Microwave spectrum,conformation, barrier to internal rotation and dipole moment of pyruvic acid

Microwave spectra of CH₃COCOOH and CH₃COCOOD are reported. The preferred conformation of the molecule is demonstrated to possess a planar HCCOCOOH skeleton with two out-of-plane hydrogens. The two carbonyl groups are trans to each other and a weak five-membered hydrogen bond is formed between the carboxyl group hydrogen atom and the carbonyl group oxygen atom. The methyl group conformation is discussed. A computer programme based on “the principal axis method” is described in some detail and the results of a least squares analysis of the observed spectra are outlined. The barrier to internal rotation was determined as V₃ = 965±40 cal mol^?1 for both isotopic species. Stark effect measurements yielded μ_a = 2.27±0.02 D, μ_b = 0.35±0.02 D and μ_tot = 2.30±0.03 D for the dipole moment and its components along the principal axes.     

Impact of molecular conformation on barriers to internal methyl rotation: the rotational spectrum of m-methylbenzaldehyde

The ground state spectrum of m-methylbenzaldehyde (m-MBA) was measured with a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The methyl rotor on m-MBA introduces an internal rotation barrier, which leads to splitting of the torsional energy level degeneracy into A and E states. Ab initio calculations predict a low torsional barrier for both the O-cis and O-trans conformers, resulting in a large doublet splitting up to several gigahertz in the frequency spectrum. The rotational constants, distortion terms, and V(3) values for both species have been determined from the ground state rotational spectrum using the BELGI-C(s) fitting program. There are significant differences in the torsional potential for the O-cis and O-trans m-MBA conformers. Molecular orbitals and resonance structures for each conformer are analyzed to understand the difference in torsional barrier height as well as the irregular shape of the O-trans torsional potential.     

Nonbonded interactions and the barrier heights to internal rotation of the methyl group: Part I. Cis- and trans-substituted propenes

Strain energies and barrier heights for rotation of the methyl group in several cis- and trans-substituted propenes have been calculated by means of the Wiberg method. The calculations reveal that the lowering of the barriers in cis-substituted propenes, as compared with the values in trans-compounds and in propene itself, can be ascribed to steric factors. The proximity of the barriers in trans-propenes and in the parent molecule is also considered to be due to nonbonded interactions.     

Microwave spectrum,conformation, barrier to internal rotation,centrifugal distortion,and dipole moment of methylpropargyl ether

The microwave spectrum of methylpropargyl ether, CH₃OCH₂CCH, has been investigated in the 11.9–26.5 GHz region. Only the gauche rotamer with a dihedral angle of 68° ± 2° from the syn position was assigned. Other forms are not present in concentrations exceeding 10 % of the total. The barrier to internal rotation of the methyl group was determined to be 2512 ± 75 cal mol^?1. The dipole moment components are μ_a = 0.290 ± 0.003 D, μ_b = 0.505 ± 0.012 D, and μ_c = 1.016 ± 0.003 D. The total dipole moment is 1.171 ± 0.013 D. Extensive centrifugal distortion analyses have been carried out for the ground as well as for two vibrationally excited states. For the ground state, transitions up to J = 77 were assigned and a large centrifugal distortion exceeding 9 GHz enabled the determination of accurate quartic and significant sextic distortion coefficients.     

Microwave spectrum,dipole moment and barrier to internal rotation of trans-methyl glyoxal

The microwave spectrum of the trans conformer of methyl glyoxal has been investigated in the frequency range from 8 to 40 GHz. The rotational constants have been determined for the A state: A = 9102.4332(31), B = 4439.8832(27) and C = 3038.9404(22) MHz. Quantitative measurements of the Stark effect have yielded the components of the electric dipole moment: μ_a = 0.1597(11), μ_b = 0.9620(7) and μ_total = 0.9751(7) D. From the splittings of rotational transitions the three-fold barrier to internal rotation of the methyl top has been found to be V₃ = 269.1 (3) cm^?1.     

Microwave spectrum,barrier to internal rotation and dipole moment in 5-methyl-pyrimidine

The microwave rotational spectrum of 5-methyl-pyrimidine has been investigated in the region from 8 to 27 GHz, the three types of lines to be expected for a molecule of this symmetry and with a very low sixfold barrier hindering internal rotation of the methyl top have been found: m = 0; |m| ≠ 0,3; |m| = 3n. From the m = 0 (a-type transitions) the rotational constants A′ (less methyl top) = 6108.41, B = 2642.198, C = 1844.196 MHz and the dipole moment μ = 2.881D have been determined. From the wide splitting of the lines |m| = 3, |k| = 1 the potential barrier has been derived as V₆ = 11.73 cal/mole.     

Rotational spectrum and internal dynamics of tetrahydrofuran-krypton

The rotational spectrum of the tetrahydrofuran-krypton van der Waals complex has been investigated by pulsed-jet Fourier transform microwave spectroscopy. The spectra of the (84)Kr and (86)Kr isotopologues have been assigned and the krypton atom is located nearly over the oxygen atom, almost perpendicular to the COC plane. Each rotational transition is split into two component lines due to, according to the observed Coriolis coupling term between the tunneling states, the residual pseudorotational effects of the ring in the complex. The splitting between the two vibrational sublevels is 87.462(2) and 87.062(2) MHz for the (84)Kr and (86)Kr isotopologues, respectively. These splittings have been used to determine the barrier to inversion, B(2) = 67 cm(-1). The dissociation energy has been estimated to be 3.7 kJ mol(-1) from centrifugal distortion effects.     

Microwave spectrum,structure, dipole moment and barrier to internal rotation of methyltrifluorosilane

The rotational spectrum of methyltrifluorosilane in the ground and the first three excited states of the torsional mode have been investigated in the region of 12.5–40.0 GHz. The rotational transitions for the ¹³C isotopic species have also been measured. The following structural parameters have been determined: r(CH) = 1.081 ± 0.004 Å, ∠ HCSi = 111°1' ± 30', r(CSi) = 1.812±0.014 Å, r(SiF) = 1.574 ± 0.007 Å, ∠ FSiC = 112°20'± 1°6'. The structural parameters are compared to the corresponding ones for similar molecules. The dipole moment was determined to be 2.33 ± 0.10 D. From relative intensity measurements, the barrier to internal rotation was found to be 0.93 ± 0.09 kcal mol⁻¹; this value is consistent with the values obtained for other methylfluorosilanes.     

The multiphoton ionization spectrum of toluene in a supersonic free jet: internal rotation of the methyl group

The S₁← S_o electronic transitions of toluene involving also some internal rotational levels were observed for the first time in the multiphoton ionization spectrum in a supersonic jet. A large population in several low-lying internal rotational levels and a strong coupling between electronic motion and the internal rotation are suggested.     

The influence of basis set on the ab initio prediction of internal rotation barrier heights in ethene thiol

In this report the effects of basis set size and electron correlation on the internal rotation barrier heights in ethene thiol are investigated and compared with experimental data. At all levels of theory reasonable agreement is obtained for the barrier tosyn/anti rotation (experimental value: 9.6 kJ/mol), however, theanti barrier (experimental value: 0.14 kJ/mol) is consistently overestimated by approximately a factor of 10. A comparison of ab initio predictions of torsional energy distributions and rotational constant variations as a function of torsional state with the corresponding experimental quantities is presented.     

Microwave spectrum and internal rotation of nitroethane

The rotational spectrum of nitroethane has been measured between 7.5 and 40 GlIz. It was found tvpicai for an asymmetric rotor with nearly free internal rotation of the nitro grouop. Several low-J R-branch transition series have been assigned.     

Far-IR spectrum,conformation stability,barriers to internal rotation,vibrational assignment,and ab initio calculations of 3-chloro-2-methylpropene

The far-IR spectrum from 375 to 30 cm⁻¹ of gaseous 3-chloro-2-methylpropene, CH₂=C(CH₃)CH₂Cl, has been recorded at a resolution of 0.10 cm⁻¹. The fundamental asymmetric torsional mode for the gauche conformer is observed at 84.3 cm⁻¹ with three excited states falling to lower frequency. For the higher energy s-cis conformer, where the chlorine atom eclipses the double bond, the asymmetric torsion is observed at 81.3 cm⁻¹ with two excited states falling to lower frequency. Utilizing the s-cis and gauche torsional frequencies, the gauche dihedral angle and the enthalpy difference between conformers, the potential function governing the interconversion of the rotamers has been calculated. The determined potential function coefficients are (in reciprocal centimeters): V₁=189±12, V₂=−358±11, V₃=886±2 and V₄=−12±2 with an enthalpy difference between the more stable gauche and s-cis conformers of 150 ±25 cm⁻¹ (430 ± 71 cal mol⁻¹). This function gives values of 661 cm⁻¹ (1.89 kcal mol⁻¹), 1226 cm⁻¹ (3.51 kcal mol⁻¹) and 812 cm⁻¹ (2.32 kcal mol⁻¹), for the s-cis to gauche, gauche to gauche, and gauche to s-cis barriers, respectively. From the methyl torsional frequency of 170 cm⁻¹ for the gauche conformer, the threefold barrier of 678 cm⁻¹ (1.94 kcal mol⁻¹) has been calculated. The asymmetric potential function, conformational energy difference and optimized geometries of both conformers have also been obtained from ab initio calculations with both the 3–21G^* and 6–31G^* basis sets. A normal-coordinate analysis has also been performed with a force field determined from the 3–21G^* basis set. These data are compared with the corresponding data for some similar molecules.     

Size-selected methyl lactate clusters: fragmentation and spectroscopic fingerprints of chiral recognition

A comprehensive experimental study of the OH stretching vibrations of size-selected clusters of enantiopure and racemic methyl lactate is presented. For the size selection, we measured angular dependent mass spectra and time-of-flight distributions at the different fragment masses. In this way the fragmentation of these clusters upon electron impact ionization is obtained. The largest fragment masses of the neutral (MLac)n clusters are the protonated (MLac)n-1H+ ions. The results of a pressure dependent study in an FTIR jet experiment are compared with completely size-selected experiments based on atomic beam deflection and depletion spectroscopy. The size assignments and spectra agree for dimers and trimers. Structures and spectral information for the trimer and the tetramer at density functional and MP2 level are provided. Selective self-aggregation and chiral recognition was observed for homochiral trimers. They exhibit a ring structure bound by OH...OH hydrogen bonds. A spectacular switch in the hydrogen bonding topology was observed for the tetramer. The homochiral enantiomer exhibits cooperative OH...OH bonding, while the heterochiral version shows isolated OH...O=C bonding in a symmetric SRSR arrangement. The crucial ingredients for this identification are the size-selective IR spectra with their different shifts and line patterns which are reproduced by the calculations.     

Magnetic hyperfine coupling of a methyl group undergoing internal rotation: a case study of methyl formate

The hyperfine structure of methyl formate was recorded in the 2-20 GHz range. A molecular beam coupled to a Fourier transform microwave spectrometer having an instrumental resolution of 0.46 kHz and limited by a Doppler width of a few kHz was used. A-type lines were found split by the magnetic hyperfine coupling while no splittings were observed for E-type lines. Symmetry considerations were used to account for the internal rotation of the methyl top and to derive effective hyperfine coupling Hamiltonians. Neglecting the spin-rotation magnetic coupling, the vanishing splittings of the E-type lines could be understood and analyses of the hyperfine patterns of the A-type lines were performed. The results are consistent with a hyperfine structure dominated by the magnetic spin-spin coupling due to the three hydrogen atoms of the methyl group.     

Vibrational spectrum and internal rotation in 2,5-dimethylpyrazine

The infrared and Raman spectra of 2,5-dimethylpyrazine have been recorded and assigned on the basis of a C_2h molecular geometry previously determined by MINDO/3. The potential energy function corresponding to the internal rotation of both methyl groups has been used to solve the Schrödinger equation, and to obtain the energy levels of that motion on the basis of a molecular symmetry G₃₆. The rotation of each substituent is found to be almost independent of the other.     

A study of the ethane internal rotation barrier

In an effort to deduce the source of the ethane internal rotation barrier, we have investigated the contributions of exchange energy and orthogonality: two effects that are required by the Pauli principle. Fully antisymmetrized, partially antisymmetrized and non-antisymmetrized optimized orbital product wavefunctions were determined. Results show that the exchange energy contribution to the barrier is negligible only when it is evaluated from energy-localized orbitals; even in this case the small total results from cancellation of large contributions. The barrier is apparently caused by the orthogonality that is required between CH orbitals on opposite ends of the molecule. The CC bond has insignificant participation in this effect.