ELECTRON-DENSITY AND ELECTROSTATIC POTENTIAL DISTRIBUTIONS IN NUCLEOSIDE ANALOGS .1. AN EXPERIMENTAL-STUDY OF 3'-O-ACETYL-2'-DEOXY-5-METHOXYMETHYLURIDINE

The experimental electron-density distribution and the distribution of electrostatic potential for 3'-O-acetyl-2'-deoxy-5-methyoxymethylurid ine have been determined from high-resolution X-ray diffraction data c ollected at 145 (4) K using graphite-monochromated Mo Kalpha radiation to sin theta/lambda = 1.08 angstrom-1. Crystal data: C13H18N2O7, M(r) = 314.297, triclinic, P1, a = 5.9701 (7), b = 6.7357 (6), c = 9.5770 (10) angstrom, alpha = 77.21 (1), beta = 79.95 (1), gamma = 75.45 (1)- degree, V = 360.63 (6) angstrom3, Z = 1, F(000) = 166, D(x) = 1.447, D (m) = 1.43 (2) Mg m-3, mu = 0.1278 mm-1, lambda = 0.71069 angstrom. Th e integrated intensities of 15 087 reflections were measured and reduc ed to 7067 independent reflections with I greater-than-or-equal-to 3si gma(I). The partial charges on the atoms of the molecule were determin ed and are in reasonable agreement with ab initio values reported for fragments of the molecule. The acetal bond, O'(4)-C'(1), shows higher electron density than the O'(4)-C'(4) bond does, which is more typical of an ether bond. The presence within a molecule of methyl H atoms in the vicinity of an O atom that might serve as the acceptor of a hydro gen bond appears to significantly perturb the electrostatic potential around the O atom and make it less acceptable for hydrogen bonding. Th e atoms N(3), O(52) and O(4) combine to create a molecular surface wit h negative electrostatic potential that extends over part of one face of the 5-methoxymethyluridine ring.