R O T A T I O N A L I S O M E R I S M O F

M E T H Y L D I C H L O R O T H I O P H O S P H A T E

R . R . S h a g i d u l l i n , O. A . R a e v s k i i , a n d I . I . V a n d y u k o v a

UDC 541.63:541.62:547.1'118

The s t r u c t u r e of methyld ichloro th iophosphate (MCP) has a l r eady been d i scussed in the l i t e r a t u r e [1- 7]. The author of [2] sugges t s the p r e s e n c e of a plane of s y m m e t r y in both the c o n f o r m e r s par t i c ipa t ing in the equi l ibr ium. At the s a m e t ime, in [6] cons idera t ions were advanced in suppor t of an equi l ibr ium of f o r m s with shielding of the methyl group by sulfur and chlor ine a toms . In this communica t ion , data on the dipole momen t s (DM) [8] a r e enl is ted for the cha r ac t e r i z a t i on of the s t r u c t u r e of MCP, toge the r with sup- p l e m c n t a r y s p e c t r o s c o p i c informat ion .

The s p e c t r o g r a m s that we obtained, in acco rdance with the a l r e ady ci ted s tudies , c l e a r l y indicate t h e p r e s e n c e of a conformat iona l equi l ibr ium. Moreover , it was found that in nonpolar med ium (solution in CC14) at 298~ the c o n f o r m e r r e spons ib l e for the absorp t ion band in the IR s p e c t r a with m a x i m a 474, 554, 702, and 827 cm - I ( i somer (H) [2]) p r edomina te s quant i ta t ively (70%). With inc reas ing d ie l ec t r i c p e r m e a b i l i t y of the medium, the pos i t ion of the equi l ibr ium is somewhat shif ted in the d i rec t ion of ano ther c o n f o r m e r , r e spons ib l e for the absorp t ion band with m a x i m a 452, 534, 720, and 820 c m -1 [ i somer (I)]. Thus, in CH3CN solution a p redominance of the l a t t e r c o n f o r m e r is a l r e ady obse rved . This gives a bas i s for bel ieving, in acco rd with [6], that the c o n f o r m e r (I) p o s s e s s e s a r e l a t i ve ly l a rge DM. It should be noted that in the gas phase an equi l ibr ium of two i s o m e r s is obse rved , with a c l ea r p r edominance of i s o m e r (II), while in the IR and R aman s p e c t r a of the sol id s ta te , only the bands of the i s o m e r (I) a r e p r e s e n t . The t h e r m o - dynamic p a r a m e t e r s of the conformat iona l equi l ibr ium in n-hexane , which we found, were : AHII- I = -- 650 �9 50 c a l / m o l e , ASII- I = - -3 .6 • 0.3 c a l / m o l e �9 deg, which is in quant i ta t ive a g r e e m e n t with the r e su l t s of [6].

A compar i son of the mo lecu l a r DM of the compounds PC13 (0.8 D), PSC1 s (1.4 D), CHa- -C- -CH2--O--p

/ (4.15 D), CH3--C--CH2--O--P = S (6.77 D) shows that g P = S depends subs tan t ia l ly on the subst i tuent . Since

t he r e a r e two chlor ine a toms and one e s t e r oxygen at the phosphorus a tom in MCP, we a s s u m e d p p = S = 2.3 D. For the r ema in ing bonds we used the va lues : PlJ--CI = 0.56, P P = O = --0 .6 and gOCH~ = --1.13D [9]. The calcula t ions gave the following values of the DM for four poss ib le s t e r i c f o r m s :

S CHs

CI CI

2.43 D

S

Cl l-is CL

2,88 D"

S i? .CHs :/ !!

b C l ' ~ ' ~ C t

2,73D

S v

d c i ~ d l ! CHa

:~,.46

A. E. Arbuzov Inst i tu te of Organic and Phys ica l Chemis t ry , Kazan ' Branch of the Academy of Sciences of the USSR. T r a n s l a t e d f rom Izves t iya Akademi i Nauk SSSR, Ser iya Khimicheskaya , No. 1, pp. 80-84, January , 1975. Original a r t i c l e submi t ted March 18, 1974.

�9 1975 Plenum Publishing Corporction, 227 West 1 7th Street, New York,'N. Y. 100ll. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

71

TABLE 1. Energies of van der Waals Interact ions with Methyldichlorothiophosphate

Confor- ZE,kcal ]Confor- I~:E, kcal mation /mole lmation i/mole

a %o7 t ] 2,77 b --0,t2 ~ 0,00

The experimental value of the DM in a solution of CC14 at 298~ proved equal to 2.82 • 0.05 D, i . e . , in termediate between a and d. Since we know f rom the IR spec t ra that under conditions of de te rmina- tion of the experimental dipole moment an equilibrium of the composi- tion 7 : 3 is real ized, with predominance of the less polar form, we should consider the following var ia t ions : 0~ a with 0.3 b, c, d; 0.7 b, with 0.3 c, d; and 0.7 c with 0.3 d. According to the equation

~ x p = ~z 2x + ~rI" (t - - z)

the experimental value is best approached by the variat ion of equilibrium a ~- d or b ~ d. For the subse- quent select ion it is important that during the var ia t ion of the t empera tu re a large energy stabili ty of the i s o m e r (I) is detected; at the same t ime, there is only 30% at 298~ For the var ia t ion a ~= d this leads to a contradict ion to the Bol tzmann distr ibution law. F rom the standpoint of the equilibrium b ~ d the experimental data can be explained by the different s ta t is t ical probabil i ty of s t e r i c forms part icipating in the equilibrium. Actually, as a resu l t of t hep re sence of two identical substituents at the phosphorus atom, in the case of complete rotat ion re la t ive to the P- -O bond, conformation b is encountered twice:

r C]-13 CH

L " CI / ~ "CI C I / ~ \CI

Consequently, the formation of this fo rm is preferent ia l with respec t to entropy. On the basis of this, of the two var ia t ions remaining we selected the equilibrium b ~ d. Let us note that it agrees with the shape of the potential energy curve of the van der Waals interact ions of the nonbonded atoms [10] (Table 1).

Thus, an analysis of the mater ia l that we obtained permi t s us to support the fact that MPC is char - ac te r i zed by an equil ibrium of gauche- and t r a n s - e o n f o r m e r s (we have in mind the orientation of the P = S and O--CH 3 bonds). Since we know that the i s o m e r (I), s tabil ized in the solid phase, posses ses a r e l a - t ively la rge DM, we should a s sume that this is a symmet r i ca l t r a n s - f o r m .

We were in teres ted in calculating the frequencies and fo rms of normal vibrations of the MPC mole- cule, considering our identification of the i s o m e r s . A t r ans -eonformat ion posses ses s y m m e t r y Cs, and of 21 normal vibrat ions, 15 belong to type A' , six to type A". The tors ional vibrations around the 1)--O and C--O bonds were neglected. All the angles were assumed to be te t rahedra l . The calculation was pe r - formed according to the method of [11, 12]. The force constants cited in [7] were taken as the null approxi- mation. For be t te r ag reement of the calculated and experimenta! ly obse rved frequencies of the t r a n s - i somer , chiefly the diagonal force constants were varied. The best ag reemen t with the experiment was obtained at the following values of the force constants: Kii/[Q1] = 7.65; [Q2] = 7.39; [qi] = 7.8; [61] = 1.8; [62] = [53] = 1.7; [71] = [~/2] = 1.65; [T3] = 0.95; [~/4] = 1.1; [~i] = 1.3; [c~ i] = 0.6; Aij/[Q2t/i] = 0.4. The remaining coefficients were unchanged. The vibrational coordinates of MCP a re given in Fig. 1.

On the assumpt ion of constancy of the selected force field, we calculated the frequencies of the gauche - i somer , possess ing s y m m e t r y C1. The theore t ica l difference in the frequencies of the two i somers obtained is r a the r close to the experimental value (Table 2). Our calculated data confirmed and refined the ass ignments in the spec t rum of the compounds studied accepted in the l i te ra ture [2, 7]. From Table 2 it is evident that the P = S bond p r imar i ly par t ic ipates in the vibrations 8 and 8', and the P--C1 bond in

6

U

Q~

{11

~ H z q t ~ x 2 / a 3

Fig. 1. Vibrational coordinates of methyldichlorothiophosphate.

72

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vibrations 9 and 9', 17, and 17'. Moreover, the angles adjoining the phosphorus atom participate in these vibrations.

E X P E R I M E N T A L M E T H O D

Methyldichlorothiophosphate was synthesized according to the method of [13]. The II~ spectra were taken on a UR-10 instrument, slit program 4, ra te of development of the pr ism 32 cm-1/min; the Raman spectra were taken on a Coderg instrument in the interval 298-103~ The measurement of the capaci- tances of the solutions was performed on an IDM instrument [14]. The indices of refract ion were deter- mined on an IRF-23 re f rac tometer . The position of conformational equilibrium and the thermodynamic parameters were determined just as in [6].

The authors would like to express their gratitude to V. K. Khairullin for synthesizing the investi- gated compound.

CONCLUSIONS

1. An equilibrium of conformers with gauche- and trams-orientation of the P = S and OCH 3 groups in the gas and liquid phases follows from the vibrational spectra and dipole moments of methyldichloro- thiophosphate. A trans-conformer is realized in the crystalline state.

2. The IR and Raman spectra of methyldichlorothiophosphate were interpreted on the basis of a calculation of the frequencies and forms of the normal vibrations considering possible rotational isomers .

L I T E R A T U R E C I T E D

1. R . A . Nyquist and W. W. Muelder, Spectroehim. Acta, 22, 1563 (1966). 2. R . A . Nyquist, Spectrochim. Acta, 23A, 1499 (1967). 3. F. Feher and A. Blumcke, Chem. Ber . , 90, 1934 (1957). 4. Von V. Hornung, O. A. Wafa, A. Lentz, andY. Goubeau, Z. Anorgan. undAllgem. Chem., 380,

137 (1971). 5. R . A . McIvor, G. A. Grant, and G. E. Hubly, Canad. J. Chem., 34, 1611 (1956). 6. A. F. Vasil 'ev, Zh. Prikl . Spektroskopii, 6, 485 (1967). 7. A. F. Vasil 'ev, Zh. Prikl . Spektroskopii, 5, 524 (1966). 8. O . A . Raevsky, J. Molec. Structure, 19A, 275 (1974). 9. O . A . Raevskii, F. G. Khalitov, and T. A. Zyablikova, Izv. Akad. Nauk SSSR, Ser. Khim., 348

(1972). 10. E. Elliel, N. Allinger, S. Angyal, and G. Morrison, Conformational Analysis [Russian translation],

Mir (1969). 11. M . V . Vol'kenshtein, L. A. Gribov, M. A. El'yashevich, and B. L Stepanov, Molecular Vibrations

[in Russian], Nauka (1972). 12. L . S . Mayants, Theory and Calculation of Molecular Vibrations [in Russian], Izd-vo AN SSSR (1960). 13. I. Houben-Weyl, Organische Phosphor Verbindung, Stuttgart, Vol. 2 (1964), p. 590. 14. R. Sh. Nignmtullin and M. R. Vyaselev, Zavod. Lab., 30, 500 (1964).

74