No abstract

The circular dichroism data on 38 pyrimidine nucleosides, selected to provide a basis for a reliable index of furanose conformation as a function of its optical activity, are presented. The functional dependency of the B2u Cotton effect on the sugar-base torsion angle and on the specific pentose conformation is determined from theory and compared with experimental data. The theoretical description of the rotational strength as a function of the torsion angle agrees in every respect with available experimental data. Substituent effects on the signed magnitudes of the B2u rotational strength generally correlate quite well with theoretical expectations. The success of the theoretical calculations depend on giving up the Kirkwood-Tinoco coupled oscillator scheme where the transition moment vector is located at the center of gravity of the chromophore and, instead, breaking down the electric transition moment into bond contributions.In paper VIII of this series,1 it was shown that an analysis of the -* * Cotton effects of cyclopyrimidine nucleosides and several other rigid systems predicts their absolute stereochemical configuration. The method of analysis is an extension of the classical coupled oscillator theory of optical activity of Kuhn,2 which was reformulated in quantum mechanical terms

SynopsisThe optical and conformational properties of certain 7-ribosyl purines have been studied using several experimental and theoretical methods. The CD, MCD, and absorption spectra of the 7-ribosyl purines and their cations have been determined experimentally with some solvent effects being noted. Theoretically we have used the SCF-CI and CNDO molecular orbital calculations to determine the optical properties of the bases; Simpson's bond exciton theory to determine the optical properties of the ribose moiety; the optical activity was determined by the dipole coupling and electricmagnetic coupling equations; and the conformational energy calculations are included as a basis for excluding highly improbable conformations. On the basis of these calculations, we concluded that the oxy derivatives and the amino derivatives are predominately in the "extreme anti" conformation and "standard anti" conformation, respectively. However, the conclusions are tentative since at the present time the band assignments are not unequivocal. At least one n--?r* transition has been identified in the spectra which is not notably sensitive to pH and solvent effects. Several implications of this observation are discussed.

A method of calculating rotational strengths by a bond-bond coupled oscillatory theory is described. The technique is applied to a variety of specially chosen molecules to check the theory at various levels of approximation. Good agreement with experiment, particularly in the case of the cyclonucleosides, suggests that coupled oscillator theory accounts for most of the observed optical activity in the pyrimidine nucleosides. The calculation is particularly useful for structure determination because of its simplicity and complete dependency on bond orientations within a molecule.

The absorption and magnetic circular dichroism (MCD) spectra of the n-tetrabutylammonium salts of OsI6 ~-and trans-IrC14Br22-, OsCl~Br22-and OsBr~C122-and of the trans-tetraethylammonium salts of IrBr4C12 ~-, OsC14122-, and OsI4C122-were measured in KBr pellets at liquid helium temperature. The bands in the absorption spectra, which are very similar for analogous Ir 4+ and Os 4+ mixed halides, are assigned with the help of MCD data to ligand-to-metal charge-transfer transitions using the j-j model previously found successful in interpreting the OsC16 ~-and OsBr6 ~-spectra. The signs and, where possible, the magnitudes of the tetragonal distortion parameters are estimated for these mixed halides by fitting the spectra, and the signs are compared with those predicted by simple molecular orbital arguments. The principal bands of OsI62-are assigned to chargetransfer transitions which correspond to the analogous ones in OsBr6!-and OsC162-. The importance of second-order ligand spin-orbit coupling is demonstrated.