R O T A T I O N A L I S O M E R I S M IN T H E T R I A L K Y L A R S E N A T E S

R . R . S h a g i d u l l i n , F . G. K h a l i t o v , L . V. A v v a k u m o v a , N. A. C h a d a e v a , a n d K. A. M a m a k o v

UDC 541.62:547.1'119

So far as we are aware, the l i terature contains no information on rotational i s o m e r i s m in four -coord i - nated a rsen ic compounds. The present paper will discuss rotational i s o m e r i s m in (MeO)3As(=O) (I) and (EtO)3As(=O) (II), drawing on data f rom vibrational spec t roscopy and measurements of dipole moments (DM) and Ker r constants (KC). The experimental conditions were the same as those descr ibed in [1, 2]; the con- stants of the compounds agreed with those repor ted in [3].

It is well-known fact that rotational i s o m e r i s m affects the vibrational spec t rum of a compound, the resul t being that the spec t rum contains more fundamental vibrations than would be anticipated theoret ical ly, the vi- brat ion frequencies change in passing f rom one phase to another, and the intensity distribution var ies with the t empera tu re and the dielectr ic constant (~) of the solvent.

An interpretat ion of the experimental ly developed vibrational spect ra of the arsenic acid es te r s has been given in [4], while the resul ts of theoret ical calculations on the spec t rum of (I) with assumed C3v symmet ry have been repor ted in [5, 6]. Judging f rom [4-6] and the cesults obtained in studies on rotational i somer i sm in (CH30)3P(--O) [7] and tr ialkyl a rseni tes [1], it would seem that conformational changes in compounds (I) and (II) could be best investigated through the As -----O and AsO 3 group vibrat ions. According to [4-6], the spectra l range for VAsO3 is f rom 600 to 700 cm- i . Working at optimal resolving power, the bands in ~AsO3 region of the spect ra of (I) and (II) could be separated into a number of different components. The IR spec t rum of compound (I) in nonpolar solvents showed three low-intensity bands (641, 651, and 663 cm -i) and two high-intensity bands (672 and 678 cm-1). The Raman spec t rum showed two low-intensi ty depolarized lines at 671 and 679 cm -1, and two high-intensi ty completely polar ized lines at 644 and 652 cm - i (Fig. 1). Here, and in what follows, all the f requencies given for compound (I) are those measured in CC14 solution. If compounds (I) and (II) exis tedin a single conformational form, each spec t rum would show either two bands (molecular symmet ry C 3 or C3v ), or three bands (C s or C 1 symmetry) , in the VAsO3 range. The spect ra of (I) and (II) showed at least five VAs Q bands, some of these being bands for "superfluous" vibrations [8].

The As = O group vibrations appear in the neighborhood of 980 cm -1 in the spectra of the tr ialkyl a r se - nates [4]. Since the IR spec t rum of (I) shows a s trong VAsO. C absorption band in this same region, identifica- tion of the VAs-O band is a mat te r of some difficulty here. On the other hand, the intensity of the VAsO_ C line is quite low in the Raman spec t rum of this compound [4]. The Raman spec t rum of (I) shows two polarized lines at 968 and 978 cm -1, these corresponding to the "shoulders" at 967 and 978 cm -1 in the IR spec t rum (cf. Fig. 1). In (II), the VASE)_ C band is displaced toward higher frequencies . As a result , the IR bands in the presumed VAs= O region appear as a doublet of moderate intensity with Vma x at 970 and 983 cm -1 (in n-hexane). The Raman spec t rum of liquid (II) shows two intense, s trongly polarized lines at 965 and 971 cm -1. It can be as- sumed that VAs= O has the form of a doublet band in the spectra of (I) and (II). This supposition is confirmed by noting that gradual addition of phenol to n-hexane solutions of (I) and (II) shifts each of these bands toward lower frequencies . Here the spec t rum of (II) shows peaks for both free molecules and H-complexes . The simul- taneous shift of both frequencies during formation of the As =:O ... H - O P h H-complex [9] gave indication that both bands were of the same type and should be assigned to VAs= O. Heating solutions of (II) in n-decane to 373~ brought about no al terat ion in the intensity rat io for VAs= O doublet bands. This eliminates the possibility of explaining the appearance of the doublet by supposing such solutions to contain comparable amounts of di- p o l e - dipole d imers and monomers .

A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Branch of the Academy of Sciences of the USSR. Transla ted f rom Izvest iya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1812-1815, August, 1977. Original a r t ic le submitted June 9, 1976.

This material is protected by copyright registered in the name o f Plenum Publishing Corporation. 227 West 17th Street, New York, N.Y. 1001I. No part [ o f 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 o f the publisher. A copy o f this article is available from the publisher for $ 7.50.

1672

/ ' a . / \ c~ 1 - , t \ J ~ , "~'!

�9 I , , f ~ ' ~ ~ _ . _ i I . 3

f~O0 .q70 930 ~80 6qO B cm ' l

Fig. i. Raman spectrum (a) and IR spectrum (b) of (CH30)3AsO in CCI~.

Cooling compounds (I) and (II), or solutions of these compounds, brought about a change in the ~As=O doublet, the low-frequency component becoming relat ively more intense.

Under al terat ion of the polari ty of the solvent and change in the value of s, the "As=O bands o:[ (I) and (II) shifted and changed in relat ive intensity. Thus, in going f rom C~HI~ to CS 2 to CH2C12 solutions, the value of e rose and, at the same time, the intensity ofthe high-frequency component of "As=O increased and that of the low-frequency component diminished.

Study of the conformational changes in compounds (I) and (II) was complicated by the fact that a reduction in the tempera ture brought about crys ta l l iza t ion and associat ive t au tomer i sm in the molecule, the resul t being that the spect ra completely r ea r ranged and the VAs= O band disappeared [19]. This made for difficulty, both in est imating the relative content of the various conformers in the sys tem and in evaluating the thermodynamic pa rame te r s - H and ~S for the conformational equilibrium. On the other hand, the conformer ratio at equilib- r ium can be est imated if one is willing to assume equality of the VAs= O absorpt ion coefficients for the various forms. On this basis , the ratio of intensities of the VAs= O band components would indicate that the conformers a r e present in essential ly equal proport ions in equilibrated solutions of (I) and (II) in nonpolar solvents.

Judging f rom the fact that the spec t ra of solutions of (I) and (II) showed "superfluous" VAs= O and VAs O bands, each affected by changes in tempera ture and the medium die lect r ic constant e in the manner descr ibe~ above, it would seem that one has here to deal with equilibrium mixtures of two rotational i somers J~bothbasis . On the basis of the variat ion of the intensity of the VAs= O doublet in passing f rom one solvent to another, the high-frequency band was assigned to the polar i somer with the la rger molecular DM. The IR bands of (I) (VAsO3 641 and 672 cm -I) were also assigned to this i somer . Study of t empera ture effects showed the i somer assigned to the low-frequency "As=O band to be energet ical ly prefer red , the VAsO3 bands appearing at 651 and 678 cm -1. Definitive ass ignment of the fifth band in the VAsO3 region of the spec t rum of (I)(663 am ' l ) could n o t b e made.

It has a l ready been noted that each conformer should be assigned either two or three of the v ~s r, vibra- tions, depending on the symmet ry . By comparing intensities in the Raman and IR spectra , on the one hand, and degrees of depolarizat ion of the VAsO3 lines, on the other, it was concluded that the 641, 651, and 663 cm -1 bands in the spec t rum of compound (I) a r i se f rom symmet r i ca l vibrat ions, and the bands of 672 and 678 cm -1 f rom an t i symmet r ica l vibrat ions. The presence of two depolarized VAs Q lines was an indication that each of the conformers has cer tain elements of symmet ry . In fact, one form must have Cs symmet ry and the other either C 3 or C3v s y m m e t r y [8]. Lacking a decision as to which of the i somers (the one with higher or lower polarity) has the third VAsO3 frequency, an ass ignment by symmet ry types could not be made.

An at tempt was made to work out the details of the i somer s t ruc tures on the basis of DM and KC data~ Measured at 298~ the dipole moments of (I) and (II), the f i rs t in CC14 solution and the second in Cr solution, were 2.94 and 2.95 D. At this same tempera ture , the measured KC value for compound (I) was - 1.10 -12. In calculating the DM's of the proposed conformations, use was made of the following bond moments: ~O_CH3 = --1.13 and PAs-O = 0.55 D [1] and PAs=O = 4.43 D, the last obtained f rom the experimental ly determined mo- ments (CH3)3As , 0.86 D and (CH3)3As(----O), 5.11 D [11]. Use was also made of the following valence angles: OAsO, 105~ O----AsO, 113~ and AsOC, 118 ~ Dipole moments for the five forms of molecule (I), each calcu- lated vectorial ly, are shown in Table 1. The angle of rotation of the O - C bond around the AsO bond was cal- culated by the right-hand screw rule, the values listed being measured with respec t to the shielding position

1673

TABLE 1. Calculated Dipole Moments and K e r r Constants (KC) for Compounds (I) and (II)

Iso met No.

(Pl

60 60

-60 -60

r ~, D

o

180

i,68 3,05 3,59 4,53 3.it

KC

+64 +i20 +283,3 +403,8 +49,3

Sym- metry

C~v C~ Co C, C,

relat ive to the ars inyl bond (~, 0~ Other possible conformations of compounds (I) and (II) were not considered here, since they e i ther did not sat isfy the s y m m e t r y conditions imposed by the spect ra l data or were ruled out by energy and s te r ic considerat ions [2].

Even a c u r s o r y examination of the calculated DM's (cf. Table 1) shows that i somer 1 with C3v symmet ry , or something approximating thereto, must be present in the equil ibrium state of the sys tem. By comparing the DM's for the possible conformations with the resul ts of calculation on the conformer rat ios in the equilibrated sys t em it was decided that i somer 3 with C s s y m m e t r y would give the next best approximation to the experi- mentally measured DM's for compounds (I) and (II). Values of the KC's for conformations 1-5 were calculated f rom an additive scheme, assuming the C - H bonds to be isotropical ly polar ized [12]. For the C - O bond: b L = 0.89, bT = b v = 0.46 .~3; for the O3As{=O ) group: b 1 (along the axis of symmetry) = 5.45 A; b 2 = b 3 = 3.61 ~3 (according to the data of [13]). Table 1 shows the calculated KC value for each of these forms to be g rea te r than the exper imental ly determined value for compound (I). This indicated that ei ther the additive scheme used here was incapable of giving quantitative information concerning the s t ruc tu res of these compounds, or that in- co r r ec t values had been chosen for the basic pa rame te r s , the situation here being s imi lar to that met with (CH30)3P(=O) [14]. In view of its KC value and minimal depar ture f rom the measured moment, it can be said that fo rm 1 would certainly come into play. In other words, the less polar i somer of compound (I) is of form 1 containing three RO groups, c i s -or ien ted relat ive to the As =O bond with C3v symmet ry . It is interest ing to note that this same conformation has been assigned to the t r ialkyl a r sena tes on the basis of parachor measure - ments [15]. The s y m m e t r y of this i somer could actually be somewhat lower than C3~ since the method employed here does not pretend to give exact values of the angle q). The same could also be true of compound (II) and the higher homologs. The data of depolarizat ion of the lines in the Raman spectra suggest that i somer 3 with C s s y m m e t r y should be the form of highest polari ty.

The authors would like to thank A. N. Vereschagin for a discussion of this work.

C O N C L U S I O N S

1. Study of vibrational spect ra , dipole moments , and Ker r constants show the molecules of the tr ialkyl a r sena tes (CH30)3AsO and (C2HsO)3AsO to each exist as two rotational i somers , the equilibrium sys tem in each case containing these i somer s in essent ial ly equal proport ions, under ordinary conditions.

2. In one of these i somers the three RO groups are a r ranged in something close to a cis configuration with respec t to the As =O bond [C~v(C 3) symmet ry ] ; in the other, one RO group is in a trans position (Cs sym- metry}. This last i somer is more polar and less favored energet ical ly .

i .

2. 3.

4.

5.

6.

7.

8.

9.

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

R. R. Shagidullin, F. G. Khalitov, L. V. Avvakumov, Z. A. Tukaev, N. A. Chadaeva, and D. A. Bagaveev, Dokl. Akad. Nauk SSSR, 228, 857 (1976). O. A. Raevskii , A. N. Vereshchagin, and F. G. Khaltiov, Izv. Akad. Nauk SSSR, Ser. Khim., 353 (1972). Gi l 'm Kamai and K. I. Kuz 'min, Tr . Kazansk. Khim.-Tekhnol. Inst. , No. 17, 7 (1952). R. R. Shagidullin and I. A. Lamanova, Izv. Akad. Nauk SSSR, Set . Khim., 1238 (1969). I. A. Lamanova and R. R. Shagidullin, Izv. Akad. Nauk SSSR, Ser. Khim., 2675 (1972). F. K. Vansant and B. J. Van der Verken, J. Mol. Struct. , 2_22, 273 {1974). L. S. Mayants, E. P. Popov, and M. I. Kabachnik, Opt. Spektrosk., 7, 170 (1959). L. M. Sverdlov, M. A. Kovner, and E. P. Krainov, Vibrational Spectra of Polyatomic Molecules [in Rus- sian], Nauka (1970), p. 24. L. V. Avvakumova, R. R. Shagidullin, I. A. Lamanova, V. S. Gamayurova, and Z. G. Daineko, Zh. Prikl . Spektroskop., 2__33, 177 (1975).

1674

10. R . R . Shagidullin, Izv. Akad. Nauk SSSR, Ser. Khim., 1677 (1975). 11. A . L . McClellan, Tab les of Expe r imen ta l Dipole Moments , Vol. 2, Raha ra E n t e r p r i s e s (1975). 12. R . J . W . Le Fevre , Adv. Phys . Org. Chem.,_3, 1 (1965); R. J . W. Le Fevre , B. J . Or r , and

G. L. D. Ritchie, J . Chem. Soc., B, 273 (1966). 13. R . J . W . Le F ev re , A. Sundaram, and R. K. P i e r ens , J . Chem. Soc., 479 (1963); M. J . Aroney,

R. J . W. Le F ev re , and J . D. Saxby, J . Chem. Soc., 4938 (1963); R. S. Arms t rong , M. J. Aroney, R. J. W. Le Fevre , and R. K. P i e r ens , J~ Chem. Soc., 2735 (1969).

14. M . J . Aroney, L. H. L. Chia, R. J . W. Le Fevre , and J. D. Saxby, J. Chem. Soc., 2948 (1964). 15. K . I . Kuz 'min , Collection of P a p e r s on Genera l Chemis t ry of the Academy of Sciences of the USSR [in

Russian] , Vol. 1, Izd. Akad. Nauk SSSR, (1953), p. 223.

1675