F.t.i.r. and Raman spectra of triphenylphosphine, triphenylarsine, triphenylstibine, and dibenzylsulphide

Specfrochimicu Acre. Vol. 43A, No. 6, pp. X05-816, Printed m Great Britain.

0584-8539

1987.

CQ 1987 Pergamon

87 $3.00 + 0.00 Journals

Ltd.

F.t.i.r. and Raman spectra of triphenylphosphine, triphenylarsine, triphenylstibine, and dibenzylsulphide ROBIN

J. H. CLARK,*

COLIN

D. FLINT?

and

ANDREW

J. HEMPLEMAN*~

*Christopher lngold Laboratories, University College London, 20 Gordon Street, U.K.; and TDepartment of Chemistry, Birkbeck College, Malet Street, London

(Received 24 October

1986; infinalform

London WC 1H OAJ, WClE 7HX, U.K.

8 November 1986; accepted 11 November

1986)

Abstract-The f.t.i.r. and Raman spectra of triphenylphosphine, triphenylarsine, triphenylstibine, and dibenzylsulphide in the solid state at ca 80 K have been recorded over the ranges 350&40 cm ’ (infra-red, 1 cm-’ resolution) and 1650-30 cm-’ (Raman, 2 cm-’ resolution). The data, particularly those in the low wavenumber region, are more extensive, more complete, and of higher quality than those obtained in previous studies. Detailed band assignments are given.

INTRODUCTION

[3] on monohalogenobenzenes. WHIFFEN expected from a planar PhX molecule into two groups. The first group contains essentially 24 phenyl vibrations which can be further subdivided from descriptions of their normal modes to C-C stretching, out-of-plane CH deformations, etc. The second group contains the remaining six vibrations which are dependent on the mass of the substituent X and are hence called X-sensitive vibrations. These are denoted by the letters q, r, ):, t, u, and x.

WHIFFEN

divided

In the course of a systematic vibrational study of dirhodium tetracarboxylate complexes involving the axially coordinated ligands triphenylphosphine, triphenylarsine, triphenylstibine, and dibenzylsulphide [l, 21, it became necessary to reinvestigate the vibrational spectra of the axial ligands themselves. The published data on these ligands are some 1620 years old and, apart from the fact that they were recorded at room temperature only and at relatively low resolution, their chief deficiency lay in the low wavenumber regions of both the infra-red and Raman spectra. This is a crucial region for dirhodium complexes, since it embraces the important skeletal modes v(RhRh) and v(Rh0) [Z]. Hence the definite identification and assignment of bands attributable to the axial ligands became a matter of major concern. The present study has yielded high quality f.t.i.r. and Raman spectra of the four named ligands in the solid state at ca 80 K and at good resolution. It is more extensive and detailed than any previously undertaken, and has led to the identification of ligand modes over the wide range 35W30 cm-‘. EXPERIMENTAL

F.t.i.r. spectra were recorded on a Bruker 113 V interferometer at 1 cm _ 1resolution. The hgands were dispersed as pellets in wax (melting point 49°C) (B.D.H. Chemicals) and potassium chloride for the ranges 660-40 cm- I and 350%500cm _ ‘, respectively. All samples were held at ca 80 K in a Beckmann RIIC cryostat cooled by liquid nitrogen. Raman spectra were recorded on a Spex 14018(R6) spectrometer at a slit width of 2 cm _ ’ using the 647.1 nm excitation of a Coherent Radiation 3000 K krypton ion laser. Small crystals of the ligands were mounted on the cold tip of a liquid nitrogen cooled cryostat using conducting grease. Samples of tripbenylphosphine, triphenylarsine and triuhenvlstibine were obtained from B.D.H. Chemicals; hibenzylsulphide was obtained from Pfaltz and Bauer. All samples were recrystallised from ethanol prior to use. RESULTS

The assignments band wavenumbers SA43:6*-F

AND DISCUSSION

of the characteristic vibrational are based upon the classic work of

the 30 vibrations

Triphenylphosphine,

triphenylarsine,

triphenylstihine

The assignment of the vibrational wavenumbers of a polyphenylorganometallic compound Ph,M on the basis of assignments for the corresponding monohalogenobenzene PhX (where M and X have similar masses) is well established [4-S]. The interpretation of the spectra in terms of the number of observables in the various symmetry classes has been attempted by a number of workers [4,8]; however, in the case of triphenylphosphine this is a complex problem. First, the crystal structure [9] shows inequivalence between the three phenyl rings, one of which is twisted about 30” from the position that would give the molecule a three-fold axis, and hence the molecule possesses no symmetry. Furthermore there are four molecules per unit cell which will cause further band splittings due to correlation effects. The structure of triphenylarsine is also known [lo] and similar problems arise, but in this case there are eight molecules per unit cell and four different triphenylarsine molecular geometries are observed. There are no structural data available for triphenylstibine. Confirmation of these effects comes from solution vibrational work [4] where the number of observables is reduced. The Raman spectra in the region 160&30 cm- I are shown for PPh,, AsPh,, and SbPh, in Figs l-3, respectively. Sample fluorescence was encountered with all three compounds, thus giving rise to high backgrounds. Fluorescence has been observed by other workers [4,7] and this is probably due to a trace impurity. It can be seen from these spectra that SbPh3 805

806

ROBINJ. I-I.CLARKet al.

00

12ca

800

400

0

Wovonunber/cm-'

Fig. 1. The Raman spectrum (1610-3Ocm-‘) of a single crystal of PPh, at ca 80 K, & = 647.1 nm.

single crystak

ca.60 K

Fig. 2. The Raman spectrum (MOO-30~x11~‘)of a single crystal of AsPh, at cu 80 K, 1, = 647.1 nm.

is the strongest Raman scatterer, followed by AsPh3, with PPh, being the weakest; however, even in this case reasonably good quality spectra were obtained. Full listings of the observed wavenumhers and their assignments are given in Tables 1-6. The spectra are discussed in terms of each significant wavenumber region. 3LOO-3000cm-‘. The infra-red spectra in the 3100-3OOOcm- l region display a large number of bands; the Raman spectra in this region could not be obtained owing to the low sensitivity of the spectrometer at 3OOOcm-’ red-shifted from the IzO = 647.1 nm exciting line, and to the fluorescence with exciting lines of shorter wavelength. Previous workers

have assigned bands occurring at 3058, 3052, 3040, 3018 and 3004cm-’ for PPhj to v(C-H) vibrations [4]. This region is further complicated by a series of absorptions arising from combinations of the various bands in the 1600-1400 cm- l region, although these combination bandsshould be much weaker than those arising from the CH fundamentals. Mono-substituted benzenes appear to give rise to a triplet of bands in this region [I 11, and in monohalogenobenxenes these are found in the m 3065, _ 3050, and N 3030 cm-’ regions [3]. On this basis the band assignments for these compounds are made. These assignments are tentative as there is also a number of bands, in particular around 3000 cm-‘, which are of com-

Vibrational spectra of phenyl compounds

SbPh,

Fig. 3. The Raman spectrum (l-30

single crystal

807

ca.80 K

cm-‘) of a single crystal of SbPh, at CLI80 K, A0= 647.1 nm.

parable intensities to the higher wavenumber bands. Some of the former bands can be attributed to summations of the ring stretching vibrations k, 1,m and n, their intensities being much higher than expected due to strong Fermi interactions. 2000-1650 cm-i. This study was also restricted to the infra-red region only and a number of weak bands were observed. These are due to overtones and combinations of the CH out-of-plane fundamentals which occur in the 1000-700 cm-’ region [12]. As these fundamentals are sensitive to the type of substitution so in turn this higher wavenumber region can be used to identify the type of benzene ring substitution. Mono-substituted benzenes generally give rise to a series of four bands of gradually diminishing intensities towards higher wavenumber. In the case of chloro- and bromobenzenes the intensity of the first of these is actually a little less than that of the second [ 131. Four bands have been previously observed at 1958, 1890,1817 and 1665 cm-i in the infra-red spectrum of PPh3 [4] and these were attributed to the combinations h +j, h + i, h + g, andf+ i, respectively. For all three compounds four groups of bands are observed in the 2000-1750 cm-i region and, as found for chloroand bromobenzene, the first band is slightly weaker than the rest. A fifth group of bands is also observed around 1650cm-‘. Instead of observing broad unresolved bands such as for the halogenobenzenes all the bands show structure, presumably due to the splitting of the out-of-plane fundamentals as a result of the solid-state effects mentioned earlier. If SbPh, is taken as an example, the five ranges observed for these bands are as follows: 19761958, 1903-1878, 18361817, 1773-1760, and 16581639 cm- ’ Now, from the assignments of the out-ofplane CH fundamentals the maximum number of possible combinations is 10, e.g.

combination

maximum range

h+j

197&1954 18961876 1839-1819 1722-1699 1912-1894

h+i h+g

h+f j+i

j+g

j+f g+i

s+f i+f

1855-1837 1738-1717 1781-1759 1607-1582 1664-1639

observed

Yes Yes Yes No possibly slight overlap with h + i No No Yes No Yes

Consequently the five groups of bands are interpretable in terms of the combinations h + j, h + i, h + g, g+i, and i+& These combination bands are not observed in the Raman spectrum owing to the low Raman intensities of the out-of-plane fundamentals. 165&1300 cm- ‘. This is the region where the ring stretching vibrations are found. Benzene possesses three ring stretching vibrations [14] at 1596 (e,,), 1486 (e,,), and 1310 (b,,) cm-‘. The degeneracy of the first two vibrations is lifted upon ring substitution, and the band at 1596cm-i gives rise to the k and I bands, whereas the band at 1486 cm-’ gives rise to them and n bands. Usually the 1596cm-’ band of benzene is not greatly perturbed upon substitution and the resulting k and I bands lie close together. In all three cases they were readily identifiable in the infra-red and Raman spectra, the 1 band being the weaker; however, this is not always the case as the intensities of these bands are dependent upon the polarity of the ring substituent [ll]. k and 1 are found in the overall infra-red and Raman ranges 15891573 and 1575-1567cm-‘, respectively, for all three compounds.

808

ROBINJ. H. CLARK et al.

The m and n bands also both occur close to the e,, band of benzene (1486 cm- ‘). Ranges have been set in mono-substituted benzenes of 1510-1480 and Table 1. Wavenumbers/cm- ‘ of bands observed in the infrared spectrum of PPh, ca 80 K Assignment

i 3079vw 3076~~ 3066m 3061m 3054w,sh 3048m,sh 3046m 304Ow 3026~ 3021~~ 3014w.sh 30llw;sh 3009w

\

v(C-H)

’ v(C-H) I 1

k+n

3OOlw,sh 2995~ 2984~~

1973vw,sh 1969~~ 1965~~ 1962~~ 1955vw,sh 1952~~. 1940vw 1938~~ 1917vw 1914vwgh 191 lvw,sh 1908~~ 1891~~ 1886vw 1882~~ 1878~~ 1875~~ 1844~~ 1839vw,sh 1837~~ 1827~~ 1820vw 1818vw 181Ovw

2j

h+j I

p+i

or j+i I I

h+i

t

J

1 )

1771vw,sh 1768~~ 1757vw 1754vw,sh 1752~~ 1261~~ 1258vw.sh 1252~~. 1203~~ 1192~~ 119Ovw.sh 1184vw;sh 1177w 1159w 1153w I 1135vw 1126~~ 1123~~ 1115vw 1109vw,sh 1106vw

h+a 2i

-

g+i

f+Y w+g orf+Y “+Y = B(C-H) c B(C-H)

v+t r+toru+t u+w

i 1675~~ 1671~~ 1668~~ 1665vw,sh 1656vw,sh 1653~~ 1608~~ 1606vw,sh 1603vw 16OOvw,sh 1596~ 1591vw 1589vw 1583m I 1575vw 1571w 157Ow,sh 1567~ I 155Ovw 1538~~ 15OOvw 1499vw,sh 1492vw,sh 1487vw,sh 1476m 1473m 1469m,sh > 1459vw 145Ovw,sh 1436m,sh 1435m 143Om 1 1427m.sh I 1424mgh 1422~ 1398~~ 1393vw I 1391vw 1377vw 133lvw 1329vw,sh 1323~~ 1317vw 1309m 1297~~ I 1284w 1281w,sh I 1273~~ 1270w 7 910vw . 907vw 905vw,sh 1 857~ 7 854~ 849~ 844vw 757m 755wsh 1 75om 745m,sh 743m 742m,sh 74Omyh 729vw 725vw

Assignment

f+i

u+i

v+r

k v(C-C)

I v(C-c) v+g

j+Y

m v(C-C)

Table 1. (Coned.)

i, 1097w 1092m 1087m 1072m 107Om 1067m 1029vw,sh 1028~ 1024m 1018vw,sh 1014vw lOlhw,sh 1006vw 1002w lOOOw,sh 998~ 996w 994w 992w,sh 988vw 98%~ 972vw,sh 971vw 941vw 938vwph 927vw 919w 917w

Assignment

q X-sens d B(C-H)

1

b NC-H)

I

s+w I p-ring I

j y(C-H) 1 h Y(C-H)

1

f+ x

>

i y(C-H)

I

i+y 2u or p+ w 2v or j -I-w

h+w 0 v(CC) V+S w+i g+t e B(C-H)

i y(C-H)

g Y(C-H)

f Y(C-H)

Assignment

703m 7Oom,sh 698~s 695m,sh 692s,sh 691s 683~ 6%~ 618vw.sh 544vw’ 541vw 515m 51ow 5OOm 492m 433w 421~ 420w 409vw 402vw 399vw 396vw 273vw 255vw 247vw 217vw 197vw 18%~

0 4F-c) I r X-sens 1

.

s a(C-C-C) 2u

1 I

y X-sens

J

t X-sens 1 r-uorw w $(C-c) 1 i 1

u X-sens w-xorx x X-sens

Table 2. Wavenumbers/cm-’ of bands observed in the Raman spectrum* of PPh, cu 80 K Assignment

i n v(C-C)

i

1589m 1585m 1573w 1568~ 1436~~ 1431vw 1324~~ 1310vw 1182vw,sh 1178~ 1161~ 1154w 1098m 1095w,sh 1088w 107Ovw 103Om 1024~ 1004s 1OOlm 998~s 996m,sh 993m 985~ 928vw 923vw 908vw 854vw 85Ovw 758vw 752vw 745vw

I

k v(C-C)

I v(C-c) n v(CC) 0 v(C-C) w+i I I

o NC-H) c B(C-H)

q X-sens I d &C-H) I b B(C-H)

1 I

p-ring

j Y(C-H)

i y(C-H) I

g Y(C-H)

f y(C-W

*647.1 nm excitation.

I

Assignment

706w 702~ 694~ 683~ 619w 617~ 513w 500w 491w I 434w 419w 410w > 406~ 274~ > 255m 249m 214m 198m 196~ 188w I 12Om,sh 114vs 92~s 83~s 69m 61m 55m 46vs 40s I 35vs

” #(C--C) r X-sens s a(C-C-C) y X-sens

t X-sens w 9F-C) u X-sens

x X-sens

1

predominantly lattice and torsional modes

Vibrational

Table 3. Wavenumbers/cmred spectrum 3 3077vw 3065~ 3060~ 3055w 3047w 3044w 3036~ 3029~ 3024~ 3019w 3OOgw 3005w 2995~ 2985~~ 2982~~ 2654~~ 1988vw 1970vw 1966~~ 1961~~ 1953vw 1947vw 1890~~ 1886vw 1876~~ 1871vw 1830~~ 1822~~ 1814~~ 1804~~ 1774vw 1767~~ 1749vw 1657~~ 1646~~ 1617~~ 1598vw 1578m 1572vw,sh 157Ovw 1480m 1475m,sh 1469~ 1465~ 1462~ 1436w,sh 1434m,sh 1432m 1425m,sh 857vw 853~ 851vw 850~ 843w 749m 748m,sh 744s,sh 742s 139s 737s 734vs 701m 697s,sh 694s,sh 692s 670w,sh 667~ 617vw 555vw

Assignment

I

v(C-H)

v(C-H) 1

I

I+n

k+q 2j h+j I

2h h+i

1

I

h+g

g+i I I i

f+i v+i k v(C-c)

1 I

I v(CC) m v(C-C) j+v

I h+y n v(C-C)

spectra

’ of bands observed in the infraof AsPh, ca 80 K it

Assignment

1388~~ 1378~~ 1335vw,sh 1331vw 1320vw 1317vw 1314vw 1305w 1303w,sh 130lw,sh I 1299w,sh 1297w.sh 1287vw 1274vw 1271vw,sh 1267vw,sh 1257vw 1184~ ) 118lw,sh 1 l75vw 1169~~ 1157w 1154w 1 l50w,sh 1084w 1075m I 1068m 1066m 1064m,sh I 1025m,sh i 1022m 1013w looom 998m 995w,sh 992vw 990vw 987vw 984vw 976vw 974vw 971vw 968vw I 919vw 917vw 915vw 910vw I 907w

2v w+j 0 v(C-C)

w+i j+t j+torh+t r+sorh+t c &C-H)

a B&-H) g+t c BGW q X-sens d B(C-H) b NC-H)

p-ring

j YK-W

9 YGH) 1

f Y(C-f-f)

I

i y(C-H)

r eYC-C) I i

r X-sens s a(C-C-C) t+lA

I

w de-C)

I I

t X-sens

J u X-sens w---X I x X-sens I

3 1584w,sh 1582w,sh 1580m 1574w,sh 1572~ 1482~~ 1434vw 1335vw 1306~~ 1275~~ 1186w 1 l83w,sh

1158w,sh 1155w 1084~ 1077w 1076~ 1068~~ 1066vw,sh 1026m 1024m,sh 1003vs looos 993w 987w 92Ovw 914vw 909vw 748vw 738vw 735vw

of bands observed of AsPh, ca 80 K

Assignment

S

r 4(C-C)

669wh 1v(C-C) I m v(C-C) n v(C-C) 0 v(C-C) w+i e NC-H) a B(C-H)

I

c B(C-H)

I

q X-sens

’

d B(C-H)

I

b B(C-H)

1

} P-ring

‘

j Y(C-H)

I )

i y(C-H)

in the

Assignment

701vw 694vw

k v(CYJ)

667m 618w 319w,sh 317w 312~ 252~ 249~ 245m 240m 235m 199m 196m 192m 188m 177w 117m,sh 104s 93s 77s 67s 60s 46s 38s,sh 32s 23s

’I

r

x_sens

s a(C-CC) j t X-sens

u X-sens I ‘r

.

x X-sens

predominantly k lattice and torsional modes

/ Y(C-H)

*647.1 nm excitation.

Table 5. Wavenumbers/cmred spectrum

i

y X-sens

I

Table 4. Wavenumbers/cm-’ Raman spectrum*

Assignment

’ of bands observed in the infraof SbPh3 ca 80 K i

Assignment

h YGW

I 482vw 478m,sh 477m 474m 471m,sh 469m 466m 462m 398vw 395vw 329~ 320m 318m 317m 313m 310m 237vw 204vw 195vw 186vw 179vw

809

of phenyl compounds

3134vw 3078~~ 307Ow 3062m 306Om,sh 3055w 3044m 3038m 3036m,sh 3026w,sh 3021w,sh 3019w 3005vw,sh 3002~ 2988~ 2985~ 2979vw,sh 2975~~ 2946~~ 2591~~ 1976~~ 1958vw 1950vw 1903vw 189&w 1892~~ 1883~~ 1878~~ 1871vw

21

v(C-H) k+m

v(C-H) I k+n k+n

2m n+c I

h+j 2h

h+i I

or I+n

1478m 1477m 1475m,sh 1431m,sh 1429s 1427m,sh 139Ovw 1385vw 1381~~ 1376~~ 1334w 1330w 13lOw,sh 1305~ 1302~ 1298vw,sh 1267~ 1264~ 1184~ 1181~ 1176~~ 1158~ 1155w 1153w 1071m 1066m 1063m 1052~~ 1045vw

m q-q

n v(C-C) I j+w I i+y I

0 v(C-c) w+i

I

J

e B(C-H) a &C-H)

I

,I

c &C-H) d N-H)

I

q X-sens w+r

810

ROBIN J.

H. CLARKet al.

Table 5. (Contd.)

Table 6. (Contd.)

Assignment 1836~~ 1831vw 1824~~ 1817~~ 181lvw 1773vw 1769vw 1764vw 176Ovw 1658~~ 1651~~ 1644vw 1639vw 1591vw 1578w,sh 1575m 1573m 1571w 1567m 1560vw 914w 908w 862vw 859w 857~ 851~ 745m 739s 737s 735m 731vs 701m 698m 697s 694s 656~ 655w,sh 653~ 652w,sh 619vw 616vw 464m

i 1024~ 1022w 102lw,sh 1019m I 1018m 1007vw lOOlw,sh 999m 997m I 993vw 991w 988vw 986vw I 977vw 975vw 973vw 971vw I 968w 919w 918w,sh I 460m 457m 1 455m 450m 1 447w,sh 444m J 397vw 277m > 271m 260m 257m 254mph 252m 23Ovw 226vw 220vw 213vw 181vw 176vw 170vw 166vw

h+g

J 2i g+i I f+i I

Assignment

f+s k v(C-C)

1v(C-c) u+o i y(C-H) 6 YGH)

J fy(C-H) I s 4G-c)

r X-sens i s a(C-C-C)

1187~~ 1182~

1577m 1572~ 1568~ 1561~~ 1478~ 1432~~ 1428~~ 1335vw 1331vw 1329vw 1328~~ 1303vw 1269vw 126&w 1191vw

Assignment

c B(C-H)

v-l-t p-ring

j TV-H)

h YGH)

k v(C-C)

1v(C-c) s+g m v(C-c) J ‘I

n v(C-C) 0 v(C-c)

J

w+i e B(C-H)

736vw 733vw 702vw 698vw 696vw 657s 655m,sh 653m 65Ovw 618w 617~ 464vw 461vw 457vw 444vw

I

y X-sens

1

I

i 976vw

Y(C-H)

h Y(C-HI

97tvw

920vw w &C-c)

917vw 912vw

i y(C-H)

9O!hW

6 Y(C-HI

t X-sens 746vw 740vw u X-sens

x X-sens

in the

.fy(C-H) c &C-c)

I

1072~~ S 1067vw.sh 1 d N-H) 1066w,sh . 1064W q X-sens 1062w I 1023m 102lw,sh b NC-H) 102Om > 1002vs 1OOOm p-r&3 998s 1

i y(C-H)

Assignment

i

/ a M-H)

h B(C-H)

Table 6. Wavenumbers/cm-’ of bands ObSeNed Kaman spectrum* of SbPhJ ca 80 K 9

Assignment

i

r X-sens

s a(C-C-C) y x-sens

I

f

YC-H)

9

Assignment

395vw

277w,sh 275m 1 259m 257m 253m 249m 231~ 227m 225m 221m 218m 2llw 182m 7 176m 170m 164m 124m 115m lOOnI 90s 85m 80m 73s 64m 59m 52s 48m 46m 38s 32s : 28m

w W-c) t X-sens

IIX-sens

x X-sens

J

predominantly lattice and torsional modes

* 647.1 nm excitation.

1470-1439 cm-’ for m and n, respectively [15]; however the halogenobenzenes occupy the lower ends of both of these ranges. (When group wavenumbers are quoted for mono-substituted benzenes, halogenobenzenes typically occupy the extremities of these ranges. This is attributable to the polarity of the halogen atom.) As in turn the compounds under discussion are related to halogenobenzenes, so their bands in this region occupy the extremities of the proposed wavenumber ranges. These modes give rise to little or no Raman intensity, but occur as two groups of infra-red bands of medium to strong intensity with ranges 1482-1469 and 14361425cm-’ for m and n, respectively. The remaining ring stretching vibration, o, occurs close to that observed for the bzu ring stretch of benzene (1310 cm-‘). It gives rise to a weak band in both the infra-red and Raman spectra. Problems arise with its assignment owing to the close proximity of summation bands, in particular w+i, which is of comparable intensity. It is found in the range 1335-1323 cm-‘. Workers [6, 161 have commented on the sensitivity of these vibrations to the mass of the ring substituent, and some [16] have suggested that the n mode is associated with a phenyl group perturbed by a heavy

811

Vibrational spectra of phenyl compounds atom substituent. These ring vibrations are likely to be more mass-sensitive than those involving the hydrogen atoms. Certainly on going from fluorine to iodine in the monohalogenobenzenes this band shifts from 1457 to 1439 cm-‘, whereas on going from chlorine to iodine it only shifts by 6 cm- ‘. Similarly from PPh, to SbPh, the shift is a maximum of 9 cm ‘. Nevertheless these shifts are small by comparison with those of the six X-sensitive modes [3] (c?ide infra). 130@990 cm-- ‘. In this region of the Raman spectra of all three compounds the internal mode giving rise to the greatest intensity is observed, namely the “ring breathing” mode p. This is derived from the b, u vibration of benzene (lOlOcm_i), this being essentially a contraction (expansion) of the ring at the triangle formed by the 1,3,5-carbon atoms and an expansion (contraction) of the ring at the triangle formed by the 2,4,6-carbon atoms. Upon monosubstitution the motion of the two triangles is decoupled and the vibrational amplitudes at positions 1,3, and 5 are damped, the substituent barely moving. In all three compounds it is found in the range 1006997 cm i. The band appears as a weak feature in the infra-red spectra. RANDLE and WHIFFEN [ 171 showed that the five inplane CH fundamentals a, b, c, d, and e also occur in this region. Their infra-red intensity is generally weak but they give rise to slightly stronger bands in the Raman spectra. This intensity reversal between infrared and Raman spectra is a general feature of the vibrational spectroscopy of substituted aromatics. Band b, 103&1018 cm-‘, is readily observable in the Raman and infra-red spectra, and is characteristic of mono-substituted aromatics [ll]. Assignment of d, 1072-1064cm-‘, is more difficult owing to its weak intensity in both the infra-red and Raman spectra as well as to the close proximity of the highest X-sensitive vibration q, which is particularly close for AsPh3. Bands c, u, and e are found in the ranges 1161-l 150, 1187-l 175, and 127551264 cm-‘, respectively. 99&700 cm- ‘. In this region there occur five modesf, g, h, i, and j, which are attributable to out-ofplane CH deformations. Only j gives rise to any appreciable Raman intensity, occurring as a shoulder on the intense band, p, at ‘Y 1000 cm- ‘. In the infrared spectra their combinations give rise to the characteristic bands in the 2000-1600cm-’ region (uide supra). In turn the fundamentals themselves are characteristic of the ring substitution pattern, most monoaround 750 materials absorbing substituted i. In the spectra of all three compounds this +15cmband, denotedf, falls in the range 758-731 cm- ’ and is the only band in this grouping to show dependence upon the substituent mass. Bands g and i arise from the erg CH deformation of benzene (849 cm ‘) and occur in the ranges 864-843 and 920-905 cm ‘, respectively. Finally, band h, which is not greatly perturbed from the e2” CH deformation of benzene (975 cm-‘), in contrast to its X-sensitive counterpart y, occurs in the range 977-968 cm- ‘.

The X-sensitive 700 cm- ’

vibrations and the region

below

In this region five of the six X-sensitive modes are found as well as the three ring-deformation modes. Due to the inherent practical problems involved in the low wavenumber regions in earlier work as well as to the low intensities of some bands, this is the area where most progress has been made in this study. The far infra-red spectra (66&140 cm-‘) are shown in Fig. 4. The in-plane ring deformation, s, usually occurs within the range [lS] 625-605 cm- ’ for monosubstituted benzenes, being at 606 cm- ’ for benzene itself. For all three compounds it appears at 618 + 2 cm- ‘, being readily identifiable in the Raman spectra but very weak in the infra-red spectra. The other two ring deformations are the out-ofplane modes u and w. The former band is derived from the inactive b,, mode of benzene (698 cm- ’ for monodeuterobenzene [19]); it appears in the infra-red spectrum as a group of intense bands in the 706691 cm-i region, but is very weak in the Raman spectrum. The band w is extremely difficult to locate in both spectra owing to its low intensity. It is derived from the eZu ring deformation of benzene (404 cm ‘) and occurs in the range 420-390 cm-’ for monosubstituted benzenes [20]. It is often located from its summation bands, e.g. the combination w + i is easily

1

PPh,

--J---k AsPh,

SbPh,

Wovenumber/cm

-1

Fig. 4. F.t.i.r. spectra (66140 cm- ‘) of PPh3, AsPh,, and SbPh, at ca 80 K.

812

ROBINJ. H. CLARK et al.

identifiable in the infra-red spectra of all three compounds; thus knowing the value of i, the position of w can be inferred. However, it was observed directly in the range 406-395 cm-’ in the infra-red spectra of all three compounds. Of the six X-sensitive vibrations, modes q, r, and tall involve appreciable C-X stretching, whereas modes y, II, and x involve C-X deformations. Some authors [7,21] have tried to attribute specific bands to C-X stretching or C-X bending; however, this is incorrect as several internal coordinates contribute to each normal coordinate. The highest X-sensitive mode, q, occurs at 1098-1087, 1084-1075, and 1066-1062~m-~ for PPhJ, AsPh,, and SbPh,, respectively. Problems arise with its assignment due to the close proximity of the inplane CH deformation d. This is particularly so in the case of AsPh, and SbPh, where q occurs virtually on top of d, the same situation happening for both bromoand iodobenzene [3]; as a result incorrect assignments have been made [22]. It is believed that the greater intensity of q relative to d in both the infra-red and Raman spectra should be the criterion on which to base the assignment. Assignment of r is difficult from the infra-red spectrum as it lies close to the strong out-of-plane ring deformation u. Fortunately the intensities are reversed in the Raman spectrum, in which r gives rise to a strong band. It is found at 683,670-667, and 657-652 cm-’ for PPh,, AsPh3 and SbPh3, respectively. The y mode, like q, is not greatly mass sensitive and occurs as a group of bands in the range 515-491, 482-462, and 464444 cm- ’ for PPh3, AsPh,, and SbPhJ, respectively. It can easily be identified in the infra-red spectrum as giving rise to a group of bands of medium intensity, unlike in the Raman spectrum where the bands are very weak; for AsPh, they are unobserved. In contrast the t mode is much more masssensitive and is relatively strong in the Raman spectrum. This occurs in the range 434-410,320-310, and 277-249 cm-’ for PPh3, AsPh,, and SbPh,, respectively. Both t and y cover a larger wavenumber range than either q or r, and it is possible that combinations such as 2u, t + x or r - x occur in these regions. The remaining two modes u and x are easily identified as giving rise to bands in both the infra-red and Raman spectra, there being several components in each case. The u vibration occurs in the range 274-247, 245-235, and 231-211 cm-’ for PPh,, AsPh,, and SbPh3, respectively, while x occurs in the ranges 214185, 199-179, and 182-164 cm-‘, respectively. A number of very intense bands occur below 150 cm- * in the Raman spectra of all three compounds. They can only be attributable to lattice modes or torsional modes of the phenyl rings. Dibenzylsulphide

The approach to the vibrational assignment is essentially the same as for the Mph3 compounds

except that the situation is further complicated by the presence of two methylene group Tables 7, 8. 3100-2800 cm-‘. The wavenumbers of the aromatic C-H stretching vibrations are closely similar to those observed for the three triphenyl ligands, namely three groups of bands in the range 3083-3025 cm-i. The assignment of the CH2 stretching modes is more difficult. Few data are available on CHr stretching wavenumbers, although they probably closely parallel those observed for methyl groups. In toluene the methyl group interacts with the ring hydrogen atoms in the or& positions, causing a lowering of the methyl stretching wavenumber [23]. Methyl groups as well as methylene groups with an adjacent nitrogen atom show a characteristic band at 2800 cm-’ provided the nitrogen lone pair lies wartsto the C-H bond [24], and a similar situation is possible in dibenzylsulphide. On this basis the bands at 2920 and 2809 cm-’ are assigned to v, CHI and v, CH,, respectively. JOSHIet al. [25] have previously assigned bands at 2922 and 1599 cm-’ for dibenzylsulphide to v,, and v, CH2, respectively, although the latter band can be attributed to k, v(C-C) and must be a misprint. For chloromethylbenzene VARSANYI[26] has assigned bands at 2968 and 2874 cm-i to v,CH, and v,CHI, respectively, and it is possible that the band at 2954 cm-’ for dibenzylsulphide could also be due to CHI stretching, particularly if there is inequivalence between the two groupings or they are close enough to each other in the solid state to interact. The crystal structure of dibenzylsulphide is not known; however the spectra would be expected to show fewer bands than those of the MPh, compounds owing to the presence of one fewer phenyl ring. 2000-1650 cm-‘. It is upon examination of this region that significant differences between the spectra of dibenzylsulphide and the MPhJ compounds become apparent. As opposed to the five distinct groups of bands arising from the combinations of the out-of-plane CH deformations h +j, h + i, h + g, g + i, andf+ i observed for the Mph3 compounds, there is no clear pattern for dibenzylsulphide. This indicates that the out-of-plane CH deformation modes which give rise to these combinations must also be displaced from their wavenumbers for the Mph3 compounds. Nevertheless, these bands can be accounted for by combinations of bands occurring in the lOOO700 cm- l region. 1650-1300 cm-‘. The five ring stretching modes k, 1, m, n, and o are easily identified in the infra-red spectrum as occurring at ca 1600, ca 1580,1492,1454, and ca 1325 cm- ‘, respectively. Apart from o, these values are typically 10 cm-’ higher than those observed for the Mph3 compounds. One problem with their identification arises from the occurrence of CHr deformation modes in this same region. There are few data available on CH2 deformation wavenumbers although they are known to be sensitive to the polarity of the substituents, particularly when attached to the methylene group [23]. With carbonyl

813

Vibrational spectra of phenyl compounds substitution the intensity of the CH2 deformations of adjacerit methylene groups is considerably increased and there is a shift towards lower wavenumber, e.g. acetic acid absorbs at 1418cm-’ compared to 1465 cm- ’ for hydrocarbons. COLTHUP et al. [27] assign the CH, deformation for the (RbCH,-(S-C) linkage, where R is a hydrocarbon group, to a band of medium intensity occurring in the 144c-1415 cm-’ region. On this basis the bands at 1413 and 1410 cm- ’ in the infra-red spectrum and at 1418 and 1415 cm-’ in the Raman spectrum are assigned to the CH, deformation for dibenzylsulphide. The assignment was further confirmed by the Raman spectrum (Fig. 5). As opposed to the infra-red spectrum the m and n v(C-C) modes are expected to give rise to weak bands, hence any strong-to-medium intensity bands occurring in the 15O(r1400 cm-’ region, in this case at 1418 and 1415 cm-‘, are likely to be due to CH, deformations. 1300-10OOcm~‘. The most characteristic band in this region is the ring breathing vibration p, occurring as a very strong doublet in the Raman spectrum at 1004 and 1001 cm-‘. The five in-plane CH deformation vibrations are less easily assigned. For PPh3, a and c occur in the ranges 1184-1177 and 1161-1153 cm-‘, respectively, although in the Raman spectrum of dibenzylsulphide there are no two groupings of bands, just a spread of eight bands between 1181 and 1156 cm-‘. The infra-red spectrum helps the assignment as the band groupings are more localised. One such group occurring in the range 1165-l 156 cm-’ is assigned to c and another two groups of bands between 1184-l 171 cm- 1 are tentatively assigned to a. The reason for the large number of bands in this region may be due to combinations such as u + t or r + t. JOSHI et al. [25] assign a further infra-red band of medium

S(CH,,Ph), h,:647~1nm

intensity at 1138 cm-’ (1136 cm-’ this work) to c; however this assignment would extend the range of c unacceptably to 27 cm - 1and hence the band has been reassigned to an overtone of y (2 x 568 cm- ‘). The most characteristic band in this grouping, b, is clearly observed at 1027 cm-’ in the infra-red spectrum and at 1030 cm- I in the Raman spectrum. The assignment of d is less straightforward. In the MPh, series it showed little sensitivity to the heteroatom but its assignment was complicated by the close proximity of the highest X-sensitive mode q. For dibenzylsulphide JOSHI et al. [25] assign to q the bands of medium to strong intensity at 12OOcm-’ in the infra-red and 1202 and 1247 cm-’ in the Raman spectrum. The latter Raman assignment is unusual in the large range given for q, the 1247 cm - ’ band being more favoured as the CH2 wag (uide infra). Nevertheless the ca 1200 cm-’ assignment for q is favoured on intensity grounds as d usually gives rise to a very weak band in the Raman spectrum compared to q. Hence the very weak band at 1076 cm- ’ in the Raman spectrum and the two medium intensity bands at 1075 and 1071 cm-’ in the infra-red spectrum are assigned to d. The remaining in-plane CH deformation e probably gives rise to the very weak Raman band at 1264 cm- I, although no infra-red counterpart is observed, or it may lie to lower wavenumber close to the CH2 wagging vibration (vide infra). JOSHI et al. [25] assign an extremely weak band at 1282 cm-’ in the infra-red spectrum and at 1274 and 1280 cm-’ in the Raman spectrum to this vibration, but none of these bands was observed in this work. Weak bands near 1300 and 1240 cm ’ have been assigned to CH, wagging vibrations [28] of long chain paraffins. DOLLISH et al. [14] give the narrow range of 1310-13OOcm-’ for the CH2 twist and wag for an

single

crystal

ca.80 K

Fig. 5. The Raman spectrum (161G30 cm-‘) of S(CH,Ph)2 at ca 80 K, 1, = 647.1 nm.

814

ROBINJ. H. CLARK et al.

alkyl substituent attached to a benzene ring. The only band that is close to this for dibenzylsulphide is the very weak Raman band at 1296 cm- ‘, there being no infra-red counterpart. In contrast, COLTHUP et al. [27] point out that CH2 wagging modes show the same sensitivity in terms of wavenumber and intensity, to substituent electronegativity as does the CHB symmetric deformation vibration. This is because the CH2 wagging vibration is mechanically similar to the symmetrical CH, deformation. From correlation studies they concluded that the CH2 wagging vibrations in C-CH,-X compounds are about 50-75 cm- 1 lower than the CH, symmetrical deformation in the corresponding X-CHJ compounds. As the S-CH3 deformation occurs in the range 1330-1290 cm-’ [27] this would put the corresponding CH, wagging vibration in the slightly larger 1280-1215 cm-’ range. In dibenzylsulphide the medium intensity infra-red band at 1235 cm-’ , and the medium intensity Raman band at 1253 cm-‘, previously assigned to q by JOSHI et al. [25] would seem best attributed to the CHI wag. The occurrence of two bands may be due to a number of factors, such as the inequivalence of the two methylene groups or to the in- and out-of-phase wagging vibrations. lOOO-7OOcm-‘. The most easily assigned modes are j and h, giving rise to bands in the ranges 995-987 and 976-968 cm- ’ , respectively, which closely parallel the values found for the Mph3 compounds. Mode f shows the greatest sensitivity to the mass of the heteroatom in the MPhJ series @de infia) and usually gives rise to a band at 750 + 15 cm- * for mono-substituted benzenes. There are exceptions though, e.g. toluene absorbs at 729cm-’ and benzoic acid at 808 cm- ’ [ 111. The band is most easily recognised in the infrared spectrum, being the strongest in the region (711, 707 and 705cm-’ for dibenzylsulphide; the corresponding Raman bands occur at 716 and 710 cm- ‘). It is here that the assignment deviates strongly from that of JOSHI et al., who assign to f the strong band at 775 cm- l. Whilst this is a possibility, this work favours this band as the X-sensitive vibration, r, again a difference with JOSHI et al. [25] who assign the 705 cm- ’ group of bands (703 cm-’ their work) to the CH2 rock. This latter assignment is considered unlikely for the following reason. The in-phase CH2 rock of four or more adjacent groups in a chain is known to give rise to absorption at around 720 cm-’ [29]; the band increases in wavenumber and decreases in intensity the fewer the number of adjacent CH2 groups. This would then favour the assignment of the weak bands at 809, 807, and 805 cm-’ in the infra-red spectrum and at 811 and 806cm-’ in the Raman spectrum to this mode. The strong band at 775 cm-’ is then attributed to the X-sensitive mode r, although its intensity is much greater than that observed for the MPh, compounds. These three assignments are more in agreement with VARSANYI’S work on benzylchloride [26], although they are still considered tentative. The out-of-plane CH deformation g is assigned to

the doublet in the infra-red spectrum at 843 and 853 cm-l, there being no coincident bands in the Raman spectrum. Finally band i is usually found at Table 7.

Wavenumbers/cm~ ’ of bands observed in the infrared spectrum of S(CH,Ph), ca 80 K

i 3103vw 3083~ 3074vw 3058~ 3051w 3038~ 303Ow 3025~ 3002~~ 2977vw 2954~ 2943vw 2920w

Assignment

J 1

2m m+n

v, CHz 2n WA

1959w 1919vw 1908vw 18%~ 1888vw I 1884~~ 1867~~ 1845~~ 1838~~ 1 1821~~ 1817~~ 1812vw,sh 1779vw 1770vw I 1763~~ 1706vw 1601W

1

1583~ 1577vw.sh I 1568vw. 1559vw 1492m 1454s 1419vw 1413w,sh 1410m 1399w,sh 1391vw,sh 1326~~ 1322~~

v(C-H) v(C-H)

2902vw 2865~~ 2809~ 2230vw 198Ow

1593vw

v(C-H)

j+i h+i lx+897 2i i+897 h+g i+897 g+i 2g k v(C-C) 1 v(C-C)

1

g+f

’

m v(C-C) n v(C-C)

2f I

SCHl {:” 0 v(C-C)

Assignment

I 1251vw 1243~~ 1235m 1204vw 1198~ 1184~~ ‘I 1180vw 1173vw 117lvw,sh 1165~ 1163w,sh 1161~ 1156vw,sh i 1136~ 1075m 1071m 1 1027~ 1026w.sh I 1004vw 1002vw 995w 993w,sh 972vw 923~ 915m

1 I I

CH2 wag q X-sens a B(C-H)

c B(C-H) 2Y d B(C-H) f~&C-H) p-ring i Y(C-H) h YGH) i y(C-H)

y+u

897m 853~ 843vw 823vw,sh 821~ 809vw 807vw,sh 805vw 775s 747vw 7llm,sh 707s,sh 705vs 696s 683m 675m 670m 62Ovw 618vw 57Ow,sh 568m 566m 564m 476vw 469m 412yw 408vw 406vw 346vw 335w 309vw 281vw 278vw 169~ 165vw 81vw 55vw

“+Y

; Y(C-H)

I I I

0 9(C-C) v(C-S) s a(C-cC)

y X-sens I I

t X-sens w K-C)

f

1

’

u X-sens

x X-sens t-x

Vibrational Table 8. Wavenumbers/cm’ of bands Raman spectrum* of S(CH,Ph),

i 1602m 1595w 1585m 1497vw 1456~~ 1454vw 1418vw,sh 1415m 1296~~ 1264vw 1253m 1245~~ 1236vw,sh 1234~ 1205w,sh 1203m 1189w,sh 1187~ 1181~~ 1176~~ 1172~~ 1168vw 1164vw 1156~ 1076~~ 1038~ 103Om 1004vs 1OOlm 993w 989vw 987vw 976vw 968vw 930vw 823vw 811~ 806m

Assignment

I k v(C-C) 1 v(C-C) m v(C-C)

I i

n v(C-C)

6CHz t+823 e B(C-H) r+Y

1 I

q X-sens

f+f f+t or a B(C-H)

I c B(C-H) d B(C-H) b B(C-H) p-ring i Y(C-H)

t h Y(C-H) i y(C-H) CH, rock

is 779m 771w 768m 716m 710w 698m N;;w,sh 673w,sh 667vw,sh 621m 615vw 568~ 56%~ 472vw 466m 409vw 349w 336~ 309m 306vw,sh 284vw 187m 184s 126s 121s lllm 105m 97m g7m 80m 73s 7Om,sh 58s 45s 4lm

observed CQ 80 K

in the

Assignment

r X-sens

1

f Y(C-H)

1 :(~~;‘) 2u s a(C-C-C)

I t

y X-sens t X-sens

’ w 4(C-C) t

815

spectra of phenyl compounds

u X-sens

x X-sens

predominantly lattice and torsional modes

900 cm-‘. There are, however, three bands observed at 923, 915, and 897 cm-’ in the infra-red spectrum and only one very weak band at 930 cm- ’ in the Raman spectrum. These bands account for a number of the combination bands observed in the 1800 cm- ’ region. The lowest band at 897 cm-’ can be accounted for by the combination y + u, leaving the two bands at 9 15 and 923 cm - 1tentatively assigned to i. Below 700 cm-‘. The assignment of the ring deformation modes is straightforward, u, s, and w all occur at wavenumbers close to those observed for the Mph3 compounds. The four remaining X-sensitive vibrations, y, t, u, and x, occur at 570-564, 476466, 349-335, and 284-278 cm- ‘, respectively. The assignment of the weak band at 309 cm-’ in both the infra-red and Raman spectra is unclear. All six X-sensitive vibrations occur at higher wavenumbers than for PPh, and more closely parallel those of fluorobenzene. The remaining bands expected in this region are those attributable to C-S stretching. These are found in the 7OG-600 cm- ’ region and give rise to medium to weak bands in the infra-red spectrum but to strong bands in the Raman spectrum. SHEPPARD [30] proposed various sub-ranges depending upon the coordination of the attached carbon atom, primary sulphides of the type RCH2-S- giving rise to bands between 660 and 630 cm-‘. The strong band at 678 cm-’ in the Raman spectrum and the bands at 675 and 670 cm- ’ in the infra-red spectrum of dibenzylsulphide are assigned to this mode. This is similar to the assignment of JOsHI et al. [25] although the actual band wavenumbers differ from theirs by as much as 28 cm-‘. The infra-red spectrum in this region is shown in Fig. 6.

around

* 647.1 nm excitation

SKH,Ph),

Fig. 6. F.t.i.r. spectrum (660-140 cm-‘)

of S(CH,Phb

at

ca

80 K.

816

ROBINJ. H. CLARKet al. CONCLUSION

The previously published data on all of these compounds have been improved upon considerably. For the Mph3 series (M = P, As, or Sb) the greatest improvement is to the low wavenumber regions of both the infra-red and Raman spectra; the best previous spectra on these compounds are those of SHOBATAKEet al. [31] and JONES and POWELL [32]. The Raman work of JOSHI et al. [25] on dibenzylsulphide was restricted to the region > 670 cm- ’ whereas the present work extends well below this to 30 cm-‘.

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