Π-bond Order Research Articles

The mechanism of thermal eliminations. Part 18. Relative rates of pyrolysis of 2-ethoxypyrazine, 3-ethoxypyridazine, 2- and 4-ethoxypyrimidine, 3-chloro-6-ethoxypyridazine, and 2-chloro-4-ethoxypyrimidine: the effect of the aza ‘substituent’ and π-bond order on the elimination rate

A kinetic study has been made of the first-order thermal decomposition of the title compounds into ethvlene and the corresponding aza-substituted pyridines, between 650 and 713 K. The relative elimination rates at 650 K are (2-ethoxypyridine = 1): 0.545, 10.0, 1.03, 1.12, 9.68, and 3.28, respectively. The electronic effects of the aza ‘substituent’ are small, and a more important factor appears to be the C–N π-bond order; this latter accounts for the high reactivity of the pyridazines. The effects of the chloro substituent and of the aza ‘substituent’ are explicable in terms of a balance between electron withdrawal from the C–O bond (producing activation) and from the nitrogen involved in the cyclic transition state (producing deactivation). The effects of the chloro substituents confirm that the most important step of the reaction is breaking of the C–O bond. The statistically corrected rate (per ring nitrogen) of 2-ethoxypyrimidine is unexpectedly low. This may reflect difficulty in achieving the coplanar transition state in which the lone pairs in the s-orbitals of oxygen and the nitrogen not involved in the elimination are brought into close proximity.

The mechanism of thermal eliminations. Part 21. Rate data for pyrolysis of 2-ethoxyquinoline, 1- and 3-ethoxyisoquinoline, and 1-ethoxythiazole: correlation of reactivities with π-bond order of the CN bond

We have measured the rates of thermal elimination of ethylene from the title compounds between 587.3 and 722.9 K. The reactivities relative to 2-ethoxypyridine at 650 K are: 3-ethoxyisoquinoline (0.21), 2-ethoxyquinoline (3.13), 1-ethoxyisoquinoline (6.47), 2-ethoxythiazole (63.1). These reactivities parallel the π-bond order of the CN bond, though the exceptional reactivity of 2-ethoxythiazole is attributed to additional acceleration through +M electron release from sulphur to nitrogen. This emphasizes the greater relative importance of nucleophilic attack by the nitrogen upon the β-hydrogen atom as compared with the analogous mechanism for pyrolysis of esters. Because of the semi-concerted nature of the reaction, interruption of aromaticity is much less significant than in, for example, electrophilic aromatic substitution. Thus retention of the benzenoid character of the ring not involved in the elimination is not an important rate-determining feature, as shown by the lower reactivity of 3-ethoxyisoquinoline relative to 2-ethoxypyridine. The unimportance of the interruption of aromaticity of the benzenoid ring means that conjugative effects are better relayed to nitrogen in the β-naphthalene-like position (isoquinoline) than in the α-naphthalene-like position (quinoline). This is the reverse of the familiar pattern for reactions of naphthalene-like systems where full charges are involved, and may be an additional factor contributing to the higher reactivity of 1-ethoxyisoquinoline than of 2-ethoxyquinoline, as may also be the –I effect of the benzo substituent. The conclusions are used to predict elimination rates for alkoxyheterocycles not yet studied.

Electronic structure and conformation of ethene when adsorbed on transition and noble metals

Using the geometry of the ethene molecule when adsorbed on transition metal surfaces, as predicted by Felter and Weinberg [Surface Sci. 103 (1981) 265], but omitting molecular twist, a tight-binding model has been employed to predict the effect of chemisorption on the σ-levels of the molecule. The energy of the σ CC-level changed appreciably from its free space value, and based on these calculations we have reinterpreted the experimental data and have revised the prediction of the geometry. The qualitative nature of the distortions, namely an increase in the C-C bond length and reduction of the C-C-H and H-C-H angles, remains intact. Secondly, the interaction of the π-orbitals of ethene with the d-band states of the metal surface has been studied, using a model of localized bonding between the metal and the molecule, within the Hartree-Fock approximation using an Anderson Hamiltonian. The conclusions are that as the strength of the metal-molecule interaction increases, the π-bond order decreases. Using experimental values of the chemisorption energy the model predicts that, for Ni and Cu, electrons are transferred from the molecule to the metal. This is in accord with the observed decrease in the work function for both these systems.

Hydrogen bonding between liganded CO and heme-proteins

Perturbations of the CO bond in carbon monoxide bound to iron in different heme-proteins are discussed. Properties evaluated from Hartree-Fock-Slater electronic structure calculations are correlated with experimental observations. The net charge on oxygen (HFS) is related to the Fe(III)/Fe(II) reduction potential (EMF). Consequently, proteins with low EMF, e.g. peroxidases, have more ability to form FeCOH…H. protein hydrogen bonds than, for example, oxidases with higher EMF. This can explain anomalous vibrational frequencies, ν CO, observed for acidified peroxidases. Calculations reveal that direct interaction between CO and polar groups on the protein can cause peturbations of the CO π-bond order similar to those from EMF shifts. Dioxygen and carbon monoxide are compared in regard to hydrogen bonding to heme-proteins.

Effect of conjugation beyond multiple bonds on geometry and acidity

A quantum chemical quantity referred to as the π-resonance integral contribution to energy is introduced as a measure of the quantum mechanical interaction between two atoms in the π-system of a molecule. The geometries of simple molecules related to gas-phase acidity as well as the protonated and the deprotonated species were calculated by the CNDO/2 method, which turns out to be fairly good in reproducing relative geometries. The resulting bond lengths are discussed in relation to π-bond order and π-resonance integral contribution to energy. Acidities are also discussed.

A 1H and 13C nuclear magnetic resonance study of nucleosides with methylated pyrimidine bases

Alkylated pyrimidine bases are of interest from the viewpoint of mutagenesis and carcinogenesis. 1H nuclear magnetic resonance data are presented for a series of ribosides and arabinosides alkylated at the O2,O4, N3, and C5 positions of the pyrimidine base. The data provide information about the stereochemical effects of base methylation. The J(5—6) proton coupling constants show that O-alkylation leads to a decrease in the π-bond order of the C5—C6 bond. The 13C chemical shifts are related to the tautomeric changes effected by O-alkylation.

ChemInform Abstract: VIBRATIONAL SPECTRA, FORCE FIELDS AND ELECTRONIC STRUCTURES OF SYDNONES

Abstract Normal vibrational analyses of 3-methylsydnone and 3-methylsydnone-d4 have been performed. On the basis of the set of force constants obtained, the electronic distribution within the mesoionic ring is evaluated and compared to the results of MO calculations. The π-bond order of the sydnone carbonyl group is shown to be lower than that in alicyclic esters and the unusually high “carbonyl stretching” frequency is due to the contributions from other coordinates to this mode. The splitting of the “carbonyl stretching” band observed in the spectra of 3-methylsydnone and of related derivatives is explained by the strong kinematic coupling between mesoionic bonds.

Vibrational spectra, force fields and electronic structures of sydnones

Normal vibrational analyses of 3-methylsydnone and 3-methylsydnone-d 4 have been performed. On the basis of the set of force constants obtained, the electronic distribution within the mesoionic ring is evaluated and compared to the results of MO calculations. The π-bond order of the sydnone carbonyl group is shown to be lower than that in alicyclic esters and the unusually high “carbonyl stretching” frequency is due to the contributions from other coordinates to this mode. The splitting of the “carbonyl stretching” band observed in the spectra of 3-methylsydnone and of related derivatives is explained by the strong kinematic coupling between mesoionic bonds.

Gekreuzt-konjugierte polyene: II. Semiempirische Rechnungen zur Konformationsanalyse des Bi-(4,4- dimethyl-2,5-cyclohexadien-1-yliden)

CNDO/2- and MNDO-calculations of bi-(4,4-dimethyl-2,5-cyclohexadiene-1-ylidene) have been performed in order to investigate its conformation. It is demonstrated that the compound has a chair-like shape with folded six-membered rings in the fundamental state. As opposed to biphenyl a torsion around the central bond is not found. To determine the structure-specific π-bond order distributions of linear and of cross-conjugated polyenes results of PPP-caIculations are presented.

Two-dimensional hückel molecular orbital theory

The conventional Hückel molecular orbital (HMO) theory is extended to include hydrocarbons with sp-hybridized C atoms in addition to the sp 2-hybridization. MOs are constructed as linear combinations of 2 p-orbitals in two dimensions perpendicular to each other. The resulting two parts of the π system (viz. π′ and π″) are assumed to be independent. The total π-bond order is assumed to be obtained by the addition of contributions from π′ and π″. The two-dimensional HMO theory covers hydrocarbons with acetylenic (CC) bonds and cumulated CC bonds. The theory is applied to vinylacetylene, diacetylene, allene, butatriene and divinylacetylene. CC bond lengths calculated from the theoretical π-bond orders are compared with experimental data. Agreements between calculated and observed bond lengths within ±0.03 Å are found.

INDO MO calculations of the electronic structures of pyrolle, imidazole, and derivatives

INDO MO calculations on a series of N-substituted pyrroles and imidazoles have been analysed for substituent effects. Some of the basic characteristics of the σ I and σ R 0 parameters are reflected in the calculated electron densities of the compounds studied. For example, good correlations are obtained between Δ qσ N(1)/ΣΔ qσ parameters and σ I for the R substituted compounds, as well as between ΣΔ qπ values and σ R 0 for the + R derivatives. The + R substituents lead to an increased localization of the π-bonds, whereas R substituted derivatives show an increased delocalization, i.e., the π-bond orders across C(2)C(3) [or C(2)N(3)] and C(4)C(5) decrease and those across other bonds in the ring increase.

Vanadium-51 and oxygen-17 nuclear magnetic resonance study of vanadate(V) equilibria and kinetics

Approximately 15 vanadate(V) and polyvanadate(V) species have been identified by 51V n.m.r. in the range pH 7–14, mostly for the first time. Oxygen-17 n.m.r. spectroscopy confirms in several cases that the vanadium coordination is tetrahedral, and that the 17O shifts are correlated closely with formal π-bond order. Most linear and cyclic catenations of VO4 tetrahedra are found up to V6, but there is no branched or triprotonated species or cyclic trimer. Several equilibrium constants and pKa values are reliably measurable. The ion [HVO4]2– rather than OH– is shown to be the dominant nucleophile in the two main ring-breaking reactions, and a simple kinetic pathway is proposed for the formation of decavanadate ion.

The Effect of Configuration Interaction on the π-Bond Orders, and the Molecular Geometry of Coronene in Its Ground State

Abstract Semiempirical MC-LCAO-MO calculations have been made on the ground-state coronene. The inclusion of the off-diagonal terms in the MC bond-order formula is, in this case, essential to improve the bond orders and the bond lengths over the usual single configuration calculation.

The absolute configuration of organometallic compounds : VII. An x-ray structural study of (−) 579436-(η 5-C 5H 5)Mo(CO) 2[(C 5H 5N)C(=S)NR] prepared from H 2NR = ( S)-(−)-α-phenylethylamine

The reaction of (η 5-C 5H 5)Mo(CO) 3Cl with pyridine-2-carboxylic acid [( S)-1-phenylethylthioamide] yields two diastereoisomers having composition (η 5-C 5H 5)Mo(CO) 2(thioamide) which are in equilibrium with one another in the ratio of 58/42 (toluene; 90°C). The diastereoisomers are separable by chromatography, and that (V; see text) whose CD spectrum shows (−) and (+)-bands, respectively, at 579 and 436 nm was used in the current crystallographic study. The space group is P2 12 12 1 and the cell parameters are: a 6.950(4), b 14.038(4) and c 20.709(8) Å; V 2020.4 Å 3; d(obs; flotation in aqueous ZnBr 2) 1.51 g cm −3; d(calc; Z = 4) 1.507 g cm −3. A total of 3321 data were recorded in one octant of reciprocal space (2θ max 60.0°) of which 2134 were considered observed ( I > 2σ( I)) and used in the solution of the structure and in the least-squares refinement. The molecules consist of a central Mo atom surrounded by an approximately square-pyramidal array of five ligands: the bidentate thioamide, two carbonyls and the Cp ring. The binding points of the thioamide are the nitrogen of the pyridine and the sulfur of the thioamide moiety. Thus, unlike previous examples of CpMo(CO) 2(thioamides) studied here, the nitrogen of the thioamide is not part of the chelate ring; instead, it is part of an unusual, three atom, conjugated system (SCN) never observed before in thioamide-metal chelates. The SMoN, SMoC(O), NMoC(O) and (O)MoC(O) angles are 76.9(2), 75.2(3), 81.4(4) and 75.6(5)°, values which are typical of square-pyramidal compounds in which the metal is above the plane defined by the basal ligands. The bonds to the carbons of the C 5H 5 ring are normal but the two MoC(O) and the two CO distances are slightly different, and the differences make sense if we attribute them to inequalities in trans effect between S and N ligands opposite the carbonyls. The MoN(pyridine) bond (2.233(8) Å) is a little longer than those observed in thioamides but nearly the same as that observed in Schiff base derivatives of related CpMo(CO) 2 compounds studied here earlier. Concomitantly, in V there is a shorter MoS bond than those observed in the thioamides containing normal, four-membered (MoSCN) chelate rings. Parallel with this, V contains longer SC bonds showing that an increase in the MoS bond order leads to a reduction of the SC π-bond order, which in this case is estimated to be only 0.47. Finally, the plane of the chelate ring (MoSCCN) and that of the pyridine ring make an angle of 8.7°, even though the two are fused to each other. The absolute configuration at the Mo site of the [(−) 579, (+) 436] diastereoisomer has been established as ( S).

Internal rotation of the NN-dimethylamino-group in aromatic and heteroaromatic systems

The origin of the rotational barrier of the dimethylamino-group in several aromatic and heteroaromatic systems has been analysed by a comparison of the results of MO calculations and experimental energies. For most of the compounds considered theory predicts that the dimethylamino-group should be coplanar with the attached ring in the ground state, except in a few cases where steric effects are present. For NN-dimethylaniline nitrogen inversion was also studied for several conformations (different angles of rotation θ around the C–N bond) by ab initio MO calculations with a limited basis set. It was thus found that, while the pyramidal structure is stable both for the planar and perpendicular conformations, the ground state of the molecule should correspond to the fully planar conformation averaged over two rapidly equilibrating pyramidal structures. The rotational behaviour for the planar-averaged dimethylamino-group was calculated for several angles of rotation by different MO approaches, namely extended Hückel, CNDO/2, INDO, PCILO, and ab initio STO-3G. The different MO approaches provide qualitatively similar results with the planar conformation corresponding to the ground state, except extended Hückel which indicates the perpendicular conformation to be more stable. The π-bond order given by CNDO/2 shows a linear dependence on the experimental free-energy of activation (ΔG*), and it appears that the resulting equation can be usefully employed for predicting the ΔG* values for different systems. Even the energy difference, ΔE, are roughly proportional to the corresponding ΔG* values, but the correlation is less satisfactory than that obtained with the π-bond order. Provisions of energy barriers with the above equations for a number of protonated and alkylated forms show that in molecules where conjugation is expected the barriers should increase. This has been verified experimentally in two molecules where measurements were made possible.

Conformational behaviour of organic carbonyl compounds. Part 1. A molecular orbital approach to the study of internal rotation in conjugated aldehydes and ketones

Semi-empirical and ab initio MO methods are employed in a study of internal rotation of conjugated aldehydes and ketones. In general CNDO/2 does not provide a correct pattern for the energy as a function of the angle of internal rotation. The results from the semi-empirical PClLO method are more consistent with experiment. The calculated energy barriers are noticeably lower than the experimental ones, but roughly proportional. The energy barriers calculated by the ab initio MO approach with the minimal STO-3G basis set behave correctly as regards the relative stability of planar and perpendicular conformations, but quantitatively are not comparable with experimental values. The π-bond order for the C–C bond joining the carbonyl group to the unsaturated system for the planar conformation shows a linear correlation with the experimental free energy of activation for a large number of molecules and this is true of the values obtained both by CNDO/2 and by ab initio STO-3G.

13C, 1H-Spin-Spin Kopplungskonstanten Teil III: Naphthalin / 13C, 1H Spin-Spin Coupling Constants Part III: Naphthalene

Abstract Using a newly developed technique that employs highly deuterated compounds and 2H broadband decoupling the 13C, 1H spin-spin coupling constants for naphthalene have been measured from the 13C NMR spectra of α-and β-H-[D7]naphthalene. A linear dependence of the 3J(13C, 1H) data on the central π-bond order is indicated.

A microstructural model of solvatochromism

Abstract A new microstructural model of solvatochromism is suggested and used to describe the solvent induced changes of both the electronic spectrum and structure of a simple pentamethinemerocyanine dye molecule. The electrostatic potential on the surface of a molecule is calculated from the net charges at the atomic centres, which are accessible from semi-empirical all-valence electron MO calculations. The “real” molecular potential thus obtained generates, in a solvent continuum, a reaction field. The re-action of this field on the molecule is included in the Hamiltonian of a quantum-chemical calculation reduced to the π-electron orbitals. The strength of the solvent interaction is proportional to a perturbation parameter λ and includes dispersion, polarisation and specific interaction forces such as hydrogen bonds. The results of the model are in good agreement with the solvent dependent experimental data such as transition energies, oscillator strengths, π-electron densities, and π-bond orders. Another consequence of the model is that the dipole moments of solvatochromic dyes must be strongly solvent dependent already in the ground state.

Crystal and molecular structure of tetramethyl- N, N′-diphenylcyclodisilazane

The crystal structure of tetramethyl- N, N' -diphenylcyclodisilazane is reported. The molecule has a centre of symmetry and the phenyl groups are almost coplanar with the cyclodisilazane ring. SiN bond distances are 1.739(3) and 1.749(3) Å, the mean SiC bond length is 1.849 and the CN bond length is 1.382(4) Å, while the NSiN and SiNSi bond angles are 85.7(1) and 94.3(1)°, respectively. Considerable bathochromic shifts are found in the UV spectra of the compound compared with that of N-phenyl(hexamethyl)disilazane. Structural and spectroscopic data show enhanced delocalization. The π-bond orders calculated by the PPP method are p(CN) 0.325 and p(SiN) 0.346.

Theoretical study of charge distribution in the benzoyl and alkynoyl cations

SCF-MO calculations, in the lNDO approximation, have been carried out on benzoyl (1), propynoyl (2), but-2-ynoyl (3), 3-phenylpropynoyl (4), and 3-fluoropropynoyl cations (5). In each case the geometries were optimized to minimum energy. In aggreement with 13C n.m.r. observations, significant charge delocalization into the phenyl rings occurs in (1) and (4). From an examination of the charge distributions, π-bond orders, and π- and σ-electron distributions it is shown that extensive delocalization of charge occurs in (2), (3), and (5), contrary to previous interpretations of 13C spectra. Thus all the mesomeric forms RCC–CO+↔RCC–[graphic omitted]O↔R[graphic omitted]z.dbd6;CCO contribute significantly in (2), (3), and (5) as well as in (4), and the charge is not substantially localized on oxygen as previously claimed.