The absorption spectra of triiodides - Royal Societyrspa.royalsocietypublishing.org/content/royprsa/158/893/167.full.pdf · 167 The Absorption Spectra of Triiodides By C. B. Allsopp,

167

The Absorption Spectra of Triiodides

By C. B. A l l so pp , M.A., Ph.D.

(Communicated by T. M. Lowry, —Received 8 , 1936)

The absorption spectra of the triiodides are of particular interest in view of the striking fact, to which attention was directed ten years ago by Professor Lowry,* that the same twin maxima of intensity occur at about 3500 A. and 2900 A. respectively in the absorption spectra both of iodoform CHI3 and of potassium triiodide K I3. This similarity could not be accounted for by the conventional theories of valency, which would assign three covalent C—I bonds to the former compound, whereas the most plausible formula for the other contains an I3~-ion, with the prob-

_|_-- 1--able configuration K [III].

Subsequent investigations revealed similar maxima in the spectra of a-dimethyltelluronium diiodide TeMe2I2 and of the so-called “ (3-diiodide ” [TeMe3] TeMeI4,f in those of the corresponding diethyl compounds TeEt2I2 and [TeEt3] TeEtI4,$ and in that of the diiodide of cyc/o-telluropentane C5H10TeI2.§ The formulae of these compounds have been written as containing either an I_-ion (in the diiodides) or covalently bound iodine (in the tetraiodides).|| Two bands, with a much wider separation than in CHI3 or K I3, have also been recorded in the spectrum of thallium triiodide T1I3 ;̂ [ they were used as evidence that the triiodide is neither a completely ionized thallic iodide nor a thallous periodide, but must contain at least a part of the iodine linked covalently to the metal. Finally, the original bands were again recorded in the spectra of Csl3 and CsI2Br and of the quaternary ammonium halides Qml3 and QmI2Br, in which Qm represents the p-bromophenyltrimethyl- ammonium radical. ** There is strong chemical evidence that the halogen atoms in these compounds form complex ions in which one atom has a positive charge and the two others negative ones, the electropositive atom

* ‘ J. Chem. Soc.,’ p. 622 (1926).f Lowry, Goldstein, and Gilbert, ‘ J. Chem. Soc.,’ p. 307 (1928).X Gilbert and Lowry, ‘ J. Chem. Soc.,’ p. 3179 (1928).§ Gilbert and Lowry, ‘ J. Chem. Soc.,’ p. 2658 (1928).|| See Lowry and Hiither, ‘ Rec. Trav. chim.’ Pays-Bas, vol. 55, p. 688 (1936);

Lowry and Miss ter Horst, ibid.,p. 697.1 Berry and Lowry, ‘ J. Chem. Soc.,’ p. 1748 (1928).** Gilbert, Goldstein, and Lowry, ‘ J. Chem. Soc.,’ p. 1092 (1931).

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168 C. B. Allsopp

always being the heaviest halogen in the complex.* This evidence is supported by the X-ray analysis of CsICl2, which indicates for the complex anion a linear structure with the iodine centrally placed between two chlorinesf; corresponding electronic configurations for the ions have been formulated on the basis of measurements of their magnetic susceptibilities.! On these grounds, the formulae for the I3~- and [I2Br]~-

--- j--- --- 1---ions could be written [III] and [IIBr] respectively. Comparison with the absorption spectra of the ions IBr2, IBrCl, and IC12, in which the iodine is always electropositive, then showed that the twin bands only appear when negative iodine is linked to positive iodine, and not at all when positive iodine alone is present.

The same bands have recently been discovered by de Boer§ in quite different circumstances, namely in the spectrum of iodine adsorbed as a unimolecular layer on the surface of a CaF2 crystal, when the maxima have wave-lengths 3450 A. and 2840 A. respectively, and are nearly ten times more intense than those in the spectra of KI3 or Csl3. Chilton and Rabinowitsch|| also found two similar maxima, but at longer wavelengths ( c. 4975 A. and 4375 A.) in the spectrum of iodine adsorbed on chabasite. De Boer’s interpretation of these spectra is as follows: The energy difference between the two maxima is about 0*8 volt, which is almost the difference between the energies of the iodine atom in its normal and its excited, metastable states (2P4 and 2P&). The absorption spectrum of the iodine molecule I2, in the vapour or in its violet solutions, also contains two bands, the first at about 5000 A. corresponding with dissociation into one excited and one unexcited atom,j[ and the second at 7320 A. corresponding with dissociation into two normal atoms.** The separation between these two bands is also equal to about 0-8 volt. The twin bands of the spectrum of adsorbed iodine may therefore be identified with those of the free molecule with a displacement of 1 • 9 volts towards shorter wave-lengths on account of the change in binding energy on adsorption. A similar explanation would be valid for the spectrum observed by Chilton and Rabinowitsch. Alternatively, it might be that through the strong polarization of the adsorbed I2 molecules a complex

* McCombie and Reade, ‘ J. Chem. Soc.,’ vol. 123, p. 142 (1923); vol. 125, p. 148 (1924); p. 2528 (1926).

f Wyckoff, ‘ J. Amer. Chem. Soc.,’ vol. 42, p. 1100 (1920).+ Gray and Dakers, ‘ Phil. Mag.,’ vol. 11, p. 81 (1931).§ ‘ Z. phys. Chem.,’ B, vol. 14, p. 163 (1931); vol. 21, p. 208 (1933).|| ‘ Z. phys. Chem.,’ B, vol. 19, p. 107 (1932).If Franck, ‘ Z. phys. Chem.,’ vol. 144, p. 120 (1926).** Brown, ‘ Phys. Rev.,’ vol. 38, p. 1187 (1931).



The Absorption Spectra o f Triiodides 169

Ion containing fluorine is formed, with a structure [FII] identical with that of the polyiodide ions described above. In either case, the primary act on absorption of light must produce both neutral and excited iodine atoms, the band of longer wave-length corresponding with the former process and that of shorter wave-length with the latter. The same effect has since been found in the spectrum of bromine adsorbed on a CaF2 surface, when the same explanation is again valid.*

Twin bands, at much shorter wave-lengths but with a similar difference of energy, also occur in the absorption spectra of the vapours and of the aqueous solutions of the alkali iodides,f and LederleJ has shown that in aqueous solutions of Lil, Nal, KI, Srl2, Mgl2, Znl2, and Cdl2 their appearance is independent both of the cation and of concentration, so that they are characteristic of hydrated I- . The difference of energy between the maxima again led to the association of these bands with the production of a neutral or of an excited I atom, and the first step in the absorption is then the loss by the iodine ion of its electron, either to the cation or to a water molecule attached to the anion, producing either two separated neutral atoms or a free iodine atom in the two cases respectively. The wave-lengths of these maxima are 2262 A. and 1935 A., and they have the same intensity, logs = 4-13. Lederle also found them in alcoholic solutions of Hgl2 and Cdl2, but they are then displaced towards longer wave-lengths and their relative intensities are changed.

The occurrence of two bands of equal intensity and the same difference of frequency (about 6000 cm.-1) in all these spectra suggests at once that they have a common origin, i.e., in the simultaneous production of a normal and an excited (metastable) iodine atom during the absorption process. Experiments which are described in the present paper reveal that the same twin bands are also characteristic of the triiodides of the tervalent elements As, Sb, and Bi, the intense colour of which (brick red, orange red, and black) indicates the presence of covalently bound iodine, since the iodine ion is colourless, and that they also occur in the spectra of iodine compounds of quadrivalent tin; and some properties of the bands which have now been observed give no reason for supposing that any alternative postulate as to their origin is necessary.

* Custers and de Boer, ‘ Physica,’ vol. 1, p. 265 (1934).t Franck, Kuhn, and Rollefson, ‘ Z. Physik,’ vol. 43, p. 155 (1927); Hilsch and

Pohl, ‘ Z. Physik,’ vol. 57, p. 145 (1930); Franck and Scheibe, ‘ Z. phys. Chem.,’ A, vol. 139, p. 22 (1928); Franck and Haber, ‘ S.B. preuss. Akad. Wiss. Berl.,’ p. 250 (1931).

t ‘ Z. phys. Chem.,’ B, vol. 10, p. 121 (1930).



log

e70 C. B. Allsopp

1— A bsorption Spectra of A sI 3, SbI 3, a n d B iI 3

The maxima in the absorption curves for alcoholic solutions of Asl3 Sbl3, and Bil3, which are reproduced in fig. 1, were as follows:—

A sI 3 : log s = 4*21 at 3560 A .; log e = 4-34 at 2940 A .;Sbl3: log s = 4 • 07 at 3570 A .; log e = 4 • 20 at 2920 A .;Bil3: log s = 4 -02 at 3560 A .; log e = 4 • 19 at 2940 A.

F ig 1—Molecular extinction coefficients of (1) Asl3; (2) Sbl3; (3) Bil3; (4) SnCH3I3; in alcohol; (5) product of reaction of Asl3 with ether.



The frequency differences were 5920, 6240, and 5920 cm.-1 respectively. The similarity of the curves, and the value of the difference of frequency, suggests that the bands have the same origin as those already described, but a number of phenomena observed during the measurements seemed to indicate that the spectra of the solutions might not be characteristic of the triiodide molecules themselves. Thus (i) the crystals dissolve very slowly in alcohol and the yellow colour of the solutions is not developed immediately; (ii) the original material could not be recovered from the solutions, which, even on slow evaporation in the cold in vacuo, yielded only tarry residues; (iii) the twin bands for the three compounds are extremely similar to one another, so that they might easily have been due to some common product of interaction of the tervalent iodides with the solvent. They could not, however, be attributed to iodoform, the bands of which are from four to seven times less intense, and are also separated slightly more widely both in wave-length and in intensity; moreover, the smell of iodoform was never detected in the solutions. The bands are also more intense than those of K I3 in water; but the solutions of Csl3 and Qml3 in alcohol give maxima of still greater intensity. These observations would therefore be compatible with the formulation of the three triiodides as univalent periodides, but this hypothesis is excluded by the well-established valencies of the elements in question. The suggestion that HI3 might be formed by some process analogous to its reversible formation in alcoholic solutions of elementary iodine* leads to even more serious difficulties, since the absence of a visible band at 4470 A. showed that there was no free iodine in the solutions. It can therefore only be supposed that if alcoholysis does occur, it is incomplete and leaves some at least of the halogen atoms bound directly to the metal, although the gradual development of the colour of the solutions might also be due to the disintegration of polymeric complexes.

Decomposition by Ether—More definite conclusions were reached when attempts were made to observe the absorption spectrum of Asl3 in ether. For this purpose 0*0057 gram of Asl3 was dissolved in 25 cc. of ether, which had been dried over sodium and distilled from phosphoric oxide. The solution had a yellow colour which faded after a few hours. It then showed a single maximum, log £ = 3*21 at 2570 A. (fig. 1, curve 5). This is at the same wave-length as that of methyl iodide, namely log s = 2 *7 at 2600 A.,f but is three times more intense. It was therefore

* Batley, ‘ Trans. Faraday Soc.,’ vol. 24, p. 438 (1928).t Lowry and Sass, ‘ J. Chem. Soc.,’ p. 622 (1926).




172 C. B. Allsopp

attributed to the formation of ethyl iodide in accordance with the equation 2AsI3 + 3EtsO = As20 3 + 6EtI.

Experiments in Carbon Tetrachloride—An attempt to obtain a spectrum of Sbl3 under conditions in which no interaction with the solvent can occur was finally made by allowing metallic antimony and iodine to interact in presence of carbon bisulphide, and diluting with a large bulk of carbon tetrachloride, in which solvent alone the crystalline iodide is insoluble. For this purpose, a solution of iodine (0-5 gm.) in CS2 (25 c.c.) was shaken intermittently for some hours with an excess of metallic antimony until the colour of the iodine was discharged. The greenish-grey solution was then diluted with 100 volumes of CC14, whereby the concentration of CS2 was reduced to a point at which it no longer prevented a comparison of the absorptive power of the solution with that of the solvent. The almost colourless solution thus obtained gave the molecular extinction coefficients recorded in fig. 2. The curve (1) now shows a single maximum logs = 4-63 at 3150A., which is twice as strong as the twin maxima of the alcoholic solutions, and is intermediate in wave-length, with a step-out at 3850 A. Fig. 2 also shows by means of the broken curves the progressive development of the twin maxima when solutions of Sbl3 in CS2 were diluted with alcohol instead of with carbon tetrachloride. Immediately after dilution, the solution (2) showed the same maximum at 3150 A. as in CC14, although it was ten times less intense. After 36 hours it had disappeared completely and had been replaced by twin maxima at 3450 A. and 2900 A. separated by a shallow minimum as shown in curve (3). The final appearance of the spectrum, when the solution had developed a characteristic yellow colour after standing for a week, is shown in the thin continuous curve (4).

In view of the fact that Sbl3 can be recrystallized without change from CS2, this striking result might be held to indicate conclusively the secondary nature of the spectra containing the twin bands which were obtained from the alcoholic solutions, but closer inspection of curve (1) shows that such a conclusion is unnecessary, since the maximum at 3150 A. may be identified with the band at 2900 A. in curve (4), and the step-out at 3850 A. with the band at 3600 A., the whole spectrum being displaced by about 250 A. on passing from CC14 to C2H5OH as solvent. This phenomenon can be explained satisfactorily if it is assumed that (i) both spectra are due to Sbl3, and (ii) the twin bands have the same significance as those in the spectra which were described above. Sbl3 is insoluble in CC14 alone but remains in solution in CS2 when this is flooded with CC14. This may be attributed to the formation of a molecular complex




xSbI3 . yCS2, which is not necessarily stable, and which still gives rise to the spectrum of Sbl3. Evidence for such a complex may be found in the readiness of the triiodides to form molecular compounds (see below) and in the high polarity of the CS2 molecules. On the addition of alcohol, in which the Sbl3 is soluble, the triiodide would distribute itself between the CS2 and the alcohol, the decrease of intensity of the maximum in curve (2) indicating that about 90% goes to the alcohol (which is in large

4500 4000 3500 3000 2500X(A)

Fig . 2—Influence of solvent on the absorption spectrum of Sbl3. (1) CS2; (2), (3),and (4), alcohol.

excess) immediately; and the subsequent attainment of equilibrium then corresponds with the transition from curve (2) to curve (4). Association of Sbl3 with an excited CS2 molecule (CS2 itself absorbs at almost identical wave-lengths) would be expected to favour the release during irradiation of an iodine atom in an excited rather than in an unexcited state, so that the band of shorter wave-length would be enhanced and the intensity of the other diminished, just as is actually observed; whilst the displacement



174 C. B. Allsopp

of the whole spectrum could be attributed to the slightly lower energy required to remove iodine from the activated complex.

There is thus no conclusive evidence to show that the twin bands in the spectra obtained with the alcoholic solutions of the triiodides are not characteristic of the molecules of these compounds; and this conclusion has now received complete confirmation from the measurements of the absorption spectra of the vapours of Sbl3 and Bil3 made by Butkow, who records the following values for the wave-lengths of the maxima of theb a n d s * B i l 3. 4150j 3386> and 28I0 A .;

Sbl3: 3430, and 2770 A.

No discrete band of longer wave-lengths was found in the spectrum of the solutions of Bil3, although the absorption curve does not fall off so rapidly towards longer wave-lengths as do those of the other two compounds ; but the other two pairs of maxima clearly correspond with those found for the alcoholic solutions, the displacement of each spectrum being about 100 A. on passing from the vapour to the dissolved state.

2—In flu en c e of Solvent o n th e A b so r ptio n Spectru m o f

Iodoform

In view of the marked influence of CS2 on the absorption spectrum of antimony triiodide, the absorption spectrum of iodoform was re-examinedunder similar conditions. The bands now observed for solutions invarious solvents are shown in fig. 3, the positions and intensities of themaxima being as follows

Solvent CurveBand I Band II

No. ^■max. log Smax. Xmax. log Smax.

C2H5O H ......................... 1 3490 3*23 2960 3-26CC14 ............................... 2 3510 3-31 3065 3-20CC14 + CS2 .................. 3 3490 3-27 2985 3-04C2H5OHf (irradiated) . . . . 4 3600 3-52 2920 3-95

t In the course of these measurements, a marked effect of irradiation on the absorption spectrum of an alcoholic solution of CH I3 was observed. Curve (4) was obtained after a solution had been exposed for several hours to the intense ultra-violet radiation from a mercury arc. The twin bands have moved apart, the intensities of both have increased (that at shorter wave-lengths about fivefold), whilst the band of longer wave-length is much wider. These irradiated solutions do not obey Beer’s law. The effects may all be attributed to the presence of free iodine, which could be identified in the solutions, and serve to indicate the care necessary in recording absorption spectra of this type.

* ‘ Z. Physik,’ vol. 90, p. 810 (1934). The experiments described in the present paper were carried out in 1932.



These wave-lengths must be compared with those of the same pair of maxima, at 3450 A. and 2940 A. respectively, observed for the vapour of iodoform by Iredale.* The agreement is so close as to leave no doubt that the spectra obtained with the solutions are those characteristic of the iodoform molecule.

On passing from alcoholic solutions to those obtained by making a saturated solution of iodoform in CS2 and then diluting 100 times in


4500 4000 3500 3000 2500x(A)

F ig . 3—(1) Absorption spectrum of iodoform in alcohol; (2) CC14; (3) CC14 + CS2;(4) alcohol after irradiation.

CC14, there is again a striking change in the appearance of the absorption curve, but in a manner quite different.from that for Sbl3.under similar conditions, since it is now the maximum of shorter wave-length which is changed, both its intensity and its width being diminished (curve 3), whilst the other band remains unaffected and the spectrum as a whole is hardly displaced. A much larger change in position was, in fact, observed

* ‘ Z. phys. Chem.,’ B, vol. 20, p. 340 (1933).



176 C. B. Allsopp

with a solution in CC14 alone (curve 2). This difference in behaviour in the presence of CS2 might be due to a difference in the origin of the two spectra, but the similarity of the curves and of the wave-lengths makes this unlikely; and there is also a slight difference in the conditions of measurement in the two cases, since iodoform is freely soluble in CC14 and it is therefore not necessary to postulate the formation of a complex molecule with CS2 in order to account for the stability of the solutions in the mixed solvent. The change might then be attributed to a kind of Stark effect in the presence of activated CS2 molecules. It is interesting to relate the diminution in the area of the band, and thus presumably the diminution in probability of production of an excited I atom, in the presence of CS2, with the observed retardation of the photochemical oxidation of iodoform in the presence of this compound.*

3—M o lecular C o m pou nd s of th e T r iio dides a n d of Iodoform

In addition to the similarity of their absorption spectra, the triiodides have in common with iodoform the property of forming molecular compounds.! Martin! has isolated the complexes KI3 . 2C6H5CN and HI3 . 4C6H5CN, and Kleinboldt and Schneider§ have established the existence of Asl3 . 3S8, Sbl3 . 3S8, Snl4 . 2S8, CHI3 . 3S8, and C2I4 . 4S8, in which one S8 molecule is attached to each iodine atom. This structure was confirmed by observations of Hertel|| on the X-ray spectra of the crystals of CHI3 . 3S8 and Asl3 . 3S8, which have similar symmetry (space group most probably = C ^1), and readily form mixed crystals,, although AsI3 and CH13 themselves are quite unlike.

This readiness to form molecular compounds has been cited above as evidence for the existence of complexes of the type xSbI3 . jC S 2, and it therefore seemed desirable to examine the absorption spectrum of Sbl3 . 3S8 and to compare it with that of the isomorphous CHI3 . 3S8.̂ [ Measurements were made with the following solutions: Sbl3 . 3Sg and CHI3 . 3S8 in CC14; Sbl3 or CHI3 in 3S8 + CS2 + CC14; Sbl3 in 12S8 + CS2 + CC14. These were prepared by mixing the elements in the correct stoichiometrical proportions in carbon bisulphide, when the

* Cf. Dubrissay and Emschwiller, ‘ C.R. Acad. Sci., Paris,’ vol. 195, p. 660 (1932).t See, for instance, Auger, ‘ C.R. Acad. Sci., Paris,’ vol. 146, p. (1908).} ‘ J. Chem. Soc.,’ p. 2640 (1932).§ ‘ J. prakt. Chem.,’ vol. 120, p. 238 (1929).|| ‘ Z. phys. Chem.,’ B, vol. 15, p. 51 (1931).51 Martin’s benzonitrile compounds are unsuitable for this purpose in view of the

disturbing influence of the phenyl group; he was not able to isolate complexes containing aliphatic nitriles.




compound could either be crystallized out and redissolved in CC14 or the solution diluted directly to 100 times its volume with this solvent in the same way as for the corresponding solutions of Sbl3 and CHI3. The antimony solutions required an excess of sulphur for stability, and the comparison solvent used in the measurements always contained the equivalent amount of sulphur in order to compensate for the increase in absorption due to this. The range of wave-lengths over which observations could be made was thus limited, but the following results could be established: CHI3 in 3S8 + CS2 + CC14 gives a band with Amax = 3490 A., log£,nax. = 3-32; CHI3 . 3S8 in CC14 alone gives a step-out at the same wave-length, logs = 3-45, on the side of longer wave-lengths of the intense absorption due to sulphur; Sbl3 in 12S8 + CS2 + CC14 gives a very flat maximum at 3560 A., log s = 3 -28; and Sbl3 . 3S8 in CC14 alone also has a step-out at 3580 A., log e = 3 • 15. From this it is clear that, in contrast to the behaviour of Sbl3 alone in CS2 + CC14, the presence of the sulphur, attached to the iodine atoms, retains the band at longer wave-lengths with an intensity still comparable with that of the same band for CHI3 under similar conditions, which would be expected if the saturation of the iodine valencies by the sulphur had prevented the formation of a complex of Sbl3 with the CS2 and had so prevented the change in the intensities of the bands which could be attributed to the excitation of the CS2 molecules when that occurred.

4— Io din e C o m pou nd s of T in

The absorption spectrum of a specimen of CH3SnI3 has also been measured. A solution of this compound in CC14 had a faint yellow colour which rapidly changed to that of free iodine on exposure to visible light; but the almost colourless alcoholic solutions remained stable long enough for the curve (4) of fig. 1 to be recorded. The two maxima are again observed, at 3600 A., log s = 2-52, and at 2910 A., log e = 2*80. Their separation (6580 cm.-1) is a little greater than for the tervalent triiodides, and their intensity lower, but their relative intensities are similar.

These results may be compared with those which have been recorded elsewhere* for a solution of Snl4 in hexane, when maxima were found at 3560 A. and 2850 A. (separation = 7690 cm.-1) with an intensity about log e = 4-0. In both cases, the spectra are again consistent with the simultaneous production of unexcited and excited iodine atoms.

* Grant, ‘ Trans. Faraday Soc.,’ vol. 31, p. 433 (1935).

VOL. CLVIII.— A. N



178 C. B. Allsopp

D iscussion

The results of the experiments which have been described, together with those of other observers, are collected in Table I. Allowing for the fact that the frequency differences are based on the wave-lengths of

T able I— A bsorptio n Spectr a of C o m po u n d s C o n ta in in g

I o din e

Band I Band II FrequencySubstance Solvent

^max.A.

log Smax. ^max.A.

log Emax.

differencecm.-1

I- ...................................... Water 2262 4-13 1935 4-13 7650k i 3 ....................... EtOH 3550 3-9 2900 3-9 5020

(EtOH 3600 4-46 2900 4-60 6700Csl3 ...................... l Water 3520 3-3 2900 3-44 6070Qml3 ................... EtOH 3580 4-38 2900 4-55 6550CsI2B r ................ . EtOH 3580 4-02 2900 4-22 6550QmI2Br ............ . EtOH 3600 4-16 2900 4-38 6700I 2 on CaF2 ............ — 3500 c. 5 0 2850 c. 5 0 6520TeMe2I 2 ................. . EtOH 3570 3-7 2840 4-05 7200TeEt2I 2 ............ • c 6h 12 3350 4-36 2700 4-65 7190[TeMe3] TeMeI4. c 6h 12 3360 4-49 2640 4-70 8120[TeEt3] TeEtI4. .. c 6h 12 3350 4-51 2670 4-80 7600C5H10TeI2 ........ . EtOH 3560 4-28 2880 4-45 6630T1I3 .................... . MeOH 4000 3-9 2600 4-35 13460

(EtOH 3490 3-23 2960 3-26 5230c h i 3.................... . 'cci4 3510 3-31 3065 3-20 4240

(Vapour 3450 — 2940 — 5020Asl3 .................... . EtOH 3560 4-21 2940 4-34 5920

Sbl3 ......................(EtOH 3570 4-07 2920 4-20 6240(Vapour 3430 — 2770 — 6950

Bil3 ....................1 EtOH 3560 4 02 2940 4 1 9 59201 Vapour 3386 — 2810 — 5950

SnCH3I3 ............ . EtOH 3600 2-52 2910 2-80 6580Snl4 .................... . CC14 3650 c. 4 0 2850 c.4-1 7690

maxima of absorption, and not on the limits of the continuous bands, the constancy of the separation is good,* and, in view of the similarity of the absorption curves, in all these cases (except T1I3) leads to the conclusion that the spectra have a common origin, as has been suggested above, in the simultaneous production of excited (metastable) and normal iodine

* The relationship of the separation of the maxima to the separation of the limits is discussed by Iredale, ‘ Z. phys. Chem.,’ B, vol. 20, p. 340 (1933).




atoms, in 2P4 and 2Pt states, during the absorption process. None of the effects now observed is out of accordance with this point of view. It remains, therefore, to investigate whether there are probable mechanisms by which this could occur in each of the following systems: (i) a hydrated I- -ion; (ii) a triatomic I3~- or (I2Br)~-ion; (iii) an adsorbed I2 molecule; and (iv) iodides of tervalent and quadrivalent elements in which the iodine is linked covalently to the metal. In case (i), the mechanism has already been established quantitatively {see above): the ion loses its charge, leaving an iodine atom which may be released in an excited state. A similar process might be postulated to account for the appearance of the bands in the spectra of the tellurium diiodides, if the formulae of these compounds are to be written as containing I- . In case (iii), as de Boer has suggested, the adsorbed I2 may first be released as a neutral molecule which then dissociates either into two normal atoms or into one normal and one excited atom. This mechanism could also apply to the polyhalide ions,

--1-- — _|--when [III] or [Bril] could each release I2 (from I+ + I- ) ; and this is in accordance with the observation that the two iodine bands do not occur in

the spectrum when only I+ is present, as in [BrIBr], when the corresponding product would be IBr.

There remain only the compounds in which the iodine is covalently bound. The photochemical decomposition of iodoform has been studied by Iredale,* who also concludes that the primary process results in the liberation of an iodine atom, either in its normal or in its excited state, according as the energy is absorbed in the band of longer or shorter wave-length. It must be noted that the twin bands do not appear in the spectrum either of methyl iodide or of methylene diiodide, each of which contains only a single band, with maxima at 2600 A. and 2960 A. respectively, t With methyl iodide, however, the primary process is the liberation of an excited I atom,]; and the same is probably true for CH2I2, so that of the two bands of the iodoform spectrum only that of higher energy would be expected to appear in the other two spectra. In CH3I this band should have rather shorter wave-length in view of the greater energy required to disrupt the C—I bond when only a single halogen atom is present.

* Iredale, ‘ Z. phys. Chem.,’ B, vol. 20, p. 340 (1933); Gibson and Iredale, ‘ Trans. Faraday Soc.,’ vol. 32, p. 571 (1936).

t Lowry and Sass, ‘ J. Chem. Soc.,’ p. 622 (1926); see also Gregory and Style, ‘ Trans. Faraday Soc.,’ vol. 32, p. 724 (1936).

X Herzberg and Scheibe, ‘ Trans. Faraday Soc.,’ vol. 25, p. 716 (1929); ‘ Z. phys. Chem.,’ B, vol. 7, p. 390 (1930); Iredale, ‘ J. phys. Chem.,’ vol. 33, p. 290 (1929).



180 C. B. Allsopp

In the four iodine derivatives of methane, the tendency to decompose freely on irradiation increases rapidly from CH3I to CI4, as the probability that a normal iodine atom will be produced increases. The spectrum of CI4, therefore, should contain the twin bands, and this has been found to be the case.* It may therefore be expected that the primary act in the irradiation of the tervalent iodides or of stannic or tellurium compounds containing three or four iodine atoms all bound covalently would also involve the liberation of a neutral iodine atom. Thus, the first stage in the decomposition of Asl3 could be Asl3 -> Asl2 + I (normal) accompanied by Asl3 -> Asl2 + I (excited) with about the same probability of either process occurring. In this way, the similarity of the spectra which have been described can be completely accounted for, and it may reasonably be concluded that the origin to which they have been attributed is correct.

E xperim ental

Materials—I am indebted to Baron Uzumasa for the following preparations. Arsenic triiodide was prepared by the method of Oddo and Giachery ;f the red crystals were recrystallized from carbon bisulphide and washed with ether. Antimony triiodide was prepared by the method of Vour- nasos;J the orange-red product was sublimed, and washed with ether and then with carbon bisulphide until the red colour of free iodine could no longer be seen in the extract. The crystals obtained by direct combination in presence of carbon bisulphide (see above) were shown by Mrs. Wooster to give the same X-ray spectrum as those prepared by the method of Vournasos. Bismuth triiodide was prepared by the method of Vour- nasos;J the grey crystalline precipitate was freed from mother liquor and sublimed in a current of carbon dioxide at about 200°; the black sublimate was then washed with ether and with carbon bisulphide until free from iodine.

Method—The absorption spectra were recorded by the methods previously described.* The points plotted in figs. 1-3, with the exception of * * * §

* These measurements were not included above as the data are not considered to be sufficiently accurate. A solution of CI4 in CC14 gave bands with maxima at 3860 A. and 3040 A. (frequency difference = 6980 cm.-1); but as the iodine band at 5200 A. was also developed strongly, no reliance can be placed on intensity measurements.

t ‘ Gazz. chim. ital.,’ vol. 53, p. 63 (1923).X ‘ C.R. Acad. Sci., Paris,’ vol. 166, p. 526 (1918).§ Allsopp, ‘ Proc. Roy. Soc.,’ A, vol. 143, p. 618 (1934).



those in the “ transitional ” curves of fig. 2, represent the means of several readings.

The author wishes to express his gratitude to Professor T. M. Lowry, F.R.S., for suggesting this problem, and for his continual interest and advice while the work was being carried out; he also thanks Professor J. E. Lennard-Jones, F.R.S., and Dr. G. B. B. M. Sutherland for helpful suggestions, and Professor K. Fajans for valuable discussion of the data.


Summary

Twin bands, which are now observed in the absorption spectra of alcoholic solutions of Asl3, Sbl3, Bil3, and SnCH3I3, are very similar to those found in the spectra of solutions of iodoform. The influence of solvents on the absorption spectra of the triiodides and of iodoform is investigated and discussed; and it is shown that the twin bands in these cases can have the same origin as those found in the spectra of the iodide ion, of adsorbed iodine molecules, of the polyhalides of the alkalis, and of a number of iodine derivatives of quadrivalent and hexavalent tellurium, namely in the simultaneous production during irradiation of iodine atoms in the normal and in the excited, metastable state.



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The absorption spectra of triiodides - Royal Societyrspa.royalsocietypublishing.org/content/royprsa/158/893/167.full.pdf · 167 The Absorption Spectra of Triiodides By C. B. Allsopp,