Benzene ground state

Thakur, S. N., Goodman, L and Ozkabak, A. G. (1986), The Benzene Ground State Potential Surface. I. Fundamental Frequencies for the Planar Vibrations, J. Chem. Phys. 84, 6642. 

Fig. 18 Low-energy transitions from the benzene ground state to excited states B, and E, , giving the secondary and primary bands.

It is instructive to touch briefly on the TPA spectroscopy of benzene [4] since we have discussed its one-photon absorption spectroscopy in Chapter 7. For TPA from the Ajg benzene ground state to some final vibronic state/,... 

The first study was made on the benzene molecule [79], The S ISi photochemistry of benzene involves a conical intersection, as the fluorescence vanishes if the molecule is excited with an excess of 3000 crn of energy over the excitation energy, indicating that a pathway is opened with efficient nonradiative decay to the ground state. After irradiation, most of the molecules return to benzene. A low yield of benzvalene, which can lead further to fulvene, is, however, also obtained. 

The concept of two-state systems occupies a central role in quantum mechanics [16,26]. As discussed extensively by Feynmann et al. [16], benzene and ammonia are examples of simple two-state systems Their properties are best described by assuming that the wave function that represents them is a combination of two base states. In the cases of ammonia and benzene, the two base states are equivalent. The two base states necessarily give rise to two independent states, which we named twin states [27,28]. One of them is the ground state, the other an excited states. The twin states are the ones observed experimentally. 

In a photochemical experiment, irradiation of benzene leads to Sj, which connects to the ground-state surface via the conical intersection shown. Benzene, the much more stable species, is expected to be recovered preferentially, but the prebenzvalene structure which hansfomis to benzvalene is also fomied. Another possible route from the prebenzvalene, along a different coordinate, will lead to fulvene [90, p.357] after a hydrogen-atom transfer from... 

The potential surfaces of the ground and excited states in the vicinity of the conical intersection were calculated point by point, along the trajectory leading from the antiaromatic transition state to the benzene and H2 products. In this calculation, the HH distance was varied, and all other coordinates were optimized to obtain the minimum energy of the system in the excited electronic state ( Ai). The energy of the ground state was calculated at the geometry optimized for the excited state. In the calculation of the conical intersection... 

The electronic theory provides by these means a description of the influence of substituents upon the distribution of electrons in the ground state of an aromatic molecule as it changes the situation in benzene. It then assumes that an electrophile will react preferentially at positions which are relatively enriched with electrons, providing in this way an isolated molecule theory of reactivity. 

Katritzky and Topsom have reviewed the information available, largely from infrared and n.m.r. studies, concerning the distortion of the tt-electron system in the benzene ring brought about in the ground state by substituents. Of particular interest is the observation that both n.m.r. studies (of m- F and chemical shifts) and infrared investigations (of the intensities of bands due to certain skeletal vibrations) suggest that the value of Taft s [Pg.226]

In a molecule with electrons in n orbitals, such as formaldehyde, ethylene, buta-1,3-diene and benzene, if we are concerned only with the ground state, or excited states obtained by electron promotion within 7i-type MOs, an approximate MO method due to Hiickel may be useM. 

The resolution of the ZEKE-PE spectmm of 1,4-difluorobenzene can be compared with, for example, that of the ultraviolet photoelectron spectmm of benzene in Figure 8.12. The greatly increased resolution in the ZEKE-PE spectmm is attributable mostly to the fact that only photoelectrons with zero kinetic energy are being detected. It is also partly attributable to the molecules being in a supersonic jet this has the effect of sharpening the bands because of the restricted rotational populations in the ground state of the molecule. 

The total 7t-eIectron energy of benzene is 6a -I- 8)3, corresponding to a DE of 2)3. Cyclobutadiene is predicted to have a triplet ground state (for a square geometry) and zero... 

The charge-tranter concept of Mulliken was introduced to account for a type of molecular complex formation in which a new electronic absorption band, attributable to neither of the isolated interactants, is observed. The iodine (solute)— benzene (solvent) system studied by Benesi and Hildebrand shows such behavior. Let D represent an interactant capable of functioning as an electron donor and A an interactant that can serve as an electron acceptor. The ground state of the 1 1 complex of D and A is described by the wave function i [Pg.394]

The first generalization is illustrated by the behavior of the 2- and 4-vs. the 3-derivatives of pyridine, the second by the reactivity of 4- vs. 2-substituted pyridines, the third by the relation of 4- vs. 2-derivatives of pyrimidine, and the fourth by the appreciable reactivity of 3-substituted pyridines or 5-substituted pyrimidines compared to that of their benzene analogs. Various combinations of azine-nitrogens in other poly-azines supply further examples. Theoretical aspects of (1), (2) and (3) are discussed in Section II, B, 2. The effect involved in (4) is believed to be more the result of the inductive stabilization of an adjacent negative chaise in the transition state (cf. 251) than of the electron deficiency created in the ground state (cf. 252). The quantitative relation between inductive stabihzation and resonance stabilization is not precisely defined by available data. However, a... 

Recently, a kinetic study has been made of the substitution of diazotised sulphanilic acid in the 2 position of 4-substituted phenols under first-order conditions (phenol in excess) in aqueous buffer solutions at 0 °C131a. A rough Hammett correlation existed between reaction rates and am values, with p about -3.8 however, the point for the methoxy substituent deviated by two orders of magnitude and no explanation was available for this. The unexpectedly low p-factor was attributed to the high reactivities of the aromatic substrates, so that the transition state would be nearer to the ground state than for reaction of monosubstituted benzene derivatives. 

FIGURE 3.39 The molecular orbital energy-level diagram for the ir-orbitals of benzene. In the ground state of the molecule, only the net bonding orbitals are occupied. 

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