Energetics, proton transfer rates, and kinetic isotope effects in bent hydrogen bonds

N-H...N H-bonds which occur in intermolecular contacts and in intramolecular situations where the bond is bent are investigated by ab initio methods. Distortion of the ��(N-H...N) angle of up to 40° from linearity results in modest increases in the barrier to proton transfer while a much higher barrier occurs in NH2CH2NH3+ where this angular distortion is some 100°. The rate of transfer is orders of magnitude slower in the latter system. In contrast to earlier suppositions that systems containing nonlinear H-bonds are subject to smaller kinetic isotope effects, kH/kD for the most highly strained system is larger than in the other complexes. This distinction is attributed in part to the difference in zero-point vibrational energies but primarily to the contribution of tunneling. Not only the value of kH/kD but also the temperature sensitivity of this parameter is enhanced by nonlinearity of the H-bond.