Spectrochimica Acta, 1965, Vol. 21, pp. 775 to 781. Pergamon Press Ltd. Printed in Northern Ireland

Infrared and Raman spectra of P(CN), and As(CN)**

FOIL A. MILLER, STEPHEN G. FRANKISS and OSW~LDO SALAT Mellon Institute, Pittsburgh, Pennsylvania

(Received 15 June 1964)

Abstract-Infrared spectra from 33 to 4000 cm-l, and Raman spectra, are reported for P(CN), and As(CN),. C,, symmetry was assumed. The fundamental frequencies suggested for P(CN), are: a,-2206, 620, 468, 145; e-2202, 581, 452, 314, and 159 cm-l. The a2 frequency is un- known. For As(CN), all nine of the allowed fundamental frequencies have probably been observed, but it w&s not possible to assign them to their correct species.

As A continuation of our interest. in cyano compounds [l-3], a study has been made of the vibrational spectra of P(CN), and As(CN),. For P(CN), the only earlier work is a note by STAATS and MORGAN [4] giving four infrared bands (which do not agree well with ours), and a paper by GOUBEAU, HAEBERLE and ULMER [5]. It seemed desirable to make a further study because the infrared and Raman values in the latter paper do not agree well below 460 cm-l, and because there are no Raman polarization measurements. EMERSON and BRITTON [6] recently published an X-ray study of P(CN),. They found that the molecule is pyramidal with C,, symmetry. A surprising feature is that the P-C=N bonds are bent slightly in the (I~ planes to give a bond angle at the carbon atom of 171 f 3”. We suspect that this results from the requirements of packing in the crystal lattice, and would not be true for the isolated molecule.

For As(CN), no spectroscopic work has been reported. EMERSON and BRITTON [7]

have shown by X-ray diffraction that it has approximate C,, symmetry in the crystal-approximate because one C--hT group is closer to neighboring molecules than the other two. The data are not good enough to demonstrate a small bending within the As-C--N groups.

P(CN), Experimental

1. Preparation of the sample. P(CN), was prepared by the reaction of PCl, and AgCN as described in Inorganic Syntheses [8]. White crystals were obtained by resubliming the crude compound in vacua. Due to the great sensitivity of P(CN),

* This work was supported by the United States Atomic Energy Commission under Contract AT(30-l)-1993.

t Permanent address: Department of Physics, University of Sao Paulo, Sao Paula, Brazil.

[l] H. STA~MREICH and 0. SALA, 2. Elektrochem. 64, 741 (1960); 65, 149 (1961). [Z] F. A. MILLER and W. K. BAER, Spectrochim. Acta 19, 73 (1963). [3] F. A. MILLER, 0. SALA et al., Spectrochim. Acta 20, 1233 (1964). [4] P. A. STAATS and H. W. MORGAN, Appl. Spectrosc. 13, 79 (1959). [5] J. GOUBEAU, H. HAEBERLE and H. ULMER, Z. Anrrg. AUgem. Chem. 311, 110 (1961). [6] K. EMXRSON and D. BRITTON, Acta Cryst. 17, 1134 (1964). [7] K. EMERSON and D. BRITTON, Acta Cryst. 16, 113 (1963). [S] P. A. STAATS and H. W. MORGAN, Inorganic Syntheses 6, 84 McGraw-Hill (1960).

775

776 F. A. MILLER, S. G. FRANKISS and 0. SALA

to water, it must be handled in a vacuum or in a good dry box, and any solvent must be carefully dried. CH,CN proved to be a suitable solvent. If it was treated with P,O, and distilled in the absence of moisture, it gave clear, colorless solutions. Without drying, a yellow solution resulted which fluoresced strongly during the Raman exposures.

2. Spectroscopic procedures. The infrared spectrum was measured from 33 to 4000 cm-l with Beckman IR-11 and IR-9 grating spectrophotometers. The sample was examined in three forms: ( a as Nujol and halocarbon mulls, (b) in CH,CN ) solution, and (c) as a solid deposited from the vapor onto a cooled CsBr or poly- ethylene plate in a conventional low temperature cell at roughly 100°K.

Raman spectra were obtained with a Cary Model 81 recording Raman spectro- photometer. The sample was studied as a crystalline powder and as a solution of about 15 wt.% in dry CH,CN. P(CN), is a weak Raman scatterer, and there was great difficulty in getting acceptable spectra. Qualitative polarization measurements were made on the solution by the two-exposure method of CRAWFORD and HORWITZ [9].

Results

The experimental results are shown in Table 1 and Fig. 1. Frequencies of typical bands should be accurate to f 1 cm-l in the infrared, and to & 2 cm-i in the Raman

I~IlIIIlll/ ,,I, 2040 2200

, / , I, , , , , / , / I,, , , , ,

600 500 400 300 200 100 Cm-’

Fig. 1. Infrared spectra of solid P(CN), (upper) ctnd As(CN), (lower). Raman frequencies are indicated by vertical bars. The cnrves are re-drawn composites

from several original recordings.

spectrum. The spectral slit width (i.e. one half the instrumental band pass) was less than 2 cm-l for the infrared, and was 10 cm-l for the Raman spectrum. The table also includes the data of GOUBEAU et ab. [5] for comparison. We did not find their 267 and 276 cm-l infrared bands. For the Raman spectrum below 460 cm-l there is no agreement, which confirms the suspicions that they themselves expressed about their low-frequency Raman data.

[9] B. L. CRAWFORD. Jr. and W. HOR~ITZ, J. Chem. Ph.ys. 15, 268 (1947).

::

Tab

le

1. R

aman

an

d in

frar

ed s

pect

ra

of P

(CN

),

GO

UB

~A

U et

al.

T

his

wor

k

Ram

an

Infr

ared

R

aman

In

frar

ed

Sol

id

CH

sCN

Sol

n.

Ass

ign

men

t S

olid

S

olid

C

H,C

N

Sol

n.

Sol

id

CH

,CN

S

oln

.

cm-l

I

cm-r

I

cm-l

I

cm-l

I

cm-l

I

P cm

-l

I cm

-l

I

288

1 28

7

355

2 32

4 42

8 46

3 2

577

3 57

9

607

2 59

9 62

9 2

627

2200

10

21

82

2 22

04

VS

267

mw

1

276

W

2 31

2 2

451

464

2 58

5 2

605

2 63

3

m

w,

sh

W

8 vs

8

110

*5

1 14

5 *5

1

315

3 31

2 1

451

1 45

3 3

468

5 46

3 3

P

577

3 -5

81

1, s

h

604

4 60

3 4

P

630

1 63

0 7

P

2206

10

22

08.5

0,

eh

22

10

0, s

h

130

f 5

0

2206

10

P

110

VW

14

5 m

15

9 m

303t

31

4 m

45

2 46

8 ,”

586

m

603

vs

630

vs

2201

.5

s

* ?

* r4

*

VI0

*

Imp

* Im

p 14

5 +

15

9 =

30

4 *

r 0

r s

463

vw

575

VW

, sh

I

%

V7

601

vw,

sh

630

vs

1

vs,

in r

eson

. w

ith

46

8 +

14

5 =

61

3 or

45

2 +

15

9 =

61

1 21

97

s V

S

Vl

2210

.5

w,

sh

* ?

2214

.5

w,

sh

* 9

w,

m,

s =

w

eak

, m

ediu

m,

stro

ng

v =

ve

ry

sh

=

shou

lder

p =

pol

ariz

ed

Imp

=

impu

rity

*

= n

ot e

xam

ined

in

th

is r

egio

n

t 30

3 w

as n

ot o

bser

ved

at r

oom

tem

pera

ture

. A

t cu

. lO

O”K

, 30

3 (w

),

316

(9).

778 F. A. MILLER, S. G. FRANKISS and 0. SALA

Our infrared band at 303 cm-l was resolved from the much stronger 314 cm-l band only at low temperature. The Raman line at 110 cm-’ is a shoulder on the side of the exciting line, and the one at 145 cm-l overlays a weak arc line; hence each has an unusually large limit of error.

It is also our belief that the two frequencies for the solid at 577 cm-l (Raman) and 586 cm-l (infrared) really belong to two different transitions. The difference is well outside of the combined experimental errors. (Note that Raman and infrared values for all the other frequencies of the solid below 700 cm-l agree within 1 cm-l. Furthermore, both values are in excellent agreement with those reported by GOUBEAU

et aa.). Table 2. Fundamental vibrations of P(CN), and As(CN),

P(CN), (Solid) c

Spe%s Goubeeu This

Activity No. Schematic description et al. work As(CN),

a1 R(P), IR 1 Cd stretch 2204 2206 2199 2 M-C stretch 585 [6201 416 3 M-C- bend 464 468 140 4 MC, deformation 312 145 106

% - 5 M-C--hi bend e R;;), IR 6 c=_N stretch (2zo4) - - 2202 2210

7 M-C stretch 605 581 461 8 M--Cti bend - 452 280 9 M-C!=N bend 451 314 122

10 MC, deform&m 278 159 80

M = P 01 As. ( ) Used twice. [ ] Estimated from Fermi ~esmmnc~ pair 603 and 630 cm-‘.

Discussion

Table 2 summarizes the fundamental vibrations and assignments for P(CN),. C,, symmetry is assumed, and seems to be satisfactory. Unfortunately only two things may be said about the assignments with complete certainty: 2206 cm-l is the symmetric C&N stretch, and 468 cm-l is another of the a, fundamentals.

Identifying the degenerate C=N stretch is a problem. It may be the 2202 cm-l infrared frequency, or alternatively it may be the two weak bands at 2210 and 2214 cm-l with the degeneracy removed by the crystalline state. The former choice is adopted arbitrarily.

One next notices that the remaining bands divide into the following groups on the basis of frequency: (a) 630, 603, 586 and 577; (b) 468 and 452; (c) 314 and 303; (d) 159, 145 and 110 cm-l. From these are to be selected two P-C stretches, three P--Cd bends, and two PC, deformations.

It is generally believed that in compounds containing an M---C&N group, the M-C stretch has a higher frequency than the M-C&N bend, There are surprisingly few molecules for which this has really been established, but ClCN and BrCN [IO] are two which are also reasonably good analogues to P(CN),. The two P-C stretches are therefore to be chosen from the group of four higher bands. Which two are to be selected ? We believe that 630 and 603 cm-l are due to an a, fundamental and a combination tone in Fermi resonance. Their infrared intensities suggest this since

[lo] W. 0. FREITA~ and E. R. NIXON, J. Chem. Phys. 24, 109 (1956).

Infrared and Raman spectra of P(CN)B and As(CN), 779

both are very intense in the solid, whereas in solution 630 cm-l is very strong while 601 cm-1 is very weak. It is unlikely that two fundamentals would show this drastic change, but it is well known that Fermi resonance pairs may do so because the coupling is extremely sensitive to solvent effects [Ill. The Raman intensities and polarizations also support this suggestion. One possibility is 2 x 314 = 628 in resonance with a fundamental near 610 cm-l. However the intensities in solution suggest that the combination is lower than the unperturbed fundamental rather than higher. Therefore a better possibility is resonance between an a, fundamental postulated near 620 cm-l and the combination 468 + 14.5 = 613 cm-l or the combination 452 + 159 = 611 cm- l. The observed 630 and 604 cm-l bands could readily result from such a close coincidence. Since the postulated 620 cm-l value is the highest of all the frequencies except the -C=N stretches, it must be due to a P-C stretch. The polarization then requires that it be vg.

Because the sum 468 + 145 = 613 must have symmetry a, to give the observed polarized line, and since 468 itself is polarized, it follows that 145 cm-l must also be a,. We therefore assign 468 cm-l to va, and 145 cm-l to vq, thus completing species a,. Similarly the combination 452 + 159 = 611 cm-l could be totally sym- metric and partake in the resonance if both 452 and 159 are e fundamentals.

For the second P-C stretch, v,, we choose the 577-586 cm-l pair for reasons to be discussed later. This leaves three good possibilities for the three P--Cd bends: 468, 452 and 314 cm-l. The first has already been assigned to va, and the latter two are reasonable for va and vg. (Note that 303 and 314 cannot be difference tones be- cause their intensities do not decrease on cooling.) Finally 159 cm-l is assigned to vn,.

This leaves two problems: what to do with 110 cm-l, and with one member of the 577-586 cm-l pair. None of these three frequencies seems to be suitable for the forbidden a2 P-Cd bending fundamental. Furthermore we believe that 110 cm-l cannot be an allowed fundamental. The 145 and 159 cm-l values are both needed to explain combination tones (Table l), and they fill the need for low-frequency fundamentals. 110 cm-i may be due to a lattice vibration; we have no better explanation.

Turning to the 577 and 586 cm-l bands, we believe that these are due to separate transitions for the reasons given in the experimental section. The 577 cm-l Raman line could be explained as 468 + 110 = 578, except that it is too intense to be a combination tone. The only explanation we can think of is that in the crystal the degeneracy of v, is broken, and that for some reason one component is only Raman- active and the other is only infrared-active. It will be seen later that the analogous vibration in As(CN), may also be split into two components. For lack of any better alternative, then, both 577 and 586 are assigned to v,.

Finally, no explanation is advanced for the two weak bands at 2210 and 2214 cm-i.

Experimental ASK%

-As(CN) a was prepared by the method used by EMERSON [ 121. Like him, we found that the obvious preparation-refluxing AsCl, and AgCN with presumed careful

[ll] C. L. ANQELL, P. J. KRUEGER, R. LAUZON, L. C. LEITCH, K. NOACK, R. J. D. SMITH and R. N. JONES, Spectrochim. Acta 15, 926 (1959).

[12] K. E~RSON, Thesis, University of Minnesota (1960); Dias. Abstr. 23, ‘91 (1961).

780 F. A. MILLER, S. G. FRANKISS and 0. SALA

exclusion of moisture-gave a product which was badly contaminated with As,O,. Infrared bands of As,O, were found at 257 (w), 350 (a), 479 (m), 489 (w, sh), 803 (vs), 840 (w, ah) and 1047 (w, vb) cm-l, even after resubliming the sample. A better product resulted from Emerson’s sealed tube reaction between the same reactant,s at -100°C. A slow resublimation of the crude product in VCLCUO gave white crystals. Even then small quantities (~10%) of As,O, were obtained, as identified by both its infrared spectrum and its X-ray diffraction pattern. The As,O, may be produced by reaction between As(CN), and small quantities of 0, in the N,-purged dry box. The problem was not studied, but considerable care was taken to ensure that none of the reported absorptions was due to As,O,.

No suitable solvent could be found for infrared or Raman measurements. Among those tried were CHCl,, Ccl,, diethyl ether, CH,CN, AsCl,, liquid HCN and liquid SOa.

The spectroscopic equipment was the same as that used for P(CN),. Infrared spectra were measured on Nujol and halocarbon mulls prepared in a dry box. The Raman measurements were made on a 1 to 2 mm thick layer of the crystals after the manner of FERRARO et al. [13]. As(CN), is a very weak Raman scatterer, and only four lines were observed. We found it impossible to go below 200 cm-l, and polariza- tion data could not be obtained. The experimental results are given in Table 3 and Fig. 1.

Table 3. Reman and infrared spectra of solid As(CN),

REUIl&ll em-’ 1nt.

416 3

443 b 454 10,

2204 5

Infrared cm-’ 1nt.

m m

122 w, sh vb sh

406 w

446 451

B 1 8

2199 m 2203 w 2210 m

Tentative assignment

VI0 V4 VB

Real? 2 : 140 = 280

122 + 223 = 402)

V,

V7

Vl ?

V8

w, m, s = weak, medium, strong ” = very b = broad sh = shoulder

A tentative assignment is proposed in Tables 2 and 3. The attribution of a given frequency to a particular mode, or even to a symmetry species, is unfortunately little more than a guess because Raman polarizations are lacking.

There are three candidates for the two As-C stretching frequencies : 457,446 and 415 cm-l. (Although 415 is very weak in the infrared spectrum, its Raman intensity requires that it be a fundamental.) Neither 457 nor 446 cm-l can be explained as a binary combination, and they therefore do not seem to be a Fermi resonance pair.

[13] J. R. FERRARO, J. S. ZIOMEE and G. MACK, Spectrochina. Acta 17, 802 (1961).

Infrared and Raman specta of P(CN), and As(CN), 781

We suggest that the degenerate v, vibration has been split into two components as a result of the crystal structure, just as was postulated for P(CN),. In As(CN),, however, both components are intense in both the infrared and Raman spectra. The X-ray diffraction work [7] showed that the symmetry in the crystal is not exactly Ca+,, which makes this suggestion more palatable.

The remaining assignments are only reasonable guesses. It seems probable that all the allowed fundamentals have been observed, but there is no reliable basis for making specific assignments. Further discussion is therefore not worth while.