The molecular structure and curious motions in 1,1-difluorosilacyclopent-3-ene and silacyclopent-3-ene as determined by microwave spectroscopy and quantum chemical calculations.

The molecules 1,1-difluorosilacyclopent-3-ene (3SiCPF2) and silacyclopent-3-ene (3SiCP) have been synthesized and studied using chirped pulse, Fourier transform microwave (CP-FTMW) spectroscopy. For 3SiCP this is the first ever microwave study of the molecule and, for 3SiCPF2, the spectra reported in this work have been combined with that of previous work in a global fit. The spectra of each contain splitting which has been fit using a Hamiltonian consisting of semirigid and Coriolis coupling parameters. A refit of the original 3SiCPF2 work was also carried out. All fits and approaches are reported. Analyses of the spectra provide evidence that the molecule is planar which is in agreement with the high-level calculations, but the source of the splitting in the spectra has not been determined.

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

The conformations of 2-cyclopenten-1-ol (2CPOL) have been investigated by high-level theoretical computations and infrared spectroscopy. The six conformational minima correspond to specific values of the ring-puckering and OH internal rotation coordinates. The conformation with the lowest energy possesses intramolecular π-type hydrogen bonding. A second conformer with weaker hydrogen bonding has somewhat higher energy. Ab initio coupled-cluster theory with single and double excitations (CCSD) was used with the cc-pVTZ (triple-ζ) basis set to calculate the two-dimensional potential energy surface (PES) governing the conformational dynamics along the ring-puckering and internal rotation coordinates. The two conformers with the hydrogen bonding lie about 300 cm-1 (0.8 kcal/mole) lower in energy than the other four conformers. The lowest energy conformation has a calculated distance of 2.68 Å from the hydrogen atom on the OH group to the middle of the C=C double bond. For the other conformers, this distance is at least 0.3 Å longer. The infrared spectrum in the O-H stretching region agrees well with the predicted frequency differences between the conformers and shows the conformers with the hydrogen bonding to have the lowest values. The infrared spectra in other regions arise mostly from the two hydrogen-bonded species.

Anomeric Effect in Five-Membered Ring Molecules: Comparison of Theoretical Computations and Experimental Spectroscopic Results.

As demonstrated in previous spectroscopic studies of 1,3-dioxole [ J. Am. Chem. Soc., 1993, 115, 12132-12136] and 1,3-benzodioxole [ J. Am. Chem. Soc., 1999, 121, 5056-5062], analysis of the ring-puckering potential energy function (PEF) of a "pseudo-four-membered ring" molecule can provide insight into understanding the magnitude of the anomeric effect. In the present study, high-level CCSD/cc-pVTZ and somewhat lower-level MP2/cc-pVTZ ab initio computations have been utilized to calculate the PEFs for 1,3-dioxole and 1,3-benzodioxole and 10 related molecules containing sulfur and selenium atoms and possessing the anomeric effect. The potential energy parameters derived for the PEFs directly provide a comparison of the relative magnitudes of the anomeric effect for molecules possessing OCO, OCS, OCSe, SCS, SCSe, and SeCSe linkages. The torsional potential energies produced by the anomeric effect for these linkages were estimated to range from 5.97 to 1.91 kcal/mol. The ab initio calculations also yielded the structural parameters, barriers to planarity, and ring-puckering angles for each of the 12 molecules studied. Based on the refined structural parameters for 1,3-dioxole and 1,3-benzodioxole, improved PEFs for these molecules were also calculated. The calculations also support the conclusion that the relatively low barrier to planarity of 1,3-benzodioxole results from competitive interactions between its benzene ring and the oxygen atom p orbitals.

Theoretical Calculations, Microwave Spectroscopy, and Ring-Puckering Vibrations of 1,1-Dihalosilacyclopent-2-enes.

High-level theoretical CCSD/cc-pVTZ computations have been carried out to calculate the structures and ring-puckering potential energy functions (PEFs) for 1,1-difluorosilacyclopent-2-ene (2SiCPF2) and 1,1-dichlorosilacyclopent-2-ene (2SiCPCl2). The structure and PEF for 1,1-dibromosilacyclopent-2-ene (2SiCPBr2) were obtained by ab initio MP2/cc-pVTZ computations. The parent silacyclopent-2-ene (2SiCP) is puckered with a 49 cm-1 barrier to planarity, 2SiCPF2 has a planar ring system, 2SiCPCl2 has a calculated tiny 4 cm-1 barrier but is essentially planar, and the dibromide has a calculated barrier of 36 cm-1. Microwave spectra of seven isotopic species of 2SiCPF2 were recorded on a chirped pulse, Fourier transform microwave (CP-FTMW) spectrometer in the 6-18 GHz region. The a-type and b-type transitions were observed. The rotational constants and three quartic centrifugal distortion constants were determined for the parent, 29Si, 30Si, and all singly substituted 13C isotopologues in natural abundance. This allowed for the determination of the heavy-atom structure of the ring and showed the ring to be planar. The experimentally determined rotational constants and geometrical parameters agree very well with the theoretical values and confirm the planarity of the five-membered ring. A comparison of the PEFs for the silane and the three dihalides shows the silane to have the stiffest puckering motion and the dibromide to be the least rigid.

Spectroscopic and Theoretical Study of the Intramolecular π-Type Hydrogen Bonding and Conformations of 3-Cyclopentene-1-amine.

The infrared and Raman spectra of 3-cyclopentene-1-amine (3CPAM) have been recorded and analyzed, and the experimental investigations have been complemented by theoretical calculations. Ab initio coupled cluster theory with single and double excitations (CCSD) was used with the cc-pVTZ (triple-ζ) basis set to calculate the conformational energy and geometrical parameters for each of the six conformers of this molecule. MP2/cc-pVTZ and B3LYP/cc-pVTZ computations were utilized to calculate the vibrational frequencies of the conformers. Both the spectra and theoretical calculations verify the presence of the conformers and show that the conformer at the lowest energy has intramolecular π-type hydrogen bonding involving the NH2 group. The hydrogen bonded conformer is about 2 kJ/mol lower in energy than the other conformers. The potential energy topographical contour map for the ring-puckering and NH2 internal rotation coordinates has been calculated, and this shows how the different conformers can interconvert into each other. The far-infrared spectrum in the 190 to 280 cm-1 region shows several NH2 internal rotation bands for each of the different conformers, and these are consistent with one-dimensional representations for their torsional motions.

Theoretical Study of Structures and Ring-Puckering Potential Energy Functions of Bicylo[3.1.0]hexane and Related Molecules.

Ab initio computations using the MP2/cc-pVTZ method have been carried out to calculate the structures and relative energies of the different conformations of five bicyclic molecules including bicyclo[3.1.0]hexane, 3-oxabicyclo[3.1.0]hexane, 6-oxabicyclo[3.1.0]hexane, 3,6-oxabicyclo[3.1.0]hexane, and bicyclo[3.1.0]hexan-3-one. Theoretical ring-puckering potential energy functions (PEFs) in terms of the ring-puckering coordinate have been calculated for each of the molecules and these were compared to those determined experimentally from spectroscopic data. Each potential function is asymmetric and has an energy minimum corresponding to where the five-membered ring is puckered in the same direction as the attached three-membered ring. In contrast to the experimental result, the calculations predict that bicyclo[3.1.0]hexane has a second shallow energy minimum. All of the other molecules have a single conformational minimum and their experimental and theoretical PEFs agree very well. The wave functions for the lower ring-puckering energy levels have been computed.

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+.

Microwave Spectra, Structure, and Ring-Puckering Vibration of Octafluorocyclopentene.

The rotational spectra of octafluorocyclopentene (C5F8) has been measured for the first time using pulsed jet Fourier transform microwave spectroscopy in a frequency range of 6 to 16 GHz. As in the molecule cyclopentene, the carbon ring is nonplanar, and inversion through the plane results in an inversion pair of ground state vibrational energy levels with an inversion splitting of 18.4 MHz. This large amplitude motion leads to the vibration-rotation coupling of energy levels. The symmetric double minimum ring-puckering potential function was calculated, resulting in a barrier of 222 cm-1. The rotational constants A0 = 962.9590(1) MHz, B0 = 885.1643(4) MHz, C0 = 616.9523(4) MHz, A1 = 962.9590(1) MHz, B1 = 885.1643(4) MHz, C1 = 616.9528(4) MHz, and two centrifugal distortion constants for each state were determined for the parent species and all 13C isotopologues. A mixed coordinate molecular structure was determined from a least-squares fit of the ground state rotational constants of the parent and each 13C isotopologue combined with the equilibrium bond lengths and angles from quantum chemical calculations.

Vapor-Phase Raman Spectra and the Barrier to Planarity of Cyclohexane.

The vapor-phase Raman spectra of an atmosphere of cyclohexane vapor heated to 90 and 110 °C collected over a large period of time and utilizing a high laser power of 4 W show hot band series starting at 380.8 cm-1 and corresponding to the v6(A1g) ring-inversion vibration. Fitting this data with a one-dimensional potential energy function allows the barrier to planarity of 8600 cm-1 (24.6 kcal/mol) to be calculated. Ab initio calculations (MP2/cc-pVTZ) predict a value of 10 377 cm-1 (29.7 kcal/mol), while DFT (B3LYP/cc-pVTZ) calculations predict 8804 cm-1 (25.2 kcal/mol).

Internal Rotation of Methylcyclopropane and Related Molecules: Comparison of Experiment and Theory.

The internal rotation about the single bond connecting a cyclopropyl ring to a CH3, SiH3, GeH3, NH2, SH, or OH group was investigated. Both CCSD/cc-pVTZ and MP2/cc-pVTZ ab initio calculations were performed to predict the structures of these molecules and their internal rotation potential energy functions in terms of angles of rotation. The barriers to internal rotation for the CH3, SiH3, and GeH3 molecules from the calculations agree well with the experimental ones, within -11% to +1% for CCSD/cc-pVTZ and -4% to +9% for MP2/cc-pVTZ. Comparisons between theory and experiment were also performed for propylene oxide and propylene sulfide, and the agreements were very good. Theoretical calculations were performed to compute the internal rotation potential energy function for cyclopropanol, and these were used to guide the determination of a potential function based on experimental data. This molecule has two equivalent synclinal (gauche) conformers with an estimated barrier of 759 cm(-1) (9.1 kJ/mol) between them. The minima are at internal rotation angles of the OH group of 109° and 251°. The theoretical potential functions for cyclopropanethiol and cyclopropylamine were also calculated, and these agree reasonably well with previous experimental studies.

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.

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.

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.

Vapor-phase Raman spectra, theoretical calculations, and the vibrational and structural properties of cis- and trans-stilbene.

The vapor-phase Raman spectra of cis- and trans-stilbene have been collected at high temperatures and assigned. The low-frequency skeletal modes were of special interest. The molecular structures and vibrational frequencies of both molecules have also been obtained using MP2/cc-pVTZ and B3LYP/cc-pVTZ calculations, respectively. The two-dimensional potential map for the internal rotations around the two Cphenyl-C(═C) bonds of cis-stilbene was generated by using a series of B3LYP/cc-pVTZ calculations. It was confirmed that the molecule has only one conformer with C2 symmetry. The energy level calculation with a two-dimensional Hamiltonian was carried out, and the probability distribution for each level was obtained. The calculation revealed that the "gearing" internal rotation in which the two phenyl rings rotate with opposite directions has a vibrational frequency of 26 cm(-1), whereas that of the "antigearing" internal rotation in which the phenyl rings rotate with the same direction is about 52 cm(-1). In the low vibrational energy region the probability distribution for the gearing internal rotation is similar to that of a one-dimensional harmonic oscillator, and in the higher region the motion behaves like that of a free rotor.

Fluorescence excitation and ultraviolet absorption spectra and theoretical calculations for benzocyclobutane: vibrations and structure of its excited S(1)(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.

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.

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 pi-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 pi-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.