An efficient method to study the dynamics of radiationless transition in large molecular systems is proposed. It is based on the use of the whole set of normal coordinates of vibration and allows for taking properly into account both the displacements and the mix of the normal modes upon transition between two electronic states. The Hamiltonian matrix elements are written in terms of generalized Franck-Condon integrals and are analytically evaluated by recursion formulae. Applications to the S 2 → S 1 internal conversion in pyrazine and to long range electron transfer between quinones in photosynthetic reaction centres are given. * This paper is dedicated to Prof. Attilio Immirzi on the occasion of his 65th birthday.

The effect of complementary base pairing on the oxidation potential of a guanosine derivative has been determined by cyclic and differential pulse voltammetry in CHCl3. The formation of the Watson-Crick H-bonded complex lowers the oxidation potential of the free molecule by 0.34 V, which compares well with the value obtained by DFT/B3LYP/6-311++g** computations.

The calculation of radiationless transition rates and of their temperature dependence from first principles is addressed by combining reliable electronic compu-

A generating function approach to the calculation of spectral band shapes including Duschinsky and Herzberg-Teller effects is proposed and applied to the computation of the free-base chlorin Q absorption bands, using molecular geometries and normal vibrations obtained by density functional theory computations. The results clearly show that non-Condon effects can significantly affect the relative intensities of the weakest Q(y) and, to a lesser extent, Q(x) bands. The proposed approach is extremely powerful and can be used in the cases where the molecular size makes the direct calculation of Franck-Condon integrals by recurrence formulas prohibitive.

The vibrational progressions of the N-->V electronic transition of ethylene--a test case for the computation of Franck-Condon factors between electronic states exhibiting very different equilibrium geometries--have been calculated by using both the Cartesian and the curvilinear internal coordinate representations of the normal modes of vibration. The comparison of the theoretical spectra with the experimental one shows that the Cartesian representation yields vibrational progressions which are not observed in the experimental spectrum, whereas the curvilinear one gives a very satisfying agreement, even in harmonic approximation.