Benzene electronic transitions

The aforementioned conditions make an analysis of the effect of substituents in thiophene on the UV spectrum more difficult than in the benzene series. In benzene there are two widely separated areas of absorption with different intensities. In thiophene there are instead two or three absorption bands due to electronic transitions which overlap and are of similar intensity. Finally, two very low-intensity bands at 313 and 318 mja have been found in thiophene. ... 

The lowest excited states of benzene are actually of B2u and Bi symmetry, and they can be reached directly by neither one-photon nor two-photon purely electronic transitions (the 0-0 band at energy Eq-o. the origin of the transition, is absent from the spectra). However, excitation into these states can be obtained through vibronic coupling (VC), if a vibrational mode of an appropriate symmetry is coupled to the electronic transition. The IPA or 2PA spectra can then show a series of narrow peaks shifted from the 0-0 band... 

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. 

The Kekule mode of benzene is the skeletal b2u mode a i4, which has been discussed all along and is shown in Scheme 34a. Another b2u mode is the hydrogenic movement shown in in Scheme 34 b and is labeled as co 15. These modes are both IR and Raman inactive for DSh benzene in the ground state. The electronic transition to the 11 B2u excited state is disallowed, and hence, the observations of the b2u... 

The two bands appear very different. Their rotational structure is quite symmetrical but that of aniline shows a pronounced gap near the band centre whereas that of aniline Ar shows a grouping of intense lines. The reason for the difference is that the band of aniline is a type B band of a prolate asymmetric rotor (see Section 6.2.4.4) whereas that of aniline Ar is a type C band of an oblate asymmetric rotor. The electronic transition moment in aniline itself is directed along the b axis which is in the plane of the benzene ring and perpendicular to the C—N axis (which is the a axis). In the aniline Ar molecule, the argon atom sits on the benzene ring, attracted by the n electrons. The fact that the argon atom is relatively heavy causes a rotation of the principal axes on inertia ... 

Since the end of the seventies, interest in cyclotriveratrylene has moved towards the use of its cone shaped structure for applications in various fields, including investigations of the electronic transitions of the benzene chromophore via UV and CD spectroscopy, studies in the area of host-guest chemistry, synthesis of new types of liquid crystals, and searches for new three-dimensional organic charge-transfer materials. These works have been made possible because efficient synthetic... 

Fig. 25 The sign of the molar rotation (MR) and of the CD of the first three electronic transitions of benzene induced by a point charge of -O.le located 1.3 A above the plane of the ring. The distortions along the edges of the sectors (particularly evident in the center of the ring) are caused by the coarseness of the grid. MR calculated with PBEO/SVP, CD with CC2/SVP. Data to prepare the figure were taken from [155]...

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