G. Winnewisser

The torsion-rotation microwave spectrum of 12CH318OH and the structure of methanol

Abstract The microwave and millimeter wave spectrum of 12 CH 3 18 OH has been observed in the frequency region 7.9–200 GHz. Both a - and b -type transitions have been assigned and measured. This spectrum was analyzed using the method of Lees and Baker, and rotational constants, torsional constants, centrifugal distortion constants, the barrier to internal rotation and moments of inertia have been evaluated. The barrier has been found to be 374.91 ± 0.18 cm −1 , in good agreement with that of 12 CH 3 16 OH. The moments of inertia were combined with those of other isotopic species to give a full substitution structure. To assist searches for this molecule in interstellar space a table of predicted frequencies of astrophysically interesting transitions is presented.

Microwave spectra and centrifugal distortion constants of formic acid containing 13C and 18O: Refinement of the harmonic force field and the molecular structure☆

Abstract Pure rotational spectra of H 13 COOH, HC 18 OOH, and HCO 18 OH have been measured in the frequency region 8–185 GHz. Analysis of the spectra has given improved rotational constants and quartic and sextic centrifugal distortion constants. The quartic distortion constants have been combined with previously published distortion constants of four other isotopic species, and with the vibrational wavenumbers of seven isotopic species, to produce a refined harmonic force field. An improved substitution structure and the ground state average structure have been obtained. Some unmeasured transition frequencies which may be of importance in radioastronomy are also presented.

Ground state spectroscopic constants of H15NCS, HN13CS, and HNC34S, and the molecular structure of isothiocyanic acid

Abstract The rotational spectrum of isothiocyanic acid was measured for the isotopically enriched species H 15 NCS and HN 13 CS as well as HNC 34 S in natural abundance. In the frequency range from 8 to 240 GHz the a -type R -branch transitions were measured for all three isotopic species. The q Q 1 transitions were identified in the microwave region for H 15 NCS and HN 13 CS. The rotational and centrifugal distortion constants were determined using Watsons Hamiltonian in the S reduction, extended empirically to higher order terms in the angular momentum. The molecular structure of isothiocyanic acid was reevaluated using a modified substitution method and the NCS chain was found to be bent: r( Nue5f8H ) = 0.993 A , r( Nue5f8C ) = 1.207 A , r( Cue5f8S ) = 1.5665 A , ∠HNC = 131.7°, and ∠NCS = 173.8°. The molecule has the trans conformation.

Rotational spectra of the 13C and 15N isotopic species of cyanoacetylene

Abstract Ground vibrational state rotational transitions have been measured in the frequency region 8 to 220 GHz for the following four isotopic species of cyanoacetylene: H 13 CCCN, HC 13 CCN, HCC 13 CN, and HCCC 15 N. The improved rotational constants were used for accurate frequency predictions for any transition not measured in this frequency range. Cyanoacetylene was produced efficiently by a radiofrequency discharge in a mixture of HCCH and HCN; this technique provides a convenient method for specific isotopic enrichment of this molecule.

A precise study of the rotational spectrum of formaldehyde H212C16O, H213C16O, H212C18O, H213C18O

Abstract The rotational spectra of formaldehyde, H212C16O and its isotopic species H213C16O, H212C18O, and H213C18O have been investigated in the ground vibrational state in the frequency region between 8 and 460 GHz. For most cases in which measurements of the a-type R- and Q-branch transitions already existed the accuracy of the line position has been improved to about 10 kHz. For H212C16O and H213C16O a large number of ΔKa = ±2 transitions were measured with similar accuracy. These new data when combined with all other available data and appropriate weightings lead to a set of ground state parameters which for the first time are compatible with infrared and ultraviolet data. The rotational constants (and 3σ standard deviations) obtained using Watsons A-reduced Hamiltonian are: H212C16O H213C16O H212C18O H213C18O A/MHz 281 970.572 (24) 281 993.258(135) 281 961.94 (39) 281 985.00 (93) B/MHz 38 836.0456(13) 37 811.0887(25) 36 904.1693(66) 35 859.256(10) C/MHz 34 002.2034(12) 33 213.9790(25) 32 511.5311(63) 31 697.868(10) This paper reports the first observations of the H213C18O rotational spectrum.

A precise study of the rotational spectrum of formaldehyde H212C17O and H213C17O

Abstract The pure rotational spectra of H 2 12 C 17 O and H 2 13 C 17 O have been investigated in the frequency region between 8 and 360 GHz in the ground vibrational state. For both isotopic species the 17 O nuclear quadrupole coupling constants and spin-rotation constants have been obtained. From both Q - and R -branch transitions a set of rotational constants and several distortion constants could be derived employing Watsons formalism in A reduction. The obtained rotational constants are in Megahertz: H 2 12 C 17 O H 2 13 C 17 O A 281 965.0 (30) 281 987.3 (19) B 37 812.287(45) 36 776.790(25) C 33 214.523(31) 32 412.920(19) This study reports the first measurement of the H 2 13 C 17 O rotational spectrum.

Ground state spectroscopic constants of isothiocyanic acid, HNCS, from its microwave and millimeter wave spectra combined with far infrared data

Abstract The microwave and millimeter wave spectra of isothiocyanic acid, HNCS, in the ground vibrational state have been investigated in the frequency region 8–300 GHz. The a-type R-branch transitions have been assigned up to J = 25 and Ka = 4, and the a-type qQ1 branch transitions up to J = 45. No b-type transitions could be identified in the frequency region covered. The far infrared data reported by Krakow, Lord, and Neely [J. Mol. Spectrosc., 27, 148 (1968)] were combined with our millimeter wave data in order to determine reliable spectroscopic constants. The rotational Hamiltonian, Watsons formalism with S reduction, has been extended empirically to higher order to facilitate the fitting of the large centrifugal distortion effects. The obtained constants are: A = 1357.3 GHz; B = 5883.4627 MHz; C = 5845.6113 MHz; D J = 1.19393 kHz; D JK = −1025.37 kHz; D K = 51.57 GHz; d 1 = −13.781 Hz; d 2 = −4.59 Hz. The 14N quadrupole coupling constant has also been determined: χaa = 1.114 MHz.

Inversion of the Ka = 0 and 1 rotational levels in the lowest excited vibrational state of HNCS

Abstract Fairly strong, regularly spaced absorption lines have been observed in the microwave spectrum of HNCS and assigned to b -type, K a = 0 ← 1, Q -branch transitions arising from molecules in the lowest excited vibrational state. The Fortrat diagram of these lines has the appearance of a c -type Q branch, which is impossible in HNCS because of its symmetry. This anomalous b -type Q -branch spectrum is caused by strong a -type Coriolis interactions among the three low-lying bending modes; the K a = 1 levels of the lowest excited vibrational state are perturbed and shifted lower in energy than the K a = 0 levels for each J . This interpretation has been confirmed by the observation of P - and R -branch transitions associated with this Q branch. The band origin has been determined to be −40 104.287 MHz (−1.3377 cm −1 ). The inversion of the K a = 0 and 1 energy levels is consistent with the interpretation of HNCS as a quasi-linear molecule.

Rotational Spectrum of Methylcyanoacetylene A New Millimeter Wave Spectrometer

Abstract The ground state rotational spectrum of methylcyanoacetylene, CH3CCCN, has been observed between 8 and 90 GHz, yielding precise rotational constants and a determination of the nuclear hyperfine costant eqQ = -4.0 ± 0.2 MHz. The millimeter wave spectra (70 - 90 GHz) were obtained with a newly constructed spectrometer, employing a synthesizer - controlled reflex klystron as source.

Diode-laser study of rotational temperature and absorption line shape in supersonic free jets

Infrared spectra of NNO and OCS have been studied in a supersonic free jet by using a diodelaser spectrometer. The sample gas of the molecules of 1 bar pressure was injected into a vacuum chamber (p ≈ 10−5 mbar) through a nozzle with a magnetic valve. The molecular beam was pulsed ( ≈ 82 Hz) by controlling the magnetic valve. Near the nozzle and for low values of the quantum number J the rotational temperature was determined to range between 29 and 3 K. For NNO we have observed the unusual line shapes in detail. They are caused by superposition of absorption signals of molecules in the jet and the background.