Equalization Of Bond Lengths Research Articles

Photoswitching neutral homoaromatic hydrocarbons

Homoaromatic compounds possess an interrupted π system but display aromatic properties due to through-space or through-bond interactions. Stable neutral homoaromatic hydrocarbons have remained rare and are typically unstable. Here we present the preparation of a class of stable neutral homoaromatic molecules, supported by experimental evidence (ring current observed by NMR spectroscopy and equalization of bond lengths by X-ray structure analysis) and computational analysis via nucleus-independent chemical shifts (NICS) and anisotropy of the induced current density (ACID). We also show that one homoaromatic hydrocarbon is a photoswitch through a reversible photochemical [1, 11] sigmatropic rearrangement. Our computational analysis suggests that, upon photoswitching, the nature of the homoaromatic state changes in its perimeter from a more pronounced local 6π homoaromatic state to a global 10π homoaromatic state. These demonstrations of stable and accessible homoaromatic neutral hydrocarbons and their photoswitching behaviour provide new understanding and insights into the study of homoconjugative interactions in organic molecules, and for the design of new responsive molecular materials.

New expanded porphyrinoids: Synthesis, structure and properties of hemihexaphyrazines and their reduced metal containing derivatives

Hemihexaphyrazines can be considered as hexamembered porphyrinoids of the ABABAB type, where thiadiazole rings (A) and pyrrole-containing subunits (B) joined via aza bridges to form a macrocyclic system with an expanded coordination cavity capable of holding at once three atoms of transition metals that are formed a triangular cluster. It was established for the first time that unsubstituted hemihexaphyrazine and its metal complexes can be reduced to form crystalline salts. These salts were characterized by UV-Vis-NIR and IR spectroscopy, MS, and X-Ray diffraction data. The magnetic properties of these compounds were studied for the first time using EPR and SQUID techniques within the [Formula: see text] K temperature range. It was shown that the three-electron reduction of starting [Formula: see text] yields [Formula: see text] and [Formula: see text] crystalline salts. An unusual doublet-quartet transition was found for the [Formula: see text] dianions. Salt [Formula: see text] also shows an unusually strong low energy NIR absorption, which was observed at 1000–2200 nm with maxima located at 1283 and 1980 nm. A phenomenon consisting of equalization of bond lengths among exocyclic nitrogen atoms ([Formula: see text] and neighboring atoms C(pyrrole) and C(thiadiazole) was observed under reduction conditions.

The nature of one-dimensional carbon: polyynic versus cumulenic.

A question of both fundamental as well as practical importance is the nature of one-dimensional carbon, in particular whether a one-dimensional carbon allotrope is polyynic or cumulenic, that is, whether bond-length alternation occurs or not. By combining the concept of aromaticity and antiaromaticity with the rule of Peierls distortion, the occurrence and magnitude of bond-length alternation in carbon chains with periodic boundary conditions and corresponding carbon rings as a function of the chain or ring length can be explained. The electronic properties of one-dimensional carbon depend crucially on the bond-length alternation. Whereas it is generally accepted that carbon chains in the limit of infinite length have a polyynic structure at the minimum of the potential energy surface with bond-length alternation, we show here that zero-point vibrations lead to an effective equalization of all carbon-carbon bond lengths and thus to a cumulenic structure.

Density Functional Theory Study on Vinyl Thiophene Group Conjugated Spirooxazines

We carried out a theoretical study on the geometries, electronic structures, and frontier molecular orbitals of vinyl thiophene group conjugated spirooxazines (SO-SO3) using density functional theory (DFT) at the B3LYP/6-31G* level. The calculated results show that the equalization of bond lengths at the left and right parts of the open-forms occurred during the ring-opening process. A large conjugated system was formed and this significantly narrowed the energy gap. The conjugated system became larger and its electrons flowed easily because of the introduction of different lengths of vinyl thiophene conjugation moieties into the spirooxazine molecule. The electrons and energy efficiently transferred from the vinyl thiophene to naphthoxazine. The orbital contribution rate of the vinyl thiophene group in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) increased obviously. Time-dependent DFT (TD-DFT) calculations showed that as the conjugated vinyl thiophene unit reached 2-3 the first singlet excited state of SO2 and SO3 resulted from the electron transition from the HOMO to the LUMO, which were also assigned to the π-π transition. Meanwhile, λmax 1847 Acta Phys. ⁃Chim. Sin. 2011 Vol.27 was between 466 and 540 nm with an obvious red-shift while the λmax of O-SO2 and O-SO3 reached 605 and 647 nm, respectively.

Theoretical studies on the aromaticity of η 5-cyclopentadienyl cobalt dithiolene complexes

Some η 5-cyclopentadienyl cobalt dithiolene complexes CpCoS 2C 2R 2 have been optimized at B3LYP/6-311++G(d) level. The optimized geometries agree well with experiment. The analyses of nature bond orbital and nucleus-independent chemical shift (NICS) at B3LYP/6-311++G(d) and GIAO-B3LYP/6-311++G(d) levels reveal the aromatic character of the η 5-cyclopentadienyl cobalt dithiolene complexes. However, their aromaticity is weaker than that of the isolated CoS 2 C 2 + 1 . There are two reasons for the change of heterocyclic aromaticity of the metal dithiolene in the η 5-cyclopentadienyl cobalt dithiolene complexes with respect to that of the isolated CoS 2 C 2 + 1 . The better equalization of bond lengths in the isolated cation CoS 2 C 2 + 1 is the first reason. The other reason is that the contribution to the NICS from the metallic cobalt atom is much larger in the isolated cation CoS 2 C 2 + 1 . The planar character of cyclopentadienyl is destroyed slightly in the complexes. At the same time, the size of cyclopentadienyl (Cp) becomes bigger than the isolated Cp −1 and this is caused by the cobalt atom in the pentagon. The π-electron delocalization causes stronger aromaticity of the Cp in the complexes than that of the isolated Cp −1.

Cation distribution and interatomic interactions in oxides with heterovalent isomorphism: X. Structure of the Ho2SrAl2O7 oxide at 100, 298, and 673 K

Crystallochemical data for the Ho2SrAl2O7 oxide at 100, 293, and 673 K were obtained by full-profile X-ray analysis. Analysis of the thermal expansion anisotropy showed that increased temperature gives rise to equalization of metal-oxygen bond lengths in all oxygen polyhedra at the expense of lengthening of some bonds and virtual invariability of others.

Cyclic aromatic systems with hypervalent centers.

Strong attractive intramolecular X<--Y(S, NR') interactions of 1,5-type occurring between terminal electron-rich main-group 15-17 centers (X = pnictogen, chalcogen, or halogen) and carbonyl, thione, or imine groups (Y = O, S, NR') incorporated into the conjugated -X-CH=CH-CH=Y fragments result in significant stabilization of cis-s-cis structure of these molecules, which may be viewed as the five-membered pseudo-heterocycles with the hypervalent arrangements across the pnictogen, chalcogen, or halogen centers. In derivatives of beta-chalcovinylaldehydes and isoelectronic chloronium cations, one of two lone electron pairs at X = S, Se, Te, Cl+ possesses pure p character and is involved in conjugation with the pi-system of the rest of the molecule, which leads to an appreciable contribution of the aromatic stabilization of the 6 pi-electron ring closed by the X<--Y bond. The aromaticity of these structures evaluated through calculation of the homodesmotic stabilization energies (HSE) increases in parallel with an increase in the strength of the intramolecular coordination bonds, i.e., in the order X = S, Se, Te and with an increase in electronegativity of a substituent attached to the chalcogen atom. Clear manifestation of the aromatic character of the cis-s-cis isomers of beta-chalcovinylaldehydes, their imines, as well as congeneric thiones and chloronium cations comes from the theoretical and experimental evidence for the pronounced equalization (as compared with the open trans-s-trans isomers) of the CC bond lengths. The cooperative effects of the hypervalent bonding and aromaticity are most distinct in the bicyclic structures of 1,6,6a lambda 4-trichalcapentalenes, their 1,6-dioxa(aza) analogues, and 1,6-dioxa-3a-aza-6a lambda 4-pnictapentalenes, in which two conjugated -X-CH=CH-CH=Y fragments are fused together via the symmetric -Y-X-Y- triad. The assessments based on the HSE values indicate that the aromaticity of these 10 pi-electron compounds is estimated as about 30-50% of the aromatic character of the most aromatic bicyclic structure of naphthalene. As shown by extensive ab initio calculations, substantial hypervalent bonding effects also operate in the case of beta-pnictovinylaldehydes formed by second and lower row pnictogen atoms (X = P, As, Sb, Bi), providing for the enhanced stability of their cis-s-cis configuration with respect to the free-of-strain trans-s-trans structures. The bicyclic 1,6,6a lambda 5-dioxapnictapentalene structure is also the most energy favorable form of these compounds. However, by contrast with the chalcogen and halonium analogues, the pseudo-cyclic structure of beta-pnictovinylaldehydes and the bicyclic structure of 1,6,6a lambda 5-dioxapnictapentalenes are maintained through primarily the occurrence of the hypervalent bonding across the pnictogen centers, whereas pi-delocalization is not (or weakly) operative in these compounds. A possible explanation for this finding is the low p character of the single lone electron pair at the pnictogen and orientation of the axis of its orbital, which is unfavorable for the optimal overlap with the pi-system of the rest of a molecule. As first proposed by Arduengo and Dixon, configurational isomerizations at the second and lower row tricoordinate pnictogen centers may proceed through the T-shaped hypervalent structures. Such a structure is strongly stabilized in the aromatic 1,6-dioxa-3a-aza-6a lambda 4-pnictapentalenes. It was found by calculations that the aromatic stabilization of the T-shaped hypervalent arrangement at the tricoordinate pnictogen center in 2H-1,3,2-dioxaphosphole and arsole arising due to withdrawal of two electrons from the pi-system of the ring onto the hypervalent electron-excessive center facilitates the rearrangement and directs it along the reaction path with the aromatic transition-state structure. Investigation of the influence of cooperative effects of hypervalent bonding and aromaticity on stability and mechanisms of reactions of main-group 14-17 compounds not considered in this review is the subject of our continuing research.

4,8-Dibenzyl-2',4',6'-trinitrospiro[1,3-dioxaazulenium-2,1'-cyclohexadienide

In molecules of the title compound, C 27 H 19 N 3 O 8 , the ' nitro groups have almost equivalent geometries. Similar 1 observations were made for some related anionic σ complexes. However, equalization of bond lengths within , the conjugated system of the cyclohexadienide ring is stronger than that in other Meisenheimer complexes.

Aromaticity Today: Energetic and Structural Criteria

Aromaticity appears to be among the most controversial concepts in modern chemistry. The present paper addresses current controversies in development and applications of the two main criteria of aromaticity, namely, energetic and structural criteria. Various types of resonance energy with an emphasis on homodesmotic and isodesmic stabilization energies have been discussed. Reliability of such structural criteria of aromaticity as equalization of carbon-carbon bond lengths and stability with respect to out-of-plane distortions of a molecular structure has been analyzed using various examples.

Crystallographic studies of intra‐ and inter‐ molecular interactions. Part VI. Crystal and molecular structure of N,N′‐dimethyl‐N′‐phenylsulphonylformamidine. Equalization of CN bond lengths in the amidine fragment as a result of substituent effects due to push–pull

AbstractThe crystal and molecular structure of N,N‐dimethyl‐N′‐phenylsulphonylformamidine is solved by direct methods and refined to R = 0.041 for 935 reflections, Equalization of CN bond lengths in the amidine fragment is discussed in terms of a push—pull effect operating between the N,N‐dimethyl and SO2Ph groups. A significant role of the substituent at the functional carbon on the degree of π‐electron delocalization on the NCN fragment is interpreted by use of the HOSE model.

Effect of Bond-Length Alternation on the Aromaticity of Benzene

Abstract Dewar-de Llano PPP-SCF-MO calculations were performed to examine the effect of bond-length alternation on the σ and π frames of benzene. It was confirmed that the equalization of the CC bond lengths in benzene was caused primarily by a high degree of aromaticity. Benzene still is highly aromatic even if it is deformed artificially. Adiabatic and vertical resonance energies were defined for this compound.

Structure of cyclooctatetraene at 129 K

CsH s, Mr= 104.2, orthorhombic, Aba2, a = 7.664 (6), b = 7.650 (3), c = 10.688 (6) A, V= 627 (1)/~3, T= 129 K, Z = 4, D x = 1.10 Mg m -3, F(000) = 224, 2(Mo Ka) = 0.71069 A, /~= 0.06 mm -1, final R = 0.036 (wR = 0.043) for 529 observed reflections (I> 2o(/)) (4525 (+h, +k, +l) measured, 597 averaged reflections (Rint 0.05) and 52 refined parameters). The molecular structure of COT in the solid state confirms earlier electron-diffraction results for the title compound suggesting a boat conformation. This is in accordance with recent structural investigations of COT derivatives. Introduction. Cyclooctatetraene (COT) ring systems are capable of three structural changes, which involve ring inversion, bond shifting and valence isomerization (Paquette, 1975). Whereas the first process requires a planar transition state only, all bond-shifting mechan- isms involve additionally an equalization of bond lengths, and consequently slightly higher energy barriers are found for the latter. As a ligand to transi- tion metals, COT usually adopts a boat form or a twisted boat conformation (Brauer & Krfiger, 1976); frequently the ligand shows fluxionality (Mann, 1982). As a dianionic ligand the COT skeleton is planar (Streitwieser et al., 1973; R6sch & Streitwieser, 1978). For free, non-substituted COT a non-planar ring shape was found by electron diffraction (Bastiansen, Hedberg & Hedberg, 1957; Tr~etteberg, 1966); preliminary results of a crystal structure analysis of COT have been quoted in the literature (Bordener, Parker & Stanford, 1972). A low-temperature X-ray diffraction study of COT was undertaken to reveal basic structural data for this important compound in the solid state.

Crystallographic Studies and Physicochemical Properties of π-Electron Systems, Part 7 Refinement of the Crystal and Molecular Structure of p-N,N-Dimethylaminobenzoic Acid, Full ab initio STO-3G Optimization of its Molecular Geometry, and Empirical Analysis of Substituent Effects on the Geometry of Benzene Rings in p-Substituted Benzoic Acids

Abstract The crystal and molecular structure of N,N -dimethylaminobenzoic acid (DABA ) was redetermined by means of X-ray diffraction. Crystal data are: Mr = 165.2, triclinic, P1̄, a = 6.651(2) Å , b = 8.045(2) Å , c = 8.609(2) Å , α - 88.97(3)°, β = 94.72(3)°, γ = 113.76(3)°, V = 420.13 Å3, Z = 2, Dm = 1.28 g cm-1, Dx = 1.306 g cm-1; CuKα: λ = 1.54178 Å, μ = 6.74 cm -1, F(000) = 176.00, T - 293 K; R = 0.0684, Rw = 0.0780 for 1185 observed reflections. The DABA molecules are almost planar and form cyclic dimers exhibiting no orientational disorder with separation of oxygen atoms RO...O - 2.627(2) Å . The pattern of molecular geometry in DABA suggests a relatively strong through-resonance effect between the NMe2 and COOH groups and shows non-additivity of substituent effects on valence angles and bond lengths in the ring. Mesomeric equalization of CO bond lengths in the carboxylic groups of 10 well solved p-substituted benzoic acids is found to depend linearly on the RO...O distance in H-bridged dimers. Full abinitio STO -3G optimizations of molecular geometries for monomers of DABA , p-aminobenzoic acid, benzoic acid, and formic acid as well as for aniline and N,N -dimethylaniline show resonable agreement with experimental data. Changes of molecular geometries on formation of planar H-bridged dimers have been obtained from calculations for benzoic acid dimers, benzoic - formic acid complex, and formic acid dimers. The latter was also optimized using the 6 -31 G basis set.

Variable-beta Hückel studies of the linear polyenes

The “variable beta” modification of the Hückel technique is applied to the series of linear polyenes, and the results are shown to be in accord with those of more elaborate approaches. Correlations regarding interatomic distances, spectral properties, ionization potentials and delocalization energies are presented. Incidentally, it is found that, contrary to current statements, the Hückel method does not predict a rapid equalization of bond lengths in long chain polyenes.