Author Institution: Institute of Applied Physics of RAS, 46 Ulyanov str., 603950 Nizhny Novgorod, Russia; Institute of Metalloorganic Chemistry of RAS, 49 Tropinin str., 603950 Nizhny Novgorod, Russia; Institute of Physical Chemistry and Electrochemistry, Leibniz University of Hannover, Callinstrasse 3A, 30167 Hannover, Germany; Technical Faculty of Christian Albrecht University of Kiel, Kaiserstrasse 2, 24143 Kiel, Germany

Detailed studies of the internal motions of dark clouds using spectral lines of many molecules require a laboratory frequency accuracy of the order of a few m s−1. Based on our laboratory studies of the HNCO rotational spectrum in the ground vibrational state, we have increased significantly the accuracy of frequency calculation in a wide range of quantum numbers. We have achieved an (1σ) uncertainty for rotational transitions in the K a = 0, 1 states recalculated to the Doppler velocity scale ≤2 m s−1 for all frequencies <1.1 THz. This value allows radio-astronomical measurements with an accuracy comparable to that of the highest-precision observations based on spectral lines of other molecules.

The pure rotational spectra in the ground vibrational state of (1,2- 13 C)ketene, H 2 13 C= 13 CO, (D 2 ,1- 13 C)ketene, D 2 C= 13 CO, (D 2 ,2- 13 C)ketene, D 2 13 C=CO, and (D 2 , 18 O)ketene, D 2 C=C 18 O, have been observed in the frequency region 200 - 350 GHz. All the spectral lines have been measured in natural abundances with a source modulated millimeterwave spectrometer. From the measured R-branch transitions a set of rotational and centrifugal distortion constants for each isotopomer could be derived, using the Watson S-reduction formalism. Further, the rotational spectra of the two isotopomers (4,5-D)ketene, D2CCO, and (4-D)ketene, DHCCO, which were already measured several years ago, have been extended to higher J-values and higher frequencies, as it is the case for all investigated isotopomers of this work. As a result of these studies a calculation of a mass-dependent structure will be the topic of a next paper.

The capability, accuracy and precision of broadband spectrometers developed in the Microwave Spectroscopy Lab in Nizhniy Novgorod on a basis of backward wave oscillators (BWOs) are presented. Examples of high accuracy laboratory measurements of molecular transition frequencies, shifts, and line shapes spanning many orders of magnitude in pressure are given. The importance of sensitive broadband line measurements in the millimeter and submillimeter wavelength range to both atmospheric and astronomical observations is discussed in the context of the laboratory precision and accuracy required for their interpretation. The demands on the quality of laboratory measurements have become very stringent. The present state of broadband BWO-based spectroscopy is shown to satisfy these demands. Accurate measurements are also needed for further studies of the physics of the molecules and their interactions. The capabilities and prospects for further research and development are discussed.

The pure rotational spectra of [O-18]ketene, H2C=(CO)-O-18, [1-C-13]ketene, H2C=(CO)-C-13, and [2-C-13]ketene, H-2 C-13=CO, have been revisited in the frequency region 200-350 GHz in the ground vibrational state. From more than 100 R-branch transitions for each isotopomer a set of rotational and centrifugal distortion constants could be derived using the Watson S-reduction formalism. The values obtained for the rotational constants B and C agree very well with results of former investigations. The agreement is worse with respect to the A constants, but our newly determined A values agree well with the corresponding values of the main species and the O-17 isotopomer.

The pure rotational spectra of [18O]ketene, H2C=C18O, [1-13C]ketene, H2C=13CO, and [2-13C]ketene, H213C=CO, have been revisited in the frequency region 200 - 350 GHz in the ground vibrational state. From more than 100 R-branch transitions for each isotopomer a set of rotational and centrifugal distortion constants could be derived using the Watson S-reduction formalism. The values obtained for the rotational constants B and C agree very well with results of former investigations. The agreement is worse with respect to the A constants, but our newly determined A values agree well with the corresponding values of the main species and the 17O isotopomer.

The pure rotational spectrum of fluoroethyne (fluoroacetylene, HCCF) has been reinvestigated. Rotational transitions have been measured in the frequency range from 230 up to 510 GHz and assigned to the excited vibrational states (υ3υ4υ5)=(000), (001), (002), (003), (010), (011), (012), (020), and (100). The analysis of the spectrum extends the spectroscopic data for HCCF by correcting and refining the rotational and rovibrational constants determined in our previous investigation.

The collisional relaxation of the J=5←4 rotational transition of CO induced by carbon monoxide, nitrogen, and oxygen has been studied at room temperature. Pressure-broadening parameters were determined as 3.29(2), 2.61(2), and 2.30(2) MHz/Torr for CO, N2, and O2 buffer gases, respectively. Experimental deviations from the Voigt line shape profile have been observed which are mostly the effect of a narrowing in the spectral line core. The difference between the model profile and the experimental profile is less than 0.5% of the maximum line amplitude in the investigated pressure range 0.2–5 Torr. In addition, a small positive collision-induced shift of the line center frequency was observed for the pure gas, corresponding to a pressure self-shift parameter of 6(3) kHz/Torr.

The rotational spectrum of (H2CCO)-O-17 in the ground vibrational state has been investigated between 20 and 330 GHz. From 82 R-branch transitions a set of rotational constants and several centrifugal distortion constants could be derived, employing the Watson S-reduction formalism. The obtained rotational constants in MHz are: A = 282071.6(223), B = 10013.4764(28), C = 9655.9118(24). The nuclear quadrupole coupling structure of the J ' (K alpha ' Kc) <-- J(K alpha Kc) = 1(01) <-- 0(00) line has been recorded by means of molecular beam Fourier transform microwave spectroscopy allowing the determination of the nuclear quadrupole constant chi (alpha alpha) = -1.534(54) MHz (without considering the spin-rotation interaction). A recalculation of the r(s)-structure has also been carried out, using the constants of the new isotopomer. The result agrees with the values reported by East et al. in 1995. This is, to our knowledge, the first reported investigation of the (H2CCO)-O-17 rotational spectrum.

$^{a}$H.M. Pickett, Effects of velocity averaging on the shape of absorption lines, J. Chem. Phys. 73 (12), 6090-6094, 1980. $^{b}$M. Danos, S. Geschwind, Broadening of Microwave Absorption Line Due to Wall Collision, Phys. Rev., 91(5), 1159-1162, 1953.

Rotational transitions of the linear molecule 3-isocyano-2-propynenitrile, NCCCNC, have been measured in the frequency range from 75 up to 120 GHz and assigned to the excited vibrational bending states (upsilon(6) upsilon(7) upsilon(8) upsilon(9)) = (0002), (0003), (0004), (0011), and (0020). The analysis of the spectrum extends the spectroscopic data for NCCCNC by adding various newly determined rotational and rovibrational constants. (C) 2000 Elsevier Science B.V. All rights reserved.

New microwave and millimeter-wave spectra up to 200 GHz of isocyanoacetylene (HCCNC) have been recorded. About 675 lines have been assigned to 111 vibrational states with energies up to 1700 cm(-1). In previously recorded infrared spectra in the 600-2000 cm(-1) domain, 53 new infrared bands have been assigned and analyzed. These concern upper vibrational states up to energies close to 1900 cm(-1), which are combinations of the three bending modes (nu(5) to nu(7)) and the lowest energy stretching mode nu(4). The assignments were made possible thanks to a global rovibrational analysis progressively applied to all available data. Good agreement has been obtained, and a set of 155 molecular parameters has been determined. Our analysis suggests that the term directly coupling 2 nu(6) and nu(5) + nu(7) states (k(5667)) is to be added to our model, and also Coriolis interactions between nu(6) and 2 nu(7) states on one hand, and between nu(5) and nu(6) + nu(7) states on the other hand. (C) 2000 Academic Press.

Propynyl isocyanide, CH3C2NC, has been prepared by vacuum pyrolysis of pentacarbonyl-(1,2-dichloropropenyl isocyanide) chromium, (CO)5Cr–CN–C(Cl)=C(Cl)CH3, and its ground state millimeter and microwave spectrum has been observed for the first time. rs structural parameters of this molecule with a C3v symmetry could be obtained from the rotational constants of several isotopomers: r(C1–C2)=1.456(2)Å, r(C2–C3)=1.206(2)Å, r(C3–N)= 1.316(2)Å, r(N–C4)= 1.175(2)Å, r(H–C1)= 1.090(1)Å, >HCC=110.7(4)°. The nitrogen quadrupole coupling constant has been determined to be 878(2)kHz and measurements of the Stark effect allowed to obtain an electric dipole moment of 4.19(3)Debye. The results fit well into a series of related compounds and are in good agreement with data from ab initio calculations.

The collisional broadening of the J '← J = 1 ← 0 rotational line of carbon monoxide by the buffer gases He, Ne, Ar, Kr, CO, N2, O2 , and air has been studied at room temperature. Two different experimental techniques in time- and frequency-domain, respectively, were used. The obtained data are in good agreement. Time-domain investigations on the temperature dependence of the foreign gas broadening parameters are also presented.

The millimetre-wave spectrum of 2-isocyano-3-propynenitrile, NC3NC, was observed in the vibrational ground state (0000) and in the excited bending states (v6v7v8v9)=(1000), (0100), (0010), and (0001). Pure rotational transitions J+1←J were recorded in the ranges 40–120 and 240–315 GHz. The analysis of the spectra, aided greatly by accompanying ab initio calculations, yielded precisely determined rotational, centrifugal distortion, and l-type doubling constants. They are in excellent agreement with the results of coupled-cluster calculations. Wavenumbers and infrared (IR) intensities of various stretching vibrational transitions were calculated variationally. Good agreement with results from high-resolution IR spectroscopy is observed for the highest fundamentals ν1–ν3, with deviations amounting to 15.4 cm−1 (ν1), 1.2 cm−1 (ν2), and 7.0 cm−1 (ν3).

The millimeter-wave spectrum of deuteroethynylisocyanide has been observed and analyzed in the ground and in the first excited vibrational states (v4, v5, v6, v7) = (0100), (0010), and (0001). Rotational, centrifugal distortion, and l-type doubling constants are given. Copyright 1998 Academic Press.