Pergamon SpectrochimicaActa, Vol. 50A, No. 5, pp. 961-971, 1994 Elsevier Science Ltd. Printed in Great Britain 0584-8539/94 $6.00 + 0.00 Normal coordinate treatment of l-methylthymine in the crystalline state: use of the ultraviolet resonance Raman intensities to improve the vibrational force field P. LAGANT* and G. VERGOTEN Inserm U279, 1 rue du Pr Caimette, 59019 Lille Cedex, France; Groupement Scientifique IBM-CNRS "Mod61isation Mol6culaire" et Cerim Facult6 de M6decine, Place de Verdun, 59045 Lille, France R. EFREMOV Shemyakin Institute of Biorganic Chemistry, Russian Academy of Sciences, 117871 Moscow, Ui Miklukho-Maklaya 16/10, Russia and W. L. PETICOLAS Department of Chemistry, University of Oregon, Eugene, OR 97403, U.S.A. (Received 25 March 1993; in final form 26 July 1993; accepted 28 July 1993) Abstract--A general valence force field for 1-methylthymine (1MET) in the isolated state has been unequivo- cally determined from the calculation of both the frequencies and the relative ultraviolet resonance Raman intensities arising from the first lowest lying vibronic transitions. In a second step, the normal modes were derived for the crystalline state using the previously determined in-plane force field and an out-of-plane force field was deduced. INTRODUCTION FOR rigid planar molecules such as nucleic acid bases with low point group symmetry, the number of force constants, in the case of a general valence force field, exceeds by far the number of observed frequencies. So, it is always possible to deduce by normal coordinate treatments several possible force fields, each one reproducing quite satisfactorily the vibrational frequencies but giving a different description of the potential energy distribu- tion (PED). In this work, we have selected the set of in-plane force constants which leads to the best fit between the frequencies and the ultraviolet resonance Raman intensities (UVRRI). Pyrimide moieties such as uracil, thymine or their methyl derivatives, have a lowest energy absorption band with a maximum lying in the 260-280 nm range. This band proceeds primarily from a H-->H* electronic transition involving a vibronic coupling with these vibrational modes which are concerned by a double bond character. In spite of this electronic transition, the C5=C6 bond length is substantially increased and the corresponding bond order is greatly reduced in the excited state relative to the ground state. In the case where the change of the molecular geometry is small (Franck- Condon approximation), it has been shown that a linear relation exists between the bond lengths and the corresponding bond orders [1]. Under this approximation, it is then possible to calculate the displacement in the normal coordinates during the transition from the ground to the first electronic transitions using the inverse matrix of the eigenvectors arising from the secular equation (LR1). In normal coordinate treatments, LR is the eigenvector (expressed in the internal coordinate space) derived from E. B. Wilson Jr's relation GFLR = LR2, where G is the inverse kinetic energy matrix and F is the matrix of force constants; 2 is the associated eigenvalue. If AR~ is a displacement of the ith bond length from its equilibrium position and AQ~ is the corresponding displacement in the jth normal coordinate, then we have the relationship A Q~= * Author to whom correspondence should be addressed. 961