Hypersensitive transition

Hypersensitive transitions are electric dipole transitions whose shape and intensity display large dependence on the point group symmetry of the metal ion, as well as on the pH, temperature, and ligand type. These transitions obey the following selection rules. 

While over the years several explanations have been proposed for hypersensitivity, including symmetry arguments, vibronic, charge-transfer and electric-quadrupole transition contributions [57,58], the most successful to date has been the dynamic coupling mechanism proposed by Mason et al. [60]. These authors suggest that the intensity of the hypersensitive transitions results from a non-zero electric dipole transition, which arises from an electric dipole in the ligand induced by the/orbitals of the metal ion. By analogy 

In contrast to the situation with the 3d transition metals in particular, the 4f-4f transitions in the electronic spectra of lanthanide complexes rarely serve any diagnostic purpose. It may be noted, however, that the spectra of the octahedral [LnXe] ions (X = Cl, Br) have particularly small extinction coefficients, an order of magnitude lower than the corresponding aqua ions, due to the high symmetry of the environment. 

Some transitions are hypersensitive to changes in the symmetry and strength of the ligand field as a result, they display shifts of the absorption bands, usually to longer wavelength, as well as band splitting and intensity variation. It is most marked for Ho +, Er + and particularly for the 19/2 H9/2, Es/2 and 19/2,- 05/2, G7/2 transitions in the case of the Nd + ion. 

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These hypersensitive transitions have been studied in a large number of carboxylate and amino-acid complexes of the lanthanides (45—49) and also in biological systems (50, 51) where Ca+2 frequently can be replaced by Ln+8 of... 

In a recent paper,609 the hypersensitive transitions 2Him or 4Gim of Er3+ have been... 

In practice, the hypersensitive transitions are often used for the determination of stability constants in aqueous solution. Lanthanide absorption bands in solution do not normally change in position on complexation to such an extent that bands due to the complexed and uncomplexed ion can be clearly observed independently, as is often the case for d transition metal ions, but the marked change of intensity of the hypersensitive bands is sufficient to allow determination of K values, for example as demonstrated for various adducts of [Ho(dpm)3].6U... 

The absorption spectra of some rare earths and actinides in a molten LiNOs—KN03 eutectic have been measured [590, 591]. A comparison of the oscillator strength of the 4650 A band 7Fo 5Dz) in DCIO4 and the molten nitrate eutectic shows an increase [510] of —18 times in the latter solvent. Incidentally, the 7Fo 02 transition is a hypersensitive transition [580]. Carnall et al. [591] were able to observe some transitions between the ground state multiplets viz. 7Fo - 7F, 7F - 7F and 7Fo 7F at 4610,4930 and 3910 cm-1 respectively in the nitrate eutectic. 

However, a theoretical treatment by J0rgensen and Judd [580] points out that these hypersensitive transitions occur due to the inhomogeneity of the dielectric and obey the selection rule AJ = 2. These authors believe that these transitions are pseudoquadrupole in nature. The author [581] has pointed out that the selection rule proposed by J0rgensen - Judd is not rigorously obeyed as they have claimed. Some comments on the validity of J0rgensen - Judd s theory may also be found elsewhere [582, 583. Recently Judd [584] tried to air a certain uneasiness caused by the theory [580], and pointed out that only the following symmetry classes will give rise to hypersensitive transitions. 

According to Jorgensen and Judd, hypersensitivity may occur due to pseudoquadrupole transitions [66]. Consequently an ion embedded in an inhomogeneous dielectric would exhibit hypersensitive behavior. These normally weak electric quadrupole transitions are probably intensified and become hypersensitive transitions. Hypersensitivity can also occur in symmetries of spherical harmonics (Ymk, with k = 1) which form totally symmetrical representations. Thus this permits their inclusion in the crystal field potential. 

In the case of Pr(III), the transition 3H4 —> 3F2 occurs at 5200 cm 1 and shows hypersensitivity. The other two transitions 3H4 -> 3P2 and 3FLt - 1D2 also show a marked sensitivity towards slight changes in the coordination environment of Pr(III) [73]. The latter two transitions in Pr(III) do not obey the selection rules and hence cannot be considered as hypersensitive transitions. 

In the case of Nd(III) both 4l9/2 — 4Gs/24G7/2, and 4Kn/2 transitions [74] have been observed to be hypersensitive transitions although the later transitions have been questioned as to their hypersensitivity [73]. Another transition 4l9/2 —> 4Fs/2 of Nd(III) has been found to be hypersensitive, sometimes showing sensitivity higher than 4l9/2 — 4G5/2 and the reason for this anomaly is not clear. It is suggested that ligand mediated pseudohypersensitivity might explain the unusual hypersensitivity of the transition 4l9/2 —> 4F5/2 [65],... 

One possible viewpoint is that the oscillator strength of the hypersensitive transitions might be affected by changes in the coordination sphere of the lanthanide ion. The aquo lanthanide ion on replacement of coordinated water by another solvent might affect the transition as in the case of substitution by a chromophore. 

Thus solvent exchange might affect the oscillator strength of the transitions and in particular the hypersensitive transitions. 

Some salient observations on hypersensitive transitions are as follows ... 

It has been suggested [77] that this order shows that the iodide complexes are the least distorted from true octahedral symmetry. The hypersensitive transitions showed the sequence... 

The covalent model of hypersensitivity was developed by Choppin and coworkers [70,74] to explain (i) the observed order of sensitivity of the parameters to the environment is 72 > 74 > 7f, >, (ii) the general trend of the hypersensitive transition oscillator strength increases with estimated covalency, (iii) the observed hypersensitivity in octahedral complexes, (iv) the simultaneous occurrence of hypersensitivity and large splittings of... 

J = 3/2 levels, (v) the correlation between oscillator strength of hypersensitive transitions and ligand basicity. 

It is a usual practice to assume that the oscillator strengths of hypersensitive transitions are proportional to the Ti parameter. This assumption is reasonable for aqueous systems. In some hypersensitive transitions, Ui4) and I/<6) matrix elements are significant relative to the magnitude of U(2). Thus it is necessary to be careful when dealing with absolute values of oscillator strength alone, since symmetry may effect a significant contribution to the intensity of both hypersensitive and non-hypersensitive transitions. Caution should be... 

It has been noted that the 7Fo — 5D2 hypersensitive transition in Eu3+ shows greater sensitivity to the environment than any other lanthanide ion. This phenomenon can be explained by possible mixing of the electron transfer character into the hypersensitive transition. The oscillator strength of an allowed electron transfer transition is given by... 

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