Microwave spectroscopic study of the conformations,relative energies,and dipole moments of ethyl cyanoformate

The microwave spectrum of ethyl cyanoformate displays a-type band spectra from three nearly prolate conformers. High-resolution spectra of the two more stable species have been assigned. One form, designated extended, has rotational constants A″ = 6453.3(4) MHz, B″ = 1500.47(6) MHz, C″ = 1236.36(6) MHz, which are consistent with a syn-anti [τ₁ (OCOC) = 0°, τ₂ (COCC) = 180°] structure. The second form, labeled compact, has rotational constants A″ = 6787.8(7) MHz, B″ = 1549.38(8) MHz, C″ = 1406.80(8) MHz, which are consistent with a syn-gauche [τ₁ (OCOC) = 0°, τ₂ (COCC) ～ 90°] structure. The extended form is marginally more stable, ΔE = 55 ± 27 cm^?1. The extended conformer has dipole moment components μ_a = 4.44(7), μ_b ～ 0 D and the compact conformer has dipole moment components μ_a = 4.25(7), μ_b = 0, μ_c = 1.08(23) D. The third conformer (relative energy 600 ± 140 cm^?1) has the most intense band series even at ?63°C. the bands of this conformer are unresolvable into individual rotational transitions.     

Microwave spectrum of 3,4-epoxy-1-butene

The microwave spectrum of 3,4-epoxy-1-butene has been studied in the region 26.5–40 GHz. For the ground-state molecule, 170 lines have been assigned up to J = 34. From these the rotational constants and the centrifugal distortion constants were determined by least-squares fitting. The rotational constants are (in MHz): A = 17367.284 ± 0.011, B = 3138.186 ± 0.004, C = 3043.697 ± 0.004. The dipole moment has been determined from the Stark effect as (in Debye): μ_a = 0.72 ± 0.01, μ_b = 1.688 ± 0.003, μ_c = 0.39 ± 0.02, μ = 1.875 ± 0.005. The rotational constants and dipole moment components indicate that the assigned conformer is the s-trans form. A rotational assignment has also been made for the first excited state of the torsional mode. The fundamental frequency of the torsional mode has been estimated as 142 ± 20 cm^?1 from relative intensity measurement.     

Conformation and dipole moment of tetrahydropyran-4-one by microwave spectroscopy

The microwave spectrum of tetrahydropyran-4-one has been studied in the frequency region 18 to 40 GHz. The rotational constants for the ground state and nine vibrationally excited states have been derived by fitting a-type R-branch transitions. The rotational constants for the ground state are (in MHz) A = 4566.882 ± 0.033, B = 2538.316 ± 0.003, C = 1805.878 ± 0.004. From information obtained from the gas-phase far-infrared spectrum and 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 and ～ 185 cm^?1 for the first excited state of the ring-twisting mode. Stark displacement measurements were made for several transitions and the dipole moment components determined by least-squares fitting of the displacements: (in Debye) |μ_a| = 1.693 (0.001), |μ_b| = 0.0, |μ_c| = 0.300 (0.013) yielding a total dipole moment μ_tot = 1.720 (0.003). A model calculation to reproduce the rotational parameters indicates that the data are consistent with the chair conformation.     

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.     

High-resolution measurements of the 1 ← 0 infrared absorption band of hydrogen iodide

The ν₁ fundamental band of FNO has been studied by the technique of CO laser Stark spectroscopy. The band origin was determined to be 1844.099 cm^?1, and values for the rotational and centrifugal distortion constants of the (100) excited vibrational state were found. The ground state dipole moment components were determined to be μ_a = 1.690 and μ_b = 0.370 D, for a total dipole moment of 1.730 D, and a relatively large reduction (5%) was found in μ for the (100) state relative to the ground state.     

Microwave spectrum,conformation, and dipole moment of divinyl ether

The microwave spectrum of divinyl ether has been observed and a, b, and c type rotational transitions of one conformer assigned. This conformer has rotational constants closely related to the cis-trans planar form. The inertial defect and dipole moment reveal that it is not planar. This nonplanarity almost certainly results because of strong repulsion between the β hydrogen of the cis vinyl group and the α hydrogen of the trans vinyl group. The c type transitions are split 53 MHz by inversion tunneling. The dipole moment has been obtained from Stark effect measurements and is 0.782 debye.     

Microwave spectrum of CH3OD

The microwave spectrum of CH₃OD has been observed in the frequency region between 14 and 92 GHz. All the ground-state transitions with J ≤ 8 and J = 2 ← 1, a-type transitions in the excited torsional states (v = 1 and v = 2) have been observed. The spectrum has been analyzed and rotational constants, torsional constants, torsion-vibration-rotation interaction constants, and centrifugal distortion constants have been evaluated. The Stark effect measurements have been made and the dipole moment components have been determined as μ_a = 0.833 ± 0.008 D and μ_b = 1.488 ± 0.015 D.     

Microwave spectrum,dipole moment,and conformation of 3-methylcyclopentanone

The rotational spectrum of 3-methylcyclopentanone has been observed in the frequency region from 18.0 to 26.5 GHz. Both a-type and b-type transitions in the ground vibrational state and a-type transitions in five excited states have been assigned. The ground state rotational constants are determined to be A = 5423.32 ± 0.18, B = 1949.51 ± 0.01, and C = 1529.59 ± 0.01 MHz. Analysis of the measured quadratic Stark effects gives the dipole moment components ∥μ_a∥ = 2.97 ± 0.02, ∥μ_b∥ = 1.00 ± 0.03, ∥μ_c∥ = 0.18 ± 0.06, and the total dipole moment ∥μ_t∥ = 3.14 ± 0.03 D. These data are consistent with a twisted-ring conformation with a methyl group in the equatorial position.     

The microwave spectra,conformations, and dipole moments of 4-cyanocyclopentene

Microwave spectra have been observed and assigned for the axial and equatorial conformations of 4-cyanocyclopentene. For the axial species the rotational constants in megahertz are A = 5095.77, B = 2185.81, and C = 1936.50; for the equatorial species the values are A = 6762.66, B = 1916.72, and C = 1590.60. Dipole moment measurements yielded |μ_a| = 3.48 D and |μ_c| = 2.51 D for the axial form and |μ_a| = 3.85 D and |μ_c| = 1.10 D for the equatorial form. Relative intensity measurements showed the equatorial conformer to be 400 ± 60 cal mole^?1 lower in energy. Several sets of vibrational satellites were observed and natural abundance C¹³ spectra were obtained for the equatorial conformer.     

Microwave spectrum,dipole moment,and conformation of 3,6-dioxabicyclo[3.1.0.]hexane

The microwave spectrum of 3,6-dioxabicyclo[3.1.0.]hexane has been obtained. The rotational lines of one ring conformation only have been observed and assigned. Ground state rotational constants are A₀ = 6287.302 ± 0.011 MHa, B₀ = 4683.546 ± 0.008 MHz, and C₀ = 3358.517 ± 0.089 MHz. The diploe moment components obtained from Stark effect measurements are μ_a = 0.276 ± 0.010 D and μ_c = 2.47 ± 0.04 giving μ = 2.485 ±0.040 for the dipole moment of the molecule. The rotational constants and dipole moment components obtained experimentally can be satisfactorily explained only if the boat form is the most stable ring conformation.     

Microwave spectral studies of rotational isomerism in substituted ethanethiols: 2-Aminoethanethiol and 2-chloroethanethiol

Microwave spectra were observed and analyzed for 2-aminoethanethiol and 2-chloroethanethiol. The amino compound exists in two gauche rotameric conformations, one exhibiting an intramolecular SH?N hydrogen bond. The hydrogen-bonded conformer lies higher in energy by 274 ± 90 cal mole^?1 and has the following rotational constants (in MHz): A = 12 040.1 ± 11.3, B = 3352.24 ± 0.03, and C = 2881.99 ± 0.03. For the non-hydrogen-bonded conformer the rotational constants (in MHz) are A = 11 929.9 ± 10.2, B = 3395.01 ± 0.03, and C = 2877.82 ± 0.03. Dipole moment measurements for the H-bond conformer led to μ_a = 2.68 D, μ_b = 0.88 D, and μ_c = 0.37 D, while for the non-H-bond form the values are μ_a = 1.51 D, μ_b = 0.0 D, and μ_c = 0.62 D. In the case of chloroethanethiol, the only assigned spectral lines were the unresolved J → J + 1 a-type bands of a trans conformation. For this molecule the combination rotational constant B + C has the value 2955.17 ± 0.02 MHz for the ³⁵Cl species and 2879.73 ± 0.02 MHz for the ³⁷Cl species.     

Microwave spectrum and conformation of vinyl mercaptan: The anti rotamer

Microwave spectra of the anti rotamer of vinyl mercaptan and its SD isotopic species have been studied in the frequency range 12–60 GHz. For the normal species rotational and centrifugal distortion constants have been obtained for the ground and first three excited states of the SH torsional mode, the ground state values being A = 49 422.75(5) MHz, B = 5 897.215(9) MHz, C = 5 279.436(9) MHz, D_J = 3.12(11) kHz, D_JK = ?38.50(1.71) kHz, and δ_J = 0.498(51) kHz. An approximate potential function for the SH torsion in the vicinity of the anti conformation, derived using the observed variation of rotational constants with vibrational quantum number, reveals the presence of a small potential barrier of 19 cm^?1 at the planar conformation. The v = 0 state lies above this barrier so the molecule is essentially planar in the ground state in spite of the observed negative value for the inertia defect (?0.1976(2) a.m.u.Ų). The anti rotamer is found to be 50 ± 25 cm^?1 less stable than the syn rotamer. The dipole moment has the ground state values μ_a = 0.425(10), μ_b = 1.033(10), and μ_total = 1.117(14) D and is shown to vary considerably with vibrational quantum number. Evidence for significant structural changes in going from the syn rotamer to the anti rotamer is also presented.     

Microwave spectrum,conformation, and dipole moment of 4-thiacyclohexanone

The microwave spectra of 4-thiacyclohexanone in the ground state and eight vibrationally excited states have been studied in the frequency region 18.0–40.0 GHz and the corresponding rotational constants have been determined. The following values of the ground-state rotational constants (MHz) were obtained from the analysis of the a-type transitions: A = 3935.149 (0.031), B = 1829.444 (0.001), and C = 1364.609 (0.001). Analysis of the Stark effect gives for the dipole components (in Debye units) μ_a = 1.409 (0.002), μ_c = 0.391 (0.064). These data are consistent with a chair conformation for the ring. A phisically reasonable set of structural parameters which reproduce the ground-state rotational constants has been derived. A qualitative estimate of the low-frequency vibrational modes was obtained from relative-intensity measurements. The lowest vibrational frequency is believed to be a ring-bending mode and it occurs at 77 ± 22 cm^?1 while the ring-twisting mode is at 204 ± 27 cm^?1.     

Microwave spectrum of phenyl isocyanate

The microwave spectra of the ground state and three excited states of the most abundant species of phenyl isocyanate have been recorded between 8 and 40 GHz. From ^aR-type transitions the ground-state rotational constants were calculated. The A value showed clearly a tilt of the NCO group from the C_2v axis. They yielded the r₀-type parameters. A centrifugal distortion treatment confirmed the validity of the rigid rotor approximation. The dipole moment components μ_a and μ_b were derived from the field strength dependence of six Stark lobes of five transitions. The values found were μ_a = (2.50 ± 0.02) D, μ_b < 0.2 D. From relative intensity measurements, the lowest vibrational excitation energies were determined. We assigned the lowest one to the NCO group torsion. All ^aR-type transitions of excited states were found unsplit by the internal rotation of the NCO group. The weakness of the μ_b dipole moment component and of the overall spectrum intensity did not allow us to find μ_b-type transitions and so, no splitting was observed on the ground-state spectrum. An evaluation of the V₂ high barrier is given.     

The microwave spectrum and dipole moment of 1-cyano-1,3-cyclopentadiene

The microwave spectrum of 1-cyano-1,3-cyclopentadiene has been assigned and the rotational constants obtained are (in megahertz): A = 8356, B = 1904.24, and C = 1565.36. The dipole moment components were measured and are (in debye) μ_a = 4.25, μ_b < 0.3, μ_total = 4.25.     

Infrared,Raman, and microwave spectra of ethaneselenol and the determination of the barriers to internal rotation

The infrared, Raman, and microwave spectra of gaseous ethaneselenol have been investigated. The rotational constants for both the more stable gauche and for the trans conformers are reported for the Et⁷⁸SeH, Et⁷⁸SeD, Et⁸⁰SeH, and Et⁸⁰SeD isotopic species. A proposed structure has been derived from a least-squares analysis of the moments of inertia. Dipole moment components have been obtained from each conformer using second-order Stark effects. For the gauche conformer, they are μ_a = 1.42 ± 0.01, μ_c = 0.37 ± 0.03, and μ_total = 1.47 ± 0.01 D. For the trans isomer they are μ_a = 1.217 ± 0.002, μ_b = 0.850 ± 0.001, and μ_total = 1.485 ± 0.002 D. The methyl barrier to internal rotation was calculated using observed frequencies obtained from the infrared and Raman spectra; a value of 3.59 ± 0.01 kcal/mole was obtained. Asymmetric potential functions have been calculated for both the EtSeH and EtSeD isotopic species. For the light species the potential constants for internal rotation around the CSe bond are V₂ = ?96.4 ± 1, V₃ = 432 ± 4, and V₆ = ?20 ± 2 cm^?1. The difference between ground-state energy levels of the two conformers was found to be 66 cm^?1. A vibrational assignment based on infrared and Raman spectra of the gaseous phase is presented.     

Electric dipole moments of acrylonitrile and of propionitrile measured in supersonic expansion

New determinations of the ground-state electric dipole moments of acrylonitrile and propionitrile have been made from Stark effect measurements at conditions of supersonic expansion. The measurements were made on selected Stark lobes of fully resolved hyperfine components of several lowest-J rotational transitions. The results are μ_a = 3.821(3) D, μ_b = 0.687(8) D, μ_tot = 3.882(3) D for acrylonitrile, and μ_a = 3.816(3) D, μ_b = 1.235(1) D, μ_tot = 4.011(3) D for propionitrile. The new value of μ_b for acrylonitrile is appreciably different from those reported previously and it has been substantiated by both ab initio calculations and relative intensity measurements. The new dipole moment implies a considerable revision in the calculated intensities of the strongest THz-region rotational transitions of acrylonitrile, to 59% of previous values.     

Microwave spectrum of DNO

The microwave spectrum of deuterated nitroxyl DNO has been observed and analyzed. The molecule was produced by the reaction of D with NO in a flow system. Both a-type and b-type transitions have been observed and the resulting rotational constants, A = 315450.3 ± 4.8 MHz, B = 38731.5 ± 1.5 MHz, and C = 34354.0 ± 1.5 MHz, are in good agreement with those of the lower electronic state ¹A′ for the electronic transition of DNO observed by Dalby. The quadrupole coupling constants for nitrogen are χ_aa = 1.03 ± 0.40 MHz, χ_bb = −6.13 ± 0.26 MHz, χ_cc = 5.10 ± 0.26 MHz. The components of the electric dipole moment of DNO have been determined to be μ_a = 1.18 ± 0.04 D and μ_b = 1.22 ± 0.04 D, giving a total dipole moment μ_total = 1.70 ± 0.05 D. The half lifetime of the molecule varies from 1 to 40 sec, depending on the condition of the surfaces of the absorption cell, which is much longer than the values reported previously.     

The microwave spectrum of piperidine: Equatorial and axial ground states

The microwave spectra of piperidine and N-deuterated piperidine were investigated between 8 and 40 GHz. The ground states of both equatorial and axial conformers have been identified by both type-A and type-C transitions, and the substitution coordinates of the imino hydrogen have been determined for both conformers. Dipole-moment components for the equatorial conformer are μ_a = 0.178 D, μ_c = 0.80 D, μ = 0.82 D, and for the axial conformer are μ_a = 1.07 D, μ_c = 0.521 D, μ = 1.19 D. The quadrupole coupling constants for the axial conformer are: χ_aa = ?3.80 MHz, χ_bb = 2.91 MHz, χ_cc = 0.83 MHz and for the equatorial conformer χ_cc = ?4.83 MHz. The rotational constants indicate a significant flattening of the ring in axial piperidine compared with equatorial piperidine. The equatorial conformer is the more abundant; intensity measurements on several sets of lines indicate the excess energy of the axial conformer to be 3.1 ± 0.3 kJ mole^?1. This represents a significant change from our earlier reported value and is now more in line with measurements obtained by other methods.     

Microwave determination of the conformation and dipole moment of 1,3-difluoroacetone

The microwave spectra of the ground state and several low-lying vibrational modes of 1,3-difluoroacetone have been assigned and analyzed. The assigned form has a molecular conformation in which one fluorine atom lies cis and the other trans to the oxygen atom. The rotational constants of the ground state species were determined using a centrifugal distortion analysis: A = 6024.843 ± 0.006 MHz, B = 2454.414 ± 0.001 MHz, C = 1783.897 ± 0.001 MHz. The molecular dipole moment components of the ground state species lie along the a and b principal axes with μ_a = 2.38 ± 0.03 D, μ_b = 0.89 ± 0.03 D, and μ_T = 2.54 ± 0.03 D. Comparative intensity measurements with OCS microwave lines indicate that the assigned form constitutes only 20% to 30% of the total gas mixture, the remainder presumably consisting of one or more other conformers, perhaps the gauche-gauche form. The lowest vibrational frequency (82 ± 12 cm^?1) is attributed to the trans-CH₂F torsion, while the next-higher vibrational frequency (127 ± 15 cm^?1) is believed to be the cis-torsion. A low-frequency in-plane bending motion is found at 285 ± 25 cm^?1.