Anomalous H/D Isotope Effect on 35Cl NQR Frequencies and H/D Isotope Effect on 1H MAS NMR Spectra in Pyrrolidinium p-Chlorobenzoate

An anomalous isotope effect was observed in the 35 CI NQR frequency of pyrrolidinium p-chlorobenzoate (C 4 H 8 NH 2 + •ClC 6 H 4 COO - ) by deuterium substitution of hydrogen atoms which form two kinds of N-H···O type hydrogen bonds. Large negative frequency shifts of the 35 CI resonance lines, reaching 309 kHz at 77 K and 267 kHz at 293 K, were obtained upon deuteration, although the CI atom in the molecule formed no hydrogen bonds in the crystal. 1 H MAS NMR lines showed significant changes by the deuterium substitution, while in contrast, small shifts of 13 C CP/ MASNMR signals were obtained. Our measurements of 1 HNMR spin-lattice relaxation times (T 1 ) suggested that the H/D isotope shifts detected from the 35 CI NQR frequencies and 1 H NMR spectra are due to structural changes rather than molecular dynamics. Single-crystal X-ray diffraction measurements showed two remarkable H/D isotope differences in the molecular arrangements, (i) the N-H length along the crystallographic a axis became 1 pm shorter, and (ii) the dihedral angle between benzene and the pyrrolidine ring changed by 1.1(2)° upon deuteration. Using density functional theory estimations, the anomalous 35 Cl NQR frequency shifts and 1 H MAS NMR line-shape changes could be explained by the dihedral angle change rather than the N-H length difference.