With the aid of a reported inversion splitting value, the far-infrared spectrum resulting from the ring-puckering vibration of coumaran has been reassigned and the one-dimensional potential energy function has been determined. The barrier to planarity is 155 +/- 4 cm(-1) and the dihedral angle is 25 degrees . These results agree well with the millimeter wave spectra values of 152 cm(-1) and 23 degrees , which utilized different data and a different type of potential function for the calculations. The MP2/cc-pvtz ab initio values of 238 cm(-1) and 26.5 degrees agree more poorly. If the benzene ring is assumed to remain rigid, the calculated barrier drops to 204 cm(-1). The puckering potential functions for the ring-flapping and ring-twisting vibrationally excited states were also determined and the barriers were found to be 149 and 156 cm(-1), respectively.

The laser-induced fluorescence spectra and dispersed fluorescence spectra of jet-cooled 1,2-dihydronaphthalene have been analyzed to investigate the ring inversion process in both the S0 and S1(π,π*) excited states. Ultraviolet absorption, infrared, and Raman spectra were also recorded to complement the analyses. Ab initio calculations predict the inversion process to involve four out-of-plane ring motions, and linear combinations of these were made to model the inversion process. The data show the barrier to inversion in the ground state to be 1363±100 cm−1 (the triple-zeta ab initio value is 1524 cm−1). The experimental data indicate that the barrier increases substantially in the excited state, for which the calculated barrier is 1526 cm−1 with a CIS/6-311+G(d) basis set.

The most effective way to obtain high quality vapor-phase Raman spectra is to heat the samples to increase their vapor pressure. Many samples can be heated to 350 °C and higher without decomposition. We have designed a simple Raman cell to allow these high temperature studies to be carried out. The high-temperature Raman spectra of nine molecules will be presented and discussed. Most of these are non-rigid molecules containing aromatic rings for which vibrational potential energy surfaces have been determined from their spectra. Two molecules (p-cresol and 3-methylindole) are model compounds for amino acids and their vapor-phase spectra are characteristic of environments with no hydrogen bonding.

A dual-pump, electronic-resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) technique for the measurement of minor species concentrations has been demonstrated. The frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of nitric oxide (NO) to create a Raman polarization in the medium. The second pump beam is tuned into resonance with rotational transitions in the (1,0) band of the A2Σ+–X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. We observe significant resonant enhancement of the NO CARS signal and have obtained good agreement between calculated and experimental spectra.

The laser-induced fluorescence excitation spectra of jet-cooled 4,4'-dimethyl-trans-stilbene have been recorded, and the phenyl and methyl internal rotation levels have been assigned for the S-0 and the S-1(pi,pi*) electronic states. Dispersed fluorescence spectra and vapor-phase Raman spectra were utilized to help assign the low-frequency vibrations in the electronic ground state. The methyl torsions have energy levels and potential functions very similar to those of m-xylene. The S-0 state has V-6 = 15+/-15 cm(-1) (negligible barrier), while S-1 has V-3 = 85 +/- 10 cm(-1) and V-6 = -30 +/- 20 cm(-1). The barrier to simultaneous internal rotation of the phenyl groups in So was found to be 3100 cm(-1). This is identical to that of trans-stilbene within experimental error. For the S, state, the barrier is 3140 cm(-1) as compared to 3000 cm(-1) for trans-stilbene. The barrier to rotation about the C=C bond in the S-0 state was estimated to be 41 +/- 5 kcal/mol based on one assigned frequency. In the S-1 state the trans-twist barrier was estimated to be approximately 2400 cm(-1).

The laser induced fluorescence excitation spectra of jet-cooled indan and related moelcules along with their ultraviolet absorption spectra have been used to study their S-1(pi,pi*) excited states. Far-infrared, Raman, and dispersed fluorescence were utilized to obtain the vibrational data for the S ground states. This allowed the potential energy surfaces (PESs) of these molecules to be determined in terms of the ring-puckering and ring-flapping coordinates for both states. These PESs provide barriers to planarity and conformational structures for these bicyclic molecules. Phthalan has a barrier of 3 5 cm(-1) in So but no barrier for S-0 Coumaran has an S-0 barrier of 279 cm(-1), while 1,3-benzodioxole has barriers of 171 and 264 cm(-1) for S-0 and S-1, respectively, due to the anomeric effect. The barriers for indan are 1077 cm(-1) for S-0 and 698 cm(-1) for S-0. Ab initio calculations in general provide good barrier values for the ground state. (0 2002 American Chemical Society.

The vapor-phase far-infrared, mid-infrared, ultraviolet, Raman, and laser-induced fluorescence spectra of indan have been recorded and analyzed. The far-infrared spectra, which are very similar to those previously reported, together with the Raman and dispersed fluorescence (SVLF) spectra of the jet-cooled molecules were used to reassign the ring-puckering and ring-flapping energy levels for the S-0 ground state. These were then utilized to calculate a two-dimensional vibrational potential energy surface (PES) which nicely fits all of the assigned puckering and flapping levels. The PES has a barrier of 488 cm(-1) as compared to a previously reported value of 1979 cm(-1), which was based on a one-dimensional analysis and earlier assignments. The dihedral angle of puckering is +/-30degrees. Fluorescence excitation spectra of jet-cooled indan together with ultraviolet absorption spectra were used to assign the flapping and puckering levels in the S-1(pi,pi(*)) electronic excited state. The PES for this state has a barrier of 441 cm(-1) and the energy minima correspond to puckering angles of +/-39degrees. The flapping frequency and the stiffness of the PES along the flapping coordinate both decrease substantially in the excited state. The barriers to planarity for both states are higher than those for analogous molecules due to the two -CH2-CH2- torsional interactions. Ab initio calculations do a fairly good job of predicting the experimental barriers for indan and related molecules in their S-0 and S-1 states. (C) 2002 American Institute of Physics.

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.

The mid-infrared spectra (4000-400 cm(-1)) of complexes between the cis and gauche conformers of 3-fluoropropene, CH2=CHCH2F, and HCl have been recorded in liquefied argon at temperatures ranging from 103 to 123 K. From this data, the complexation enthalpy for the complex with cis-3-fluoropropene was determined to be -9.6(3) kJ mol(-1), while that for the complex with gauche 3-fluoropropene was estimated, from the observed HCl stretching frequency, to be -10.4(5) kJ mol(-1). The stoichiometry of these complexes was determined from constant temperature, variable concentration experiments to be 1:1. A few weak bands in the spectra are assigned to higher order complexes. An ab initio study of the structures and vibrational frequencies of the complex was performed at the B3LYP/G-311++G(d,p) level. These calculations show that HCl can form a van der Waals bond with either the carbon-carbon double bond or the fluorine atom. Comparison of the experimental frequencies with the calculated ones shows that all observed complex bands were due to complex formation with the fluorine atom.