Electronegativity and resonance parameters from the geometry of monosubstituted benzene rings

Reference values of the structural substituent parameters, S _E and S _R, measuring the electronegativity and resonance effects, respectively, of functional groups (Campanelli et al. J Phys Chem A 107:6429–6440, 2003) have been determined from the benzene ring geometries of 100 Ph–X species, including different conformations of the same molecule. Geometries have been obtained by quantum chemical calculations at the HF/6-31G*, HF/6-311++G**, and B3LYP/6-311++G** levels of theory. The substituent parameters from HF/6-311++G** calculations are in close agreement with those determined at the HF/6-31G* level. Using the B3LYP density functional yields S _E and S _R values which—in general—correlate well with the corresponding HF values. Exceptions occur with some charged groups, and, in the case of S _E, with a few dipolar groups having very high or low electronegativities. S _R values from B3LYP calculations are about 22% smaller than the corresponding HF values. The variations of the benzene ring geometry caused by electronegativity, resonance, and steric effects are illustrated in some detail.     

An Ab Initio Study and NBO Analysis of the Stability and Conformational Properties of Hexakis(trimethylelementhyl)benzene (Element = C,Si, Ge,and Sn)

Ab initio molecular orbital and density functional theory were used to investigate energetic and structural properties of the various conformations of hexa-tertbutylbenzene (1), hexakis(trimethylsilyl)benzene (2), hexakis (trimethylgermyl)benzene (3), and hexakis(trimethylstannyl)benzene (4). HF/3-21G//HF/3-21G and B3LYP/3-21G//HF/3-21G results revealed that the Twist-Boat (TB) conformer of compound 1 is more stable than the 1-Chair (C), 1-Boat (B), and 1-Planar (P) conformers. B3LYP/3-21G//HF/3-21G results show that the 1- TB conformer is more stable than 1- C, 1- B, and 1- P conformers of about 1.13, 4.34, and 99.94 kcal mol^?1 , respectively. Contrary to the stability order of compound 1 conformers, the C conformer of compounds 2–4 is more stable than TB, B, and P conformations, as calculated by B3LYP/3-21G//HF/3-21G and HF/3-21G//HF/3-21G levels of theory. The energy gap between the C and P conformers in compounds 1–4 is decreased in the following order: ΔE(4: C, P) < ΔE (3: C, P) < ΔE(2: C, P) < ΔE (1: C, P). This fact can be explained in terms of the increase of C _aromatic -M (M═C, Si, Ge, and Sn) bond lengths and the decrease of steric (van der Waals) repulsions in the previously discussed compounds. For compounds 1–3, the calculations were also performed at the B3LYP/ 6-31G*//HF/3-21G level of theory. However, the comparison showed that the results at B3LYP/3-21G//HF/3-21G methods correlated well with those obtained at the B3LYP/6-31G*// HF/6-31G method. Further, NBO analysis revealed that in compounds 1–4, the resonance energy associated with the σ_M-C1 to σ*_C2-C3 delocalization is 5.20, 9.68, 11.15, and 12.27 kcal mol^?1, respectively. These resonance energy values could explain the easiness of the ring flipping processes of C, B, and TB conformers of compounds 4 to 1. Also, the NBO results showed that by an increase of the σ_M-C1 → σ *_C2-C3 resonance energies in compounds 1–4, the σ_M-C1 bonding orbital occupancies decrease. This fact could fairly explain the increase of the C_aryl-M bond length from compound 1 to 4. The NBO results are also in good agreement with the calculated energy barriers for the ring flipping of the chair conformations in compounds 1–4, as calculated by B3LYP and HF methods.     

Steric and Electronic Structure of Selenoanisole: A Quantum-Chemical Study

The potential functions of internal rotation around the C_sp2-Se bond in selenoanisole were ob- tained by quantum-chemical calculations in the approximations HF/3-21G(d), HF/6-31G(d), MP2(f)/6-31G(d), and B3LYP/6-31G(d). The calculations were performed in the range of variation of the torsion angle (between the planes of the benzene ring and C_sp2-Se-C_sp3 bonds) from 0° to 90° with 15° step. The energy minimum is in the region of the orthogonal conformation ( 90°), and the energy maximum, in the region of the planar form ( 0°). The rotation barriers (kJ mol^{- 1}) are as follows: HF/3-21G(d), 9.20; HF/6-31G(d), 13.13; MP2(f)/6-31G(d), 10.25; and B3LYP/6-31G(d), 6.41. The geometric parameters, Koopmans ionization potentials, and dipole moments are given. The energies, degrees of hybridization, populations of the lone electron pairs of Se, energies of their interaction with the antibonding * orbitals of the benzene ring, and electron density distributions were determined in terms of the natural bond orbital approach.     

Origin of threefold methyl torsional potential in methylindoles

In this article, we present a systematic study on mono-methylindoles to investigate the electronic origin of the threefold symmetric component (V ₃) of the methyl torsional potential barrier in the ground electronic state (S ₀). The structures and the torsional potential parameters of these molecules were evaluated from ab initio calculation using Hartree-Fock (HF), second order Mollar Plesset perturbation (MP2) and B3LYP density functional level of theories and Gaussian type basis set 6-31G(d, p). Natural bond orbital (NBO) analysis of these molecules were carried out using B3LYP/6-31G(d, p) level of calculation to understand the formation of the threefold V ₃ term arising from the changes of various non-covalent interactions during methyl rotation. Our analysis reveals that the contributions from π orbitals play a dominant role in the barrier height determination in this class of molecules. The threefold term in the barrier arises purely from the interactions non-local to the methyl group in case when the methyl group has two single bonds vicinal to it. On the other hand, it is the local interaction that determines the potential energy barrier when the methyl group has one single bond and one double bond vicinal to it. However, in all these cases, the magnitude of the energy barrier depends on the resonance structure formation in the benzene ring frame upon rotation of the methyl group and, therefore, the energetics of the barrier cannot be understood without considering the molecular flexing during methyl rotation.     

Fluorinated heterocyclic compounds: Selective chlorine/fluorine exchange reactions on pyrimidines

Selective chlorine/fluorine exchange reactions on tetrachloropyrimidine, 6-methyl-, 6-chloromethyl-, 6-dichloromethyl-, and 6-trichloromethyl-2,4,5-trichloropyrimidine are described. Sodium fluoride, potassium fluoride, hydrogen fluoride, and antimony trifluoride were used as the fluorinating agents. It was found that NaF and KF fluorinate only in the heterocyclic nucleus, HF in the nucleus and in the chlorinated methyl group, and SbF₃ only in the chlorinated methyl group. In the first stage of fluorination with NaF only chlorine bound in position 4 of the pyrimidine ring is exchanged. The HF reaction is an equilibrium reaction in which the substitution of the fluorine for the first chlorine atom occurs preferentially in position 2. The behaviour of partly fluorinated pyrimidines in nucleophilic exchange reactions is also discussed.     

Complexation of Cu(II) with Hetarylformazanes in Solutions,Polycrystals, and Modified Anionite

The Cu(II) complexes with hetarylformazanes (HF) formed in solutions, polycrystals, and AN-18 polymeric ionite modified with HF were studied by EPR method and were found to have almost identical structures. The configuration of the Cu²⁺ coordination surrounding (square-planar or pseudotetrahedral) is specified by the substituents at N¹ in formazane fragments. Complexes with coordination unit Cu(4N) were formed mainly, when the ligands were taken in excess, while in the case of the ligand deficiency and with the coordinating OH group in benzene ring, the complexes with the Cu(3NO) coordination unit were formed. The covalence of in-plane metal-ligand σ-bond was found to be affected by the substituents. The formation of binuclear copper complexes in solutions and polycrystals was established in the case of deficiency of the ligand with SO₃H substituent in benzene ring.     

Radiochemical and Quantum-Chemical Study of the Migration of the Cationic Center in the SiC6H 7 + Ion

The reactions of silylphenyl and phenylsilyl cations with methyl tert-butyl ether were studied radiochemically. The phenylsilylium ion is more stable thermodynamically than its isomers with the charge localized on carbon atoms of the benzene ring. Because of the high activation barriers to migration of the cationic center over the benzene ring (according to B3LYP/6-31G* calculations), only the ortho isomer can transform into the silylium ion.     

Structural variations and electronic substituent effects in phenylcubane derivatives: a quantum chemical study

Electronic substituent effects in 4-substituted 1-phenylcubane derivatives, Ph–C₈H₆–X, have been investigated from the structural changes caused by the substituent X. The molecular structures of 34 derivatives with charged or dipolar substituents have been determined from quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. Geometrical variations caused by substitution appear both in the cubane framework and in the benzene ring, but the two kinds of changes show no correlation. The rather small changes in the benzene ring geometry are caused by long-range polar effects (field effects), while the larger changes in the cubane cage are controlled primarily by electronegativity effects. A structural parameter measuring the long-range polar effect of the substituent, S _F^CUB, has been derived from the geometry of the phenyl group acting as a probe. This parameter correlates well with the calculated gas-phase acidities of 4-substituted cubane-1-carboxylic acids, HOOC–C₈H₆–X, and with other indicators of long-range polar effects obtained from bicyclo[2.2.2]octane derivatives. The correlations can further be improved by introducing a resonance parameter as an additional explanatory variable. This indicates that the electron delocalization resulting from hyperconjugative interactions between substituent and cage modifies the long-range polar effect of the substituent. Strong hyperconjugative interactions between some charged substituents and the cubane cage result in remarkable variations in the cage geometry, superimposed onto those ascribed to electronegativity effects.     

Proton Migration in Benzene Complexes of Methyl and Silyl Cations

Quantum-chemical calculations at the B3LYP/6-31G(d,p) level of theory were used to optimize stationary points in the C₆H₆ + H₃X⁺ (X = C, Si) systems. At X = Si, the adduct of the cation with benzene is the most stable isomer, whereas at X = C, the para isomer is more stable than the adduct (ipso isomer). This difference is explained in terms of charge distribution in the benzene ring in toluene and phenylsilane: In the latter, the negative charge on the carbon atom attached to silicon is much higher than on the other carbon atoms, unlike toluene in which the highest negative charge is on the carbon atom para to the methyl group. Proton migration from the ipso to para position requires the lowest (X = C) and highest (X = Si) barriers to be overcome compared with the other barriers to proton migration over the benzene ring. These barriers and relative stabilities of the isomers correlate well with the charge distribution on benzene carbon atoms in toluene and phenylsilane.     

Model porphyrin precursors: 1,2,4,5‐tetrakis(cyanomethyl)­benzene,methyl 3,4,5‐triacetoxy­benzoate and 2‐(N‐phthalimidomethyl)benzoic acid

In the crystalline state, the centrosymmetric mol­ecule 1,2,4,5‐tetrakis­(cyano­methyl)­benzene, C₁₄H₁₀N₄, has one cyano­methyl group in the benzene plane and one cyano­methyl group rotated 67.2 (2)° out of the benzene plane. Molecules of methyl 3,4,5‐tri­acetoxy­benzoate, C₁₄H₁₄O₈, form chains with each mol­ecule twisted 89.6 (1)° from the preceding mol­ecule. In this orientation, a close C—H?O contact is formed, with an H?O distance of 2.34 Å. The structure of 2‐(N‐phthalimido­methyl)­benzoic acid, C₁₆H₁₁NO₄, reveals hydrogen‐bonded dimers linked by the carboxyl groups of adjacent mol­ecules. The O4?O3 distance is 2.636 (2) Å and the O4—H?O3 angle is 171 (2)°.     

Darstellung sowie Kristall- und Molekülstruktur von Triphenylphosphinoxidhydrogenfluorid, (C6H5)3PO · HF

Preparation, Crystal and Molecular Structure of Triphenylphosphineoxide Hydrogen - fluoride (C₆H₅)₃PO · HF (C₆H₅)₃PO · HF was prepared from hydrofluoric acid (40%) and (C₆H₅)₃PO in benzene. It crystallizes in the monoclinic space group P2₁/c with a = 1 032.8(3), b = 1 051.0(7), c = 1695.5(2) pm, β = 121.95(2)° and Z = 4; d (calc./obs.) 1.27/1.26 g ° cm^?3. The structure was determined by direct methods from 2 709 independent reflections and has been refined by full matrix least squares methods to R = 0.049. In the compound HF and (C₆H₅)₃PO are linked by a short H-bond. Some distances: O? F 238.4(5), O? H 142.3, H? F 99.8, P? O 149.5(4) pm. Angle O? H? F 159.8°.     

The Dowd-Beckwith ring expansion: a theoretical study

UB3LYP/6-311++G(d,p) and ROMP2/6-311++G(d,p)//UB3LYP/6-311++G(d,p) calculations including the effect of benzene solvent through the PCM-UAHF method render a concerted mechanism without fragmentation as the most favourable one for the Dowd-Beckwith radical ring expansion of the bromomethyl adduct of methyl cyclopentanone-2-carboxylate to yield methyl cyclohexanone-3-carboxylate. The corresponding concerted TS is a bicyclic alcoxy radical.     

Cooperative intramolecular hydrogen bond and conformations of thiocalix[4]arene molecules

The joint FTIR spectroscopic study and ab initio quantum-chemical calculations (HF/3-21G and PBE/TZ2P methods) showed that thiocalix[4] arene molecules adopt the cone conformation in CCl₄ solutions. The weakening of the cooperative intramolecular H bond in thiocalix[4]arenes compared to the corresponding calix[4]arenes can be due to the larger thiocalixarene macrocycle, bifurcated hydrogen bond in it, and electron density transfer from the bridging S atom to the benzene ring.     

Oxidation of aromatic hydrocarbons with H2O2 catalyzed by a nano-scale tubular aluminosilicate, Fe-containing imogolite

Imogolite is a nano-scale tubular clay mineral with the typical chemical composition (OH)₃Al₂O₃SiOH. The Fe-containing imogolite was synthesized from FeCl₃, AlCl₃ and Na₄SiO₄ aqueous solutions. The fibrous morphology observed by AFM in this sample is similar to that of synthetic imogolite. In the catalytic oxidation of aromatic hydrocarbon compounds such as benzene, phenol and chrolobenzene, the aromatic ring was oxidized. When the side-chain is a hydrocarbon group such as methyl, both benzene ring and the side-chain group were oxidized. It was found that the side-chain group was preferentially oxidized.     

Kinetics of the hydrogen atom reactions with benzene,cyclohexadiene and cyclohexene: hydrogenation mechanism and ring cleavage

The hydrogen atom reaction with benzene and the subsequent elementary reactions with H-atoms were studied in detail, using a fast gas flow in a linear reactor at pressures in the mbar region, with a mass spectrometer for the product analysis. The rate-constant determinations were based on a kinetic model, which includes the strong catalytic H-atom recombination on the wall, caused by adsorbed reactant molecules, and also corrects for the pressure drop within the reactor. The H-atom concentration was determined by scavenging with NO₂. The method was checked by determining the rate constant k(H + trans-butene-2) = (4.6 ± 1.2) × 10⁸ M^?1 s^?1, which agrees with the literature value of Daby et al. within experimental error limits. The rate constants determined are:

H + benzene is the rate determining step for benzene hydrogenation. From the rate constant for H + C₆D₆ it is concluded, that benzene is reformed from some intermediate reaction products (C₆H^*₇ and/or C₆H^*₈). These back reactions should be suppressed at high pressures, in agreement with results by Sauer and Ward (1–54 bar). The mass spectra show that H + benzene at mbar pressures predominantly initiates ring cleavage to form methyl radicals, methane, and C₂-hydrocarbons as the main products. However for H + cyclohexene 85% of the products is cyclohexane. The results for H + cyclohexadiene are intermediate to these extremes. It is argued that accumulation of vibrational energy over two consecutive reactions must be responsible for the ring cleavage, which most likely occurs from C₆H^**₈ and C₆H^**₁₀.     

Synthesis, biological activity of salidroside and its analogues

Salidroside is a phenylpropanoid glycoside isolated from Rhodiola rosea L., a traditional Chinese medicinal plant, and has displayed a broad spectrum of pharmacological properties. In this paper, about 18 novel salidroside analogues were prepared through Koenigs-Knorr method, the effects of these compounds over PC12 was assessed with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. The novel compounds differ in the substituents attached to the benzene ring or in the glycosyl donor. According to the data, compounds (3,5-dimethoxyphenyl)methyl β-D-glucopyranoside and (3,5-dimethoxyphenyl)methyl β-D-galactopyranoside with methoxy group at 3 and 5-positions of the benzene ring were the most viability at concentration of 300 μmol/l and 60 μmol/l, respectively.     

Steric and Electronic Structure of C6H5XCF3 Molecules (X = O or S): A Quantum-Chemical Study

The potential functions of internal rotation around the Csp ²-X bond in C₆H₅XCF₃ molecules (X = O or S) were obtained by quantum-chemical calculations in the HF/6-31G(d), MP2(f)/6-31G(d), and B3LYP/6-31G(d) approximations. The calculations were performed in the range of torsion angles (angle between the planes of the benzene ring and Csp ²-X-Csp ³ bonds) from 0° to 90° with a 15° step. The barriers to rotation around the Csp ²-X bonds (kJ mol^{- 1}) were evaluated: for C₆H₅XCF₃, 7.60 (HF), 3.04 (MP2), and 1.04 (B3LYP); for C₆H₅XCF₃, 16.57 (HF), 14.67 (MP2), and 8.73 (B3LYP). The geometries (bond angles and bond lengths), Koopmans ionization potentials, and dipole moments of the molecules were calculated. The hybridization, energy, and population of the lone electron pairs of the heteroatoms, and also the energy of their resonance interaction with antibonding orbitals and the natural atomic charges were evaluated using the NBO approach.     

Conformational Flexibility of Benzannelated Derivatives of Six-Membered 1,2- and 1,4-Dihydroaromatic Rings Containing No Saturated Carbon Atoms

The effect of benzannelation on the conformational flexibility of the six-membered 1,2- and 1,4-dihydroaromatic rings containing no saturated carbon atoms is studied by the ab initio HF/6-31G(d, p) method. It is shown that replacement of the C=C bond by the benzene ring leads to enhanced conformational flexibility of the ring due to weaker conjugation interactions involving the bridging carbonyl group or the heteroatom. The dihydro ring remains flexible even when included in the tetracene fragment. In all cases, the 1,2-dihydroaromatic rings are less rigid than their 1,4-dihydroanalogs.     

Theoretical Investigation of the Additivity of Structural Substituent Effects in Benzene Derivatives

The additivity of substituent effects in 1,3- and 1,4-disubstituted C₆H₄X₂, and 1,3,5-trisubstituted C₆H₃X₃ (X=F, Cl, CN, NO₂, CH₃, CF₃, NH₂, OH) benzene derivatives on the ring geometry has been investigated. The analysis is based on ab initio calculations at the MP2/6-31G** level of theory. The substituent impacts on the benzene ring are generally in good agreement with the results reported in earlier experimental and lower level theoretical studies. The impacts determined in the monosubstituted benzenes were used to estimate the ring distortions in the di- and trisubstituted derivatives. The estimated ring CC bond distances agree generally within 0.001 Å and the estimated CCC bond angles within 0.3 degree, with the optimized ones. The best agreement (deviations up to only 0.0003 Å and 0.03 deg.) between the estimated and optimized geometrical parameters was obtained for the CH₃ derivatives. Generally, the para-disubstituted derivatives showed the best compliance with additivity, somewhat poorer agreement characterized the meta derivatives while the trisubstituted derivatives showed angular distortions of up to about 0.4°.