Ham effect

In some aromatic molecules that have a high degree of symmetry, i.e. with a minimum D2h symmetry (e.g. benzene, triphenylene, naphthalene, pyrene, coronene), the first singlet absorption (So —> Si) may be symmetry forbidden61 and the corresponding oscillator strength is weak. The intensities of the various forbidden vibronic bands are highly sensitive to solvent polarity (Ham effect). In polar solvents, the intensity of the 0-0 band increases at the expense of the others. 

In connection with the Jahn-Teller effect the role of A may be well defined. It has been shown that if a dynamic Jahn-Teller effect is operative, very substantial quenching of orbital angular momentum may take place — the Ham effect. In the case of T ground terms, if the dynamic Jahn-Teller frequencies bear the right relationship to spin-orbit coupling and temperature, A may be reduced to values well below 0.5.113119... 

The Jahn-Teller community has lost one of our illustrious members with the death of Frank Ham on December 12, 2002. Phrases such as The Ham effect , Ham factors and Ham quenching are terms that frequently appear in any article in which the Jahn-Teller effect or vibronic coupling is involved. I would like to record my own personal thoughts about Frank and on behalf of the whole Jahn-Teller community. 

Frank s research career began at a time when Solid State Physics was a new topic in Physics. He contributed very much to what became an all-embracing topic in both basic physics and in its numerous applications in chemistry as well as physics in areas now frequently described as Condensed Matter . He pointed out how random strains can force a dynamic Jahn-Teller system to reflect distorted static behaviour in certain cases particularly in EPR spectra. This involved considerations of the orbital angular momentum of the magnetic ions present in these systems and he showed how the Ham Effect , as it became known, could explain why the electronic angular momentum could be quenched in many systems. 

A fundamental contribution was given successively by Ham who demonstrated the possibility of quenching off-diagonal electronic operators by dynamic JT coupling. The Ham effect can be visualized by the following considerations. Let us express the complete wavefunction of the system (r, Q) as... 

Spin-orbit coupling, g-factors, nuclear quadrupole splitting, hyperfine interaction are some of the magnitude affected by the Ham effect and one can say that Ham s papers have opened up the wide field of the different spectroscopic techniques to investigate the JT effect. 

The spectroscopic behavior of a JT system is to a large extent governed by the quenching of electronic operators (Ham effect), which causes a shift of the absorption (or emission) bands and a modification of their shapes. Moreover the vibronic mixing of different electronic states can strongly affect relaxation processes, which also modify spectral band shapes. 

Symmetry-lowering effects of the solvent are referred to as the Ham effect (Ham, 1953 Platt, 1962). Thus, in rigid or fluid solutions, symmetry-forbidden vibrational components of the Bju ( Lb) absorption band of benzene appear with increasing intensity as the polarizability and the polarity of the solvent increase. The fact that for pyrene and 2-methylpyrene the direction of the Lb transition moment is inclined on the average 40° and 20° away from the y axis, respectively, has also been ascribed to a symmetrylowering perturbation by the environment, related to the Ham effect (Lang-kilde et al., 1983). The associated intensification of otherwise weak vibronic peaks in the Lb band can be used for an investigation of microenvironments such as micelles. 

The chapter contains a detailed description of two methods of determining the JT energy (1) the spin-orbit splitting quenching (the Ham effect) and (2) analysis of the potential energy surfaces in the excited electronic state. 

Example of Estimations of the Jahn-Teller Energy from the Second Order Ham Effect... 

The present chapter was devoted to the detailed consideration of the dynamic JT effect in the orbital triplet states for the 3d ions in a cubic crystal field, which included analysis of the spin-orbit splitting quenching (Ham effect) and geometry of the excited states (deformation of the equilibrium ligands configuration and cross-section of the potential energy surfaces). All necessary equations involved into such an analysis were given and explained. Theoretical description has been supported by... 

The magnitude of the Jahn-Teller coupling in the [Ti(OH2)6] " and [V(OH2)6] " cations is weak compared to the [Cu(OH2)6] and [Cr(OH2)6] " cations that we will consider in Sect. 3.3. Nevertheless, the effect on the electronic coordinates are no less profound since spin-orbit coupling acts to first order in orbital triplet ground terms and is greatly affected by the Jahn-Teller interaction. The Ham effect... 

Transition metal complexes have represented, for a long time, the archetypical system for which the JT effect plays a crucial role, especially with regard to crystal field splitting and spin-orbit interaction (Ham effect). This affects optical as well... 

From the vibrational structure in the monomer fluorescence spectrum of Py, the Ham-effect (54-58), information was obtained on the polarity of the surface where the excimer and dimer formation takes place. It appears that Py on silica/decanol resides in an environment similar to a homogeneous decanol solution (36), whereas for Py on dry silica a considerably higher polarity was found between that of methanol and water, see Fig. 8. For rever-sed-phase Si-C.jg in contact with methanol/water 3/1 (Section 4.2.3), it was concluded that Py finds itself in a nonpolar medium (21). 

The sensitivity of the Ham effect is strongly reduced (49,58) for 1-all ylpyrenes, such as 1Py(3)1Py and PPS/PDS. Therefore, information on environmental polarity cannot easily be obtained with these molecules. 

The vibrational structure of the fluorescence of aromatic molecules is also sensitive to solvent polarity. Particularly the ratio of the intensity of the first over the fourth peak of the pyrene monomer fluorescence increases with the polarity of the solvent (Ham effect). This was used by Nakajima (190) and by Kalyanasundaram and Thomas (191) to probe the environment of pyrene in SDS and CTAC micelles, which turned out to be similar to short chain alcohols. This polarity must be attributed to the surface region of the micelles, since it was concluded from NMR investigations that pyrene in these micelles is solubilized in the surface region (192). 

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