Dipole–Dipole Coupling Constant for a Directly Bonded CH Pair—A Carbon-13 Relaxation Study

Abstract

Multiple-field (4.7, 9.4, 14.1 T) carbon-13 relaxation data are reported for hexamethylenetetramine (HMTA) in the cryosolvent D 2O/DMSO at 243 K. Under these conditions, the reorientational motion of HMTA is outside of the extreme narrowing range and the relaxation data can be subjected to a quantitative interpretation. Because of the high symmetry of the HMTA molecule, the reorientation must be isotropic. Treating the reorientation as a small-step rotational diffusion of a rigid body, we obtain a rotational correlation time of 1.0 ns and a carbon–proton dipole–dipole coupling constant corresponding to an effective internuclear distance of 114. 2 pm. The harmonic vibrational correction to the dipole–dipole coupling constant, based on a known force field, yields an NMR estimate of the r α distance of 110.8±0.3 pm.