Petros Valtazanos

Stability and physicochemical reactions of light dications

The authors present theoretical results on the stability of Be22+, BeH2+, BH2+, BeHe2+ and BeLi+. The wavefunctions and potential energy curves have been obtained with large basis sets to account properly for the ionic structures and the correct shape of the curves. The ground-state potential curve of the first three molecules is dissociative with a local minimum. However, the previously predicted BH2+ does not seem to exist since its potential is found not to support any vibrational levels. BeHe2+ and BeLi+ are thermodynamically stable. The tunnelling widths of the BeH2+ and Be22+ lowest rovibrational resonances are negligible at room temperatures. Using the stable light dications as reactants, highly exothermic physicochemical reactions are possible.

Local minima of ground hypersurfaces. The cases of He2+3 and of the BeH2+2 nonclassical dihydrogen complex

Abstract We report the existence of special features in the bonding of the molecules He 2+ 3 and BeH 2+ 2 . Their hypersurfaces have only local minima at 0.4 and 5.2 eV below the energies of (H 2+ 2 1 Σ + g + He) and (BeH 2+ 2 Σ + + H), respectively. In C 2v , symmetry the molecules are bound. Vibrational analysis reveals that symmetry breaking turns the He 2+ 3 minimum into a saddle point, and thus the molecule fragments into 2He + + He. By contrast, the BeH 2+ 2 minimum is shown to hold up under symmetry breaking. Being the ground state, it can trap energy whose only escape is via multidimensional tunneling. The bonding characteristics of BeH 2+ 2 depict a structure of the type Be “2+” …H 2 . The formation of this “dihydrogen complex” is due to the vacant metal-ion orbitals, something which is verified by additional computations on BeH + 2 . These results are compared with analogous bonding situations in transition-metal compounds.

Hydrogen complexes of Be2+2

Abstract We predict the existence of hydrogen complexes of Be 2+ 2 whose energy, geometry and vibrational frequencies are determined from CI and MCSCF calculations. [Be 2 H 4 ] 2+ and [Be 2 H 8 ] 2+ were investigated. Upon reaction, neither the metalmetal nor the H 2 bond breaks. Instead, addition of H 2 molecules stabilizes Be 2+ 2 which, when alone, exists in a local minimum. Their formation is due to induced σ-bonding coupled with π back-bonding to the σ* H 2 orbital. These findings suggest possibilities for chemical synthesis of complexes beyond those which are currently pursued in transition-metal chemistry and are of the form L n M(H 2 ) (L = ligand, M = transition-metal atom).

Chemical trapping of molecular hydrogen by BeO

Abstract It is shown via CI and MCSCF calculations that OBeH 2 binds H 2 in molecular form. The equilibrium geometry corresponds to a local minimum on the repulsive ground hypersurface. The choice of this compound was made following our recent prediction of the existence of H 2 complexes in ionized molecules such as Be 2+ … (H 2 ) or Be 2+ 2 … (H 2 ) 2 . Analogous calculations for OLiH 2 and OBH 2 did not yield bound geometries, although the corresponding ions LiH + 2 and BH 2+ 2 are hydrogen complexes. The dissociation energy — computed between zero-point energies — for the release of H 2 by OBeH 2 is 1.0 eV. The present results suggest the possibility of reversible storage of H 2 by a simple ionic solid such as BeO.

Excited molecules and clusters in solid media. Hydrogen and tetrahydrogen in ionic crystals

Abstract We present accurate results from full CI calculations on ground and excited states of H 2 and (H 2 ) 2 embedded in AgF and RbI solids. It is found that the effect of these crystals on the spectra and on the energy surface characteristics is considerable. This finding suggests that, with a suitable selection of solid media, it may become possible to manipulate substantially the electronic spectroscopy and the energy storage and dissipation of certain classes of molecules and clusters.

The OBeH2 hypersurface: Local and global minima, transition states, and reaction paths

The lowest singlet potential energy surfaces of OBeH2 and its isomers are calculated, using geometry optimization techniques at the Fermi–Sea‐multiconfiguration‐self‐consistent‐field level. The reaction paths going from one isomer to the others are mapped and the appropriate transition states are located and verified. It is shown that, although the transition from BeO+H2 to OBeH2 is virtually barrierless, it can be achieved by an approach of molecular hydrogen to BeO from the Be side at right angles only. It is also shown that, should OBeH2 be formed, the transition to other, energetically lower points on the potential energy surface, involves twisting of the H2 moiety, thus making subsequent formation of linear H–O–Be–H the only option. The energy release upon this isomerization is 71 kcal/mol, while the activation barrier is 10 kcal/mol.

The (H2O)2* cluster at a geometry of intramolecular charge transfer

Abstract We have computed the potential energy surfaces (PES) of a chemically bound excited state of the (H 2 O) 2 ∗ cluster (10.0 eV above the energy of two H 2 O molecules) and of the corresponding dissociative ground state at a geometry which was predicted by applying the maximum ionicity of excited state (MIES) theory of bonding. These PES confirm the existence of an avoided region which is caused by intramolecular charge transfer and is characteristic of the MIES structures. Three-dimensional PES plots show the overall repulsive nature of the lower state, which breaks into (H 2 O) + (H 2 O). Also, by taking a slice of the two surfaces along the fragmentation coordinate for , the two-dimensional MIES feature of an avoided crossing is brought out and connection is made with structure and PES characteristics of certain diatomic molecules where bonding due to charge transfer is recognized from the charged atomic dissociation products. The present results, together with our earlier results on clusters such as (H 2 ) n and XH 2 (X = He, Ne, Ar), suggests that the features associated with MIES structures should form part of our description of the electronic structure and intramolecular dynamics of a number of non-reactive closedshell species.

Inversion of Molecular-Spectra by Solid Environments

Abstract It is demonstrated via full-CI cluster calculations of H 2 in a NaF crystal that it is possible for spectra of molecules embedded substitutionally in ionic crystals to be inverted as compared to those of the free state. Such inversions imply, in principle, the possibility of switching mechanisms for large amounts of molecular energy as a function of crystal distortion.

Structure and Vibrational Analysis of Protonated Hydrogen-Peroxide

Abstract We have determined the geometry and vibrational spectrum of the protonated hydrogen peroxide cation by ab initio calculations at the SCF, FORS-MCSCF and second-order Moller-Plesset (MP2) levels. The methods used were tested by similar calculations on H 2 O 2 and comparison of these results with experimental values. It was found that both the FORS-MCSCF and MP2 methods gave very good (and similar) results while - predictably - the SCF results were poor. The H 3 O 2 + geometry turns out to be almost that which can be generated from H 2 O 2 by a reflection in one of the OOH planes.

Molecules with “Volcanic” Ground Hypersurfaces. Structure, Stability and Energetics.

There exist molecular species whose ground state repulsive potential energy surfaces (PES) contain a local chemical minimum which may be stable upon symmetry breaking, thus trapping energy in vibrational levels lying above the dissociated products. Because of their special form, we have named these ground PES “volcanic”. Volcanic ground PES owe their appearance to the geometry-dependent interaction of covalent-ionic structures and the concomitant (avoided) intersection between the repulsive ground and the attractive first excited singlet PES. This article contains our recent findings from MCSCF and CI calculations on the structure, stability and energetics of clusters and complexes which exhibit these features, such as (H20)2 *, He3 ++, BeH2 ++, BH2 ++, [Be2H8]++, OBeH2 and FBeH2. Except for FBeH2, the other H2 compounds constitute “nonclassical” hydrogen complexes, whereby the H2 bond is weakened but not broken.