Turning Low Barrier Hydrogen Bonds On and Off

The hydrogen bond in the [HDihH-Fda](+) ion (HDih: 4,5-dihydro-1H-imidazole, Fda: 1-fluoro-N,N-dimethylmethaneamine) can be an ordinary single well hydrogen bond (HB) or a low barrier hydrogen bond (LBHB) depending on the orientation of the CH2F group in the Fda molecule. Both domains on the cluster's potential energy surface are well separated from each other by a 10 kcal/mol high barrier, which turns the CH2F group into a switch for the LBHB. The conformers with a LBHB are generally higher in energy than their counterparts with a standard HB, which suggests that the LBHB is not necessarily the more stable bond.