Conformation of 4-methylcyclohexanone by microwave spectroscopy

The microwave spectrum of 4-methylcyclohexanone has been observed in the frequency region from 18.0 to 26.5 GHz. Both a-type and c-type transitions in the ground state and a-type transitions in four excited states have been assigned. The ground state rotational constants are determined to be A = 4034.39 ± 0.06 MHz, B = 1455.46 ± 0.01 MHz, and C = 1174.06 ± 0.01 MHz. From these data, it is shown that the most stable conformer exists in the chair form with the methyl group in the equatorial position.     

Far infrared Fourier-transform spectroscopy of mono-deuterated hydrogen peroxide HOOD

We present the gas phase spectrum of singly deuterated hydrogen peroxide, HOOD, in its vibrational ground state, recorded by the high resolution Fourier-transform interferometer located at the AILES synchrotron beamline connected to SOLEIL. More than 1000 transitions in the range from 20 to 143 cm^?1 were assigned, leading to a set of preliminary rotational and centrifugal distortion constants determined by least squares fit analysis. All transitions are split by the tunneling motion of a hindered internal rotation. The splitting has been determined to be 5.786(13) cm^?1 in the torsional ground state and it shows a dependence on the rotational quantum number K_a. Some perturbations were not treated yet, but the present analysis permits to obtain a preliminary set of parameters.     

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.     

Far-infrared spectrum and ground state constants of methyl amine

The far-infrared spectrum of methyl amine has been studied in the region 40 to 350 cm⁻¹ by Fourier transform spectroscopy with an apodized resolution of 0.005 cm⁻¹ or better. Both the pure rotational spectrum and the spectrum of the fundamental torsional band have been assigned. This paper reports the ground state constants obtained from a global fitting of a data set including ground state microwave transitions from the literature, as well as far-infrared pure rotational ground state transitions and ground state combination differences from the torsional band obtained in this work. Slightly over 1000 energy differences for the ground state with 0 ≦ K ≦ 19 and K ≦ J ≦ 30 were fit to 30 molecular parameters from a group theoretical formalism developed earlier, and a standard deviation of ±0.00063 cm⁻¹ was obtained. An ambiguity (noted earlier in the microwave literature) in the determination of the structural parameter ϱ, which arises when two large amplitude motions are present in the molecule, can be understood and resolved using the present formalism.     

Microwave spectrum of 3-bromopropene in the excited torsional states

The microwave spectra in the excited states of the CC torsion for the ⁷⁹Br and ⁸¹Br isotopic species of 3-bromopropene were measured in the frequency region 15.3–23.7 GHz. The a-type R-branch and b-type Q-branch rotational transitions in the first and second excited states of one conformer, skew, have been assigned and analyzed. Analysis of the spectrum yields the rotational constants and the nuclear quadrupole coupling constants. From relative intensity measurements the energy differences associated with the CC torsion, between the ground and first excited state, the ground and second excited state have been found to lie 109 and 206 cm^?1, respectively.     

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.     

The microwave spectra of ortho-fluorotoluene with the asymmetric internal rotors CH2D and CD2H

The rotational spectra of αd₁- and αd₂-ortho-fluorotoluene in the ground state of the methyl group torsion have been measured. The evaluation of the spectra has been based on the theory for the internal rotation of an asymmetric internal top formulated earlier by several authors. The barrier potential being threefold symmetric (V₃), each torsional level consists of three nondegenerate substates, designated as sy and ±asy. The sy-state is assigned to the conformation with the unique methyl hydrogen isotope within the molecular heavy-atom plane (sy-rotamer), while the ±asy-states belong to the respective out-of-plane conformation (asy-rotamer). In the torsional ground state the level spacing between the ±asy substates is very small and numerous accidental close degeneracies are present between the rotational level systems based on these torsional substates. The rotational levels involved are strongly perturbed by the coupling between molecular overall rotation and internal rotation. Large deviations from a rigid rotor spectrum and (+) ? (?) intersystem (“tunneling”) transitions are observed. The spectrum of the asy-rotamer can be well reproduced by a “two-dimensional” Hamiltonian containing 11 “rotational constants,” 9 of which are determined by a fit to the spectrum. Several are sufficiently barrier-dependent to derive V₃. We obtain (in cal/mole) 567 ± 48 for αd₁-ortho-fluorotoluene, 711 ± 40 for the αd₂-isotope. The deviations from 649 cal/mole for the normal isotope are appreciable, probably indicating shortcomings of the semirigid model. The sy-rotamer presents a rigid rotor spectrum.     

Microwave spectrum and molecular structure of bromoacetonitrile

The ground state microwave spectrum of BrCH₂CN has been observed and assigned in the P- and R-band regions. For two isotopic species a series of strong Q-branch transitions, several clusters of μ_a-typeR-branch transitions and scattered μ_b-type and R- and P-branch transitions were fit using both the rigid rotor approximation and the reduced hamiltonian of Watson. The hyperfine structure due to the bromine atom has been analyzed and the complete field gradient tensor determined. A least-squares structure consistent with the observed rotational constants has been determined, has been shown to exhibit internal consistency and has been found to be in agreement with the structures of similar molecules.     

Rotational Spectra of Argon Acetone: A Two-Top Internally Rotating Complex

The rotational spectra of the argon acetone weakly bound complex was studied by pulsed jet Fabry-Perot Fourier transform microwave spectroscopy. Over 500 transitions of the complex were measured between 5.5 and 26 GHz from J=2-1 to J=12-11. The two methyl groups undergo hindered internal rotation resulting in four or five internal rotation states. The microwave transitions are within these states, resulting in a splitting of each rotational transition into four and sometimes five distinct transitions. The three-fold barrier to internal rotation is determined to be 260 cm⁻¹, 2% less than the 266 cm⁻¹ barrier in acetone itself. The structure of the complex has the argon atom above the heavy atom plane of the acetone, 3.52 Å from the CO bond and approximately in the C_s plane, which is perpendicular to the CCC plane of acetone.     

The microwave spectrum,dipole moment and ring planarity of 1,2-dimethylenecyclobutane

The microwave spectrum of 1,2-dimethylenecyclobutane has been studied in the range 26.5–40 GHz using a Hewlett-Packard 8400C Stark-modulated spectrometer. The rigid rotor constants have been derived for the ground state (in MHz: A = 4925.22, B = 4089.88, C = 2301.67) and four excited states of the ring-puckering vibration. That the ring skeleton is planar is indicated by the smooth variation of the rotational constants with vibrational state and by the value of 1/2(I_a + I_b ? I_c) which is consistent with only 4 hydrogen atoms out of the plane of the remaining atoms.Analysis of the Stark effect yields a dipole moment lying along the b-axis; μ_b = 0.457 ± 0.002D. A physically reasonable set of structural parameters which reproduce the ground state rotational constants has been derived by adjustment of the carbon skeleton parameters by a diagnostic least-squares procedure.     

Fourier transform microwave spectrum of the CO-dimethyl sulfide complex

The rotational spectrum of the CO-dimethyl sulfide (DMS) complex was measured in the frequency region from 4.8 up to 25 GHz by Fourier transform microwave spectroscopy. For the normal species 27 a-type and 57 c-type transitions were observed, while 16 and 8 c-type transitions were assigned for the species with ³⁴S and ¹³C in the DMS moiety, respectively, in natural abundance. In addition, 7 a-type and 48 c-type transitions were assigned for the complex with the ¹³CO enriched species as a component and 9 a-type and 42 c-type transitions for the complex with enriched C¹⁸O. No splitting was observed, which could be ascribed to the tunneling motion of the CO between two possible potential minima around DMS, while many transitions were split by the internal-rotation of the two methyl tops of the DMS unit. In cases where the K-type splitting was close to the methyl internal-rotation splitting, forbidden transitions were observed which apparently followed b-type selection rules. All of the observed transition frequencies for the normal species were analyzed simultaneously using a two-top internal-rotation and rotation Hamiltonian. The potential barrier height V₃ to internal rotation of the methyl groups of the DMS was determined to be 745.5 (30) cm⁻¹. The transition frequencies observed for all the isotopomers were analyzed using an asymmetric-rotor rotational Hamiltonian, to determine rotational and centrifugal distortion constants. The r_s coordinates calculated from the observed rotational constants led to the conclusion that the CO moiety was located in a plane perpendicular to the skeletal plane of the DMS and bisecting its CSC angle. This structure of the CO-DMS is very much different from that of the CO-DME, in which the CO is located in the DME skeletal plane. The distance between the centers of gravity of the two moieties, R_cm, was calculated to be 3.789 Å for the CO-DMS, which is longer by only 0.11 Å than that in the CO-DME complex: 3.68 Å, in spite of the fact that the van der Waals radius of the S atom is much larger than that of the O atom. The small difference in R_cm is, in part, ascribed to the location of the CO relative to the DMS/DME. The more important reason is that the intermolecular bonding of the CO-DMS is stronger than that of CO-DME; by assuming a Lennard-Jones-type potential, the force constant of the van der Waals stretching mode and the dissociation energy were estimated to be 2.7 Nm⁻¹ and 3.3 kJ mol⁻¹, respectively, which were larger than those of the CO-DME: 1.4 Nm⁻¹ and 1.6 kJ mol⁻¹.     

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.     

Hydrogen migration tunneling effects in the rotational and vibrational spectrum of protonated acetylene C2H3+

The influence of a possible hydrogen atom migration in protonated acetylene, C₂H₃⁺, on the high-resolution vibration-rotation spectrum of this ion is considered. The migration model, which derives from ab initio calculations, consists of a planar structure with the three hydrogen atoms moving on an approximately elliptical path around the two carbon atoms. Symmetry considerations for this ion are discussed in terms of the permuation-inversion group G₂₄. A method for calculating energy levels for a highly idealized model of this ion can be found in the early microwave literature. In this idealized model, an equilateral triangle of three hydrogen atoms performs internal rotation about a dumbbell of two carbon atoms, with all atoms remaining in the same plane. This model gives rise to a symmetric rotor top, an asymmetric rotor frame, and a sixfold barrier to the internal rotation motion. An alternative method of calculating the migrational tunneling splittings in this ion, involving a more recently proposed algebraic formalism, shows that qualitatively similar splitting patterns are obtained even when the hydrogens are not constrained to form a rigid equilateral triangle during their migration motion. Energy level diagrams, nuclear spin statistical weights, selection rules for electric dipole transitions, etc., are presented. It is hoped that the patterns described in the present work will aid in identifying and interpreting any spectrum of C₂H₃⁺ obtained as a result of ongoing efforts elsewhere in high-resolution ion spectroscopy.     

High-Resolution Visible Laser Spectroscopy of Cobalt Monochloride

The electronic spectrum of cobalt monochloride has been investigated from 415 to 725 nm using a laser-ablation/molecular-beam laser-induced fluorescence spectrometer. Two separate electronic systems with origins near 483.3 and 470.3 nm were observed. Data have been recorded for these two transitions at both low and high resolution. These transitions are now characterized as the [20.7]³Φ₄−X³Φ₄ and [21.3]³Φ₄−X³Φ₄ transitions. A low-resolution vibrational analysis and a high-resolution rotational analysis have been carried out for each system, resulting in accurate values for the ground and excited state vibrational spacings and effective rotational constants. In addition, the magnetic hyperfine structure resulting from the spin of the Co nucleus was resolved and the hyperfine constants were determined. Comparison of the CoCl spectrum with that of CoH and CoF has allowed the ground state electron configuration of (core)(10σ)²(4π)⁴(1δ)³(5π)³(11σ)² to be determined. The hyperfine constants support the electron promotion 11σ→12σ for the observed transitions.     

Some realistic calculations on the migration process of hydrogen isotopes in bcc metals

By adopting the empirical interaction potential between hydrogen and metal atoms determined in our previous paper, calculations have been performed of the 2T configuration, in which two neighboring tetrahedral (T) sites are equally shared by a hydrogen (H) atom. The tunneling matrix element J and the activation energy are estimated by calculating the excitation energy of the H atom for the 2T state and the energy difference between the 2T and 1T state, respectively.From these calculations, it is suggested that H atoms in V and Fe migrate between the ground states of neighboring T sites by adiabatic transitions, whereas in the lower temperature region in Ta by non-adiabatic tunneling process.     

The rotational spectrum of the ground state of methylamine

Recent progress is reported in measuring, assigning, and fitting the rotational spectrum of the ground vibrational state of methylamine, CH₃NH₂, a spectrum complicated both by internal rotation of the methyl top and by inversion of the amino group. New measurements of 513 rotational transitions with J up to 30 and K up to 9 were carried out between 49 and 326 GHz using the millimeter-wave spectrometer in Kharkov. After removing the observed quadrupole hyperfine splittings, these new data along with previously published measurements were fitted to a group-theoretical high-barrier tunneling Hamiltonian from the literature, using 53 parameters to give an overall weighted standard deviation of 0.80 for 850 far-infrared and 673 microwave transitions in the ground state. The root-mean-square deviation of 0.018 MHz obtained for 346 millimeter-wave transitions measured with 0.020 MHz uncertainty represents an approximately 30-fold improvement in fitting accuracy over past attempts.     

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.     

The microwave spectrum of CHD2Cl

The ground vibrational state microwave spectrum of CHD₂Cl has been studied in the region 26.5–40.0 GHz. From the observation of weak c-type transitions the A₀ rotational constants of CHD₂³⁵Cl and CHD₂³⁷Cl have been determined to be 95 426.08 ± 0.06 and 95 425.23 ± 0.11 MHz, respectively. The observed a-type and c-type transitions have been used to obtain A, B, C, all five quartic and one sextic distortion constants present in the reduced Hamiltonian of Watson for the ³⁵Cl and ³⁷Cl isotopic modifications of CHD₂Cl.     

Analysis of the microwave spectrum of hydrazine

Microwave measurements in the interval from 6 to 133 GHz, consisting of 444 rotational transitions in the vibrational ground state of hydrazine with J ≤ 31 and K_a ≤ 6 were fit to an effective rotational Hamiltonian containing 9 asymmetric rotor constants, 14 NH₂ inversion parameters, and 1 internal rotation parameter, with an overall standard deviation of the fit of 0.40 MHz. This set of parameters contains: (i) the three rotational constants; (ii) tunneling splitting constants for NH₂ inversion at one end of the molecule, for NH₂ inversion at both ends of the molecule, and for internal rotation through the trans barrier; (iii) two K-type doubling constants affecting the K = 1 levels; (iv) an a-type Coriolis interaction with matrix elements linear in K; and (v) various centrifugal distortion corrections to the above parameters. A consistent group theoretical formalism was used to label the energy levels and to select terms in the phenomenological rotational Hamiltonian. The Hamiltonian matrix, which is set up in a tunneling basis set, is of dimension 16×16 and contains only ΔK_a = 0 matrix elements, asymmetric rotor effects being taken into account on the diagonal by terms from a Polo expansion in bⁿ. Hyperfine splittings and barrier heights are not discussed.     

The microwave spectrum and structure of the argon trifluoroacetonitrile complex

The rotational spectrum of argon trifluoroacetonitrile complex has been studied by pulsed-nozzle, Fourier transform microwave spectroscopy. Both a-type and b-type transitions have been observed. The rotational constants are A = 3053.0903(2) MHz, B = 1039.9570(2) MHz, and C = 895.5788(1) MHz. The ¹⁴N nuclear quadrupole hyperfine components of the rotational transitions have been resolved, the ¹⁴N nuclear quadrupole coupling constants are χ_aa = 1.746(1) MHz, and χ_bb − χ_cc = −6.426(2) MHz. The complex is T-shaped, with the argon atom located 3.73 Å from the center of mass of the trifluoroacetonitrile molecule.