Search

Theoretical studies of the retro-Diels-Alder reaction of 1,4-cyclohexadiene, 4H-pyran 4H-thiopyran, 1,4-dioxine, and 1,4-dithiine in the gas phase were carried out using DFT methods at the B3LYP/6–311 + G(d,p) levels of theory. The barrier height (ΔE‡) and thermodynamic parameters (ΔG‡ and ΔH‡) were estimated. The progress of the reactions was followed by means of the Wiberg bond indices. The synchronicity values of the reactions were calculated. The kinetic parameters were calculated for both reactions in 300–1200-K temperature range. Also, fitted equations to the gas phase Arrhenius equation were determined. Effect of the character and number of heteroatoms were illustrated on the thermodynamic and kinetic parameters.

Densities and thermal expansion coefficients of pure methylcyclohexane, 1-methylcyclohexene, 4-methylcyclohexene, 1-methyl-1,4-cyclohexadiene, and toluene were measured by means of a vibrating tube densimeter. A wide range of temperature from 283.15 to 358.15 K was selected for the measurements. The densities and thermal expansion coefficients of binary mixtures of methylcyclohexane and toluene were also measured. Under the conditions of experimentation, positive values of excess molar volumes are observed for the methylcyclohexane-toluene system. Comparing the measured densities of pure components with the densities calculated from the five widely used density equations, the Costald equation is proved to be the most reliable in representing the experimental density data.

To study the relationship between the [2 + 2] photocycloaddition reactivities of 1,4-cyclohexadiene derivatives (1,4−CHDs) and their structures, photophysical properties of a series of 1,4−CHDs were studied experimentally and by performing theoretical calculations. Specifically, UV–vis absorption spectra of these compounds in diluted solutions were acquired and the theoretical calculations were performed at the density functional theory (DFT) level. Their UV–vis absorption maxima were found to be related to the substituents on the 1,4-cyclohexadiene ring. To describe the [2 + 2] photocycloaddition reactivities of the 1,4−CHDs, time-dependent density functional theory (TDDFT) was used to optimize their ground- and excited-state structures, and their electronic excitation energies were calculated at the M062X/def-TZVP level. Frontier molecular orbitals and electron-hole distribution analyses were used to illustrate the electron transition modes of the 1,4−CHDs. The differences between the ground- and excited-state structures of the different 1,4−CHDs were characterized by carrying out a root-mean-square-deviation (RMSD) analysis. The results showed that the photophysical properties of 1,4−CHDs are meaningful for explaining their [2 + 2] photocycloaddition reactivities.

Platinum nanoparticles (NPs) capped with polyvinylpyrrolidone (PVP) were studied with sum frequency generation (SFG) vibrational spectroscopy under reaction conditions during cyclohexene (CH) and 1,4-cyclohexadiene (1,4-CHD) hydrogenation at 295 K. Despite similar vibrational features observed during reaction, CH and 1,4-CHD proceed through mutually exclusive pathways on 1.7 and 4.6 nm Pt-PVP NPs unlike Pt(111) studied previously. The intense red-shifted C–H stretch of adsorbed 1,4-CHD at 2770 cm–1 was monitored for both reactions. SFG and kinetic experiments show CH hydrogenation is active and reversible, while 1,4-CHD hydrogenation poisons the surface.

The adsorption thermodynamics of 1,4-cyclohexadiene on Si(001)-2 × 1 are studied in the framework of the lattice gas model with the Monte Carlo and transfer-matrix methods. Adsorption isotherms, heat capacity, entropy as function of chemical potential and adlayer density were calculated. It is shown that there are two ordered phases for the model under consideration. The first phase consists of the tetra-σ complexes and there is a low surface concentration of the adsorbed molecules. The second phase consists of the π-complexes and there is a high surface concentration. The ordered phase consisting of the di-σ complexes is absent within the constructed model. The obtained set of ordered phases and the sequence of its arising with growth of chemical potential are in close correspondence with the experimental data. In the framework of the model under consideration the observed behavior of 1,4-cyclohexadiene adsorption layer on Si(001)-2 × 1 is explained by the presence of three adsorption states with different surface area per molecule and repulsive lateral interactions. Thus, the transition between π- and di-σ configurations and the formation of the π-complex phase can be the thermodynamically driven effect.

Electron ionization of the two cyclohexadiene isomers has been found to produce similar ion populations with comparable total cross sections. The total cross sections for 1,3-cyclohexadiene and 1,4-cyclohexadiene were measured to be at maxima of 1.42 × 10−15 and 1.41 × 10−15 cm2, respectively, at 80 eV. Results show that the major product ions are the four largest ions, C6H5–8+, with combined intensities contributing more than three quarters of the ion populations. Minor product ions include C5H3,5+, C4H2–6+, C3H2,3,5+ and C2H3+. The branching ratios for the major product ions from the two isomers match each other well at low energies near thresholds but differ slightly at elevated energies; while the branching ratios for the minor ions are approximately identical between the two isomers within error limits.

The ozonolysis reactions of 1,3- and 1,4-cyclohexadiene have been studied using a combination of matrix isolation, infrared spectroscopy, and theoretical calculations. Experimental and theoretical results demonstrate that these reactions predominantly do not follow the long-accepted Criegee mechanism. Rather, the reaction of O3 with 1,4-cyclohexadiene leads to the essentially barrierless formation of benzene, C6H6, and H2O3. These two species are then trapped in the same argon matrix cage and weakly interact to form a molecular complex. There is also evidence for the formation of a small amount of the primary ozonide as a minor product, formed through a transition state that is slightly higher in energy. The reaction of O3 with 1,3-cyclohexadiene follows two pathways, one of which is the Criegee mechanism through a low energy transition state leading to formation of the primary ozonide. In addition, with a similar barrier, ozone abstracts a single hydrogen from C5 while adding to C1, forming a hydroperoxy intermediate. This study presents two of the rare cases in which the Criegee mechanism is not the dominant pathway for the ozonolysis of an alkene as well as the first evidence for dehydrogenation of an alkene by ozone.

Rate constants for the reactions of Cl atoms with two cyclic dienes, 1,4-cyclohexadiene and 1,5-cyclooctadiene, have been determined, at 298 K and 800 Torr of N2, using the relative rate method, with n-hexane and 1-butene as reference molecules. The concentrations of the organics are followed by gas chromatographic analysis. The ratios of the rate constants of reactions of Cl atoms with 1,4-cyclohexadiene and 1,5-cyclooctadiene to that with n-hexane are measured to be 1.29 ± 0.06 and 2.19 ± 0.32, respectively. The corresponding ratios with respect to 1-butene are 1.50 ± 0.16 and 2.36 ± 0.38. The absolute values of the rate constants of the reaction of Cl atom with n-hexane and 1-butene are considered as (3.15 ± 0.40) × 10−10 and (3.21 ± 0.40) × 10− 10 cm3 molecule−1s−1, respectively. With these, the calculated values are k(Cl + 1,4-cyclohexadiene) = (4.06 ± 0.55) × 10−10 and k(Cl + 1,5-cyclooctadiene) = (6.90 ± 1.33) × 10−10 cm3 molecule−1 s−1 with respect to n-hexane. The rate constants determined with respect to 1-butene are marginally higher, k(Cl + 1,4-cyclohexadiene) = (4.82 ± 0.80) × 10− 10 and k(Cl + 1,5-cyclooctadiene) = (7.58 ± 1.55) × 10− 10 cm3 molecule−1 s−1. The experiments for each molecule were repeated three to five times, and the slopes and the rate constants given above are the average values of these measurements, with 2σ as the quoted error, including the error in the reference rate constant. The relative rate ratios of 1,4-cyclohexadiene with both the reference molecules are found to be higher in the presence of oxygen, and a marginal increase is observed in the case of 1,5-cyclooctadiene. Benzene is identified as one major product in the case of 1,4-cyclohexadiene. Considering that the cyclohexadienyl radical, a product of the hydrogen abstraction reaction, is quantitatively converted to benzene in the presence of oxygen, the fraction of Cl atoms that reacts by abstraction is estimated to be 0.30 ± 0.04. The atmospheric implications of the results are discussed. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 431–440, 2011

A quantitative understanding of the thermochemistry of cyclohexadienyl radical and 1,4-cyclohexadiene is beneficial for diverse areas of chemistry. Given the interest in these two species, it is surprising that more detailed thermodynamic data concerning the homolytic C-H bond enthalpies of such entities have not been forthcoming. We thus undertook an experimental and computational evaluation of (a) the enthalpy of formation of cyclohexadienyl radical (C(6)H(7)), (b) the homolytic C-H bond enthalpy of 1,4-cyclohexadiene (C(6)H(8)), and (c) the enthalpy of the addition of a hydrogen atom to benzene. Using laser photolysis experiments coupled with highly accurate ab initio quantum mechanical techniques, a newly recommended enthalpy of formation for C(6)H(7) is determined to be 208.0 +/- 3.9 kJ mol(-1), leading to a homolytic bond dissociation enthalpy of 321.7 +/- 2.9 kJ mol(-1), almost 9 kJ mol(-1) higher than previously determined enthalpies that used less certain experimental values for the C(6)H(7) enthalpy of formation.

A method for the rapid, safe hydrogenation of alkenes and deprotection of benzyl ethers and carboxybenzyl amides is described using catalytic transfer hydrogenation under microwave heating conditions. Commonly available Pd/C catalyst is extremely effective with 1,4-cyclohexadiene as the hydrogen transfer source. In general, the reactions are complete within five minutes at 100 °C.

The adsorption states of 1,4-cyclohexadiene on Si(100) were studied with low-temperature scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS). We newly observed two kinds of molecular images by STM, which clearly differed from the previous STM results obtained at room temperature. Molecular protrusion was observed between the down Si dimer atoms of the neighboring dimer rows on Si(100)c(4×2) at 80 K. After heating the substrate, this species converted to a new species that was located on the Si dimer. The UPS results indicate that adsorbed 1,4-cyclohexadiene has two π bonds at 100 K. After annealing at 140 K, one of the two π bonds reacts with the dimer. Thus, we conclude that 1,4-cyclohexadiene is adsorbed on Si(100)c(4×2) as a π-complex species at ∼100 K and that the π-complex species is transformed to a di-σ species after the substrate is heated beyond 140 K. The present results agree with a recent report by Kato et al. [J. Phys. Chem. C 2007, 111, 2557], and we must rev...

A modular approach to novel 6-amino-7-hydroxysubstituted hexahydro-benzo[1,4]oxazin-3-ones has been developed. The method involves a sequential ring-opening of trans-1,4-cyclohexadiene dioxide with amino nucleophiles. The resultant mono-epoxide from benzylamine was converted to a general electrophilic precursor, which enables the parallel treatment with amino nucleophiles to obtain a series of cyclohexane-fused morpholin-3-ones.

Dithioacetals were reduced by 1,4-cyclohexadiene in the presence of a catalytic amount of Lewis acid to afford the corresponding sulfides in good yields.

To elucidate the thermal chemical processes of 1,4-cyclohexadiene (C6H8) on Si(001), the adsorption states were characterized by temperature-programmed desorption (TPD), low-energy electron diffrac...

The intermolecular dynamics of five six-membered-ring molecular liquids having different aromaticities-benzene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, cyclohexene, and cyclohexane-measured by femtosecond Raman-induced Kerr effect spectroscopy have been compared in this study. The line shapes of the Fourier transform low-frequency spectra, which arise from the intermolecular vibrational dynamics, are trapezoidal for benzene and 1,3-cyclohexadiene, triangular for 1,4-cyclohexadiene and cyclohexene, and monomodal for cyclohexane. The trapezoidal shapes of the low-frequency spectra of benzene and 1,3-cyclohexadiene are due to the librational motions of their aromatic planar structures, which cause damped nuclear response features. The time integrals of the nuclear responses of the five liquids correlate to the squares of the polarizability anisotropies of the molecules calculated on the basis of density functional theory. The first moments of the low-frequency spectra roughly linearly correlate to the bulk parameters of the square roots of the surface tensions divided by the densities and the square roots of the surface tensions divided by the molecular weights, but the plots for cyclohexene deviate slightly from the correlations. The picosecond overdamped transients of the liquids are well fitted by a biexponential function. The fast time constants of all of the liquids are approximately 1.1-1.4 ps, and they do not obey the Stokes-Einstein-Debye hydrodynamic model. On the other hand, the slow time constants are roughly linearly proportional to the products of the shear viscosities and the molar volumes. The observed intramolecular vibrational modes at less than 700 cm(-1) for all of the liquids are also assigned on the basis of quantum chemistry calculations.

International audience; While a series of carbo-mers of 1,3-cyclohexadienes was reported through the use of a specifically developed synthetic strategy, no example of their 1,4-regioisomers was known. Inspired by the methodology elaborated for the preparation of the 1,3-isomers, the synthesis of the first example of carbo-mer of 1,4-cyclohexadiene is presented. Comparison of physico-chemical properties of this first representative with those of the recently described 1,3-regioisomer, especially UV-vis absorption properties, is also addressed.

Isomerization induced by ultraviolet radiation due to intramolecular hydrogen-atom migration in 3,6-diimino-1,4-cyclohexadiene-1,4-diamine (DCD) was studied by matrix-isolation Fourier transform infrared (IR) spectroscopy and density functional theory (DFT) calculation. Two isomers produced by single and double inversion of the hydrogen atom(s) around the C N bond(s) were identified using the wavelength dependence of the IR band-intensity changes. In addition, the IR bands of an intermediate produced by single hydrogen-atom transfer from the amino to the imino group ( C – NH 2 ⋯ NH C ) in the hydrogen bond were measured and identified by a comparison with the calculated spectral patterns. These observations confirm stepwise photoisomerization of DCD.

In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the interaction process (i.e. adsorption and dissociation) of cyclic hydrocarbons (cyclopentene, cyclohexene, and 1,4-cyclohexadiene) with the silicon ( 0 0 1 ) surface. Our first-principles calculations suggest that all considered cyclic hydrocarbons are adsorbed on the Si( 0 0 1 ) surface via a [2+2] cycloaddition reaction with adsorption energies of 35, 23, and 31 kcal/mol for cyclopentene, cyclohexene, and 1,4-cyclohexadiene, respectively. Possible reaction pathways for the dissociation of the methyl radicals and the breaking of the Si–Si dimers of the surface are considered. In addition, we present theoretical STM images of possible adsorbed systems, with a view to contribute to further experimental investigations.

The adsorbed states of 1,4-cyclohexadiene molecules on Si(1 0 0)(2 × 1) at 300 K have been investigated as a function of coverage in real space by means of scanning tunneling microscopy (STM). Two kinds of intermolecular interaction have been studied (i) between the neighboring molecules on the neighboring dimer rows and (ii) between the neighboring molecules on the same dimer row. By analyzing the observed arrangements of adsorbed molecules, we have concluded that the intermolecular interaction between the molecules on the neighboring dimer rows is very small. On the other hand, the intermolecular interaction between the neighboring molecules on the same dimer row becomes repulsive when the distance between two adsorbates is shorter than 2a (a=3.85 A: the distance between the Si dimers on the same row). Various types of arrangements are discussed, based on the steric repulsion. The proposed models can explain the alternate height variations in consecutive molecular protrusions in STM images and the saturation coverage of 0.6 molecules per Si dimer at 300 K.

The adsorption reactions and binding configurations of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7 were studied using high-resolution electron energy loss spectroscopy (HREELS), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and DFT calculation. The covalent attachments of these unsaturated hydrocarbons to Si(1 1 1)-7 × 7 through the formation of Si–C linkages are clearly demonstrated by the observation of the Si–C stretching mode at 450–500 cm −1 in their HREELS spectra. For chemisorbed cyclohexene, the involvement of π CC in binding is further supported by the absence of CC stretching modes and the disappearance of the π CC photoemission. The chemisorption of both 1,3-cyclohexadiene and 1,4-cyclohexadiene leads to the formation of cyclohexene-like intermediates through di -σ bonding. The existence of one π CC bond in their chemisorbed states is confirmed by the observation of the CC and (sp 2 )CH stretching modes and the UPS and XPS results. DFT calculations show that [4 + 2]-like cycloaddition is thermodynamically preferred for 1,3-cyclohexadiene on Si(1 1 1)-7 × 7, but a [2 + 2]-like reaction mechanism is proposed for the covalent attachment of cyclohexene and 1,4-cyclohexadiene.

The gas-phase reactions of OH radicals with 1,4-cyclohexadiene, 1,3,5-cycloheptatriene, and 2,3-dimethylpentanal have been investigated to determine the importance of H-atom abstraction at specific positions in these molecules. Benzene was observed as a product of the reaction of OH radicals with 1,4-cyclohexadiene in 12.5 ± 1.2% yield, in good agreement with a previous study and indicating that this is the fraction of the reaction proceeding by H-atom abstraction from the allylic CH bonds. In contrast, no formation of tropone from 1,3,5-cycloheptatriene was observed, suggesting that in this case H-atom abstraction is not important. For the reaction of OH radicals with 2,3-dimethylpentanal, formation of 3-methyl-2-pentanone was observed in 5.4 ± 1.0% yield (after correction for reaction of 3-methyl-2-pentanone with OH radicals), and this product is predicted to be formed after initial H-atom abstraction from the 2-position CH group. Acetaldehyde and 2-butanone were also observed as products, with initial yields of ∼90% and ∼26%, respectively, and their formation appeared to involve, at least in part, an intermediary acyl peroxy radical. Using a relative rate method, the measured rate constants for the reactions of OH radicals with 2,3-dimethylpentanal, 3-methyl-2-pentanone, and tropone are (in units of 10−12 cm3 molecule−1 s−1) 2,3-dimethylpentanal, 42 ± 7; 3-methyl-2-pentanone, 6.87 ± 0.08; and tropone, 42 ± 6. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 415–426, 2003

The adsorption of 1,4-cyclohexadiene (CHD14) on Pt(1 1 1) was analysed using first-principle density functional theoretical calculations. CHD14 adsorbs at the bridge site via quadra-σ-type bonding. The adsorption energy was calculated to be 146 kJ/mol. CHD14 can also adsorb at the hollow site via the formation of a di-σ- and a π-bond with an adsorption energy of 142 kJ/mol. Upon adsorption CHD14 adopts a boat conformation. Strong π-electron donation to the platinum surface in combination with a hyperconjugative interaction of these π-orbitals with the axial σCH orbital helps to explain the observed red shift of the axial C–H stretching frequency and the corresponding C–H bond lengthening.

The adsorption of 1,4-cyclohexadiene on the Si(001) surface is studied by first-principles density-functional calculations within the generalized gradient approximation. The “pedestal” structure where the two C=C double bonds react with different Si dimers is found to be more stable than the “upright” structure where only one of the two C=C bonds reacts with a Si dimer. However, the [2+2] cycloaddition reaction can easily form the upright structure but not the pedestal one. The latter structure can be obtained from the former through a high energy barrier of ∼0.95 eV, indicating a small reaction rate at room temperature. Our results provide the theoretical basis for the interpretation of recent low-energy electron diffraction and photoelectron spectroscopy data in which the upright structure was seen.

The complex photochemistry of 1,4-cyclohexadiene (CHDN) in solution and in the gas phase is analyzed using a model postulating the involvement of conical intersections. Several conical intersections are found for this system, using a method based on the Longuet–Higgins phase-change rule. The energies and structures of the conical intersections and of some key intermediates are calculated using quantum chemical methods. A single conical intersection, the H/allyl one, is shown to account qualitatively for all the products observed in solution, and also for the cleavage of a H-atom observed in the gas phase. The model is also consistent with the isotopic distribution in the products of deuterated CHDN. The helicopter-type motion of the H2 product observed in the collision-free photolysis of CHDN by E. V. Cromwell, D.-J. Liu, M. J. J. Vrakking, A. H. Kung and Y. T. Lee (J. Chem. Phys., 1991, 95, 297 []), is not compatible with the properties of this conical intersection. A different one, which is shown to lead naturally to such motion is proposed, and its properties are calculated.

Reaction of 1,4-cyclohexadienyl-3-carbonyl chloride 17 with 1,8-bis(dimethylamino) naphthalene 6 gave the divinylketene ketenylidene-1,4-cyclohexadiene 14 as identified by the IR band at 2101 cm. Reaction of 14 with tetramethylpiperidinyloxyl (TEMPO, TO•) gives N2,2,6,6-tetramethylpiperidinyl benzoate 18 formed from the radical 19 resulting from TEMPO attack on the carbonyl carbon of 14.

In order to elucidate the nature of the interface bonding between different molecules and a silicon surface, high resolution Si 2p photoelectron spectroscopy measurements were performed. After adsorption of unsaturated hydrocarbon molecules (ethylene, cyclopentene, and 1,4-cyclohexadiene), the peaks corresponding to the up and down atoms of surface asymmetric dimers vanished, while new peaks appeared between 215 and 398 meV relative to the bulk Si peak. These peaks are assigned to the di-σ Si–C bonds at the interface between the molecules and the surface. We can also estimate the amounts of reacted asymmetric dimers and the charge transfers from the peak intensities and the relative binding energies, respectively.

The adsorption state of 1,4-cyclohexadiene on a Si(100)(2×1) surface is studied with synchrotron radiation photoelectron spectroscopy (PES), high-resolution electron energy loss spectroscopy (HREEL...

In order to fabricate atomically controlled organic materials on Si surfaces, the electronic states of unsaturated cyclic hydrocarbons chemisorbed on Si(1 0 0)(2×1) surfaces were investigated by synchrotron radiation ultraviolet photoelectron spectroscopy (UPS) as a fundamental study. For cyclopentene and cyclohexene, the CC bond interacts with the Si dimer, resulting in the formation of di-σ SiC bonds. In the case of 1,4-cyclohexadiene, on the other hand, one of two CC bonds reacts with the Si dimer and another CC bond remains unreacted with the Si surface. This intact CC bond may introduce the possibility of subsequent functionalization of the organic film, and for the fabrication of atomically controlled and highly functional organic film/Si devices.

The adsorption state of 1,4-cyclohexadiene on a $\mathrm{Si}(100)(2\ifmmode\times\else\texttimes\fi{}1)$ surface is studied using low-energy electron diffraction (LEED) and photoelectron spectroscopy (PES). LEED shows a sharp $2\ifmmode\times\else\texttimes\fi{}1$ pattern at saturation after the adsorption of 1,4-cyclohexadiene indicating that the Si dimer structure is maintained. Judging from the PES results, almost all the dangling bonds react on adsorption of 1,4-cyclohexadiene, and one of the two \ensuremath{\pi} bonds in 1,4-cyclohexadiene reacts with the Si dimer. Thus, the molecule adsorbs on a $\mathrm{Si}(100)(2\ifmmode\times\else\texttimes\fi{}1)$ dimer one to one with $di\ensuremath{-}\ensuremath{\sigma}$ bonding, and the ideal saturation coverage is expected to be 0.5 monolayer. It is concluded that the molecules are anisotropically aligned and the molecular plane is inclined from the surface normal, with the remaining \ensuremath{\pi} bond located at the vacuum side.

The title complexes were tested in the hydrogenation of hex-3-yne and of 1,3- and 1,4-cyclohexadiene (CHD) under solid–gas conditions. The clusters were deposited on three “standard” supports, that is, pyrex glass, alumina, and silica. All the clusters, particularly (μ-H)Ru3(CO)10(PPh2), show hydrogenation activity. However, they are not particularly selective toward the formation of monoenes; “disproportionation” of 1,3- and 1,4-CHD to hydrogenated products and benzene also occurs. The hydrogenation activity of the clusters is dependent on their nature, the type of substrate, and the characteristics of the supporting material; silica and pyrex glass are usually more active than alumina. Attempts at detecting the formation of organometallic intermediates or by-products (through IR spectroscopy) were made. HRTEM was used to check for eventual decomposition on some supports.

The recently defined concept of “ultradiagonal” strain energy is applied to cyclopropene, cyclopropenone, and methylenecyclopropene and their respective diaza and diphospha derivatives using ab initio (DFT) calculations. In the relatively few cases where comparisons could be made between theory and either experiment or earlier calculational studies, very good agreement for both energies and structures were found. This gives us confidence for the remaining species. The results were also qualitatively explained, and the concept of ultradiagonal strain gibbs energy is briefly discussed.

Flash-photolysis of 1,4-cyclohexadiene with ArF laser at 193 nm is investigated. Cyclohexadienyl radical with λ max at 300 nm is observed as direct evidence for the simultaneous occurrence of a radical dissociation channel along with the concerted molecular channel to form benzene and hydrogen. The radical decays with first-order kinetics with a rate coefficient of (3.0±0.2)×10 4 s −1 . The activation barrier and enthalpy of reaction are evaluated using semi-empirical PM3MM and HF/STO-3G theories and compared with available experimental results.

The electronically excited states of the two isomers 1,3- and 1,4-cyclohexadiene are examined by means of multiconfigurational second-order perturbation (CASPT2) theory with extended ANO-type basis...