Gabriele Cazzoli

Lineshape measurements of rotational lines in the millimeter-wave region by second harmonic detection

Abstract Lineshape measurements of OCS rotational transitions have been carried out by using frequency modulation and comparing the experimental second harmonic signal to the computed second Fourier component of a modulated Voigt profile. The asymmetry of the spectral lines has been accounted for by adding to the lineshape model a suitable distortion term. The modulation amplitude values which provide good agreement between the experimental and the computed spectra have been found. The self-broadening coefficients of the J = 8 ← 7, 12 ← 11, and 16 ← 15 ground state transitions of OC 34 S and O 13 CS have been determined.

Equilibrium structure and force field of nitrosyl chloride

The a-type rotational spectra of 16O15N35Cl in the (100) excited vibrational state (ν1 ⋍ 1770 cm−1) and of 18O15N35Cl and 18O14N37Cl in the ground state were observed. The equilibrium structure of nitrosyl chloride was calculated from the values of the equilibrium rotational constants Be and Ce of the two isotopic species 16O14N35Cl and 16O15N35Cl. The ground-state rotational constants of seven isotopic species of nitrosyl chloride were used to calculate Watsons rm structure, which was found to be in satisfactory agreement with the re structure. The quadratic and cubic force fields of nitrosyl chloride were simultaneously refined by optimizing the least-squares fit to a set of 48 experimental data which includes the sextic centrifugal distortion constants, with one quadratic and three cubic terms of the potential energy function constrained to the values computed by SCF/ab initio methods. All remaining quadratic and cubic SCF force constants were found to be in reasonably close agreement with the empirically determined constants.

Observation of crossing resonances in the hyperfine structure of the J = 1 ← 0 transition of DC15N

Lamb-dip spectroscopy in the millimeter-wave region has been performed using a conventional free space cell. Crossing resonances in the hyperfine spectrum of the J = 1 ← 0 transition of DC15N have been observed. The analysis of the profile of the spectrum has provided accurate values of the hyperfine constants of deuterium.

Direct l doublet transitions for the 0110 state of cyanogen iodide

Abstract Direct l doublet transitions for the 01 1 0 state of 127 I 12 C 14 N and 127 I 13 C 14 N were observed between 4 and 25 GHz using a computer-controlled Stark modulation spectrometer. Values of the l doubling constants and the asymmetry parameters of the 127 I nuclear quadrupole coupling tensors were derived. Some details of the construction of the spectrometer are also given.

A new experimental absolute nuclear magnetic shielding scale for oxygen based on the rotational hyperfine structure of H2O17

The hyperfine structure in the rotational spectrum of water containing (17)O has been investigated experimentally and by means of quantum-chemical calculations. The Lamb-dip technique has been used to resolve the hyperfine structure due to spin-rotation as well as spin-spin interactions and allowed the determination of the corresponding hyperfine parameters with high accuracy. The experimental investigation and, in particular, the analysis of the spectra have been supported by quantum-chemical computations at the coupled-cluster level. The experimental (17)O isotropic spin-rotation constant of H(2)(17)O has been used in a further step for the determination of the paramagnetic part of the corresponding nuclear magnetic shielding constant, whereas the diamagnetic contribution as well as vibrational and temperature corrections have been obtained from quantum-chemical calculations. This joint procedure leads to a value of 325.3(3) ppm for the oxygen shielding in H(2)(17)O at 300 K, in good agreement with pure theoretical predictions, and in this way provides the basis for a new absolute oxygen shielding scale.

Precise Laboratory Frequencies for the J = 1-0 and J = 2-1 Rotational Transitions of C18O

Precise rest frequencies for the J = 1-0 and J = 2-1 rotational transitions of C18O have been measured in the laboratory using the Lamb-dip technique with an accuracy better than 1 kHz (2 σ uncertainty). In contrast to the main CO isotopomer, interstellar C18O lines are significantly narrower and very often have Gaussian line shapes. Taking into account their relatively high intensities in interstellar clouds, the new laboratory measurements allow us to consider C18O as the best frequency standard in radio astronomical spectroscopy.

Precise Laboratory Frequencies for the J ? J-1 (J=1, 2, 3, 4) Rotational Transitions of 13CO

Precise rest frequencies for the first four rotational transitions of 13 CO have been measured in the laboratory using the Lamb-dip technique with an accuracy better than 1 kHz. Taking into account available frequencies for the higher J transitions, the new measurements allowed us to predict rotational transitions up to 1 THz with 2 � � 1ms � 1 in a Doppler velocity scale. We found that the hyperfine (hf) splitting constant due to 13 C magnetic spin-rotation interaction, estimated from Lamb-dip measurements, is in excellent agreement with the results from molecular-beam experiments. In contrast to recent studies, no rotational dependence on the J quantum number has been determined with significantly high accuracy. Since hf splitting is comparable to the thermal line broadening at Tk¼ 10 K, its analysis can be important for 13 CO observations in dark clouds without nonthermal broadening. Subject headingg ISM: molecules — methods: laboratory — molecular data — radio lines: ISM

Rotational spectra of 1-chloro-2-fluoroethylene. II. Equilibrium structures of the cis and trans isomer

Equilibrium structures for the cis and trans isomer of 1-chloro-2-fluoroethylene are reported. The structures are obtained within a least-squares fit procedure using the available experimental ground-state rotational constants for various isotopic species of both forms. Vibrational effects were eliminated before the analysis using vibration-rotation interaction constants derived from computed quadratic and cubic force fields with the required quantum chemical calculations carried out using second-order Moller-Plesset perturbation as well as coupled-cluster (CC) theory. The semiexperimental or empirical equilibrium geometries obtained in this way agree well with the corresponding theoretical predictions obtained from CC calculations [at the CCSD(T) level] after extrapolation to the complete basis-set limit and inclusion of core-valence correlation corrections. The present results allow a detailed analysis of the geometrical differences between the two forms of 1-chloro-2-fluoroethylene. They are also compared to the structural data available for other halogenated ethylenes.

Microwave spectrum, structure and electric quadrupole coupling constants of HOCl

Abstract The rotational “a” type spectra of H 16 O 35 Cl, H 16 O 37 Cl, D 16 O 35 Cl, D 16 O 37 Cl and D 18 O 35 Cl have been measured. The rotational constants B and C and the quadrupole coupling constants χ aa and χ bb were obtained for each isotopic species. The values of D J and D JK for H 16 O 35 Cl were also determined from centrifugal distortion analysis of the spectrum. The following values of the structural parameters were obtained: r OH = 0.959 ± 0.005 A from I a and I b , r OH = 0.975 ± 0.003 A from I b and I c , r OCl = 1.689 5 ± 0.003 5 A and

Pure rotational spectra of 32SD+3 and 34SD+3: Determination of the substitution structure of the sulfonium ion

Abstract The pure rotational spectrum of the 32 SD + 3 ion was observed in the 152–457 GHz frequency region using a magnetically confined negative glow discharge as ion production source. Moreover, the J=1←0, K=0 transition of the isotopic species 34 SD + 3 was measured at 151858.506 MHz. The rotational constants B 0 determined from the analysis of the transition frequencies, along with the B 0 constant of 32 SH + 3 obtained by Lee et al. [S.K. Lee, H. Ozeki, S. Saito, S. Yamamoto, Chem. Phys. Lett. 224 (1994) 21] and the C 0 axial rotational constants of 32 SH + 3 and 32 SD + 3 from high-resolution infrared spectroscopy [T. Nakanaga, T. Amano, J. Mol. Spectrosc. 133 (1989) 201; C. Xia, M.M. Sanz, S.C. Foster, J. Mol. Spectrosc. 188 (1998) 175], were used to calculate the substitution structure of the ion.