Wojciech P. Oziminski

Toward a Physical Interpretation of Substituent Effects: The Case of Fluorine and Trifluoromethyl Groups

The application of ab initio and DFT computational methods at six different levels of theory (MP2/cc-pVDZ, MP2/aug-cc-pVTZ, B3LYP/cc-pVDZ, B3LYP/aug-cc-pVTZ, M06/cc-pVDZ, and M06/aug-cc-pVTZ) to meta- and para-substituted fluoro- and trifluoromethylbenzene derivatives and to 1-fluoro- and 1-trifluoromethyl-2-substituted trans-ethenes allowed the study of changes in the electronic and geometric properties of F- and CF3-substituted systems under the impact of other substituents (BeH, BF2, BH2, Br, CFO, CHO, Cl, CN, F, Li, NH2, NMe2, NO, NO2, OH, H, CF3, and CH3). Various parameters of these systems have been investigated, including homodesmotic reactions in terms of the substituent effect stabilization energy (SESE), the π and σ electron donor-acceptor indexes (pEDA and sEDA, respectively), the charge on the substituent active region (cSAR, known earlier as qSAR), and bond lengths, which have been regressed against Hammett constants, resulting mostly in an accurate correspondence except in the case of p-fluorobenzene derivatives. Moreover, changes in the characteristics of the ability of the substituent to attract or donate electrons under the impact of the kind of moiety to which the substituent is attached have been considered as the indirect substituent effect and investigated by means of the cSAR model. Regressions of cSAR(X) versus cSAR(Y) for any systems X and Y allow final results to be obtained on the same scale of magnitude.

Aromatic character of heptafulvene and its complexes with halogen atoms

Geometry optimization of heptafulvene–halogen complexes (halogens: F, Cl, Br, I, and At) carried out at the B3LYP/6-311+G(d,p) level of theory allowed us to estimate the geometry-based aromaticity index HOMA, the magnetism-based indices NICS, NICS(1), and NICS(1)ZZ, as well as the energy of complex formation. Application of the NBO method allowed us to estimate the pEDA characteristics of the π-electron distribution in complexes (i.e., the electron excess/deficiency of the π-electron system in the ring). All of the characteristics of the complexes were found to be mutually interrelated, exhibiting good or at least acceptable correlation coefficients. It was also noted that halogen atoms with greater radii yielded weaker complexes and lower aromaticities, as shown by the HOMA, NICS, and pEDA indices. The energy of complex formation was observed to be linearly correlated with the degree of aromatization of the heptafulvene ring, as expressed by these indices.

Sigma- and pi- electron structure of aza-azoles.

The reasons behind changes of aromaticity in 10 unsubstituted aza-azoles were analysed by employing the natural bond orbital (NBO) approach at the MP2/6-311+G(d,p) level of theory. Sum of occupations of pz orbitals at atoms in the ring correlates well with the magnetism based aromaticity index NICS as well as with the number of nitrogen atoms in the ring. Changes of NICS depend strongly in a linear way on the number of NN bonds. Classification of azoles based on the number of pyridine-type nitrogen atoms vicinal to NH is supported by plotting the relative occupation of π orbitals (πocc) against the relative occupation of σ orbitals (σocc) for all individual atoms in rings.

Natural bond orbital approach to the transmission of substituent effect through the fulvene and benzene ring systems

Electronic structure of 22 monosubstituted derivatives of benzene and exocyclically substituted fulvene with substituents: B(OH)2, BH2, CCH, CF3, CH3, CHCH2, CHO, Cl, CMe3, CN, COCH3, CONH2, COOH, F, NH2, NMe2, NO, NO2, OCH3, OH, SiH3, SiMe3 were studied theoretically by means of Natural Bond Orbital analysis. It is shown, that sum of π-electron population of carbon atoms of the fulvene and benzene rings, pEDA(F) and pEDA(B), respectively correlate well with Hammett substituent constants

A Quantitative Analysis of Factors Influencing Ease of Formation and σ-Bonding Strength of Oxa- and Thia-N-Heterocyclic Carbenes

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The influence of substituent field and resonance effects on the ease of N-heterocyclic carbene formation from imidazolium rings

and aromaticity index NICS. The substituent effect acting on pi-electron occupation at carbon atoms of the fulvene ring is significantly stronger than in the case of benzene. Electron occupations of ring carbon atoms (except C1) in fulvene plotted against each other give linear regressions with high correlation coefficients. The same is true for ortho- and para-carbon atoms in benzene. Positive slopes of the regressions indicate similar for fulvene and benzene kind of substituent effect – mostly resonance in nature. Only the regressions of occupation at the carbon atom in meta- position of benzene against ortho- and para-positions gives negative slopes and low correlation coefficients.

Aromaticity of substituted fulvene derivatives: substituent-dependent ring currents

AbstractInteractions between the NO2 group and 13 different substituents (BF2, BH2, CF3, CH3, CHO, CN, F, NH2, NMe2, NO2, NO, OH, OMe) were investigated computationally for bicyclo[2.2.2]octane (BCO) and benzene substituted at 1,4 and 1,3 positions in the ring. Three methods were employed to estimate the character and strength of the substituent effect: substituent effect stabilization energy (SESE), sigma/pi electron donor acceptor index (sEDA/pEDA) and substituent active region (cSAR) parameter. For the first time the sEDA/pEDA parameters were calculated not for the ring but for the NO2 group. All calculations were performed at the B3LYP/6-31G(d,p) level of theory. For 1,4 derivatives, a direct comparison of slopes of linear regressions between BCO and benzene reveals a much better transmission of the substituent effect in the latter. The ratio of slopes (benzene over BCO) is always larger than 4. It follows that the resonance effects, which are absent in the BCO, dominate in this case. For 1,3 derivatives, because of much lower correlation coefficients, estimated standard deviations (ESD) were used to calculate the ratio instead of the slopes. For these systems the ratio is much closer to the unity, which indicates that only the sigma/through space effects are present and they are of similar magnitude in benzene and BCO. It follows from natural population analysis (NPA) charges that the substituent effect in the studied systems is due mainly to through-space interactions. FigureWhat is the nature of the substituent effect

Aromatization of fulvene by complexation with lithium.

The index described previously (carbene relative energy of formation) has been extended to oxygen and sulfur heterocycles. This provides a quantitative overview of factors determining ease of formation of (i) neutral N-heterocyclic carbenes (NHCs) by deprotonation of heterocyclic salts and (ii) anionic NHCs by deprotonation of heterocyclic mesomeric betaines. The influence of the nature and ring position of oxygen and sulfur is discussed for a range of known and unknown systems. Attention is directed to unexplored systems of potential interest.