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The fluoroborane (HBF) free radical has a large vibronic interaction which splits the orbitally degenerate 2Π state in the linear configuration into two separate electronic states, one strongly bent and one linear. The observed vibrational structure of the electronic transition between the Renner–Teller pair of states is very complex. As an aid to understanding the spectrum, the vibronic energy levels of the ground and first excited states have been calculated from high-level ab initio potential energy surfaces using a variational method. The vibrational frequencies and anharmonicities have been derived from these energy levels and the boron and hydrogen isotope shifts have been predicted. Although the ground state energy levels are for the most part well behaved, the excited state levels show substantial Renner–Teller mixing with nearby ground vibrational states. The calculations in the present work have been successfully used in the companion paper to make vibrational assignments of the laser-induced fluorescence spectra of HBF and DBF. [ABSTRACT FROM AUTHOR]

The global potential energy surfaces for both the ground (X1A′) and excited (Ã1A′′) electronic states of the HNO molecule have been constructed by three-dimensional cubic spline interpolation of more than 17 000 ab initio points, which have been calculated at the internal contracted multi-reference configuration interaction level with the Davidson correction using an augmented correlation-consistent polarized valence quadruple zeta basis set. The low-lying vibrational energy levels for the two electronic states of HNO have also been calculated on our potential energy surfaces including the diagonal Renner-Teller terms. The calculated results have shown a good agreement with the experimental vibrational frequencies of HNO and its isotopomers. [ABSTRACT FROM AUTHOR]

Through a variety of highly correlated methods combined with large basis sets we have studied the electronic structure of FeO, FeO+, and FeO-. In particular, we have constructed complete potential energy curves for 48, 24, and 4 states for the FeO, FeO+, and FeO- species, respectively, at the multireference level of theory. For all states examined we report energetics, common spectroscopic parameters, and dipole moments. Overall our results are in good agreement with experiment, but we have encountered as well interesting differences between experiment and theory deserving further investigation. [ABSTRACT FROM AUTHOR]

A new potential energy surface involving the antisymmetric Q3 normal mode of CO2 for the He–CO2 van der Waals complex is constructed at the coupled-cluster singles and doubles with noniterative inclusion of connected triple [CCSD(T)] level with augmented correlation-consistent quadruple-zeta (aug-cc-pVQZ) basis set plus bond functions. Two vibrationally adiabatic potentials with CO2 at both the ground and the first excited vibrational states are generated from the integration of the three-dimensional potential over the Q3 coordinate. The potential has a T-shaped global minimum and two equivalent linear local minima. The bound rovibrational energy levels are obtained using the radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm. The observed band origin shift of the complex (0.0946 cm-1) is successfully reproduced by our calculation (0.1034 cm-1). The infrared spectra of the complex are also predicted. The fundamental band is in excellent agreement with the experiment. Most of the transitions corresponding to the observed hot band [M. J. Weida et al., J. Chem. Phys. 101, 8351 (1994)] are assigned reasonably. [ABSTRACT FROM AUTHOR]

We report here the first observation of the D 1Δg state of the C3 radical, which provides the first comprehensively analyzed example of the dynamic Renner-Teller splitting in Δ symmetry. Two color double resonance spectroscopy via the à 1Πu state was employed to experimentally probe an extensive range of vibronic levels in this D 1Δg state, covering all three modes of vibration of C3. The analysis was supported by ab initio potential energy surface calculations on the C3 radical to outline the lowest eight singlet electronic states. Two methods were used to analyze the Renner-Teller effect. The first method is an empirical Hamiltonian based on normal modes, using harmonic oscillator functions as a basis, with Renner-Teller and other terms added as required, which allows conventional vibrational parameters to be determined. The second is a much larger program that uses the exact kinetic energy operator for a triatomic molecule to calculate vibronic energy levels directly from the Renner-Teller pair of potential energy surfaces. Both methods give a good fit to the experimental results, with only a small adjustment to the ab initio surfaces required for the latter. One of the overall conclusions is that the Renner-Teller effect is rather smaller in the D 1Δg state than in the à 1Πu state. [ABSTRACT FROM AUTHOR]

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Δ symmetry and three states of Π symmetry of the molecule within 15 000 cm-1. The lowest 2Δ state is identified as the ground state of NiI, and the lowest 2Π state is found at 2174.56 cm-1 above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] 2Π and [14.6] 2Δ are 3 2Π and 3 2Δ electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally. [ABSTRACT FROM AUTHOR]