Comparison of Conventional and Nonconventional Hydrogen Bond Donors in Au^- Complexes

Although gold has become a well-known nonconventional hydrogen bond acceptor, interactions with nonconventional hydrogen bond donors have been largely overlooked. In order to provide a better understanding of these interactions, two conventional hydrogen bonding molecules (3-hydroxytetrahydrofuran and alaninol) and two nonconventional hydrogen bonding molecules (fenchone and menthone) were selected to form gas-phase complexes with Au^-. The Au^-[M] complexes were investigated using anion photoelectron spectroscopy and density functional theory. Au^-[fenchone], Au^-[menthone], Au^-[3-hydroxyTHF], and Au^-[alaninol] were found to have vertical detachment energies of 2.71 ± 0.05, 2.76 ± 0.05, 3.01 ± 0.03, and 3.02 ± 0.03 eV, respectively, which agree well with theory. The photoelectron spectra of the complexes resemble the spectrum of Au^- but are blueshifted due to the electron transfer from Au^- to M. With density functional theory, natural bond orbital analysis, and atoms-in-molecules analysis, we were able to extend our comparison of conventional and nonconventional hydrogen bonding to include geometric and electronic similarities. In Au^-[3-hydroxyTHF] and Au^-[alaninol], the hydrogen bonding comprised of Au^-···HO as a strong, primary hydrogen bond, with secondary stabilization by weaker Au^-···HN or Au^-···HC hydrogen bonds. Interestingly, the Au^-···HC bonds in Au^-[fenchone] and Au^-[menthone] can be characterized as hydrogen bonds, despite their classification as nonconventional hydrogen bond donors.