Anisotropic reorientation of 9-methylpurine and 7-methylpurine molecules in methanol solution studied by combining 13C and 14N nuclear spin relaxation data and quantum chemical calculations.

Reorientation of 9-(trideuteromethyl)purine and 7-(trideuteromethyl)purine molecules in methanol-d4 solutions has been investigated on the basis of the interpretation of the nuclear spin relaxation rates of their 14N (or 1H) and 13C nuclei. The transverse quadrupole relaxation rates of 14N nuclei have been obtained from the line shape analysis of their 14N NMR spectra. Alternatively, the information on the longitudinal 14N relaxation rates has been obtained via the scalar relaxation of the second kind of protons coupled to 14N. The longitudinal dipolar relaxation rates of the protonated 13C nuclei in the investigated molecules have been determined by measuring their overall relaxation rates and NOE enhancement factors. The molecular geometries, scalar coupling constants, and EFG tensors needed for quantitative interpretation of the above data have been calculated theoretically [DFT B3LYP/6-311++G(2d,p) or B3PW91/6-311+G(df,pd)] including the impact of the solvent by using discrete solvation and the polarizable continuum model. The reorientation of the investigated purines has been described as rotational diffusion of an asymmetrical top. It has been found that to get a fully consistent interpretation of the relaxation data, effective C-H bond lengths being 3% longer than the calculated ones had to be used in analysis to compensate for the ground-state vibrations. The obtained rotational diffusion coefficients and orientations of the principal diffusion axes show that the investigated molecules reorient anisotropically and that the mode of their solvation is remarkably different, in spite of their structural similarity.