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The energetics associated with the photoequilibrium (Formula: see text) are measured at 77 K by using pulsed-laser photocalorimetry and a range of excitation wavelengths and relative starting concentrations. Enthalpies for the photochemical transformations R hv----B and I hv----B are measured to be delta HRB = 32.2 +/- 0.9 kcal mol-1 and delta HIB = 27.1 +/- 3.2 kcal mol-1, respectively. Although the value of delta HRB is slightly lower than that reported previously by Cooper of 34.7 +/- 2.2 kcal mol-1 [Cooper, A. (1979) Nature (London) 282, 531-533], the two values are in agreement within experimental error. The energy difference delta HRB - delta HIB = 5.1 +/- 3.3 kcal mol-1 is identical within experimental error with the difference in enthalpies of isorhodopsin and rhodopsin [5.2 +/- 2.3; Cooper, A. (1979) FEBS Lett. 100, 382-384]. We suggest that this result is consistent with the theory that bathorhodopsin is a single, common photochemical intermediate connecting rhodopsin and isorhodopsin.

The point dipole interaction model for molecular polarizability proposed by Applequist, Carl, and Fung [J. Am. Chem. Soc. 94, 2952 (1972)] is modified by replacing the point dipole interaction tensor with a descaled distributed charge interaction tensor. Our procedure is based on the descaled tensor algorithm proposed by Thole [Chem. Phys. 59, 341 (1981)] and uses a Slater-type orbital (STO) function to represent the charge distribution. The resulting STOPDI formalism calculates mean molecular polarizabilities and the components of the molecular polarizabilities with errors comparable to experimental uncertainty. Furthermore, these procedures require only one optimized parameter per atom, the average atomic polarizability. The formalism is invariant to coordinate transformations and avoids the discontinuities and/or false resonances that are characteristic of previous classical and semiclassical formalisms. The STOPDI algorithm requires less parameterization and computation time than the anisotropic atom point dipole interaction (AAPDI) model of Birge [J. Chem. Phys. 72, 5312 (1980)] and is more reliable for the calculation of polarizability derivatives and Raman cross sections. We demonstrate, however, that none of the above formalisms are reliable for calculating absolute Raman cross sections for normal modes involving significant bond stretching components. This is an inherent limitation of any formalism which does not explicitly account for electron density redistribution accompanying changes in the internuclear distances of covalently bonded atoms.

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeCl and DGeCl have been recorded by laser excitation of selected bands of the A 1A"-X 1A' electronic transition. Twenty-six ground state vibrational levels of HGeCl and 42 of DGeCl were measured, assigned, and fitted to standard anharmonicity expressions, which allowed all the harmonic frequencies to be determined for both isotopomers. A normal coordinate least squares analysis obtained by fitting the harmonic frequencies yielded reliable values for five of the six force constants. The ground state effective rotational constants and force field data were combined to calculate average (rz) and approximate equilibrium (re z) structures, with re z(GeH)=1.586(1) A, re z(GeCl)=2.171(2) A, and the bond angle fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.9 degrees . Comparisons show that the derived bond lengths are consistent with those of the appropriate diatomic molecules in their ground electronic states and the bond angle is similar to that of germylene (GeH2). A Franck-Condon simulation of the vibrational intensities in the 0(0) (0) band emission spectrum of HGeCl using ab initio force field data shows good agreement with experiment, lending credence to the vibrational analysis of the observed spectra.