Electric transition moment

FIGURE 14. (a) Representation of charge excess, Q, and defect, 0> on the four carbon atoms for the two orbitals n. (left) and tr (right) for 1,3-butadiene, (b) Electric transition moments arising from the 7T - n transition... 

Figure 1-17. Exciton chirality of acyclic allylic benzoates and the sign of the predicted benzoate Cotton effects. The thick line denotes the electric transition moment of the benzoate group. Reprinted with permission by Am. Chem. Soc., Ref. 61. 

A chemical interconversion requiring an intermediate stationary Hamiltonian means that the direct passage from states of a Hamiltonian Hc(i) to quantum states related to Hc(j) has zero probability. The intermediate stationary Hamiltonian Hc(ij) has no ground electronic state. All its quantum states have a finite lifetime in presence of an electromagnetic field. These levels can be accessed from particular molecular species referred to as active precursor and successor complexes (APC and ASC). All these states are accessible since they all belong to the spectra of the total Hamiltonian, so that as soon as those quantum states in the active precursor (successor) complex that have a non zero electric transition moment matrix element with a quantum state of Hc(ij) these latter states will necessarily be populated. The rate at which they are populated is another problem (see below). 

The Cotton effects may be classified into three types168 those arising from chirally perturbed local achiral chromophores (ketones, /i.y-unsaturated ketones, double bonds, benzoates, aromatic compounds) those arising from inherently achiral chromophores, such as conjugated dienes or a,/3-unsaturated ketones those arising from interaction of the various electric transition moments when two or more chromophores which are chirally disposed are positioned nearby in space, intra- or intermolecularly (exciton chirality method)169. 

The signs of the exciton-split Cotton effect reflect the absolute configuration of the molecule if the direction of the transition moment in the chromophore is established. For example, chiral 9,9 -spirobifluorene derivative 1 has R configuration as indicated by a strong positive CD couplet (A = +1111.7) due to the LBb band of the anthracene chromophores, the electric transition moment of which is polarized along the long axis of the chromophore (Figure 14)100... 

In most of these studies, hydroxyl has been the typical resident functional group, which is derivatized with appropriate acids containing chromophores suitable for exciton coupling. The ideal chromophore would have a very intense UV-visible transition, located in a convenient spectral window, and with the orientation of its electric transition moment being well-defined relative to alcohol R—OH bond. One of the most successful has been p-dimethylaminobenzoate, which has an intense (s ca 30, 000) transition in an... 

Therefore, the transition probability Wab between states a and b is proportional to the time f, to the square of the electric transition moment... 

In equation 23 Im means to take the imaginary part of the corresponding expression. In a series of related compounds for a particular magnetic dipole transition, as is the (n, tt ) excitation of 152-154, 156, 157, the magnetic transition moment is approximately constant, but the electric transition moment may vary widely. Then, if 0 represents the angle between the directions of the electric and magnetic transition moments, an equation... 

Finally, it should be noted that for states and that are produced by mutual magnetic mixing, the contributions to the B terms are of equal magnitude but of opposite sign. Whether the contribution of the energetically lower or the hi er transition to the B term is positive depends on the orientation of the electric transition moment relative to the magnetic transition moments and... 

Although the description of electronic states by means of the perimeter model is somewhat less satisfactory for molecules that can be derived formally from an antiaromatic AN perimeter than for aromatic molecules, simple statements about MCD signs are still possible. While nothing can be said about the S and D bands, which according to the perimeter model have zero electric transition moments and which experimentally are found to be very weak (the latter is normally inobservable), predictions are possible for the strong absorptions that are referred to as the N, N2, P, and P2 bands according to the nomenclature given in Section 2.2.7. The parameters that are essential for the MCD spectra of systems derived from a 4)V-electron perimeter are... 

Inherently chiral chromophores Chromophore itself is twisted and chiral and, therefore, the 7t—7t and/or n—it transitions become CD active, because both electric transition moment and magnetic transition moment < M 0> take nonzero values. The CD intensity of this group is stronger than that of group (a). 

Exciton-coupled CD8, The systems have two or more chromophores, which are placed in chiral positions to each other. Each chromophore exhibits intense tt—tt transition generating a large electric transition moment , and at the same time, these moments make a large magnetic transition moment... 

Figure 40 (a) Vinblastine 14 consists of two half-molecules, cleavamine and vindoline the CD spectra of the component molecules, (b) Left The sum CD spectrum (dotted line) = CD(deavamine) + CD(vindoline) and CD spectrum of vinblastine Center Electric transition moments of indole and indoline chromophores Right Difference CD = CD(vinblastine) - [CD(cleavamine) + CDfvindoline)]. Redrawn from C. A. Parish J.-G. Dong W. G. Bornmann J. Chang K. Nakanishi N. Berova, Tetrahedron 1998, 54, 15739-15758. 

Figure 52 Enantiomeric conformations P and M of bilirubin 128 with electric transition moments. Redrawn from R. V. Person B. R. Peterson D. A. Lightner, J. Am. Chem. Soc. 1994, 116, 42-59.

The dipole coupling requires two chromophoric groups having transitions with significant electric transition moments that couple via dipole-dipole interactions. 

Charge transfer transitions are generally electric dipole allowed, with a corresponding electric transition moment... 

The interaction terms are defined in equation 9. The simplest interpretation of this term is that the radiation field interacts directly with the last two moments (it sees the whole molecule effectively as a point) with the quantity in curly brackets providing a strong perturbative glue of dipolar interactions between electric transition moments (each of which is symmetry allowed)on A and B, so that the radiation field sees the two chromophores as a single system. Further discussion of this term is given elsewhere (5). ... 

Coupling may also occur between an electric transition moment in one chromophore and the transition quadrupole moment generated by an n - 7t transition in a neighbouring chirally disposed chromophore (Tinocco, 1962). This m-p mechanism may give rise to substantial rotatory strengths, e.g. in chiral diketopiperazines (Hooker et al., 1974). 

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