Solvent and Solvent Isotope Effects on the Vibrational Cooling Dynamics of a DNA Base Derivative

Chris T. Middleton, Boiko Cohen,† and Bern Kohler*
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
J. Phys. Chem. A, 2007, 111 (42), pp 10460–10467
DOI: 10.1021/jp0740595
Publication Date (Web): October 2, 2007
Copyright © 2007 American Chemical Society
†

 Current address:  Departamento de Quimica Fisica, Universidad de Castilla La Mancha, Avda. Carlos III, S.N., 45071 Toledo, Spain.

,
*

 Corresponding author:  E-mail:  kohler@chemistry.ohio-state.edu.

Abstract

Vibrational cooling by 9-methyladenine was studied in a series of solvents by femtosecond transient absorption spectroscopy. Signals at UV and near-UV probe wavelengths were assigned to hot ground state population created by ultrafast internal conversion following electronic excitation by a 267 nm pump pulse. A characteristic time for vibrational cooling was determined from bleach recovery signals at 250 nm. This time increases progressively in H2O (2.4 ps), D2O (4.2 ps), methanol (4.5 ps), and acetonitrile (13.1 ps), revealing a pronounced solvent effect on the dissipation of excess vibrational energy. The trend also indicates that the rate of cooling is enhanced in solvents with a dense network of hydrogen bonds. The faster rate of cooling seen in H2O vs D2O is noteworthy in view of the similar hydrogen bonding and macroscopic thermal properties of both liquids. We propose that the solvent isotope effect arises from differences in the rates of solute−solvent vibrational energy transfer. Given the similarities of the vibrational friction spectra of H2O and D2O at low frequencies, the solvent isotope effect may indicate that a considerable portion of the excess energy decays by exciting relatively high frequency (≥700 cm-1) solvent modes.

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History

  • Published In Issue October 25, 2007
  • Received May 25, 2007
    Revised July 21, 2007

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