Transformation of potential energy surfaces for estimating isotopic shifts in anharmonic vibrational frequency calculations

Zero-Point-Energy Driven Isotopic Exchange of the [H3O]- anion Probed by Mid-Infrared Action Spectroscopy

We present the first observation of vibrational transitions in the [H3O]- anion, an intermediate in the anion-molecule reaction of water, H2O, and hydride, H-, using a laser-induced isotopic H/D exchange reaction action spectroscopy scheme applied to anions. The observed bands are assigned as the fundamental and first overtone of the H2O-H- vibrational stretching mode, based on anharmonic calculations within the vibrational perturbation theory and vibrational configuration interaction. Although the D2O·D- species has the lowest energy, our experiments confirm the D2O·H- isotope to be a sink of the H/D exchange reaction. Ab initio calculations corroborate that the formation of D2O·H- is favored, as the zero-point-energy difference is larger between D2 and H2 than between D2O·H- and D2O·D-.

Ab initio calculations on the X∼2B1 → X∼1A1 photoelectron spectrum of thioformaldehyde negative ion

The equilibrium structures, multidimensional potential energy surfaces, and anharmonic vibrational frequencies of thioformaldehyde (H2CS) and its 2B1 radical anion were obtained at the (U)CCSD(T)-F12a/cc-pVTZ-F12 level; and the photoelectron spectrum of H2CS-X∼2B1→ H2CS X∼1A1 was simulated by calculating the Franck-Condon factors (FCFs). The additional electrons cause a significant change in the bond length of the C = S bond of H2CS, which affects the potential energy surface and the C = S bond stretching mode, while the interaction between electrons turns the originally planar structure of H2CS into a slightly out-of-plane umbrella structure, resulting in an unusual change in the vibrational frequency of the out-of-plane bending mode. Finally, a time-independent eigenstate-free Raman wave function approach (RWF) was used to calculate the photoelectron spectrum of H2CS and to point out the changes in the band under the influence of different temperatures. These data provides a theoretical basis for interstellar observations and experimental studies.

The Molpro quantum chemistry package

Molpro is a general purpose quantum chemistry software package with a long development history. It was originally focused on accurate wavefunction calculations for small molecules but now has many additional distinctive capabilities that include, inter alia, local correlation approximations combined with explicit correlation, highly efficient implementations of single-reference correlation methods, robust and efficient multireference methods for large molecules, projection embedding, and anharmonic vibrational spectra. In addition to conventional input-file specification of calculations, Molpro calculations can now be specified and analyzed via a new graphical user interface and through a Python framework.

Refined analysis of the X̃ 2A2←X̃ 1A1 photoelectron spectrum of furan

The X̃ ²A₂←X̃ ¹A₁ photoelectron spectrum of furan has been studied by a time-independent eigenstate-free Raman wave function approach based on multi-dimensional potential energy surfaces obtained from explicitly correlated distinguishable clusters calculations. Individual vibronic transitions with the most significant Franck-Condon factors were determined by our recently developed residual-based algorithm for the calculation of eigenpairs in conjunction with the formalism of contracted invariant Krylov subspaces. The account of anharmonic and temperature effects allowed us to explain most bands in an experimental high-resolution zero kinetic energy photoelectron spectrum. This led to the reassignment of many spectral features, as well as a refined interpretation of the intensity mechanism for the corresponding transitions.

Eckart-Sayvetz conditions revisited

It is shown that vibrational displacements satisfying the Eckart-Sayvetz conditions can be constructed by projection of unconstrained displacements. This result has a number of interesting direct and indirect ramifications: (i) The normal coordinates corresponding to an electronic state or an isotopologue of a molecule are transformed to those of another state or isotopologue by a linear and, in general, non-orthogonal transformation. (ii) Novel interpretation of axis switching. (iii) One may enhance the separation of rotational-large-amplitude internal motions and the vibrational motions beyond that offered by the standard use of the Eckart-Sayvetz conditions. (iv) The rotational-vibrational Hamiltonian given in terms of curvilinear internal coordinates may be derived with elementary mathematical tools while taking into account the Eckart conditions with or without enhancement.