Benzene oxide, calculations

MINDO/3 calculations, 7, 500 Benzene oxide dipole moment, 7, 553 Benzenes substituted... 

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

Consequently, the benzene oxidation mechanism was further developed by considering additional decomposition and oxidation steps. Sethuraman et al. proposed that phenyl radical decomposition can occur by either of two key pathways (3-scission of phenyl radical or by breakdown of the phenylperoxy radical formed by the oxidation of phenyl radical (Fig. 9). Using PM3 calculations,which were ultimately verified by DFT studies,Carpenter predicted that another species, 2-oxepinoxy radical (3 in Fig. 9b), is an important intermediate due to its relative stability, formed via a spirodioxiranyl intermediate (2 in Fig. 9b) from phenylperoxy radical. Pathway A in Fig. 9b is the thermodynamically preferred pathway at temperatures increasing up to 432 K, while pathway B has an entropic benefit at higher temperatures. While pathway B essentially matched the traditional view of benzene combustion, pathway A introduced a new route for phenylperoxy radical, which could resolve discrepancies observed using previous models. 

The spontaneous oxepin-benzene oxide isomerization proceeds in accordance with the Woodward-Hoffmann rules of orbital symmetry control and may thus be classified as an allowed thermal disrotatory electrocyclic reaction. A considerable amount of structural information about both oxepin and benzene oxide has been obtained from theoretical calculations using ab initio SCF and semiempirical (MINDO/3) MO calculations (80JA1255). Thus the oxepin ring was predicted to be either a flattened boat structure (MINDO/3) or a planar ring (SCF), indicative of a very low barrier to interconversion between boat conformations. Both methods of calculation indicated that the benzene oxide tautomer... 

The dipole moments of oxepin and benzene oxide have been calculated to be in the range 0.76-1.36 D and >1.5 D respectively using the ab initio SCF and MINDO/3 methods (80JA1255). The lower calculated dipole moment would be in accord with experimental observations where the equilibrium was found to favor oxepin (7) in less polar solvents. Coordination between the oxirane oxygen atom and polar solvent molecules would also strengthen the C—C bond of the epoxide and thus lead to a preference for the benzene oxide isomer <72AG(E)825). Thus the proportion of oxepin (7) was found by UV spectral analysis to be higher in isooctane solvent (70%) than in water-methanol (10%). 

Similarly, the fusion of an aromatic ring to the oxepin-benzene oxide system was found to drive the equilibrium toward extremes in either direction. The calculated resonance energies for oxepins (26), (27) and (28) were 4.81, 78.46 and 81.72 kJ mol-1 respectively (70T4269). These calculated values concur with experimental observations since oxepins (27) and (28) have been synthesized and are relatively stable compounds. The formation of 2-benzoxepin (26) from naphthalene 1,2-oxide would involve a considerable loss in resonance energy to the system and has not been detected spectroscopically (67AG(E)385). 

Using both semi-empirical and ab initio calculations the study of oxepin (6), benzene oxide (7), and their equilibrium (Scheme 2) has been conducted. The fully optimized geometry (90MI902-01) agrees with that experimentally found for several substituted oxepines. The carbon skeleton of benzene oxide is practically planar while the angle between the epoxide ring and the adjacent plane is ca. 106°. The oxepin molecule is boat-shaped with a fold angles between C2—C7 and C3—C6 of ca. 137 and 159°, respectively. 

Substitution of methyl groups on the oxirane ring tilts the stability of the tautomers in favor of oxepin. Thus 1-methylbenzene oxide (154) exists as 2-methyloxepin (155), in rapid equilibrium with the benzene oxide tautomer 154.74 The AH has been calculated as 0.4 0.02 kcal/mol, i.e., 1.3 kcal/mol... 

Valence tautomers, benzene oxide 1 and oxepine 2 (Equation 1), as well as relative tautomeric systems, benzene sulfide-thiepine and o-xylene-2,7-dimethyloxepine, have been studied by a post-Hartree-Fock (HF) ab initio QCISD(r)/6-31G //MP2/6-31G method. In particular, the enthalpy calculated for a benzene oxide-oxepine system is 0.59 kj moF1 <1997PCA3371>. The calculated molecular orbital (MO) energies are in linear relationship to those from the photoelectron (PE) spectra <1996JCF1447>. Barrier to tautomerization for a benzene oxide-oxepine system is 29.4 kj mol-1. Protonation stabilizes the oxide form versus the oxepine <1997PCA3371>. 

Thermodynamic parameters for the benzene oxide-oxepine system are calculated at MP4(SDQ)/6-31+G //HF/ 6-31G level of theory. The effect of solvent polarity on the above equilibrium is studied using the isodensity polarized continuum method. Low polar solvents favor the oxepine formation, whereas medium to high polar solvents lead to benzene oxide formation. The transition state for the tautomerization is fully characterized and the activation energies for the forward and reverse reaction are estimated to be ca. 9.5 and 11.0 kcal mol-1, respectively. The solvent polarity exerts a reasonable effect decreasing the activation energies up to 4 kcal mol-1 <2001MI471>. 

Additional evidence for the rapid equilibration in the benzene oxide-oxepin system la-lb has accrued from a range of molecular orbital (MO) calculations. ... 

Preferred geometry of the benzene oxide-oxepin system can be predicted by molecular orbital methods. Thus benzene oxide la is predicted to be markedly non-planar (with the epoxide ring at an angle of 73° to the benzene ring), while the oxepin lb has been predicted to prefer a shallow boat structure (MINDO/3) or a planar structure ab initio) As previously mentioned, the proportion of each tautomer present at equilibrium is both temperature and solvent-dependent. Molecular orbital calculations have been used to rationalize the solvent effects, both in terms of the more polar character of the arene oxide that is favored in polar solvents and the strengthening of the oxirane C-C bond upon coordination of the oxygen atom lone pair in polar solvents. Thus values in the range 1.5-2.0 D and 0.76-1.36 D for the dipole moments of arene oxide la and oxepin lb have been calculated. 

Semiempirical and ab initio calculations were carried out to investigate the relative stabilities of O-protonated benzene oxide and its related carbenium ions and to obtain further insight into the mechanism of the acid-catalyzed isomerization of the benzene... 

Acid-base properties of oxide surfaces are employed in many fields and their relationship with PZC has been often invoked. Adsorption and displacement of different organic molecules from gas phase was proposed as a tool to characterize acid-base properties of dry ZnO and MgO [341]. Hammet acidity functions were used as a measure of acid-base strength of oxides and some salts [342]. Acidity and basicity were determined by titration with 1-butylamine and trichloroacetic acid in benzene using indicators of different pAg. There is no simple correlation between these results and the PZC. Acid-base properties of surfaces have been derived from IR spectra of vapors of probe acids or bases, e.g. pyridine [343] adsorbed on these surfaces. The correlation between Gibbs energy of adsorption of organic solvents on oxides calculated from results obtained by means of inverse gas chromatography and the acceptor and donor ability of these solvents was too poor to use this method to characterize the donor-acceptor properties of the solids [344],... 

Such a complex is not stable enough to be isolated but its occurrence was inferred by the IR spectra of the solutions. Drago s suggestion is confirmed by ah initio calculations. Therefore, the improved selectivity observed upon carrying out benzene oxidation in sulfolane may be due to the formation of this large species, which can not enter the titanium silicalite pores, thus allowing phenol to remain relatively protected against further oxidation. 

Hammar (107) studied benzene oxidation over catalysts consisting of V20B on supports, and found that the rates of maleic anhydride, CO, and C02 formation were proportional to benzene concentration (the reaction order was close to first, as calculated from the degree of conversion as a function of the contact time). The activation energies were equal and amounted to 24 kcal/mole. The reaction order with respect to benzene, as determined from initial rates, varied from zero to second order. 

Semiempirical (AMI) and ab initio (6-31G) calculations of naphthalene oxides (3)-(5) show that the benzene nucleus retains benzenoid character while in both benzene oxide and oxepin a polyenic nature is found <91 Ml 902-01). Molecular mechanics calculations were used in conformational studies of some complicated oxepanes, i.e. 3,3,6,6-tetramethyloxepan-4,5-dione <83JCS(P2)i35l) and a 14/ -steroid possessing an oxepan ring as a 17j -substituent <86ZN(C)297). 

Keywords Zeolites Bivalent metal cation stabilization ZnZSM-5 FeZSM-5 Ethane dehydrogenation Benzene oxidation DFT cluster and periodical calculations Carbon oxide Carbon dioxide Carbonate Carbonilation... 

Our quantum-chemical simulation of benzene oxidation reaction based on pseudospinel iron center (see Fig. 20.36, bottom) reveals the same structure. The characteristic feature of such intermediate is the presence of C(sp )-H bond. The presence of the C(5/7 )-H bond intermediate was confirmed by in-situ IR experiment of Panov et al. [84]. The IR band at 2874 cm appeared immediately after benzene was fed to the FeO catalyst. At the same time no phenol signals were detected. Heating of the sample resulted in complete disappearance of this band. According to our quantem-chemical simulation only the a-complex structure has the characteristic of this IR band. For benzene oxide, which also has two C(sp )-H bonds, this band is not present, since all of the vibrational frequencies are within narrow range of 3182-3218 cm . In the case of the benzene o-complex the calculated IR frequency for the C(sp )-H vibration is 2930 cm , while the other C-H vibrations are within 3178-3215 cm . Applying anharmonic scaling factor/= 0.96 one may obtain quite reasonable agreement 2813 em and 3050-3086 cm (theory estimation) versus 3037-3090 cm and 2874 em (experimental data). 

A/s have been calculated for a series of luild and total oxidation reactions involving molecules withu = 1-8. A/s range from 0.15 to 2.01 eV in the case of mild oxidation, and up to 5.3 eV when combustion of phenol is considered [33,34]. When the number of carbons differs during the reaction (e.g., benzene oxidation to maleic anhydride), the following correction is applied ... 

Phenyls and polyhalogenated phenyls are metabolized to their hydroxy derivatives, each presumably through a benzene oxide intermediate (e.g., references 134, 152, 180, 189, and 272) (Figure 9). Aromatic hydroxylation also occurs in the metabolism of Oxazepam 457) as evidenced by the production of hydroxyphenyl metabolites in the urine of the rat and (to a lesser extent) pig and man. ° Lutz and Schlatterhave demonstrated covalent binding in vivo of a benzene metabolite with the DNA of livers of rats exposed to isotopi-cally labeled benzene in an inhalation chamber. In terms of alkylated nucleotides per mole of DNA phosphate, the potency of benzene calculates to... 

For toluene-l,2-oxide/2-methyl oxepin, photolysis and OH reaction are again competing daytime losses (Klotz et al., 20(X)). The OH reaction is calculated to occur on a timescale of about 0.5 h, while the photolysis lifetime is about 40 min. with overhead Sun. Products of the OH-initiated oxidation are the E, E- and , Z- isomers of 6-oxohepta-2,4-dienal, analogous to the 2,4-hexadiendial products of the benzene oxide/oxepin case. The main photolysis product identified was o-cresol, albeit only in about 10% yield. The reaction with NO3 is also very rapid, and should this species be present at nighttime, its removal via reaction with NO3 would be expected to occur in less than 1 min. in heavily polluted conditions. 

---