J. R. Villarreal

Low‐frequency vibrational spectra and ring puckering of cyclopentene‐d8

More than 25 ring‐puckering transitions have been observed in the vapor‐phase Raman and far‐infrared spectra of cyclopentene‐d8. The ring‐puckering potential energy function in reduced form was determined to be V=18.20(Z4−6.88 Z2) cm−1 which represents a barrier to inversion of 215 cm−1 (0.61 kcal/mole). The barrier differs from that of the undeuterated cyclopentene by 17 cm−1 owing to mixing of other motions (presumably CH2 rockings). The equilibrium value of the molecule is calculated to have a dihedral angle of 26°.

Vibrational spectra and normal coordinate analysis for cyclopentene, cyclopentene-1-d1, cyclopentene-1,2,3,3-d4 and cyclopentene-d8

Abstract The vibrational spectra of cyclopentene, cyclopentene-1-d 1 , cyclopentene-1,2,3,3-d 4 and cyclopentene-d 8 have been measured between 4000 and 100 cm −1 in all three physical states. In conjunction with a normal coordinate analysis, vibrational assignments are proposed on the basis of isotopic shift ratios, group frequency considerations, relative band intensities and shapes as well as depolarization ratios. The vibrational assignments and coupling effects are discussed in terms of the calculated potential energy distribution. A total of thirty-seven valence force constants was used to calculate the one hundred and twenty-eight frequencies for the four isotopic derivatives of cyclopentene. The correspondence between the observed and calculated frequencies is reflected by the overall percentage error of about 1%.

Vibrational spectra and potential energy surface for the ring bending and ring twisting of 5,6-dihydro-2H-thiopyran

5,6‐Dihydro‐2H‐thiopyran, CH2CH2CH=CHCH2S, has been synthesized and its far‐infrared and Raman spectra recorded. Two series of sharp bands were observed originating from 139 and 235 cm−1 in the infrared spectrum for the out‐of‐plane ring‐bending and the ring‐twisting vibrations, respectively. A detailed energy level diagram including numerous excited states was determined for the two coupled vibrations. The two‐dimensional potential energy surface, which satisfactorily fits more than two dozen observed transitions, was calculated to be V=2.431×104 x41 −0.383×104x21 +2.258×104x42 −1.966×104 x22 +1.026×105x21 x22 , where x1 is the ring‐bending coordinate and x2 is the ring‐twisting coordinate. The minimum energy on the potential surface corresponds to a twisting angle of 37.8° (the half‐chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 4130 cm−1 above the twisted conformation on the potential energy surface. The results are compared to analogous molecules and to m...

Spectroscopic investigations and potential energy surfaces of the ground and excited electronic states of 1,3-benzodioxan

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.

Low‐frequency vibrational spectra and conformations of bicyclo [3.2.0] hept‐6‐ene and 2‐oxabicyclo [3.2.0] hept‐6‐ene

The low‐frequency (100–500 cm−1) infrared and Raman spectra of bicyclo [3.2.0] hept‐6‐ene and its 2‐oxa analog have been analyzed. The large collection of transitions involving the four out‐of‐plane ring modes were assigned and complex interaction was found. For the oxa compound a detailed energy level diagram was determined from the 30 transitions observed. Analysis of the ring‐puckering spectra demonstrated that the asymmetric potential energy functions had only one energy minimum within 4 kcal/mole of the ground state, which corresponds to a boat species. Whether a second minimum is present at higher energies could not be determined.

Far‐infrared spectra and hindered pseudorotation of 1,3‐oxathiolane

The far‐infrared spectra arising from the two out‐of‐plane ring vibrations of 1,3‐oxathiolane have been recorded and analyzed. Fifteen pseudorotational (ring‐bending) transition frequencies were observed in the 25–125 cm−1 region and were fit with the one‐dimensional potential function V(cm−1)=−541(1−cos 2φ)+139(1−cos 4φ). The molecule is twisted with a barrier to pseudorotation of 541±20 cm−1 (1.54±0.06 kcal/mol). The spectra recorded include transition frequencies occurring above the barrier, and this is the first molecule (other than a nearly free pseudorotor) for which such bands have been seen. Seven radial transitions arising from three different pseudorotational states have also been observed in the 297–317 cm−1 region. Those in the pseudorotational ground state can be fit with a one‐dimensional double‐minimum function with a barrier to planarity of 2720 cm−1. However, this value for the one‐dimensional approximation is believed to be too high. Difference bands (180–240 cm−1), sum bands (398–426 cm...

Vibrational Spectra, Theoretical Calculations, and Two-Dimensional Potential Energy Surface for the Ring-Puckering Vibrations of 2,4,7-Trioxa[3.3.0]octane

2,4,7-Trioxa[3.3.0]octane (247TOO) is an unusual bicyclic molecule which can exist in four different conformational forms which are determined by the directions of the two ring- puckering motions. The vibrational assignments of 247TOO have been made based on its infrared and Raman spectra and theoretical density functional theory (DFT) calculations. The two ring-puckering motions (in-phase and out-of-phase) were observed in the Raman spectra of the liquid at 249 and 205 cm(-1) and these values correspond well to the DFT values of 247 and 198 cm(-1). Ab initio calculations were utilized to calculate the structures and conformational energies for the four energy minima and the barriers to interconversion and the data was utilized to generate a two-dimensional potential energy surface (PES) for the two ring-puckering motions. The resulting quantum state energies for this PES were then calculated in order to better understand the patterns that are produced when the PES has four energy minima at different energy values. The wave functions corresponding to the different quantum states were also calculated. The NMR spectrum of 247TOO showed the presence of the two lowest energy conformations, consistent with the results of the ab initio calculations.

Far‐infrared spectra and two‐dimensional potential energy surface for the ring‐bending and ring‐twisting vibrations of 5,6‐dihydro‐4H‐thiopyran

5,6‐Dihydro‐4H‐thiopyran has been synthesized and its far‐infrared spectrum has been recorded. Eleven ring‐bending bands originating at 120.7 cm−1 and four ring‐twisting bands originating at 274.5 cm−1 were observed. Twelve sum and difference bands in the 383–397 and 148–166 cm−1 regions were also observed and these facilitated the construction of a detailed energy map including numerous excited vibrational states of the two coupled vibrations. The two‐dimensional potential energy surface, which satisfactorily fits the observed data, was determined to be V=9.48 ×104x4−4.13×104x2+1.37×104τ4−1.82×104τ2+1.10 ×105x2τ2, where x and τ are the bending and twisting coordinates, respectively. The minima on the potential energy surface correspond to twisting angles of ±48° (half‐chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 1500 cm−1 above the twisted conformation on the potential energy surfaces, and the barrier to planarity was estimated to be 6000 cm−1. Both of the...

Numerical Modeling of a Turbulent Gas Jet Flame in a Swirling Air Stream

A numerical simulation of a turbulent natural gas jet diffusion flame at a Reynolds number of 9000 in a swirling air stream is presented. The numerical computations were carried out using the commercially available software package CFDRC. The instantaneous chemistry model was used as the reaction model. The thermal, composition, flow (velocity), as well as stream function fields for both the baseline and airswirling flames were numerically simulated in the near-burner region, where most of the mixing and reactions occur. The results were useful to interpret the effects of swirl in enhancing the mixing rates in the combustion zone as well as in stabilizing the flame. The results showed the generation of two recirculating regimes induced by the swirling air stream, which account for such effects. The present investigation will be used as a benchmark study of swirl flow combustion analysis as a step in developing an enhanced swirl-cascade burner technology.Copyright

FAR-INFRARED SPECTRA AND POTENTIAL ENERGY SURFACE FOR 5,6-DIHYDRO-4H-THIOPYRAN

The far infrared spectrum of 5,6-dihydro-4H-thiopyran shows eleven ring bending bands (near 120 cm−1), four ring-twisting bands (near 275 cm−1), and twelve sum and difference bands in the 383–397 cm−1 and 148–166 cm−1 regions. From these frequency data a detailed energy map was constructed for many of the excited vibrational states of the two coupled out-of-plane ring vibrations. A two-dimensional potential energy surface, which satisfactorily fits the observed data, was determined in terms of the bending and twisting coordinates respectively. The minima on the potential energy surface correspond to twisting angles of ±48° (half-chair conformation). The lowest-energy bent (boat) conformation correspnds to a saddle point 1500 cm−1 above the twisted conformation on the potential energy surfaces, and the barrier to planarity was estimated to be 6000 cm−1.