Characteristic NMR spectra of proton transfer in protonated water clusters

Abstract

Characteristic NMR spectra of proton transfer in protonated water clusters were studied using the H+(H2O)n complexes, n=2−5, as model systems, and ab initio calculations at the RIMP2/TZVP level and BOMD simulations as model calculations. Based on the concept of presolvation, two-dimensional potential energy surface of proton in the smallest, most active intermediate complex (the Zundel complex) was constructed as a function of the H-bond distance (RO–O) and the asymmetric stretching coordinate (ΔdDA). The low-interaction energy path and the path with ΔdDA=0Å were analyzed and discussed in comparison with the model systems. The two proton transfer paths associate with the characteristic IR frequencies namely, the structural diffusion and oscillatory shuttling frequencies, respectively. RIMP2/TZVP calculations showed that the proton moving on the oscillatory shuttling path is characterized by the 1H NMR shielding constant (σH+corr) varying in a narrow range, whereas on the structural diffusion path, σH+corr changes exponentially with RO–H. The energetic, dynamic and spectroscopic results obtained from BOMD simulations in the temperature range between 350 and 450K validated the presolvation model and revealed that the activation energies for the proton exchange in the smallest, most active intermediate complex, computed from the Arrhenius equation, IR spectra and a simple 1H NMR line shape analysis, are consistent and in good agreement with experiments in aqueous solution. Based on the presolvation model and the outstanding characteristics of the IR and 1H NMR spectra of the transferring protons, the present theoretical study suggested framework and steps to investigate structural diffusion processes in strong, protonated H-bond systems.