The microwave spectrum of HGeCl

The J = 1₀₁-0₀₀ and 2₀₂-1₀₁ transitions of nine isotopomers of chlorogermylene, H⁷⁴Ge³⁵Cl, H⁷⁴Ge³⁷Cl, H⁷²Ge³⁵Cl, H⁷²Ge³⁷Cl, H⁷⁰Ge³⁵Cl, H⁷⁰Ge³⁷Cl, H⁷⁶Ge³⁵Cl, H⁷⁶Ge³⁷Cl, and H⁷³Ge³⁵Cl are measured at 8-9 and 16-18 GHz. The effective rotational constants, the nuclear quadrupole coupling constants of ³⁵Cl, ³⁷Cl, and ⁷³Ge, and the nuclear spin-rotation coupling constants of ³⁵Cl and ³⁷Cl are determined.     

The microwave spectrum of urea

The microwave spectrum of urea has been recorded between 5 and 50 GHz. The rotational constants obtained from a least-squares fit of 47 b-type transitions are A = 11233.333, B = 10369.369, C = 5416.668 MHz. Centrifugal distortion constants were evaluated subject to the planarity constraints, giving satisfactory agreement up to at least J = 20. They are: τ_aaaa = ?0.04423, τ_bbbb = ?0.04075, τ_aabb = 0.02268, τ_abab = ?0.02806. The diagonal elements of the nuclear quadrupole coupling tensor are χ_aa = 2.16, χ_bb = 1.88, χ_cc = ?4.04. The dipole moment is μ = μ_b = 3.83 D (12.8 × 10^?30 Cm).     

The microwave spectrum of trioxaadamantane

The microwave spectrum of 2,8,9-trioxaadamantane has been investigated in the region from 12.4 to 26.5 GHz. The observed spectrum exhibited the expected symmetric top pattern, with the rotational constant B₀ = 1848.64 MHz. Numerous weaker lines were observed and were attributed to vibrational satellites of the main rotational transition. The transitions from J = 3 → 4 through J = 6 → 7 were studied and no centrifugal distortion effects were observed.A structure is derived that is consistent with the observed rotational constants of the normal and one isotopic species by use of the method of diagnostic-least-squares.The second order stark effect for the K = 0 state yielded a dipole moment of 3.01 ± 0.03 D.     

The microwave spectrum of tetrazole

Microwave spectra of tetrazole and the two singly deuterated tetrazole species have been assigned. Dipole moment measurements suggest that the tautomeric form observed in normal tetrazole and N-deuteriotetrazole is 2H-tetrazole while that observed in C-deuteriotetrazole is 1H-tetrazole. All three species are planar.     

The microwave spectrum of pyrrolidine

The rotational spectra of the normal and N-D isotopic species of pyrrolidine have been measured. The molecule exists in an envelope conformation, with the nitrogen atom out of the plane containing the carbon atoms and with the imino hydrogen in the axial position. The analysis of the rotational spectra of two and three vibrationally excited states for the normal and N-D isotopic species, respectively, indicates that both barriers to pseudorotation and to the imino hydrogen inversion are relatively high. Additionally, the N(14) quadrupole coupling constants have been determined.     

The microwave spectrum of vinylcyclopropane

Frequencies of rotational transitions in the microwave region are reported for the ground and three torsionally excited states of the species of vinylcyclopropane in which the vinyl group is trans to the cyclopropane ring and there is a plane of symmetry. Transitions in the species with ¹³C substitution at the out-of-plane carbon site are also reported. An extensive search for transitions belonging to another species was not successful. The torsional excitation energy is estimated from relative intensity measurements to be 125 cm^?1. The dipole moments determined from the Stark effect ar μ_a = 0.486 ± 0.007 D, μ_c = 0.110 ± 0.010 D and μ = 0.498 ± 0.007 D.     

The microwave spectrum of cyanoformamide

The spectrum of cyanoformamide, NCCONH₂, has been measured between 18 and 40 GHz using a Hewlett-Packard spectrometer with Stark modulation. The molecule is somewhat unstable and could most conveniently be measured in a flow system. The quadrupole hyperfine structure due to the two nitrogen nuclei has been analyzed for the ground state, and quadrupole coupling constants, rotational constants, and centrifugal distortion constants have been determined for the ground state. A rough determination of the components of the electric dipole moment was possible from the Stark shifts of suitable transitions.     

The microwave spectrum of vinyl azide

Vinyl azide has been shown to exist in both cis and trans conformations with the cis more stable by 460 cal/mole. The rotational constants in Megahertz are: cis, A = 14 698.23, B = 4088.44, C = 3195.64; trans, A = 46 761.19, B = 2751.40, C = 2598.83. The dipole moment components (in debye) are: cis, μ_a = 1.337, μ_b = 0.326, μ_total = 1.376; trans, μ_a = 1.156, μ_b = 0.900, μ_total = 1.465.     

The microwave spectrum of bromoacetic acid

The microwave spectrum, rotational constants, centrifugal distortion parameters, and nuclear quadrupole coupling constants of bromoacetic acid are reported. Deviations from the second-order perturbation theory for quadrupole splittings are discussed, and the principal axes of the nuclear quadrupole tensor are determined. From the a-substitution coordinates of the Br atom and the carboxyl H atom the molecular conformation is found to be trans with respect to the atoms BrCCOH.     

The microwave spectrum of methylene ketene

The microwave spectrum of methylene ketene has been observed from 8 to 35 GHz and assigned. Rotational constants and centrifugal distortion constants have been determined for CH₂CCO, CHDCCO, and CD₂CCO. The dipole moment was found to be μ_a = 2.14 ± 0.06 D (7.07 ± 0.2 × 10^?30 Cm). We were unable to detect propynal among the pyrolysis products of acrylic anhydride. This pyrolysis proved to be the most convenient route for generating methylene ketene for the present investigation.     

The microwave spectrum of methyl isocyanate

The microwave spectrum of methyl isocyanate, CH₃NCO, has been measured in the range from 8 to 40 GHz. More than 200 lines have been assigned to the a-type J + 1 ← J rotational transitions (J = 0 to 3) arising from molecules in the ground state and excited torsional and CNC bending states. The observed spectrum has been analyzed and successfully interpreted in terms of the five-dimensional quasi-symmetric top molecule model accounting explicitly for the large-amplitude CNC bending motion, and internal and overall rotation. In the least-squares fitting to the observed transition energies the values of the parameters of the CNC bending-torsional potential function have been determined.     

The microwave spectrum of methyl isothiocyanate

The microwave spectrum of methyl isothiocyanate, CH₃NCS, has been measured in the range from 5 to 26 GHz. Almost 300 lines have been assigned to the a-type J + 1 ← J rotational transitions (J = 0 to 4) arising from molecules in the ground state and excited torsional and CNC bending states. The observed spectrum has been analyzed and successfully interpreted in terms of the five-dimensional quasi-symmetric top molecule model accounting explicitly for the large-amplitude CNC bending motion, and internal and overall rotation. In the least-squares fitting to the observed transition energies the values of the parameters of the CNC bending-torsional potential function have been determined.     

The microwave spectrum of 1-pyrazoline

The microwave spectrum of 1-pyrazoline has been observed from 18 to 40 GHz in the six lowest states of the ring-puckering vibration. It is an a-type spectrum of a near oblate asymmetric top. Each vibrational state has been fitted to a separate effective Hamiltonian, and the vibrational dependence of both the rotational constants and the quartic centrifugal distortion constants has been observed and analyzed. The v = 0 and 1 states have also been analyzed using a coupled Hamiltonian; this gives consistent results, with an improved fit to the high J data. The preferred choice of Durig et al. [J. Chem. Phys.52, 6096 (1970)] for the ring-puckering potential is confirmed as essentially correct, but the A and B inertial axes are shown to be interchanged from those assumed by Durig et al. in their analysis of the mid-infrared spectrum.     

The microwave spectrum of methyl isothiocyanate

New measurements of microwave transitions of CH₃NCS in the region 14–40 GHz are reported. Assignments of several sets of lines in terms of the transitions expected for a pseudosymmetric-top molecule are suggested and a tentative extension of the analysis to cover all the k = 0 lines in our spectra and in the region 5–11 GHz is proposed. Preliminary calculations using a two-dimensional anharmonic oscillator model lead to a bending potential with a central hump of some 160 cm^?1, and a minimum-energy configuration at a CNC bond angle of about 147°. A correlation between rovibrational energy levels of a two-dimensional anharmonic oscillator and those of an asymmetric-top molecule with a bent skeleton and freely rotating methyl group is proposed.     

The microwave spectrum of difluoroacetic acid

Transitions due to two conformations have been observed in the microwave spectrum of difluoroacetic acid. In both rotamers the carboxylic hydrogen atom is cis to the CO bond, whereas the other hydrogen atom is either trans or gauche with respect to the CO bond. By means of isotopic substitution of the hydrogen and oxygen atoms a serious deviation of local symmetry in the CHF₂ group is deduced for the gauche form. This is thought to be an artifact of the substitution method, although no satisfactory explanation can be given. In torsionally excited states of the gauche conformation, doublet splittings were observed due to tunneling through a barrier of 190 ± 10 cm^?1. For the second excited state a satisfactory treatment could be given, but for the third excited state large differences between observed and calculated transition frequencies remained. A computer program to account for the internal rotation in asymmetric molecules was written, and it was shown qualitatively that interactions with excited states of the trans conformation could very well be responsible for the discrepancies. From intensity measurements the energy difference E(gauche) - E(trans) was found to be 370 ± 180 cm^?1. The large uncertainty is due to the failure of attempts to determine the dipole moments experimentally; they were estimated from an improved set of bond moments which was derived from related compounds. Both energy difference and dihedral angles are at variance with the results of an earlier electron diffraction investigation.     

The microwave spectrum of lactic acid

The microwave spectrum of lactic acid has been identified with aid of double resonance techniques. Satellite spectra have been tentatively assigned to the two CC torsional modes. From deuterium substitution into the two hydroxyl groups the conformation was determined: the molecule shows a hydrogen bond from the α-hydroxyl group to the carbonyl oxygen atom. The c coordinate of the hydrogen atom involved has the appalling imaginary value 0.31i, thus once more casting doubt on the reliability of the substitution method for accurate structure determination.     

The microwave spectrum of HCN dimer

The microwave spectrum of the hydrogen-bonded HCN dimer was observed over the range 28–93 GHz. Improved values of the spectral parameters B = 1788.214(16) MHz, D_J = 2.582(16) kHz were obtained. The electric dipole moment was found to be 6.0(15) D (2.0(5) × 10^?29 cm).     

The microwave spectrum of germyl isocyanate,a quasi-symmetric top

The microwave spectrum of germyl isocyanate in the K, P, and R bands (12.4–40.0 GHz) has been investigated using natural isotopic samples and samples containing specific germanium isotopes, mass numbers 72 and 74, and samples containing GeD₃ groups. The very complex spectrum is analyzed as that of a quasi-symmetric top, in which the skeletal bending motion has an energy maximum at the linear configuration; the height of the maximum is some 339 cm⁻¹. Many vibrational states of the two-dimensional bend lie close in energy, resulting in strong interactions between rotational levels of different vibrational states. The program used for the analysis has been used previously to account for similar spectra of methyl and silyl isocyanates and methyl isothiocyanate; the behavior of the germyl compound is more like that of the methyl isocyanate than that of the silyl compound, where the height of the maximum is much less. Although we did not include centrifugal distortion operators in the Hamiltonian the program successfully predicts k = 0 line positions in J + 1 ← J bunches with J up to 9, using only lines in the J = 3 bunch to define the parameters of the model, which include the height of the hump, the bend angle of the skeleton at the energy minimum (142.18(5)°), the force constant for the bending motion at the minimum (0.1094(7) mdyn Å), and the GeN bondlength at equilibrium (1.8257(5) Å). The threefold barrier to internal rotation of the GeH₃ group at the equilibrium position is about 3 cm⁻¹, much lower than the value found for methyl isocyanate. The fit to the observations, about 100 lines in the 4 ← 3 bunch, is improved by allowing for a variation in the GeN bondlength as the bend angle changes. The quality of the least-squares fit corresponds to an estimated standard deviation for an observation of about 2 MHz, but not all observed lines can be accounted for precisely, showing that additional interactions are involved that are not yet included in the model.