Molecular-Level Understanding of Ground- and Excited- State O-H???O Hydrogen Bonding Involving the Tyrosine Side Chain: A Combined High-Resolution Laser Spectroscopy and Quantum Chemistry Study

摘要

The present study combines both laser spectroscopy and ab initio calculations to investigate the intermolecular O-H???O hydrogen bonding of complexes of the tyrosine side chain model chromophore compounds phenol (PH) and para-cresol (pCR) with H_2O, MeOH, PH and pCR in the ground (S_0) state as well as in the electronic excited (S_1) state. All the experimental and computational findings suggest that the H-bond strength increases in the S_1 state and irrespective of the hydrogen bond acceptor used, the dispersion energy contribution to the total interaction energy is about 10-15% higher in the S1 state compared to that in the S0 state. The alkyl-substituted (methyl; +I effect) H-bond acceptor forms a significantly stronger H bond both in the S_0 and the S_1 state compared to H_2O, whereas the aryl-substituted (phenyl; + effect) H-bond donor shows a minute change in energy compared to H_2O. The theoretical study emphasizes the significant role of the dispersive interactions in the case of the pCR and PH dimers, in particular the C-H???O and the C-H???p interactions between the donor and acceptor subunits in controlling the structure and the energetics of the aromatic dimers. The aromatic dimers do not follow the acid-base formalism, which states that the stronger the base, the more red-shifted is the X-H stretching frequency, and consequently the stronger is the H-bond strength. This is due to the significant contribution of the dispersion interaction to the total binding energy of these compounds.