Detecting hydrogen bonding by NMR relaxation of the acceptor nuclei

The formation of hydrogen bends (HB) between phenol or N-methyltrifluoroace tamide and several accepters (pyridine, carbonyl compounds, nitriles, amide s) in CCl4 or CHCl3 been investigated through the analysis of NMR relaxatio n times (T-1) of the heteronuclei (N-14 and O-17) directly involved in the HE interaction. Thus. a comparison is made between such T-1 values (correct ed for changes in molecular dynamics and motional anisotropy) and electric field gradients calculated by ab initio methods for the acceptor molecules, both isolated and in 1:1 or 2:1 hydrogen-bonded complexes. When other effe cts are properly accounted for, there is a good agreement between theoretic al and experimental electric field gradient (efg) changes. The noticeable d ifference found between CCl4 or CHCl3 as solvents is discussed in relation to the presence of phenol oligomers, and the non-negligible HE donor power of CHCl3.