J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 297-303 IR and NMR Studies of Hydrogen Bonding in Hexan-1 -01-Tetrabutylammonium Iodide Solutions in the Temperature Range 28-145 *C and in Tetrachloromethane Oleg N. Kalugin, Dmitry A. Nerukh, Ivan N. Vyunnik, Elena G. Otlejkina, Yurij N. Surov and Nikolaj S. Pivnenko Department of Inorganic Chemistry, Kharkov State University, 4 Freedom Sq.,310077 Kharkov, Ukraine IR spectra of (Bu,NI)-hexan-1-01 solutions at 25, 55, 85, 115 and 145°C in the OH stretching region have been investigated. The OH stretching spectra and the 'H NMR chemical shifts of the hexanol OH-group were obtained from Bu,NI-hexanol-CCI, solutions in the alcohol concentration range 3 x 10-3-7.8 mol dm-3. The relationship between absorbance and wavenumber is represented as the product of a Lorentzian and a Gaussian curve.Using this dependence deconvolution of the OH-band was carried out by the Simplex method. From these data, it was established that Bu,NI is a structure-breaker at moderate temperatures and/or low concentrations of CCI,. At higher temperatures or in very dilute solutions of Bu,NI-hexanol in CCI,, Bu,NI is observed to be a structu r e-maker . Based upon the statistical-mechanical theory of electrolyte solutions the square-mound potential d,/k, T for some 1 : 1 electrolytes in alcohol solutions was calculated in our pre- vious work.'V2 The perturbation energy d, could be approached as the Gurney cosphere overlap Gibbs energy, and is the sum of many effects relating to the relative energies of interaction of the free ions and ion pairs with the sur- rounding ~olvent.~.~ For most 1 : 1 electrolyte alcoholic solu- tions, especially those of higher alcohols, d,/k, T increases with temperature' and corresponds to the energy gain of an ion in alcohol medium as compared with an ion-pair (without considering the Coulombic part of the interionic potential). From a comparison of the temperature dependences of square-mound potentials and the Gibbs energy of intermo- lecular interaction calculated using the theory of dielectric liquids, the sign and values of the temperature change of dJk, T are determined by the orientational correlations of dipoles of the solvent molecules.In alcoholic solutions the most important role is played by chain association units.Starting from these conclusions, the purpose of the present paper may be formulated as follows: to investigate the influ- ence of electrolyte on alcohol association over a wide tem- perature range by a direct experimental method. For this purpose the IR spectra of Bu,NI-hexan-1-01 solutions at 25, 55, 85, 115 and 145 "C in the OH stretching region have been investigated. The OH stretching spectra and 'H NMR chemi- cal shifts of the hexan-1-01 (hexOH) OH-group were also obtained from Bu,NI-hexOH-CC1, solutions in the concen- tration range 3 x 1OP3-7.8 mol dm-3 at room temperature. In contrast to lower homologues the relatively high boiling temperature of hexan-1-01 (157.1 "C) allows one to carry out the investigation at an elevated temperature without special instrumentation.Furthermore, the temperature range is in accord with our previous studies.'V2 A literature overview has demonstrated that the majority of spectral investigations of alcohol solutions involve studies of lower alcohols at room or low temperatures.'-' ' The use of Bu,NI as an electrolyte enables us to simplify the interpretation of the spectra because of the absence of any influence of the Bu,N+ ion on the IR and NMR OH-group spectra.', The choice of this salt was dictated by its good solubility not only in hexOH, but also in the inert solvent CCl,, which was used as an analogue of the effects of tem- perature on the alcohol self-association. Experimental Hexan-1-01 was dried for 10 days over freshly made K,CO, , then fractionally distilled under a pressure of 100 Pa.The water content in the alcohol was determined by the Karl- Fisher method and did not exceed 0.01%. The Bu,NI used was of 'pure' grade and was recrystallizated six times from a benzene-hexane mixture. The final drying was performed at 55°C and 1 Pa over P,O,. CC1, was boiled with P,O, for 5 h and distilled. IR spectroscopic measurements were made on a double-beam Specord M80 spectrometer in NaCl, LiF and CaF, cells. CaF, cells were used at room temperature (28°C) for pure hexOH and Bu,NI-hexOH solutions, NaCl cells for measurements in CCl, medium. Our home-made LiF cell was used for multi-temperature investigations.Nuclear magnetic resonance studies were made on a Tesla BS-487-B 80 MHz spectrometer at room temperature. 'H chemical shifts were measured relative to hexamethyldisilane, which was added to samples in trace amounts. The following OH stretching spectra of hexOH were studied: (a) pure hexOH (I) and solutions of Bu,NI in hexOH (11) (molality 0.56 mol kg- ') at 28, 55, 85, 115 and 145 "C;(b)solutions of (I) in CCl, (111) and solutions of (11) in CCl, (IV) with a minimum concentration of alcohol of 4 x lop3 mol drn-,. All hexOH-Bu,NI-CC1, solutions were made by dilution of (11) in pure CCl,. This made it possible to keep the alcohol :electrolyte molar fraction constant (18 : 1) in all the solutions. NMR spectra were obtained from systems (111 and IV) at 28 'C.Results and Discussion Experimental spectra in the OH stretching region are shown in Fig. 1 and 2. Both for pure alcohol and for electrolyte solution the polymeric alcohol band shifted to higher wave- numbers and the intensity of the monomer band increased with temperature or CCl, concentration. In a qualitative sense these conclusions have long been known.'-'o* ',-' However, there are no quantitative descriptions of a simulta- neous influence of higher temperature and electrolyte on the OH band profile resulting from alcohol self-association by hydrogen bonding. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 1 3 4 (4 I"r I I 80 -70 --Q) 60 C me g 50-A -40 -30 -20 -10 .., .. .. ...... ... ... ... .... .., , __ ........ ... . . . ....-.. ... .... . ......__. ... .. I I I .. I I 03700 3600 3500 3400 3300 3200 3100 wavenumber/cm -1 2 3 I I I 80 70 60 Q)0 5 50 %' 40 30 IV 2c 1c I.. .. . ..... . I I 1 IC I 3700 3600 3500 3400 3300 3200 3100 wavenumber/cm-l Fig. 1 Experimental (circles) and calculated (lines) IR spectra of hexOH-CC1, (111) and hexOH-Bu,NI-CCl, (IV) solutions at 28°C with alcohol concentrationsof (a) 3.5 and (b)0.015 mol dm-3. The numbers on the top of each figure correspond to calculated bands. We used an algorithm suggested by Symons6 to compute (4)the approximate band shape: In eqn. (1)-(4) Acalc is the calculated intensity, Ai is the (1) absorbance of each unit, B is the base line, &,, is the band height, Liand Ciare Lorentzian and Gaussian terms, respec-tively, vma, is the band position in abscissa units, a is the (2) half-bandwidth at half-height, rn is the number of bands producing the overall envelope, is a Gaussian contribution,1 v is the abscissa value at which the absorbance is to be calcu-(3) lated.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 3I' 1 .o 0.8 Q) C 0.6 2 % 0.4 0.2 I I I0 3600 3500 3400 3300 3200 3100 wavenumber/cm-' 1 3 4 1.2 I I I 1 .o 0.8 Q) C e d 0.6 0.4 0.2 I I I I I 3600 3500 3400 3300 3200 3100 wavenumber/cm-' Fig. 2 Experimental (circles) and calculated (lines) IR spectra of pure hexOH (I) and hexOH-Bu,NI (11) solutions at (a) 55 and (b)115 "C Deconvolution of the experimental band shape was carried fitted parameters the diagonal elements of the covariation out by minimizing the sum of square deviation Q for k matrix were used: AXi = (COV Wii= J[s~(H-')~~] (6) s2 = -Q (7)m-n The individual band parameters (Ao,i,v,,,, i, ai) and the where H is the matrix of the second derivatives of Q with common parameters B and B were altered until a good fit respect to X and s is the approximation dispersion.with the experimental band was obtained. Eqn. (5)was mini- The results of data handling by the algorithm outlined mized by the Simplex method.I6 As a statistical evaluation of above are presented in Table 1 (for I11 and IV) and Table 2 J.CHEM. SOC. FARADAY TRANS., 1994,VOL. 90 Table 1 IR spectra of OH-groupvibration in hexOH-CCl, (111) and hexOH-CC1,-Bu,NI (IV) solutions at 28"C n" band height band width band position I11 c = 0.004,b/3 = 0.05, s = 0.017 1 154 f 2 11.7f0.2 3639(f9 x lop4) c = 0.02,/3 = 0.17,s = 0.005 1 145 f5 12.7f0.5 3639( k2 x lo-,)2 5.9f0.6 37 f5 3511( f1 x c = 0.04, /3 = 0.16,s = 0.025 1 140f5 13.1 f0.5 3639(+ 1 x 2 13.2f0.7 51 f5 352qf 1 x 3 4.9f0.5 70 f 11 3377(+5 x 10-6) c = 0.07, /3 = 0.29,s = 0.086 1 121 f 16 13 f 1 3639(f3 x lop3)2 15 f 3 49 f 6 3509(f9 x 3 10f 2 102 + 15 3366(f3 x c = 0.97,/3 = 0.53,s = 0.002 1 22 f 2 13 f 2 3636(+5 x lov6)3 241 f 21 119 f 8 3343(f2 x 4 17 f 58 69 f 16 3055(+2 x c = 3.42,/3 = 0.65,s = 0.003 1 6.0f0.5 15f2 363qf4 x 3 180f 12 120 f6 3338(f 1 x 4 15 f 14 76 f9 3068( f9 x c = 6.68,/3 = 0.68,s= 0.03 1 2.4f0.3 15f2 3637( f4 x 3 133 f 8 120 f 5 3335(f3 x 10-5) 4 12 f6 106 f 11 305qf 1 x 10-5) c = 7.98,/3 = 0.66,s = 0.005 3 144f7 119 f4 3331(f8 x 4 12 f4 126 f 16 3067(f 1 x IV c = 0.003/3 6,/3 = 0.14,s = 0.011 1 119 f 6 12.8f0.6 3639(+4 x lo-,)3 0.5 f 1 106 f 347 3422(f 1 x lo-') c = 0.01, = 0.19,s = 0.14 1 103 11 13 f 1 3639(+5 x 2 2.7f0.6 99 f 106 353qf 1 x 10-5) 3 3f2 49 f 21 3402(+3 x 10-5) c = 0.03,/3 = 0.21,s = 0.017 1 109f8 12.7f0.8 3639(+5 x 2 6.1f0.8 51 f9 353q+ 1 x 10-5)3 7fl 63 f9 3402(_+3x 10-5) c = 0.06,/3 = 0.29,s = 0.059 1 9991 f 11 13f 1 3639(f2 x 2 9fl 38 f 5 3508(f5 x 3 14f2 62 f 6 3392(+5 x c = 1.16,/3 = 1.00,s = O.ooOo6 1 17f 1 16 f 1 3636(f 1 x 3 192 _+ 16 119 f7 3361(_+3x lop6)4 18 f64 106 f23 3076(+ 1 x c = 3.71,/3 = 1.00,s = 0.001 1 5.7f0.2 18.8+_ 0.9 3635(+2 x s 3 155 f3 113 f 1 335qf3 x 10-5) 4 16f4 107 f7 3139(f2 x c = 7.45,/3 = 1.00, = 0.005 3 117 f 26 109 f 12 336qf5 x lop4)4 22 f 19 126 f29 3209(+ 1 x lop4) " Band number.Concentration in mol dm-3. (for I and 11). Typical resolutions of IR spectra for all of these er than those formed by terminal molecules (S,) in polymer systems is shown in Fig. 1 and 2. The resolved bands are or dimer units because of the reinforcing effect of the bond to labelled 1, 2 etc. from the high-frequency end. The band oxygen.We assume that absorption band 3 in the fundamen- labels correspond to the band number in Tables 1 and 2. tal (vOH) is due to doubly bonded hexOH molecules [see (ii)]. We now focus on the bands matched to the condition of Except for chain association units, alcohol molecules form the hexanol OH-group in (I-IV). Band 1, with v,,, at ca. branched or 'bush' (group) associated units where triply 3640 cm-l, is the typical band of the fundamental vibration bonded molecules (S,) are necessarily present [Scheme 1 of the monomeric alcohol 0H-gr0up.l~ At ambient tem-(iii)]. peratures its intensity is vanishingly small in samples contain- For these molecules the reinforcing effect will naturally be ing pure alcohol or in Bu,NI-hexOH solutions. Only at much more than for S,.Since the shift to low frequency is higher temperature, and particularly in dilute CCl, solutions, approximately proportional to the hydrogen-bond strength, is this band very much more pronounced (see Fig. 1 and 2 we conclude that band 4 corresponds to vibration of the OH-and Tables 1 and 2). groups of the S, molecules. Furthermore, following Symons' results for methanol,6 we It has been determined from dielectric measurements l7 suggest that band 2 at ca. 3570-3540 cm-' corresponds to that chain-type association is dominant for normal higher the fundamental vibration of the OH-group in terminal alcohols. This fact is responsible for the greater intensity of hexOH molecules in polymeric or dimeric association units. band 3 as compared with band 4.The results of dielectric It may be represented by equilibria (i) and (ii), where S, investigations" make it possible to establish that, in spite of denotes a solvent that forms n hydrogen bonds. the relatively long length of the hydrocarbon chain, the mean The wide low-frequency band in system (I), according to level of association of hexanol at 25 "Cis as great as 11 [i.e. our calculations, can be reconstructed with minimum of two n = 11 in eqn. (ii)-(iii)]. These two factors complicate the symmetrical bands, as indicated in Fig. 1 and 2, with v,,, at detection of terminal molecules in spectra of pure hexOH. In ca. 3330-3430 cm-' (labelled 3) and ca. 3045-3270 cm-' this case, the spectra are reconstructed with two (at low (labelled 4).It should be pointed out that the hydrogen bonds temperatures) or three bands, 1, 3 and 4. for average doubly bonded hexOH molecules (S,) are strong- Note that v,,, for bands 2-4 is dependent on the presence J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 30 1 Table 2 IR spectra of OH-group vibration in pure hexOH (I) and hexOH-Bu,NI (11) solutions at various temperatures ~~ ~~ ~~ nu band height band width band position t = 28,b fi = 0.66, s = 0.0052 3 144f7 119 + 4 3331(f9 x lo-') 4 12 f4 126 f16 3067(f2 x lo-') t = 55, #I = 0.78, s = O.OOO9 1 0.8 f0.1 11 f2 3638(f2 x lop6) 3 409 f4 122 f9 335qf9 x 10-6) 4 5 f10 134 + 29 304qf5 x 10-5) t = 85, = 1.00, s = 0.0009 1 1.3 f 0.2 11 f2 3640(+2 x 3 32 + 5 126 f10 3378(f2 x 4 5f 15 185 f11 3055( f6 x t = 115, #I = 1.00, s = 0.0013 1 2.4 f0.4 11 +_2 3641(+5 x 2 30 f7 119 f12 3425(f3 x lop5) 3 6f 19 157 f51 3228(f8 x lop6) t = 145, fi = 1.00, s = 0.0003 1 1.5 f0.9 16 + 6 3~3(f8 10-7) 3 7+3 109 & 51 3596(f8 x 4 4f4 133 f43 3270(+8 x 5 6 f74 97 f69 3084(+8 x I1 t = 28, p = 1.00, s = 0.0052 3 117 & 26 109 f12 336qf5 x lo-,) 4 22 f19 126 + 29 32oq+i x 10-4) t = 55, #I = 1.0, s = 0.001 1 1.4 +_ 0.1 27 3 3aq+_8x 10-7) 3 32 + 3 108 f3 3397(f2 x 4 7f7 153 f22 3209( +6 x t = 85, p = 1.0, s = O.OOO4 1 1.4 f0.1 27 f3 3~+810-7)x 3 32 f3 108 f3 3397(+2 x 4 7f7 153 f22 3209( f6 x lop6) t = 155, fi = 1.00, s = 0.0009 1 2.7 f0.1 15.6 f0.9 3639( +6 x lop6) 3 41 + 1 115 f1 3423(f2 x 10-5, 4 6f2 154 & 20 3164(f2 x r = 145, fi = 1.00, s = 0.001 1 4.0 & 0.2 12.1 f0.5 3639( +6 x 3 46.1 f0.7 113.7 f 0.5 3423(&2 x 4 8k1 147 k 12 3164(+2 x loW6) Band number.Temperature in "C. hex2\ -hex hex known empirical rule:1','4,18 0-H \\ li20-H---0-H AHH-bond = C(vmax, monomer -'ma,, polymer) SO Sl s, where C is a constant, i.e. the enthalpy change, AH, of H-bond formation depends upon the polymer band shift. Note that AHH-bond can result from (a) a change in the energy (strength) of a fixed number of hydrogen bonds; (b)a change in the extent of hydrogen bonding with a fixed energy. For this reason, in our opinion, it is more proper to discuss v,,,(ii) in terms of structure-making and structure-breaking effects. Taking into account eqn.(8), the position of polymer band 3 has been chosen as the quantitative characteristic of self- association of hexanol by hydrogen bonding . Our results (Fig. 3 and Tables 1 and 2) indicate that there is no need for the introduction of an additional band that corresponds to the OH-group H-bonded with I-ion in any of the spectra (large Bu,Nf ions are unlikely to form hydro- gen bonds with alcohol molecules and do not greatly modify ,O'H the total hydrogen bonding in alcohols6,' 2). The experimen- hex tal overlapping curve may be represented by as many bands as for pure hexOH without any essential difference in disper- sion (Table 1, 2). Moreover, as shown in Fig. 3, within the standard deviation, the width of band 3 does not depend on Scheme 1 the presence of electrolyte.That is, the bands 3 and 4 in systems with and without Bu,NI are the same. of Bu,NI, the CCL, concentration and the temperatures, Summarizing, we can contend that the influence of tem- unlike v,,, for monomeric band 1. This derives from the fact perature, inert solvent and electrolyte on the state (strength) that, in effect, these bands are the sum of individual closely of hydrogen bonds in liquid hexanol can be described with related bands due to the energetic states of the OH-group. the aid of a single qualitative characteristic, namely, the posi- That is, the influence of temperature and inert solvent leads tion of the polymer band 3 (vmax)(at least for an electrolyte to the distribution of the molecules amongst the possible con- such as Bu,NI).formations in the polymer, which is reflected in the intensities There is another important reason for this conclusion. As of the corresponding OH bands. As a result, the positions of follows from quantum-chemical calculations" of the the polymer bands change. This serves as a basis for the hydrogen-bonded complex of ethanol with Br-, the alcohol 302 150 150 r c I 120 E \k 120 * b" b" 90 90 60 L 60 -1.0 -0.5 0 0.5 1 .O 30 60 90 120 150 log c tl" c 400 300 300 7 9-4 50' I I-, I 8' I -1.04.5 0 0.5 1.030 60 90 120 150 log(c/mol dm-') t/T Fig. 3 Width of (a)polymer band 3 and (b) polymer band 4 of OH- group at various temperatures (on the right: 0, 11) and alcohol I; .,concentrations (on the left: 0,II; 0,IV) molecule is brought into the strong negative field of the anion.As this takes place, about 10% of the negative electro- static potential of Br- is retained at a range of three intervals between the hydrogen of the alcohol molecule and the Br- ion (1.0-1.5 nm). In the condensed phase we conclude that the magnitude of this field offers the possibility of a large influence of the anion on reformation of hydrogen bonds between alcohol molecules at long distances from the anion. This deduction correlates well with the appreciable change in v,,, of band 3 in spite of the low content of electrolyte mol- ecules (1 :18). For example, at 28 "C vmaX of band 3 for pure hexOH is 3331 cm-' and that for Bu,NI solution is 3360 cm-'; this corresponds to an increase of temperature of 50 "C for pure hexOH.The main results of this work may be represented by the dependence of polymer band position (vmax) us. concentration of an inert solvent [CCl, Fig. qa)] or temperature [Fig. qb)] for all solutions investigated (1-IV). Conclusions When the intrinsic structure of the solvent is clearly defined (low temperature, low inert solvent concentration) an electro- lyte, such as Bu,NI, plays a structure-making role. In contrast, if some conditions (higher temperature, near the boiling point, very dilute solutions in the inert solvent) led to significant destruction of the set of hydrogen-bonds in alcohol, the electrolyte (even one such as Bu,NI) could act as a structure-maker, that is it could lead to the reinforcement of hydrogen bonds in the solvent.This result is supported by the results of NMR investiga-tions (Table 3). It is known13 that hydrogen-bond formation leads to a downfield shift of the hydroxy proton chemical J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3425 -3400 5--. Xm ,E 3375 3350 3325 I I 1 I -3 -2 -1 0 1 log(c/mol d~n-~) ,,? (b) 3475 3450 -c I5 3425 ---. x sz 3400 --3375 -3350 3325 I I I I I I I 20 40 60 80 100 120 140 160 t/"c Fig. 4 Polymer band 3 position us. concentration of alcohol (a) for hexOH-CC1, (11) and hexOH-Bu,NI-CC1, (IV) solutions and us. temperature (b) for pure hexOH (I)and hexOH-Bu,NI (11)solutions shift.The hydroxy proton peak for hexOH in Bu,NI solu-tions (IV) is upfield compared with systems without electro- lyte, i.e. S(II1) > 6(IV), whereas in dilute solutions G(II1) < 6(IV) (Table 3). This difference corresponds to an alcohol concentration of ca. 0.06 mol dm-3 (cf: 0.04 mol dm-3 from IR measurements). Table 3 'H Chemical shifts (6) of hexOH OH-group for hexOH-CC1, (111) and hexOH-CC1,-Bu,NI (IV) solutions at 28 "C as a function of alcohol concentration (c) c/mol dm-3 6 I11 0.005 0.93 1 0.009 1.288 0.018 2.000 0.048 3.188 0.094 3.863 0.191 4.344 0.492 4.813 0.741 5.044 1.Ooo 5.200 IV 0.005 1.013 0.010 1.388 0.018 2.150 0.05 1 3.250 0.096 3.848 0.187 4.225 0.483 4.650 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 We thank Dr. S. Yarmolenko and Dr. A. Doroshenko for their support in the performance of the experiment and helpful remarks. The work was financed by Ukrainian State Committee on Science and Technology and Fund of Funda-mental Investigations (section 3, ‘Chemistry’, project N3/89). References 1 0.N. Kalugin, S. M. Gubsky, I. N. Vyunnik, M. Grigo and R. Saendig, J. Chem. SOC., Faraday Trans., 1991,87,63. 2 0.N. Kalugin, Dissertation, Kharkov State University, 1987. 3 (a)J. C. Rasaiah and H. L. Friedman, J. Phys. Chem., 1968, 72, 3352; (b) J. C. Rasaiah, J. Phys. Chem., 1970,52,704. 4 H. L. Friedman and C. V. Krishnan, in Water: A Comprehensive Treatise, ed. F. Franks, Plenum Press, New York, 1973, vol.111, p. 17. 5 I. M. Strauss and M. C. R. Symons, J. Chem. SOC., Faraday Trans., 1978, 74, 2146. 6 H. L. Robinson and M. C. R. Symons, J. Chem. Soc., Faraday Trans., 1985,81, 2131. 7 I. M. Strauss and M. C. R. Symons, J. Chem. SOC., Faraday Trans., 1977, 73, 1796. 8 T. R. Grifliths and M. C. R. Symons, Mol. Phys., 1960,3,174. 9 I. M. Strauss and M. C. R. Symons, Chem. Phys. Lett., 1977, 45, 423. 10 S. E. Jackson, I. M. Strauss and M. C. R. Symons, J. Chem. SOC., Chem. Commun., 1977, 174. 11 Problems of Solution Chemistry. Ionic solvation, ed. G. A. Krestov, Nauka, Moscow, 1987. 12 B. S. Krumgalz, J. Chem. Soc., Faraday Trans., 1983,79,571. 13 G. C. Pimentel and A. L. McClellan, The Hydrogen Bond, W. H. Freeman, San Francisco and London, 1960. 14 L. J. Bellamy, The Infrared Spectra of Complex Molecules, Methuen, London, 1954. 15 V. I. Malyshev, Usp. Phys. Nauk, 1957,63, 323. 16 D. M. Himmelblau, Applied Nonlinear Programming, McGraw-Hill, New York, 1972. 17 M. I. Shahparonov, Mechanisms of Fast Processes in Liquids, Vyshaya shkola, Moscow, 1980. 18 L. V. Vilkov and Yu. A. Pentin, Physical Methods in Chemistry. Structure Methods and Optical Spectroscopy, Vyshaya Shkola, Moscow, 1987. Paper 3/03 1565 ;Received 3rd June, 1993