May, 1961
53TRUCTURAL
DEPENDENCE O F ABSORPTION SPECTRA
evaporation coefficients, the AF from the Knudsen experiments would be more positive than those from the Langmuir experiments. If molecular vaporization were significant, the calculated AF's would be more positive than the correct value. I n this case, the error would be greater in the Kriudsen experiments because of the suppression of Pzr by the excess graphite. Thus, departures from the two assumptions would lead to errors of opposite sign, but the results of the Knudsen experiments would be the more positive for both cases and the two effects would be cumulative Therefore agreement of the observed Knudsen and Langmuir AF values implies that both assumptions must be substantially correct. If the experimental uncertainty in the free energy determinations is taken as 1.5 kcal. a t 2675, then the lower limit for the evaporation coefficient must be 0.75 or alternatively the upper limit of the fraction of molecular zirconium in the vapor is about one-fourth P,,in the Knudsen experiments, or about 2.5 X atmosphere. The free energy values in Tables I1 and V are thermodynamically consistent in that they do not show a,ny apparent trend with temperature, as may be expected from the fact that the entropy change for reaction 2 is only about -3 e.u. A more reliable tech for significant errors may be
OF
/%DIKETONE CHELATES
735
made by use of the "Third Law" to obtain a heat of formation a t 298OK. which may then be compared with that obtained by Mah and Boyle from combustion calorimetry. Thus if AFo(f, 2675OK.) = 38.9 koal. is combined with the change of free energy function for 2675'K. from Table I, (AFOT - AH0~9e)/T = -3.3, the heat of formation a t 298OK. is found to be -47.7 kcal., in good agreement with Mah and Boyle's value -44.1 kcal. This agreement supports the validity of thi. conclusions drawn concerning the vaporizatioil processes. I n summary, zirconium carbide vaporizes at 2620 to 2747'K. predominantly by dissociation to the elements with evaporatioii coefficients equal or nearly equal to unity. Thicarbide also can vaporize congruently at compositions which are near that of the stoichiometric compound. Acknowledgments.-The author wishes to express indebtedness to Dr. R. L. lLIcKisson for his suggestions and criticism both during the work and in the preparation of the manuscript. He also wishes to thank G. W. Dollman and Ami Margrave of the Analytical Chemistry Unit for performing the necessary analyses, and Paul Romo of the Solid State Metallurgy Group for the crystal parameter measurements.
STRUCTURA.L DEPEXDESCE OF ,4BSORPTIOX SPECTRA OF o-DIKETOSE CHELATES. 11. ULTRAVIOLET' BY J. CHARETTE, G. NEIRYNCK AND PH. TEYSSI~ Departments of Physics and Chemistry, Lovanium University, Lkopoldville, Congo Received August $2, 1960
An interpretation of the absorption spectra of @-diketonechelates is presented, based on new experimental results and on previously published data. The absorption band characteristic of the T-T* transition in the ligand is found to be drpendent on the structure of the metal, of the ligand and of the solvent: these effects are discussed in terms of theories of resonance, crystal field and free electron model. The lack of significance of all the data obtained with concentrations lower than a critical value, specific of the chelate studied, is emphasized.
Introduction I n the past decade, numerous attempts have been made to correlate the molecular structure of the diketone chelates with a number of their physicochemical properties, namely magnetism, absorption spectra, X-ray diffraction, dipole moments, etc. . . . In the first part of this study,' a definite relationship was found t)etmeen the perturbed carbonyl absorption frequency and the stability of the chelate. A sirnilw relationship vias believed to he found in the ultraviolet absorption spectra by Yamasaki and Sone2s3; but a more detailed investigation revealed that other factors are determinant of the absorption frequencies, and that in many cases, their effects are inseparablc. The problem appears still more intricate orving ( 1 ) For the preceding paper, see: J. ChRrotte and P. TeyxsiB' Spectrochmm. Acta. 1 6 , 080 (1960) 12) K Yainabaki a n 1 K Sone, n'ature, 166, 998 (19fO) ( 3 ) I
