Tadeusz M. Krygowski

Separation of the energetic and geometric contributions to the aromaticity of π-electron carbocyclics.

Separation of the aromaticity index HOMA, based on experimental geometry, into energetic and geometric contributions is proposed for carbocyclic π-electron systems. The decrease of the aromatic character may be due either to an increase of bond length alternation (geometric term) or to the lengthening of the mean bond lengths of the ring (energetic term). On the basis of 169 sets of data for benzene rings in benzenoid hydrocarbons, 116 in para-disubstituted benzene derivatives and 90 in TCNQ molecules in EDA complexes and salts, as well as 48 for five-membered rings in cyclopentadienyl complexes with Rh it is shown that these two terms are uncorrelated, supporting the view of Katritzky et al.1–4 and Jug et al.5 that aromaticity is a multidimensional phenomenon. The separation is applied to five-, six- and seven- membered rings in typical π-electron systems (benzenoid hydrocarbons, fullerenes, fulvene and heptafulvene derivatives). Aromaticity index (HOMA) = 1 - [dearomatization term due to bond length alternations (GEO) + energetic term (EN)]. All terms are accessible from experimental bond lengths.

Aromaticity from the viewpoint of molecular geometry: application to planar systems.

to Planar Systems Tadeusz M. Krygowski,*,† Halina Szatylowicz,*,‡ Olga A. Stasyuk,‡ Justyna Dominikowska, and Marcin Palusiak †Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland ‡Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Department of Theoretical and Structural Chemistry, Faculty of Chemistry, University of Łod́z,́ Pomorska 163/165, 90-236 Łod́z,́ Poland

EMPIRICAL PARAMETERS OF LEWIS ACIDITY AND BASICITY FOR AQUEOUS BINARY SOLVENT MIXTURES

Abstract Empirical parameters of Lewis acidity, E N T , introduced by Reichardt et al., and Lewis basicity, B KT , introduced by Kamlet and Taft, have been determined for mixtures of water with ten organic solvents. In the case of water/alcohol mixtures a distinct dependence between these acidity and basicity parameters have been found. For the other solvent mixtures the E N T on B KT dependence is more complex even if these parameters are purified from non-specific solute/solvent interactions.

Separation of the energetic and geometric contributions to the aromaticity. Part IV. A general model for the π-electron systems

Application of the Pauling bond number1 to the aromaticity index HOMA2–3 allowed us to extend its separation into the geometric and energetic contributions for the hetero π-electron systems. The energetic term (EN) represents the part related to resonance energy of a given system. The geometric term represents dearomatization due to an increase of bond length alternation (localisation of double bonds). The Bird indices I5 and I64 well represent aromatic character due to its geometric features. In most cases geometric contributions play a significant role and variation in GEO-term is the most important factor in variation of HOMA values and their correlation with Birds indices. Aromaticity indices for 24 five- and six-membered, most typical, heterocyclic systems are presented and discussed. Aromaticity index (HOMA) = 1 - [dearomatization term due to bond length alternations (GEO) + energetic term (EN)] is extended for hetero-π-electron systems. All terms are accessible from experimental bond lengths.

Global and Local Aromaticity in Porphyrins: An Analysis Based on Molecular Geometries and Nucleus‐Independent Chemical Shifts

Two pyrrole rings participate in the aromatic structure of porphyrin. Hence, a 22 π-electron description is better than the usual [18]annulene representation. The dianion and the metal complex system favor different electronic structures.

Facts and artifacts about aromatic stability estimation

Abstract The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of CC or CX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G∗∗.

Separation of the Energetic and Geometric Contributions to Aromaticity. 2. Analysis of the Aromatic Character of Benzene Rings in Their Various Topological Environments in the Benzenoid Hydrocarbons. Crystal and Molecular Structure of Coronene

Statistical analysis of the aromatic character and its geometric and energetic contributions of 167 benzene rings embedded in various topological environments in 26 benzenoid hydrocarbons leads to the following conclusions:  the aromatic character of benzene rings with three or fewer fused rings is due mostly to geometric contributions, whereas in other cases energetic contribution is decisive. Aromaticity indices for individual rings (local aromaticity) depend strongly on the kind of topological environment. Terminal rings always exhibit a strong aromatic character, whereas those fused to many rings are often weakly aromatic. The study is based on precisely solved X-ray or neutron crystal structure determination retrieved from Cambridge Structural Database supplemented by our own precise determination of coronene.

Global and Local Aromaticity of Linear and Angular Polyacenes

Abstract Ab initio (B3LYP/6-311G∗∗) optimisation of seven linear and five angular polyacenes gave us an opportunity to study aromaticity in terms of HOMA index and its components EN and GEO, the energy of the carbon skeleton of these hydrocarbons was calculated from CC bond lengths. Independently, the Cohen–Benson group additivity values were used to estimate the thermochemical aromatic stabilisation energy (ASE). For individual rings the NICS values were computed and compared with HOMA. An increase of size of both linear and angular polyacenes is associated with a substantial decrease in their aromaticity, with a greater decrease for the linear polyacenes. This is well documented by both HOMA and Cohen–Benson ASE.

Crystallographic studies on sterically affected chemical species Part II. Molecular and crystal structure of 1,8-bis(dimethylamino)- naphthalene tetrafluoroborate. Analysis of distortion of geometry in the aromatic part due to intramolecular hydrogen bonding

Abstract The crystal and molecular structures of 1,8-bis (dimethylamino) naphthalene tetrafluoroborate have been determined by X-ray diffraction. The compound crystallizes in the triclinic system, space group P 1 , with a=8.073(1), b=8.1200(1), c=12.256(1)A, α=92.25(1)°, β=106.23(1)° and γ=79.23(1)°. The structure has been solved by direct methods and refined to R=0.06 for 2370 reflections. Strong intramolecular H-bonding causes out-of -plane deformations in the region of peri substituents and very strong in-plane deformations of the valence angles at carbons C1, C8 and C9. The H position is symmetrical and the N⋯N distance is 2.562 A.

The rotational spectra, electric dipole moments and molecular structures of anisole and benzaldehyde

The rotational spectra of anisole and of benzaldehyde were investigated in supersonic expansion at frequencies up to 41 GHz, and at room temperature in the millimetre-wave region, from 170 to 330 GHz. Accurate spectroscopic constants for the parent isotopomers in the ground vibrational state and for the first excited torsional state were determined for both molecules. The supersonic expansion spectrum allowed measurement, in natural abundance, of all singly substituted 13C isotopomers, as well as of the 18O isotopomer for both anisole and benzaldehyde. The rotational constants were used to determine the r(s) and the r(m)(1) gas-phase geometries, which are found to be consistent with prediction of bond length alternation in the phenyl ring induced by the asymmetric substituent. Stark measurements were made on the supersonic expansion spectrum resulting in electric dipole moment determination, /mu a/ = 2.9061(22) D, /mu b/ = 1.1883(10) D, /mu tOt/ = 3.1397(24) D for benzaldehyde and /mu a/ = 0.6937(12) D, /mu b/ = 1.0547(8) D, mu tOt = 1.2623(14) D for anisole. During the investigation it was found that use of a carrier gas mixture consisting of 30% Ar in He carries significant advantages for studies of weak lines, and pertinent experimental details are reported.