Petr Nachtigall

Investigation of the benzene-dimer potential energy surface: DFT/CCSD(T) correction scheme.

A novel method, designated as the density functional theory/coupled-cluster with single and double and perturbative triple excitation [DFT/CCSD(T)] correction scheme, was developed for precise calculations of weakly interacting sp(2) hydrocarbon molecules and applied to the benzene dimer. The DFT/CCSD(T) interaction energies are in excellent agreement with the estimated CCSD(T)/complete basis set interaction energies. The tilted T-shaped structure having C(s) symmetry was determined to be a global minimum on the benzene-dimer potential energy surface (PES), approximately 0.1 kcal/mol more stable than the parallel-displaced structure. A fully optimized set of ten stationary points on the benzene-dimer PES is proposed for the evaluation of the reliability of methods for the description of weakly interacting systems.

Coordination of Cu+ and Cu2+ ions in ZSM-5 in the vicinity of two framework Al atoms

A combined quantum mechanics/interatomic potential function ntechnique has been used to examine Cu+/Al− nsites in the vicinity of H+Al− sites and nCu2+ in the vicinity of two framework Al atoms under dehydrated nconditions. The symmetrical coordination of Cu2+ ions to nfour framework oxygen atoms is the most stable site and Cu2+ nis hard to reduce at these sites. In agreement with EPR measurements a square-pyramidal ncoordination of Cu2+ at these sites is suggested. However, nthis symmetrical coordination is attained only for specific positions of nthe framework aluminium pair. Coordination changes upon reduction of divalent nto monovalent copper ions and upon excitation of Cu+ ions nin the first triplet excited state are also investigated. The coordination n of the copper ions does not significantly change during reduction in the ndehydrated state. Coordination changes of Cu+ upon excitation nin the triplet state and calculated transition energies for Cu+/Al− nsites in the vicinity of H+Al− sites are nidentical to those observed for isolated Cu+/Al− nsites.

The vibrational dynamics of carbon monoxide in a confined space—CO in zeolites

Based on theoretical calculations, and a survey of infrared spectra of CO adsorbed on different cation exchanged zeolites, a model is proposed to explain the influence of the zeolite framework on the vibrational behaviour of CO confined into small void spaces (zeolite channels and cavities). The concepts developed should help to understand a number of details relevant to both, precise interpretation of IR spectra and a better understanding of the vibrational dynamics of small molecules in a confined space.

RI-MP2 calculations with extended basis sets—a promising tool for study of H-bonded and stacked DNA base pairs

The interaction energies of 9-methyladenine···1-methylthymine H-bonded and 9-methyladenine···9-methylguanine stacked pairs were evaluated at the MP2 and resolution of the identity MP2 (RI-MP2) levels. The interaction energies are almost identical for both methods. The RI-MP2 method is about one order of magnitude faster than the exact MP2 method and, therefore, this method is well suited for the study of large models of biological importance. The basis set dependence of both base pairs was studied and reasonable relative energy characteristics were obtained only if at least the valence double-ζ-polarisation SVP basis set with modified (diffuse) polarisation functions on non-hydrogen and hydrogen atoms was used. However, reliable absolute stabilisation energies of both base pairs were only obtained if at least the aug-SVP basis set is applied. Stack n interaction energies and three- and four-body energy terms for selected base-pair steps of B-DNA were calculated for the first time using the RI-MP2 method and the aug-SVP basis set.

FTIR spectroscopic and computational studies on hydrogen adsorption on the zeolite Li–FER

The interaction, at a low temperature, between molecular hydrogen and the zeolite Li-FER was studied by means of variable temperature infrared spectroscopy and theoretical calculations using a periodic DFT model. The adsorbed dihydrogen molecule becomes infrared active, giving a characteristic IR absorption band (H-H stretching) at 4090 cm(-1). Three different Li(+) site types with respect to H(2) adsorption were found in the zeolite, two of which adsorb H(2). Calculations showed a similar interaction energy for these two sites, which was found to agree with the experimentally determined value of standard adsorption enthalpy of DeltaH(0) = -4.1 (+/-0.8) kJ mol(-1). The results are discussed in the broader context of previously reported data for H(2) adsorption on Na-FER and K-FER.

Coordination of alkali metal ions in ZSM-5: A combined quantum mechanics/interatomic potential function study

The siting and coordination of alkali metal ions (Li+, Na+, and K+) in zeolite ZSM-5 were studied by a combined quantum mechanics/interatomic potential function method, which employed the B3LYP density functional and core–shell model potentials. New interaction parameters for Li+ and K+ ions with the zeolite were parameterized based on ab initio data. Several different sites of extra-framework M+ ions in the ZSM-5 were found and classified. Relative stabilities of particular sites depend (i) on the location of the framework Al atom and (ii) on the size of the alkali metal ion. The Li+ ion preferably occupies the sites on top of the six-membered ring on the channel wall while K+ ion preferably binds on the channel intersection.

Calculations of the site specific stretching frequencies of CO adsorbed on Li+/ZSM-5

Interaction of the CO molecule with Li+ within ZSM-5 was investigated by means of the combined quantum mechanics/interaction potential function method. Both, C-on and O-on species were considered. The scaling method based on the linear correlation between CO bond length and stretching frequency has been applied to calculate CO frequencies in CO(OC)–Li+/ZSM-5 adsorption complexes. Three types of C-on adsorption complexes with different r(CO) bond lengths, ν(CO) frequencies, and CO binding energies were identified. The calculated IR spectra of CO adsorbed on the Li+/ZSM-5 system show three distinctive bands at about 2194 cm−1, 2187 cm−1 and 2183 cm−1 for C-on complexes and at about 2116 cm−1, 2114 cm−1 and 2104 cm−1 for O-on complexes, in excellent agreement with experimental data. Calculated adsorption energies and CO stretching frequencies were used for the simulation of the IR spectra at various CO coverages.

Carbon monoxide adsorption on low-silica zeolites-from single to dual and to multiple cation sites

Infrared spectra of CO adsorbed on the Al-rich Na-A zeolite were analysed by using a combined theoretical and experimental approach, showing that such spectra cannot be interpreted by assigning each IR band to CO interacting with a specific type of single cation site. This concept, which usually works well for high-silica zeolites, should not be uncritically extended to Al-rich zeolites that are crowded with cations in configurations which lead to preferential formation of CO adsorption complexes involving more than one cation site.

On the site-specificity of polycarbonyl complexes in Cu/zeolites: combined experimental and DFT study.

The preferred Cu(+) sites and formation of mono-, di-, and tricarbonyl complexes in the Cu-FER were investigated at the periodic density functional theory level and by means of FTIR spectroscopy. The site-specificity of adsorption enthalpies of CO on Cu-FER and of vibrational frequencies of polycarbonyl complexes were investigated for various Cu(+) sites in Cu-FER. Large changes in the Cu(+) interaction with the zeolite framework were observed upon the formation of carbonyl complexes. The dicarbonyl complexes formed on Cu(+) in the main channel or on the intersection of the main and perpendicular channels are stable and both, adsorption enthalpies and CO stretching frequencies are not site-specific. The fraction of Cu(+) ions in the FER cage, that cannot form dicarbonyl can be determined from IR spectra (about 7% for the Cu-FER with Si/Al = 27.5 investigated here). The tricarbonyl complexes can be formed at the Cu(+) ions located at the 8-member ring window at the intersection of main and perpendicular channel. The stability of tricarbonyl complexes is very low (DeltaH degrees (0 K)>or=-4 kJ mol(-1)).

The calculation of the vibrational states of SO2 in the C̃1B2 electronic state up to the SO(3Σ−)+O(3P) dissociation limit

Abstract In a previous paper [P. Nachtigall, J. Hrusak, O. Bludsky, S. Iwata, Chem. Phys. Lett. 303 (1999) 441], we reported an investigation of the stationary points along the dissociation path of C 1 B 2 SO 2 , carried out with high-level ab initio methods. Here we calculate the vibrational energy levels up to the SO( 3 Σ − )+O( 3 P ) dissociation limit using a scaled ab initio potential energy surface. The scaled potential energy surface is of near-spectroscopic accuracy below the dissociation limit and has a realistic behaviour along the dissociation path.