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PHYSICAL CHEMISTRY Registered in
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VOLUME 72, NUMBER 8 AUGUST 15,1968
Evidence for Molecular Dimers, (1.J2,in Iodine Vapor by A. A. Passchier and N. W. Gregory Department of Chemistry, University of Washington, Seattle, Washington 98105 (Received April 4, 1968)
It has been confirmed that the temperature and concentration dependence of an absorption band in iodine vapor (around 265 mp) can be explained by assuming the existence of the equilibrium 21, = 14, with both Iz and 1 4 contributing to the absorbance. Equilibrium constants over the range 150-420' and the entropy (123 & 2 eu at 605°K) and enthalpy of formation (AH'6os = -2.9 f 0.4 kcal mol-l) of L(g) have been deduced from spectrophotometric data. The thermodynamic results are in accord with those expected for a complex formed by the van der Waals interaction in which the two 1 2 partners move relatively freely with respect to each other. The absorption appears to be associated with a charge-transfer transition. I n the vicinity of 267 mp, Kortum and Friedheim' have observed a band in the spectrum of iodine vapor for which the apparent molar absorptivities are concentration dependent; they did not make a quantitative study of this dependence and its variation with temperature. A similar band for iodine in carbon tetrachloride has been reported by de Maine2j3and de Maine, et ~ 1 . ;they ~ account for the behavior by assuming the existence of the equilibrium 212 = 14 and deduce an equilibrium constant and an enthalpy change for the reaction. Evidence for the existence of 14 in iodine absorbed on silica gel has also been reported by Kortum and Koffer;6 various other investigators have suggested that molecular species larger than diatomic molecules may exist for iodine in both the gas phase and in solut i ~ n . ~ We - ~ have made a systematic study of the concentration and temperature dependence of the iodine vapor-phase absorption spectrum over the wavelength interval 225-300 mp, between temperatures of 150 and 420', and over the concentration interval (1.3-60) X mol I.-'. The observations may be explained if it is assumed that the equilibrium 212(g) = 14(g)exists, with both species absorbing light. Thermodynamic properties of L(g) have been deduced. Results are compared with those of the other investigators2-6 and with those which may be predicted from PVT10,1' and gas viscosity data.12 The spectral behavior of iodine
vapor in this wavelength interval is quite similar to that reported recently for bromine.laJ4 After this paper had been prepared, a report by Tamres, Duerksen, and Goodenow of a similar spectrophotometric study of this Iz-14band was published.16 I n their study, at lower temperatures,
