Excited State Chemistry in the Condensed Phase

QM/EFPl scheme was used for investigating a variety of chemical processes in aqueous environment, including chemical reactions, amino acid neutral/zwitterion equilibrium, solvent effects on properties of a solute such as changes in dipole moment and shifts in vibrational spectrum, and solvatochromic shifts of electronic levels [36, 56, 59-60, 71-79]. Applications of a general QM/EFP scheme were limited so far to studies of electronic excitations and ionization energies in various solvents [56-58]. Extensions of QM/EFP to biological systems have been also developed [80-85]. 

Para-nitroaniline (pNA) is an organic chromophore that is often used as a tag in UV/Vis, Raman, and second-harmonic generation spectroscopies [86-91], pNA possesses electron donor amino group and electron acceptor nitro groups that give rise to a bright charge- 

QM/EFP methods were applied to understand solvent effects and relaxation dynamics of the CT state of pNA in three different solvents water, dioxane, and cyclohexane [57], Specifically, pNA was described by the configuration interaction singles with perturbative doubles [C1S(D]] method [92] in 6-31-t-G basis, while solvent molecules were represented by the EFP fragments. For each system, pNA molecule was solvated by 64 solvent molecules. Configurational space of each system was sampled with EFP MD (in which pNA was also represented as an EFP fragment) with periodic boundary conditions, using NVT ensemble at 300 K. Snapshots from the 

MD trajectories were used for QM/EFP excited state calculations. Further details of these simulations can be found in Ref. [57]. 

An example of solvent-induced solvatochromic shifts (calculated at a characteristic snapshot from the MD trajectory for each solvent) on different electronic excited states is shown in Fig. 5.5. Inspection of this plot reveals that the electronic states with the dipole moments that are larger than the dipoles in the ground state (shown as solid red curves in Fig. 5.5) become increasingly stabilized (red-shifted) in polar solvents. For example, l Ai, l Bi, 2 Bi states, which dipoles are larger than in the ground state dipole (7.7 Debye), demonstrate systematic red shifts upon solvation. The red shift increases in more polar solvents (in the order of c-hexane, dioxane, and water). The most dramatic red shift is experienced by the experimentally observed l Ai charge-transfer state with the (gas-phase) dipole moment of 12.9 D. It is quite intriguing that this state (the lowest red state in Fig. 5.5) is only the third lowest excited state in the gas phase but becomes the lowest excited state in water. On the 

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Tautomerism may be of many kinds as illustrated in this book and in Ref. [1]. However, tautomerism may be less obvious depending on the rates of interconversion or the equilibrium constant may be very much in favor of one form. A typical example of the latter is acetone and other ketones. Nevertheless, tautomerism plays a major role in the chemistry of alkyl ketones. Tautomerism may occur both in the gas phase, namely the liquid and condensed phases, and in the solid state, namely in the ground state as well as in the excited state. As tautomerization often involves the movement of light atoms, proton transfer is the usual case and hydrogen bonding will often be involved. Tautomerization can of course be both intra- and intermolecular, and in the latter case a solvent molecule may also be involved. For some characteristic cases, see Ref. [1, Chapter 3]. 

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