Unexpected Suppression of Double-Proton Tunneling Induced by Quantum Barriers from Zero-Point Energy

To assess tunneling, we studied the guanine-cytosine (GC) base pair tautomerization in the gas phase. We applied multidimensional semiclassical reaction path methodology with microcanonically optimized multidimensional tunneling (μOMT) using POLYRATE. The minimum energy path (MEP) has a single saddle point for the double proton transfer. Addition of vibrational zero-point energy (ZPE) to the MEP gives the vibrationally adiabatic ground state curve, V_a^G, which is the barrier through which tunneling occurs. Unexpectedly, V_a^G has not one but two well-separated barriers in the transition state region. The first is near the saddle point. The second barrier is entirely due to a large amount of ZPE associated with local reaction path curvature. We refer to it as a quantum barrier. Its height and width reduce the tunneling transmission coefficient. In other words, GC tautomerization has two competing quantum effects, tunneling and ZPE, that have opposite effects on the reaction rate. The transmission coefficient κ is 1.57, and tunneling constitutes 36% of the rate constant at 298 K. Our computed kinetic isotope effects (KIE) are lower than expected, e.g., KIE = 5.05 at 298 K. In the discussion, we show that the quantum barrier is a consequence of reaction path curvature as the tautomer begins to form.