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 (3SiCPF) 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 3SiCPF, 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.

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.

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 [ , 1993, 115, 12132-12136] and 1,3-benzodioxole [ , 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.

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 (2SiCPF) and 1,1-dichlorosilacyclopent-2-ene (2SiCPCl). The structure and PEF for 1,1-dibromosilacyclopent-2-ene (2SiCPBr) were obtained by ab initio MP2/cc-pVTZ computations. The parent silacyclopent-2-ene (2SiCP) is puckered with a 49 cm barrier to planarity, 2SiCPF has a planar ring system, 2SiCPCl has a calculated tiny 4 cm barrier but is essentially planar, and the dibromide has a calculated barrier of 36 cm.

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.

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.

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.

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

The rotational spectra of octafluorocyclopentene (CF) 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.

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 and corresponding to the v(A) ring-inversion vibration. Fitting this data with a one-dimensional potential energy function allows the barrier to planarity of 8600 cm (24.

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.

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.

Anharmonic Vibrational Analysis of the Infrared and Raman Gas-Phase Spectra of s-trans- and s-gauche-1,3-Butadiene.

A quantum-mechanical (hybrid MP2/cc-pVTZ and CCSD(T)/cc-pVTZ) full quartic potential energy surface (PES) in rectilinear normal coordinates and the second-order operator canonical Van Vleck perturbation theory (CVPT2) are employed to predict the anharmonic vibrational spectra of s-trans- and s-gauche-butadiene (BDE). These predictions are used to interpret their infrared and Raman scattering spectra. New high-temperature Raman spectra in the gas phase are presented in support of assignments for the gauche conformer.

Infrared, Raman, and Ultraviolet Absorption Spectra and Theoretical Calculations and Structure of 2,3,5,6-Tetrafluoropyridine in its Ground and Excited Electronic States.

Infrared and Raman spectra of 2,3,5,6-tetrafluoropyridine (TFPy) were recorded and vibrational frequencies were assigned for its S electronic ground states. and density functional theory (DFT) calculations were used to complement the experimental work. The lowest electronic excited state of this molecule was investigated with ultraviolet absorption spectroscopy and theoretical CASSCF calculations.

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.

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.

Infrared and Raman spectra, theoretical calculations, conformations, and two-dimensional potential energy surface of 2-cyclopenten-1-one ethylene ketal.

The infrared and Raman spectra of the bicyclic spiro molecule 2-cyclopenten-1-one ethylene ketal (CEK) have been recorded. Density functional theory (DFT) calculations were used to compute the theoretical spectra, and these agree well with the experimental spectra. The structures and conformational energies for the two pairs of conformational minima, which can be defined in terms of ring-bending (x) and ring-twisting (τ) vibrational coordinates, have also been calculated.

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.

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.

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.

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.

Infrared, Raman, and ultraviolet absorption spectra and theoretical calculations and structure of 2,6-difluoropyridine in its ground and excited electronic states.

The infrared and Raman spectra of 2,6-difluoropyridine (26DFPy) along with ab initio and DFT computations have been used to assign the vibrations of the molecule in its S0 electronic ground state and to calculate its structure. The ultraviolet absorption spectrum showed the electronic transition to the S1(π,π*) state to be at 37,820.2 cm(-1).

Trans effect in halobismuthates and haloantimonates revisited. Molecular structures and vibrations from theoretical calculations.

Ab initio and density functional theory computations have been carried out to calculate the structures and vibrational spectra of halobismuthates and haloantimonates of formulas MX6(3-), M2X10(4-), and M2X9(3-) for M = Bi or Sb and X = I, Br, or Cl. The results have been compared to experimental crystal structures and the infrared and Raman spectra of these species as well as the (MX5(2-))n and (MX4(1-))n anions. Even though the calculations neglect the effect of which cation is present, they do a good job in verifying the observed trends in bond distances and bond stretching vibrational frequencies.

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.

Gas-phase Raman spectra of s-trans- and s-gauche-1,3-butadiene and their deuterated isotopologues.

The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d(2),1,1,4,4-d(4) and d(6) isotopologues have been recorded using intense (6 W) green laser excitation and sensitive CCD detection. Hundreds of bands have been observed and assigned for each isotopologue. These spectra provide the best data to date for the s-trans conformer and also provide the first direct observation of the gas-phase Raman bands of the s-gauche conformer.

Gas-phase Raman spectra and the potential energy function for the internal rotation of 1,3-butadiene and its isotopologues.

The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d(2), 1,1,4,4-d(4), and -d(6) isotopologues have been recorded with high sensitivity in the region below 350 cm(-1) in order to investigate the internal rotation (torsional) vibration. Based on more accurate structural information, the internal rotor constants F(n) were calculated as a function of rotation angle (ϕ). The data for all the isotopologues were then fit using a one-dimensional potential energy function of the form V = (1)/(2)∑V(n)(1 - cos ϕ).