AMINO-GROUPS IN NUCLEIC-ACID BASES, ANILINE, AMINOPYRIDINES, AND AMINOTRIAZINE ARE NONPLANAR - RESULTS OF CORRELATED AB-INITIO QUANTUM-CHEMICAL CALCULATIONS AND ANHARMONIC ANALYSIS OF THE ANILINE INVERSION MOTION

The amino group nonplanarity in nucleic acid bases, aniline, aminopyri dines, and aminotriazine was investigated by ab initio methods with an d without inclusion of correlation energy utilizing medium and extende d basis sets. For all the systems studied, the amino group was found t o be nonplanar and the coupled cluster method [CCSD(T)] ''nonplanariti es'' and inversion barriers slightly higher than their second-order ma ny-body perturbation-theory (MP2) counterparts. To assess the reliabil ity of the calculations, inversion splittings for aniline and aniline- ND2 were evaluated by solving a two-dimensional vibrational Schrodinge r equation for the large-amplitude inversion and torsion motions, whil e respecting the role of small-amplitude C-N stretching and H-N-H bend ing motions. Because a large number of points is required for the desc ription of the aniline potential energy surface, the Hartree-Fock (HF) method with 6-31G basis set was utilized. The vibrational calculatio ns were performed within the framework of the semirigid bender Hamilto nian of Landsberg and Bunker. Excellent agreement between experimental and theoretical inversion-torsion frequencies for fundamental, overto ne, and combination modes was found, which gives strong evidence for t he adequacy of the theoretical model used in general, and potential en ergy surface in particular. Similarity between the HF, MP2, and CCSD(T ) aniline inversion barriers and amino group nonplanarities gives us c onfidence that the MP2 and CCSD(T) inversion barriers and amino group nonplanarities of the DNA bases, aminopyridine, and aminotriazine, are close to the actual values which are still experimentally unknown. (C ) 1996 American Institute of Physics.