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Sarka J.,Eotvos Lorand University | Sarka J.,MTA ELTE Complex Chemical Systems Research Group | Fabri C.,ETH Zurich | Szidarovszky T.,MTA ELTE Complex Chemical Systems Research Group | And 5 more authors.
Molecular Physics | Year: 2015

One-dimensional (1D) and two-dimensional (2D) models are investigated, which help to understand the unusual rovibrational energy-level structure of the astronomically relevant and chemically interesting astructural molecular ion H+5. Due to the very low hindering barrier characterising the 1D torsion-only vibrational model of H+5, this model yields strongly divergent energy levels. The results obtained using a realistic model for the torsion potential, including the computed (near) degeneracies, can be rationalised in terms of the model with no barrier. Coupling of the torsional motion with a single rotational degree of freedom is also investigated in detail. It is shown how the embedding-dependent rovibrational models yield energy levels that can be rationalised via the 2D vibrational model containing two independent torsions. Insight into the complex rovibrational energy level structure of the models and of H+5 is gained via variational nuclear motion and diffusion Monte Carlo computations and by the analysis of the wavefunctions they provide. The modelling results describing the transition from the zero barrier limit to the large barrier limit should prove to be useful for the important class of molecules and molecular ions that contain two weakly coupled internal rotors. © 2015 Taylor & Francis.

Sarka J.,Eotvos Lorand University | Sarka J.,MTA ELTE Complex Chemical Systems Research Group | Csaszar A.G.,Eotvos Lorand University | Csaszar A.G.,MTA ELTE Complex Chemical Systems Research Group
Journal of Chemical Physics | Year: 2016

Variational nuclear motion computations, employing an exact kinetic energy operator and two different potential energy surfaces, are performed to study the first 60 vibrational states of the molecular ion H5 + ≡ [H2-H-H2]+ and all of its deuterated isotopologues and isotopomers, altogether 12 species. Detailed investigation of the vibrational wavefunctions mostly results in physically intuitive labels not only for the fundamentals but also for the overtone and combination states computed. The torsional motion associated with the left and right diatomics appears to be well separated from the other vibrational degrees of freedom for all species. The unusual structure of the higher-lying bending states and the heavy mixing of the internal motions is partly due to the astructural character of all these molecular ions. The existence of distinct isotopomers in the H5-nD5 +, n = 1-4 cases, in the energy range studied, is confirmed. Two rules determine the stability order of the isotopomers: first, when possible, H prefers to stay in the middle of the ions rather than at the sides, and, second, the isotopomer with a homonuclear diatomic at the side is always lower in energy. The large number of precise vibrational energies of the present study, as well as the detailed assignment of the states, should serve as benchmarks for future studies by more approximate nuclear-motion treatments, such as diffusion Monte Carlo and multiconfiguration time-dependent Hartree. © 2016 Author(s).

Csaszar A.G.,MTA ELTE Complex Chemical Systems Research Group | Csaszar A.G.,Eotvos Lorand University | Furtenbacher T.,MTA ELTE Complex Chemical Systems Research Group | Furtenbacher T.,Eotvos Lorand University
Journal of Physical Chemistry A | Year: 2015

An additive, linear, atom-type-based (ATB) scheme is developed allowing no-cost estimation of zero-point vibrational energies (ZPVE) of neutral, closed-shell molecules in their ground electronic states. The atom types employed correspond to those defined within the MM2 molecular mechanics force field approach. The reference training set of 156 molecules cover chained and branched alkanes, alkenes, cycloalkanes and cycloalkenes, alkynes, alcohols, aldehydes, carboxylic acids, amines, amides, ethers, esters, ketones, benzene derivatives, heterocycles, nucleobases, all the natural amino acids, some dipeptides and sugars, as well as further simple molecules and ones containing several structural units, including several vitamins. A weighted linear least-squares fit of atom-type-based ZPVE increments results in recommended values for the following atoms, with the number of atom types defined in parentheses: H(8), D(1), B(1), C(6), N(7), O(3), F(1), Si(1), P(2), S(3), and Cl(1). The average accuracy of the ATB ZPVEs is considerably better than 1 kcal mol-1, that is, better than chemical accuracy. The proposed ATB scheme could be extended to many more atoms and atom types, following a careful validation procedure; deviation from the MM2 atom types seems to be necessary, especially for third-row elements. © 2015 American Chemical Society.

Arendas P.,MTA ELTE Complex Chemical Systems Research Group | Arendas P.,Eotvos Lorand University | Furtenbacher T.,MTA ELTE Complex Chemical Systems Research Group | Furtenbacher T.,Eotvos Lorand University | And 2 more authors.
Journal of Mathematical Chemistry | Year: 2016

Spectroscopic networks (SNs), where the vertices are discrete, rovibronic energy levels and the edges are transitions among the levels allowed by quantum mechanics, serve as useful models helping to understand high-resolution spectra of molecular systems. The experimental SNs of the (Formula presented.), and (Formula presented.) molecules, containing a single copy of the known measured and assigned transitions, are investigated via the corresponding network representation matrices, including the Ritz matrix (Formula presented.), the adjacency matrix (Formula presented.), the combinatorial Laplacian matrix (Formula presented.), and the normalized Laplacian matrix (Formula presented.). Using elements of graph (network) theory and the eigenvalue spectra of the matrices mentioned, several interesting results relevant for high-resolution molecular spectroscopy are revealed about the structure of the investigated SNs. For example, as long as the parity selection rule of molecular rovibrational transitions is not violated, the experimental SNs investigated not only contain, with the exception of (Formula presented.), two principal components but they are all bipartite networks, as proven by the symmetry of the eigenvalue spectrum of (Formula presented.) about the origin. Furthermore, the PageRank ordering system is introduced to molecular spectroscopy to identify the most important vertices of SNs. The rankings provided by the degree of the levels and by PageRank may differ significantly; it appears that PageRank provides the more useful ranking. The connectors of relatively dense clusters of the SNs are identified and analysed via spectral clustering techniques based on (Formula presented.) and (Formula presented.). The identification of connectors becomes especially important when judging the true accuracy of the experimental rovibrational energy levels obtained through the Measured Active Rotational-Vibrational Energy Levels (MARVEL) approach, built with the help of the Ritz matrix (Formula presented.). © 2016, Springer International Publishing Switzerland.

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