methods [9, lo]. All manipulations were carried out under an atmosphere of dry argon. A commercial sample of DTCH (Aldrich Chemicals) was used without further purification.

Infrared spectra were obtained using a Perkin-Elmer 521 spectrometer (4000-250 cm-‘). Liquid film samples were held between KBr or CsI windows. and. for DTCH. solid samples as mulls in Nujol or hexachlorobutadiene. Spectra were calibrated using known wavenumbers of CH4, HBr, CO, NH3 and HZO. All of the observed wavenumbers are accurate

to f 2 cm- ’ ( f 5 cm- ’ for very weak and/or very broad features).

The Raman spectra were obtained using a Spex Ramalog spectrometer, with a SCAMP data processor, and a Coherent Radiation CR3OOOK krypton ion laser, exciting line 647.1 nm, output ca 3 W at the laser. All samples were held in glass capillaries. Spectra were calibrated using liquid indene as standard; peak positions are accurate f 2 cm- ’ ( f 5 cm- ’ for very weak and/or broad features). Polarization data were obtained by examining the spectrum with the incident tight respectively parallel and perpendicular to the axis of a Polaroid analyser. Depolarization ratios so obtained were proportional to the true values.

RESULTS AND DISCUSSION

The i.r. and Raman spectra of DTCH and the DTBCH derivatives studied are listed in Tables l-4, together with the assignments discussed below.

Vibrational analysis: 1,3-dithiacyclohexane

An electron-diffraction study of gaseous DTCH has shown that the molecule exists solely in the chair conformation (of C, symmetry) [ 111. X-Ray diffrac- tion by crystalline 2-phenyl- 1,3-dithiacyclohexane [12] and solution-phase dipole moment measure- ments on DTCH [13] also point to a chair confor- mation. The classification of normal modes for DTCH under C, symmetry is given in Table 5.

Raman polarization data enabled an almost com- plete vibrational assignment to be made for 1,3- DTCH, although Raman features in the range 1500-1050cm-1 were rather weak, leading to some tentative assignments for methylene deformations.

(i) Vibrations ofthe methylene groups. Twenty-four of the 36 normal vibrational modes of 1,3-DTCH are associated with the methylene groups. Of the eight CH2 stretches (6A’ + 2A”), six should give polarized Raman bands. In 1,4-DTCH, the CH2 stretches occur in the range 296&2800 cm- ’ [14], and in l-thiacyclo- hexane 2958-2850cm-’ [S]. Seven i.r. features are seen in 1,3-DTCH: 2958,2940,2900,2860,2848,2838 and 2818 cm- ’ (CS2 solution). Those at 2900, 2860, 2838 and 2818 cm- ’ have polarized Raman counter- parts and are hence A’ modes. The 2958 and 2940cm-’ bands are probably antisymmetric, A”, modes, while the 2848 cm- ’ feature can be assigned as

913

914 GEORGE DAVIDSON and KENNETH P. EWER

Table 1. Vibrational wavenumbers (/cm- ‘) and assignments for 1,3-dithiacyclohexane

Infrared (solid)

Raman (CS, soln) (Cl, soln) (solid) (liquid) (Ccl, soln) Assignment

2950 ms 2925 s 2900s

2820 w

2958 ms 2940 m

2900 vs 2860 sh’ 2848 sh’ 2838 ms 2818m 2760 w

1428s 1424 vs 1418 m,sh’ 1419m.sh’ 1388m 1387 ms 1340m 1342 VW 1285 ms 1285m 1272 ms 1274 ms 1244 ms 1244ms 12lOvvw 1210wm 12OOvw 12OOwm 1180m,sh’ 1170ms 1172~s 1150ms 1lSOw 1090 ms 1090 w

1009ms 1OlOms

919s 922 vs 887 ms 889 ms 820 wm 817wm 798 w 792 w 749vs 750vs 730 shr m, 680 m, sh’ 672 vs 677 s

640s 642 wm 472 w

2960 ms 2948 ms

2900 vs 2862 sh’ 2850sh’ 2840 ms 2820 m, sh’

1432 w, sh’ 1426 vs 1418 m,sh’ 1387m

1287 m 1276 ms 1245 ms 1208 wm 1203 wm

1175vs 1152w 109Ow

1OlOm

921 vs 888 m

675s

642wm 472 w

2962 m 2950 s 2932 m 2900 vs 2859 w

2820 w, br

1419wm 139Owm 1338 VW 129Ovw 1272 VW 1244w 1204w 12cQw 1180m 1175w,sh’ 1152~~ 1090 w 1047 wm 1009m

916wm 888 w, br 818w,br 798 w

738 m 679 ms 672 w, sh’ 638 vvs 466 w 332~ 316m 312s

80m

1424 m?dp 1418 wm?dp 1387 w, dp

1243 w,dp

1009 w, pol

918wm 886 VW, dp

792w,pol 748 w, dp

*CSz solution.

an overtone (2 x 1424); of course there will be Fermi resonance involving this band and those at 2838 and 2818cm-’ [lS, 161. Two A’ CH2 stretches remain unassigned; they are likely to be accidentally de- generate with two of the above modes.

HITCH and Ross [S] have assigned i.r. bands at 1426 and 1388 cm- 1 to CH2 scissors deformations in 1,3- DTCH. Four such modes are expected (3A’ + A”), and Raman bands (Ccl, solution) at 1417 cm- ’ (pal) and 1387 cm-’ (dp) are confidently assigned as A’, A”

modes, respectively. The remaining A’ features are believed to be at 1426 cm-’ (i.r.-with Raman counterpart of undetermined polarization) and 1432 cm-’ (i.r. only). An i.r. band at 1340cm-’ is at too low a wavenumber for such an assignment.

Four features observed in both i.r. and Raman spectra (1285,1272, 12441210 cm- ’ ) are suggested as CH2 wags. The Raman band at 1272 cm- ’ is definitely polarized and is therefore the A’ mode; the others are

678 m, dp 640 vs. pol 470 m, pol 334 m, pol 316 m,dp

2958 m CHr stretch (?A”) 2940 m CH, stretch (?A”)

2900 ms, pot 2860 wm, pol

2835 wm, pol 2818w,pol

1424 m?dp 1417w,pol 1387w,dp

CHZ stretch (A’) CHZ stretch (A’) 2 x 1424 = 2848 CH2 stretch (A’) CH; stretch (A’) ?2 x 1387 = 2774 ?CH2 scissors (A’) CH, scissors (A’) CH2 scissors (A’) CH, scissors (A”) ?

1272 w,pol 1241 w 1204w

CHZ wag (A”) CH2 wag (A’) CHZ wag (A”) CH, wag (A”)

1044 vw?dp 1008 w, pol

CHZ twist (A’ or A”) CHZ twist (A’ or A”) CHI twist (A’ or A”) CH2 twist (A’ or A”) ?

918 wm,pol 884 VW, dp

792 w* 748 w, dp

CHI rock (A’)+C-C stretch (A”)

C-C stretch (A’) CH2 rock (A”) CH2 rock (A”) CH2 rock (A’) C-S stretch (A”) ?

679 m, dp

636 s, pol 470 m, pal* 336 m, pal* 315m,dp*

C-S stretch (A’ + A”)

C-S stretch (A’) Ring deformation (A’) Ring deformation (A’) Ring deformation (A”) Ring deformation (A’) Ring deformation (A’)

-

the 3A” modes expected. No specific assignments to CH2 twisting modes can be made in the-absence of Raman polarization measurements, but bands at 1180, 1175, 1152 and 1090cm-’ are all in the region expected for such vibrations.

It is possible to be more specific in assigning CH2 rocking modes. A strong i.r. band at 887 cm- ‘, with a depolarized Raman counterpart, is clearly the A”

mode. Polarized Raman bands are seen at 1002, 918 and 792 cm- ’ , but the 918 cm- I feature is in the range expected for a symmetric C-C stretch, and so the A’

CH2 rocks are assigned to 1009 and 792 cm- I, with the third believed to be an i.r. band at 817 cm- r , with a weak Raman counterpart (seen only in the solid phase spectrum).

(ii) Vibrations of the ring skeleton. The six ring stretches (3A’ + 3A”) can be divided into predomi- nantly C-C stretches (A’+ A”; expected range 1050-900 cm- ‘) and C-S stretches (2A’ + 2A”; found

1,3-Dithiacyclohexane and 2-substituted 1,3dithia-2boracyclohexanes 915

Table 2. Vibrational wavenumbers (/cm- ‘)and assignments for Zchloro-1,3dithia-2boracyclohexane

Table 3. Vibrational wavenumbers (/cm- i)and assignments for 2-bromo-1,3-dithia-2-boracyclohexane

Infrared (liquid)

Raman (liquid) Assignment

Infrared (liquid)

Raman (liquid) Assignment

2965 w, shr 2950 ms 2930 ms 2910 m 2880 VW

2840 VW 1441 m 1430 ms

1290 wm,shr 1282 m 1262 w 124Ow

119Ow

1118~~ 1058 VW 1018 ms

980 vs 937 w 910 m, shr 895 s

875 w,sh’ 857 s 824 w 815 w,br 770 w, br 750 w, br 670 ms 645 ms 522 VW 490m

442m

410 w

2973 m, pol 2969 m, dp 2944 ms, ? pol 2928 s, pol 2911 ms,pol 2880 m, pol 2854 VW 2841 ms, pol 1442 w,?dp 1429 s, dp 1360 VW 1293 w, dp 1283 w,pol 1266 VW 1242 m, dp 1212 VW 1191 m,dp 1154 VW 1117 w,dp 1057 w, dp

1007 s,pol

981 VW

941 ms, pol 895 s, dp

864 w,?pol 858 w,?pol 827 VW, pol 819 w,br

665 w, s’, dp 646 vs, pol 520 m, pol 489 s, pol 468 w,? pol 441 vs,pol

407 m, pol 383 w,pol 327 ms, dp 269 s, pol 206 w,dp 136~

CH2 stretch (A’) CH2 stretch (A”) CH2 stretch (A’) CH2 stretch (A’) CH2 stretch (A’) 2 x 1441 = 2882 2 x 1430 = 2860 CH2 stretch (A’) CH2 scissors (A’) CH2 scissors (A’ + A”)

CH2 wag (A”) CH2 wag (A’)

CHs wag (A”)

CH2 twist (A”) CHZ twist (A’) CHI twist (A”)

“ES stretch (A”) CH2 rock (A’)

+ CC stretch (A”) “ES stretch (A”)

CC stretch (A’) CH2 rock (A”) + “‘EC1

stretch (A’) ? “ECIstretch (A’) ? CH2 rock (A’) 2 x 410 = 820 442 x 327 = 769 442 + 310 = 752 C-S stretch (A”) C-S stretch (A’) ?2+269 = 538 Ring deformation (A’) ?269+206 = 475 ES stretch (A’) + ring

deformation (A’) EC1 deformation (A’) ? Ring deformation (A”) Ring deformation (A’) EC1 deformation (A”) ? Ring deformation (A’)

2970 m 2945 s 2935 s 2915 m 2880 VW 2842 m 1444 ms 1430 s 1370 m, br 1352 w, shr 1290 ms 1282 m, shr 126Ovw, sh’ 1244m 1215 w, br 1192~

1120 VW 106Ovw 1018 ms 1010 m, shr

985 s 915vw 883 ms 860 VW 840 ms

820 m, shr 806 vs

670 m 645w 490w

415 m

390 VW

315 w,br

2968 ms, dp 2940 m, shr 2920 vs, pol 2912 s,pol 2878 w, pol 2840 m, pol 1444 ms,?pol 1428 s, dp

1349 VW, pol 1288 m, ? pol 1280 m, pol

1241 m,?dp

1190 w,dp 1164 VW, pol 1114 w,dp 1055 w, dp

1006 m, pal

986 w, ? dp 915 m,pol 881 ms,?dp

834 VW

804 m, pol 776 w, br 664 vs, ? dp 642 vs, pol 490 s, pol 469 VW, pol 447 m, pol 443 w, sh’ 410 vs, pol

387 m,?pol 356 VW, pal 326 s, dp 3 10 m, shr, dp 227 vs, pol 167 w,dp

100 w,sh’

CH2 stretch (A” +?A’) CH2 stretch (A”) CH2 stretch (A’) CH2 stretch (A’) 2 x 1444 = 2888 CH2 stretch (A’) CH2 scissors (A’) CH2 scissors (A’ + A”) 9

2 x 670 CH2 wag (A”) CH2 wag (A’)

CH2 wag (A”)

CHZ twist (A”) CH2 twist (A’) CH2 twist (A”) 643 + 410 = 1053 “ES stretch (A”) CH2 rock (A’)

+ CC stretch (A”) “B-S stretch (A”) CC stretch (At) CHI rock (A”) 642 + 228 = 870 CHI rock (A’)

+ “‘B-Br stretch (A’) . I

7

” EBr stretch (A’) 326 •)- 447 = 773 C-S stretch (A”) C-S stretch (A’) Ring deformation (A’) ? Ring deformation (A’) ?2x227=454 ES stretch (A’) + ’ 'EBr

deformation (A’) ’ 'B-Br deformation (A’) 257 + 100 = 357 Ring deformation (A”) ? Ring deformation (A’) Ring deformation (A”) + EBr deformation (A”)

Table 4. Vibrational wavenumbers (/cm- ‘) and assignment for 2-phenyl-1,3dithia-2- boracyclohexane

Infrared (liquid)

3140 w 3095 m 3076 s 3050 s 3030 ms, shr 3015 ms 2962 ms, shr 2940s 2925 ms, sh’ 2910 ms,sh’ 2880 wm 2840 s

Raman (liquid)

3052 s, pol

3012 VW 2967 m, dp 2940 ms,?vol 2924 s, pol 2910 m, shr, ? pol 2880 w,pol 2843 wm, pol

Assignment

2910+226 = 3136 Phenyl CH stretch (B,) Phenyl CH stretch (Es) Phenyl ring stretch (A,) Phenyl ring stretch (A,) Phenyl ring stretch (Al) CHZ stretch (A”) CH2 stretch (A’ or A”) CHI stretch (A’) CH2 stretch (A” or A’) CHs stretch (A’) CH2 stretch (A’)

SA(A)42:8-E

916 GEORGE DAVIDSON and KENNETH P. EWER

Table 4. (Concd.)

Infrared (liquid)

Raman (liquid) Assignment

2798 w 2438 VW 2402 w 2242 w 2100 w 206Ow 1970 wm 1908 wm 1890 wm 1829 wm 1770 w, br 1670 wm 1618 wm 1595 s 1570 w 1490 w 1442 ms 1432 vs 1382 w 1370 w 1350 w 1338 m 1312 m

1288 s 1255 w 1244 wm 1222 ms 1205 vs 119Om,shr 116Om 1118 wm 1105 VW 1068 m 1058 VW 1034 m 1001 m

970 vs, br 920 ms 909s

897 s 857 vs 830 m 820 w,shr 753 vs 700 vs 680 ms 660s 620 s

549 ms 537 s 493 wm 445 VW 400w

1616 w,?pol 1594 vs, dp

1490 w,? dp 1442 w,?pol 1429 m, dp

1333 ww,dp

1300 vvw, dp 1286 vw, ? dp 1256 vw, sh’ 1242 w,?dp 1219 m,shr 1204 s,pol 118 m, shr, dp 1158 wm,dp 1120 vvw, dp

1066 vw,?pol 1056 ww, dp 1034 Ins, pol 1001 vs, pol 987 w, shr

918 vw,dp 908 vs,?pol 891 vs, dp 857 w,?dp

752 vw, dp

678 ww,? dp 656 wm, dp 616 m,dp 613 m,pol 542,542 VW 532 w, dp 484 m, pol 440 w, pol 396 s, pol 328 wm, dp 280 w, br, dp 226 m, pal 168 m, dp

1595 + 1205 = 2800 2 x 1222 = 2444 1338 + 1068 = 2406 1244 + 1001 = 2245 1205+897 = 2102 1205 + 857 = 2062 1001+ 970 = 1971 987 + 920 = 1907 970 + 920 = 1890 ?909 + 920 = 1829 920+857 = 1777 920 + 153 = 1673 Phenyl ring stretch (A,) Phenyl ring stretch (B,) 909+660= 1569 Phenyl ring stretch (A,) Phenyl ring stretch (B,) + CHZ scissors (A’) CHZ scissors (A’ + A”) 700+680 = 1380 753 + 620 = 1373 ?2 + 680 = 1360 Phenyl ring stretch (B,) 2 x 660 = 1320 CHI wag(A’+A”)+ Phenyl CH deformation (&) “B-phenyl stretch CHI wag (A”)

‘IB-ohenyl stretch (A’) CH2twisi (A”) + pdenil CH deformation (A’) CH, twist (A’) + phenvl CH deformation (82) CH; twist (A”‘) _ _ ? Phenyl CH deformation (BZ ) 660+400= 1060 Phenyl CH deformation (A,) Phenyl ring “breathing” (A, ) + CHI rock (A’) CC stretch (A”) +phenyl CH deformation (AZ) “ES stretch (A”) + phenyl CH deformation (B,) Phenyl CH deformation (B, ) CC stretch (A’) CH2 rock (A”) ?Phenyl CH deformation (A, ) CH2 rock (A’) 4943 + 328 = 821 Phenyl CH deformation (B,) Phenyl ring deformation (B,) Phenyl ring stretch (X-sensitive) (A,) C-S stretch (A”) Phenyl ring deformation (B2) + C-S stretch (A’) Phenyl-“‘B deformation (A’/B,) Phenyl-’ ‘B deformation (A’/B, ) Ring deformation (A’) Ring deformation (A’) ES stretch (A’) + phenyl-B deformation (A”/B,) Ring deformation (A”) ?Phknyl ring deformation (AZ/B,) Ring deformation (A’) Ring deformation (A”)

in 1,CDTCH in the range 7W620 cm- ’ [ 143). A very the A’ CH2 rock at 1010 cm- ‘. These are close to the strong i.r. band, with a weak but polarized Raman figures for analogous modes in I-thiacyclohexane [SJ. counterpart, at 921 cm -I, is confidently assigned to One symmetric C-S stretch gives the extremely the symmetric (A’) C-C stretch, with the antisym- strong, polarized Raman band at 636 cm- ‘. Two metric (A”) C-C stretch accidentally degenerate with depolarized Raman bands, at 748 and 679 cm- ’ (with

1,3-Dithiacyclohexane and 2-substituted 1,3dithia-2-boracyclohexanes 917

Table 5. Classification of vibrational modes of 1,3-dithiacyclohexane under C, symmetry

CHr stretches CH, deformations (scissors) CH; deformations (wag) CH2 deformations (twist) CH, deformations (rock) Ring stretches Ring deformations

6A’ + 2A” 3A’+ A”

A’+ 3A” A’+ 3A”

3A’+A” 3A’ + 3A” 4A’+2A”

Activities: A’, Raman (pot) + i.r.; A”, Raman (dp) + i.r.

very strong i.r. counterparts), must be A” C-S stretches. The remaining A’ mode appears as a shoulder (672 cm- ‘) to the strong solid-phase Raman band at 679 cm- *.

Four of the six (4A’ + 2A”) ring deformation modes were detected by HITCH and Ross [5] in the i.r. spectrum of DTCH, at 465335,312 and 217 cm-‘. In the present work, i.r. data were unavailable below 400 cm- ‘, but three of these features were seen in the Raman spectrum (470, 336 cm-‘, both polarized, 3 12 cm- ‘, only in the solid). In addition, a depolarized Raman band was observed at 315 cm-‘, and solid- phase only Raman band at 80 cm- ‘. The Raman band at 315 cm- ’ is clearly an A” mode, while those at 470 and 336 cm- ’ are A’ modes. It is tentatively suggested that 312 and 80 cm-’ are also A’ modes, with that at 217 cm-’ (i.r. only) as the remaining A” feature.

Vibrational analysis: 2-substituted-1,3-dithia-2-bora- cyclohexanes

There is no structural information on these species, but the similarity of their vibrational spectra to that of 1,3-DTCH gives good grounds for assuming a molecu- lar symmetry of C,. The classification of normal modes for 2-X-DTBCH is summarized in Table 6. Vibrational assignments will be given, except where otherwise noted, for 2-Cl- or 2-Br-DTBCH, as those for 2-Ph- DTBCH are often more uncertain, due to the presence of numerous internal modes of the phenyl group.

(i) Vibrations of the methylene groups. Comparison with assignments for 1,3-DTCH (above), and with those for 2-substituted 1,3_dithiacyclopentanes (2-X- DTBCP) [2], makes the assignment of methylene modes in 2-X-DTBCH relatively straightforward.

Table 6. Classitication of vibrational modes of 2- substituted-1,3-dithia-2-boracyclohexanes under

C, symmetry

CH2 stretches CH2 scissors CHz wags CH, twists CHI rocks Ring stretches

4A’+2A” 2A’+A”

A’+2A” A’+2A”

2A’+ A” 3A’ + 3A” 4A’+ZA”

A’ A’+A”

Ring deformations B-X stretch EX deformations

Activities: A’, Raman (pal) + i.r.; A”, Raman (dp) + i.r.

Six Raman bands for 2-Cl-DTBCH in the range 2841-2973 cm- ’ are due to vCHz modes. Definitely polarized features at 2973,2928,2911 and 2841 cm- ’ are the A’ modes, with a depolarized band at 2960 cm-i and one of uncertain polarization at 2944 cm- ’ due to A” modes. All of these except that at 2973 cm- ’ have i.r. counterparts. Bands at 2880 and 2854 cm- I can reasonably be assigned as overtones of CH2 scissors modes involved in Fermi resonance with A’ fundamentals in this region [15,16].

Only two bands can be assigned as CHI scissors modes (ca 1440, ca 1430 cm-‘). In 2-Br-DTBCH the higher wavenumber is polarized in the Raman spec- trum, and so is an A’ mode. The lower wavenumber feature is depolarized and thus assigned as an A” mode. The second A’ modecannot be seen in these molecules. A band near 1370 cm- ’ is at too low wavenumbers; the mode is tentatively assigned as being accidentally degenerate with the A“ mode.

Assignments to the three methylene wagging modes are easy to make for 2-Cl-DTBCH. Depolarized Raman bands at 1293 and 1241 cm-’ are A” modes, with a polarized band at 1283 cm- ’ as the A’ mode- all have i.r. counterparts. There is similarly little uncertainty about the CHr twisting mode assignments. The clearest data are for the 2-Br-DTBCH molecule, with one polarized (1164 cm- ‘)-and two depolarized Raman bands (1190, 1114 cm- ‘) as the A’ and 2A” modes, respectively.

For the CHI rocking modes, difficulties in assign- ment arise from the presence of skeletal stretching modes in the same region of the spectrum. However, analogy with 1,3-DTCH and 2-X-DTBCP suggests that the A’ modes are at 1007 and 827 cm- ’ (2-Cl- DTBCH), with the A” mode at 895 cm- ‘. The other DTBCH molecules also give analogous figures.

(ii) Vibrations of the ring skeleton. The six ring stretches can be regarded as predominantly C-C (A’ + A”), C-S (A’ + A”) or ES (A’ + A”) motions. The C-C stretches and antisymmetric ES are ex- pected to be above 900 cm- ‘. In 2-Cl-DTBCH a polarized Raman band at 914cm-’ is the A’ C-C stretch; the antisymmetric (A”) CX stretch is thought to be accidentally degenerate with the A’ rocking mode at 1007 cm-‘.

The antisymmetric B-S stretch in the boracyclopen- tanes gave a very strong i.r. band and a much weaker Raman feature (ca 92&950 cm- ‘) [2]. In the cyclo- hexanes a very similar pattern was found, but with the bands at somewhat higher wavenumbers [in 2-C]- DTBCH at 980 cm-’ (“B-S) and 1018 cm-’ (‘Q-S)].

The C-S symmetric and antisymmetric stretches are expected in the range 600-700 cm- ‘. In both 2-Cl- and 2-Br-DTBCH a strong polarized Raman band ca 645 cm-’ (weaker i.r. counterpart) is the A’ mode, a weak depolarized Raman band ca 665 cm-‘, with a much stronger i.r. band, the A” mode. In 2-Ph- DTBCH the A” mode is at similar wavenumbers, but the A’ mode is at ca 615 cm- ‘. The lower value is

918 GEORGE DAVIDSON and KENNETH P. EWER

probably due to mixing with phenyl modes, such as the X-sensitive ring mode at 680 cm- r.

The remaining ring stretch (symmetric vB-S) is characteristically strong in the Raman spectrum, and much weaker in the i.r. Such a mode is seen in 2-Cl- DTBCH at 441 cm- ‘, and 410 cm- ’ in 2-Br-DTBCH, and 396 cm- ’ in 2-Ph-DTBCH. This marked depen- dence on substitution is undoubtedly due to extensive coupling with VI&X or a symmetric ring deformation.

Assignment of the ring deformations to specific modes is not easy. However, in 2-Cl-DTBCH strong, polarized Raman bands at 489 and 269 cm-’ are clearly A’ modes, a third is thought to be accidentally degenerate with v,,,B-S (441 cm- ‘) and a fourth as a weak Raman band at 136 cm- ‘. An A” mode gives rise to a moderately strong, depolarized Raman band at 327 cm- ‘. The last mode is not seen in 2-Cl-DTBCH. Several of the ring deformations involve motion of the SLBX unit, and would be expected to be substituent- dependent. For 2-Br-DTBCH, the A’ modes are at 490, 447,227 and (probably) 100 cm- ‘, the A” modes at 326 and 167 cm- ‘. In 2-Ph-DTBCH, the A’ modes are at 484, 440 and 226 cm- ’ (no low wavenumber band detectable), the A” at 328 and 168 cm- ‘.

(iii) The B-Cl stretch nnd deformations. In 2chloro- 1,3dithia-2boracyclopentane, the l1 B-Cl stretch is at 967 cm- 1 (strong i.r. band, weak Raman counterpart) [2]. In 2-Cl-DTBCH, however, the only available feature with these characteristics is at 857 cm- ’ (with a contribution from v”B-Cl to the CHI rocking mode at 895 cm- ‘).

The B-Cl deformations (A’ + A”) are at 206 cm- ’ (depolarized Raman band; A”) and at 407cm-’ (polarized Raman band; A’).

(iv) The B-Br stretch and deformations. A medium- intensity polarized Raman band at 804 cm- ’ in 2-Br- DTBCH appears to be v”B-Br (v”B-Br contribu- ting to the CH2 rock at 834cm-‘). As for the v&Cl modes in the cyclopentane and cyclohexane deriva- tives, the R-Br stretch is at significantly lower wave- numbers in the cyclohexane than in the cyclopentane system. In bothcases the differences may be ascribed to changes in the extent of mixing between vB-X and ring vibrations between the five- and six-membered ring systems.

The symmetric B-Br deformation can easily be assigned to a medium intensity Raman band at 387 cm-‘, which appears to be polarized (very weak i.r. counterpart at 390 cm- ‘). The antisymmetric B-Br deformation is not observed-it may be accidentally degenerate with the A” ring deformation at 167 cm- ‘.

(v) The B-Ph stretch and deformations. EPh stretches are generally in the wavenumber range 1200-1300 cm- ‘. A strong polarized Raman band at 1204 cm- 1 in 2-Ph-DTBCH, together with an i.r. band at 1205 cm-‘, can be assigned as v’ ’ B-Ph. The v’OB_Ph may be assigned to a Raman band of undetermined polarization at 1256 cm- ‘. With 2- phenyl-1,3-dithia-2-boracyclopentane one would sug_ gest wavenumbers of ca 540 cm-’ (symmetric) and ca

400 cm- ’ (antisymmetric). The latter is almost cer- tainly hidden by the strong, polarized Raman band at 396 cm- ’ due to v,BS. The former appears to give a weak Raman band at 532 cm-‘, which seems to be depolarized. This A’ mode ought to be polarized, but it is derived from a B1 mode in a local (PhB$) symmetry of CZ”, and hence the degree of polarization will be very small.

(vi) Internal vibrations of the phenyl ring. A reason- ably complete assignment can be made of the internal phenyl modes for 2-Ph-DTBCH, using a local sym- metry approximation of C2”. The assignments are included in Table 4, and are generally close to those for the cyclopentane analogue [2].

CONCLUSION

Analysis of the i.r. and Raman spectra (especially using the polarization data for the latter) have enabled very many vibrational assignments to be made for 1,3- dithiacyclohexane and 2-X-1,3-dithia-2-boracyclo- hexane (where X = Cl, Br or Ph). For the latter system, the assignments are all very close to those found earlier for 2-X-1,3-dithia-2-boracyclopentane derivatives, except for the vB-S modes, which are at higher wavenumbers in the six membered ring complexes. In all cases, however, it must be remembered that exten- sive vibrational coupling will occur among some of the modes, as shown by recent ab initio force field calcu- lations [ 171.

REFERENCES

Cl1

VI

c31

c41

CSI

C61

c71

C81

191 Cl01

Cl11

Cl21

Cl31

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