W. Lewis-Bevan

The microwave spectrum and structure of the unstable molecule bromine isocyanate, BRNCO

Abstract The microwave spectra of two isotopic species of the unstable molecule bromine isocyanate, BrNCO, have been observed from the flow reaction of Br 2 vapor with silver cyanate. The spectra were observed in the frequency range 8–54 GHz. They contain strong a -type transitions, as well as b -type transitions, with the latter so weak they initially could not be assigned. The transitions show Br and N quadrupole hyperfine structure; many perturbations in the former have been found, and have been used in a novel method to evaluate accurately all the rotational constants and the Br quadrupole coupling tensor entirely from a -type R branches. This has been done using a global least-squares fitting program developed especially for the purpose. The resulting constants permitted the b -type transitions to be identified. The molecule has been shown to be planar, the BrNCO configuration has been confirmed, and a partial r 0 structure has been determined. The principal values of the Br quadrupole tensor have been evaluated and the BrN bond has been shown not to be bent.

The infrared spectrum of gaseous aminoborane, H2NBH2: Location of the fundamentals and rotational structure in the 401 band (BN stretching vibration at 1337 cm−1)

Abstract The infrared spectrum of gaseous aminoborane, H2NBH2, has been observed for the first time, following controlled thermal decomposition of NH3BH3 in a flow system. The positions of the nine vibrational fundamentals lying above 800 cm−1 have been determined from medium-resolution Fourier transform spectra, and the rotational structure of the ν4 fundamental (the BN stretching vibration at 1337.474 cm−1) has been studied at high resolution. The 41 level is extensively perturbed, and analysis of the perturbations has permitted the positions of the three lowest-lying fundamentals (which are either infrared inactive or below the wave number range of our spectrometer) to be estimated. An accurate set of rotational constants for the ground level of the molecule has been obtained.

Fourier transform spectroscopy of the CO‐stretching band of C‐13 methanol in the torsional ground state

The Fourier transform spectrum of the CO‐stretching fundamental band of 13CH3OH has been investigated at 0.002 cm−1 resolution. For the n=0 torsional ground state, 35 subbands containing some 2300 lines have been identified for K values from 0 to 10 for the three torsional symmetries A, E1, and E2 up to J values typically well over 30. The assigned transitions have been fitted to J(J+1) power‐series expansions to obtain subband origins and sets of phenomenological state‐specific parameters describing the J dependence of the subbands. These parameters reproduce the observed wave numbers to within the experimental uncertainty of ±0.0005 cm−1 for unblended lines, except when particularly large asymmetry shifts or perturbations due to Fermi or Coriolis resonances are present. The subband origins have been fitted to effective molecular constants for the excited CO‐stretching state and a torsional barrier of 378.7(8) cm−1 is found, representing a 1.3% increase over the ground state value. A wide variety of glob...

The microwave spectrum of an unstable molecule: Chloroketene ClHCCO

The microwave spectra of four isotopic species of the unstable molecule chloroketene ClHCCO have been observed from the flow pyrolysis of monochloroacetyl chloride and from the pyrolytic reaction of dichloroacetyl chloride with zinc metal. The spectra, which consist only of b‐type transitions, were observed in the frequency range 26.5–80 GHz. Accurate rotational constants and quartic and sextic centrifugal distortion constants have been obtained for all isotopes. Large numbers of perturbations have been observed in the chlorine quadrupole hyperfine structure; consequently, the complete quadrupole coupling tensor has been evaluated. The dipole moment has been measured. The molecule has been shown to be planar, and a partial r0 structure has been determined.

The high-resolution infrared spectrum of the 201 band of carbonyl fluoride: Determination of the far infrared laser frequencies

Abstract The first high-resolution (0.0024 cm −1 ) spectrum of the 2 0 1 band of COF 2 at 963 cm −1 is reported. Nearly 4000 rotational transitions have been observed and assigned in the band between 936 and 990 cm −1 . The line positions are estimated to be accurate to 0.0001 cm −1 (relative). The spectrum was calibrated using two adjacent bands of OCS. Approximately 1560 infrared transitions have been fitted simultaneously with the previously reported microwave data. The wave numbers of far infrared laser lines recently observed by Tobin and by Temps and Wagner have been calculated from the 2 1 energy levels, with estimated uncertainties of between 10 −5 and 10 −6 cm −1 .

Determination of rotational and centrifugal distortion constants using quadrupole hyperfine structure: The microwave spectrum of lodine isocyanate, INCO☆

Abstract The microwave spectrum of the unstable molecule iodine isocyanate, INCO, has been observed from the flow reaction of I 2 vapor with silver cyanate. The spectrum, which was measured in the frequency range 18–54 GHz, contains strong a -type and very weak b -type transitions. The transitions show I and N quadrupole hyperfine structure; many perturbations in the I structure have been observed. Our novel method for using such perturbations to evaluate otherwise unobtainable rotational constants has been extended to include centrifugal distortion constants. This has been done using a global least-squares fitting program developed especially for the purpose. The molecule has been shown to be planar, and a partial r 0 structure has been determined. The principal values of the I quadrupole tensor have been evaluated, and are consistent with a straight IN bond.

The microwave spectrum of bromine isocyanate, Brnco: Determination of rotational constants from quadrupole hyperfine structure

Abstract A novel method has been developed to evaluate accurate rotational constants from the microwave spectrum of the unstable molecule bromine isocyanate, using perturbations in nuclear quadrupole hyperfine structure. It has been applied to this prolate near-symmetric rotor to determine A v and x ab accurately, entirely from a -type R branches. The method has been made possible by the development of a special computer program for global least-squares fitting to rotational and centrifugal distortion constants, along with all components of the Br nuclear quadrupole coupling tensor.

Fourier transform spectroscopy of torsionally excited C-13 methanol in the CO-stretching band

Torsionally excited n=1 and n=2 subbands of the CO‐stretching fundamental have been assigned in the Fourier transform infrared spectrum of 13CH3OH recorded at a resolution of 0.002 cm−1. For the n=1 torsional state, subbands with K values up to 7 have been identified for A torsional symmetry and up to 5 for E symmetry. For n=2, four subbands have so far been assigned. Subband origins have been obtained by fitting the observed wave numbers to J(J+1) power‐series expansions. The expansion parameters represent the data compactly to close to the experimental uncertainty in the absence of line blending or perturbations. Significant downshifting of the n=1 origins is observed, correlated closely with proximity to the n=0 OH‐bending state. J‐localized perturbations have been observed and characterized for several subbands. Two of the far‐infrared laser lines optically pumped by the 10R(14) CO2 laser line have been assigned.

The microwave spectrum and centrifugal distortion constants of difluoroborane, HBF2

Abstract The microwave spectra of two isotopic species of the unstable molecule difluoroborane have been measured between 8 and 36 GHz. Transitions have been measured up to J = 56 and K a = 9 for H 10 BF 2 , and up to J = 66 and K a = 11 for H 11 BF 2 . Improved values for the rotational constants, boron nuclear quadrupole coupling constants, and quartic and sextic centrifugal distortion constants have been evaluated for both species.

The microwave spectrum of the unstable molecule bromoketene, BrHCCO☆

Abstract The microwave spectrum of the unstable molecule bromoketene, BrHCCO, has been observed for the first time, using the flow pyrolysis of dibromoacetyl chloride with zinc metal. Although, because of experimental difficulties, only a few transitions were measured, accurate rotational and Br quadrupole coupling constants, including the off-diagonal term χ ab , have been evaluated for two isotopic species, along with a limited number of certrifugal distortion constants. Several structural parameters have been obtained, and the quadrupole coupling constants are discussed in relation to them.