Two-dimensional Flexible Model Research Articles

Theoretical characterization of dimethyl carbonate at low temperatures.

Highly correlated ab initio methods (CCSD(T) and RCCSD(T)-F12) are employed for the spectroscopic characterization of the gas phase of dimethyl carbonate (DMC) at low temperatures. DMC, a relevant molecule for atmospheric and astrochemical studies, shows only two conformers, cis-cis and trans-cis, respectively, of C2v and Cs symmetries. cis-cis-DMC represents the most stable form. Using RCCSD(T)-F12 theory, the two sets of equilibrium rotational constants have been computed to be Ae = 10 493.15 MHz, Be = 2399.22 MHz, and Ce = 2001.78 MHz (cis-cis) and to be Ae = 6585.16 MHz, Be = 3009.04 MHz, and Ce = 2120.36 MHz (trans-cis). Centrifugal distortions constants and anharmonic frequencies for all of the vibrational modes are provided. Fermi displacements are predicted. The minimum energy pathway for the cis-cis → trans-cis interconversion process is restricted by a barrier of ∼3500 cm(-1). DMC displays internal rotation of two methyl groups. If the nonrigidity is considered, the molecule can be classified in the G36 (cis-cis) and the G18 (trans-cis) symmetry groups. For cis-cis-DMC, both internal tops are equivalent, and the torsional motions are restricted by V3 potential energy barriers of 384.7 cm(-1). trans-cis-DMC shows two different V3 barriers of 631.53 and 382.6 cm(-1). The far-infrared spectra linked to the torsional motion of both conformers are analyzed independently using a variational procedure and a two-dimensional flexible model. In cis-cis-DMC, the ground vibrational state splits into nine components: one nondegenerate, 0.000 cm(-1) (A1), four quadruply degenerate, 0.012 cm(-1) (G), and four doubly degenerate 0.024 cm(-1) (E1 and E3). The methyl torsional fundamentals are obtained to lie at 140.274 cm(-1) (ν15) and 132.564 cm(-1) (ν30).

Isomer-Specific Ultraviolet Spectroscopy of m- and p-Divinylbenzene

The ultraviolet spectroscopy of m- and p-divinylbenzene isomers (mDVB and pDVB) was studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), laser-induced fluorescence (LIF), UV-UV hole-burning spectroscopy (UVHB), and single vibronic level fluorescence spectroscopy (SVLF). In mDVB, there are three low-energy conformations, cis-cis, cis-trans, and trans-trans whose S1 <-- S0 origins occur at 31,408, 31,856, and 32,164 cm(-1), respectively, as confirmed by UVHB spectroscopy. There are two possible conformations in pDVB, cis and trans. UVHB studies confirm the S1 <-- S0 origin of trans-pDVB occurs at 32,553 cm(-1), and the corresponding cis-pDVB origin is tentatively assigned to a transition at 32 621 cm(-1). SVLF studies were used to determine several of the vinyl torsional levels of the isomers of mDVB and pDVB. A two-dimensional flexible model was used to fit these levels in mDVB to a potential form and determine the barriers to isomerization.

The aerodynamic performance of paragliders

AbstractAn analysis of paraglider performance has revealed that wing section drag is the most significant contribution to overall drag. Wind tunnel measurements performed on two-dimensional hollow models indicate that intake drag is less significant than previously thought. An experimental investigation into the characteristics of a ‘quasi ’ -two-dimensional flexible model consisting of solid ribs covered with a fabric skin was performed at realistic Reynolds numbers. The main cause of performance deterioration was found to be a significant reduction in section lift coefficient when compared to a similar solid wing section. This is believed to be mainly due to two factors: a large trailing edge separation and the deformation of the wing between ribs. The deformation was measured and it was shown that the deformed shape is less capable of generating high lift coefficients than the design section. It is thought that the extent of the trailing edge separation is increased due to the presence of streamwise grooves caused by the shape deformation of the wing. The shape of the separated region was found to be strongly three-dimensional with the separation point being about half a chord-length further upstream along the ribs. A small separation bubble was also observed immediately behind the lip of the intake, due to the fabric ‘flaring’ open. Based on the observations presented a number of suggestions for improved wings have been made.

Millimeter-Wave Absorption Free Jet Spectrum, Barriers to Internal Rotation, and Torsional Relaxation inp-Anisaldehyde

The rotational spectrum ofp-anisaldehyde in a supersonic expansion has been investigated in the frequency range 60–78 GHz. Transitions up toJ= 57 measured for theantiandsynconformers have been used to determine complete sets of fourth-order centrifugal distortion constants. Methyl group internal rotation splittings have also been observed for some of the lines and have yielded the respective barriers for both conformers. A vibrational satellite observed for each of the two conformers has been assigned to the respective first excited methoxy torsional state. The jet conditions, in particular the stagnation pressure of the carrier gas, can be adjusted to control the degree of cooling achieved in the expansion. This possibility has been exploited in the present work to enhance the intensities of the observed vibrational satellites. Applying a two-dimensional flexible model to the methoxyl and aldehydic groups torsions, the potential energy and structural deformation parameters transferred from anisole and benzaldehyde have been found to be suitable to describe these motions inp-anisaldehyde. Mixing ofantiandsynconformations has been found to be insignificant within the first 82 calculated torsional eigenstates and therefore less likely to explain the intermediate bands observed in the low-resolution microwave spectrum than the internal vibrational relaxation suggested by R. K. Bohn, M. S. Farag, C. M. Ott, J. Radhakrishnan, S. A. Sorenson, and N. S. True (1992,J. Mol. Struct.268,107–121). A qualitative discussion of relaxation among the torsional states and its effect on the rotational spectrum is given.

Microwave and Infrared Spectra,ab InitioCalculation, and Two-Dimensional Model of Amino Group Inversion and Ring Puckering in 2,5-Dihydropyrrole

The microwave spectra of 2,5-dihydropyrrole and 2,5-dihydropyrrole-1-d1have been measured with Stark and Fourier transform spectrometers in the range 10–39 GHz. Rotational constants, centrifugal distortion constants, and14N quadrupole coupling constants have been determined from the observed transition frequencies for the ground vibrational state. In addition, two satellites of the normal species and one satellite of the deuterated species have been identified and measured. Splittings of the rotational transitions due to amino group inversion tunneling have been observed and analyzed. Infrared transitions of the amino group inversion mode have been measured in the range 490–720 cm−1. The effect of ring puckering on the inversion motion of the amino group in 2,5-dihydropyrrole and 2,5-dihydropyrrole-N-d1has been investigated byab initiocalculations and two-dimensional flexible model calculations from the results of microwave and infrared spectroscopy. The observed molecular properties have been reproduced by a model which involved adjustable parameters for the potential energy surface and the structural relaxation of the CCC valence angles. Additional parameters have been transferred from theab initiocalculations. The adjustment of the model to the experimental data has yielded an equatorial equilibrium conformation with slightly larger CCC valence angle than in the most stable axial conformation. Excitation of the first ring puckering state has been found to enhance the inversion tunnel splittings.

ChemInform Abstract: Microwave Spectrum and Torsional Potential Energy Surfaces of cis‐Crotyl Alcohol

Abstract The microwave spectrum of cis-crotyl alcohol is reported for the normal and -OD isotopic species. The assigned transitions are consistent with a hydrogen-bonded conformation having the oxygen atom skew with respect to the CC and the hydroxylic hydrogen gauche with respect to the CC bond. The valence angles CCC and OCC are a few degrees larger than in trans-crotyl alcohol. From measurements of Stark lobes displacements the electric dipole moment has also been determined. Analysis of the A-E doublings in the ground and some torsionally excited states allowed us to infer the potential energy surfaces of the methyl and skeletal torsions and to determine kinetic and potential energy interaction between the two motions through the use of a two-dimensional flexible model.

Microwave spectrum and torsional potential energy surfaces of cis-crotyl alcohol

The microwave spectrum of cis-crotyl alcohol is reported for the normal and -OD isotopic species. The assigned transitions are consistent with a hydrogen-bonded conformation having the oxygen atom skew with respect to the CC and the hydroxylic hydrogen gauche with respect to the CC bond. The valence angles CCC and OCC are a few degrees larger than in trans-crotyl alcohol. From measurements of Stark lobes displacements the electric dipole moment has also been determined. Analysis of the A- E doublings in the ground and some torsionally excited states allowed us to infer the potential energy surfaces of the methyl and skeletal torsions and to determine kinetic and potential energy interaction between the two motions through the use of a two-dimensional flexible model.

High resolution microwave spectrum and torsional potential energy surfaces oftrans-crotyl alcohol

The high resolution microwave spectrum of trans crotyl alcohol, CH3-CH=CH-CH2-OH, is reported for the normal and -OD isotopic species. The assigned transitions are consistent with a hydrogen bonded conformation having the oxygen atom skew with respect to the C=C bond and the hydroxylic hydrogen gauche with respect to the C-C bond. The analysis of the A-E doublings in the ground and some torsionally excited states allowed the potential energy surfaces of the methyl and skeletal torsions to be inferred and the kinetic and potential energy interaction between the two motions, through the use of a two-dimensional flexible model, to the determined. The electric dipole moment has also been determined.

Hydrogen bond and torsion-torsion interaction in 2-methylallyl alcohol from the microwave spectrum

The microwave spectrum of 2-methylallyl alcohol, CH 2C(CH 3)CH 2OH, has been investigated for the normal and −OD isotopic species. The assigned transitions are consistent with a hydrogen-bonded conformation having the oxygen atom skew with respect to the CC bond and the hydroxylic hydrogen gauche with respect to the CC bond. From the Stark effect measurements on several transitions the electric dipole moment has been determined. The V 3 barrier to the internal rotation of the methyl group has been obtained in first approximation from the ground state AE rotational splittings. The analysis of the AE doublings has been extended to the first excited state of the methyl group internal rotation and to the first excited state of the CC skeletal torsion. By using a two-dimensional flexible model the potential surface of the two motions has been described with the following parameters V 3 = 605(5) cm −1; V 6 = −13(6) cm −1 for the methyl torsion and k = 1.76(2) cm −1 deg −2; a = −0.0022(4) deg −1 for the skeletal torsion harmonic and cubic force constants.