Ab initio and DFT calculations with MP2/cc-pVTZ and B3LYP/cc-pVTZ basis sets have been carried out for cyclohexene and four of its oxygen analogs. All of the molecules possess a twisted structure while the bent forms represent saddle points in two-dimensional surfaces. The structures, relative energies, and frequencies for the lowest energy vibrations of the twisted, bent, and planar forms were calculated and compared to experimental results. The calculated results agree very well with the microwave data but the computed barriers are somewhat less than those based on low-frequency infrared data.

The laser-induced fluorescence spectra (both fluorescence excitation and dispersed fluorescence) of jet-cooled 1,3-benzodioxan along with its ultraviolet absorption spectra have been recorded and analyzed in order to determine the vibrational quantum levels in both the ground and S-1(pi,pi(*)) electronic excited states. A detailed energy map of the vibrational levels involving the six lowest frequency vibrations was established and utilized to better understand the structural and conformational differences between the ground and excited electronic states. The energies of more than a dozen vibrational excited states involving the out-of-plane ring twisting (nu(47)) and the out-of-plane ring bending (nu(48)) modes were determined for both S-0 and S-1 electronic states. Ab initio and density functional theory (DFT) calculations were also carried out to complement the experimental work. The data allowed one-dimensional potential energy functions in terms of the ring-twisting coordinate to be calculated. These show the molecule to have a twisting angle of 33 degrees and a barrier to planarity of 4300 +/- 500 cm(-1) for the S-0 ground state and an angle of 24 degrees and a barrier of 1500 +/- 200 cm(-1) for the S-1(pi,pi(*)) excited state.

The Raman and infrared spectra of dipicolinic acid (DPA) and dinicotinic acid (DNic) and their salts (CaDPA, Na2DPA, and CaDNic) have been recorded and the spectra have been assigned. Ab initio and DFT calculations were carried out to predict the structures and vibrational spectra and were compared to the experimental results. Because of extensive intermolecular hydrogen bonding in the crystals of these molecules, the calculated structures and spectra for the individual molecules agree only moderately well with the experimental values. Theoretical calculations were also carried out for DPA dimers and DPA·2H2O to better understand the intermolecular interactions. The spectra do show that DPA and its calcium salt, which are present in anthrax spores, can be distinguished from the very similar DNic and CaDNic.

Author Institution: Department of Chemistry, Texas A\M Department of Natural Sciences, Fayetteville State University, Fayetteville, NC 28301; School of Biological Sciences, University of Missouri-Kansas City, Kansas City, MO, 64110-2499

Several calculations with different basis sets have been carried out to better understand the unusual vibrational frequencies of cyclopropenone. It is shown that the bands at 1840, 1483, and 1026 cm−1 are predominantly the CO, CC, and symmetric C–C stretches. However, for the first and last of these there is strong interaction between the CO and C–C stretches. The results differ quantitatively from a previous normal coordinate calculation and interpretation.

Abstract: The heating of samples sealed in special heatable glass cells [1] allows samples with high boiling points to be studied as vapors by Raman spectroscopy. The investigation of the potential energy surfaces (PESs) of non-rigid molecules in our laboratory [2,3] is typically complemented by the Raman spectra of the vapors as these nicely provide data not available from infrared and ultraviolet absorption and fluorescence measurements. Examples of molecules recently investigated are 2cyclohexenone, tetralin (TET), 1,4-benzodioxan (14BZD), and 3-methylindole, which are shown below in that order.

3-Methylindole (3MI), which serves as a structural model for the tryptophan side chain in proteins, has been investigated using vapor phase Raman spectroscopy. The vapor phase spectrum of 3MI identifies the Raman signature of the indolyl moiety free of intermolecular interaction and extends previously reported solution Raman studies of 3MI and related tryptophan derivatives. The Raman spectrum of 3MI vapor is also complemented here with newly obtained vapor phase infrared data and ab initio calculations to refine and extend previous vibrational assignments. The present results provide an improved basis for assessing the dependence of the indolyl Raman signature on the local environment of the tryptophan side chain of proteins. The principal conclusions of this work are the following. (i) The vapor phase 3MI molecule exhibits Raman bands at 3506, 1585, 1409, 1349/1341 (Fermi doublet) and 881cm−1, which differ greatly from their counterparts in the Raman spectrum of 3MI liquid and thus serve as spectral markers of the indolyl ring environment. (ii) The Fermi doublet relative intensity ratio (I1/I2, where I1 and I2 are, respectively, the Raman intensities of the higher and lower wavenumber components of the doublet) is highly sensitive to the state of 3MI condensation, consistent with the previously reported sensitivity of I1/I2 to solvent polarity. The maximum value of the intensity ratio (I1/I2=3.0) is observed for 3MI vapor, while the minimum value (I1/I2=0.43) is observed for 3MI in CHCl3 solution. Implications of the present results for Raman analysis of hydrogen bonding states, hydrophilic interactions and hydrophobic interactions of tryptophan residues in proteins are considered.