Benzene electronic states

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... 

Depending on the electronic state of azafulvalene and the reaction conditions, simple nucleophiles such as amines or alcohols show a different behavior. Upon heating methanol reacted with azafulvalenes as electron-rich olefins by addition to the central double bond (64BSF2857 67LA155). Using the TAF 77 (Ar = Ph), the addition reaction in a neutral benzene-ethanol solution required several days to obtain a minor amount of 147, while the reaction proceeded rapidly in the presence of a catalytic amount of potassium hydroxide (79JOC1241). Tlie yellow-colored adduct 147 can be reconverted to the quinoid starting material by irradiation (Scheme 58). 

Spectroscopy of the PES for reactions of transition metal (M ) and metal oxide cations (MO ) is particularly interesting due to their rich and complex chemistry. Transition metal M+ can activate C—H bonds in hydrocarbons, including methane, and activate C—C bonds in alkanes [18-20] MO are excellent (and often selective) oxidants, capable of converting methane to methanol [21] and benzene to phenol [22-24]. Transition metal cations tend to be more reactive than the neutrals for two general reasons. First, most neutral transition metal atoms have a ground electronic state, and this... 

These selection rules are affected by molecular vibrations, since vibrations distort the symmetry of a molecule in both electronic states. Therefore, an otherwise forbidden transition may be (weakly) allowed. An example is found in the lowest singlet-singlet absorption in benzene at 260 nm. Finally, the Franck-Condon principle restricts the nature of allowed transitions. A large number of calculated Franck-Condon factors are now available for diatomic molecules. 

Figure 2.6 gives the UV-visible spectrum of a very dilute solution of anthracene (Figure 2.7) in benzene, which clearly shows small fingers superimposed on a broader band (or envelope). These fingers are called the vibrational fine structure and we can see that each finger corresponds to a transition from the v = 0 of the ground electronic state to the v = 0, 1, 2, 3, etc. vibrational level of the excited electronic state. 

The entire approach presupposes the separation of electronic degrees of freedom. As already noted, for the higher electronic states of polyatomic molecules, there can be important couplings with both spectroscopic and dynamic implications. The vibronic spectroscopy of benzene is reviewed by Ziegler and Hudson (1982). 

Figure 2. Diabatic (left) and adiabatic (right) population probabiUties of the C (fuU line), B (dotted line), and X (dashed line) electronic states as obtained for Model II, which represents a three-state five-mode model of the benzene cation. Shown are (A) exact quantum calculations of Ref. 180, as well as mean-field-trajectory results [(B), (E)] and surface-hopping results [(C),(D),(F),(G)]. The latter are obtained either directly from the electronic coefficients [(C),(F)] or from binned coefficients [(D),(G)].

Figure 25. Diabatic and adiabatic population probabilities of the C (fuU line), B (dotted hne), and X (dashed line) electronic states as obtained for a five-state 16-mode model of the benzene cation.

Figure 11 shows some of the geometrical parameters (computed at the MCSCF level of theory) for the five electronic states of 33. The local geometries of the carbene and nitrene subunits (bond angle at the carbene center, C—bond length of the carbene and the bond lengths between the diradical centers and the benzene ring) of the quintet state ( A ) are very similar to that of triplet phenylcar-bene and phenynitrene computed at the same level of theory (Fig. 12). The situation is less clear for the other two A states ( A and A ), but the geometry of...

These simple molecular orbital pictures provide useful descriptions of the structures and spectroscopic properties of planar conjugated molecules such as benzene and naphthalene, and heterocychc species such as pyridine. Heats of combustion or hydrogenation reflect the resonance stabilization of the ground states of these systems. Spectroscopic properties in the visible and near-ultraviolet depend on the nature and distribution of low-lying excited electronic states. The success of the simple molecular orbital description in rationalizing these experimental data speaks for the importance of symmetry in determining the basic characteristics of the molecular energy levels. 

In substituted benzenes, the symmetry is lowered and the transitions into the states that correlate to the B2u and B u states of benzene become allowed by IPA, 2PA, or both. However, when the substituents induce only a weak perturbation on the benzene yr-electron system, the IPA or 2PA spectra of the substituted compounds often closely resemble the spectrum of the unsubstituted parent molecule. Various theoretical models have been developed in an attempt to predict the type of change in the band intensity and characteristics in the 2PA spectra of substituted benzenes and, more generally, of alternant hydrocarbons [34-36]. It was found that the effect of a perturbation is quite different for IP and 2P allowed transitions. In particular, 2P transitions to the state correlated to the benzene B2u state (Lb) are affected more by vibronic coupling than transitions to the state correlated to the benzene Biu state (La, in Platt notation [31,32]). In contrast, inductive perturbations enhance the La band more than the Lb band. The effects of vibronic coupling and inductive substituents are reversed for IP transitions into these states. Experimental... 

B terms of the 1st and 2nd n - jt transitions of the mono-substituted benzenes are summarized in Table A3. The major parts of the B terms of the 1 st and 2nd t - it transitions may come from the mixing with the lowest four or five excited electronic states, because the terms with larger denominators in Eq. (A-3) are considered to be negligibly small. The contribution to the B terms comes from the mixing between the first two excited states, resulting in the opposite signs of the MCD bands, as shown in Table A4 and Fig. A2. 

We note that the induced absorption bands mentioned are associated with single or double electronic transitions. Such induced bands are not limited to oxygen similar bands of a few other systems (benzene, for example) were known for some time. However, most of the common diatomic molecules have electronic states that are many eV above the ground state. As a consequence, for those molecules, electronic induced transitions commonly occur in the ultraviolet region of the spectrum where interference from allowed electronic spectra may be strong. Electronic induction is not nearly as common as rovibro-translational induced absorption. 

In this section we present experimental results for the lifetime of individual rovibronic states of benzene at different excess energies in the Si electronic state. In this way the dependence of the lifetime of the states on their excess energy and their rotational quantum number is studied. A general model for the underlying coupling mechanism is presented, and the influence of a van der Waals bound noble-gas atom on the intramolecular dynamics is investigated. 

Some properties of palladium deposited on different amorphous or zeolitic supports were determined, including catalytic activity per surface metal atom (N) for benzene hydrogenation, number of electron-acceptor sites, and infrared spectra of chemisorbed CO. An increase of the value of N and a shift of CO vibration toward higher frequencies were observed on the supports which possessed electron-acceptor sites. The results are interpreted in terms of the existence of an interaction between the metal and oxidizing sites modifying the electronic state of palladium. 

Since parallel variations were observed in turnover number for benzene hydrogenation and in CO vibration frequency, interaction between metal and oxidizing supports does exist. This interaction modifies the electronic state and catalytic properties of palladium. 

The situation at wavelengths below 2000 A has improved recently. The upper electronic state of benzene excited by absorption of the 1849-A line of mercury would be the 1 lu state.47 No light emission from this state has been reported and no evidence exists relative to crossover from this state to any of the various triplet states of lower energy. 

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