Andrea Miani

Vibrational energy levels for symmetric and asymmetric isotopomers of ammonia with an exact kinetic energy operator and new potential energy surfaces

A new vibrational Hamiltonian operator for ammonia is presented. The potential energy part is expressed in terms of symmetrized bond-angle valence coordinates and an inversion coordinate, which is a function of the bond angles. In the exact kinetic energy operator, the stretching part is instead given in terms of unsymmetrized bond displacement coordinates. Six-dimensional ammonia potential energy surfaces are calculated using high-level ab initio tools, the CCSD(T) method with aug-cc-pVQZ and aug-cc-pVTZ basis sets. The potential energy functions are constructed in two, two-dimensional steps. The surfaces are expressed as a Taylor-type series with respect to the doubly degenerate asymmetric stretching and bending coordinates. This representation is given along a two-dimensional surface of the totally symmetric stretching and inversion coordinates of ammonia. Vibrational energies are calculated variationally in a finite basis representation. Employing successive basis set contractions, it is possible to o...

Six-dimensional ab initio potential energy surfaces for H3O+ and NH3: Approaching the subwave number accuracy for the inversion splittings

New potential energy surfaces are calculated for the hydronium ion using high-order coupled cluster ab initio methods. Large basis sets are used especially for the inversion part of the full surface. Electronic energies obtained with different correlation consistent basis sets are extrapolated to the infinite basis set limit. Core-valence and first order relativistic effects are also included. The influence of these two contributions and basis set sizes on both the inversion barrier height and equilibrium geometry are investigated thoroughly. The same methods are also adopted for ammonia in order to further improve a recently published surface [J. Chem. Phys. 118, 6358 (2003)]. The vibrational eigenvalues are calculated variationally both for the symmetric and asymmetric isotopomers using exact six-dimensional kinetic energy operators and successive basis set contractions. With the new surfaces, the mean absolute deviations obtained for all experimentally observed inversion splittings for different isotop...

Can ortho–para transitions for water be observed?

The spectrum of water can be considered as the juxtaposition of the spectra of two molecules, with different total nuclear spin: ortho-H2O, and para-H2O. No transitions have ever been observed between the two different nuclear-spin isotopomers. The interconversion time is unknown and it is widely assumed that interconversion is forbidden without some other intervention. However, weak nuclear spin-rotation interaction occurs and can drive ortho to para transitions. Ab initio calculations show that the hyperfine nuclear spin-rotational coupling constants are about 30 kHz. These constants are used to explore the whole vibration-rotation spectrum with special emphasis on the coupling between nearby levels. Predictions are made for different spectral regions where the strongest transitions between ortho and para levels of water could be experimentally observed.

New inversion coordinate for ammonia: Application to a CCSD(T) bidimensional potential energy surface

A new inversion coordinate is defined for ammonia as a function of the valence angles. Its square is similar to the often used totally symmetric bending displacement coordinate for the pyramidal XY3–type molecules. We have used this in a two-dimensional calculation including the totally symmetric stretching and the inversion mode. A conventional symmetrized internal coordinate is employed for the symmetric stretch. A two-dimensional potential energy surface is calculated using the ab initio CCSD(T) method together with the aug-cc-pVTZ, cc-pVQZ, and aug-cc-pVQZ basis sets. The corresponding eigenvalues are calculated variationally using a Morse oscillator basis set for the stretch and a harmonic oscillator basis set for the inversion. A good agreement is obtained between the calculated and 22 experimental inversion levels, 9 of 14NH3 and the others involving 4 other isotopomers (14ND3, 15NH3, 15ND3, and 14NT3). With the aug-cc-pVTZ basis, a mean absolute error of 5.0 cm−1 is obtained whereas with the aug-c...

Anharmonic force field for methanol

An ab initio quartic anharmonic force field for methanol has been calculated at the equilibrium position using the CCSD(T) method for the structure and the harmonic potential energy surface, and the MP4(SDQ) method for the anharmonic part of the surface. A triple zeta basis set was employed with symmetrized curvilinear internal valence coordinates in all calculations. The internal coordinate force field constants have been transformed into force constants in the dimensionless normal coordinate representation for various isotopomers. Vibrational term values for CH3OH, CH3OD, CD3OH, and CD3OD have been obtained using second order perturbation theory. Particular care has been devoted to the inclusion of Fermi resonance interactions between different vibrational states. A good accuracy has been achieved in the calculation of the fundamentals for all the isotopomers, the mean absolute error being 5.8 cm−1.

Six-dimensional variational calculations for vibrational energy levels of ammonia and its isotopomers

Results of six-dimensional variational calculations of vibrational energy levels are presented for ammonia using a Hamiltonian expressed in curvilinear internal bond coordinates. A two-dimensional potential energy surface, which was introduced in a preliminary study on the inversion motion, is combined with a surface by Martin et al. Both surfaces are calculated at the aug-cc-pVTZ/CCSD(T) ab initio level. The exact kinetic energy operator is an enlargement of the one used in the previous two-dimensional calculations. Eigenvalues are computed variationally using successive basis set contractions for some symmetric and asymmetric isotopomers of ammonia.

VIBRATION-ROTATION FLUORESCENCE SPECTRA OF WATER IN THE GROUND ELECTRONIC STATE

Laser-induced fluorescence has been used to investigate overtone states of water. Specific rovibrational states have been excited by a high-resolution Titanium:sapphire ring laser in the near-infrared region. The resulting fluorescence light around 3000–4000 cm−1 has been dispersed by a Fourier transformation interferometer. Rich collision-induced spectra are obtained. The specific nuclear spin states chosen by the laser excitation are conserved in collisional processes. Interesting vibrational intensity effects have been observed. Vibrational calculations with an exact kinetic energy operator in the curvilinear internal coordinate representation have been performed to compute emission intensities.

CCSD(T) inversion spectrum for H3O

Mass independent bidimensional potential energy surfaces (including the inversion and the symmetric stretch) for H3O+ have been calculated using the CCSD(T) ab initio method with augmented triple and quadruple zeta basis sets. All surfaces have been fitted using high order Chebyshev polynomials, and the vibrational inversion spectra have been calculated using similar methods we recently introduced in the study of ammonia. A good agreement with the experimental inversion values: a mean absolute error of 11.9 cm−1 with the triple zeta basis set, and of 5.6 cm−1 with the quadruple zeta basis set was obtained.

Bidimensional vibrational Hamiltonian for asymmetric ammonia-like pyramidal molecules: application to the calculation of the inversion spectrum of NH2D and NHD2

Abstract A bidimensional vibrational Hamiltonian expressed in terms of internal coordinates has been developed to describe the symmetric stretch and the inversion of asymmetric pyramidal ammonia-like molecules. This work is an extension of the approach we have recently presented on NH 3 and on some of its C 3v isotopomers [J. Chem. Phys. 115 (2001) 1243]. A new set of internal coordinates, which reflects the symmetry of the problem, has been defined and used to obtain a new bidimensional kinetic energy operator. Inversion levels for NH 2 D and NHD 2 have been calculated variationally using an ab initio CCSD(T) bidimensional potential energy surface (PES). A good agreement with experimental values has been obtained.