Peter Botschwina

Ab initio calculation of near‐equilibrium potential and multipole moment surfaces and vibrational frequencies of H+3 and its isotopomers

H+3 potential energies and multipole moments are calculated from a full CI with a 10s, 4p, 2d GTO hydrogen basis. 69 calculated energy points with energies of up to 25 000 cm−1 above the minimum are fitted by a power series expansion in terms of a Morse‐type coordinate with a mean square error of less than 1 cm−1. Rotationless vibrational states with energies of up to 12 000 cm−1 above equilibrium are calculated variationally for ten isotopomers. The resulting band origins for the seven analyzed fundamental transitions show a mean deviation of less than 2 cm−1. For the other predicted frequencies, the errors are expected to be below 0.1% also. The equilibrium bond length of H+3 is predicted to be 0.8732(2) A.

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...

Collisions of excited Na atoms with H2 molecules. I. Ab initio potential energy surfaces and qualitative discussion of the quenching process

Potential energy surfaces have been calculated for the four lowest electronic states of Na (3 2S, 3 2P)+H2(1Σ+g) by means of the RHF–SCF and PNO–CEPA methods. For the so‐called quenching process of Na (3 2P) by H2 at low initial translational energies (E–VRT energy transfer) the energetically most favorable path occurs in C2v symmetry, since—at intermediate Na–H2 separation—the ? 2B2 potential energy surface is attractive. From the CEPA calculations, the crossing point of minimal energy between the ? 2A1 and ? 2B2 surfaces is obtained at Rc = 3.57 a.u. and rc = 2.17 a.u. with an energy difference to the asymptotic limit (R = ∞, r = re) of −0.06 eV. It is thus classically accessible without any initial translational energy, but at low initial translational energies (∼0.1 eV) quenching will be efficient only for arrangements of collision partners close to C2v symmetry. There is little indication of an avoiding crossing with an ionic intermediate correlating asymptotically with Na+ and H2− as was assumed in ...

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.

Infrared intensities of polyatomic molecules calculated from SCEP dipole-moment functions and anharmonic vibrational wavefunctions. I. Stretching vibrations of the linear molecules HCN, HCP and C2N2

Abstract Infrared intensities of transitions between a variety of stretching vibrational states of various isotopomers of HCN, HCP and C 2 N 2 have been calculated from SCF and SCEP dipole-moment functions and anharmonic vibrational wavefunctions. Best agreement with experiment, usually with deviations of the order of 10% for the fundamentals, is obtained by CEPA when the dipole moment is calculated as an energy derivative. The performance of the familiar “double-harmonic approximation” is investigated and found to work well in the absence of vibrational resonances and strongly curved dipole-moment functions.

Vibrational frequencies from anharmonic ab initio/empirical potential energy functions. III. Stretching vibrations of hydrogen cyanide and acetylenes

Abstract Stretching vibrational states are calculated for various isotopes of hydrogen cyanide, acetylene, fluoroacetylene, chloroacetylene and cyanoacetylene by diagonalizing an approximate vibrational hamiltonian which neglects the anharmonic coupling between stretching and bending motions. Empirically corrected ab initio potential energy functions are constructed from which the lower lying vibrational transition frequencies are obtained with an overall accuracy of about 20 cm −1 . The influence of electron correlation on different potential energy terms is investigated for hydrogen cyanide and acetylene by means of the coupled electron pair approximation (CEPA) within the framework of the self-consistent electron pairs theory (SCEP). Equilibrium geometries are estimated for the substituted acetylenes and quartic and sextic centrifugal distortion constants are calculated for all molecules under investigation.

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.

Structure of the CCCN and CCCCH radicals: Isotopic substitution and ab initio theory

The millimeter‐wave rotational spectra of the 13C isotopic species of the CCCCH and CCCN radicals and CCC15N were measured and the rotational, centrifugal distortion, and spin‐rotation constants determined, as previously done for the normal isotopic species [Gottlieb et al., Astrophys. J. 275, 916 (1983)]. Substitution (rs) structures were determined for both radicals. For CCCN, an equilibrium structure derived by converting the experimental rotational constants to equilibrium constants using vibration–rotation coupling constants calculated ab initio was compared with a large‐scale coupled cluster RCCSD(T) calculation. The calculated vibration–rotation coupling constants and vibrational frequencies should aid future investigations of vibrationally excited CCCN. Less extensive RCCSD(T) calculations are reported here for CCCCH. The equilibrium geometries, excitation energies (Te), and dipole moments of the A2Π excited electronic state in CCCN and CCCCH were also calculated. We estimate that Te=2400±50 cm−1 ...

An ab initio calculation of the frequencies and IR intensities of the stretching vibrations of HN2

The dependence of the potential energy and electric dipole moment of HN 2 + on the stretching coordinates has been calculated from highly correlated CEPA wavefunctions. The origin of the ν 3 band of H 14 N 2 + is predicted to lie at 2254 cm −1 . The ν 1 band of H 14 N 2 + is very strong, with S = 2682 atm −1 cm −2 at 298 K, while ν 3 has only a very small intensity. HN 2 + is predicted to be an excellent IR emitter.

Vibrational frequencies from anharmonic ab initio/empirical potential energy functions. I. Method and application to H2O, HNO, HOF and HOCl

Abstract Near-equilibrium anharmonic potential energy functions have been constructed for some non-linear triatomic molecules (H 2 O, HNO, HOF and HOCl) from SCF/gradient calculations and little experimental information (equilibrium geometries and fundamentals of the most abundant isotope). Vibrational band centers are calculated by the variational method of Whitehead and Handy. Good agreement (mostly better than 10 cm −1 ) is obtained for a variety of vibrational states of water isotopes. The H/D isotope shifts for the stretching vibrations involving hydrogen, which are strongly affected by anharmonicity, are calculated with an accuracy of 11, 6 and 6 cm −1 for HNO, HOF and HOCl, respectively. The anomalous H/D shift for ν 3 (mainly OF stretch) of HOF is well reproduced by the calculations and is found to be mainly due to a relatively large negative quartic stretch—stretch coupling constant. Several overtone and combination band centers are predicted for the four molecules.