Benzene ultraviolet transitions/spectrum

Benzene. The ultraviolet absorption spectrum of benzene is characterized by the low intensity L band at 256 nm and the more intense La band at about 200 nm, both bands being 77 77 transitions. Changes in the absorption spec-... 

A calculated transition energy used to assess the polarity of a solvent. The solvent ionizing capability directly affects the position of a peak, easily measured, in the ultraviolet region of the spectrum of the complex of an iodide ion with 2-methyl- or l-ethyl-4-carbomethoxypyridinium ion. Water has a Z value of 94.6, ethanol has a value of 79.6, dimethyl sulfoxide s value is 71.1, and benzene has a value of 54. A similar polarity scale, known as x(30) values, is related to the Z value scale Z = 1.41 t(30) -E 6.92. See Solvent Effects... 

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 addition to transitions in the ir manifold discussed above, some attention has been given recently to possible o-ir states in the ultraviolet nonradlative transitions from the B2jj state. Callomon et al. (27) have postulated the transitions from higher vibrational levels of B2y to a E2u(high resolution absorption spectrum of vapor phase benzene in the 5.0 eV region. initio (12,28) calculations have predicted the presence of at least one a state (e2 ) lying between two filled ir-molecular orbitals. Photoionization (29,30) experiments, and other evidence (31) appear to agree with the conclusion that a-o-ir transition may be... 

Figure 7.12 Ultraviolet spectrum of the S, state of benzene with partially resolved rotational structure. Although the K levels are not resolved, the transition probability is peaked for K = /. The absorption of one additional photon produced the benzene ion. Taken with permission from Kiermeier et al. (1988).

Benzene displays three bands in the ultraviolet region around 185, 210 and 260 nm (Fig. 17), which correspond to transitions from the ground state to the three lower-lying excited states Bj and Bj respectively (Fig. 18) (Murrell, 1963). Transitions to B excited states are symmetry-forbidden, but, on account of the symmetry-Ejg molecular vibrations (1595 and 606 cm in the infrared spectrum), which distort the benzene n-electron cloud, they are allowed to a certain extent, appearing as a medium-intensity band and a weak (e 150) band respectively (Fig. 17). 

The absorptions that result from transitions within the benzene chromophore can be quite complex. The ultraviolet spectrum contains three absorption bands, which sometimes contain a great deal of fine structure. The electronic transitions are basically of the n — - n type, but their details are not as simple as in the cases of the classes of chromophores described in earlier sections of this chapter. 

Figure 7.17a shows the molecular orbitals of benzene. If you were to attempt a simple explanation for the electronic transitions in benzene, you would conclude that there are four possible transitions but each transition has the same energy. You would predict that the ultraviolet spectrum of benzene consists of one absorption peak. However, owing to electron-electron repulsions and symmetry considerations, the actual energy states from which electronic transitions occur are somewhat modified. Figure 7.17b shows the energy-state levels of benzene. Three electronic transitions take... 

Figure 12 Raman-REMPI (resonantly enhanced multiphoton ionization) spectrum of the °Qi(AJ = 0, AK = -2,K- 1) transitions of the Vie e2g) mode of benzene in a molecular beam. An energy-level diagram is shown for the double-resonance experiment. The ultraviolet source was tuned to 36,474 cm and the Raman wave-number calibration is adjusted to match the / = 6 line reported in Ref. 109. The expansion was 13% benzene in argon at 80 kPa and the sampling was done at XfD = 175 (D = 0.20 mm nozzle diameter) using pump and Stokes laser energies of 2 and 0.5 mJ. (From Ref. 117, with permission.)...

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