The vibrational spectra and structure of tropylium hexachlorophosphate, C7H7PCl6

Spectrochimica Acta, Vol. 34A, pp. 881 to 887 © Pergamon Press Ltd., 1978. Printed in Great Britain

0584-8539/78/0901-0881502.00/0

The vibrational spectra and structure of tropylium hexachlorophosphate, C7HTPCI6 C. SOURISSEAU (with the technical collaboration of J. HERVIEU) Laboratoire de Spectrochimie Infrarouge et Raman, C.N.R.S., 2 rue Henri Dunant, 94320, Thiais, France (Received 21 July 1977)

Abstract--The i.r. and Raman spectra of CTHTPCI6 in both solid state (at 300 K and 100 K) and solution have been studied in the 4000-30 cm- 1 range. For the first time, all normal modes of the C7H¢ cation have been observed and a complete assignment of vibrations in the D7h group symmetry is proposed. A normal coordinate analysis has been performed and the valence force constants have been calculated: both aromatic systems CTH~" and C6H6 are compared. The spectroscopic results are consistent with the ionic character of this tropylium salt.

INTRODUCTION A study of tropylium cation has been undertaken as a part of the program in research on delocalized rt electrons systems [-1]. Curiously enough, few structural data for this cation have been reported [-2], although stable crystalline tropylium salts have been available for 20 years [-3-18]. FATELEYet a l. [,19-21] have already reported the vibrational spectra of tropylium bromide and have concluded that the cation has a D7h symmetry. However, several tropylium fundamentals remain unknown or have not been determined. We have thus investigated the i.r. and Raman spectra of CTH7PC16 compound in order to identify all the fundamental vibrations even those which are forbidden under D7h symmetry. Our previous experience has namely shown that the complexation of a molecule like benzene breaks down the selection rules and gives rise to more or less intense fundamentals without appreciable shifting of the frequencies from their "unperturbed" values [1]. The new data on C7H~- could thus be used for a more accurate determination of valence force constants which can be compared with those of benzene molecule.

as shown by a weak i.r. absorption near 586 cm-~ due to PCI5 [24]. Infrared spectra were recorded on Perkin-Elmer 225 and 180 spectrometers and on a Polytec FIR 30 interferometer in the far i.r. In the different spectral ranges, CaF2, NaCl, CsI (with polyethylene films to avoid halogen reaction), polyethylene and polymethylpentene windows were used. Mulling of solid samples in Nujol and fluorolube have been prepared. Raman spectra were recorded on Coderg PH 1 and T 800 instruments with the exciting wavelength 6471 A of a Spectra-Physics Krypton laser (150 mW). Spectra were obtained from crystalline powders sealed in small glass tubes. Infrared and Raman spectra at 100 K were recorded with a classical liquid nitrogen cryostat. For the solution spectra we have used saturated solutions (C ~ 10- l M/l) in CH3CN. All frequencies are reported with an accuracy of + 1 cm- ~ for thin bands. RESULTS AND DISCUSSION First of all we shall discuss the characteristic bands of PCI~ anions. After we shall localize all vibrations of tropylium cations and report the results of normal coordinate calculations. 1. Vibrations of PCl6 anions

For an XY6 group of Oh symmetry, the distribution of the vibration species is, Fvib = 1Alg(R) + lEg(R) + Because of the great sensitivity of the compound towards 2Flu(i.r.) + IF2g(R) + 1F2u (forbidden). We expect oxygen and moisture all experiments have been carried out three Raman active vibrations (one polarized) and two under argon in glove-bags. Chemicals of the best available i.r. activities. In agreement with the vibrational results commercial grade were used, recently distilled 1.3.5-cycloreported by BEATTIE et al. [-25] on the tetraethylheptatriene (Merck-Schuchardt) and just sublimed phosphorus pentachloride (J. T. Baker). In agreement with ammonium and pyridinium salts, Et4N+PCI6 and DILLON et al. [22], tropylium hexachlorophosphate was prepyH ÷ PC16, we find again the characteristic f r e q u e n c i e s pared by a modification of the first literature method for of this octahedric anion: the three intense Raman C~4HI4PCIT [23]: after filtration, washing with carbon bands at 355, 272 and 240 cm-~ (Fig. 1) are assigned tetrachloride and drying in vacuo at room temperature a colorless and pulverulent solid, stable in inert atmosphere, to A~g, Eg and F2g vibrations, the first one being has been obtained (yield ~ 100~o). The results of several polarized; also the i.r. absorptions near 450 and 284 microanalyses are in agreement with the proposed formula c m - 1 correspond with FI~ modes. The weak absorpCTHTPCI6 (found: C, 25.8~o, H, 2.2~o, P, 8.9~o and CI, 63~o; C7HTPCI6 requires C, 25.1~o, H, 2.1~o, P, 9.2~o and CI, tion at 214 c m - ~ could be assigned to the F2u vibration which is allowed by crystal effects; this fundamental 63.6~o). This compound can dissociate in CTH7CI and PCIs 881 EXPERIMENTAL DETAILS

882

C. SOURISSEAU a n d J. HERVIEU

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The vibrational spectra and structure of tropylium hexachlorophosphate, CTHTPCI6

883

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3000

3100

1600

1400

~,

1200

1000

800

600

400

200

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Fig. 1. Infrared and Raman spectra of CTH~ PCI~- compound in the solid state at 300 K. * This band is assigned to PCI5 impurity (see text).

mode has yet never been observed but it is expected at a slightly lower frequency than that of the F2~ vibration 1-26]. At last, bands below 100 cm-1 are assigned to lattice modes (Table 1). There are no coincidences between Raman and i.r. frequencies for lattice and anion vibrations; so it comes out'that CTH7PC16 compound must crystallize in a centrosymmetric space group in which PClg anions are located on at least Ci symmetry sites. In agree-

ment with 3,p N M R and 35C1 N Q R investigations [22], only a very weak distortion of the PC16 octahedra must take place in the crystal structure.

2. Vibrations of CTH~- cations When tropylium ion is assumed to have D7h symmetry, one expects seven Raman active vibrations, 2A~ + 1E~ + 4 E ~ and four i.r. modes 1A~ + 3E'z (Table 2). The Raman spectrum between 3100 and

Table 2. Comparison of the fundamental frequencies of benzene and tropylium ion C7H7+ DTh

act.

A'

R I

C7~+Br Vibrations I vC-H vC-C 6CH

A' 2 A" 2

IR

E' I

IR

E" I

R

E' 2

R

E" 2

E' 3

E" 3 * Frequency twice assigned.

yC-H vC-H vc-c 6C-H yC-H

{

I vC-H VC-C 6C-H 6CCC yC-H yCCC

{

vC-H VC-C 6C-H

Exp. Freq. (21)

C7H7+PCI6 -

Cal. Freq. (30)

3085 868 -

~ 645 3020 1477 992 830(cal) 3045 159h 1210 433 -

-

6CCC

-

yC-H yCCC

-

1288

This work

C6H 6 Freq.

D6h

3072 869

3062 992

A1g

1309

1346

A2g

~ 648 3020 1477 993 892

673 3057 1482 1037 845

A2u

914 389

3040 1596 1224 430 1o25 353

3047 1596 1178 606 967 398

3024 1598 1207 1148 1211 625

3085 1282 1100 768 I025~ ~ 648 ~

3048 1309 1146 1010 990 707

855925ou

E1u E1g

E2g

E2u ~lu B2 u B9 u lu B2g

The vibrational spectra and structure of tropylium hexachlorophosphate, CTHTPCI6 400 cm-1 is relatively simple, there being observed only few bands but we note the weak scattering of CTH~ cations compared to PC16 anions. On the other hand, we observe many i.r. absorption peaks* and the most intense ones at about 3020, 1480, 990 and 650 cm-1 can be assigned as fundamental frequencies of the tropylium ion. These frequencies can be well compared with those found for the ionic compound CTH~Br- [19,21]: CTH7PC16 is really a tropylium salt and because of the crystal effects all vibrations become i.r. active. Such effects may explain why the i.r. spectrum of one KBr pellet of CTH7Br [3] presents more bands than that of one HBr solution [21]. Also, this feature does not come from a strong interaction with a symmetry lowering of the ring, as in n-cycloheptatrienyl metallic complexes [27, 29], because one would expect not only a decrease of vibrational frequencies for vC--C (at 1480cm -a) and 6C--H (at 990cm -1) modes but also a large increase for the 7C--H (at 650 cm- 1) vibration.

885

All fundamental frequencies of the tropylium ion are listed in Table 2 : for the active vibrations in DTh symmetry a comparison can readily be made of the corresPonding frequencies proposed by FATELEVet al. [19-21] excepting the 7CH (E'~) mode which has not :yet been observed but was calculated near 840 or 925 cm-1 by different normal coordinate treatments [21, 39]. We assigned this vibration at 892 cm- 1 since this mode, observed at 845 cm -1, is also weak in the benzene Raman spectrum. In agreement with FATELEY et al. [21] we note a great similarity to the assignments for benzene (Table 2). Also the assignments of other fundamental frequencies of A~, E~, E~ and E] symmetry species can be made with comparison to benzene and they have been checked by a study of the i.r. spectrum of tropylium iodide in the solid state [31]. For example, we find again, with this salt, an i.r. absorption near 350 cm-1 assigned to the 7CCC (E~) vibration. In general, the inactive vibrations have quite different frequencies from those predicted by AIDA[30] using a set of force constants derived from benzene's * In the 3200-600 cm- 1 range, most of the bands have values [32]. However, in the past decades, many similar frequencies to those reported for C14H14PC17 com- authors [33-38] have determined the force field of pound [23] : in agreement with NMR and NQR studies of DILLONet al. [22] and with the i.r. investigation of BEAYrIE benzene molecule with more accuracy than in the first and WEBSTER[24], this result corroborates the proposed analysis of CRAWFORDand MILLER[32]. So, with the double salt structure, CTH~-PCIg--C7H~C1-. help of our new experimental results we have carried Table 3. Valence force symmetry constants and comparison of calculated and observed frequencies for tropylium ion Species (D7h)

A' I

Valence force symmetry constants a)

F s = 5.130 mdynes/~ F t = 7.750

"

A'2

F8 = 0.943 mdynes.A/rad 2

A"2

Fy = 0.349

E'

F

I

C7~ ! Cal. Freq. Obs. Freq.

3071.6

3072

869.1

869

1309

1309

648

648

3020.6

3020

1477.5

1477

991.9

993

892

892

F s = 5.030 mdynes/~

3040.3

3040

F t = 5.890

1596.2

1596

1223.9

1224

429.6

430

F "= 5.182 mdynes/~

3085.5

3085

F t = 3.848

1282.8

1282

1099.8

1100

767.8

768

s

"

= 4.940 mdynes/~

F t = 14.910

"

~

F 8 = 0.962 mdynes.A/rad 2 Ft~ = 2.41 mdynes/rad E"

I

E' 2

Fy = 0.574 mdynes.A/rad 2

"

F B = 1.286 mdynes.A/rad 2 Fm = 1.005 FtB= E' 3

"

0.390 mdynes/rad

S

"

F B = 0.924 mdynes.A/rad 2 F

= 0.843

Ft8 = 0.798

"

mdynes/rad

a) The notation system is the same as that used in Ref. [21]. * This Ft value is twice as large as that obtained for benzene (see Ref. [21]).

886

C. SOURISSEAUand J. HERVIEU

Table 4. Valence force constants for (CH)4 and (CH)6 +

a)

c7~ Fateley (21)

C6H 6

Aida (30)

This work

Snyder (37)

8.h05

6.019

6.415

~C-C ftI mdynes/~

8.292

f t2

2.118

1.605

1.070

0.772

f t3

(0.565)

-0.252

-0.198

-0.319

f t4

-1.937

-1.822

-0.007

0.289 5.066

~C-H f sI

5.160

5.064

5.062

s2

-0.0721

0.018

-0.0408

(0)

f s2

0.0343

-0.031

0.0471

(0)

f s3

(0.0343)

-0.036

0.0278

(0)

1.016

1.041

1.030"

6CH f~! mdynes ~ / r a d 2 f~e

1.10

-0.018

-0.014

(0)

f~3

(o)

o.o11

-0.093

-o.o21 x

f p4 ~ccc

(o)

-0.034

0.057

o.o18 ~

1.02

1.31

1.058

0.857

(0)

-0.26

0.119

(0)

fa I b) 2 f b)

-0.123

a) Same symbols as those used in Ref. [21]. b) With f~3 = f~4 = 0. * Value twice as large as that reported because of the different choices in coordinates. ( ) Fixed values. out a normal coordinate calculation of the tropylium ion.

3. Normal coordinate analysis of the C7H~ ion The FG technique of WItSON [39] was used to obtain the secular equations for the different symmetry species. In many respects the analysis is similar to that described by FATELEYet al. [21] but with one exception for the geometric parameter t(C--C): instead of 1.390 A we have used the value 1.400 A which is closer to that determined for benzene molecule, 1.397 A [40], and to the value calculated for C7H~-, 1.405 A, by the MINDO/3 method [41]. The values of valence force symmetry constants are listed in Table 3. The E~ and E~ symmetry blocks have not been calculated because of the uncertainty of some frequencies. In the A], E] and E~ species we find again values in good agreement with those reported by FATELEY et al. [21]. On the other hand, the Fr force constants in A~ and E'~ blocks have different values which are larger than for benzene molecule [32, 38]: it must be easier to bend the carbon hydrogen bond out-of-benzene-plane than out of tropylium plane. Moreover, the secular equation for E~ species being solved for the first time, the values of f(vC--C), f(vC--H) and f(3C--H) valence force constants and of the corresponding interactions can be well determined. We obtain the valence force constants from the

valence force symmetry constants by solving different systems of the following equations; for example we get for the carbon-carbon stretching force constants: A'

1

F, =ft + 2 f z + 2f,a + 2f," F,E"=f 1 + 2cos q~'f z + 2 cos 2q~'f a + 2 cos 3q~ .f4 F('~= ffl + 2 cos 24).f2 + 2 cos 34) .fa + 2cos q~ .f4

F , % = f , + 2cos3~b'fz + 2cos ~b .fa + 2cos2q~.f4 where q~ = 360°/7, f / is the f(C--C) diagonal constant, andftZ, f 3, f 4 are interaction constants between C - - C bonds. Solutions of these equations are given in Table 4 where they are compared with the values previouslyproposed for CvH4~ [21, 30] and for C6H 6 [37]. Since the bond order is 1.5 in benzene instead of 1.43 in tropylium, the latter carbon-carbon stretching diagonal constant is expected to be lower. Only our value, 6.019 ndyn/A, compares favorably with that of benzene. The carbon-hydrogen stretching and bending force constants are also comparable in values with those expected for an aromatic ring. At last, the carboncarbon-carbon angle bending constants are larger in tropylium than in benzene. The CCC angle has increased from 120° in benzene to 128° in tropylium ion. This change results in an angle for which, apparently, more energy is required to distort the ring. In conclusion vibrational spectra and force constants of benzene molecule and tropylium ion indicate a

The vibrational spectra and structure of tropylium hexachlorophosphate, C7H 7PCl6 striking similarity between b o t h structures. This confirms the stability of tropylium ion in the planar aromatic D7h structure [41]. Acknowledoement The authors wish to express their sincere thanks to Dr A. NOVAK for helpful discussions throughout the course of the present work. REFERENCES

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