Imidazole and its saturated derivatives vs pyrazole and it saturated derivatives : a computational study on relative stability

Possible structures of imidazole, pyrazole, and their semi saturated and fully saturated derivatives have been studied at the DFT and ab initio computational levels. Calculations have been performed using several computational schemes (BLYP, PBE0, CAM-B3LYP, wB97XD, M06, MP2, CBS-QB3, and G4 methods have been employed) and the 6-311++G** basis set. The most stable structures for each group of studied compounds (tautomers of native imidazole and pyrazole, tautomers of their semi saturated derivatives, and fully saturated derivatives) have been determined. In general species with imidazole ring are more stable than those with pyrazole one. The discussion of this phenomenon origin, especially the influence of aromaticity as well as distributions of total charge and energy among atoms are thoroughly discussed

Imidazole and its Saturated Derivatives Vs Pyrazole and it Saturated Derivatives: A Computational Study on Relative Stability

Possible structures of imidazole, pyrazole, and their semi saturated and fully saturated derivatives have been studied at the DFT and ab initio computational levels. Calculations have been performed using several computational schemes (BLYP, PBE0, CAM-B3LYP, wB97XD, M06, MP2, CBS-QB3, and G4 methods have been employed) and the 6-311++G** basis set. The most stable structures for each group of studied compounds (tautomers of native imidazole and pyrazole, tautomers of their semi saturated derivatives, and fully saturated derivatives) have been determined. In general species with imidazole ring are more stable than those with pyrazole one. The discussion of this phenomenon origin, especially the influence of aromaticity as well as distributions of total charge and energy among atoms are thoroughly discussed

Avoiding pitfalls of a theoretical approach : the harmonic oscillator measure of aromaticity index from quantum chemistry calculations

The concept of the harmonic oscillator measure of aromaticity (HOMA) is based on comparing the geo-metrical parameters of a studied molecule with the param-eters for an ideal aromatic system derived from bond lengths of the reference molecules. Nowadays, HOMA is routinely computed combining the geometries from quantum chem-istry calculations with the experimentally based parameter-ization. Thus, obtained values of HOMA, however, are bound to suffer from inaccuracies of the theoretical methods and strongly depend on computational details. This could be avoided by obtaining both the input geometries and the parameters with the same theoretical method, but efficiency of the error compensation achieved in this way has not yet been probed. In our work, we have prepared a benchmark set of HOMA values for 25 cyclic hydrocarbons, based on the all core CCSD(T)/cc-pCVQ(T)Z geometries, and used it to investigate the impact of different choices of the exchange– correlation functionals and basis sets on HOMA, calculated against the experimentally based (HOMA EP) or the consis-tently calculated (HOMA CCP) parameters. We show that using HOMA EP leads to large and unsystematic errors, and strong sensitivity to the choice of XC functional, basis set, and the experimental data for the reference geometry. This sensitivity is largely, although not completely attenuated in the consistent approach. We recommend the most suitable functionals for calculating HOMA in both approaches (HOMA EP and HOMA CCP), and provide the HOMA parameters for 25 studied exchange–correlation functionals and two popular basis sets

Aromaticity of benzenoid hydrocarbons with inserted –B=B– and –BH–BH– groups : a comparison

Structures of selected polycyclic conjugated hydrocarbons with -B=B- and -BH-BH- moieties inserted in different places were calculated at the B3LYP/6-311++Glevel and their aromatic properties evaluated. HOMA, NICS(0), NICS(1)zz, Λ and PDI indices were used for studying their aromatic properties. Both optimized planar (as in parent hydrocarbons) and non-planar structures were taken into account. It is shown that insertion of both types of boron groups disturbs and decreases the aromaticity of the corresponding hydrocarbons. The decreasing effect of the -BH-BH- group is much stronger. What is quite intriguing is that it appears that non-planar structures of the studied compounds have a little higher aromaticity than the strictly planar ones. Mutual correlations between results obtained by different aromaticity indices are calculated and thoroughly discussed.We thank the Ministerio de Ciencia e Innovación (Project No. CTQ2012-35513-C02-02) and the Comunidad Autónoma de Madrid (Project MADRISOLAR2, ref. S2009/PPQ-1533) for their continuing support. Thanks are also given to “ICM” Warsaw (Project Number G17-8) and to the CTI (CSIC) computer centers for an allocation of computer time.Peer Reviewe

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