Ab initio molecular dynamics (AIMD) simulations for the excited-state hydrogen transfer (ESHT) reaction of 7-azaindole (7AI−(H2O)n; n = 1, 2) clusters in the gas phase and in water are presented. The effective fragment potential (EFP) is employed to model the surrounding water molecules. The AIMD simulations for 7AI−H2O and 7AI−(H2O)2 clusters show an asynchronous hydrogen transfer at t ∼ 50 fs after the photoexcitation. While the ESHT mechanism for 7AI−H2O in water does not change appreciably compared with that in the gas phase, the AIMD simulations on 7AI−(H2O)2 in water solution exhibit two different mechanisms. Since the tautomer form is lower in energy compared to the normal form in the S1 state, 7AI and (H2O)n fragments separate from each other after the ESHT. With the use of the results of the AIMD trajectories, the minimum energy conical intersection point in the tautomer region has also been located. Disciplines Chemistry CommentsReprinted (adapted) ReceiVed: May 17, 2008; ReVised Manuscript ReceiVed: July 16, 2008 Ab initio molecular dynamics (AIMD) simulations for the excited-state hydrogen transfer (ESHT) reaction of 7-azaindole (7AI-(H 2 O) n ; n ) 1, 2) clusters in the gas phase and in water are presented. The effective fragment potential (EFP) is employed to model the surrounding water molecules. The AIMD simulations for 7AI-H 2 O and 7AI-(H 2 O) 2 clusters show an asynchronous hydrogen transfer at t ∼ 50 fs after the photoexcitation. While the ESHT mechanism for 7AI-H 2 O in water does not change appreciably compared with that in the gas phase, the AIMD simulations on 7AI-(H 2 O) 2 in water solution exhibit two different mechanisms. Since the tautomer form is lower in energy compared to the normal form in the S 1 state, 7AI and (H 2 O) n fragments separate from each other after the ESHT. With the use of the results of the AIMD trajectories, the minimum energy conical intersection point in the tautomer region has also been located.

Ab initio molecular dynamics (AIMD) simulations are performed to investigate the excited state dynamics of coumarin 151 (C151) in the gas phase and in water solution at the CASSCF level of theory with segmented DZP basis sets, where in the latter case effective fragment potentials (EFP) are used. The dipole moment of an isolated C151 molecule increases considerably upon electronic vertical excitation, from 5.0 D (S 0 state) to 11.1 D (S 1 state). Two equilibrium structures have been identified in the S 1 state, i.e., a charge-transfer state with a planar amino group and a deformed structure of the six-membered ring with the carbonyl group, and a structure that is similar to the S 0 equilibrium structure. In AIMD simulations for an isolated C151 molecule (presumably similar to dynamics in nonpolar solvents), C151 decays from S 1 to S 0 via a crossing point of the charge-transfer state in some trajectories, while in the AIMD simulations for C151-EFP (including solvent), the S 1 and S 0 energies show an almost parallel energy variation with structural changes, and no crossing point is observed. This result is in good agreement with the experimental observation.

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