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Simultaneously measured absorption (ABS) and magnetic circular dichroism (MCD) spectra of the Q-bands of chlorophyll-a (Chl-a) in ether over 150-186 K reveal that the species that forms at low temperature is a chlorophyll hydrate rather than a diether complex. We have recently proposed a new assignment paradigm for the spectra of chlorophillides which, for the first time, quantitatively accounts for a wide range of observed data. Observations performed at low temperature in ether have historically been very important for the interpretation of the spectra of Chl-a. While our assignment for this system initially anticipated only small spectral changes as the temperature is lowered, significant changes are known to occur. Extensive CAM-B3LYP time-dependent density-functional theory (TD-DFT) calculations verify that the observed spectra of the hydrated species conforms to expectations based on our new assignment, as well as supporting the feasibility of the proposed hydration reactions.

A unified picture is presented of water interacting with pyridine, pyridazine, pyrimidine, and pyrazine on the S(1) manifold in both gas-phase dimers and in aqueous solution. As (n,π*) excitation to the S(1) state removes electrons from the ground-state hydrogen bond, this analysis provides fundamental understanding of excited-state hydrogen bonding. Traditional interpretations view the excitation as simply breaking hydrogen bonds to form dissociated molecular products, but reactive processes such as photohydrolysis and excited-state proton coupled electron transfer (PCET) are also possible. Here we review studies performed using equations-of-motion coupled-cluster theory (EOM-CCSD), multireference perturbation theory (CASPT2), time-dependent density-functional theory (TD-DFT), and excited-state Monte Carlo liquid simulations, adding new results from symmetry-adapted-cluster configuration interaction (SAC-CI) and TD-DFT calculations. Invariably, gas-phase molecular dimers are identified as stable local minima on the S(1) surface with energies less than those for dissociated molecular products. Lower-energy biradical PCET minima are also identified that could lead to ground-state recombination and hence molecular dissociation, dissociation into radicals or ions, or hydration reactions leading to ring cleavage. For pyridine.water, the calculated barriers to PCET are low, suggesting that this mechanism is responsible for fluorescence quenching of pyridine.water at low energies rather than accepted higher-energy Dewar-benzene based "channel three" process. Owing to (n,π*) excitation localization, much higher reaction barriers are predicted for the diazines, facilitating fluorescence in aqueous solution and predicting that the as yet unobserved fluorescence from pyridazine.water and pyrimidine.water should be observable. Liquid simulations based on the assumption that the solvent equilibrates on the fluorescence timescale quantitatively reproduce the observed spectral properties, with the degree of (n,π*) delocalization providing a critical controlling factor.

The hydrogen bonding between water and pyrazine in its ground, lowest (n,pi*), and lowest (pi,pi*) states is investigated using density-functional theory (DFT), time-dependent density function theory (TD-DFT), coupled-cluster singles and doubles (CCSD) theory and equation-of-motion coupled cluster (EOM-CCSD) theory. For all states, the minimum-energy configuration is found to be an orthodox linear hydrogen-bonded species, with the bond strength increasing by 0.4 kcal mol-1 upon formation of the (pi,pi*) state and decreasing by 1.0 kcal mol-1 upon formation of the (n,pi*) state. The calculated solvent shifts for the complexes match experimental data and provide a basis for the understanding of the aqueous solvation of pyrazine, and the excited-state complexes are predicted to be only short-lived, explaining the failure of molecular beam experiments to observe them. Quite a different scenario for hydrogen bonding to the (n,pi*) excited state is found compared to those of H2O:pyridine and H2O:pyrimidine: for pyridine linear hydrogen bonds are unstable and hydrogen bonds to the electron-enriched pi cloud are strong, whereas for pyrimidine the excitation localizes on the nonbonded nitrogen leaving the hydrogen-bonding unaffected. For H2O:pyrazine, the (n,pi*) excitation remains largely delocalized, providing a distinct intermediary scenario.