Conservation of Vibrational Excitation During Hydrogen-Bonding Reactions

We report novel transient infrared “tag and probe” investigations of the equilibrium kinetics and vibrational energy transfer between the free acid Et 3 SiOH and its 1:1 hydrogen-bonded complex with acetonitrile, Et 3 SiOH···N⋮CCH 3, in dilute (0.1 mol/dm 3 acid) CCl 4 solutions at 293 K. Picosecond time-resolved infrared double-resonance spectroscopy measures the vibrational population of the OH-stretching mode (via v = 1 → 2 absorption) after IR excitation of either the free acid or the homogeneously broadened OH( v = 0 → 1) absorption of the complex. The free acid and 1:1 complex OH( v = 1) population lifetimes ( T 1 ) and equilibrium reaction rate constants affecting hydrogen-bond formation and dissociation are unambiguously determined for this system. Tagged free acid OH( v = 1) leads to complex formation with OH( v = 1) excitation, indicating OH( v = 1) population is maintained during collision and formation of hydrogen bonds. This is the only known example where OH( v = 1) vibrational population of a polyatomic reactant is prepared and that excitation is followed during a condensed-phase bimolecular reaction which leads to an excited hydrogen-bonded product.