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Perturbation of some vibrational bands in solution
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Perturbationof some vibrational bands in solution D. 33. CUNLIFFE-JONES Distillers Chemioals and Plastioa Ltd. Reseamh Department Saltend, Hedon, Hnll (Received 8 May 1966; rev&d 4 September 1968) .&&&--Shifts of the infrared active stretching vibrations of four non-polar molaoufes in solution have been measured. The resultad&agreestrikingly in some instanues with the predir&ions of “dielectric theories”, and it appears that the type of long-range i&era&ion envisaged by the ‘%&ion field” model plays no sign&ant r&e. This supports the view, given by COUWON[l], that the fluctuating electrostatio field of the solute molecule is greatly screened by the first layers of eleutronio charge that it meets. The shifts obtained using carbon tetrachloride end carbon tetrabromide as solutes are attributed to the effects of inters&ions involving the peripheral atoms of the solute and neighbouring solvent atoms. Doublet absOrptions in some aliphatio ether solvents and the halfband width variation of vs of cartron tetrabromide provide evidenoe of separate contributions to the shifts, arising from collisional interactions involving different parts of the solvent moleoules. Quite different behaviour is shown by the linear solute carbon disulphide, and this is attributed to additional contributions which arise from the alignment of solute and solvent bonds. The alignment of bond dipoles, rather than interactions between whole molecules, also explains the large shifts often observed using polar solutes in solvents such es p-dioxane, which by virtue Of their symmetry have small dipole moments, low dielectric ~0~~~ and yet are not highly polarisable. CsrbOn dioxide does not show these additional effects to any marked degree at normal temperatures. This is attributed in part to the retention of rotational energy by oarbon dioxide in solution, for which some evidence is given. In solvents where the width of the absorption of carbon dioxide was sufficiently narrow, an additional absorption was observed approximately 12 cm-i to lower frequency. This is assigned to the hot band vs +- vs - vs. INTRODUCTION
attempts have been made to interpret the changes in vibrational frequencies observed when molecules pass from the gaseous state to solution in terms of some hind of solute-solvent interaction. The most attractive theory is that of BUCKINQEAM [Z-d], in which the solvent shifts are found to be proportional to an expression involving the fbst and second derivatives of the interaction energy, U, with respect to the normal co-ordinates. For diatomic molecules the fractional shift is given by SEVIZW
and for polyatomic molecules an expression of similar form is obtained. If as a simplification the term in 77”is neglected, it has been shown (HE&D C. [2] A, (33 A. 141 A. [&I C. [I]
A. COULSON, Proc. Roy. ~5’00.. &%6, 09 (1960). D. &JQ9nuoIEaaa, Proc. Roy. Soo. Af3Bs, 169 (1968). D. &WKINQlUX, Proc. Roy. 800. #66, 32 (1969). D. BUCXINQEAM,True. &WU&Z~AS%.#, 753 (1960). E&m snd 11. w. TEOXl’sON, PTOG. &ye SOG. A@& 39 (1962). 779
and
D. B.
780
CumE-Jams
THOXPSON [5] and ROTRSCHI.LD [S]) that pre~~tions based on the theory, when~pp~ed to symmetrical linear polyatomic molecules, fail. Yet the theory as it stands is difficult to apply in terms of several types of intermolecular forces, and it would be convenient if the use of a simple model describing the interaction were found satisfactory. Buckingham therefore introduced the “reaction field” model, which had been used in the earlier treatment of BAUER and M&+AT [7], where the field, .F, acting on the solute molecule due to the charge distribution of its neighbours is taken to be proportional to its dipole moment. Thus, F = gs . m, + gn . Am where m, and Am are the eq~b~urn dipole moment and the oscillatory dipole resulting from the vibration, and gE,g, are consents which are simple function of the dielectric co~tant and refractive index of the solvent medium. Assuming that terms arising from “non-dipolar” interactions are the same for all solvents, and neglecting higher terms in g as a rough approximation, the frequency shifts in polar solvents are then given by the equation: Ahw 0
-
==c+Gh(&JJ -tG&$)
According to this “dielectric theory” the variation in the shift from solvent to solvent arises from a general non-specific type of long range dipolar interaction. However, considerable doubt has been cast on the validity of the reaction field model evidence (Bw (LAS~LOand &&JSHXREs]), and there is much expe~en~l et d. [9] ; BE~AMY and WAITS [lo] ; BEX,L.UVXY and HAIUM [ll] ; BIUZAXY and WIUIAMS [12]) which suggests that speoiflo, relatively short range, interactions are usually of dominant importance. It is now common to take some intermediate position, saying that both local interactions and bulk dielectric effects may be significant, their relative importance changing considerably from case to case (BEY&ANY and WIIUAMS [12]; CBLDOW et al. [13]; HALLAMand RAY [14]; RATAJCUK and ORKCLLE-THOMAS [ 151. However, it has been pointed out by COUIXJON [I] that the fluctuating eleotrostatic field of the solute molecule will be greatly screened by the iI.rstlayers of electronic charge that it meets. Since these will be by far the most polarizable parts of the neighbo~ng solvent atoms (ABBOT and BOLTOX[16]) it should be expected that this field will be damped in the outer parts of the first co-ordination shell. It follows that the relatively long range dieleotric approach will in general play no [Sl W.
G. ROTHNYHXLD,J. Chem. Hays 49,694 (1966). [7] E. BAUER and M. MAUAT, J. Phye. Radium 9, 319 (1938). [8] P. LASZLO and J. I. MUSHER, J. Chem. P&p. 41, 3906 (1964). [Q] L. J. BELUAXY, H. E. HALLAM md R. L. Wmm, Pram. Faraday Sot. &1120 (1958). [lo] L. J. BELLAIKY and R. L. WILLIAMS, Trace. Yamday Boo. & 14 (1969). [ll] L. J. BELLAXY and H. E. -AM, Tram. Far&y Soo.&,220 (1959). [12] L. J. BELLA~ and R. L. Wnmamas, Pvoo. Roy. Sot. A$@, 22 (1960). [13-j G. L. CALDOW, D. CUNLIIFFE-JONESand H. W, TEIOMPSON,PPOC.Roy. Sot. A%%, 17 (1960). [14] H. E. HALLAM and T. C. RAY, Trans. Far&f/ Soo. ris, 1299 (1962). 1161 H. RATAJCUK and W. J. ORVILLE-THOMAS, Tram. Fcwatfag Sw. 81,2603(1965). [I61 J. A. AESBOPIY and R. C. BOLTON, Pmt. Roy. Sot, A916,477 (1963).
Perturbation of some vibrational bands in solution
781
s~~c&nt role, md it would seem more appropriate to attempt an ~~rp~t&tion solely in terms of the derivatives of short range intermolecular forces. The results described in this paper provide strong support for this view. It is proper first to enquire what evidence there is to have suggested any contribution from “dielectric effects”. BELUMY snd WIGWAMS [ 121 suggested that some dielectric contribution is implied by the fact that many plots of the relative frequency shifts of one compound against those of another give straight lines which do not pass through the origin. But if the shifts result from the effects of several types of short range interaction, unless the different contributions vary in the same proportion for each solute, on passing from solvent to solvent, an exact fit to a straight line would not be expected. There should, in general, be some scatter of the points about the best mean line, and it would be fo~~~us if this line passed through the origin. The Bellamy scatter in theseplots is infact~equen~y~a~rth~n theexpe~en~le~r. and Williams also consider the reduction of ~20 om-l in the OH stretching frequency of 2%di-tet$-butyl p-ores01 on passing from v&pour to hexane solution must be attributed to dieleotric effects. But, although there may be no dimerization or other interaction directly involving the hydroxyl group of the hindered phenol, the aasumption that short range iutercactionsat other sites in the molecule cannot lead to this frequency shift is doubtful. More generally, some contribution of dielectric effects has been argued from the approximate fit which is sometimes observed with the Kirkwood-Bauer-M&g&t, or Buckingham relations. This however could well be fortuitous, since contributions arising from short range directions dependent on solvent bond polarities and pol~~b~~s will be crudely related to the bulk properties of dielectric constant and refractive index. Agreement has also been clf$med occasionally between changes in band intensities rendcalculations based on simple dielectric field theories (YAMADA end PERSON[17], RATAJCZLLK and ORVILLE-THOMAS [15]. But the interpretation of intensity changes in terms of dielectric effects has been disputed in recent work by IOCUNSEN et al. [18]. There thus appears to be no wholly convincing evidence for the contribution of bulk medium effects. Much of the experimental data reported so far has been concerned with the shifts of stretching vibrations of polar solutes, but a better assessment of the different interpretations may be obtained by making similar measurements using non-polar solutes, with which strong specific &ssoci~tionsare not expected. Shifts &sing from the effect of the much red&d reaction field (now related only to the oscillstory dipole of the molecular vibration) should increase in a uniform manner according to s, function of the solvent refractive index, and there seems no reason to expect, from this approach, any radical differences in the behaviour of non-polar solutes of different geometry. However, if the shifts arise mainly from short range interactions some differences may be expected. For compact, nearly spherical non-polar solutes the shifts should arise simply from interactions between the peripheral solute atoms and neighbouring solvent atoms. These interactions will probably include coulombic, inductive and dispersive forces. Both inductive and dispersive contributions to the shift will increase with solvent atom polariz:ability, but, in view of the anisotropy of [I77 H. YAMADA ssd W. B. PZSCSON, J. Chepn.phg8.4&309 [18] A. V. IOUAXSEN, E. V. BROaS a.ndG. T.3.~TOV~NKO,
(1964). et.
&e&y
18,18 (1965).
D. B. a-JONES
782
polarizability of most solvents, the behaviour should be less regular than expected from the reaction field model. However, for linear non-polar solutes (as for polar solutes), the alignment of solute and solvent polar bonds in temporarily attractive complexes may also be important, and this should lead to a more radical departure from the expected behaviour of the reaction field model. This contribution should be appreciably affected by steric considerations and will also be temperature dependent. Further complications may also be expected in some cases, if the vibration involves appreciable motion of other atoms not involved in the aligning solute bond, and also when interactions involving different sites in the molecule interfere. It is of some interest that both carbon tetrachloride (RATAJCZAE and ORVILLETHOM.M[ 161)and carbon disulphide (YAMADAand PEISSON[ 171)have been described as “ideal solutes” which illustrate the predictions of the dielectric theories. The present paper describes measurements of the infrared active stretching vibrations of carbon tetrachloride, carbon tetrabromide, carbon dioxide and carbon disulphide in a wide variety of solvents. EXPERIMENTAL METHOD
All the solutes and solvents used were commercial samples, 99% pure or better. This purity was considered quite sufficient and no further purification was attempted. A stream of carbon dioxide gas was obtained by evaporation of solid carbon dioxide, and allowed to bubble through the solvents used. Bands of suitable intensity were then obtained with this solute using pathlengths of O-13 mm; in some cases where the solubility in the solvent was less the pathlength was increased to 0.5 mm. Some of the measurements were obtained using a Hilger H800 spectrophotometer fitted with a sodium chloride prism, but the measurements on the ys bands of carbon tetrabromide and carbon dioxide were made using a Grubb-Parsons Spectromaster using 1600 and 7600 line/in. gratings respectively. The absolute accuracy of the measurements using the Hilger instrument was f 1 cm -l. Repeated measurements suggested a reproducibility within the tier limits of f0.6 cm-l. The accuracy and reproducibility of the Spectromaster were at least ,tO*4 cm-l. The frequency shifts are quoted to the nearest half wave number, except in the case of broad or overlapped bands where the shifts are given approximately. Solvent absorption was eliminated in all cases by the use of a variable pathlength compensating cell in the reference beams of the spectrometers. RESULTSANDDISCUSSION 1. Tetrahedral and linear non-polar solutes
Table 1 lists the shifts of frequency on passing from gas to solution of the antisymmetric stretching vibrations of two tetrahedral and two linear molecules. The Fermi-resonance doublet of carbon tetrachloride, of which the antisymmetric stretching vibration, va, is the high frequency component, has been measured before in a number of solvents (TUOMIICOS~I [19], LISITSAand OVANDER[ZO]), but attention was directed mainly to intensity changes and the unusual %olvent order” followed [I91 P. TUO~~~I~OEIKI,J.C~~~.P~~~.~,
2083(1956). [ZO] M. P. LISITEJA and L. A. OVANDER,Opt. Spectry 7, 383 (1969).
Perturbation of some vibrational bands in solution
783
by the shifts of rs seems to have escaped notice. No previous rn~~rnen~ appear to have been recorded in the case of carbon tetrabromide. In calculatiug the shifts the v&pour &equencies of the two tetrahedral solutes were taken as the band absorption maxima 795.5 cm-* for oarbon tetrachloride and 6804 cm-l fox carbon tetrabromide. Table 1. Shifts of the antisymmetrio atretohingvibrations of tetrahedral and linear solutes to lower frequenoieson pressingfrom gas to solution (cm-l)
w&mitri1e
nitromwthellw
CCI, (Yt 796.6 w-1) 3.6 4
se&me methyld
3.6 -
1: 2-di~~orne~e di~~ommeth~e diethyl ether d&isopropyl ether I&diohloroethene II-pentane ohlomform pdioxane ethyl bmmide dimethyl sulphoxide di-n-butyl ether n-hexaue oyolopentane triethylamina carbon totraahloride benzene di-Wt. butyl diiulphide I : Z-dibmmoetbane diethyl disulpbido dimethyi diaolpbide methyl iodide dibmmo methane ayctohexane bmmoform a-tetrabromoethane carbon disulpbide di-iodomethane
6.6 6 -
8” 46 7 6.6 I 7.6 7.6 7 7.6 8 8.6 9 7.6 8 8 9.6 11.6
CBr, (Y*680-6 om-1) 4 -
6.6 ~7.6
-10
(Y*2S?cm-1, 7 7
% (v5 1636 on+) 16 -
8 9.6
18*6 -
10 10.6 10.6
16 l&6 14.6 12.6 16.6 12.6 17 17 16 20.6 14.6 11.6 12 16.5 19.6 13.6 17 l&b 19 18 13.5 19 20.5 -
-9.6
10~6
7 ~6 8.6 -9.5 7-6 8 12.6 8 10.6 10 11 11*S 11.6 8.6 12 16
IO.5 IO.6 11 11 11 11.6 11.6 12 12.6 12.6 13 13 13 13 13.6 :3*5 13.6 14 146 16 17.6
Some frequency shifts of ya of carbon dioxide in solution have been reported by and THOMPSON [5] who used a siugie beam Perkin-Elmer instrument with a Iithium fluoride prism. These measurements have now been repeated using a grating iustrument and the data extended to other solvents. Where comparisons are possible the agreement is generally satisfac~~. The vapor frequencies used in ~alc~at~g the shifts for carbon dioxide and carbon ~s~phide are 2349 and 1536.5 cm-l respectively. The frequency given for vs of carbon disulphide in the vapour state differs from that used by YAMADA and PERSON[17] who claimed that the frequency shift of carbon disulphide on passing from gaseous to condensed states demonstrated its ideal behaviour in accord with “dielectric theory”. The frequency they used WM that quoted by HERZBERU [22], probably derived from BAILEY snd CASSIE [23], and HEALD
[Zl] H.
YAMADA and W. B. PEBBON,J. Ciwn. Phys. 41,247s (1904). [22] G. EEERZBERQ, hfra-Red ar& Bamm Spectra of PoZgatmic MoZewZ~, Nostrand (1945). [23] C. R. BLILEY and A. B. D. CASSIE,Ppoc. Rag. Sot. 140,605(1933).
p. 277.
Van
D. B. Cum-JONES
784
it seems doubtful that this was checked. The frequency was later observed by PLYLER and HTJMPHREYS [24] to be 1535 cm-l, and measurements made for the present work using both prism and grating spectrometers confirm this value. The measurements of YAMADA and PERSONon carbon disulphide in condensed states therefore lead to an opposite conclusion, namely that the frequency shifts observed on passing to the condensed states are considerably different from those predioted by the theory. (a) Carbon tetrddoride and carbon tetrabrornide. As might be expected, the two tetrahedral solutes behave in a very similar manner. The “solvent order” of the shifts differs considerably from that shown by the shifts of the stretching vibrations of
Fig. 1. Relativefrequencyshiftsof carbontetrachlorideand caxbontetrabromide oomparedwith a fun&ionof solventrefractiveindex. typical polar solutes. In particular, polar solvents such as aoetonitrile and acetone give the smallest shifts to lower frequency. It is probable that this is attributable more to their low polarizability than to their polarity, for the most polar solvent used, dimethyl sulphoxide, does not in fact give the smallest shift, and this solvent has a higher refractive index and presumably higher average peripheral polarizability than acetonitrile or acetone. Although there seems to be a general increase of shift with solvent polarizability, plots of AV/Yagainst (nz - l/2%%+ 1) (Fig. 1) show considerable scatter. This does not suggest the uniform behaviour to be expected from the reaction field model, but could be accounted for in terms of interactions between the peripheral atoms of the solute and neighbouring solvent atoms. The fractional shifts Av/v are not greatly dissimilar in magnitude from those of s+-._~of acetone. This may be considered remarkable in view of the large reduction in the reaction field on passing from a polar to a non-polar solute, though it must be [24] E. K.
l?LWR
and C. J. Thx~~
YS, J. Rex. Nat1 Bur. Std. 89,
69 (1947).
Perturb&ion of some vibrational bands in solution
785
admitted that the shifts are not directly related to the interaotion energy but rather to its derivatives. A comparison of the data for carbon ~tra~~o~de and acetone (given in Table 2) sugg~~ that (1) in some cases short range ~~ra~tions make similar contributions to the fkactional shifts for these two solutes, and (2) in other cases there are probably additional coutributious to the shift of vo+, of acetone. The latter arise either from association of the oarbonyl group with solvent bond dipoles or ?r-electron systems (methy iodide, ethyl bromide, benzene, acetone, dimethyl sulphoxide, acetonitrile, nitromethane) or from hydrogen bonding effects (bromoform). [It is interestiug to note that the gre&est relative fkequenctyshifts quoted for both solutes occur with ~-i~omethane. The difference between these figures is rather Table 2. Gompazison of rehtive fmquenoy &ifts (Avjv) shown by VW acetone and pa of carbon tetraohloride (w=C n-hexene cyclohexane dietbyl ether triethylamine benzene carbon disulphide ethyl bromide a_dioxene aoetene I : 2.diehloroethene methyl iodide aoetimitrile nitromethane d~e~yl~pho~de bromoform di-iodomethene
Aaetane 1737 em-‘) 8.4’ 8.4. @*2? s.t3* 11*8* 11.w 11.8 12.7: 12*7t 13*3* 13.3+ 13-w 14.1. 16.6 18.7’ 17.3’
Carbon tetmohloride (pa 79&b am-1) 8.3 s-4 7.6 9.2 8.8 12 8-8 6.7 4.4 6.9 11.3 4.4 6 6‘9 l&l 14.6
of
A -0.4 -X*0 Cl.7 SO.6 43 -@2 I; -f-8+3 +a*4 +2 +s*4 s.B.1 +e+ 46.8 j-2.8
* JIBWEIL and Txo~onr [26]. t JOSIEN~~~ LA&lCOW%E[~6]. 2 BELLAMY and WELLXAMS[lo].
small suggesting that dispersion forces are domiuant and that the large shift of the carbonyl band of acetone in this case owes very little to the low polarity of the solvent (N1D) or to its weak hydrogen bonding capability.] Detailed study of the V, abso~tion of carbon ~trac~o~de is complicated by Fermi-resonance; the ya absorption of carbon tetrabromide however is not complicated by any overlapping bands and it is possible to study changes in band width and shape (Pig. 2, Table 3). It is seen in Fig. 2 that, although the ~~~a~ band intensity remains almost cogent, there is a large variation in the half band-width. If the perturbation of the absorptiou is the result of short range interactions, the half band-widths will reflect the structural anisotropy of the solvent and increase with the difference in the perturbing effects of different parts of the solvent molecule. This certainly appears to be true, for the half band-widths are least in hydrocarbon solution (5 cm-l), where the immediate environment should be reasonably uniform, and Hearst in solvents such as a~~~t~e, ~-iodomethane, dioxane and acetone (16-24 om-1) where large di~e~n~s between the effects of the different groups are suggested by the frequency measurements.
The separate ~ntribution of short range perturbiug ~~ra~tio~ involviug diEerent pafts of the solvent molecule, which this half band-width variation suggests, is illustrated more dramatically by the us absorption of carbon tetrabromide in a series of aliphatio ether solvents. The absorption in some cases appears as an overlapped doublet. The decrease in intensity of the higher frequency component of this doublet through the solvent series p-dioxane, methylal, dðyl ether, d&isopropyl ether, di-n-butyl ether clearly establishes that this arises from perturbing interactions involving the ether oxygen atoms. This component appears to be only very slightly displaced from the vapour frequency, whereas the second component shows a shift comparable to that obtained in aliphatic hy~~arbon solutions. In dioxane or methyl solution, where theresho~d be agreater incidence of eo~sional in~raetio~ involving solvent oxygen atoms, the two components are of comparable strength and overlap extensively to give broad bands similar in shape and position to that obtained in acetone (see Fig. 2). It is clear from these results that no interpretation solely in terms of the reaction field model is adequate for solutes such as carbon tetrachloride and carbon tetrabromide, but the results can be explained quite simply in terms of short range interactions. Finally, mention should be made of the unexpectedly large shift found using carbon tetrabromide but not with carbon tetraobloride in triethylamine. Repeated measurements contied the results, but it was found that the carbon tetrahalides react with this solvent, the rate of reaction increasing considerably through the series CC&, CBr4, CI,. It is possible that the large ahif%obtained using ~bon~~abro~de in triethyla~e results from some ~~raction involving the highly ~la~zable s(pa lone pair orbital of the solvent prior to reaction. (b) Ca&oa &ox& and carbon &m&Aide. The shifts obtained using carbon disulphide as a solute show the marked contrast expected for a linear solute from those of carbon tetrachloride and carbon tetrabromide (see introduction above), the solvent order now roughly following that shown by the shifts of vo_o of aeetone (see Fig. 3). In particular, the shifts obtained in acetone and acetonitrile are now greater than in aliphatic hydrocarbon solution, and the shift obtained using dimethyl sulphoxide as solvent is as great as that in the highly polar&able a-tetrabromoethane. These results disagree strikingly with the predictions of the dielectric theory when applied to a solute of no permanent dipole moment. In terms of short range interactions they may be simply explained as arising from an ad~tional ~nt~bution to the total shift which becomes possible with the linear molecule, namely the alignment of solvent dipolar bonds with solute bonds, as illustrated in Fig. 4. The greater shift also observed in benzene is probably due to the anisotropy of polar&ability of this solvent, since in the event of an alignment involving the benzene r-electron cloud and the linear solute, the greater polarizability of benzene in the longitudinal direction will be appropriate. The increased shift in Ip-dioxane also is attributed to alignment involving the polar ether links at either end of the solvent molecule. The alignment of bond dipoles, rather than interactions between solute and solvent as whole molecules, provides a simple explanation of the large shifts obtaiued with many polar solutes in solvents such as p-dioxane, which merely by virtue of their
Q?-
@I-
02-
0.3-
o-
0.1 -
O-2-
a3-
CM-
05”
0.6 -
15 145
7
6 145
Linear wavelength
8
15
scde,
p
14.5
9 I5
14-5
IO 15
Fig. 2. Variation in width and contour of the vs absorption of carbon tetmbromide in different solvents. 1. oyclohexcme, 2. carbon diaulphide, 3. ethyl bromide, 4. aoetonitrile, 6. aoetone, 6. p-dioxane, 7. methylal, 8. diethyl ether, 9, di-isopropyl ether, 10. di-n-butyl ether. The solutions were all of similar concentration, 0~0006-0~0082 molar.
I45
6
/;\/-/\A\
.
1
3
D. B. ~-JONES
788
AV/VXI~ for
AVIVXI@ forF+of acetone
yof CBq
Fig. 3. Compaison of relativefrequencyshifts.
8-
S+
8-
Fig. 4. Suggestedalignment of CS, solute with solvent containing accessiblepolar bonds in temporarily attraotive collision complexes.
symmetry have a small dipole moment and low dielectric constant, and yet are not highly polarizable. The effects arising from the alignment of polar bonds will be temperature dependent and it is possible that studies of the variation of “solvent order” with temperature may be helpful in sorting out the different effects in some cases. It would be expected from the above interpretation that as the temperature is raised the solvent order obtained with both Yeof carbon disulphide and yo_o of acetone will tend to approach that shown by the ~avibration of carbon tetrachloride. It is remarkable that the shifts of va of carbon dioxide do not follow the same pattern as those of vQof carbon disulphide. Instead they closely follow the solvent order obtained using the tetrahedral solutes. HEALDand THOMPSON [5] suggested an interpretation of the shifts of V~of carbon dioxide in terms of dispersion and dipoleinduced dipole interactions involving solute and solvent as whole molecules. However, the marked contrast with carbon disulphide cannot be readily explained on this view and the results obtained with the tetrahedral solutes and carbon disulphide argue strongly in favour of shorter range interactions. Two suggestions may be made to account for the lack of similar bond alignment effects to those noticed with carbon disulphide. One arises from the considerable difference in the length of the two solutes. With the smaller carbon dioxide molecule the electrostatic field of the aligning solvent bond C-X may be cast more effectively over the whole molecule. The opposing polarities of the two solute bonds might then
Perturbation of some vibrational bands in solution
189
lead to a decrease in the attractive force. The second possibility is that carbon dioxide may retain much more rotational energy than carbon disulphide in solution and this will inhibit the alignment. Evidence for some retention of rotational energy by carbon dioxide in solution is provided by variation in the shape and width of the vaabsorption (Fig. 4). There is less evidence of completely free rotation than observed by LASCO-E et al. [27]for some diatomic molecules, but the width of the band in some solvents and the variation of half band-width with solvent (see Table 3) seems to parallel the behaviour of the Table 3. Half bandwidths of vs of oarbon dioxide md vs of oarbon t&r&bromide in various solvents (v* 23%) eoetonitrile nitromethane eoetone methylal 1: 2dichloroethane diohloromethane diethyl ether d&isopropylether 1: 1-diahloroethene n-pentene chloroform p-dioxene ethyl bromide dimethyl aulphoxide di-n-butyl ether n-hexme Gyczopentene triethylamine cerhon tetmohloride benzene di-twt. butyl dieulphide 1: 2-dibromoethane diethyl disulphide methyl iodide dibromomethane cyclohexene bromoform s-te.Wobromoethane carbon dieulphide di-iodomethane
8.7 7.4 9.3 7.7 7.1 8.0 10.6 12.1 8.0 20.6 7.7 6.7 9.0 6.6 10.6 17.1 16.4 13.4 10.9 7.4 7.8 6.3 7.6 7.6 6.7 12.2 6.6 6.0 10.8 6.8
CBr, (v, 680.6 om-‘) 16.9 24.3 21.4 18.7 14.0 6.6 22.2 11.9 12.1 4.9 6.1 12.3 8.6 14.8 11.3 6.4 7.1 16.0
di~txxGc molecules much more than that of carbon tetrebromide. The opposite behaviour of the half band-widths of vSof carbon dioxide and carbon tetrabromide indeed is striking, in view of the fact that the frequency shift beheviour of the two solutes is so similar. The widest vS absorption of carbon dioxide was obtained in hydrocarbon solution, where the solubility if the solute also appeared to be least, LASCOMBEet al. observed that for diatomic molecules on passing from non-polar to polar solvents with which the solute is capable of ctssociating, the shoulders disappear and the band assumes a simple form. Using carbon dioxide, on passing from
[25] D. J. JEKVELL and H. W. THOMPSON,Spctrochim Actu 18,254(1968). [26] M. L. JOSIEN J. LASCOMBE,J. Chim. Phys. 52, 162 (1955). [27] J. LASCOMEE,P. V. HUONU and M. L. JOSIEN, BUZZ.Sot. Chim. France, 1175 (1969). [28] W. E. OSBERU and D. F. HORNIU, J. Chem. Php. fu), 1345 (1952).
1
I
2320
23!50 cm-’
Acehe
2329
2350
p-diaxane
2320
Fig. 5. Variation in width and contour of the vs absorption of aarbon dioxide in different solvents.
2350
n-pentone
I
tjj
P
Perturbation of some vibrational bands in solution
791
aliphatic hydrocarbons to solvents more likely to impede the rotational motion, the band does become narrower, but an additional absorption is observed always ~12 cm-i to lower frequency than the main band. The appearance of this secondabsorption appears to be correlated with nothing apart from the decrease in width of the major absorption. It cannot be attributed to free rotation, nor to solute-solvent complexes, and it is here assigned to the hot band va + va - va. Using the spectrum obtained with p-dioxane as the solvent, the intensity of this band was estimated to be of the order of 5-10 % of the total absorption. At the temperature used in these measurements the fraction of the solute molecules excited to the vastate, (2eWtialkr)is also close to 10%. The assignment of this band as a “hot transition” is confirmed by the measurements of YAMADaand PEESON [21] and OSBERUand HORNIG [28] on films of carbon dioxide at liquid nitrogen temperatures. Under these conditions the half band-width is ~7 cm-l, yet there is no detectable lower frequency absorption except possibly when using very thick films, when the hot band absorption amounts at most to a small fraction of that due to the weak WO, band at 2280 cm-l. This observation is interesting in relation to the vibrational spectrum of carbon dioxide, but does not bear directly on the argument of the present paper. The main conclusion drawn is that carbon dioxide, though impeded by the action of the solvent, nevertheless retains some rotational energy in solution, and this may be one of the factors contributing to the difference between the solvent order shown by the shifts of carbon dixoide and carbon disulphide. AohmowZedgemente-Iwish to expressmy thanks to ProfessorC. A. COWLSON, Dr. L. J. BELLAMY and Mr. K. COUPL~WD for their interest and helpful oomment at various times, and to The Directors of The Distillers Chemioalsand Plastios Limited for permissionto publish this work.
Acts,VoL ZW,pp.779 to791. 3?~0npresS
1SdS.hintedin NorthantI~ei8IId
Perturbationof some vibrational bands in solution D. 33. CUNLIFFE-JONES Distillers Chemioals and Plastioa Ltd. Reseamh Department Saltend, Hedon, Hnll (Received 8 May 1966; rev&d 4 September 1968) .&&&--Shifts of the infrared active stretching vibrations of four non-polar molaoufes in solution have been measured. The resultad&agreestrikingly in some instanues with the predir&ions of “dielectric theories”, and it appears that the type of long-range i&era&ion envisaged by the ‘%&ion field” model plays no sign&ant r&e. This supports the view, given by COUWON[l], that the fluctuating electrostatio field of the solute molecule is greatly screened by the first layers of eleutronio charge that it meets. The shifts obtained using carbon tetrachloride end carbon tetrabromide as solutes are attributed to the effects of inters&ions involving the peripheral atoms of the solute and neighbouring solvent atoms. Doublet absOrptions in some aliphatio ether solvents and the halfband width variation of vs of cartron tetrabromide provide evidenoe of separate contributions to the shifts, arising from collisional interactions involving different parts of the solvent moleoules. Quite different behaviour is shown by the linear solute carbon disulphide, and this is attributed to additional contributions which arise from the alignment of solute and solvent bonds. The alignment of bond dipoles, rather than interactions between whole molecules, also explains the large shifts often observed using polar solutes in solvents such es p-dioxane, which by virtue Of their symmetry have small dipole moments, low dielectric ~0~~~ and yet are not highly polarisable. CsrbOn dioxide does not show these additional effects to any marked degree at normal temperatures. This is attributed in part to the retention of rotational energy by oarbon dioxide in solution, for which some evidence is given. In solvents where the width of the absorption of carbon dioxide was sufficiently narrow, an additional absorption was observed approximately 12 cm-i to lower frequency. This is assigned to the hot band vs +- vs - vs. INTRODUCTION
attempts have been made to interpret the changes in vibrational frequencies observed when molecules pass from the gaseous state to solution in terms of some hind of solute-solvent interaction. The most attractive theory is that of BUCKINQEAM [Z-d], in which the solvent shifts are found to be proportional to an expression involving the fbst and second derivatives of the interaction energy, U, with respect to the normal co-ordinates. For diatomic molecules the fractional shift is given by SEVIZW
and for polyatomic molecules an expression of similar form is obtained. If as a simplification the term in 77”is neglected, it has been shown (HE&D C. [2] A, (33 A. 141 A. [&I C. [I]
A. COULSON, Proc. Roy. ~5’00.. &%6, 09 (1960). D. &JQ9nuoIEaaa, Proc. Roy. Soo. Af3Bs, 169 (1968). D. &WKINQlUX, Proc. Roy. 800. #66, 32 (1969). D. BUCXINQEAM,True. &WU&Z~AS%.#, 753 (1960). E&m snd 11. w. TEOXl’sON, PTOG. &ye SOG. A@& 39 (1962). 779
and
D. B.
780
CumE-Jams
THOXPSON [5] and ROTRSCHI.LD [S]) that pre~~tions based on the theory, when~pp~ed to symmetrical linear polyatomic molecules, fail. Yet the theory as it stands is difficult to apply in terms of several types of intermolecular forces, and it would be convenient if the use of a simple model describing the interaction were found satisfactory. Buckingham therefore introduced the “reaction field” model, which had been used in the earlier treatment of BAUER and M&+AT [7], where the field, .F, acting on the solute molecule due to the charge distribution of its neighbours is taken to be proportional to its dipole moment. Thus, F = gs . m, + gn . Am where m, and Am are the eq~b~urn dipole moment and the oscillatory dipole resulting from the vibration, and gE,g, are consents which are simple function of the dielectric co~tant and refractive index of the solvent medium. Assuming that terms arising from “non-dipolar” interactions are the same for all solvents, and neglecting higher terms in g as a rough approximation, the frequency shifts in polar solvents are then given by the equation: Ahw 0
-
==c+Gh(&JJ -tG&$)
According to this “dielectric theory” the variation in the shift from solvent to solvent arises from a general non-specific type of long range dipolar interaction. However, considerable doubt has been cast on the validity of the reaction field model evidence (Bw (LAS~LOand &&JSHXREs]), and there is much expe~en~l et d. [9] ; BE~AMY and WAITS [lo] ; BEX,L.UVXY and HAIUM [ll] ; BIUZAXY and WIUIAMS [12]) which suggests that speoiflo, relatively short range, interactions are usually of dominant importance. It is now common to take some intermediate position, saying that both local interactions and bulk dielectric effects may be significant, their relative importance changing considerably from case to case (BEY&ANY and WIIUAMS [12]; CBLDOW et al. [13]; HALLAMand RAY [14]; RATAJCUK and ORKCLLE-THOMAS [ 151. However, it has been pointed out by COUIXJON [I] that the fluctuating eleotrostatic field of the solute molecule will be greatly screened by the iI.rstlayers of electronic charge that it meets. Since these will be by far the most polarizable parts of the neighbo~ng solvent atoms (ABBOT and BOLTOX[16]) it should be expected that this field will be damped in the outer parts of the first co-ordination shell. It follows that the relatively long range dieleotric approach will in general play no [Sl W.
G. ROTHNYHXLD,J. Chem. Hays 49,694 (1966). [7] E. BAUER and M. MAUAT, J. Phye. Radium 9, 319 (1938). [8] P. LASZLO and J. I. MUSHER, J. Chem. P&p. 41, 3906 (1964). [Q] L. J. BELUAXY, H. E. HALLAM md R. L. Wmm, Pram. Faraday Sot. &1120 (1958). [lo] L. J. BELLAIKY and R. L. WILLIAMS, Trace. Yamday Boo. & 14 (1969). [ll] L. J. BELLAXY and H. E. -AM, Tram. Far&y Soo.&,220 (1959). [12] L. J. BELLA~ and R. L. Wnmamas, Pvoo. Roy. Sot. A$@, 22 (1960). [13-j G. L. CALDOW, D. CUNLIIFFE-JONESand H. W, TEIOMPSON,PPOC.Roy. Sot. A%%, 17 (1960). [14] H. E. HALLAM and T. C. RAY, Trans. Far&f/ Soo. ris, 1299 (1962). 1161 H. RATAJCUK and W. J. ORVILLE-THOMAS, Tram. Fcwatfag Sw. 81,2603(1965). [I61 J. A. AESBOPIY and R. C. BOLTON, Pmt. Roy. Sot, A916,477 (1963).
Perturbation of some vibrational bands in solution
781
s~~c&nt role, md it would seem more appropriate to attempt an ~~rp~t&tion solely in terms of the derivatives of short range intermolecular forces. The results described in this paper provide strong support for this view. It is proper first to enquire what evidence there is to have suggested any contribution from “dielectric effects”. BELUMY snd WIGWAMS [ 121 suggested that some dielectric contribution is implied by the fact that many plots of the relative frequency shifts of one compound against those of another give straight lines which do not pass through the origin. But if the shifts result from the effects of several types of short range interaction, unless the different contributions vary in the same proportion for each solute, on passing from solvent to solvent, an exact fit to a straight line would not be expected. There should, in general, be some scatter of the points about the best mean line, and it would be fo~~~us if this line passed through the origin. The Bellamy scatter in theseplots is infact~equen~y~a~rth~n theexpe~en~le~r. and Williams also consider the reduction of ~20 om-l in the OH stretching frequency of 2%di-tet$-butyl p-ores01 on passing from v&pour to hexane solution must be attributed to dieleotric effects. But, although there may be no dimerization or other interaction directly involving the hydroxyl group of the hindered phenol, the aasumption that short range iutercactionsat other sites in the molecule cannot lead to this frequency shift is doubtful. More generally, some contribution of dielectric effects has been argued from the approximate fit which is sometimes observed with the Kirkwood-Bauer-M&g&t, or Buckingham relations. This however could well be fortuitous, since contributions arising from short range directions dependent on solvent bond polarities and pol~~b~~s will be crudely related to the bulk properties of dielectric constant and refractive index. Agreement has also been clf$med occasionally between changes in band intensities rendcalculations based on simple dielectric field theories (YAMADA end PERSON[17], RATAJCZLLK and ORVILLE-THOMAS [15]. But the interpretation of intensity changes in terms of dielectric effects has been disputed in recent work by IOCUNSEN et al. [18]. There thus appears to be no wholly convincing evidence for the contribution of bulk medium effects. Much of the experimental data reported so far has been concerned with the shifts of stretching vibrations of polar solutes, but a better assessment of the different interpretations may be obtained by making similar measurements using non-polar solutes, with which strong specific &ssoci~tionsare not expected. Shifts &sing from the effect of the much red&d reaction field (now related only to the oscillstory dipole of the molecular vibration) should increase in a uniform manner according to s, function of the solvent refractive index, and there seems no reason to expect, from this approach, any radical differences in the behaviour of non-polar solutes of different geometry. However, if the shifts arise mainly from short range interactions some differences may be expected. For compact, nearly spherical non-polar solutes the shifts should arise simply from interactions between the peripheral solute atoms and neighbouring solvent atoms. These interactions will probably include coulombic, inductive and dispersive forces. Both inductive and dispersive contributions to the shift will increase with solvent atom polariz:ability, but, in view of the anisotropy of [I77 H. YAMADA ssd W. B. PZSCSON, J. Chepn.phg8.4&309 [18] A. V. IOUAXSEN, E. V. BROaS a.ndG. T.3.~TOV~NKO,
(1964). et.
&e&y
18,18 (1965).
D. B. a-JONES
782
polarizability of most solvents, the behaviour should be less regular than expected from the reaction field model. However, for linear non-polar solutes (as for polar solutes), the alignment of solute and solvent polar bonds in temporarily attractive complexes may also be important, and this should lead to a more radical departure from the expected behaviour of the reaction field model. This contribution should be appreciably affected by steric considerations and will also be temperature dependent. Further complications may also be expected in some cases, if the vibration involves appreciable motion of other atoms not involved in the aligning solute bond, and also when interactions involving different sites in the molecule interfere. It is of some interest that both carbon tetrachloride (RATAJCZAE and ORVILLETHOM.M[ 161)and carbon disulphide (YAMADAand PEISSON[ 171)have been described as “ideal solutes” which illustrate the predictions of the dielectric theories. The present paper describes measurements of the infrared active stretching vibrations of carbon tetrachloride, carbon tetrabromide, carbon dioxide and carbon disulphide in a wide variety of solvents. EXPERIMENTAL METHOD
All the solutes and solvents used were commercial samples, 99% pure or better. This purity was considered quite sufficient and no further purification was attempted. A stream of carbon dioxide gas was obtained by evaporation of solid carbon dioxide, and allowed to bubble through the solvents used. Bands of suitable intensity were then obtained with this solute using pathlengths of O-13 mm; in some cases where the solubility in the solvent was less the pathlength was increased to 0.5 mm. Some of the measurements were obtained using a Hilger H800 spectrophotometer fitted with a sodium chloride prism, but the measurements on the ys bands of carbon tetrabromide and carbon dioxide were made using a Grubb-Parsons Spectromaster using 1600 and 7600 line/in. gratings respectively. The absolute accuracy of the measurements using the Hilger instrument was f 1 cm -l. Repeated measurements suggested a reproducibility within the tier limits of f0.6 cm-l. The accuracy and reproducibility of the Spectromaster were at least ,tO*4 cm-l. The frequency shifts are quoted to the nearest half wave number, except in the case of broad or overlapped bands where the shifts are given approximately. Solvent absorption was eliminated in all cases by the use of a variable pathlength compensating cell in the reference beams of the spectrometers. RESULTSANDDISCUSSION 1. Tetrahedral and linear non-polar solutes
Table 1 lists the shifts of frequency on passing from gas to solution of the antisymmetric stretching vibrations of two tetrahedral and two linear molecules. The Fermi-resonance doublet of carbon tetrachloride, of which the antisymmetric stretching vibration, va, is the high frequency component, has been measured before in a number of solvents (TUOMIICOS~I [19], LISITSAand OVANDER[ZO]), but attention was directed mainly to intensity changes and the unusual %olvent order” followed [I91 P. TUO~~~I~OEIKI,J.C~~~.P~~~.~,
2083(1956). [ZO] M. P. LISITEJA and L. A. OVANDER,Opt. Spectry 7, 383 (1969).
Perturbation of some vibrational bands in solution
783
by the shifts of rs seems to have escaped notice. No previous rn~~rnen~ appear to have been recorded in the case of carbon tetrabromide. In calculatiug the shifts the v&pour &equencies of the two tetrahedral solutes were taken as the band absorption maxima 795.5 cm-* for oarbon tetrachloride and 6804 cm-l fox carbon tetrabromide. Table 1. Shifts of the antisymmetrio atretohingvibrations of tetrahedral and linear solutes to lower frequenoieson pressingfrom gas to solution (cm-l)
w&mitri1e
nitromwthellw
CCI, (Yt 796.6 w-1) 3.6 4
se&me methyld
3.6 -
1: 2-di~~orne~e di~~ommeth~e diethyl ether d&isopropyl ether I&diohloroethene II-pentane ohlomform pdioxane ethyl bmmide dimethyl sulphoxide di-n-butyl ether n-hexaue oyolopentane triethylamina carbon totraahloride benzene di-Wt. butyl diiulphide I : Z-dibmmoetbane diethyl disulpbido dimethyi diaolpbide methyl iodide dibmmo methane ayctohexane bmmoform a-tetrabromoethane carbon disulpbide di-iodomethane
6.6 6 -
8” 46 7 6.6 I 7.6 7.6 7 7.6 8 8.6 9 7.6 8 8 9.6 11.6
CBr, (Y*680-6 om-1) 4 -
6.6 ~7.6
-10
(Y*2S?cm-1, 7 7
% (v5 1636 on+) 16 -
8 9.6
18*6 -
10 10.6 10.6
16 l&6 14.6 12.6 16.6 12.6 17 17 16 20.6 14.6 11.6 12 16.5 19.6 13.6 17 l&b 19 18 13.5 19 20.5 -
-9.6
10~6
7 ~6 8.6 -9.5 7-6 8 12.6 8 10.6 10 11 11*S 11.6 8.6 12 16
IO.5 IO.6 11 11 11 11.6 11.6 12 12.6 12.6 13 13 13 13 13.6 :3*5 13.6 14 146 16 17.6
Some frequency shifts of ya of carbon dioxide in solution have been reported by and THOMPSON [5] who used a siugie beam Perkin-Elmer instrument with a Iithium fluoride prism. These measurements have now been repeated using a grating iustrument and the data extended to other solvents. Where comparisons are possible the agreement is generally satisfac~~. The vapor frequencies used in ~alc~at~g the shifts for carbon dioxide and carbon ~s~phide are 2349 and 1536.5 cm-l respectively. The frequency given for vs of carbon disulphide in the vapour state differs from that used by YAMADA and PERSON[17] who claimed that the frequency shift of carbon disulphide on passing from gaseous to condensed states demonstrated its ideal behaviour in accord with “dielectric theory”. The frequency they used WM that quoted by HERZBERU [22], probably derived from BAILEY snd CASSIE [23], and HEALD
[Zl] H.
YAMADA and W. B. PEBBON,J. Ciwn. Phys. 41,247s (1904). [22] G. EEERZBERQ, hfra-Red ar& Bamm Spectra of PoZgatmic MoZewZ~, Nostrand (1945). [23] C. R. BLILEY and A. B. D. CASSIE,Ppoc. Rag. Sot. 140,605(1933).
p. 277.
Van
D. B. Cum-JONES
784
it seems doubtful that this was checked. The frequency was later observed by PLYLER and HTJMPHREYS [24] to be 1535 cm-l, and measurements made for the present work using both prism and grating spectrometers confirm this value. The measurements of YAMADA and PERSONon carbon disulphide in condensed states therefore lead to an opposite conclusion, namely that the frequency shifts observed on passing to the condensed states are considerably different from those predioted by the theory. (a) Carbon tetrddoride and carbon tetrabrornide. As might be expected, the two tetrahedral solutes behave in a very similar manner. The “solvent order” of the shifts differs considerably from that shown by the shifts of the stretching vibrations of
Fig. 1. Relativefrequencyshiftsof carbontetrachlorideand caxbontetrabromide oomparedwith a fun&ionof solventrefractiveindex. typical polar solutes. In particular, polar solvents such as aoetonitrile and acetone give the smallest shifts to lower frequency. It is probable that this is attributable more to their low polarizability than to their polarity, for the most polar solvent used, dimethyl sulphoxide, does not in fact give the smallest shift, and this solvent has a higher refractive index and presumably higher average peripheral polarizability than acetonitrile or acetone. Although there seems to be a general increase of shift with solvent polarizability, plots of AV/Yagainst (nz - l/2%%+ 1) (Fig. 1) show considerable scatter. This does not suggest the uniform behaviour to be expected from the reaction field model, but could be accounted for in terms of interactions between the peripheral atoms of the solute and neighbouring solvent atoms. The fractional shifts Av/v are not greatly dissimilar in magnitude from those of s+-._~of acetone. This may be considered remarkable in view of the large reduction in the reaction field on passing from a polar to a non-polar solute, though it must be [24] E. K.
l?LWR
and C. J. Thx~~
YS, J. Rex. Nat1 Bur. Std. 89,
69 (1947).
Perturb&ion of some vibrational bands in solution
785
admitted that the shifts are not directly related to the interaotion energy but rather to its derivatives. A comparison of the data for carbon ~tra~~o~de and acetone (given in Table 2) sugg~~ that (1) in some cases short range ~~ra~tions make similar contributions to the fkactional shifts for these two solutes, and (2) in other cases there are probably additional coutributious to the shift of vo+, of acetone. The latter arise either from association of the oarbonyl group with solvent bond dipoles or ?r-electron systems (methy iodide, ethyl bromide, benzene, acetone, dimethyl sulphoxide, acetonitrile, nitromethane) or from hydrogen bonding effects (bromoform). [It is interestiug to note that the gre&est relative fkequenctyshifts quoted for both solutes occur with ~-i~omethane. The difference between these figures is rather Table 2. Gompazison of rehtive fmquenoy &ifts (Avjv) shown by VW acetone and pa of carbon tetraohloride (w=C n-hexene cyclohexane dietbyl ether triethylamine benzene carbon disulphide ethyl bromide a_dioxene aoetene I : 2.diehloroethene methyl iodide aoetimitrile nitromethane d~e~yl~pho~de bromoform di-iodomethene
Aaetane 1737 em-‘) 8.4’ 8.4. @*2? s.t3* 11*8* 11.w 11.8 12.7: 12*7t 13*3* 13.3+ 13-w 14.1. 16.6 18.7’ 17.3’
Carbon tetmohloride (pa 79&b am-1) 8.3 s-4 7.6 9.2 8.8 12 8-8 6.7 4.4 6.9 11.3 4.4 6 6‘9 l&l 14.6
of
A -0.4 -X*0 Cl.7 SO.6 43 -@2 I; -f-8+3 +a*4 +2 +s*4 s.B.1 +e+ 46.8 j-2.8
* JIBWEIL and Txo~onr [26]. t JOSIEN~~~ LA&lCOW%E[~6]. 2 BELLAMY and WELLXAMS[lo].
small suggesting that dispersion forces are domiuant and that the large shift of the carbonyl band of acetone in this case owes very little to the low polarity of the solvent (N1D) or to its weak hydrogen bonding capability.] Detailed study of the V, abso~tion of carbon ~trac~o~de is complicated by Fermi-resonance; the ya absorption of carbon tetrabromide however is not complicated by any overlapping bands and it is possible to study changes in band width and shape (Pig. 2, Table 3). It is seen in Fig. 2 that, although the ~~~a~ band intensity remains almost cogent, there is a large variation in the half band-width. If the perturbation of the absorptiou is the result of short range interactions, the half band-widths will reflect the structural anisotropy of the solvent and increase with the difference in the perturbing effects of different parts of the solvent molecule. This certainly appears to be true, for the half band-widths are least in hydrocarbon solution (5 cm-l), where the immediate environment should be reasonably uniform, and Hearst in solvents such as a~~~t~e, ~-iodomethane, dioxane and acetone (16-24 om-1) where large di~e~n~s between the effects of the different groups are suggested by the frequency measurements.
The separate ~ntribution of short range perturbiug ~~ra~tio~ involviug diEerent pafts of the solvent molecule, which this half band-width variation suggests, is illustrated more dramatically by the us absorption of carbon tetrabromide in a series of aliphatio ether solvents. The absorption in some cases appears as an overlapped doublet. The decrease in intensity of the higher frequency component of this doublet through the solvent series p-dioxane, methylal, dðyl ether, d&isopropyl ether, di-n-butyl ether clearly establishes that this arises from perturbing interactions involving the ether oxygen atoms. This component appears to be only very slightly displaced from the vapour frequency, whereas the second component shows a shift comparable to that obtained in aliphatic hy~~arbon solutions. In dioxane or methyl solution, where theresho~d be agreater incidence of eo~sional in~raetio~ involving solvent oxygen atoms, the two components are of comparable strength and overlap extensively to give broad bands similar in shape and position to that obtained in acetone (see Fig. 2). It is clear from these results that no interpretation solely in terms of the reaction field model is adequate for solutes such as carbon tetrachloride and carbon tetrabromide, but the results can be explained quite simply in terms of short range interactions. Finally, mention should be made of the unexpectedly large shift found using carbon tetrabromide but not with carbon tetraobloride in triethylamine. Repeated measurements contied the results, but it was found that the carbon tetrahalides react with this solvent, the rate of reaction increasing considerably through the series CC&, CBr4, CI,. It is possible that the large ahif%obtained using ~bon~~abro~de in triethyla~e results from some ~~raction involving the highly ~la~zable s(pa lone pair orbital of the solvent prior to reaction. (b) Ca&oa &ox& and carbon &m&Aide. The shifts obtained using carbon disulphide as a solute show the marked contrast expected for a linear solute from those of carbon tetrachloride and carbon tetrabromide (see introduction above), the solvent order now roughly following that shown by the shifts of vo_o of aeetone (see Fig. 3). In particular, the shifts obtained in acetone and acetonitrile are now greater than in aliphatic hydrocarbon solution, and the shift obtained using dimethyl sulphoxide as solvent is as great as that in the highly polar&able a-tetrabromoethane. These results disagree strikingly with the predictions of the dielectric theory when applied to a solute of no permanent dipole moment. In terms of short range interactions they may be simply explained as arising from an ad~tional ~nt~bution to the total shift which becomes possible with the linear molecule, namely the alignment of solvent dipolar bonds with solute bonds, as illustrated in Fig. 4. The greater shift also observed in benzene is probably due to the anisotropy of polar&ability of this solvent, since in the event of an alignment involving the benzene r-electron cloud and the linear solute, the greater polarizability of benzene in the longitudinal direction will be appropriate. The increased shift in Ip-dioxane also is attributed to alignment involving the polar ether links at either end of the solvent molecule. The alignment of bond dipoles, rather than interactions between solute and solvent as whole molecules, provides a simple explanation of the large shifts obtaiued with many polar solutes in solvents such as p-dioxane, which merely by virtue of their
Q?-
@I-
02-
0.3-
o-
0.1 -
O-2-
a3-
CM-
05”
0.6 -
15 145
7
6 145
Linear wavelength
8
15
scde,
p
14.5
9 I5
14-5
IO 15
Fig. 2. Variation in width and contour of the vs absorption of carbon tetmbromide in different solvents. 1. oyclohexcme, 2. carbon diaulphide, 3. ethyl bromide, 4. aoetonitrile, 6. aoetone, 6. p-dioxane, 7. methylal, 8. diethyl ether, 9, di-isopropyl ether, 10. di-n-butyl ether. The solutions were all of similar concentration, 0~0006-0~0082 molar.
I45
6
/;\/-/\A\
.
1
3
D. B. ~-JONES
788
AV/VXI~ for
AVIVXI@ forF+of acetone
yof CBq
Fig. 3. Compaison of relativefrequencyshifts.
8-
S+
8-
Fig. 4. Suggestedalignment of CS, solute with solvent containing accessiblepolar bonds in temporarily attraotive collision complexes.
symmetry have a small dipole moment and low dielectric constant, and yet are not highly polarizable. The effects arising from the alignment of polar bonds will be temperature dependent and it is possible that studies of the variation of “solvent order” with temperature may be helpful in sorting out the different effects in some cases. It would be expected from the above interpretation that as the temperature is raised the solvent order obtained with both Yeof carbon disulphide and yo_o of acetone will tend to approach that shown by the ~avibration of carbon tetrachloride. It is remarkable that the shifts of va of carbon dioxide do not follow the same pattern as those of vQof carbon disulphide. Instead they closely follow the solvent order obtained using the tetrahedral solutes. HEALDand THOMPSON [5] suggested an interpretation of the shifts of V~of carbon dioxide in terms of dispersion and dipoleinduced dipole interactions involving solute and solvent as whole molecules. However, the marked contrast with carbon disulphide cannot be readily explained on this view and the results obtained with the tetrahedral solutes and carbon disulphide argue strongly in favour of shorter range interactions. Two suggestions may be made to account for the lack of similar bond alignment effects to those noticed with carbon disulphide. One arises from the considerable difference in the length of the two solutes. With the smaller carbon dioxide molecule the electrostatic field of the aligning solvent bond C-X may be cast more effectively over the whole molecule. The opposing polarities of the two solute bonds might then
Perturbation of some vibrational bands in solution
189
lead to a decrease in the attractive force. The second possibility is that carbon dioxide may retain much more rotational energy than carbon disulphide in solution and this will inhibit the alignment. Evidence for some retention of rotational energy by carbon dioxide in solution is provided by variation in the shape and width of the vaabsorption (Fig. 4). There is less evidence of completely free rotation than observed by LASCO-E et al. [27]for some diatomic molecules, but the width of the band in some solvents and the variation of half band-width with solvent (see Table 3) seems to parallel the behaviour of the Table 3. Half bandwidths of vs of oarbon dioxide md vs of oarbon t&r&bromide in various solvents (v* 23%) eoetonitrile nitromethane eoetone methylal 1: 2dichloroethane diohloromethane diethyl ether d&isopropylether 1: 1-diahloroethene n-pentene chloroform p-dioxene ethyl bromide dimethyl aulphoxide di-n-butyl ether n-hexme Gyczopentene triethylamine cerhon tetmohloride benzene di-twt. butyl dieulphide 1: 2-dibromoethane diethyl disulphide methyl iodide dibromomethane cyclohexene bromoform s-te.Wobromoethane carbon dieulphide di-iodomethane
8.7 7.4 9.3 7.7 7.1 8.0 10.6 12.1 8.0 20.6 7.7 6.7 9.0 6.6 10.6 17.1 16.4 13.4 10.9 7.4 7.8 6.3 7.6 7.6 6.7 12.2 6.6 6.0 10.8 6.8
CBr, (v, 680.6 om-‘) 16.9 24.3 21.4 18.7 14.0 6.6 22.2 11.9 12.1 4.9 6.1 12.3 8.6 14.8 11.3 6.4 7.1 16.0
di~txxGc molecules much more than that of carbon tetrebromide. The opposite behaviour of the half band-widths of vSof carbon dioxide and carbon tetrabromide indeed is striking, in view of the fact that the frequency shift beheviour of the two solutes is so similar. The widest vS absorption of carbon dioxide was obtained in hydrocarbon solution, where the solubility if the solute also appeared to be least, LASCOMBEet al. observed that for diatomic molecules on passing from non-polar to polar solvents with which the solute is capable of ctssociating, the shoulders disappear and the band assumes a simple form. Using carbon dioxide, on passing from
[25] D. J. JEKVELL and H. W. THOMPSON,Spctrochim Actu 18,254(1968). [26] M. L. JOSIEN J. LASCOMBE,J. Chim. Phys. 52, 162 (1955). [27] J. LASCOMEE,P. V. HUONU and M. L. JOSIEN, BUZZ.Sot. Chim. France, 1175 (1969). [28] W. E. OSBERU and D. F. HORNIU, J. Chem. Php. fu), 1345 (1952).
1
I
2320
23!50 cm-’
Acehe
2329
2350
p-diaxane
2320
Fig. 5. Variation in width and contour of the vs absorption of aarbon dioxide in different solvents.
2350
n-pentone
I
tjj
P
Perturbation of some vibrational bands in solution
791
aliphatic hydrocarbons to solvents more likely to impede the rotational motion, the band does become narrower, but an additional absorption is observed always ~12 cm-i to lower frequency than the main band. The appearance of this secondabsorption appears to be correlated with nothing apart from the decrease in width of the major absorption. It cannot be attributed to free rotation, nor to solute-solvent complexes, and it is here assigned to the hot band va + va - va. Using the spectrum obtained with p-dioxane as the solvent, the intensity of this band was estimated to be of the order of 5-10 % of the total absorption. At the temperature used in these measurements the fraction of the solute molecules excited to the vastate, (2eWtialkr)is also close to 10%. The assignment of this band as a “hot transition” is confirmed by the measurements of YAMADaand PEESON [21] and OSBERUand HORNIG [28] on films of carbon dioxide at liquid nitrogen temperatures. Under these conditions the half band-width is ~7 cm-l, yet there is no detectable lower frequency absorption except possibly when using very thick films, when the hot band absorption amounts at most to a small fraction of that due to the weak WO, band at 2280 cm-l. This observation is interesting in relation to the vibrational spectrum of carbon dioxide, but does not bear directly on the argument of the present paper. The main conclusion drawn is that carbon dioxide, though impeded by the action of the solvent, nevertheless retains some rotational energy in solution, and this may be one of the factors contributing to the difference between the solvent order shown by the shifts of carbon dixoide and carbon disulphide. AohmowZedgemente-Iwish to expressmy thanks to ProfessorC. A. COWLSON, Dr. L. J. BELLAMY and Mr. K. COUPL~WD for their interest and helpful oomment at various times, and to The Directors of The Distillers Chemioalsand Plastios Limited for permissionto publish this work.