Strahlungschemie von Kohlenwasserstoffen. 15. Mitteilung. Energieübertragung im System Benzol–3-Phenylcyclohexadien-(1,4)

3-Phenylcyclohexa-1,4-diene, which is a reaction product in the benzene radiolysis, is decomposed when its solutions in benzene are irradiated by γ-rays. The main products are biphenyl (20%) and dihydrobenzene (10%). They show that hydrogen from phenylcyclohexadiene is transferred to benzene. The high G-value for the disappearance is explained by energy-transfer from a high-lying state of benzene, formed with g(B*) ≥ 2, to the diene. From the concentration dependence in the range 0.015–0.25 Mol/1 can be concluded that τ · k_tA is 21 1/Mole for phenylcyclohexadiene and 4 1/mole for cyclohexene, where τ is the lifetime of the excited state of benzene involved in the reaction and k_tA is the rate constant for energy-transfer from this excited state to the acceptor. This rate constant is in the same order of magnitude for biphenyl, naphthalene and cyclohexa-1,4-diene as accepted for phenylcyclohexadiene. The nature of the energy transfer from benzene to phenylcyclohexadiene is discussed. The possibilities of charge transfer or the lowest singlet- or tripletstate as excited states are ruled out.     

Potential-energy surfaces in the lower excited states of benzene⇌Dewar–benzene isomerization process

Adiabatic potential-energy surfaces in the lower excited states following the benzene?Dewar–benzene isomerization process were calculated by the INDO /S method postulating the concerted reaction mechanism which is proved to be valid in the ground state by the calculation of the intrinsic reaction coordinate. It was concluded that the benzene molecule in the ¹B_1u and ¹E_2u excited states readily isomerizes to Dewar–benzene in the condensed phase although the quantum yield may not be large. Bryce-Smith's proposal, that the isomerization to Dewar–benzene occurs only after the intersystem crossing to the ³B_1u state in benzene molecule, will not be probable; for the ³B_1u route is not favorable to the isomerization in comparison with these singlet routes. However, the rearomatization of Dewar–benzene in the ground state may produce the ³B_1u benzene in small yield with higher yield of the ground-state benzene. The activation energy in the rearomatization is calculated to be 19.03 kcal/mol. These conclusions are consistent with the experimental facts. Molecular orbital correlations in the isomerization and the ionization potentials of Dewar–benzene were also discussed.     

A multi-configuration LCAO–MO study for complex unsaturated molecules. I. General theory and its application to the benzene anion

A multi-configuration LCAO –MO approach using a π-bond order–bond length linear relation is introduced to predict the geometrical structures for the electronic ground and excited states of unsaturated hydrocarbons. The procedure is designed to include configuration interaction in each iterative computation where the π-electron approximation is employed under the Pariser–Parr type semi-empirical treatment. The π-bond order–bond length relation is determined as r_pq = 1.523 – 0.193P_pq, when the bond lengths of ethylene, benzene and naphthalene are used and the groundstate functions including the singly and doubly excited configurations are taken into account to obtain the bond orders P_pq. The iterative calculation is applied to the ground state and the two lowest excited states of the benzene anion in both D_6h and D_2h molecular geometries. The geometrical structures and the π-electron energies are computed for the ground and excited states of the anion; for the latter, two types of configuration species are used. It is found that the first lowest excited state is not subjected to the Jahn–Teller effect and the calculated excited state energies do not agree with the observed values (c. 1.0 ～ 2.5 eV higher than the observed values). The latter point is discussed in detail. It is also found that the resultant ground state energy depression due to configuration mixing is not very large and the two types of configuration species used give different CI effects on the energy levels of the two lowest excited states of the anion. Finally, the stabilization energy due to the Jahn–Teller distortion is estimated for the ground state of the anion.     

Quantenchemische untersuchungen zum mechanismus der elektrophilen substitution. I. Zur potentialhyperfläche des systems benzol/h+

Results of semiempirical calculations (CNDO/2-FK and MINDO/2 methods) for the σ-π complex problem on protonated benzene are given and compared with previous ones. The semiempirical methods were chosen according to the agreement of their results with new theoretical energy data (E_HF + E_korrel) concerning the classical–nonclassical problem of protonated ethylene. By these methods the corresponding part of the energy surface of the benzene/H⁺ system is simulated. The stationary points of this surface are found by a gradient method with complete optimization of the geometry. On the basis of this method we determined the energy profile of a reaction coordinate between the classical (σ-complex) and nonclassical (π-complex) cation. The so called strong π-complex is a saddle point between two σ-complex minima and can be interpreted as transition state of 1,2-proton shifts. Hypotheses for possible minimum energy paths of electrophilic attacks in the given region of the surface are discussed.     

A multi-configuration LCAO–MO study for complex unsaturated molecules. II. Application to the benzene cation

The method of the MC –LCAO –MO approach, described in the preceding paper, is further applied to the benzene cation. Through the iteration process the π-electron energies and the molecular shapes are computed for the ground and two lowest excited states of the cation in both D_6h and D_2h geometries. A remarkable fact obtained is that a comparatively small variation of the geometrical structure (c. 0.010 – 0.013 Å bond length difference) brings about a considerable change of the energy value (c. 0.85 – 1.25 eV). The π-electronic excitation energies obtained from the iteration process are compared with the transition energies calculated from the usual method in which the structures of the excited states are assumed to be the same as the corresponding ground state structures. The difference in the excitation energy between the cation and the anion, and the CI effect on the excited states, are discussed. It is found that the doubly excited configurations play an important role in CI , which is somewhat different from that of the singly excited configurations. The stabilization energy due to the Jahn–Teller distortion is estimated for the ground state of the cation.     

Non-radiative decay of the benzene 1B2u and 3B1u states

Optic-acoustic measurements on high pressure benzene are presented, and are used to analyse the nature of the decay channels form the highly vibrationally excited ³B_1u state. The vibrationally relaxed benzene ³B_1u state is deactivated by n-pentane with a collisional efficiency of 3 × 10^?5. A model, introducing an intermediate state close in energy to the ³B_1u state, is shown to be in good accord with the results.     

Benzene clusters in a supersonic beam

A supersonic beam is employed to produce benzene clusters (C₆H₆) _n up ton=40. Mass analysis is achieved after two-photon ionization in a reflectron mass spectrometer. Photon energy is chosen so that the internal energy of the cluster ions is less than 700 meV and a slow decay on the µs time scale is observed. By an energy analysis with the reflecting field it is found that the elimination of one neutral benzene monomer is the favoured dissociation process of the cluster ions. Information about the dissociation pathways of the cluster ions is essential if one is to obtain neutral cluster abundances from the ion mass spectrum. Furthermore an experimental method is presented to obtain pure intermediate state (S ₁←S₀) spectra of selected clusters without interferences from the other clusters present in the molecular beam. This method is based on the observation of the metastable decay of the corresponding cluster ion. When the metastable signal is recorded as a function of photon energy it reflects theS ₁←S ₀ intermediate state spectrum. Spectra are presented for the benzene dimer, trimer, tetramer and pentamer.     

Dielectric relaxation of sulfolane in benzene solution from microwave absorption data

The electric constant (ɛ′) and dielectric loss (ɛ″) for dilute solutions of sulfolane in benzene solution has been measured at 9.885 GHz at different temperatures (25, 30, 35, and 40°C) by using standard microwave techniques. Following the single frequency concentration variational method, the dielectric relaxation time (τ) and dipole moment (μ) have been calculated. It is found that dielectric relaxation process can be treated as the rate process, just like the viscous flow. Based on the above studies, monomer structure of sulfolane in benzene has been inferred. The presence of solute-solvent associations in benzene solution has been proposed. Energy parameters (ΔH _ɛ, ΔF _ɛ, ΔS _ɛ) for dielectric relaxation process of sulfolane in benzene at 25, 30, 35, and 40°C have been calculated and compared with the corresponding energy parameters (ΔH _η, ΔF _η, ΔS _η) for the viscous flow.     

INDO-Type calculations on the ground state and various ionic states of transition metal tricarbonyls

By means of the ΔSCF and transition operator (TO) methods based on a recently developed INDO extension to the first transition metal series, the first ionization potentials of benzene—chromium tricarbonyl ( I ), cyclopentadienyl manganese tricarbonyl ( II ), the iron—tricarbonyl complexes with trimethylenemethane ( III ), and cyclobutadiene ( IV ) have been calculated and compared with experimental data. It is shown that the electronic structure of I to IV can be rationalized by Hoffmann's fragment approach in both the ground state and the cationic hole states. Within the series I—IV there are remarkable energy differences in the ground state for MOs derived from the 1a₁ and 1e orbitals of the M(CO)₃ fragment. The observation that only one band is associated with the ionization events from MOs predominantly localized at the metal site is traced back to large relaxation effects. In the cationic hole states the split of the M(CO)₃ fragment orbitals 1a₁ and 1e is minute in all four compounds.     

Nature of the water/aromatic parallel alignment interactions

The water/aromatic parallel alignment interactions are interactions where the water molecule or one of its O? H bonds is parallel to the aromatic ring plane. The calculated energies of the interactions are significant, up to ΔE_{CCSD(T)(limit)} = ?2.45 kcal mol^?1 at large horizontal displacement, out of benzene ring and CH bond region. These interactions are stronger than CH···O water/benzene interactions, but weaker than OH···π interactions. To investigate the nature of water/aromatic parallel alignment interactions, energy decomposition methods, symmetry‐adapted perturbation theory, and extended transition state‐natural orbitals for chemical valence (NOCV), were used. The calculations have shown that, for the complexes at large horizontal displacements, major contribution to interaction energy comes from electrostatic interactions between monomers, and for the complexes at small horizontal displacements, dispersion interactions are dominant binding force. The NOCV‐based analysis has shown that in structures with strong interaction energies charge transfer of the type π → σ*(O? H) between the monomers also exists. © 2014 Wiley Periodicals, Inc.     

Ab initio study on the electronic structures of styrene in the Franck-Condon region

The electronic structures of styrene in the Franck‐Condon region have been theoretically examined by means of ab initio complete active space self‐consistent field (CASSCF) and the second order multireference Møller‐Plesset calculations. The optimized structure of styrene in S₀ is planar but the torsional motion of the phenyl group is very floppy. The S₁ state is assigned to the local π–π* excitation within the benzene ring. On the other hand, S₂, above S₁ by 0.561 eV, is assigned to a state that resembles the so‐called V‐state of ethylene. The transition intensity of S₀–S₁ is weak, while that of S₀–S₂ is strong. This is in good agreement with the experimental absorption spectrum where the S₀–S₁ and S₀–S₂ transitions are in the energy range of 290–220 nm. The optimized geometry of S₁, characterized by an enlarged benzene ring and its vibrational analyses, further justifies the assignment of the S₁ state. © 2002 Wiley Periodicals, Inc. J Comput Chem 9: 928–937, 2002     

Loss of CO from the molecular ions of o-, m- and p-anisoyl fluorides,CH3OC6H4COF,with fluorine atom migration

Loss of CO from the molecular ions ([CH₃OC₆H₄COF]⁺˙) of o-, m- and p-anisoyl fluorides has been investigated by mass-analysed ion kinetic energy (MIKE) spectrometry. This reaction involves fluorine atom migration from the carbonyl group to the benzene ring. In the cases of o- and p-anisoyl fluorides, the fluorine atom migrates via a three-membered transition state to form the molecular ions ([CH₃OC₆H₄F]⁺˙) of o- and p-fluoroanisoles, respectively. On the other hand, in the case of m-anisoyl fluoride, the fluorine atom migrates from the carbonyl group to the benzene ring via a three- or four-membered transition state.     

Mechanism for the Nonadiabatic Photooxidation of Benzene to Phenol: Orientation‐Dependent Proton‐Coupled Electron Transfer

An efficient catalytic one‐step conversion of benzene to phenol was achieved recently by selective photooxidation under mild conditions with 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) as the photocatalyst. Herein, high‐level electronic structure calculations in the gas phase and in acetonitrile solution are reported to explore the underlying mechanism. The initially populated ¹ππ* state of DDQ can relax efficiently through a nearby dark ¹nπ* doorway state to the ³ππ* state of DDQ, which is found to be the precursor state involved in the initial intermolecular electron transfer from benzene to DDQ. The subsequent triplet‐state reaction between DDQ radical anions, benzene radical cations, and water is computed to be facile. The formed DDQH and benzene‐OH radicals can undergo T₁→S₀ intersystem crossing and concomitant proton‐coupled electron transfer (PCET) to generate the products DDQH₂ and phenol. Two of the four considered nonadiabatic pathways involve an orientation‐dependent triplet PCET process, followed by intersystem crossing to the ground state (S₀). The other two first undergo a nonadiabatic T₁→S₀ transition to produce a zwitterionic S₀ complex, followed by a barrierless proton transfer. The present theoretical study identifies novel types of nonadiabatic PCET processes and provides detailed mechanistic insight into DDQ‐catalyzed photooxidation.     

Quantum-chemical study of nitrosonium complexes of monocyclic aromatic compounds

With the use of Hartree-Fock and DFT methods we demonstrated that for the benzene derivatives with the substituents Me, Et, Pr, i-Pr, t-Bu, CF₃, F, and Cl π-complexes are more favorable by energy, whereas with the substituents CHO, MeCO, PhCO, CN, NO, and NO₂ n-complexes are more feasible. The affi nity of aromatic compounds to the nitrosonium-cation (A_NO+) at the formation of the π-complexes grows with the growing donor character of the substituents in the ring and with their number. The best agreement between the calculated and experimental A_NO+ values for benzene was obtained with the use of RI-MP2/L1 method.     

Evaluation of Excess Gibbs Energy of Mixing in Binary Mixtures of Di-Isobutyl Ketone (Dibk) and Nonpolar Solvents

Abstract

Excess Gibbs energy of mixing in binary mixture of Di-isobutyl ketone (DIBK) in nonpolar solvents namely n-heptane, p-xylene, cyclohexane, dioxane, benzene and tetra-chloromethane have been evaluated at 303°K. The results indicate that (ΔG_AB)_maxima is in the order, n-heptane > p-xylene > cyclohexane > benzene > dioxane > tetra-chloromethane.     

X-ray photoelectron spectroscopic characterization of the acetylene cyclotrimerization catalyst NbCl2(CnHn) (n = 10–12)

The elemental and ionic compositions of the surface of NbCl₂(C _n H _n ) (n = 10–12), an active catalyst for acetylene cyclotrimerization into benzene, have been determined by X-ray photoelectron spectroscopy. Binding energy data for the sample sputtered with argon ions E _b (Nb3d _5/2) = 203.8–204.2 eV) suggest that the oxidation state of niobium in the active catalyst is +2 or +3. The narrow C1s line indicates the equivalence of all carbon atoms, and the corresponding binding energy, E b(C1s) = 284.0 eV, is close to the BE value for cyclic unsaturated hydrocarbons with conjugate double bonds. Interacting with atmospheric oxygen and moisture during sample preparation, niobium ions on the catalyst surface oxidize to their highest oxidation state, +5, characterized by E _b (Nb3d _5/2) = 207.3–207.7 eV. These data suggest that niobium oxychlorides or oxides form ion the sample surface. The catalyst is stable in a high vacuum and undergoes slight charging under the action of an X-ray beam, showing poor dielectric properties.     

The isomers of ionized ethane

The previously reported ²A_g, ²A_1g, and ²B_g states of ionized ethane are characterized at several levels of theory. The diborane-like ²A_g state, which gives rise to the observed ESR spectrum, is predicted by SCF and CCD calculations not to exist in a separate minimum from the ²A_1g state formed by ionization of the C(SINGLE BOND)C bond. However, as reported by Lunell and Huang, second-order Moller-Plesset theory places the ²A_g lowest, provided polarization functions are included on carbon. QCISD theory predicts that both A states correspond to potential energy minima, but places the long-bond ²A_1g state lower, at least with moderately large basis sets. F orbitals on carbon stabilize the diborane structure more than the long-bond one. When a potential energy surface is generated for a series of fixed C(SINGLE BOND)C bond lengths by optimizing all variables except for the C(SINGLE BOND)C bond length with MP2 theory and calculating the energy with QCISD(T), the ²A_g state is predicted to be the lowest energy state with the ²A_1g state 1.83 kJ/mol above it. The two A states are predicted to be separated by a barrier 2.79 kJ/mol above the lower state. This barrier is above the zero-point energy in the C(SINGLE BOND)C stretch for the lower state but below the ZPE for this stretch in the upper state, which is therefore predicted not to exist as a stable species. A single quantum of vibrational excitation in the low frequency C(SINGLE BOND)C stretch is predicted to yield an ion with a poorly defined C(SINGLE BOND)C bond length. The highest levels of theory employed give poor agreement with the experimental hyperfine coupling constants. The discrepancy could either be due to neglect of vibrational effects, to poor inherent accuracy of the calculation, as one author has concluded, or to compression of the ion by the matrix as suggested by another. The ²B_g state is found to be higher in energy than the A states at all theoretical levels and is predicted to have a large (160.2–177.4 G) hyperfine coupling from four hydrogens. The transition state for simultaneous exchange of two hydrogen atoms between the carbons by a diborane structure is predicted to lie above the lowest energy fragmentation threshold, in agreement with experiment. © 1996 by John Wiley & Sons, Inc.     

Magnetic rotational strengths of higher states in benzene: evidence for the existence of an out-of-plane transition

Calculations of magneto-optical B values in equilibrium benzene, were carried out in the CNDO/S CI approximation. The calculations demonstrate that the apparent B term feature just on the low energy edge of the E_1u absorption in the MCD spectrum of benzene vapor may arise from an electric dipole allowed, out-of-plane polarized transition to an A_2u state. Although the A_1g ? A_2u transition has a very low oscillator strength, it can have a non-trivial magnetic rotational strength via magnetic coupling with the close lying E_1u state. Signs and magnitudes of the rotational strength are sensitive to details of the calculation. The B value of the E_1u state is also discussed.     

Computational Study of the Interactions between Benzene and Crystalline Ice Ih: Ground and Excited States

Ground‐state geometries of benzene on crystalline ice cluster model surfaces (I_h) are investigated. It is found that the binding energies of benzene‐bound ice complexes are sensitive to the dangling features of the binding sites. We used time‐dependent DFT to study the UV spectroscopy of benzene, ice clusters, and benzene–ice complexes, by employing the M06‐2X functional. It is observed that the size of the ice cluster and the dangling features have minor effects on the UV spectral characteristics. Benzene‐mediated electronic excitations of water towards longer wavelengths (above 170 nm) are noted in benzene‐bound ice clusters, where the cross‐section of photon absorption by water is negligible, in good agreement with recent experimental results (Thrower et al., J. Vac. Sci. Technol. A, 2008, 26 , 919–924). The intensities of peaks associated with water excitations in benzene–ice complexes are found to be higher than in isolated ice clusters. The π→π* electronic transition of benzene in benzene–ice complexes undergoes a small redshift compared with the isolated benzene molecule, and this holds for all benzene‐bound ice complexes.