Simple-harmonic-oscillator calculations of the deformation energy necessary to transform molecules with n-electron systems into their Kekul6 structures are presented. They enable estimation of (i) relative stabilization energies and (ii) Kekul6-structure contributions; both are calculated directly from experimental molecular geometries. Correlation between HOSE (Harmonic Oscillator Stabilization Energy) values and the Hess & Schaad resonance energies for alternant unsaturated hydrocarbons is very good (r = 0.991, n = 22); for non-alternant species the correlation is worse (r = 0.937, n = 12) but still acceptable. A very good correlation exists too between percentage contributions of the Kekul6 structures calculated by use of the HOSE model and those calculated by use of the quantumchemical method of Randi6 (r = 0.985, n = 65). Analysis of errors shows that only the geometries of molecules estimated with e.s.d.'s for bond lengths <0.004A are sufficiently precise for successful application of the HOSE model. The HOSE model enabled the percentage contributions of the quinoid structure to be estimated for EDA complexes of N,N,N',N'-tetramethyl-p-phenylenediamine and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ); in both cases the percentage quinoid form obtained was in line with chemical expectations. For TCNQ salts a good correlation was found between the Flandrois-Chasseau• charge at TCNQ species and percentage contribution of the quinoid form calculated by use of the HOSE model (r = 0.992, n = 11). The HOSE model may serve as a convenient method of prediction and summarization of some chemical properties of molecules of n-electron compounds, directly from experimental geometry. AbstractLattice-energy calculations in the atom-atom potential approach have been performed for observed and isostructurally derived hypothetical forms of pheno-0108-7681/83/060739-04501.50 thiazine and phenoselenazine compounds. Energy minimizations with respect to cell constants and molecular rigid-body coordinates lead to absolute minima of energy surfaces in all cases. The experimental values of cell constants for the three
Structural and spectroscopic properties of hydrogen bonding in solid 5-nitro-N-salicylideneethylamine have been investigated. This is the first example of ionic [O-···H−N+] intramolecular hydrogen bonding in a structure of Schiff base. Single-crystal X-ray diffraction and 13C magic-angle spinning NMR show that the title molecule is dominated by ionic canonical structures favored by through molecule conjugation between the nitro group and both groups of salicylidene fragment. Being involved in strong intramolecular [O-··· H−N+] hydrogen bonding, 5-nitro-N-salicylideneethylamine forms its crystal lattice by means of different types of weak intermolecular [O-···H−N+], N−H···O and C−H···O hydrogen bonds. On the basis of the solid-state NMR results, it has been suggested that the acidic proton can also stay at the oxygen atom, and this is qualitatively supported by X-ray diffraction.
Crystal structures of a series of five symmetrically substituted N,N-bisarylformamidines ArNHsCHdNAr with Ar ) X-C 6 H 4 , X ) p-OCH 3 (IV), p-CH 3 (V), p-F (VI), p-NO 2 (VII), and m-Br (VIII) have been determined by single-crystal X-ray diffraction (XRD) and complete the series studied previously where X ) H (I), p-Br (II), and p-Cl (III). In addition, the results of variable-temperature 15 N CPMAS NMR experiments performed on 15 Nlabeled I, II, and IV are reported. All compounds form cyclic dimers linked by two NsH‚‚‚N hydrogen bonds which can form two different tautomers, a and b, interconverting by fast double proton transfers. The NMR experiments indicate three types of amidines characterized by different magnitudes of the equilibrium constants K ab of the tautomerism. In dimers of type such as V-VIII, we find K ab , 1 (i.e., only a single tautomer in the temperature range between 100 and 300 K). In this case, the hydrogen-bonded protons are ordered and can be localized by XRD. Furthermore, the C‚‚‚N bond lengths and torsional and valence angles involving the two aryl groups of an amidine unit are different. For dimers such as II and III, characteristic temperature dependent 15 N CPMAS NMR line shape changes are observed indicating that K ab ) 1 within the margin of error. Rate constants of the tautomerism can in this case be obtained by line shape analysis. For this degeneracy to occur, the aryl group conformations at both amidine nitrogen atoms must be similar. XRD then observes disordered hydrogen-bond protons and, in principle, also disordered nitrogen atoms. However, in practice, the disorder of the latter is not resolved leading to the observation of equalized C‚‚‚N bond lengths. Finally, dimers (I, IV) represent an intermediate case with K ab < 1, which could be labeled as "dynamic partial order". The implications of the molecular structure and the hydrogen bond and proton transfer characteristics are discussed.
Analysis of the precisely measured geometry (retrieved from the CSD) of 21 diazobenzene derivatives revealed that the CC bond in the ring cis to NNR group is significantly longer than the trans CC bond. This effect is propagated over the whole ring, resulting in the imbalance of two Kekule structure weights equal to 54.8:45.2 for the mean geometry. 6-31G* calculation of 1,3,5-tridiazabenzene in C 3 symmetry shows the substantial enhancement of the effect. Approaching the NNH group to the C1C2 bond of the ring by bending the CNN bond and keeping the C2C1N angle fixed enhanced the effect considerably. By rotation around the C1N bond to the perpendicular conformation and keeping the C2C1N bond angle fixed, the π electron interactions are removed and the effect observed is due mostly to the strain. It is opposite to that of the optimized planar conformer. The conclusion is that the imbalance of Kekule structures for a ring is due to the combination of the strain effect and the π electron interaction of the diazo group with the ring.
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