Far Ultraviolet Solution Spectroscopy of the Iodide Ion BY MALCOLM F. Fox,* School of Chemistry, Leicester Polytechnic, Leicester LE1 9BH AND ELIE HAYON Pioneering Research Laboratory, U.S. Army Natick Laboratories, Natick, Massachusetts, 01760 U.S.A. Received 21st April, 1976 The far ultraviolet absorption spectra of iodide ion have been studied in a large range of solvents and over a large temperature range. These include both strong blue- and red-shifting solvents relative to water and D20. A number of new absorption bands have been observed, in some cases down to 165 nm. The observed absorption spectra are envelopes composed of overlapping absorption bands which have been resolved, using mainly lognormal (asymmetric) band shapes, to show sets of band pairs labelled Al /A2, Bl/B2 etc. The first characteristiccharge-transfer-to-solvent(c.t.t.s.) doublet, Al /A2, has a separation close to the doublet splitting of the 'P atomic states of the iodine radical in the gas phase. The BIB2 higher energy doublet bands have a slightly smaller separation and have c.t.t.s. character such as high solvent and temperature sensitivities. At higher energies further bands are observed. The band systems observed have been correlated on the basis of solvent shifts, doublet band splittings, oscillator strengths and temperature coefficients. The c.t.t.s. character of the higher energy bands has been evaluated and tentative assignments suggested. The charge-transfer-to-solvent (c.t.t.s.) absorption spectra of simple anions in solution are intense, broad bands which do not show structure over large changes in temperature. Theories and applications of c.t.t.s. spectra have been extensively de- scribed in reviews by Rabinowitch,l Orgel and more recently by Blandamer and Fox which show that c.t.t.s. spectra are uniquely sensitive to solvent, organic co- solvents, added electrolytes, temperature or pressure. The spectra are cation inde- pendent at suitably low concentrations and act as microprobes of ion-solvent and solute-solvent interactions. Both theoretical treatments and experiment have con- centrated on the lowest energy band of iodide, Al, because of experimental difficulties which have previously limited detailed studies to the region below 50 000 cm-l. The various theoretical models proposed for c.t.t.s. spectra have an underlying unity in the strong spectroscopic evidence for an excited electronic state which is predominantly defined by the solvent and centred on the parent anion site. The mounting weight of evidence has, however, demonstrated the need for further refine- ment of spectroscopic models for c.t.t.s. transitions. First, application of current models for c.t.t.3. spectra to those for the pseudo- halide ions has been shown by Blandamer and Fox and Fox and Hunter to give values for the ionisation potential of the ion which are often very different from those obtained using other techniques. Second, very little work has been published on band A2 for halides. The temperature coefficient, dv,,,/dT, of band A2, has been shown to be approximately the same as that for Al of iodide in aqueous solution, as is also the absorption coefficient, for band A2 is always substantially greater than that for band Al, but no systematic studies have been In other solvents 1003 Published on 01 January 1977. Downloaded by UNIVERSITY OF NEBRASKA on 01/09/2013 10:27:38. View Article Online / Journal Homepage / Table of Contents for this issue
Transcript
Far Ultraviolet Solution Spectroscopy of the Iodide Ion
BY MALCOLM F. Fox,* School of Chemistry, Leicester Polytechnic, Leicester LE1 9BH
AND
ELIE HAYON
Pioneering Research Laboratory, U.S. Army Natick Laboratories, Natick, Massachusetts, 01760 U.S.A.
Received 21st April, 1976
The far ultraviolet absorption spectra of iodide ion have been studied in a large range of solvents and over a large temperature range. These include both strong blue- and red-shifting solvents relative to water and D20. A number of new absorption bands have been observed, in some cases down to 165 nm. The observed absorption spectra are envelopes composed of overlapping absorption bands which have been resolved, using mainly lognormal (asymmetric) band shapes, to show sets of band pairs labelled Al /A2 , Bl /B2 etc. The first characteristic charge-transfer-to-solvent (c.t.t.s.) doublet, A l /A2 , has a separation close to the doublet splitting of the 'P atomic states of the iodine radical in the gas phase. The BIB2 higher energy doublet bands have a slightly smaller separation and have c.t.t.s. character such as high solvent and temperature sensitivities. At higher energies further bands are observed. The band systems observed have been correlated on the basis of solvent shifts, doublet band splittings, oscillator strengths and temperature coefficients. The c.t.t.s. character of the higher energy bands has been evaluated and tentative assignments suggested.
The charge-transfer-to-solvent (c.t.t.s.) absorption spectra of simple anions in solution are intense, broad bands which do not show structure over large changes in temperature. Theories and applications of c.t.t.s. spectra have been extensively de- scribed in reviews by Rabinowitch,l Orgel and more recently by Blandamer and Fox which show that c.t.t.s. spectra are uniquely sensitive to solvent, organic co- solvents, added electrolytes, temperature or pressure. The spectra are cation inde- pendent at suitably low concentrations and act as microprobes of ion-solvent and solute-solvent interactions. Both theoretical treatments and experiment have con- centrated on the lowest energy band of iodide, Al, because of experimental difficulties which have previously limited detailed studies to the region below 50 000 cm-l.
The various theoretical models proposed for c.t.t.s. spectra have an underlying unity in the strong spectroscopic evidence for an excited electronic state which is predominantly defined by the solvent and centred on the parent anion site. The mounting weight of evidence has, however, demonstrated the need for further refine- ment of spectroscopic models for c.t.t.s. transitions.
First, application of current models for c.t.t.3. spectra to those for the pseudo- halide ions has been shown by Blandamer and Fox and Fox and Hunter to give values for the ionisation potential of the ion which are often very different from those obtained using other techniques. Second, very little work has been published on band A2 for halides. The temperature coefficient, dv,,,/dT, of band A 2 , has been shown to be approximately the same as that for A l of iodide in aqueous solution, as is also the absorption coefficient, for band A2 is always substantially greater than that for band A l , but no systematic studies have been
In other solvents
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undertaken until recently when, in a series of powerful red- and blue-shifting solvents, we observed iodide to have further absorption bands at higher energies.6 In addition, we have reported a second band pair, designated B,/B2, analogous to the A 1 / A 2 band pair but of lower intensity.6 Because of their lower intensities these bands are not immediately obvious and we have presented separately detailed numerical analytical arguments for their existence using both asymmetric (lognormal) and symmetric (normal) band shapes, as against a single, highly asymmetric A / A 2 band pair fitting the observed band profile^.^
From our preliminary communication and the numerical analysis of the absorp- tion profiles described above,’ it is clear that the solution spectra of solvated iodide are much more complex than could be explained by extended discussion of the A 1 / A 2 band pair and should be discussed in the context of additional band pairs and transi- tions of different character. Advances in far ultraviolet spectrophotometric tech- niques have enabled us to obtain spectra to a limit of 61 000 cm-’ (164 nm) in some solvents.8* We have applied these techniques using a variety of solvents over wide temperature ranges in a systematic study of iodide solution spectra.
EXPERIMENTAL
MATERIALS
Solvents were purified using the methods described previously.8 Triethyl phosphate was technical grade (MCB) and was not improved, for our purposes, by attempts to purify it. The solvent was dried with Drierite and filtered before use. Amides were dried with Drierite and then distilled very slowly under nitrogen, retaining only the middle third frac- tion. Hexamethylphosphoramide (PCR, Inc.) was used from freshly opened bottles. The iodide salts used were the potassium, tetramethylammonium and tetrahexylammonium iodides (Eastman Kodak) and were used as received.
SPECTROPHOTOMETRY
A Cary 15 “ low UV ” model recording spectrophotometer was used as described pre- v i o ~ s l y . ~ ~ All spectra were determined in the absence of air by flushing pre-purified nitro- gen through the solvents and solutions. Cylindrical cells of 1.0,O.l and 0.05 mm pathlength cells and 25 or nominally 7 x mm pathlength demountable cells (Beckman UV-01) were used in conjunction with more conventional rectangular cells. In the range 250-318 K, the sample temperature was controlled by circulating propyl alcohol, from a thermostatted and refrigerated bath, through the Cary cell holder, good thermal contact between the optical cell and its holder being maintained by copper plates and wedges. Below 250 K a variable temperature dewar unit with a TEM-1C controller with samples in an FH-01 cell (all Beck- man). The dewar unit was positioned in the sample compartment of the Cary 15 with the outer windows removed. The sample compartment was kept slightly above atmospheric pressure by the nitrogen purge, otherwise the solvent films rapidly disappeared when a vacuum was applied. Solution temperatures were either measured with a thermocouple unit (dewar unit) or with a thermistor system (YSI).
The spectra presented here obey Beer’s Law, in some cases over three orders of iodide concentration and are independent of the electronic gain of the instrument, showing the absence of stray light effects. The highest concentration used was 5 x mol dm-3. Significantly different effects are apparent at higher concentrations and these will be reported separately. The onset of stray light was repeatedly determined and the spectra presented are terminated before that All spectra presented here are also independent of the cation where significant variation of the cation is not effected by solubility problems. The molar absorption coefficients have been corrected for thermal density variations of the solvent, using literature values where available and otherwise by extra- or inter-polation.
D E C O N V O L U T I O N OF O V E R L A P P I N G BANDS
It is clear from the results presented below that the experimentally determined absorption spectra are envelopes composed of overlapping absorption bands. We have proceeded on the assumption that the absorption profiles are linear summations of lognormal bands or, where insufficient information is available on band asymmetries, normal bands. The limited range ultraviolet spectra of iodide and bromide in acetonitrile have been successfully analysed as the sum of two normal bands by Blandamer et The lognormal band shape has been applied to the spectra of large organic molecules and the ratio of halfwidths, p , which mea- sures the asymmetry of the band, found to lie between 1.3 and 1.5.11 But for aqueous iodide p is found to be 1.024, i.e., very close to the value for a normal band shape.12
We have used two levels of analysis. At the first level we have used a du Pont Curve Resolver and plotter, using normal curves, so as to obtain a preliminary analysis. Alter- natively, we have used an analogue system on a visual display unit of a computer, the PEAK program of Money l3 or our log-diff. method (DECON).14 The results from these analyses are then used as input for either the LOGFIT (lognormal) approach of Siano and Metzler l 1 or the GSAN (normal) approach of S c h w a r ~ . ~ ~ Asymmetric band shapes have been used for the final optimisation wherever possible and almost all results are presented for this band shape. Where p is only very slightly different from unity the band parameters are found to be very close to those obtained using GSAN. Where data are limited, such as for the amide solutions, only GSAN is used. The use of GSAN and LOGFIT has been described separate- Iy at length, as have also the statistical arguments in favour of the Bl/& band pair.7
RESULTS
The general thrust of this investigation has been to lower temperatures, for this sharpens c.t.t.s. absorption bands and aids the difficult deconvolution problems which arise. Second, the liquid ranges of solvents below ambient temperature are generally longer than those above, e.g., below 298 K butyronitrile has a liquid range of 137 K but only 98 K above. We have demonstrated that the transparency (the available spectroscopic range of solvents) is considerably enhanced by decrease in tempera- ture.s*
u1crn-l x
FIG. 1.-Far U.V. absorption spectra of iodide ion, 5 x mol dm-3, as the tetramethylammonium salt in acetonitrile at 230 K. Continuous lines represent the experimental curve, broken lines the
Extension of the spectrum of iodide as either the potassium, tetramethyl- or tetra- hexyl-ammonium salt in acetonitrile further into the far ultraviolet region shows features additional to the well-characterised doublet. At 230 K iodide shows at least three additional bands, see fig. 1, all well overlapped.
Resolution of the spectrum clearly shows the A l and Az components of the iodide doublet with separation, AvA, of 7304 cm-l. AvA is always < 7603 cm-,, the gas phase doublet separation value,3 which has always been assumed to apply previously. The temperature sensitivities, duAi/dT and duA,/dT, are - 22.85 and - 23.85 cm-l K-l, in
6C
St
m
2 x 5c
E e 4
1
J
4:
4c
D
- 18.7
I I I I I I I I I I I
25 0 3 0 0
TlK FIG. 2.-Plot of q (where i = AI, A2, B1, B,, D and E respectively) in acetonitrile for tetramethyl- ammonium iodide at 5 x mol dmW3 against temperature. Slopes have values in cm-’ K-l
obtained by a least squares fit procedure.
general agreement with previous reports. The oscillator strength (f) ratio of bands A , and A2,fAz/fA1 is 1.76, substantially greater than unity over the temperature range studied. Previous reports have attributed the increased absorption at the second, higher energy, peak to overlap of yet higher energy bands. These attributions have followed the conventional wisdom thatfAz/fAl = 1 .O, founded on the oscillator strengths for iodide in water, which we will show later to be a special case. However, resolu- tion of the far ultraviolet spectrum of iodide in acetonitrile showsfA2 $- fA1, table 1. Bands A l and A2 have been observed in eleven other solvents and examined over a wide range of temperature, see table 1 and the spectra of iodide presented below.
A feature of the resolved spectrum in fig. 1 is a low intensity band, B1, between A , and A2. We have previously demonstrated that all digital methods confirm the presence of the B1 band and give essentially the same parameters for A l , B1 and A2.7 Analogue methods give very similar results. A further band is resolved at higher energies than A2 which is very similar to B1 and is thus labelled B2. The separation between these bands is of the order of 6800 cm-l. The Bl/B2 band pair has been resolved in acetonitrile solutions of iodide over the temperature range 230-3 18 K and is unaltered by dilution from 5 x to 5 x lod6 mol dm-3 iodide concentration at 298 K. The temperature sensitivity of band B,, dvB,/dT, is -29.6 cm-1 K-l, higher than for bands A l and A 2 . Fig. 2 presents the temperature sensitivities of the resolved bands of iodide in acetonitrile as solvent. Similar results are obtained for all other solvents studied.
394 438 482 526 570 614
u1cm-l x FIG. 3.-Resolution of the absorption-spectrum of either potassium or tetramethylammonium iodides
at 5 x mol dm-3 in (a) butyronitrile at 153 K, (b) acetone at 180 K.
As the band pair A1/A2 is shifted to lower energies by change of solvent, the shifts of band B1 are even more pronounced. This effect may best be seen in the spectra of iodide in a series of solvents, Thus, in fig. 3(a) the large contribution of B1 to the apparent " first peak " of iodide in acetone at 180 K can be observed. In butyronitrile at 153 K band B, can be seen in the overall experimental spectrum close to 46 000 cm-l, see fig. 3(b).
Band B1 is found between bands Al and A2 in a large number of different solvents where V A ~ < 43 000 cm-1 at 298 K. Thus, the band pair B1/B2 is not only observed in the alkyl nitriles and acetone described but also in such further diverse solvents as
hexamethylphosphoramide, triethyl phosphate and dimethoxyethane. Despite the widely differing bulk physical properties of these media, such as permittivity, the posi- tion of band B1 in these solvents is solely related to the position of band Al, see fig. 4.
At higher energies the Al band of iodide in water (or D20) has been used exten- sively as a probe of solute-solvent interactions. The positions of the A l bands require that the B1 bands occur to higher energies than the A2 bands, see fig. 5. However, the spectra of iodide in aqueous and D20 solution are anomalous when compared with those in other solvents, for the ratio of oscillator strengths, fAz/fAl, is close to unity in water, lying between 0.98 and 1.13. In other solvents we find the
1.22 I E
1.09.
0.95 '
8 OSa2: 0.66.
. A P, 2 0.54.
(d 0.41 *
0.27 *
0.1 4 .
P
(4 op'
u/cm-' x FIG. 4.-Resolution of the absorption spectrum of tetramethylammonium iodide at 5 x mol
dm-3 in (a) triethyl phosphate at 253 K and (b) dimethoxyethane at 274 K.
ratio to lie between 1.3 and 1.9. The oscillator strength ratio is low because band A2 is much less intense than expected from comparison of band A2 in other solvents. On the other hand,fA, in water and D,O correlates well withfAl in other solvents.
Fig. 6 shows the absorption of iodide in two blue-shifting solvents, hexafluoro-2- propyl alcohol and trifluorethyl alcohol, both at 274 K. In these solvents, as in water and D20, band B1 would appear at higher energies than for A2. The extent of the blue shift for the fluorinated alcohols has moved B1 into the region of the high energy, high intensity band D. Limits of resolution for bands of differing intensities, widths and varying separation have been discussed by Morrey l3 and by ourselves in con- nection with the log-diff. method, DECON.14 Given the ratios of the parameters
for the B1 and D bands of iodide in the fluorinated alcohols, it is not possible to meaningfully separate the B, band from the D band in these spectra.
THE " D" AND " E" BANDS OF IODIDE
The higher energy bands of iodide, labelled D, E, etc., are more completely ob- served in the resolved spectra when the band pair A1/A2 is strongly red-shifted, as for iodide in propionitrile at 181 K (fig. 7) , triethyl phosphate at 253 K [fig. 4(a)] and in acetonitrile (fig. 1) at 230 K. In water and DzO, the higher energy transitions can also be observed (fig. 5 ) because of the increased spectroscopic range of the work reported here.
I I l l
*I A2 = I 0
0 . 8 4 ~
(b) 3 5 0 394 438 402 526 5 7 0 614
u/cm-' x
dm-3 in (a) D20 at 278 K and (b) H 2 0 at 318 K. FIG. 5.-Resolution of the absorption spectrum of tetramethylammonium iodide at 5 x mol
The higher energy bands are rather different in character from the A or B band pairs. The bands are very intense with large half-widths, e.g. , for bands D of iodide in acetonitrile at 230 K, u+ is 4658 cm-l compared with 2732 cm-l for band A l , with an oscillator strength of 0.35. In general the bands are so overlapped that digital deconvolution methods alone cannot be used in the initial analytical procedure and recourse has to be made to analogue methods as well. The profiles in the high energy transition region have been analysed using as few bands as possible consistent with acceptable resolution. These results have been used as inputs to the least- mean-squares minimisation techniques, GSAN and LOGFIT.
The D and E bands have apparent temperature and solvent sensitivities similar to the A I / A 2 band pair of fig. 2. A caveat must be entered here on the parameters of the highest bands in each set found for an iodidefsolvent system. These are
dm-3 in (u) hexafluoro-2-propyl alcohol at 274 K and (b) trifluoroethyl aIcohol at 274 K. FIG. 6.--Resolution of the absorption spectrum of tetramethylammonium iodide at 5 x mol
necessarily more inaccurate than those for lower energy bands of the same set, because a substantial part of the highest energy band is generally beyond the limits of experiment. No band pairs similar to the A 1 / A 2 and B1/BZ sets have been convincingly demon- strated as yet, although such pairings may be possible for the DIE bands of iodide in the alkyl nitriles and phosphate systems. The difficulty is that the higher energy compoiients of any band pair will be on, or just beyond, the limit of measurement. In general accurate and unique deconvolutions become more ill-conditioned as the edge of the spectroscopic range is approached.
6 0
5 5
m
2
E 5 0
-a 3
45
4 0
FIG. 8.-Plot of U i (where i = Al, A z , B1, B2, D and E respectively) as a function of U A ~ for each solvent system and temperature. Solid lines indicate the best-fitted line to each band with the indi-
cated slopes.
DISCUSSION
The main points reported above for the solvated iodide spectra are first, the Bl /B2 band pair, and second, a series of higher energy bands. Before discussing these, the results will be correlated on the basis of solvent shifts, oscillator strengths and tem- perature coefficients found in table 1 .
A quantitative comparison of the solvent shifts for the resolved bands in iodide spectra has been made using the c.t.t.s. solvent scale proposed by Treinin.16 Fig. 8 shows a plot of the band maxima positions as a function of vA1 for each band/solvent/ temperature system. Whilst band A l has a slope of 1.0 by definition, band A2 has a slope which is the same as Al and band B1 has a slope of 2.33. The same relationship probably exists for band B2 but because fewer data are available in this case to give a meaningful Comparison, a relative slope of 2.33 is plotted for these data. The shifts of the higher energy bands D and particularly E are more difficult to comment upon as these bands occur at the high energy limit of the absorption profile. Nevertheless, the shift of band B with solvent and temperature appears to be of the same order as that for A l at 1.08.
OSCILLATOR STRENGTHS
At 274 K E , , ~ for the A l band of iodide in hexafluoro-2-propyl alcohol is 1.78 x lo3 ni2 rnol-l, 25 % more than for iodide in water. It is noticeable that as U A ~ increases, then E,,, and U+ increase similarly. The oscillator strength, f, incorporates these parameters and fig. 9 shows the variation of bothf,, andfAz with their band
Q
um&rn-l x
FIG. 9.--Correlation of f . l and fA2 against V A ~ and U A ~ respectively for solvent systems at 298 K, best fitted straight line drawn, data taken from table 1.
maxima, uA, or uAz respectively, at 298 K for the complete range of solvents studied, the variation being quite striking from 0.1 19 for iodide in hexamethylphosphoramide at 37 993 cm-1 to 0.335 in hexafluoro-2-propyl alcohol at 48 665 cm-l. Apart from the special case of fAz for H 2 0 and D20, fA2 behaves exactly as expected for a high energy A l band. This means that band A , for iodide in trifluoroethyl alcohol at 47 696 cm-1 has an oscillator strength of 0.336 whilst band A2 for iodide in aceto- nitrile at 47 712 cm-l has an oscillator strength of 0.327.
R E L A T I V E I N T E N S I T I E S OF THE A N D A2 B A N D P A I R S
The relative intensities of the bands A and A2, expressed as their oscillator strength ratio,fA2/fAi, vary from 0.98 : 1 to 1.92 : 1 according to solvent at 298 K. Assuming as an initial premise that selection rules apply to a photoionisation process, the relative density of 'P+ and 'P+ states may be calculated on a simple multiplicity basis as 2 : 1. The lower energy band for iodide is the 2Ppt. state and the predictions from the simplistic approach are quite inconsistent with experiment. The isoelectronic atom, Xe, has a first transition pair which corresponds to 5P6('SO) - 5P56S1(3P1, 'P1) with relative intensities of 2 : 1 at 67 970 and 77 160 cm-l, respectively.17
Bauman and Smyth l 8 considered the transition strengths of the two final states of SeH resulting from the photoionisation of SeH- and found a relative intensity of 1 : 1, in agreement with the results found here for iodide in aqueous solution. The interaction between solute and solvent clearly alters the intensity ratios between the atomic states which comprise the doublet, an interaction which is clearly consistent with the previously established relationships between uA and fa. Donohue and Weisenfeld have demonstrated the variability of relative I(2P+) and I(2P3) concentra- tions on the influence of alkyl group structures in the photodissociation of alkyl iodides.
THE B ~ / B z B A N D F A I R
We have shown that the B1/B2 band pair is found using various deconvolution techniques and strategies, exists unaltered over three orders of salt concentration and has a solvent sensitivity which is solely related to the c.t.t.s. scale of Treinin through the Al/A2 bands. The weight of evidence is that the band pair is real, has c.t.t.s. character and does not arise from any type of ion pair formation. In the closely related field of rare gas and alkali halide spectra, the B1/B2 band pair could have been predicted, for there is good evidence for further sets of doublet absorption bands found to higher energies of the first characteristic absorption pair." Baldini has proposed that the series of absorption bands in the xenon spectra form a progression with quantum numbers 1 ,2 and 3, etc. * 9 A similar proposal was made by Franck and Platzman 22 in their original paper on solvated halide spectra, except that they proposed solvated electron states with quantum numbers of 2 and 3, etc. Indeed, they predict a second band set of a " Balmer-like series '' to occur at 180 nm or 55 555 cm-l for iodide in water. Further, Baldini's treatment of the Xe band pairs predicts that their relative intensities should vary as r3, thus predicting the ratios offAllfsl to be 8/l, assuming that YE = 1 and 2 for the A and B bands respectively. In this work the ratiofAllfBl for iodide has generally been found to be of this order and further supports the assignment ~f the B1/B2 band pair as arising from the same atomic states as does the A I / A Z pair. This assignment is further supported by recognising that any bands additional to the A l / A 2 must arise either from further solvated electron states or from higher energy iodine radicals. The latter is ruled out by the next iodine atomic state being 49 000 cm-1 above the 2P doublet. The B1/B2 band pair are entirely consistent with a centrosymmetric electron state (probably 2s) which is further delocalised into the solvent than is the case for the A 1 / A 2 band pair solvated electron state, which is probably 1s.
We find B1 for aqueous iodide at 54 433 cm-l.
T H E D A N D E B A N D S
The yet-higher energy bands appear to have the same order of solvent and tempera- ture sensitivities as the A 1 / A 2 band pair, see fig. 2 and 8, and therefore have c.t.t.s. character. Again, as higher energy iodine radical states are excluded on energy
grounds these bands presumably arise from solvated electron states. Their solvated electron states are unlikely to have s-character for higher s-states will probably decrease in intensity as r3. The iodide D and E bands must be transitions for solvated electrons of different character.
From the correlations presented above betweenfA and V A , the ground and excited states of the iodide, c.t.t.s. transition must be more closely coupled than has been pre- viously recognised. The iodide c.t.t.s. transition can no longer be regarded as a single photoionisation, for if p -+ s transitions are rejected for the higher energy bands, then a p --+ p transition is also rejected by selection rules leaving a p --+ d transition where the initial orbital is the highest filled 5P6 level on iodide. A d-solvated electron state would not be spherically centrosymmetric yet would be extensively delocalised into the solvent. It would be less affected by variations in the potential well radius than would be expected for an s-type state. The present evidence indicates solvent and temperature sensitivities for the higher energy bands which are similar to those for the band pair represented by A 1 / A 2 . The limit of measurement has hampered the iden- tification of any band pairs so far. Further study requires extension of the spectro- scopic range of instruments, solvents and cells.
MODELS FOR C . T . T . S . TRANSITIONS
The new results presented in this work required explanations beyond those given by current models for c.t.t.s. transitions. It would be premature at this stage to pro- pose a more sophisticated model based on the results for iodine alone, which we believe to be a failing of previous approaches. Evidence for similar phenomena is currently being obtained for a series of simple inorganic anions.23 The new model which will eventually be proposed must satisfactorily explain first, the proliferation of electron states within the potential well, second, the relationships between fA and 2)A
and third, the variation of thefA2/fAl ratio with change in solvent type. Previous models have used thermodynamic and electrostatic properties to evaluate
urnax for c.t.t.s. transitions. These are bulk properties and cannot be expected to describe individual solutes interacting with a solvent in both ground and excited states. An initial step towards introducing quantum mechanics into c.t.t.s. theory has been made by applying Mulliken charge-transfer theory to c.t.t.s. transition^.^ The con- figuration coordinate model also shows promise in explaining band shapes and posi- tions.12 Through these approaches and extended studies of a range of ions, a more detailed insight into the solvation of both ground and excited states of simple ions may be obtained.
' E. Rabinowitch, Rev. Mod. Phys., 1940, 14, 112. L. E. Orgel, Quart. Rev., 1954,8,422 ; see also, G. Stein in The Chemical and Biological Actions of Radiations, ed. M. Haissinsky (Massons, Paris, 1969), vol. 13, p. 119. M. J. Blandamer and M. F. Fox, Chem. Rev., 1970, 70, 59. M. F. Fox and T. F. Hunter, Nature, 1968, 223, 177. J. Jortner, B. Raz and G. Stein, Trans. Paraday Soc., 1960, 56, 1273.
€3. E. Barker, M. F. Fox, A. Walton and E. Hayon, J.C.S. Faradzy 1, 1976, 72, 344.
M. F. Fox, Appl. Spectr., 1973, 27, 155. l o M. J. Blandamer, T. R. Griffiths, L. Shields and M. C . R. Symons, Trans. Faraday Suc., 1964,60,
1524 ; M. J. Wootten, L. A. Dunn, D. E. Clarke and H. S. Frank, Nature Phys. Sci., 1971,233, 138.
D. B. Siano and D. E. Metzler, J.C.S. Faraday I, 1972, 68,2043.
' M. F. Fox and E. Hayon, Chem. Phys. Letters, 1972, 14,442.
* M. F. Fox and E. Hayon, J. Phys. Chem., 1972, 76,2703.
'' D. E. Metzler and D. B. Siano, J. Chem. Phys., 1969, 51, 1856.
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