Pavel Rosmus

PNO–CI and CEPA studies of electron correlation effects. III. Spectroscopic constants and dipole moment functions for the ground states of the first‐row and second‐row diatomic hydrides

A systematic investigation of the ground state potential curves and dipole moment functions has been performed for the diatomic hydrides LiH to HCl on the basis of variational configuration interaction wavefunctions PNO–CI and the coupled electron pair approximation CEPA. The basis sets of Gaussian‐type orbitals are derived by simple rules from optimized atomic sets available in the literature. Between 95% (LiH) and 85% (HCl) of the valence shell correlation energies are accounted for in the CEPA calculations. Core–valence intershell correlation has been included for several members of each row, and its effect on the potential curves is analyzed. Comparison of the spectroscopic constants derived from the CEPA potential curves with experiment shows a high reliability of the theoretical values. The standard deviations over both rows are as follows: re:0.003 A, ωe:14 cm−1, αe:0.005 cm−1, and ωexe:1.5 cm−1. The errors of the calculated dissociation energies go up to 0.3 eV but behave very regularly. They are ...

Molecular properties from MCSCF‐SCEP wave functions. I. Accurate dipole moment functions of OH, OH−, and OH+

Potential energy and dipole moment functions for the ground states of OH, OH−, and OH+ have been calculated from MCSCF, MCSCF‐SCEP, and SCEP‐CEPA electronic wave functions. The stability of the dipole moments with respect to the number of configurations (up to 598) and orbitals (up to 14) simultaneously optimized in the MCSCF procedure and the number of reference configurations (up to 11) in the MCSCF‐SCEP wave functions (up to 69 830 configurations) has been investigated. The dipole moment functions obtained from the best electronic wave functions are more accurate than all previously calculated ones. The deficiencies of the former calculations have been critically analyzed. The OH− and OH+ ions are predicted to be stronger IR emitters than the neutral OH radical. The rotationless rates of spontaneous emission A10 for the fundamental transitions are calculated to be 12.2, 137, and 263 s−1 for OH, OH−, and OH+, respectively. The calculated dipole moments in the vibrational ground states are 1.65, 1.04, an...

Theoretical dipole moment functions of the HF, HCl, and HBr molecules

Potential energy and dipole momentfunctions of the HF, HCl, and HBr molecules in their electronic ground states have been calculated from highly correlated SCEP/CEPA a b i n i t i owave functions. Purely rotational transition energies are obtained with an accuracy of about 0.1 cm−1, and vibrational transition energies agree within 10–20 cm−1 with the experimental values. The SCEP/CEPA dipole moments in the vibrational ground states are calculated to be (experimental values in parenthesis) 1.807 D (1.826 D) for HF, 1.120 D (1.1085 D) for HCl and 0.829 D (0.828 D) for HBr. For HF various theoretical approaches, i.e., the SCEP/VAR (including variationally all singly and doubly excited configurations), SCEP/CEPA (accounting approximately for unlinked cluster effects), and MC‐SCF (with eight optimized valence configurations and with 66 configurations including atomic correlation) methods are compared. The spectroscopic constants and dipole momentfunctions calculated from SCEP/CEPA and MC‐SCF wave functions are of comparable accuracy. The SCEP/CEPA and MC‐SCF dipole momentfunctions of HF are in good agreement with the experimental function over a range of internuclear distances which covers approximately the nine lowest vibrational states. The theoretical potential and dipole momentfunctions have been used to calculate vibrational dipole matrix elements. The fully a b i n i t i o results of HF and HCl up to v=5 agree within about 5% with the values derived from experiments. For HBr strongly differing slopes of the dipole momentfunction have been reported in the literature. The present theoretical results are in good agreement with the most recent measurements and enable a reliable estimate of the absolute intensity for the 0–1 vibrational transition.

A variational method for the calculation of spin-rovibronic levels of Renner-Teller triatomic molecules

This paper reports an extension of our method, introduced several years ago, for the variational treatment of the rovibronic levels of Renner-Teller triatomic molecules. A more complete treatment now introduces the effect of electron spin. Thus new terms enter the Hamiltonian because (a) the total-angular-momentum operator is replaced by - Ľ - S in the kinetic-energy operator, and (b) spin-orbit effects must be introduced. Here we include the latter through a semi-empirical form AĽ · S . Expansion functions having the correct symmetry (Σg +, Σu +, Σg -, Σu -), for D∞h molecules are derived. Hence the present approach takes into account the full dimensionality, anharmonicity and rotation-vibration coupling, as well as the coupling of all relevant angular momenta. This is particularly important for open-shell molecules and/or for electronically degenerate states. For the 2Πg electronic ground state of the CO2 + ion the spin-rotation and spin-orbit split Renner-Teller levels have been calculated using three-...

The unimolecular dissociation of HCO: I. Oscillations of pure CO stretching resonance widths

The unimolecular dissociation of the formyl radical HCO in the electronic ground state is investigated using a completely new ab initio potential energy surface. The dynamics calculations are performed in the time‐independent picture by employing a variant of the log‐derivative Kohn variational principle. The full resonance spectrum up to energies more than 2 eV above the vibrational ground state is explored. The three fundamental frequencies (in cm−1) for the H–CO and CO stretches, and the bending mode are 2446 (2435), 1844 (1868), and 1081 (1087), where the numbers in parentheses are the measured values of Sappey and Crosley obtained from dispersed fluorescence excitation spectra [J. Chem. Phys. 93, 7601 (1990)]. In the present work we primarily emphasize the dissociation of the pure CO stretching resonances (0v20) and their decay mechanisms. The excitation energies, dissociation rates, and final vibrational–rotational state distributions of CO agree well with recent experimental data obtained from stim...

Dissociation of NH3 to NH2+H

Potential energy, dipole moment, and electronic transition moment surfaces for the lowest dissociative pathways of the singlet X and A states of NH3 yielding NH2 (X 2B1,A 2A1) +H(2S) products have been calculated using complete active space MCSCF ab initio wave functions. The A state dissociation proceeds via a minimum barrier at the following planar geometry: αHNH =113°, rNH =1.042 A (in the NH2 fragment), and RNH =1.323 A (in the dissociation coordinate). The barrier height is calculated to be 3226 cm−1 with an expected accuracy of about 300 cm−1. The barrier height increases with increasing out‐of‐plane angle. Close to the barrier there are strong variations of the shapes of the dipole moment and transition moment surfaces. The minimum energy path through the X–A conical intersection follows planar geometries. Along this pathway the angle αHNH decreases, but the distance rNH in the NH2 fragment hardly changes. The crossing distance RcNH of the X and A states in planar structures depends strong...

PNO‐CI and CEPA studies of electron correlation effects. VI. Electron affinities of the first‐row and second‐row diatomic hydrides and the spectroscopic constants of their negative ions

Near equilibrium potential curves for the ground states of the first‐row and second‐row diatomic hydride negative ions have been calculated using PNO‐CI and CEPA wavefunctions. The spectroscopic constants for the bound AH− ground states are given and the Franck–Condon factors for the processes AH−→AH are derived. The calculated intensity ratios support fully the assignments of the photoelectron spectra but the shifts in the re values which have been deduced from the observed intensity ratios are shown to have the wrong sign in most cases. The calculated electron affinities show systematic errors ranging from−0.15 eV to −0.35 eV. The yet unobserved BH− and and AlH− are predicted to be stable with estimated affinities of about 0.15 eV.

Theoretical A 1A‘2–X 1A1 absorption and emission spectrum of ammonia

Potential energy, electric dipole moment, and electronic transition moment surfaces have been calculated for the A and X states of NH3 from CASSCF and CEPA electronic wave functions. Anharmonic vibrational term values, Franck–Condon factors, and A–X radiative transition probabilities for the symmetric stretching and bending modes of NH3 and ND3 have been evaluated. The theoretical absorption spectra at room and low temperatures agree well with experimental data. The symmetric stretching mode in the A state has only small intensities in the A–X absorption spectrum. Emission rates from various initial vibronic levels of the A state are given. The ab initio electric dipole moment surfaces for the ground state of NH3 have been used to compute transition moments, which are in good agreement with experimental data.

THE UNIMOLECULAR DISSOCIATION OF HCO. II. COMPARISON OF CALCULATED RESONANCE ENERGIES AND WIDTHS WITH HIGH-RESOLUTION SPECTROSCOPIC DATA

We present a theoretical study of the unimolecular dissociation resonances of HCO in the electronic ground state, X1A′, using a new ab initio potential energy surface and a modification of the log‐derivative version of the Kohn variational principle for the dynamics calculations. Altogether we have analyzed about 120 resonances up to an energy of ≊2 eV above the H+CO threshold, corresponding to the eleventh overtone in the CO stretching mode (v2=11). The agreement of the resonance energies and widths with recent stimulated emission pumping measurements of Tobiason et al. [J. Chem. Phys. 103, 1448 (1995)] is pleasing. The root‐mean‐square deviation from the experimental energies is only 17 cm−1 over a range of about 20 000 cm−1 and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. The assignment of the states is discussed in terms of the resonance wave functions. In addition, we compare the quantum mechanical state‐resolved dissociation rates ...

The ultraviolet absorption spectrum of the à 1A‘2←X̃ 1A1 transition of jet-cooled ammonia

The A←X absorption spectra of NH3 and ND3, recorded in a cold molecular jet, are presented. Vibrational band progressions resolvable up to v’2=14 appear. No other vibrations are present, either alone or in combinations. Relative band intensities for v2 progressions are recorded, and the homogeneous lifetime broadenings of vibrational levels of the A state are reported. The FWHM linewidths span 34–293 cm−1 over all bands of NH3 and 30–135 cm−1 over the v’2=2 through 14 bands of ND3. In general, the rate of dissociation increases nonlinearly with vibrational energy. The band intensity alternation, previously observed only in matrix spectra below 15 K, has been observed in these very cold gas phase samples.