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For more than three decades far-infrared and Raman spectroscopies, along with appropriate quantum mechanical computations, have been effectively used to determine the potential energy functions which govern the conformationally important large-amplitude vibrations of nonrigid molecules. More recently, we have utilized laser-induced fluorescence (LIF) excitation spectroscopy and ultraviolet absorption spectroscopy to analyze the vibronic energy levels of electronic excited states in order to determine the potential energy surfaces and molecular conformations in these states. Transitions from the ground vibrational state in an S 0 electronic state can typically be observed only to several excited vibronic levels. Hence, the LIF of the jet-cooled molecules generally provides data on only a few excited state levels. Ultraviolet absorption spectra recorded at ambient temperatures, however, often provide data on many additional excited vibronic levels. However, these can only be correctly interpreted if the electronic ground state levels have been accurately determined from the far-infrared, Raman, and dispersed fluorescence studies. In this article, we will first present our results for bicyclic molecules in the indan family in their S 0 and S 1 (π,π*) electronic states. Two-dimensional potential energy surfaces in terms of the ring-puckering and ring-flapping vibrations were utilized for the analyses. Next, we review our work on trans-stilbene in its S 0 and S 1 (π,π*) states and examine the data from which two-dimensional potential energy surfaces were determined for the phenyl torsions and one-dimensional functions were calculated for the torsion about the CdC bond, which governs the photoisomerization. Finally, we consider seven cyclic ketones in their S 0 and S 1 (n,π*) states. The carbonyl wagging vibration of each was studied in its electronic excited state in order to determine the barrier to inversion and the wagging angle. The barrier to inversion was found to increase with angle strain. Conformational changes between the ground and excited electronic states were also examined in terms of the out-of-plane ring motions.

p-Cresol is a simple molecular model for the para phenolic side chain of tyrosine. Previously, Siamwiza and co-workers [(1975) Biochemistry 14, 4870-4876] investigated p-cresol solutions to identify Raman spectroscopic signatures for different hydrogen-bonding states of the tyrosine phenoxyl group in proteins. They found that the phenolic moiety exhibits an intense Raman doublet in the spectral interval 820-860 cm(-1) and that the doublet intensity ratio (I2/I1, where I2 and I1 are Raman peak intensities of the higher- and lower-wavenumber members of the doublet) is diagnostic of specific donor and acceptor roles of the phenoxyl OH group. The range of the doublet intensity ratio in proteins (0.30 < I2/I1 < 2.5) was shown to be governed by Fermi coupling between the phenolic ring-stretching fundamental nu1 and the first overtone of the phenolic ring-deformation mode nu(16a), such that when the tyrosine phenoxyl proton is a strong hydrogen-bond donor, I2/I1 = 0.30, and when the tyrosine phenoxyl oxygen is a strong hydrogen-bond acceptor, I2/I1 = 2.5. Here, we interpret the Raman and infrared spectra of p-cresol vapor and extend the previous correlation to the non-hydrogen-bonded state of the tyrosine phenoxyl group. In the absence of hydrogen bonding, the Raman intensity of the higher-wavenumber component of the canonical Fermi doublet is greatly enhanced such that I2/I1 = 6.7. Thus, for the non-hydrogen-bonded phenoxyl, the lower-wavenumber member of the Fermi doublet loses most of its Raman intensity. This finding provides a basis for understanding the anomalous Raman singlet signature (approximately 854 cm(-1)) observed for tyrosine in coat protein subunits of filamentous viruses Ff and Pf1 [Overman, S. A., et al. (1994) Biochemistry 33, 1037-1042; Wen, Z. Q., et al. (1999) Biochemistry 38, 3148-3156]. The implications of the present results for Raman analysis of tyrosine hydrogen-bonding states in other proteins are considered.

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