Vibrational and Conformational Analysis of 5-Methyl-2-Hexyne

Infrared and Raman spectra were obtained for 5- methyl-2-hexyne and wereinterpreted with the aid of normal coordinate calculations. The spectra and molecular mechanics calculations show the compound to exist in two spectroscopically distinguishable stable conformations, with the C₁ conformer being only a little more stable than the C_s, conformer. Vibrational assignments were made for both conformers.     

Vibrational and Conformational Analysis of 2-Methyl-1-butene

Normal coordinate calculations were made for 2-methyl-1-butene to aid in making vibrational assignments for both conformers. A partial solid-state IR spectrum was obtained that showed the presence of only the trans (C_s) conformer. Molecular mechanics and semi-empirical molecular orbital calculations were also made for both conformers.     

Vibrational Analysis of 1,2-Dibromopropane and 1,2-Dibromopropane-D6

Abstract

Infrared spectra were obtained for 1,2-dibromopropane-d₆ in the liquid and in the unannealed and annealed solid states. Vibrational assignments were made for the three conformers of 1,2-dibromopropane and the three conformers of 1,2-dibromopropane-d₆ with the aid of normal coordinate calculations. All three possible conformers of CD₂BrCDBrCD₃ were found to be present in the liquid and unannealed solid, but the P_HS_HH conformer was absent in the annealed solid.     

Conformational Analysis of 2-Methylhexane and 3-Methylhexane

Abstract

Infrared spectra were obtained for 2-methylhexane and 3-methylhexane, and were interpreted with the aid of normal coordinate calculations. It was shown that 2-methylhexane exists in three conformations. Evidence for the presence of three conformations of 3-methylhexane is presented, but a fourth conformer probably exists. Transferred force constant values resulted in an overall average difference between observed and calculated wavenumbers of 4.3 cm^?1, or 0.5%, for seven conformers of these two compounds.     

Conformational Analysis of Some Chloro and Bromoalkanes

Rotational isoraerism has been shown by vibrational spectroscopy to exist in l-chloro-3-methylbutane,¹ l-chromo-3-methylbutane,² 2-chloro-4-methylpentane,^3,4 2-bromo-4-methylpentane,⁴ 1,3-dichlorobutane,⁵ and 1,3-dibromobutane.⁵ In some cases, definite conclusions were drawn concerning the number and identification of the conformers present. In other cases, it was not possible to determine with certainty which conformers exist. Molecular mechanics calculations have now been made for these six compounds in order to obtain additional information about their conformational behavior, including energies and structures of the possible conformers. The calculations were done with the MM2 program written by Allinger and Yuh and converted by Petillo to run on a microcomputer.⁶     

Conforhational Analysis of 3,3-Dimethylheptane

Infrared and Raman spectra were obtained for 3,3-dimethylheptane and were interpreted with the aid of normal coordinate calculations. The presence of only three conformers could be verified in this way. Molecular mechanics calculations showed the probable presence of all eleven possible conformers, with concentrations ranging from 27% to 33%. Structural parameters and the heat of formation were obtained from the calculations.     

Conformational Analysis of Some Dichloroalkanes

Molecular mechanics calculations were made for 1,1-dichlorobutane, 2,2-dichlorobutane, and 1,2-dichloro-2-methylpropane in order to compare the results with conclusions obtained from vibrational spectra concerning the conformational behavior of these compounds. Calculations were also made for 1,2-dichloro-2-methylbutane, although vibrational spectra are not available for this compound. The structures and relative energies of the most abundant conformers are given.     

The geometry of organophosphonates: Fourier-transform microwave spectroscopy and ab initio study of diethyl methylphosphonate, diethyl ethylphosphonate, and diisopropyl methylphosphonate

The rotational spectra of diethyl methylphosphonate (DEMP), diethyl ethylphosphonate (DEEP), and diisopropyl methylphosphonate (DIMP) in supersonic expansions have been acquired using Fourier-transform microwave spectroscopy. Spectroscopic constants have been determined for five distinct conformers of the three molecules. Experimental data have been compared to ab initio calculations performed for each species. For both DEMP and DEEP, the calculations indicate the presence of several low-energy conformers (i.e., ?∼400 cm⁻¹ above the ground state) may be present at room temperature (300 K) for both DEMP and DEEP. When entrained in a supersonic expansion, the rotational temperatures of the samples are much colder (∼2 K); nonetheless, spectra from three conformers of DEEP are still observed experimentally, whereas only one conformer of DEMP is observed. In contrast, only a single low-energy conformer of DIMP is predicted by theory, and is present in the molecular beam. The relative abundance of low-energy conformers of DEMP and DEEP is attributed to the flexibility of the ethoxy groups within each molecule. The presence of multiple DEEP conformers in the supersonic beam indicates a more complex potential energy surface for this molecule that is directly related to conformational shifts of the PCH₂CH₃ group. Conversely, the absence of low-energy conformers of DIMP is attributed to steric hindrance between isopropoxy groups in the molecule. The internal rotation barrier for the PCH₃ group in DEMP and DIMP is compared to that found in DMMP and several phosphonate-based chemical weapon agents.     

Conformational Analysis of 2, 3, 3-Trimethylhexane

Infrared and Raman spectra were obtained for 2, 3, 3-trimethylhexane and were interpreted with the aid of normal coordinate calculations. It was concluded that this compound exists in three molecular conformations, but a fourth conformer may exist. Vibrational assignments were made for the three conformers.     

Microwave spectrum of 1,2-propanediol

The microwave spectrum of the sugar alcohol 1,2-propanediol (CH₃CHOHCH₂OH) has been measured over the frequency range 6.5–25.0 GHz with several pulsed-beam Fourier-transform microwave spectrometers. Seven conformers of 1,2-propanediol have been assigned and ab initio electronic structure calculations of the 10 lowest energy forms have been calculated. Stark effect measurements were carried out on several of the lowest energy conformers to provide accurate determinations of the dipole moment components and assist in conformer assignment.     

Conformational stability from Raman spectra,r0 structural parameters,and vibrational assignment of methylcyclohexane

The Raman spectra (3500–50 cm⁻¹) of the liquid and solid methylcyclohexane and the infrared spectra of the gas and solid methylcyclohexane have been recorded. The Raman band at 754 cm⁻¹ in the liquid has been confidently assigned to the less stable axial conformer and its intensity was recorded as a function of temperature from 25 to −95 °C. By the utilization of 15 different temperatures, the enthalpy difference between the more stable chair‐equatorial conformer and the chair‐axial form was determined to be 712 ± 71 cm⁻¹ (8.50 ± 0.84 kJ/mol). The ab initio predicted value of 710 cm⁻¹ (8.50 kJ/mol) from the MP2(full)/6‐311G(2d,2p) calculations with and without diffuse functions is in excellent agreement. The harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational wavenumbers have been obtained for both conformers from MP2(full)/6‐31G(d) ab initio calculations. With two scaling factors of 0.88 for the C‐H stretches and 0.9 for the remaining ones, the fundamental wavenumbers have been predicted and along with the depolarization values and infrared band contours (B‐type for A″ modes) a complete vibrational assignment has been made for the chair‐equatorial conformer. Predicted r₀ structural parameters have been provided from adjusted parameters from ab initio MP2(full)/6‐311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some similar molecules. Copyright © 2009 John Wiley & Sons, Ltd.     

Vibrational spectroscopic studies,conformations and ab initio calculations of 1,2‐bis(trifluorosilyl)ethane (SiF3CH2CH2SiF3)

Infrared spectra of 1,2‐bis(trifluorosilyl)ethane (SiF₃CH₂CH₂SiF₃) were obtained in the vapour and liquid phases, in argon matrices and in the solid phase. Raman spectra of the compound as a liquid were recorded at various temperatures between 293 and 270 K and spectra of an apparently crystalline solid were observed. The spectra revealed the existence of two conformers (anti and gauche) in the vapour, liquid and in the matrix. When the vapour was chock‐frozen on a cold finger at 78 K and annealed to 150 K, certain weak Raman bands vanished in the crystal. The vibrational spectra of the crystal demonstrated mutual exclusion between IR and Raman bands in accordance with C_2h symmetry. Intensity variations between 293 and 270 K of pairs of various Raman bands gave ΔH(gauche—anti) = 5.6 ± 0.5 kJ mol⁻¹ in the liquid, suggesting 85% anti and 15% gauche in equilibrium at room temperature. Annealing experiments indicate that the anti conformer also has a lower energy in the argon matrices, is the low‐energy conformer in the liquid and is also present in the crystal. The spectra of both conformers have been interpreted, and 34 anti and 17 gauche bands were tentatively identified. Ab initio and density functional theory (DFT) calculations were performed giving optimized geometries, infrared and Raman intensities and anharmonic vibrational frequencies for both conformers. The conformational energy difference derived in CBS‐QB3 and in G3 calculations was 5 kJ mol⁻¹. Copyright © 2009 John Wiley & Sons, Ltd.     

Millimeter-wave spectrum and ab initio DFT calculation of the C-gauche conformer of allyl isocyanate

The analysis of the ground state rotational spectrum of the C-gauche conformer of allyl isocyanate has been extended up to the frequency range of 100.0 GHz. A detailed centrifugal distortion analysis has been carried out using previously reported and newly measured transition frequencies. More accurate values of the rotational and centrifugal distortion constants are presented. Quantum chemical calculations at DFT levels of theory using large basis sets b3lyp/6-311G(d, p), have also been made to compute rotational constants, dipole moments and potential energies for different conformers of this molecule. Finally, different bond lengths and centrifugal distortion constants for the C-gauche conformer have been computed.     

Conformational Analysis of 4,4-Dimethylheptane

Abstract

Infrared and Raman spectra were obtained for 4,4-dimethylheptane and were interpreted with the aid of normal coordinate calculations. It was shown that this compound exists in three conformations. Force constant values transferred from dimethylpentanes resulted in an overall average difference between observed and calculated wavenumbers of 4.5 cm^?1 for the three conformers.     

Reaction products from a discharge of N2 and H2S: The microwave spectrum of NH2SH

Thiohydroxylamine has been identified as one of the reaction products from the discharge reaction of N₂ + H₂S. Both cis and trans conformers have been observed. The rotational spectra have been studied from 56 to 170 GHz for the normal species and several deuterated isotopic species of each conformer. The electric dipole moments of both conformers have been determined. A number of the transitions of the cis conformer exhibit splittings due to the nuclear quadrupole moment of the ¹⁴N nucleus. A least squares fit of the frequency splittings have led to an analysis of the eQq values. Ab initio calculations using a 4-31G basis set both with and without polarization functions have been carried out to aid in the analysis and to provide a final structural comparison with the microwave results.     

High-resolution spectroscopy near the continuum limit: the microwave spectrum of trans-3-bromo-1,1,1,2,2-pentafluoropropane

ABSTRACT

The microwave spectrum of 3-bromo-1,1,1,2,2-pentafluoropropane has been observed using CP-FTMW spectroscopy. Potential energy scans have been performed and confirm the existence of two conformers – trans and gauche – for which further structural optimisations and electric field gradient calculations have been performed in order to get highly accurate nuclear quadrupole coupling constants for assignment purposes. The combination of multiple conformers and large nuclear quadrupole coupling constants produce a very dense spectrum at an estimated 1?transition/MHz, near the continuum limit. This spectral density makes it necessary to have very sophisticated computational approaches in order to get geometric and electronic structures that are very close to experimental observation. Analysis of the spectrum allowed for the assignment of the trans conformer, but the gauche proved to be prohibitive, although it is believed to be present in the current spectrum. Full analysis of the rotational spectroscopic parameters of two isotopologues – the⁷⁹Br and⁸¹Br – have been observed and are reported. Geometric analysis of the experimentally observed conformer is also reported using Kraitchman coordinate and second moments arguments. Further analysis of the spectrum reveals the occurrence of dipole-forbidden, nuclear quadrupole allowed transitions with one forbidden transition possessing the first known x-type forbidden transition linkage pathway.     

Conformational stability and vibrational study of phenylacetyl chloride

The Fourier transform infrared spectroscopy and Fourier transform Raman spectra of phenylacetyl chloride were recorded and analyzed in the range 3500–400 and 3500–200 cm^?1 at room temperature, respectively. In order to obtain the structural information and conformational stabilities, a potential energy surface scan for internal rotation was carried out at the B3LYP/6‐31G(d) level. The potential energy surface reveals that the title compound has two minimal conformers (A and B). The optimized geometries, structural parameters, stabilities, energies, thermodynamic parameters, vibrational wavenumbers, infrared intensities, and Raman activities for the two conformers (A and B) have been obtained by employing B3LYP and MP2 calculations with 6‐311++G (d, p) basis sets. The conformational energy difference between A and B is very small, indicating that the B conformer coexists with the A conformer. The detailed vibrational assignments of vibrational spectra of each conformer have been made on the basis of the potential energy distributions analysis. The highest occupied molecular orbital –lowest unoccupied molecular orbital energy gap and molecular electrostatic potential of the two conformers have been also calculated for comparison of their chemical activities. Copyright © 2015 John Wiley & Sons, Ltd.     

An investigation into the relaxation of the conformers of butan-2-ol in a supersonic expansion

The behavior of the chiral secondary alcohol butan-2-ol in both helium and argon-based supersonic expansions has been investigated by a combination of high-resolution microwave spectroscopy and ab initio molecular structure calculations. The study extends and complements a previous investigation that concentrated solely on an argon-based expansion. The spectrum of the helium-based expansion has been shown to contain at least six conformers of butan-2-ol, indicating a marked difference from the spectrum of the argon-based system, which has been shown to consist of contributions from only three. This indicates a difference in the relative abilities of the two gases to produce efficient conformer relaxation. By consideration of the calculated barriers to inter-conversion between the various conformers, we are able to show that this behavior is qualitatively consistent with previous observations of similar systems and a few remarks regarding the likely cause of this behavior are also made.Finally, we have been able to identify and characterize the four ¹³C isotopomers of the lowest energy butan-2-ol conformer and to perform a Kraitchmann-type analysis to confirm the conformation of the carbon chain in this species.     

A Vibrational Force Field For 1-Alkynes and Nitriles

Abstract

Normal coordinate calculations were made for 1-butyne, propionitrile, and the two conformers each of 1-pentyne and butyronitrile, using a thirty-one parameter modified valence force field. Only the triple bond stretching force constant was assumed to be different in the two families of compounds. Twenty force constants were refined to fit 117 frequencies of the six molecules, with the average error being 5.1 cm^?1, or 0.65%.     

Vibrational Spectra,Normal Coordinate Calculations,and Molecular Mechanics Calculations for 2, 3, 3-Trimethyl-1-Butene

Infrared and Raman spectra were obtained for 2, 3, 3-trimethyl-1-butene, and a vibrational assignment was made with the aid of normal coordinate calculations. Molecular mechanics calculations were also made to determine molecular parameters of the stable conformation. Values for the force constants of a forty-one parameter modified valence force field were obtained that will be used for other 2-methyl-1-alkenes that contain substituted methyl groups.