Microwave spectrum, structure, dipole moment, and internal rotation of fluoromethyl methyl ether

Microwave spectra of fluoromethyl methyl ether and its 10 isotopically substituted species were measured. The r_s structure of this molecule was determined from the observed moments of inertia. Structural parameters obtained for this molecule, which was in the gauche form, were compared with those of the analogous molecules. Dipole moments of the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment is 1.744 ± 0.029 D making an angle of 100°54′ with the O---CH₂ bond toward the C---F direction and lies in the plane whose dihedral angles with the FCO and COC planes are 114°9′ and 44°56′, respectively. The barrier to internal rotation of the methyl group was calculated taking into account the coupling effect with the skeletal torsion using the observed splitting data of the spectra in the ground, first excited methyl torsional, and skeletal torsional states. The barrier, skeletal torsional frequency, and coupling term were determined to be V₃ = 1538 ± 40 cal/mole, ω_t = 158 ± 4 cm⁻¹, and V_s = 490 ± 500 cal/mole, respectively.     

Microwave spectrum,barrier to internal methyl rotation,dipole moment,and partial structure of dimethylketene

From the microwave spectrum of dimethylketene which has been recorded from 8 to 37 GHz, the following rotational constants were derived: A = 8 267.832 ± 0.8, B = 3 884.101 ± 0.03, C = 2 728.826 + 0.03 MHz. The dipole moment is μ_a = 1.94 ± 0.01 D. Substitution coordinates for all methyl group atoms have been obtained by investigating the spectra of six isotopic species of the molecule. The potential barrier V₃ hindering internal rotation of the methyl tops has been fitted to the multiplet width of a number of high-J ground state ^aQ-transitions which were observed as triplets. V₃ is 2065 cal/mole, keeping fixed I_α = 3.132 amu Ų and angle (methyl-top to a-axis) = 58.94° as obtained from the partial substitution studies.     

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

Microwave spectra of methylpropargylether and its nine isotopically substituted species were measured. The plausible structure of this molecule was determined from the observed moments of inertia. The r_s structural parameters of the OCH₃ part of the molecule could be obtained and were compared with the corresponding parameters of the analogous molecules. The dipole moment and its direction in the molecule were determined by Stark-effect measurements. The barrier to internal rotation of the methyl group was determined from the A-E splittings of the spectra reported by K. M. Marstokk and H. Møllendal (J. Mol. Struct. 32, 191–202 (1976) taking into account the coupling effect of the skeletal torsion.     

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

Microwave spectra of methylsilylsulfide and its three isotopically substituted species were measured and their b-type transitions were assigned. The spectra of all the species exhibit doublet structures due to the internal rotation of the methyl group. Using the internal axis method, the potential barriers were determined from the observed A- and E-component frequencies to be 1081.0 ± 3.3, 1073.9 ± 2.0, 1065.1 ± 11.4, and 1076.0 ± 1.9 cal/mol for the normal, CH₃SSiD₃, CD₃SSiH₃, and ¹³CH₃SSiH₃ species, respectively. The analysis also yielded 3°49′ as the tilt angle of the methyl top. From the rotational constants obtained, a plausible structure was estimated. The molecular electric dipole moments were determined from the second-order Stark effect of some A-component transitions with low- J quantum numbers for the normal and SiD₃ species. A comparison of the obtained parameters was made with analogous molecules.     

Microwave spectrum,structure, dipole moment,and internal rotation of the trans isomer of ethyl methyl sulfide

The microwave spectra of the trans isomer of ethyl methyl sulfide and its 10 isotopic species were measured. The r_s structure of this isomer was determined from the observed moments of inertia. The dipole moment and its direction in the molecule were determined by Stark effect measurements of low J transitions for the normal and CH₃CH₂SCD₃ species. The barrier to internal rotation of the SCH₃ group was calculated from the observed A-E splittings of the transitions. The present results were compared with those for the analogous molecules.     

Microwave spectrum,barrier for methyl rotation,methyl conformation,and dipole moment of ortho-xylene

Nine microwave ground-state spectra of seven isotopes of ortho-xylene have been measured between 9 and 29 GHz. From the rotational constants a partial substitution structure could be calculated. The dipole moment was determined from Stark-lobe shifts, μ_a = 0.640 ± 0.005 D. The high-J transitions were found split into multiplets due to the interaction of methyl top internal rotation with the overall molecular rotation; doublets through quintets with the correct nuclear spin weight dependence could be observed according to group-theoretical expectations. A weighted average, V₃ = 1490 ± 50 cal/mole, was derived for the internal rotation barrier neglecting top-top coupling and presumably small, higher than threefold barrier terms. The methyl groups both stagger the bond between the two benzene carbon atoms which carry them.     

Microwave spectrum,structure, dipole moment,and internal rotation of the GT isomer of fluoromethylethylether

Microwave spectra of fluoromethylethylether and its 13 isotopically substituted species have been measured. The r_s structure of the GT isomer of this molecule was determined from the observed moments of inertia. The structural parameters obtained are roughly close to those of fluoromethylmethylether and the GT isomer of chloromethylethylether. The dipole moments and their directions in the molecule were determined from the Stark effect measurements of several low-J transitions for the normal and two deuterated species. The dipole moment of the normal species was found to be 1.806 ± 0.012 D, making angles of 136°50′ and 107°40′ with the CF and FCH₂O bonds, respectively. From the A-E splittings of the spectra in the first excited methyl torsional state, the barrier to internal rotation of the methyl group was calculated to be 3150 ± 50 cal/mole in the one-top approximation.     

Microwave spectra of deuterated ethylenes: Dipole moment and rz structure

The pure rotational spectra of three deuterated ethylenes, CH₂CD₂, CH₂CHD, and cis-CHDCHD, were observed by microwave spectroscopy, and the rotational and centrifugal distortion constants were determined precisely. The dipole moment of CH₂CD₂ was calculated from the Stark effects to be 0.0091 ± 0.0004 D. From the observed rotational constants the average structure was calculated to be $\text{r}{}_{\mathrm{z}}\text{(CC)}\text{= 1.3391 ± 0.0013}\text{A}\text{?}$, $\text{r}{}_{\mathrm{z}}\text{(CH)}\text{= 1.0869 ± 0.0013}\text{A}\text{?}$, θ_z(CCH) = 121.28 ± 0.10°, and $\text{r}{}_{\mathrm{z}}\text{(CH)}\text{- r}{}_{\mathrm{z}}\text{(CD)}\text{= 0.00137 ± 0.00037}\text{A}\text{?}$, where the errors include one standard deviation in the fitting and errors due to an uncertainty (±0.03°) in θ_z(CCH) - θ_z(CCD).     

Microwave spectrum,structure, quadrupole coupling constants,dipole moment,and barrier to internal rotation of N-methylhydroxylamine

The microwave spectrum has been observed and analyzed for five isotopic species of N-methylhydroxylamine. For the normal species the rotational constants (in Megahertz) are A = 38 930.771 ± 0.005, B = 3939.607 ± 0.002, and C = 8690.716 ± 0.001. These data show that the molecule exists in the trans conformation, with structural parameters that include the following: CN = 1.460, NO = 1.461, NH = 1.007, and OH = 0.962. Hyperfine structure analyses have yielded the complete inertial axis ¹⁴N quadrupole coupling constant tensor, and thus the tensor values in the electric field-gradient principal axis system as follows: χ_xx = 4.41 ± 0.30, χ_yy = 1.93 ± 0.45, and χ_zz = ?6.34 ± 0.30 MHz. The total electric dipole moment has been found to have the value μ_T = 0.71 D, and the barrier to internal rotation of the methyl group is 3.55 kcal/mole.     

Utilization of simultaneous multiple substitutions in the determination of the rs structure

A new procedure is proposed to calculate the substitution structure of molecules. The procedure allows the inclusion of isotopic species with single or multiple substitutions of any type. It is equivalent to Kraitchman's method when the proper number of singly substituted isotopes is used. Some possible applications are demonstrated with the example of ethylene epoxide.     

Microwave spectra of deuterated ethanes: Internal rotation potential function and rz structure

The microwave spectra of CH₃CHD₂ in the first excited torsional state and of CH₃CH₂D and CD₃CH₂D in the ground states have been observed by a source-modulation spectrometer and analyzed to determine the two potential constants, V₃ and V₆, simultaneously and also to assess the isotopic effects on the potential function. The results obtained for C₂H₆ are V₃ = 2.882 ± 0.010 and V₆ = 0.020 ± 0.010 kcal/mole. The staggered conformation in ethane was established by observing microwave spectra of gauche CH₂DCH₂D. The r_z structure of ethane was recalculated by adding precise rotational constants obtained in this work to previous microwave and infrared data.     

Microwave spectrum,dipole moment,and structure of anti-vinyl alcohol

The rotational spectra of the anti conformer of vinyl alcohol (ethenol, H₂CCHOH) and its OD modification have been studied by microwave spectroscopy. The compounds have been generated by very-low-pressure pyrolyses of the appropriate isotopic species of 3-thietanol. In both cases the 25 measured μ_a- and μ_b-type transitions allowed the rotational constants and all five quartic centrifugal distortion constants to be determined. Stark effect measurements have yielded the electic dipole moment: μ_a = 0.547(2), μ_b = 1.702(1), and μ = 1.788(1) D. By relative intensity measurements it has been found that the vibrational ground state of the anti conformer lies 4.5±0.6 kJ mol^?1 above the syn conformer. In addition, ab initio calculations at the 6–31G⁷ level have been performed to obtain the structure, relative energy, and dipole moment of both rotamers.     

Microwave spectrum,dipole moment,and structure of 3-aminopropanol

The microwave spectra of 3-aminopropanol and three of its deuterium substituted isotopic species have been investigated in the 26.5 to 40 GHz frequency region. The rotational spectrum of only one conformer has been assigned in which presumably a hydrogen bond of the OH---N type exists. The rotational spectra of a number of excited vibrational states have been observed and assignments made for some of these excited states. The average intensity ratio for the rotational transitions between the ground and excited vibrational states indicates that the first excited state is about 120 cm^?1 above the ground state.and the next higher state is roughly 200 cm^?1 above the ground vibrational state. The dipole moment was determined from the Stark effect measurements to be 3.13 ± 0.04 D with its principal axes components as |μ_a| = 2.88 ± 0.03 D, |μ_b| = 1.23 ± 0.04 D and |μ_c| = 0.06 ± 0.01 D. The possibility of another conformer where the hydrogen bond could be of NH---O type was explored, but the spectra of such a conformer could not be identified.     

Microwave spectrum,structure, dipole moment,quadrupole coupling constant,and internal motion of thioformamide

The r_s structure of thioformamide has been determined from the microwave spectra of the normal as well as isotopic species of the molecule. The structural parameters obtained assuming the planarity of the molecule are $\text{NH}{}_{\mathrm{c}}\text{= 1.001}{}_8\text{± 0.006}\text{A}\text{?}\text{,}\text{NH}{}_{\mathrm{t}}\text{= 1.006}{}_5\text{± 0.003}\text{A}\text{?}\text{,}\text{CN}\text{= 1.358}{}_2\text{± 0.003}\text{A}\text{?}\text{,}\text{CS}\text{= 1.626}{}_2\text{± 0.002}\text{A}\text{?}\text{,}\text{CH}{}_{\mathrm{a}}\text{= 1.09}{}_6\text{± 0.08}\text{A}\text{?}\text{, ?}\text{H}{}_{\mathrm{c}}\text{NH}{}_{\mathrm{t}}\text{, = 121°42′ ± 40′, ?}\text{H}{}_{\mathrm{c}}\text{NC}\text{= 117°55′ ± 40′, ?}\text{H}{}_{\mathrm{t}}\text{NC}\text{= 120°22′ ± 30′, ?}\text{NCS}\text{= 125°16′ ± 15′ ?}\text{NCH}{}_{\mathrm{a}}\text{= 108°}{}_{\mathrm{5\prime }}\text{± 5°,}\text{and}\text{?}\text{SCH}{}_{\mathrm{a}}\text{= 126°}{}_{\mathrm{39\prime }}\text{± 5°}$.The dipole moment is calculated from the Stark effects of the three transitions to be μ_a = 3.99 ± 0.02 D, μ_b = 0.13 ± 0.25 D, and μ_total = 4.01 ± 0.03 D, where the c component is assumed to be zero.The quadrupole coupling constant of the ¹⁴N nucleus is estimated using the doublet splittings observed for six Q-branch transitions; χ_cc - χ_bb = ?5.39 ± 0.15 MHz and χ_aa = 2.9 ± 1.2 MHz.Two sets of vibrational satellites are observed and assigned to the first excited state of the amino wagging and the NCS bending vibrations, respectively. The relative intensity measurement gives the vibrational energies of 393±40 cm^?1 and 457 ± 50 cm^?1 for NH₂CHS and 293 ± 30 cm^?1 and 393 ± 40 cm^?1 for ND₂CHS. The amino wagging inversion vibration in the molecule is discussed in comparison with that in formamide. It is most probable that the thioformamide molecule is also planar without any potential hump to the amino inversion at the planar configuration.     

Microwave spectrum of the gauche and trans isomers of methylsilanethiol: Determination of molecular structure,dipole moment,and internal rotation potential functions of methyl and mercapto groups

Microwave spectra of methylsilanethiol and three of its deuterated species were measured and assigned for the gauche and trans isomers. The double minimum splitting due to internal rotation of the mercapto group in the gauche isomer was directly observed in c-type transitions for all the species measured. Rotational constants and the pure torsional energy difference, Δν, between the (+) and (−) states in the gauche isomer of the parent species were determined to be A(+) = 15 567.654 ± 0.040, B(+) = 3663.038 ± 0.004, C(+) = 3179.727 ± 0.005, ΔA = −4.328 ± 0.021, ΔB = −0.220 ± 0.012, ΔC = −0.008 ± 0.011, and Δν = 2826.371 ± 0.045 MHz, where A(+) represents the A rotational constant of the (+) state and ΔA = A(−) – A(+) and so on. For the trans isomer of the parent species, the following rotational constants were determined: A = 14 745.953 ± 0.051, B = 3841.291 ± 0.010, C = 3220.350 ± 0.010. Additional splittings due to internal rotation of the methyl group were also observed for both of the isomers. Analysis of these splittings derived barrier heights of the methyl internal rotations to be 1581 ± 26 and 1729 ± 23 cal/mol for the trans and gauche isomers of the parent species, respectively. Dipole moments were obtained from Stark effect measurement to be 1.056 ± 0.006 and 1.604 ± 0.006 D for the trans and gauche isomers of the parent species, respectively. Potential function of the mercapto internal rotation and plausible structures for both the isomers were discussed.     

1-Butyne microwave spectrum,barrier to internal rotation,and molecular dipole moment

Measurements of rotational transitions of 1-butyne have been made in the range of ～20–130 GHz. Both a-type transitions up to J = 46 and b-type transitions up to J = 42 have been measured and fitted to a rotational Hamiltonian which includes centrifugal distortion terms. In addition to the five quartic centrifugal distortion constants, three sextic coefficients had to be included to reproduce the observed frequencies to within experimental error. The results of the analysis are sufficient for the prediction of all strong transitions throughout the millimeterwave range. A barrier to internal rotation of the methyl group of 3.260 kcal/mole (1 kcal/mole = 4.18 kJ/mole) has been derived from the first excited torsional state. Analysis of the second-order Stark effect has led to an accurate determination of both μ_a and μ_b with μ_a = 0.763(3) D and μ_b = 0.170(4) D.     

The microwave spectrum,substitution structure,internal rotation barrier,and dipole moment of thioacetaldehyde,CH3CHS

The microwave rotational spectrum of the unstable species thioacetaldehyde, CH₃CHS, has been studied in a flow pyrolysis system. Eight isotopic variants have been studied allowing an accurate substitution structure to be derived. Most of the spectral lines show splittings due to internal rotation, analysis of which has allowed a barrier study to be made. For the torsional ground state of the most abundant species, V₃ = 1572 ± 30 cal/mole or 375.7 ± 7 J/mole. The dipole moment is μ = 2.33 ± 0.02 D with components μ_A = 2.26 ± 0.02 and μ_B = 0.56 ± 0.01 D.     

Microwave spectrum, structure, and dipole moment of the trans-trans isomer of methylpropylether

Microwave spectra of the trans-trans (TT) isomer of methylpropylether and its 12 isotopically substituted species were measured. The r_s structure of this isomer was determined from the observed moments of inertia. Structural parameters of this isomer were roughly equal to those of the reported r_s structures of trans-ethylmethylether and propane. Dipole moments of the TT isomer for the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment was μ_a = 0.082 ± 0.010, μ_b = 1.104 ± 0.013, and μ_total = 1.107 ± 0.013 D making angles of 4°17′ with the b-inertial axis, of 6°7′ with the bisector of the COC angle. The barrier to internal rotation of the CH₃C group was calculated to be 3300 ± 60 cal/mole from A-A splittings of the spectra in the CH₃C excited torsional state.     

Microwave spectrum and dipole moment of pentafluorobenzene

With the idea of evaluating the dipole moment of pentafluorobenzene from a lowJ transition, its microwave spectrum was investigated in the frequency region of 8,000 to 12,400 MHz. The spectrum had been earlier observed by the authors in the 12,400 to 18,000 MHz region which needs reassignment in the light of present investigations. The rotational constants areA=1480·856±0·003 MHz,B=1030·066±0·003 MHz andC=607·496±0·002 MHz. The dipole moment is 1·44±0·05 D.