Hydrogen bond studies. 89. A neutron diffraction study of hydrogen bonding in 1-methylthymine

Abstract

A neutron diffraction study of 1-methylthymine has been carried out. Intensities of 1681 independent reflections were measured at the Brookhaven National Laboratory High Flux Beam Reactor and the structure was refined to an R(F2) of 0.046. The thymine molecules link to form dimers by means of N–H ··· O hydrogen bonds. The dimers appear to be slightly disordered (∼11%) such that dimers using one or the other of the oxygen atoms as hydrogen-bond acceptors are present in the crystal structure. Evidence is found to suggest the existence of a C–H ··· O hydrogen bond; the H ··· O distance between the methine hydrogen atom in one dimer and the oxygen atom not participating in an N–H ··· O hydrogen bond in a different dimer is 2.045 (3) Å. The C ··· O distance is 3.121(2) Å and the bond is almost linear [∠C–H ⋯ O=170.9(3)°]. The bond distances between heavy atoms are determined to a precision of 0.002 Å or better; the corresponding value for heavy atom-hydrogen atom bond lengths is 0.006 Å.