Triply resonant sum frequency (TRSF) spectroscopy is a fully coherent mixed vibrational−electronic spectroscopic technique that is ideally suited for probing the vibrational−electronic couplings that become important in driving reactions. We have used cyanocobalamin (CNCbl) and deuterated aquacobalamin (D 2 OCbl + ) as model systems for demonstrating the feasibility of using the selectivity of coherent multidimensional spectroscopy to resolve electronic states within the broad absorption spectra of transition metal complexes and identify the nature of the vibrational and electronic state couplings. We resolve three short and long axis vibrational modes in the vibrationally congested 1400−1750 cm −1 region that are individually coupled to different electronic states in the 18 000−21 000 cm −1 region but have minimal coupling to each other. Double resonance with the individual vibrational fundamentals and their overtones selectively enhances the corresponding electronic resonances and resolves features within the broad absorption spectrum. This work demonstrates the feasibility of identifying coupling between different pairs of vibrational states with different electronic states that together form the reaction coordinate surface of transition metal enzymes.

Bond rearrangement, namely the dissociation of water ions into H + 2 + O (q−1)+ (q=1-4) following fast ion-impact ionization, unexpectedly occurs following multiple ionization of water in spite of the presumably fast "Coulomb explosion" of the transient molecular ion. Furthermore, the branching ratio of bond rearrangement is found to be nearly equal for each level of ionization, q. In addition, formation of H + 2 is more than twice as likely to occur from the lighter water isotopologue H2O + than D + 2 from D2O +. These findings are consistent with the ground state dissociation mechanism in which a fast projection of the ground state nuclear wave function onto the vibrational continuum of the cation potential energy surface is sometimes followed by H + 2 formation.

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