Showing papers by "Cristina Puzzarini published in 1996"

Accurate ab initio prediction of the equilibrium geometry of HCO+ and of rovibration energy levels of DCO+

Abstract: The ground state potential energy surface for HCO+ has been investigated theoretically. A large fraction of the electron correlation is included by multi reference internally contracted configuration interaction from CASSCF reference wavefunctions using large orbital expansions. The origin of the potential is then shifted and the force constants scaled to reproduce all spectroscopic data available for five isotopically substituted species. The estimated equilibrium geometry and force constants agree closely with the results of previous computations. Variational calculations of vibrational and rotational frequencies for transitions up to J = 4 ⇃ 3 have been performed, with accuracy which is typically ± 2 cm-1 for vibrations and ± 20 MHz for most rotations. From computed and experimentally available ground state rotational and l-doubling constants estimates have been made of the equilibrium geometry (r e(C-H) = 0·0919 ± 0·0009 A and R e (C-O) = 1·1055 ± 0·0003 A) and of the most important anharmonic interac...

Rovibrational energy levels and equilibrium geometry of HCP

Abstract: 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.

Millimetre-wave and infrared spectroscopy of Br13CN: anharmonic force field of cyanogen bromide from spectroscopic data and ab initio calculations

Abstract: The rotational spectra of 79Br13C14N and 81Br13C14N in the low-lying 0110, 1000, 0200 and 0220 vibrational states were observed in the millimetre-wave region, and the ν3 fundamental band of the 13C containing species was recorded from 2110 to 2170 cm−1 with a diode laser spectrometer. The anharmonic force field of cyanogen bromide was calculated using the data obtained together with those already known for the normal and 15N isotopomers. The experimental determination of the force field was supported by an ab initio calculation of the intramolecular potential energy surface, which was obtained by the CASSCF method. A further refinement of the equilibrium structure was made using the equilibrium rotational constants of six different isotopomers.

Isomerism and spin–vibronic energy levels of silicon nitroxide

Abstract: The ground-state potential-energy surface of silicon nitroxide has been investigated theoretically by the internally contracted multi reference configuration interaction (MRCI) method from complete active space self consistent field (CASSCF) reference wavefunctions using large orbital expansions. Three different linear isomers have been identified on the ground electronic state potential-energy surface, corresponding to the chemical structures SiNO, NSiO and SiON. Their geometries and relative energies have been estimated. Variational theory has been used to calculate the vibronic levels for values of the total angular momentum quantum number J=½, , including dynamic non-adiabatic interactions between electronic spin, orbital and vibrational angular momenta. The predicted vibronic frequencies are expected to be accurate for all three isomers.

Gas‐Phase IR Spectrum of 1‐Azaindolizine: Scaled Quantum Mechanical Force Field and Spectrum Assignment.

Abstract: The gas-phase IR spectrum of 1-azaindolizine has been recorded in the range from 100 to 3200 cm–1, at the resolution of 0.05 cm–1, using a multipass cell heated to 80 °C. The spectrum analysis has been performed by comparing the experimental spectrum with the theoretical one, using both frequency and intensity matching as guidelines for the assignments. The theoretical spectrum of 1-azaindolizine has been calculated from the harmonic force field evaluated at the self consistent field (SCF) level followed by empirical scaling according to the scaled quantum mechanical (SQM) approach, with pyridine and imidazole as parent molecules. The final root-mean-square deviation (RMSD) of the computed and experimental frequencies is 18 cm–1.