Time-dependent density functional theory study of the excited-state dihydrogen bond O–H⋯H–Si

Intermolecular dihydrogen bonding in the electronically excited states of a phenol–diethylmethylsilane (DEMS) complex was studied theoretically using the time-dependent density functional theory (TDDFT) method. Analysis of the frontier molecular orbitals revealed a locally excited S1 state for the dihydrogen-bonded phenol–DEMS complex in which only the phenol moiety is electronically excited. The calculated infrared spectrum of the phenol–DEMS complex is quite different from that of previously studied S1 state of a dihydrogen-bonded phenol–borane-trimethylamine complex. The O–H and Si–H stretching vibrational modes appear as intense, sharp peaks for the S1 state which are slightly red-shifted compared with those predicted for the ground state. Upon electronic excitation to the S1 state, the O–H and Si–H bonds involved in the dihydrogen bond O–H⋯H–Si lengthen slightly, while the C–O bond shortens. The calculated H⋯H distance is significantly shorter in the S1 state than in the ground state. Thus, the intermolecular dihydrogen bond of the phenol–DEMS complex is stronger in the electronically excited state than in the ground state.