Flickering dipoles in the gas phase: Structures, internal dynamics, and dipole moments of -naphthol-H₂O in its ground and excited electronic states

Described here are the rotationally resolved S₁-S₀ electronic spectra of the acid-base complex cis - naphthol-H₂O in the gas phase, both in the presence and absence of an applied electric field. The data show that the complex has a trans-linear O - HO hydrogen bond configuration involving the -OH group of cis - naphthol and the oxygen lone pairs of the attached water molecule in both electronic states. The measured permanent electric dipole moments of the complex are 4.00 and 4.66 D in the S₀ and S₁ states, respectively. These reveal a small amount of photoinduced charge transfer between solute and solvent, as supported by density functional theory calculations and an energy decomposition analysis. The water molecule also was found to tunnel through a barrier to internal motion nearly equal in energy to kT at room temperature. The resulting large angular jumps in solvent orientation produce flickering dipoles that are recognized as being important to the dynamics of bulk water.