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| - Solvent and Solvent Isotope Effects on the Vibrational Cooling Dynamics of a DNA BaseDerivative
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| - Vibrational cooling by 9-methyladenine was studied in a series of solvents by femtosecond transient absorptionspectroscopy. Signals at UV and near-UV probe wavelengths were assigned to hot ground state populationcreated by ultrafast internal conversion following electronic excitation by a 267 nm pump pulse. A characteristictime for vibrational cooling was determined from bleach recovery signals at 250 nm. This time increasesprogressively in H2O (2.4 ps), D2O (4.2 ps), methanol (4.5 ps), and acetonitrile (13.1 ps), revealing a pronouncedsolvent effect on the dissipation of excess vibrational energy. The trend also indicates that the rate of coolingis enhanced in solvents with a dense network of hydrogen bonds. The faster rate of cooling seen in H2O vsD2O is noteworthy in view of the similar hydrogen bonding and macroscopic thermal properties of bothliquids. We propose that the solvent isotope effect arises from differences in the rates of solute−solventvibrational energy transfer. Given the similarities of the vibrational friction spectra of H2O and D2O at lowfrequencies, the solvent isotope effect may indicate that a considerable portion of the excess energy decaysby exciting relatively high frequency (≥700 cm-1) solvent modes.
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