Ham bands

Gotch AJ, Garrett AW, Zwier TS. The ham bands revisited spectroscopy and photophysics of the C6H6-CC14 complex. J Phys Chem 1991 95 9699-9707. 

Fluorescence decay of 9-methylanthracene has been used by Tan and Treloar to study the coiling of poly(methacrylic acid) in water. Hara and Ware have studied the influence of solvent on the radiative probablity from the i state of pyrene. The fraction of fluorescence in the 0-0 parallels the extinction coefficient change for the 0-0 band. The Ham band also decreased with temperature, an effect correlated with the decrease in dielectric constant with temperature. There is also evidence that a solvent-solute interaction is involved in Ham band effects in alcohols and aromatic solvents. Use of the integrating sphere eliminates any uncertainty in the conventional instrumental refractive index correction. Some of the data obtained is given in Table 4. 

The first stoand of Ham s argument [11] is that V(c()) supports continuous bands of Floquet states, with wave functions of the form... 

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. 

Azulene has weak absorption in the visible region (near 7000 A) and more intense band systems in the ultraviolet. The first ultraviolet system, which commences at about 3500 A, has been examined in substitutional solid solution in naphthalene (Sidman and McClure, 1956) and in the vapour state (Hunt and Ross, 1962), and can be observed in fluorescence from the vapour (Hunt and Ross, 1956). Theory predicts that the transition is 1Al<-lAl(C2K), i.e. allowed by the electronic selection rules with polarization parallel to the twofold symmetry axis (see, e.g., Ham, 1960 Mofifitt, 1954 Pariser, 1956b). The vibrational analysis shows that the transition is allowed but does not establish the axis of polarization. The intensity distribution among the vibrational bands indicates a small increase in CC bond distance without change in symmetry. 

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. 

Gleiter et al. measured the He(Ia) photoelectron (PE) spectra of the parent heterocycle and its five mono-, di-, and tri-methyl derivatives, and assigned the PE bands by comparison with the results of MO calculations <83CPL(97)94>. They showed that the spectral assignment was consistent with the results of four very different theoretical procedures (HMO, EWMO, HAM/3, and LOGO MB). Recently, it has been reported that the PE spectra were reproduced well by the Outer-Valence Green Function approach with PM3 <91JCS(P2)1865> (see Table 3). 

B. Segall, F.S. Ham "The Green s Function Method of Korringa, Kohn and Rostoker for the Calculation of the Electronic Band Structure of Solids", in Methods of Computational Physics, Vol.8, ed. by B. Adler,... 

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