Cristina Puzzarini

ROVIBRATIONAL ENERGY LEVELS AND EQUILIBRIUM GEOMETRY OF HCP

The ground state potential energy surface for HCP has been investigated theoretically. A large fraction of electron correlation is included by multireference internally contracted configuration interaction from CASSCF reference wave functions using large orbital expansions. The origin of the potential is then shifted and the force constants scaled to reproduce all spectroscopic data available for the four isotopically substituted species. Variational calculations of vibrational and rotational frequencies for transitions up to J = 7 ← 6 have finally been performed, with accuracy which is typically ± 5 cm−1 for vibrations and ± 10 MHz for most rotations. By comparison with the results of the perturbation treatment the importance of the ν1:2ν3 Fermi interaction for vibrational frequencies and effective rotational constants has been determined. From computed and experimental ground state rotational constants, the molecular equilibrium geometry has also been estimated.

Rotational spectrum of 13C17O and 13C18O: completely resolved nuclear hyperfine structures due to 13C and 17O

Abstract The pure rotational spectrum of 13 C 17 O has been observed in the millimeter- and submillimeter-wave region using the Lamb-dip technique in order to increase the instrumental resolving power and so to resolve both the hyperfine structure due to 17 O and that due to 13 C . In this way very accurate values of the rotational and centrifugal distortion constants, the 17 O nuclear electric quadrupole coupling constant and the 17 O and 13 C spin–rotation constants have been obtained. In addition, the J =1←0 and J =2←1 transitions of 13 C 18 O have been recorded using the Lamb-dip technique. The resolved hyperfine structure of these transitions allowed us to determine the rotational and 13 C spin–rotation constants. The experimental determination of all these parameters has been supported by ab initio computations.

Millimeter- and submillimeter-wave spectrum of C17O. Rotational hyperfine structure analyzed using the Lamb-dip technique

The pure rotational spectrum of C17O has been observed in the millimeter- and submillimeter-wave region using the Lamb-dip technique in order to increase the instrumental resolving power and so to analyze the hyperfine structure of the rotational transitions. The accuracy of the measurements have been evaluated to lie within ±1 kHz. Under these conditions, it has been possible to obtain very accurate values of the B0 and D0 spectroscopic constants and of the 17O nuclear electric quadrupole coupling and spin–rotation constants.

The potential energy and dipole moment surfaces of HOBr

Abstract A theoretical study of the spectroscopy of HOBr and its deuterated isotopomer is presented. Highly accurate ab initio potential energy and dipole moment surfaces have been determined at the multireference configuration interaction level of theory, with large triple-ζ quality basis sets. From the analytic expression of the PES, a quartic anharmonic force field is derived, which, after a little empirical adjustment on the harmonic part, is used to evaluate spectroscopic parameters by means of standard perturbative formulae. Comparisons with experiment and previous computations are made.

Lamb-dip millimeter-wave spectrum, structure and dipole moment of HCCCCF

The ground state rotational spectrum of HCCCCF has been observed in the millimeter-wave region using the Lamb-dip technique and very accurate values of the spectroscopic constants have been obtained. Moreover, the observation of the ground state rotational spectra of the four 13C monosubstituted isotopomers and of the deuterated species of fluorodiacetylene have enabled the determination of the r0 and rs molecular structures. The electric dipole moment of HCCCCF has been determined by observing the Stark spectrum of the J=3�2 rotational transition. The equilibrium geometry and the dipole moment have been evaluated using different abinitio methods and comparison between experimental and theoretical results has been made.

The anharmonic force field of cis-1-chloro-2-fluoroethylene

A comprehensive anharmonic vibrational analysis of cis-1-chloro-2-fluoroethylene and its isotopomers has been performed on the basis of a complete ab initio quartic force field constructed by means of second-order Møller-Plesset perturbation theory (MP2) and the coupled-cluster singles and doubles approach, augmented for structural optimization and harmonic force field by a contribution of connected triple excitations (CCSD(T)). The theoretical force field was scaled by global least-squares fitting to all spectroscopic data and parameters experimentally determined for this molecule. This final force field, employing standard perturbation theory, yields a complete set of spectroscopic molecular constants providing a critical assessment of experimental rotational and centrifugal distortion constants, fundamentals, overtones, and combination bands determined over many years. Effects of Fermi and Darling-Dennison resonances were included by matrix diagonalization.

Theoretical molecular structure and experimental dipole moment of cis-1-chloro-2-fluoroethylene

The equilibrium geometry of cis-1-chloro-2-fluoroethylene has been evaluated using two different ab initio n methods: the coupled-cluster (CC) approach and Moller–Plesset perturbation theory. Accurate predictions have been obtained. Using both methods, the dipole moment has been estimated numerically as energy derivative with respect to an applied electric field at zero field strength. The experimental dipole moment of n cis-1-chloro-2-fluoroethylene has been determined by observing the Stark spectrum of the J=40,4←31,3 and J=41,3←40,4 n transitions. The spectrum profile has been fitted to a model function computed as n a sum of Lorentzian profiles over the hyperfine-Stark components, whose frequencies have been derived by diagonalizing the full rotational–quadrupole-Stark Hamiltonian matrix for each value of the applied electric field. Very good n agreement between experimental and theoretical dipole moment has been obtained.

Accurate structure and torsional barrier height of disilane

Very accurate structural parameters and torsional barrier heights of disilane have been determined using the CCSD(T) method with correlation consistent basis sets. Not only has the basis set incompleteness been taken into account using the appropriate extrapolation schemes, but also the core-correlation corrections have been considered.

The anharmonic force field of chlorofluoromethane

Abstract The quartic anharmonic force field of chlorofluoromethane (CH2FCl) has been investigated theoretically using second-order Moller-Plesset and Hartree-Fock self consistent field methods. Following a procedure recently described, the theoretical force field is scaled by a global least squares fitting to all spectroscopic data and parameters experimentally determined for this molecule. The final force field is used to make predictions of all important vibrational and rotational parameters, which are believed to be accurate and useful for new high resolution spectroscopic infrared and microwave investigations on this molecule.