14N NQR spectroscopy reveals the proton position in N–H⋯N bonds: a case study with proton sponges

Abstract

The position of protons in hydrogen bonds is often uncertain to some degree, as the technique most often used for structure determination, X-ray diffraction, is sensitive to electron density, which is not particularly abundant around protons. In hydrogen bonds, protons introduce an additional problem: the potential for proton motion is inherently anharmonic and thus requires the consideration of nuclear quantum effects (NQEs). Here, we demonstrate that ¹⁴N NQR spectroscopy is able to rather accurately determine proton positions in N–H⋯N bonds, in certain cases with an accuracy comparable to that of X-ray and neutron diffraction at room temperature. We first derive, using ab initio calculations considering also the NQEs, a relation between the proton distance from the bond midpoint and the difference between the quadrupole coupling constants for the two nitrogen sites. The found relation is linear with a proportionality constant of 0.108 Å MHz⁻¹ for tertiary amine nitrogens. Then, we validate our theoretical calculations experimentally, using several 1,8-bis(dimethylamino)naphthalene (DMAN) complexes.