Dipole Moment Components Μa Research Articles

Microwave spectrum, centrifugal distortion, dipole moment and conformation of 2-methyl-1,3-dioxane

Microwave spectrum of 2-methyl-1,3-dioxane has been investigated in the frequency range 8–40 GHz. Rotational a-type and c-type transitions with J≤40 have been identified. Rotational constants A = 4658.122(2) MHz, B = 2503.221(1) MHz, C = 1783.950(1) MHz and centrifugal distortion constants for the ground vibrational state have been found. Dipole moment components µa = 1.43 ± 0.01 D, µc = 1.15±0.01 D and overall dipole moment µ = 1.84±0.02 D have been determined. The data obtained are in accord with the chair conformation of the molecule having equatorial arrangement of the methyl group.

Structure and spectra of 1,3,2-dioxaphospholenes. 1. Microwave spectra of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene and its isotopomers in the ground and excited vibrational states

The microwave spectrum of 2-chloro-4,5-dimethyl-1,3,2-dioxaphospholene (1) was studied in the frequency range from 7 to 53 GHz. Rotational transitions of the parent molecule in the ground and eleven excited vibrational states and those of its mono-substituted 37Cl, 13CMe, and 13CCycle isotopomers in the ground vibrational state were identified. Rotational constants and partial rs-structure were obtained. The quartic centrifugal distortion constants, dipole moment components μa = 3.8D and μc = 0.24D (the total dipole moment μ is 3.81D), and the 35Cl quadrupole coupling constants were determined for the parent molecule. The fine structure of the microwave transitions in the parent molecule was analyzed under the assumption of noninteracting methyl groups. The height of the barrier to internal rotation (V30 = V03 = 665 cm–1) and the frequency of torsional vibrations (ν = 167 cm–1) were found. The frequencies of two lowest vibrational modes corresponding to deformation vibrations of the five-membered ring were estimated (ν∼100 cm–1) from the relative intensities of rotational transitions for different vibrational states.

Conformers of β-Vinylethanol: Internal Hydrogen Bonding to π-Bonded System

In a combined experimental spectroscopic and theoretical ab initio MP2/6-311++G(d,p) study of β-vinylethanol a new (second known) conformer (y) has been detected via heated preexpansion free jet microwave spectroscopy. The new species was found to have rotational constants A = 19928.6325(24), B = 2172.37374(91), C = 2144.28416(76) MHz and electric dipole moment components μa = 0.01284(37), μb = 1.262(11), and μc = 0.486(43) D. Species y has been identified with the theoretically predicted extended conformer CAA, having dihedral angles of ∠CCCC = −113.9°, ∠CCCO = 183.1°, ∠CCOH = 181.5° by comparison of experimental data with the ab initio predicted rotational constants and electric dipole moment components for the seven stable conformers of β-vinylethanol. A similar analysis for the first known conformer (x) originally detected by Marstokk and Møllendal in a previous conventional microwave spectroscopy study identifies this species with the theoretically predicted gauche conformer CMG, having dihedral angles of ∠CCCC = −107.4°, ∠CCCO = 64.3°, ∠CCOH = −49.7°. From an ab initio exploration of the energy barriers of the conformational potential hypersurface it has been shown that the two energetically comparable but undetected gauche conformers CMA and CMM would relax in the jet expansion to the lower energy detected species CMG and that similarly the two undetected extended conformers CAG and CAM of slightly higher energy than CAA would relax to the detected species CAA. Relaxation of the extended conformers to the lower energy gauche CGM is precluded by high intervening energy barriers. The observed energy difference between CAA and CGM is consistent with the stabilizing effect of an internal hydrogen bond in CGM linking the hydroxyl hydrogen to the π-electron cloud of the vinyl group. Bonding appears to be directed approximately along an axis perpendicular to the vinyl plane and passing through the nonterminal sp2-carbon atom.

Synthesis and Microwave Spectroscopic Study of Fluorinated Methyl Isocyanides

Abstract Difluoromethyl isocyanide has been generated by flash vacuum pyrolysis of pentacarbonyl (di-fluoromethyl isocyanide) chromium at 240 °C. Spectroscopic studies revealed the presence of several major and minor impurities like HCN, HCC -CN and HCC -NC . The pyrolysis of pentacarbonyl (fluoromethyl isocyanide)chromium did not lead to the observation of free fluoromethyl isocyanide. The rotational spectrum of difluoromethyl isocyanide has been recorded with a conventional continuous wave Stark spectrometer and a molecular beam Fourier transform microwave spectrometer resulting in A = 10.110841 (42), B = 4.550914 (19), C = 3.343550 (19) GHz, D′J = 0.41(13), D′JK = 26.72(36), D′J = - 21.3 (18) kHz. The dipole moment components μa = 1.46(1), μc= 1.83 (1) and μtotal 2.34 (1) D were obtained from the second order Stark effect. The 14N nuclear quadrupole hyperfine structure in the rotational spectrum of CHF2-NC has been resolved and the 14N nuclear quadrupole coupling constants xaa = 599.0 (10), xbb = −406.5 (14), xcc= −192.6 (14) kHz have been obtained.

Double-resonance spectra of CH_3^18OH

Two double-resonance techniques have been used to confirm previously proposed assignments of infrared transitions in the CH318OH molecule. The techniques involve a cw CO2 laser, and they complement each other. The infrared–infrared double-resonance technique is used for probing accidental coincidences between the CO2 laser lines and Stark-tuned levels of E symmetry. Two such coincidences have been assigned, and the permanent electric dipole moment components μa″ and μa′ have been determined, for the first time to our knowledge, in this molecule for the ground state and the CO stretch state, respectively. The infrared radio-frequency double-resonance technique has been used to probe a level of A symmetry. This observation has permitted a more accurate determination of molecular asymmetry parameters with (n, K) = (0, 3) in both vibrational levels.

Microwave spectrum, dipole moment, and conformation of ethanedithiol

The microwave spectrum of 1,2-ethanedithiol has been investigated. Measurements yield rotational constants having values A=9292.451±0.006, B=2239.194±0.002, C=1935.557±0.002 MHz; and dipole moment components μa=0.683±0.011, μb=1.749±0.044, μc=0.774±0.034 D. Analysis of these data shows that the molecule adopts an unsymmetrical non-hydrogen-bonded conformation.

Microwave spectrum, dipole moment, and the conformation of 7-oxabicyclo[4.1.0]hept-3-ene

The microwave spectrum of 7-oxabicyclo[4.1.0]hept-3-ene has been studied and the rotational constants for the ground state are (in MHz) A=4073.15±0.01, B=3394.54±0.01, C=2174.07±0.01. Stark effect measurements have allowed determination of the dipole moment components μa=0.775±0.002 D and μc=1.770±0.005 D for a total dipole moment μ=1.932±0.005 D. Analysis of these data indicates the molecule has a boat conformation.

Rotational Spectrum , Electric Dipole Moment, Quadrupole Coupling , and Partial r0- Structure of 3 -Cyanothiophene

Abstract The microwave rotational spectrum of the most abundant species of 3-Cyanothiophene was investigated for the ground vibrational state. Rotational constants and centrifugal distortion constants are given. The electric dipole moment components μa and μb and the 14N-quadrupole coupling constant X + = X bb + X cc were determined from the Stark-effect splittings and hfs-splittings respectively. The experimental results are compared to CNDO/2 calculations and are discussed with reference to ring distortion.

ChemInform Abstract: MICROWAVE SPECTRUM OF METHYLFLUOROGERMANE, CH3GEH2F

Microwave spectra of three isotopic species of CH3GeH2F have been investigated in the region 10 000–30 000 MHz. The height of the potential barrier hindering the internal rotation of the methyl group was determined to be 940±30 cal/mole, from A–E frequency differences, using the PAM method through fourth‐order perturbations. Dipole moment components μa= 2.32±0.10 D, μb=1.00±0.10 D, giving a resultant molecular μ= 2.53±0.15 D, were obtained from Stark effect measurements. Bond moments are derived for germanium compounds as μGe→H=1.0 D and μGe→F=3.3 D.

Microwave spectrum of methylfluorogermane, CH3GeH2F

Microwave spectra of three isotopic species of CH3GeH2F have been investigated in the region 10 000–30 000 MHz. The height of the potential barrier hindering the internal rotation of the methyl group was determined to be 940±30 cal/mole, from A–E frequency differences, using the PAM method through fourth-order perturbations. Dipole moment components μa= 2.32±0.10 D, μb=1.00±0.10 D, giving a resultant molecular μ= 2.53±0.15 D, were obtained from Stark effect measurements. Bond moments are derived for germanium compounds as μGe→H=1.0 D and μGe→F=3.3 D.

Microwave spectrum of (fluoromethyl)germane, CH2FGeH3

Microwave spectra of three isotopic species of CH2FGeH3 have been investigated in the region 8000–30 000 MHz. The height of the potential barrier hindering the germyl group (GeH3) internal rotation was determined to be 1390±40 cal/mole from measured A-E frequency differences, using a PAM calculation through fourth-order perturbations. Dipole moment components μa = 1.00 D, μb = 1.30 D, giving a resultant μ = 1.64±0.03 D, were obtained from Stark effect measurements. A technique for assigning R-branch asymmetric rotor transitions is discussed.

Vollständige rs -und r0 -Struktur, 14N-Kernquadrupol-HFS und Dipolmoment von Methylthiocyanat aus dem Mikrowellenspektrum

Abstract The microwave rotational spectra of five isotopic species of the molecule methyl thiocyanate, CH3SCN, have been observed in the region between 7 and 33 GHz. From the rotational constants of these and two other species whose spectra were previously known a complete rs-structure of the molecule could be determined which is compared with several r0-structures obtained subject to certain assumptions. The dipole moment components μa and μb could be determined inclusive of relative sign. The 14N quadrupole coupling tensor, complete with the only non-vanishing non-diagonal element χab, could be evaluated from the HFS of the lines and its change upon isotopic substitution.

Structure of the Methylamine Molecule. I. Microwave Spectrum of CD3ND2

The spectrum of CD3ND2 has been investigated in the 24 000–36 000 Mc region and about thirty lines have been assigned. The rotational spectrum exhibits a fine structure as a result of internal rotation and inversion. An approximate treatment of these internal motions is formulated and shown to give an excellent fit of the observed spectrum. A treatment of Stark effects and nuclear quadrupole interactions is developed and applied to the CD3ND2 spectrum. The relation to the CH3NH2 spectrum is discussed. Molecular constants of CD3ND2 determined from the spectral analysis are: torsional barrier height H=684.7±2 cm-1; dipole moment components μa=0.265 D, μc=1.299 D, total μ=1.326±0.015 D; N14 quadrupole coupling constants χaa=2.35 Mc, χbb=2.12 Mc, χcc=—4.47 Mc. Structural parameters calculated from data on CD3ND2 and CH3NH2 are: rCH=1.093 A (assumed), ∠HCH=109°28′ (assumed), rCN=1.474 A, rNH=1.011 A, ∠HNH=105°52′, ∠CNH=112°3′; the nitrogen atom is located 0.091 A from the CH3 symmetry axis.