Ivan Černušák

Ring currents in six-membered heterocycles: the diazaborinines (CH)2B2N2

Six of the eleven possible cyclic isomers (CH)2B2N2 are found, by geometric optimization at a correlated level of theory, to have planar closed-shell ground states. Current density maps calculated with distributed-origin coupled Hartree–Fock theory wavefunctions show that all six of these have delocalized π-electron systems supporting diamagnetic ring currents. In comparison with benzene at the same level of theory, the isomer with lowest energy, 1,3,2,4 diazadiborinine, has a 5% greater centre of ring shielding, but 20–25% smaller mean magnetizability, mean polarizability and polarizability anisotropy.

Triple and quadruple excitation contributions to the binding in Be clusters calibration calculations on Be3

We have investigated the contribution of connected triple and quadruple excitations to the binding in Be3 by comparing various coupled‐cluster (CC) and truncated configuration‐interaction (CI) treatments with multireference CI (MRCI) and full CI (FCI) calculations. The CC method with single and double excitations (CCSD) produces results that differ substantially from more elaborate treatments, but most extensions to CCSD that account approximately for connected triple excitations perform very well. In contrast, good agreement with FCI for Be2 can be achieved only with the highest level CC and MRCI methods.

Benchmark calculations of some molecular properties of O2, CN and other selected small radicals using the ROHF-CCSD(T) method

The capability of the ROHF-CCSD(T) method in obtaining accurate molecular properties in a defined and controlled way is analysed. Electron affinity, polarizability, and hyperpolarizability of the oxygen molecule in its ground state, electron affinity, electric dipole moment of the CN radical, and some other molecules serve as model cases for obtaining the ‘right result for the right reason’. Most calculated CCSD(T) data were extrapolated to the complete basis set (CBS) limit in order to minimize the basis set dependence of results. Some problems, specific to open shell systems include effects due to the spin adaptation, and details in the selection of the reference orbitals and related selection of denominators in non-iterative triples and other subtleties, which can affect the accuracy of the final ROHF-CCSD(T) results, are investigated.

A coupled-cluster study of the photoelectron spectra of C−4

Abstract Many-body perturbation theory and coupled-cluster theory have been used to compute the vertical ionization potentials of linear and rhombic C − 4 and the vertical electron affinities of linear and rhombic C 4 . The ionization potentials of linear C −4 4 give a good account of the observed photoelectron spectrum.

Activation barriers of SN2 reactions: F− + CH3F and H− + CH3F. Fourth-order MB RSPT calculations

Abstract Activation barriers of the title reactions are calculated using a DZ + P basis set with diffuse p functions on negative ions. In the correlation effects, which are large and negative, the importance of single and triple excitations is stressed. The barrier for the reaction of H − with CH 3 F is 12 kJ/mole, in good agreement with experiment, while the barrier for the reaction of F − with CH 3 F is predicted to be 14 kJ/mole.

On the way to the highest coordination number in the planar metal-centred aromatic Ta©B10− cluster: Evolution of the structures of TaBn− (n = 3–8)

The structures and chemical bonding of TaB(n)(-) (n = 3-8) clusters are investigated systematically to elucidate the formation of the planar metal-centred aromatic borometallic cluster, Ta©B10(-) (the

Toward More Efficient CCSD(T) Calculations of Intermolecular Interactions in Model Hydrogen-Bonded and Stacked Dimers

Interaction energies of the model H-bonded complexes, the formamide and formamidine dimers, as well as the stacked formaldehyde and ethylene dimers are calculated by the coupled cluster CCSD(T) method. These systems serve as a model for H-bonded and stacking interactions, typical in molecules participating in biological systems. We use the optimized virtual orbital space (OVOS) technique, by which the dimension of the space of virtual orbitals in coupled cluster CCSD(T) calculations can be significantly reduced. We demonstrate that when the space of virtual orbitals is reduced to 50% of the full space, which means reducing computational demands by 1 order of magnitude, the interaction energies for both H-bonded and stacked dimers are affected by no more than 0.1 kcal/mol. This error is much smaller than the error when interaction energies are calculated using limited basis sets.

Basis set and electron correlation effects on static electric properties of 1,3-thiazoles and 1,3-benzothiazoles as potential fragments in push-pull NLO chromophores

Dipole moments μ, static averaged polarizabilities ⟨αrang;, and hyperpolarizabilities βvec of thiazole, benzothiazole, and their dipolar substituted derivatives have been computed by means of B3LYP and MP2 theories with the Pol basis set. Basis set effects have been considered for thiazole, benzothiazole, and doubly substituted thiazole and benzothiazole derivatives at the B3LYP level of theory and in the case of thiazole also at the HF and MP2 levels. Attention has been paid to thiazole geometry effects on the electric properties. The effectiveness of thiazole and benzothiazole as a conjugative pathway between electron donor NH2 and electron acceptor NO2 substituents has been compared with benzene. With one exception, differences in geometry appear to influence only modestly the calculated electric properties. The dipole moment- and polarizability-oriented Sadlej Pol basis set and also its reduced form ZPol basis sets are able to reproduce the aug-cc-pVTZ generated μ, ⟨αrang;, and also βvec at all levels of theory (HF, B3LYP, and MP2) very satisfactorily. All approaches yield almost identical basis set trends. A very good agreement in ⟨αrang; values between the B3LYP and MP2 approach has been found. While the doubly substituted thiazole derivatives possess smaller βvec calculated at the B3LYP level than at the MP2 level, for doubly substituted benzothiazole derivatives the B3LYP estimates of βvec exceed the MP2 estimates. The trends established by the B3LYP functional parallel the trends obtained with the MP2 method. The 2-nitrobenzothiazole-6-amine and 6-nitrobenzothiazole-2-amine exhibit the largest electric properties from the investigated set of molecules. Thiazole (benzothiazole) as a bridging unit between NH2 and NO2 shows slightly reduced (enhanced) NLO characteristic than pNA. Thiazole behaves as a dipolar bridge rather than just an electron acceptor substituent in singly substituted derivatives. Concerning the charge transfer process, NO2 group interacts better with thiazole than NH2, which can result from the tendency to pyramidization of the NH2 group.

Structural changes in the series of boron-carbon mixed clusters CxB10-x− (x = 3-10) upon substitution of boron by carbon

We report a theoretical investigation on the ten-atom boron-carbon mixed clusters C(x)B(10-x)(-) (x = 3-10), revealing a molecular wheel to monocyclic ring and linear species structural change as a function of x upon increasing the number of carbon atoms in the studied series. The unbiased searches for the global minimum structures of the clusters with x ranging from 3 to 9 were conducted using the Coalescence Kick program for different spin multiplicities. Subsequent geometry optimizations with follow-up frequency calculations at the hybrid density functional B3LYP∕6-311+G(d) level of theory along with the single point coupled-cluster calculations (UCCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(d) and RCCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(d)) revealed that the C3B7(-) and C4B6(-) clusters possess planar distorted wheel-type structures with a single inner boron atom, similar to the recently reported CB9(-) and C2B8(-). Going from C5B5(-) to C9B(-) inclusive, monocyclic and ring-like structures are observed as the most stable ones on the PES. The first linear species in the presented series is found for the C10(-) cluster, which is almost isoenergetic with the one possessing a monocyclic geometry. The classical 2c-2e σ bonds are responsible for the peripheral bonding in both carbon- and boron-rich clusters, whereas multicenter σ bonding (nc-2e bonds with n > 2) on the inner fragments in boron-rich clusters is found to be the effective tool to describe their chemical bonding nature. It was shown that the structural transitions in the C(x)B(10-x)(-) series occur in part due to the preference of carbon to form localized bonds, which are found on the periphery of the clusters. Chemical bonding picture of C10(-) is explained on the basis of the geometrical structures of the C10 and C10(2-) clusters and their chemical bonding analyses.

Ring currents and magnetic properties of the cyclopropenyl cation and isoelectronic triangular 2π electron systems

A coupled Hartree-Fock method is used to compute the magnetizability tensors and carbon and hydrogen shielding tensors of the isoelectronic series of cyclic molecules consisting of the cations C3 H3 + (cyclopropenyl), C2 H2 N+ (azirinyl), CHN2 + (diazirinyl) and N3 + (triazirinyl) and the neutral molecules C2 H3 B (borirene), CH2 NB (borazirene), N2 BH (boradiazirine), and NB2 H3 (azadiboridine). A distributed-origin method is also used to map the electron current densities induced in these by an external magnetic field. Comparisons are made with the benzene molecule. The computations show that whereas the anisotropy of the magnetizability of benzene is dominated by the contribution of the π current, that in the cyclopropenyl cation is due almost entirely to the valence σ current. The current density maps of the two molecules also differ sharply. The maps for benzene show a localized circulation of σ charge in each of the bonds, and a simple circulation of π charge around the principal axis with maximum d...