A study on the anisole-water complex by molecular beam-electronic spectroscopy and molecular mechanics calculations Rotationally resolved fluorescence excitation spectra of several vibronic bands in the S 1 ←S 0 electronic transitions of styrene ͑STY͒ and phenylacetylene ͑PA͒ have been obtained. Confirming earlier low resolution results, we find that the origin band of PA is a b-type band but that the corresponding band of STY is an a-type band, showing that the S 1 state of PA is 1 L b in character ͑like that of most other monosubstituted benzenes͒ but that the corresponding state of STY is 1 L a . The observed changes in the rotational constants of PA and STY that occur when the photon is absorbed are consistent with these assignments. Reversal in the electronic character of the S 1 state in STY is attributed to the presence of the ''off-axis'' conjugating -CHvCH 2 group, a suggestion that is supported by the observed polarizations of higher vibronic bands in both molecules.

Rotationally resolved S(1) <-- S(0) electronic spectra of 1,2-dimethoxybenzene (DMB) and its water complex have been observed and assigned. The derived values of the rotational constants show that the bare molecule has a planar heavy-atom structure with trans-disposed methoxy groups in its ground and excited electronic states. The transition of DMB is polarized along the b-axis bisecting the methoxy groups, demonstrating that its S(1) state is an (1)L(b) state. Higher energy bands of DMB are also polarized along the b-axis and have been tentatively assigned to different vibrational modes of the (1)L(b) state. The water complex origin appears 127 cm(-1) to the blue of the bare molecule origin. Analyses of the high resolution spectra of DMB/H(2)O and DMB/D(2)O suggest that the water molecule is attached via two O-H...O hydrogen bonds to the methoxy groups in both electronic states. A tunneling motion of the attached water molecule is revealed by a splitting of these spectra into two subbands. Potential barriers to this motion have been determined.

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