Stepwise hydration of protonated carbonic acid: a theoretical study.

The gas-phase geometries, binding energies (BEs), and sequential binding energies (SBEs) of protonated carbonic acid (PCA)-water (W) clusters (PCAW(n), where n = 1-6) have been calculated using density functional theory (DFT) with Becke's three-parameter hybrid exchange functional and the Lee-Yang-Parr correlation functional (B3LYP) and M05-2X methods. The presence of wirelike structures of protonated water in PCAW(n)(x) clusters is evident from the results. The results indicate that a proton is transferred from PCA to its immediate water molecule in the linear and monohydroxy clusters of PCA. The involvement of the Eigen cation and Grotthuss type of mechanism in the proton transport is observed from the sequential hydration energies and from the calculated vibrational spectra. Although geometrical parameters clearly reveal the presence of the Eigen core, calculated lower-energy vibrational modes provide clues about the involvement of the sequence Eigen --> Zundel --> Eigen in the proton transfer.