Microwave spectrum and structure of isoxazole

The investigation of the microwave spectrum of isoxazole ($\text{ONCHCHC}$H) has been continued. The assignment of transitions up to J = 42 gave accurate rotational and distortion constants. The hyperfine splitting of the J = 0 → 1 transitions could be partially resolved and values for the quadrupole coupling constants were obtained.The use of microwave-microwave double resonance modulation in place of Stark effect modulation allowed the assignment of six vibrationally excited states and of the monosubstituted isotopic species containing ¹³C, ¹⁵N and ¹⁸O in their natural abundances. Kraitchman's equations for planar molecules were used to derive the r_s-structure of the isoxazole ring. Application of a least squares technique for singular systems of normal equations gave structure parameters which optimally reproduce the observed changes of all three moments of inertia under isotopic substitution.     

Microwave spectrum and the structure of benzoyl chloride

The microwave spectrum of benzoyl chloride was observed in the frequency range 12–18.6 GHz. Rotational constants have been obtained for the ground vibrational state, the first three excited torsional states of the COCl group, and one of the out-of-plane bending states. The residual inertial defect obtained from the ground and the torsional excited states indicates that the equilibrium conformation is planar. Ab initio MO calculations (STO-3G) showed the potential energy curve as a function of the COCl torsional angle to be rather flat around zero degrees.     

Microwave spectrum,structure and dipole moment of trimethylamine-borane

The ground state rotational spectra often isotopic species of trimethylamineborane, (CH₃)₃N¹⁰BH₃, (CH₃)₃N¹¹BH₃, (CH₃)₃N¹⁰BD₃, (CH₃)₃N¹¹BD₃, (CH₃)₃N¹¹BD₂H, (CD₃)₃N¹⁰BH₃, (CD₃)₃N¹¹BH₃, (CD₃)₃N¹⁰BD₃, (CD₃)₃N¹¹BD₃ and (¹³CH₃)(¹²CH₃)₂N¹¹BH₃, have been measured and the effective moments of inertia obtained. The utilization of Kraitchman's equations leads to an r_s value of the B-H distance of 1.211±0.003 Å and a NBH angle of 105.32±0.16°. By a least squares fit of the rotational constants the following structural parameters were obtained: r(NC) = 1.495 Å, r(BN) = 1.609 Å, and ∠BNC = 110.9°. The value of the dipole moment was found to be 4.59±0.13 D. A lower limit to the barrier to internal rotation of the BH₃ group was determined to be 3.4 kcal/mole.     

Microwave spectrum and structure of CF3OOCl

The microwave spectrum of chloroperoxytrifluoromethane has been recorded from 12.5 to 40.0 GHz. Only a-type transitions were observed. The R-branch assignments have been made for both the CF₃OO³⁵C1 and CF₃OO³⁷Cl species for the ground vibrational state. The rotational constants are: A=4808± 12, B=1318.55±0.02, C=1278.28±0.02 MHz for the ³⁵CI species, and A=4748±300,B=1285.28±0.96, C=1246.80±0.96 MHz for the ³⁷Cl species. From a diagnostic least-squares adjustment to fit the six rotational constants the following structural parameters were obtained: r(C-0)=1.377±0.03 Å, r(O-O)=1.445± 0.049 Å, r(Cl-O)=1.69±0.04 Å, ∠COO=108.1±4.2°, ∠ClOOC=99.5±2.0°, and ∠tilt = 6.0±0.9° with reasonable assumptions for the three other structural parameters. The relatively large uncertainty in these structural parameters results from the large uncertainty in the A rotational constants. These parameters are compared to the corresponding ones in some other peroxides. The quadrupole coupling constants have been obtained and are discussed.     

Microwave spectrum,dipole moment and structure of isopropyl cyanide

The microwave spectrum of isopropyl cyanide, (CH₃)₂CHCN, has been recorded from 26.5 to 40.0 GHz. Both A- and C-type transitions were observed. The R-branch assignments have been made for the ground and three different excited states. The following structural parameters were obtained: r(C-CN) = 1.501 Å, ∠CCC = 113.8°, and an angle between the CCC plane and the CN bond of 53.8° with reasonable assumptions made for the structural parameters for the isopropyl moiety and the nitrile bond. The dipole moment components were determined to be μ_a = 4.05±0.02, μ_c= 1.4 ± 0.2 and μ_t = 4.29 ±0.10 D. The dipole moment of t-butyl cyanide has been re-measured and found to have a value of4.34±0.04 D. From the relative intensities of the excited state lines, the two torsional modes were found to have frequencies of 200 ±20 and 249 ±10 cm^?1 which gave a periodic barrier to internal rotation of 3.3 kcal mole^?1.     

Microwave spectrum,structure and dipole moment of cyclopentadienylberyllium hydride

Microwave spectra of C₅ H₅ BeH, C₅ H₅ BeD, ¹³CC₄ H₅ BeH, and ¹³CC₄ H₅ BeD are reported. The molecule is a C_5v symmetrical top. The BeH bond length was found to be 1.32 Å with an error limit of 0.01 Å and the CC bond length was determined as 1.423 Å with one standard deviation of 0.001 Å. The distance from the beryllium atom to the centre of the cyclopentadienyl ring, h, and the CH bond length were assumed to be 1.49 Å and 1.09 Å, respectively. The dipole moment was determined through the Stark effect to be 2.08 D with one standard deviation of 0.01 D. Four different vibrationally excited normal modes were identified and their frequencies determined by relative intensity measurements.     

Microwave spectrum and substitutional structure of CH2DF

Forty-two transitions of the microwave spectrum of CH₂DF have been observed in the region between 75 and 450 GHz. The measurement of both a-type and b-type transitions makes possible the analysis of the spectrum and the accurate calculation of the rotational constants (in MHz): = 119 675.0535 ± 0.074, = 24 043.4415 ± 0.072, ? = 22959.3732 °0.072, °j = 0.049371 ±0.00011, °_jk = 0.34268 ±0.0006, 2_k = 3,3774 ± 0.0035, δ _j = 0.002329 ± 0.000045, δ_k = 0.0687 ± 0.036. These constants, in combination with the results of earlier work on the symmetric speci r_s structure calculation based entirely on high-accuracy microwave data. The structural parameters are r_CH = 1.100 Å, r_CF = 1.383 Å, and ∠HCH = 110° 37'.     

Microwave spectrum and molecular structure of arsenic tribromide1

The microwave spectrum of arsenic tribromide has been recorded in the frequency range 26.5–40.0 GHz. From the rotational constants for the symmetric top species ⁷⁹AsBr₃ and ⁸¹AsBr₃ the following structural parameters have been deduced: r_z(AsBr) = 2.324 ± 0.003 Å, θ_z(BrAsBr) = 99.8 ± 0.2°. These results are in excellent agreement with the parameters obtained by Samdal et al., in a concurrent electron diffraction study.     

Microwave spectrum, structure, dipole moment and internal rotation of allylsilane

Microwave spectra of allylsilane and its ¹³C and deuterium substituted species have been measured and assigned for the skew isomer. The r_s structure was determined with the aid of several assumptions. Some of the parameters determined are; r(C=C) = 1.328 ± 0.007 Å, r(C---C) = 1.492 ± 0.008 Å, (CCC) = 126.7 ± 0.8°, (CCSi) = 111.6 ± 0.5° and τ(CCCSi) = 106.8 ± 1.1°. Dipole moments and their components were also determined for the CH₂ = CHCH₂SiH₃ and CH₂=CHCH₂SiD₃ species. Hyperconjugation between the C=C π bond and the C---Si σ bond is discussed.     

Microwave spectrum,structure, dipole moment and internal rotation of ethyl fluorosilane

Microwave spectra of the trans and gauche isomers of ethyl fluorosilane and their eleven isotopically substituted species have been measured. The r_s structures of the two isomers were determined from the observed moments of inertia. The molecular structures found for the two isomers in the present study are compared with those of analogous molecules. Dipole moments of the two isomers were determined by Stark-effect measurements and are also compared with those of analogous molecules. The energy difference between the trans and gauche isomers was obtained from the relative intensity measurements of the spectra and the barrier to internal rotation of the methyl group for the gauche isomer was obtained from the A—E splittings of the spectra in the first excited methyl torsional state. The V₃ value was 2775 ± 25 cal mol^?1.     

Microwave spectrum, structure, tautomeric, and conformational composition of 4-vinylimidazole

The microwave spectra of the two conformers each, of the 1H and 3H tautomers of 4-vinylimidazole, have been measured in the 48-72 GHz spectral region. The 4-vinylimidazole was generated in situ by the facile decarboxylation of urocanic acid at its vaporization temperature of 220 °C. The recognition of this reaction casts doubt on the reliability of a previous published spectroscopic study apparently mistakenly thought to be of uncontaminated vaporized urocanic acid, a natural product of great interest in skin cancer etiology. Quantum chemical theoretical predictions of the structures of each of ten possible conformers∕tautomers of urocanic acid and four of 4-vinylimidazole were performed at the ab initio MP2∕cc-pVTZ level, with vibrational predictions at the B3LYP∕cc-pVTZ and M062X∕cc-pVTZ levels. The predicted values of rotational constants for all the urocanic acid species were found to be quite inconsistent with those of the four observed spectra. For the 4-vinylimidazole isomers, the calculated relative energies suggested that all four species would have substantial equilibrium mole fractions at 220 °C. The isomers were identified by matching the observed and calculated rotational constants. The resulting assignment was found to be consistent with the predicted and observed (14)N nuclear quadrupole hyperfine multiplet patterns for a suitable rotational transition, and with the observed versus empirically calculated inertial defects. With one exception, the predicted structures were found to be planar. Resembling the case of 1-vinylimidazole, where one conformer is nonplanar, one isomer of 4-vinylimidazole was found to be quasiplanar. This seems to belong to a class of spontaneous symmetry-breaking observed in the molecular structure of some otherwise planar vinyl aromatic compounds.     

Microwave spectrum of cyclohexanone

The rotational spectrum of cyclohexanone has been observed within the frequency region from 18.0 to 40.0 GHz. Transitions in the ground state and six excited states have been assigned. The ground state rotational constants are (in MHz) A = 4195.316 +- 0.059, B = 2502.627 ± 0.005 and C = 1754.443 ± 0.005.From information obtained from relative intensity measurements, these excited states are estimated to be ~ 100 cm^?1 above the ground state for the first excited state of the ring-bending mode and ~ 180 cm^?1 for the first excited state of the ring-twisting mode.     

Microwave spectrum of n-methylvinylamine

The microwave spectrum of a new transient molecule has been observed by pyrolysis of 2-methylaziridine. The rotational constants determined are A = 17413.011(14), B = 5974.342(4), and C = 4586.645(4) MHz. The analysis of the spectrum leads to the conclusion that the origin of the spectrum is cis-N-methylvinylamine. The molecular constants have been estimated by ab initio MO calculation, and they are consistent with the experimental results.     

Microwave spectrum of ethylamine-gauche

The gauche rotamer of the ethylamine molecule was successfully assigned making ample use of MW-MW-DR-techniques. The transitions of the gauche form appear as quartets due to two large amplitude motions: the CN torsion and inversion. The rotational as well as interaction constants for all four states were fitted as well as their relative energies.     

Microwave spectrum of sulfolane

The microwave spectrum of sulfolane has been investigated in the frequency region 26.5 to 40.0 GHz. The rotational constants have been derived for the ground state (in MHz: A = 3780.814 ± 0.326, B = 2047.243 ± 0.006 and C = 1899.572 ± 0.006) and three excited states of the ring puckering vibration. Possible interpretations of the effect of nuclear spin statistical weights on the relative intensity measurements and of the analysis of the variation of the rotational constants with the vibrational quantum number are discussed. Two situations may be compatible with the available data: pseudorotation with the bent conformer lowest in energy or a near planar structure with a small barrier at the planar configuration with respect to the bending mode.     

Microwave spectrum of ethynyl-cyclohexane

The ground state microwave rotational spectra of the axial and equatorial chair forms of ethynyl-cyclohexane are investigated in the frequency region 8–26.5 GHz. The spectrum of the first vibrationally excited state of the e-conformer is also assigned and intensity measurements locate this state at 130 ± 30 cm^?1. Using the ground state rotational constants obtained for the two isomers, a least-squares structural analysis is performed.     

Microwave spectrum of cyclohexanecarbonitrile

The microwave rotational spectrum of cyclohexanecarbonitrile was investigated in the frequency region 8–40 GHz. From the measured transition frequencies the rotational constants of the two molecular were derived (equatorial isomer: A = 4238.77, B = 1399.172, C = 1128.845 MHz; axial isomer: A = 3005.58, B = 1763.483, C = 1558.615 MHz). Assuming the values of 1.531, 1.096 and 1.159 Å, respectively, for the CC, CH and CN distances, and supposing that the ring structure has the same symmetry as in cyclohexane, the following structural parameters were also obtained: equatorial isomer CCC (carbon ring) = 111.40°, r(CCN) = 1.489 Å, HCCN = 107.42°; axial isomer CCC (carbon ring) = 111.65°, r(CCN) = 1.489 Å, HCCN = 105.53°.     

Microwave spectrum and conformation of n-propyltrifluorosilane

The conformational properties of gaseous n-propyltrifluorosilane (CH₃CH₂CH₂SiF₃) have been investigated by microwave spectroscopy and high-level quantum chemical calculations. The microwave spectrum was investigated in the 20–62 GHz spectral range at a temperature of −78 °C. The spectra of the ground vibrational state and three vibrationally excited states of one conformer having an antiperiplanar conformation of the C–C–C–Si chain of atoms were assigned. No evidence for the existence of the synclinal (gauche) conformer was seen in the microwave spectrum. It is concluded that the synclinal form is at least 3.5 kJ/mol less stable than the antiperiplanar conformer in the gas phase. Density functional theory calculations have been performed for the system mainly to predict the effects of centrifugal distortion. The G3 quantum chemical method has been used to test the ability of this method to predict the energy difference between the synclinal and antiperiplanar conformers.