Esther J. Ocola

Vibrational spectra and structure of cyclopentane and its isotopomers.

The infrared and Raman spectra of vapor, liquid, and solid state cyclopentane and its d(1), 1,1-d(2), 1,1,2,2,3,3-d(6), and d(10) isotopomers have been recorded and analyzed. The experimental work was complemented by ab initio and density functional theory (DFT) calculations. The computations confirm that the two conformational forms of cyclopentane are the twist (C(2)) and bent (C(s)) structures and that they differ very little in energy, less than about 10 cm(-1) (0.1 kJ/mol). The bending angle for the C(s) form is 41.5° and the dihedral angle of twisting is 43.2° for the C(2) form. A reliable and complete vibrational assignment for each of the isotopomers has been achieved for the first time, and these agree very well with the DFT (B3LYP/cc-pVTZ) computations. The ab initio CCSD/cc-pVTZ calculations predict a barrier to planarity of 1887 cm(-1), which is in excellent agreement with the experimental value of 1808 cm(-1).

Intramolecular π-Type Hydrogen Bonding and Conformations of 3-Cyclopenten-1-ol. 2. Infrared and Raman Spectral Studies at High Temperatures

The vapor-phase infrared and Raman spectra of 3-cyclopenten-1-ol (3CPOL) have been collected at temperatures ranging from 25 to 267 degrees C. These clearly show the presence of four conformations of 3CPOL with the one with intramolecular pi-type hydrogen bonding being most abundant. The spectra of all four conformations have been assigned, and these agree well with the computed values from the DFT calculation. The frequency shifts observed for the different conformations are in accord with the predicted values. In the O-H stretching region the conformer A with the pi-type intramolecular hydrogen bond has the lowest stretching frequency at 3623.4 cm(-1) while the three higher energy conformers have frequencies 14.2, 32.0, and 36 cm(-1) higher. In the C=C stretching region conformer A again has the lowest frequency at 1607.3 cm(-1) while the other conformers have bands 2.1, 8.0, and 13.4 cm(-1) lower. Both the O-H stretching and the C=C stretching force constants are decreased about 2% by the hydrogen bonding. Five of the other vibrations show significant predicted frequency shifts up to 193 cm(-1). Analysis of intensity data at different temperatures was used to calculate the energy difference between the two most stable conformers. This was found to be 435 +/- 160 cm(-1), and the result agrees reasonably well with the high level ab initio results which range from 274 to 401 cm(-1).

Intramolecular π-Type Hydrogen Bonding and Conformations of 3-Cyclopenten-1-ol. 1. Theoretical Calculations

The 3-cyclopenten-1-ol (3CPOL) molecule possesses two large-amplitude, low-frequency vibrations, namely, the ring-puckering and OH internal rotation, which can interconvert its four conformers into each other. Ab initio and density functional theory (DFT) calculations have been carried out to understand the energetics of these conformational changes. The lowest energy 3CPOL conformer possesses weak pi-type intramolecular hydrogen bonding between the hydroxyl hydrogen and the carbon-carbon double bond, and this lies 274 cm(-1) (0.78 kcal/mol) to 420 cm(-1) (1.20 kcal/mol) lower in energy than the other three conformations according to CCSD/6-311++G(d,p) computations. The two-dimensional potential energy surface for 3CPOL was computed as a function of the ring-puckering and OH internal rotation coordinates with the MP2/6-31+G(d,p) model.

Bis-trifluoromethyl Effect: Doubled Transitions in the Rotational Spectra of Hexafluoroisobutene, (CF3)2C═CH2

Rotational spectra for hexafluoroisobutene, and its (13)C isotopologues, have been recorded between 8 and 16 GHz using a chirped pulse, Fourier transform microwave spectrometer. Notably, all spectra observed are doubled with separations between the doublets being between 1 and 60 MHz. We propose that the bis-trifluoromethyl groups of the target molecule are staggered in the equilibrium configuration, and that a novel, out-of-phase rotation through a F-CCC-F planar configuration with low barrier (<100 cm(-1)), leads to the observed doubled rotational spectra.

Infrared and Raman spectra and theoretical calculations for benzocyclobutane in its electronic ground state.

The infrared and Raman spectra of vapor-phase and liquid-phase benzocyclobutane (BCB) have been recorded and assigned. The structure of the molecule was calculated using the MP2/cc-pVTZ basis set and the vibrational frequencies and spectral intensities were calculated using the B3LYP/cc-pVTZ level of theory. The agreement between experimental and calculated spectra is excellent. In order to allow comparisons with related molecules, ab initio and DFT calculations were also carried out for indan (IND), tetralin (TET), 1,4-benzodioxan (14BZD), 1,3-benzodioxan (13BZD) and 1,4-dihydronaphthalene (14DHN). The ring-puckering, ring-twisting, and ring-flapping vibrations were of particular interest as these reflect the rigidity of the bicyclic ring system. The infrared spectra of BCB show very nice examples of vapor-phase band types and combination bands.

Vibrational spectra, theoretical calculations, and structure of 4-silaspiro(3,3)heptane.

Theoretical computations have been carried out for 4-silaspiro(3,3)heptane (SSH) in order to calculate its structure and vibrational spectra. SSH was found to have two puckered four-membered rings with dihedral angles of 34.2° and a tilt angle of 9.4° between the two rings. The puckering and tilting reduce the D2d symmetry to C2. Nonetheless, the vibrational assignments can be done quite well on the basis of D2d symmetry. This is confirmed by the fact that all but the lowest E vibrations show insignificant splitting into A and B modes of C2 symmetry. However, the observed splittings of the lowest frequency modes do confirm the lower conformational symmetry. The calculated infrared and Raman spectra were compared to the experimental spectra collected for the vapor, liquid, and solid states, and the agreement is excellent.

Spectroscopic and Theoretical Study of the Intramolecular π-Type Hydrogen Bonding and Conformations of 2-Cyclohexen-1-ol.

The infrared and Raman spectra of 2-cyclohexen-1-ol have been recorded and analyzed. The experimental work has been complemented by ab initio and density functional theory computations. The calculations show that in the vapor phase the conformations with the π-type hydrogen bonding are the lowest in energy, and these findings are supported by the experimental spectra, which agree well with the theoretical predictions. The six conformers predicted result from differences between the direction on the ring-twisting angle and the -OH internal rotation angle. The lowest-energy conformer has the hydrogen of the OH group pointing to the middle of the C═C double bond. The other conformers are calculated to be 72 cm(-1) (0.21 kcal/mol) to 401 cm(-1) (1.15 kcal/mol) higher in energy. In the liquid phase, only two conformers can be identified in the spectra, and these correspond to different directions of the ring-twisting.

Theoretical calculations and vibrational potential energy surface of 4-silaspiro(3,3)heptane.

Theoretical computations have been carried out on 4-silaspiro(3,3)heptane (SSH) in order to calculate its molecular structure and conformational energies. The molecule has two puckered four-membered rings with dihedral angles of 34.2° and a tilt angle of 9.4° between the two rings. Energy calculations were carried out for different conformations of SSH. These results allowed the generation of a two-dimensional ring-puckering potential energy surface (PES) of the form V = a(x1 (4) + x2 (4)) - b(x1 (2) + x2 (2)) + cx1 (2)x2 (2), where x1 and x2 are the ring-puckering coordinates for the two rings. The presence of sufficiently high potential energy barriers prevents the molecule from undergoing pseudorotation. The quantum states, wave functions, and predicted spectra resulting from the PESs were calculated.

Ring-Puckering Potential Energy Functions for Trimethylene Sulfide and Its Monovalent Cation

The spectra and ring-puckering potential energy function for trimethylene sulfide cation (TMS+) from vacuum ultraviolet mass-analyzed threshold ionization spectra have recently been reported. To provide an in-depth comparison of the potential function with that of trimethylene sulfide (TMS) itself, we have used ab initio MP2/cc-pVTZ calculations and DFT B3LYP/cc-pVTZ calculations to predict the structures of both TMS and TMS+ and then used these to calculate coordinate-dependent ring-puckering kinetic energy functions for both species. These kinetic energy functions allowed us to calculate refined potential energy functions of the puckering for both molecules based on the previously published spectra. TMS has an experimental barrier of 271 cm-1 and energy minima at ring-puckering angles of ±29°. For TMS+ the barrier is 60 cm-1 and the energy minima correspond to ring-puckering angles of ±21°. The lower barrier for the cation reflects the smaller amount of angle strain in the ring angles for TMS+.

Fluorescence excitation and ultraviolet absorption spectra and theoretical calculations for benzocyclobutane: Vibrations and structure of its excited S1(π,π*) electronic state

The fluorescence excitation spectra of jet-cooled benzocyclobutane have been recorded and together with its ultraviolet absorption spectra have been used to assign the vibrational frequencies for this molecule in its S1(π,π(*)) electronic excited state. Theoretical calculations at the CASSCF(6,6)/aug-cc-pVTZ level of theory were carried out to compute the structure of the molecule in its excited state. The calculated structure was compared to that of the molecule in its electronic ground state as well as to the structures of related molecules in their S0 and S1(π,π(*)) electronic states. In each case the decreased π bonding in the electronic excited states results in longer carbon-carbon bonds in the benzene ring. The skeletal vibrational frequencies in the electronic excited state were readily assigned and these were compared to the ground state and to the frequencies of five similar molecules. The vibrational levels in both S0 and S1(π,π(*)) states were remarkably harmonic in contrast to the other bicyclic molecules. The decreases in the frequencies of the out-of-plane skeletal modes reflect the increased floppiness of these bicyclic molecules in their S1(π,π(*)) excited state.