Abstract
The infrared spectra of H(H(2)O)(2)(+), D(D(2)O)(2)(+), H(D(2)O)(2)(+), and D(H(2)O)(2)(+) isotopologues of the Zundel cation in the spectral range of 0-4000 cm(-1) are computed by quantum dynamics in full dimensionality using the multiconfiguration time-dependent Hartree method. The spectra present dramatic isotope effects in the middle spectral region between 600 and 2000 cm(-1). Not only the expected line shifts due to isotopic substitution take place but the intensities of the peaks and the number of absorptions with appreciable intensity vary. The most complex spectrum is the one of H(D(2)O)(2)(+), in which a group of at least four coupled vibrational modes is found in a narrow spectral range between 1000 and 1500 cm(-1) and is responsible for the three peaks found in this spectral region. The simplest spectrum of the series corresponds to D(H(2)O)(2)(+). In this case deuteration of the central position induces decoupling of the vibrational modes, especially of the asymmetric central proton mode and the ungerade water bending, leading to a spectrum which is easy to assign and interpret. Zero-point energies and low energy vibrational eigenstates of each isotopologue related to the wagging (pyramidalization) and water-water internal relative rotation are computed using the block improved relaxation algorithm. The effect of isotopic substitution on these states is discussed. The reported simulations provide detailed information on the dynamics and vibrational spectroscopy of the Zundel cation and contribute to our general understanding of protonated water clusters and the hydrated proton.