Detecting hydrogen bonding by NMR relaxation of the acceptor nuclei

The formation of hydrogen bonds (HB) between phenol or N-methyltrifluoroacetamide and several acceptors (pyridine, carbonyl compounds, nitriles, amides) in CCl4 or CHCl3 been investigated through the analysis of NMR relaxation times (T1) of the heteronuclei (14N and 17O) directly involved in the HB interaction. Thus, a comparison is made between such T1 values (corrected 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 effects are properly accounted for, there is a good agreement between theoretical and experimental electric field gradient (efg) changes. The noticeable difference found between CCl4 or CHCl3 as solvents is discussed in relation to the presence of phenol oligomers, and the non-negligible HB donor power of CHCl3.