The far infrared spectra of some Group Vb trihalides

Spectrorhirnica Acta, 1965,Vol. 21, pp. 1773to lf83. Pergamon PressLtd. Printedin h’orthern Ireland

The far infrared spectra of some Group Vb trihalides T. R. MANLEY Department

and D. A. WILLIAMS

of Chemistry, Rutherford College of Technology, Newcastle-upon-Tyne (Received

20 February

1965)

Abstract-The far infrared absorption spectra of the Group Vb trihalides AsI,, SbI,, BiBr, and BiI, have been examined in the region 500-50 cm-l using both grating spectrometers and a Michelson far infrared interferometric spectrometer. The spectra obtained by the two methods are compared and fundamental vibration frequencies are assigned. Force constants have been calculated using a 4-constant potential function.

compounds investigated are all trihalides of Group Vb and in the vapour state have been shown to exist as pyramidal molecules of symmetry C,,. The base of the pyramid is formed by the three halogen atoms arranged in an equilateral triangle and the apex is occupied by a Group Vb atom. In crystalline AsBr3, SbCl, and SbBr, the molecular structure has been shown to be very similar to that found in the gaseous state [l, 21. BACKER and IN VEGARD [3] using X-ray diffraction, have shown that the crystalline compounds AsI,, SbI, and BiI, have an MX, layer structure with hexagonal close packing, and with each Group Vb atom octahedrally surrounded by six iodine atoms. The lattice consists of two adjacent layers of halogen atoms with a layer of Group Vb atoms between them. The layer can be regarded as an infinite two-dimensional molecule since the primary valences of the M and X atoms are satisfied within the layer, there being only van der Waals forces between adjacent layers. The positions of the Group Vb atoms in AsI,, SbI, and BiI, are not accurately established but it seems probable that the layer structure for these molecules may be regarded as intermediate between a layer and a molecular structure since, in the ideal structure, the Group Vb atom has six equidistant halogen neighbours, but in AsI,, SbI, and BiI, three of these appear to be much closer than the other three [3]. We have examined the far infrared spectra of the three iodides dispersed in a polythene matrix and in Nujol. The observed bands are in good agreement with those calculated for these molecules assuming them to have a regular pyramidal configuration, with symmetry C,,. Our results for AsI, are in fair agreement with the Raman study of the molecule by STAMMREICH et al. [4] but we have modified the assignment made by them. THE

[l] I. LINDQUIST and A. NIGGLI,J. Inorg.Nucl. Chem. [2] D. W. CUSHEN and R. HTJLME, J. Chem. Sot. 2218 [3] J. BACKER and L. IN VEGARD, Skhfterutgitt av Det Nat. Naturw. Klasse No. 2. 73 (1947). 141 H. STAMMREICH,R.FORNERIS and Y.TAVARES,J. 1773

2, 345 (1956). (1962). Norske Videnskaps-Akademie Chem. Phys.

25, 580 (1956).

i Oslo, I.

T. R. MANLEYand D. d. WILLIAMS

1774

The dispersion of the iodides in the polythene matrix very probably weakens the layer structure present in the normal crystalline state by nullifying the van der Waals forces between the non-bonded metal-halogen atoms. The lattice then breaks down into essentially discrete pyramidal molecules, in solution in the polythene matrix. Force constants were calculated for the molecules on the basis of a four-constant potential function derived by HOWARD and WILSON [5]. The function assumes a general force field within the molecule consisting of valence forces and an additional force between the Group Vb atom X and one of the halogen atoms Y, when the distance of the other Y atoms from X is changed, and an additional force tending to change one Y-X-Y angle when the others are changed. The function would be the most general one if two more constants had been included, the interaction of each bond stretching with change in adjacent bond angle, and the interaction of the stretching of each bond with the change of the angle opposite. These have been neglected since VAN VLECK and CROSS[6] found such interaction to contribute very little to the potential energy of the molecule. The potential function is of the form 2v

=fdi3(A4)21 +.fa[W~~12)21 + ?fddP?W(Ad2)l + 2fol,P;(~a12N~~23)1(1)

where Ad,, Ad,, Ad, are the internal co-ordinates indicating the changes in the X-Y bond distances and Aa12, Aa,, and Act,, are the changes in the Y-X-Y inter-bond angles. The four force constants are given by fd, f,, fdd, f,,. The potential function above gives rise to two quadratic expressions in terms of fd and fdd(equations (2) and (3)) and two quadratic expressions in terms of fa,f,, (equations (4) and (5)). All four expressions contain the atomic masses M,, M,, the angle of XY with the symmetry axis of the molecule (B) and the observed vibrational fundamental frequencies vr, v2, va, v4.

(fd 4

2rr2c2

2fdd)

= ~N

(v12 + v~~)M,M, (M, + 3M, cos2 j3)

*

(M, + 3M, cos2 ,!I)” -

tfd _fddJ _4n2c2 (vs2 +

(M, + 3M,)(M,

v~~)M,M, f (2M, + 3M, sin2 /3)

N

-

M,M,2v,2v42(2M,

+ 2M, cos2 /3 + 3M, sin4 /3)

- vr2vZ2MzMar2(1 + (for- fa,) =

+ 3M, cos2 B)

(FM :QMMi?;) z Y

(2M, + 3M, sin2 p)(M,

cos2 B(M, +

II (2)

4M$Q2y12v22(M, + 3M, sin2 @) t

(v12 + v~~)~M:M,

II

+ M, cos2 B + 3M1, sin2 /?) 3 co@ 8)

1

3 cos2 p) 3M, sin2 /?)

(3)

(4)

3M,)(f, + 2fd

vS2vp2MzMy2( 1 + 16.rr4c4 3N2 (M, + M, cos2 p +

*

* (fd - fdd)

[5] J. B. HOWARDand E. B. WILSON,JR., J. Chem. Phys. 2, 630 (1934). [6] J. H. VAN VLECKand P. C. CROSS,J. Chem. Phys. 1,357 (1933).

(5)

The far infrared spectra of some Group Vb trihalides

1775

EXPERIMENTAL PROCEDURE The grating spectrometers used in the investigation were the Grubb Parsons DM4 (500-200 cm-l) and GM3 (ZOO-65 cm-l). The experiments were also performed using a Grubb Parsons-N.P.L. Michelson far infrared interferometric spectrometer, of range 500-20 cm- 1. The grating instruments were capable of a resolution better than 3 cm-l for most of their working range, and the interferometer could be used at greater resolution to examine band structure in more detail. The compounds used were obtained commercially and two techniques were employed. In the first the trihalides were well dispersed in low molecular weight polythene (“Epolene N-10”) and pressed to the desired thickness. The film specimens were kept in a vacuum desiccator. The films used in the experiments were about 300 p thick; for this thickness spectra of optimum intensity and sharpness with the DM4 grating instrument were obtained with 5-1Oo/o of trihalide, but concentrations of 1O-15o/o trihalide were necessary using the interferometer and the GM3 long wave grating spectrometer. In order to examine bands at frequencies lower than 100 cm-’ in greater detail on the interferometer, appropriate long-wave cut-on transmission filters [7] were employed. These filters eliminate radiation shorter than the cut-on wavelengths without appreciably attenuating radiation of longer wavelength. In the second method the trihalides (except BiBr, which is too hygroscopic) were examined on the grating spectrometers in Nujol dispersed between 1 mm polythene plates. (“Rigidex 35”, British Resin Products) Spectra comparable to those of the first method were obtained only when the trihalide particles were less than 10 11. Polystyrene (O-25 mm thick) which is quite transparent to frequencies lower than 100 cm-r was used for Nujol windows to examine the spectra of the trihalides around 70 cm-l where polythene absorbs strongly. ARSENIC TRIIODIDE, AsI, Arsenic triiodide is a yellowish-red solid (m.p. spectrum only has been reported previously [4]. have shown that AsI, has C,, symmetry with an I-As-I angle of 1Ol"l'.The far infrared spectrum Assignments

146’C, b.p. 403%). The Raman Electron diffraction studies [8] As-I distance of 2.55 A and an is shown in Fig. 1.

and force constants

The four vibrational fundamentals of AsI, and the corresponding force constants are summarised in Table 1. The predicted force constants and bond angles are compared in Table 5. The spectrum of AsI, in Epolene film showed three well defined absorption bands lying at 200,102 and 74 cm-l, and a weak absorption maximum at 148 cm-l. The 200 cm-l band was further resolved into a triplet having band positions 226, 216 and 201 cm-l. The band at 74 cm-l sharpened appreciably when examined in Nujol between polystyrene windows. Examination of the AsI,-Epolene film on the interferometer at 4 cm-l resolution gave the values for the triplet as 226, 217 and [7] T. R. MANLEY and D. A. WILLIAMS, Spectrochinz. Acta 21, 737 (1965). [8] H. A. SKINNER and L. E. SUTTON, Trans. Faraday Sot. 40, 164 (1944).

T. R. MANLEY and D. A. WILLIAMS

1776

60

Wavenumber,

Fig. Table

1. Force constants and vibrational fundamentals

Vibrational fundamental (cm-l) yl(al) v&al) ys(e) ~~(0)

= = = =

cm-’

1. Far infrared spectrum of AsI,.

225.7 101.6 201.2 73.6

Tntensity

Description

M’ w vvs, b S

As-I stretching AsI, deformation As-I stretching AsI, deformation

w = weak, m = medium,

s = strong,

v = very,

of AsI,

Force constants (dgne cm-l x lo-“) fd

= 1.052 0.158 0.171 0.030

fdd = fa = fola= b = broad

201 cm-i, and confirmed the 148 and 102 cm-l bands. Strong absorption at 74 cm-i was also indicated. Due to the symmetry of the pyramidal trihalides XY, the force constants of a member of the group may be predicted with reasonable accuracy from the force constants of other trihalides containing similar atoms, and of similar molecular weight [9]. Calculations of the fundamental frequencies of AsI, from the observed force constants of AsBr,, PBr, and PI, from the expression derived by VENKATESWARLU and SUNDARAM [lo] from WILSON’S group theoretical model lead to the values: 219(a,), 209(e) cm-l (stretching): 90(aJ, 72(e) cm-l (deformation). AsI, will have lower stretching fundamentals than those of AsBr, and PI, and higher values than for SbI,. The corresponding frequencies are: AsBr,: PI,: AsI,: SbI,;

279, 272 cm-l (stretching); 325 cm-l (stretching); 226, 201 cm-l (stretching); 177, 147 cm-l (stretching); 303,

128, 98 cm-l (deformation) 79 cm-l (deformation) 102, 74 cm-l (deformation) 89, 71 cm-l (deformation) 111,

[9] F. A. MILLER and W. K. BAER, Spectrochim. Acta 17, 112 (1961). [lo] K. VEN~STESWARLU and S. SUNDARAM, Proc. Phys. Sot. (Londo’l~) A69, 180 (1956).

The far infrared

spectra

1777

of some Group Vb trihalides

The relatively intense 201 cm-r band is taken to be a stretching fundamental of AsI,. As the two stretching fundamentals probably lie close together, we assume that either the 226 or 216 cm-l band (the latter being the more intense) is the other stretching vibration. However, calculation of the force constants of AsI, taking the 216 band to be either the degenerate or the symmetric stretching mode, in conjunction with the 201 cm-l band, gives rise to imaginary force constants. Only by assigning the 226 cm-l band to the symmetric stretching mode are real force constants obtained. The assignment of the 201 cm-l band to the doubly-degenerate stretching mode is also in keeping with the fact that the Raman depolarised degenerate stretching mode for most of the pyramidal trihalides appears as the broadest,, most intense vibrational fundamental in their far infrared spectra. The bands at 102 and 74 cm-r are assigned to the totally symmetric and doubly by comparison with other similar degenerate deformation modes, respectively, molecules in Table 6. The relatively weak band at 148 cm-l can only be attributed to the overtone 2vq = 2 x 73.6 cm-l

= 147.2 cm-’

No combination band will account for the 216 cm-l band; apart from improbable isotopic shifts the only explanation is the second overtone 3vq = 3 x 73.6 = 220.8 cm-l STAMMREICH etd., using Raman techniques of AsI, as follows: 216 (al), 221 (e) cm-l

(stretching);

[4], assigned the four fundamentals

94 (a,), 70 (e) cm-r (deformation).

We were unable to find any absorption maximum in the region of 90 cm-l and this assignment gives rise to an unreasonably low value for the force constant fdd. (0.05 x lo5 dyne cm-l compared with the predicted value 0.16 x lo5 dyne cm-l). We therefore prefer the complete assignment: 226 (a,), 201 (e) cm-l

(stretching);

102 (a,), 74 (e) cm-l

(deformation).

ANTIBIONY TRIIODIDE, SbI, Antimony Triiodide is an unstable tetramorphous solid, (m.p. 167’C, b.p. 4Ol’C). Electron diffraction studies [ 1 l] have shown that SbI, has C,, symmetry, with an Sb-I distance of 2.67 A and an I-Sb-I angle of 99”O’. The far infrared spectrum is shown in Fig. 2. Assignments

and force constants

Table 2 summarises the observed fundamental vibration frequencies and the calculated force constants of SbI,. Four bands were observed lying at 177, 147, 89 and 71 cm-r. The positions of the four bands obtained using the grating instruments were confirmed on the interferometer to within one wavenumber for each fundamental. The stretching frequencies for SbI, will be lower than those for SbBr, and AsI,, and higher than those for BiI,. By comparison with Table 6 our observed values appear to be in agreement. We accordingly assign the 177 and 147 cm-l bands to the two stretching [ll]S.M.

SWINGLE,

quoted

in ActaCryst.

3,

46 (1950).

1778

T. R. MANLEY and D. A. WILLIAMS

‘300

280

260

240

220

200

160

160

Wavenumber,

Fig. Table Vibrational fundamental Y1(al) ~~(a,) yg(e) Y‘&0)

120

100

SO

60

40

cm-’

2. Far infrared spectrum of SbI,.

2. Force constants and vibrational fundamentals

(cm-l)

= 177.1 = 88.6 = 146.7 = 71.0

140

Intensity 8 W

vvs, b m

Descript,ion Sb-I SbI, Sb-I SbI,

stretching deformation stretching deformation

of SbI,

Force constants (dyne cm-l x 10-5)

fd = 0.806 fdd= 0.153 fa = 0.155 faGI = 0.015

modes, and the 89 and 71 cm-l bands to the deformation modes. We further assign the 89 cm-l band to the symmetric deformation mode and the 71 cm-l to the degenerate deformation mode. Calculation [lo] of the frequencies of SbI, from the force constants of SbBr,, AsI, and AsBr, leads to the values: 177 (a,), 157 (e) cm-l (stretching); 86 (a,), 69 (e) cm-l (deformation), and the observed values are thus seen to be in quite good agreement. We next consider which of the stretching frequencies is the totally symmetric vibration and which the doubly degenerate vibration. MILLER and BAER [9] observed that for Z=XY, molecules of C,, symmetry the doubly degenerate X-Y stretching frequency is higher than the symmetric one in every case in which they had been properly distinguished by polarisation measurements. For the pyridimal trihalides XY,, however, the trend was reversed on going from I to Br to Cl for the phosphorus halides, as can be seen from Table 6. This does not appear to apply, however, to the heavier trihalides (with the possible exception of SbBr,) [12] where in every case the symmetric stretching frequency is higher than the degenerate mode. A calculation of the force constants of SbI, taking the 147 cm-l band as the totally symmetric [12] E. F. GROSS and I. M. GINSBURG, O&ka

i Spektroskopiya

1, 710 (1956).

The far infrared spectra of some Group Vb trihalides

1779

gives rise to imaginary force constants. We therefore prefer the alternative complete assignment: 177 (a,), 147 (e) cm-l (stretching); 89 (a,), 71 (e) cm-l (deformation). Further support that the 147 cm-l band is the degenerate stretching fundamental is given by its great intensity, as previously discussed under Asl,. mode

BISMUTH TRIBROMIDE, BiBr, Bismuth tribromide is an orange-yellow crystalline solid (m.p.Z18”C, b.p. 46O”C), Electron diffraction studies [ 131 indicate readily hydrolysed at room temperature. that BiBr, has C,, symmetry with a Bi-Br distance of 2.63 A and a Br-Bi-Br angle of 100’0’. Fig. 3 shows the spectrum of BiBr, in the region 300-60 cm-l. 100

.\’ 60 f E 5 F

40

‘300

260

260

240

220

200

160

180 Wovenumber.

Fig.

Assignments

140

120

100

60

60

40

cm-’

3. Far infrsred spectrum

of BiBr,.

and force constants

Table 3 summarises the observed vibrational fundamentals and force constants. Five pronounced bands at 280, 196, 169, 104 and 89 cm-l were noted, the 104 cm-l being relatively much weaker. Some absorption was also observed at 545 cm-l. By comparison with the other C,, trihalides (Table 6) we would expect the stretching Table

3. Force constants and vibrational fundamentals

Vibrational fundamental (cm-l) vl(al) Y&r) rs (e) VP(e)

= 196.2 = 103.7 = 169.0 = 90.4

Intensity 8, b VW s, b m

Description Bi-Br stretching BiBr, deformation Bi-Br stretching BiBr, deformation

of BiBrs Force constants (dyne cm-l x 10-5) fa = faa = foe = fGTo:=

[13] H. A. SKINNER and L. E. SUTTON, Trans. Faraday. Sot. 80, 681 (1940).

I.062 0.163 0.164 0.015

1780

T. R. MANLEYand D. A. WILLIAMS

vibrations of BiBr, to be higher than those of BiI, and SbI,, and lower than those of SbBr, and BiCl,. The bands at 545 and 280 cm-l are most probably combination tones as they are The position of all six bands was confirmed on the too high to be fundamentals. interferometer but the band at 104 cm-l appeared to be relatively very weak. Calculation of the frequencies of BiBr, from the force constants of BiCI,, SbBr, and SbCl, leads to the values: 195 (a,), 169 (e) cm-l (stretching); 95 (cc,), 81 (e) cm-l (deformation). These are in reasonable agreement with our observed values. We therefore assign the 196 and 169 cm-l bands to the stretching modes and the 104, 89 cm-l bands to the deformation modes by comparison with other molecules given in Table 6. Calculation of the force constants of BiBr, assuming the 195 cm-l to be the degenerate stretching mode gives rise to a negative value for fdd which is improbable. The alternative assignment 196 (a,), 169 (e) cm-l (stretching); 104 (cci) 89 (e) cm-l (deformation) leads to real and positive values of the force constants which are in agreement with those of the other trihalides. We therefore prefer this assignment. BISMUTH TRIIODIDE, BiI, Bismuth triiodide when freely sublimed consists of hexagonal greyish black crystals, isomorphous with a form of SbI,. It is hydrolysed in air extremely slowly. BiI, is assumed to be a pyramidal trihalide of C,, symmetry on theoretical grounds in the absence of published infrared and Raman spectroscopic data or The force constants are calculated from the observed crystallographic detail. vibrational data and compared with the theoretical values computed on the assumption that BiI, conforms to a regular pyramidal configuration. For this purpose we assumed a bond angle of loo”, the value found for BiCI, The angles of the trihalides of As and Sb (except SbF,), all of C,, and BiBr,. symmetry, lie close to loo”, are effectively constant for each Group Vb element, and show negligible variation in going from XC& to XI,. It is to be noted that the effect on the force constants of possible inaccuracies in the bond angle is small compared to the effect due to the inaccuracy of the infrared data. The far infrared spectrum is shown in Fig. 4. Assignments

and force constants

The observed vibrational fundamentals of BiI, and the calculated force constants are summarised in Table 4. Five bands were observed at 252, 145, 115, 90 and 71 cm-l. The stretching frequencies for BiI, were expected to be lower than those for BiBr, and SbI,. As no bands were observed below 70 cm-l using either type of instrument, we assume that the pronounced bands at 90 and 71 cm-l are the two deformation fundamentals. The 71 cm-l band was confirmed by using BiI,/Nujol between polystryrene windows on the GM3 grating instrument. It is interesting to note that there is no apparent shift in the deformation fundamentals on going from SbI, to BiI, and the values appear to converge for the triiodides on going from PI, to BiI,. The 252 cm-l band is most probably a combination band or an overtone as it is too high to be a fundamental.

1781

The far infrared spectra of some Group Vb trihalides

300

280

26 Wovenumber. Fig.

Table

= 145.4 = 90.2 = 115.2 = 71.0

cm-’

Far infrared spectrum of BiI,.

4. Force constants and vibrational fundamentals

Vibrational fundamental (cm-l) vl(al) vz(al) vs(e) vq(e)

4.

Intensity S

s, b vs, b m

Description Bi-I BiI, Bi-I BiI,

stretching deformation stretching deformation

of BiI,

Force constanm (dyne cm-l x 10P5)

fd = fdd = fa = faa =

0.651 0.141 0.183 0.024

Calculation of the frequencies from the force constants of SbBr,, SbI, and BiBr, leads to the values 148 (a,), 100 (e) cm-l (stretching); 88 (cc,), 67 (e) cm-l (deformation) and these are seen to be in quite good agreement with the observed values. We therefore assign the bands at 145 and 115 cm-l as the two stretching fundamentals. We further attribute the 145 cm-l band to the totally symmetric stretching mode and the 115 cm-l band to the degenerate stretching mode since a calculation of the force constants using the alternative assignment gives rise to imaginary force constants. Further support is given to our assignment by the fact that the 115 cm-i was much broader and more intense than the I45 cm-r band. The band observed at 252 cm-l can then only be attributed to the combination tone 2vz + vq = (180 + 72) cm-l = 252 cm-l Our complete assignment for BiI, is: 145 (a,), 115 (e) cm-l (stretching); 90 (a,), 71 (e) cm-l (deformation). In conclusion, our results would indicate that bismuth triiodide does conform to a regular pyramidal configuration. 6

T. R. MANLEY and D. A. WILLIAMS

1782

60

50320

300

260

260 Wavenumber,

Fig. Table

5. Infrared spectrum of AsBrB (1%

240

220

200

cm-’

in Epolene)

in the region 320-200

cm-l.

5. Calculated and observed fundamental vibrational frequencies, force constants and bond angles of some Group Vb trihalides Vibrational fundamentals

Molecule

AsBr, Observed Calculated ASI, Observed Celoulated SbI, Observed Calculated BiBr, Observed Celoulated BII, Observed Calculated

(dyne cm-’

.fa

x

10-5)

angle (a)

fdd

fa

1.61 1.57

0.15 0.09

0.18 0.16

0.02 0.03

100~30’ 87’28

74 72

1.05 1.14

0.16 0.15

0.17 0.14

0.03 0.01

loloo’ 81’50

147 157

71 69

0.81 0.89

0.15 0.12

0.16 0.14

0.02 0.02

104 95

169 169

89 81

1.06 1.05

0.16 0.16

0.16 0.13

0.015 0.013

100°0’ 81’4’

90 88

115 100

71 67

0.65 0.57

0.14 0.22

0.18 0.17

0.02 0.02

100°0’ 73’12’

VI(%)

%@l)

279 268

128 123

212 272

98 86

226 219

102 90

201 209

177 177

89 86

196 195 145 148

Note on the far infrared

Bond

Force constants

(cm-‘)

%(d

spectrum

vr(‘4

of arsenic tribromide,

fara

99OO’ 80%

AsBr,

In a recent paper, MILLER and BAER [9] reported on the Raman and the far infrared spectrum of arsenic tribromide and found three vibrational fundamentals at 98, 128 and 275 cm-l. They concluded that the 275 cm-l band was a doublet, but no splitting was indicated. We carried out an examination of a 1% finely divided dispersion of AsBr, in Epolene film (0.3 mm thick) and succeeded in splitting the 275 cm-l band into two components of band positions 279 and 272 cm-l, (Fig. 5) corresponding to the ~~(a~) and yQ(e) vibrations respectively.

The far infrared spectra of some Group Vb trihalides

1783

The value for or is in fair agreement with MILLER and BAER’S value of 284 cm-l, which they estimated from a combination tone. We also observed weak absorption at 255 cm-l and 226 cm-l which we attribute to the following: 2~~ = 2 x 128 cm-l = 256 cm-l yz + Ye = 128 + 98 cm-l = 226 cm-l Table 5 shows the observed and calculated vibrational fundamentals of the trihalides examined in this investigation, and summarises the observed and calculated force constants. Table

6. Fundamental

frequencies and force constants of the Group Vb trihalides

Vibrational fundamentals Molecule

Ref.

vs(e)

v,(e)

fd

511 510 507 380

531 258 251 260 162

840 484 480 494 400

486 190 190 189 115

392 303 707 405 410 412 (284) 279 216 226 360 377 355 227 244 177 288 196 145

161 111 341 194 193 194 128 128 94 102 165 164 162 110 110 89 130 104 90

392 326 644 370 370 387 275 272 221 201 320 356 318 236 226 147 242 169 115

116 79 274 158 159 155 98 98 70 74 134 128 126 92 91 71 100 90 71

4.59 2.12 2.12 2.17 1.63 1.63 1.83 1.21 3.92 2.03 2.03 2.14 (1.57) 1.60 1.17 1.05 1.78 2.10 1.78 1.51 1.47 0.81 1.19 1.06 0.65

VI(%)

PI,

[141, t151 t101, [I51 [I41 WI WI, [I51 [I41 [161 c41

AsBr,

w% P51 [I41 [I61 Ku

PF, PCl,

PBr,

AsF, A&l,

ArtI, SbCl,

SbBr, SbI, BiCl, BiBr BiI 3 a * This work.

cl43

;41 Ll PW IlO1 WI P71 * UOI *

* ( ) Estimated

Force constanta (dvn cm-’ x 1O-8)

(CII-I-‘)

890

v&d

fdd 0.39

0.26 0.27 0.09 0.12 0.03 0.03 0.06 0.34 0.18 0.19 0.19 (0.09) 0.15 0.05 0.16 0.16 0.16 0.25 0.11 0.10 0.15 0.17 0.16 0.14

fa

fau:

1.07 0.28 0.32 0.32 0.20 0.27 0.24 0.18 0.41 0.19 0.23 0.22 (0.15) 0.18 0.14 0.17 0.18 0.16 0.17 0.15 0.16 0.16 0.13 0.16 0.18

0.04 0.07 0.06 0.07 0.07 0.07 0.07 0.03 0.06 0.03 0.02 0.03 (0.03) 0.02 0.02 0.03 0.02 0.03 0.02 0.01 0.01 0.015 0.025 0.016 0.02

values.

Table 6 summarises vibrational frequencies and force constants of the pyramidal trihalides of phosphorus, arsenic, antimony and bismuth; the results refer to work employing both Raman and infrared techniques. AcknowZedgemRnts-The authors are grateful to Dr. A. E. MARTIN, Dr. G. R. SHARP and Mr. J. SHIELDS of Sir Howard Grubb Parsons for assistance with the spectroscopic measurements and to British Resin Products and Kodak Ltd., for gifts of materials. One of us (D. A. W.) is indebted to the Ministry of Aviation for a maintenance grant. [14] G. HERZBERG, Molecular Spectra and Molecular Structure (Vol. 2,) Infrared and Raman Spectra of Polyatomic Molecules, Van Nostrand, New York (1962). [15] J. CABANNES and A. ROUSSET, Ann. Phys. 19, 229 (1933). [16] P. W. DAVIS and R. A. OETJEN, J. Mol. Spectrorrcopy, 2, 253 (1958). [17] T. R. MANLEY and D. A. WILLIAMS. Unpublished.