Heteroatom Incorporation Effect in σ- and π-Electron Systems: The sEDA(II) and pEDA(II) Descriptors

The effect of heteroatom or heteroatomic group incorporation into unsaturated five- and six-membered cyclic systems was studied by means of DFT/B3LYP/aug-cc-pVDZ calculations. Two descriptors of the incorporation effect, sEDA­(II) and pEDA­(II), reflecting the influence of the incorporated atom or group on the population of the σ and π valence electrons, were constructed on the basis of natural bond orbital analysis. The sEDA­(II) and pEDA­(II) descriptors were shown to be linearly independent; the former correlated very well with electronegativity scales, whereas the latter correlated with NICS(1)_ZZ and HOMA_CC aromaticity indices. The two descriptors seem to be universal tools for analyzing different chemical and physicochemical effects occurring in unsaturated heterocyclic systems

Substituent Effect in the First Excited Singlet State of Monosubstituted Benzenes

sEDA, pEDA, and cSAR descriptors of the substituent effect were determined for >30 monosubstituted benzenes in the first excited singlet S₁ state at the LC-ωB97XD/aug-cc-pVTZ level. It was found that in the S₁ state, the σ- and π-valence electrons are a bit less and a bit more affected, respectively, than in the S₀ state, but basically, the effect in both states remains the same. In the S₀ and S₁ states, the <i>d</i>(C–X) distances to the substituent’s first atom and the ring perimeter correlate with the sEDA and pEDA in the appropriate states, respectively. The energies and the gap of the frontier orbitals in the two states are linearly correlated and for the HOMO­(S₁), LUMO­(S₁), and HOMO­(S₁)–LUMO­(S₁) gap correlate also with the pEDA­(S₁) and cSAR­(S₁) descriptors. In all studied correlations, three similar groups of substituents can be distinguished, for which correlations (i) are very good, (ii) deviate slightly, and (iii) deviate significantly. Comparison of the shape of the HOMO­(S₀) and HOMO­(S₁) orbitals shows that for case (i) HOMO orbitals exhibit almost perfect antisymmetry against the benzene plane, for case (ii) the antisymmetry of HOMO in one of the states is either perturbed or changed, and for case (iii) one HOMO state has σ-character

Structure–Spectra Correlations in Anilate Complexes with Picolines

We report a joint structural and spectroscopic study of a series of hydrogen-bonded chloranilate and bromanilate complexes with α- and β-picolines. Single-crystal structures at 100 K are provided for all the systems analyzed, which were found to form B:XA:XA:B, :(B:XA:B):XA, and B:XA:B type synthons, where XA and B stand for the acid and base molecules, respectively. By extending single-crystal X-ray crystallography onto computationally supported high-resolution solid-state spectroscopy, we provide a comprehensive analysis of spectral signatures that can possibly facilitate the design and recognition of the supramolecular architectures formed by these kinds of synthons. To this end, we employed nuclear magnetic resonance spectroscopy along with complementary optical (infrared, Raman, terahertz time-domain spectroscopy) and neutron (inelastic neutron scattering) vibrational techniques. Despite a large chemical similarity, the studied systems exhibited strikingly different spectral responses. All the spectral signatures and peculiarities arising from structural factors, intermolecular forces, and specific effects are interpreted and discussed in detail. Based on state-of-the-art first-principles calculations for solid-state, in both static and time-evolved manners, the spectral influences of long-range dipole coupling, proton transfer, symmetry-distortion, as well as anharmonicity are covered extensively. In this way, we take the necessary first step needed to gather combined structure–spectroscopy data on low-weight supramolecular synthons, which are important in crystal engineering and materials science

On Stability, Chirality Measures, and Theoretical VCD Spectra of the Chiral C₅₈X₂ Fullerenes (X = N, B)

The stability of all 23 C₅₈N₂ and C₅₈B₂ heterofullerenes in the singlet and triplet states was determined at the B3LYP/6-31G** level. In equilibrium mixture the achiral (1,4) C₅₈N₂ isomer would be populated in ca. 95.8%, the chiral (1,16) one in ca. 3.3%, and the achiral (1,4) C₅₈B₂ in 100%, whereas all triplet state isomers are less stable. Fourteen out of 23 C₅₈X₂ are chiral. Four different chirality measures were calculated by our own CHIMEA program: pure geometrical, labeled, mass, and charge. Intercorrelations between the measures for all chiral compounds indicate that the pure geometrical chirality measure is unstable and should not be used in QSAR predictions of the other molecular properties, while the labeled and mass-weighted ones are promising QSAR descriptors. For each chiral C₅₈N₂ molecule, some very strong VCD bands, of intensity comparable with that in the IR spectra, can serve in identification and characterization of the isomers

Chirality Measures of α-Amino Acids

To measure molecular chirality, the molecule is treated as a finite set of points in the Euclidean <i>R</i>³ space supplemented by <i>k</i> properties, <i>p</i>₁^(<i>i</i>), <i>p</i>₂^(<i>i</i>), ..., <i>p</i>_<i>k</i>^(<i>i</i>) assigned to the <i>i</i>th atom, which constitute a point in the Property <i>P</i>^<i>k</i> space. Chirality measures are described as the distance between a molecule and its mirror image minimized over all its arbitrary orientation-preserving isometries in the <i>R</i>³ × <i>P</i>^<i>k</i> Cartesian product space. Following this formalism, different chirality measures can be estimated by taking into consideration different sets of atomic properties. Here, for α-amino acid zwitterionic structures taken from the Cambridge Structural Database and for all 1684 neutral conformers of 19 biogenic α-amino acid molecules, except glycine and cystine, found at the B3LYP/6-31G** level, chirality measures have been calculated by a CHIMEA program written in this project. It is demonstrated that there is a significant correlation between the measures determined for the α-amino acid zwitterions in crystals and the neutral forms in the gas phase. Performance of the studied chirality measures with changes of the basis set and computation method was also checked. An exemplary quantitative structure–activity relationship (QSAR) application of the chirality measures was presented by an introductory model for the benchmark Cramer data set of steroidal ligands of the sex-hormone binding globulin