QUANTUM-CHEMICAL STUDY OF STRUCTURE AND STABILITY OF ALL 14 ISOMERS OF ISOCYTOSINE

Results of quantum chemical computations based on the HF/6-31G(d,p) (p ointwise MP2/HF/6-31G(d,p)) approximation of fully optimized structure s, electronic energies, rotational constants, electric dipole moment c omponents and electric field gradient principal values at nitrogen ato m sites of all 14 isomers of isocytosine are reported. Together with r esults of the computation of fully optimized total electronic energies of all 2- and 6-amino- and all 2- and 6-hydroxypyrimidine isomers at the same level of approximation, the data are analyzed for relative st abilities of tautomers and conversion energies of geometric isomers wi th imino and hydroxy substituents. From these quantities and analogous data for all cytosine isomers, an extended system of attractive and r epulsive contributions (''increments'') is reported, from which geomet ric isomer conversion energies may additively be reproduced (within 0. 5 kcal mol(-1)). Two exceptions to this linear additive model are list ed. The increments are used to derive estimates for conversion energie s of isocytosine and cytosine tautomers. They furthermore form a basis for interpretation of predicted non-planar structures found earlier f or two cytosine isomers, for the geometry of non-planar amino groups o f some cytosine and isocytosine isomers and for relaxation of local st ructural parameters upon interconversion of geometric isomers. A numbe r of predictions for rotational spectra of isocytosines and some corre lations of electric field gradient tensors at sites of nitrogen nuclei (with sp(2) or p(z) lone electron pairs) and of quadrupole coupling c onstants with attractive and repulsive interactions are put forward. A statistics based comparison shows predicted and crystallographic stru ctural parameters of two isocytosine isomers to be significantly diffe rent.