Strongly allowed transitions magnetic

For a Kramers doublet the strongly allowed transitions are those in which the nuclear magnetic quantum number does not change. The frequency at which the transition occurs is not altered by the presence of the term given by eq. (14), because the value of P is the same for each component of the doublet. However, in the presence of anisotropy, transitions in which the nuclear magnetic quantum number changes by 1 or 2 may be weakly allowed if these are observable, they provide additional information from which the value of P may be derived. 

For a carbonyl group in a Cjv symmetry environment, such as in formaldehyde, the dipole approximation for the n- r transition yields JWo f > 0 and Mo f = 0. Although the transition is magnetically strongly allowed and polarized along the CO axis, it is electrically forbidden. The absorption therefore is of very weak intensity and the rotational strength is equal to zero. Perturbations by vibrations or by an achiral solvent can affect and to such an extent that a small nonzero electric dipole transition moment results but again this produces only a very small absorption intensity and... 

Thus, g is 10 to 10 and the CD is easily measurable. The absorption of the nn transitions often gains intensity from the effect of vibronic coupling with allowed nn transitions via nonsym-metric vibrations induced by the perturbation of the symmetry of the chromophore by its surroundings. For an electrically allowed and magnetically forbidden dipole transition, g is 10 to 10 and thus its corresponding CD is difficult to measure because of its strong absorption e between 10" and 10 ) as found, e.g., for nn and crcr transitions. For the third type of chromophore no common rule can be given. They are often transitions between n and a or a and n orbitals. 

There are a number of methods to overcome this loss of atoms by optical pumping. For example, there could be two laser beams tuned for excitation out of each hfs state, the laser spectrum could be sufficiently broad to excite both hfs states, there could be an rf or microwave field that would induce hfs transitions to return atoms to the appropriate hfs state, or optical pumping could be inhibited by careful choice of experimental conditions. For a variety of carefully considered technical reasons, we employ the last of these alternatives. We use circularly polarized light and the axis provided by the magnetic tuning field to allow only excitations to a particular sublevel of the atomic excited state. The only strongly allowed decay process returns the atom to the original state. 

Density functional theory and MC-SCF calculations have been applied to a number of pericyclic reactions including cycloadditions and electrocyclizations. It has been established that the transition states of thermally allowed electrocyclic reactions are aromatic. Apparently they not only have highly delocalized structures and large resonance stabilizations, but also strongly enhanced magnetic susceptibilities and show appreciable nucleus-independent chemical-shift values. 

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