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

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Abstract An anomalous isotope effect was observed in the 35Cl NQR frequency of pyrrolidinium p-chlorobenzoate (C4H8NH2+·ClC6H4COO−) by deuterium substitution of hydrogen atoms which form two kinds of N–H···O type hydrogen bonds. Large negative frequency shifts of the 35Cl resonance lines, reaching 309 kHz at 77 K and 267 kHz at 293 K, were obtained upon deuteration, although the Cl atom in the molecule formed no hydrogen bonds in the crystal. 1H MAS NMR lines showed significant changes by the deuterium substitution, while in contrast, small shifts of 13C CP/MAS NMR signals were obtained. Our measurements of 1H NMR spin–lattice relaxation times (T1) suggested that the H/D isotope shifts detected from the 35Cl NQR frequencies and 1H 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 35Cl NQR frequency shifts and 1H MAS NMR line-shape changes could be explained by the dihedral angle change rather than the N–H length difference.