Transition moments direction

First, we note that the cr-cr nature of the lowest excited state of long-chain polysilanes predicted by the semiempirical calculations described in detail below implies a transition moment direction lying approximately along the chain direction rather than perpendicular to it. Evidence for such an orientation of the transition moment has been obtained in measurements on solid polysilane samples (19,36). This... 

We now report that in the region of the absorption band the flow linear dichroism of a solution of 1 is positive (Fig. 3). Assuming that the nature of the flow orientation is of the usual kind, i.e., that the polymer chains in a random coil conformation which dominates in solution (34) tend to align with the flow direction, this observation provides additional support for the absolute assignment of the transition moment direction along the chain direction, even in solution. Similar conclusions based on polarization studies on a stretched film of poly(di-n-hexyl silane) have recently been reported (36). 

For a rod-like probe with its absorption transition moment direction coinciding with the long molecular axis, the rotational motion in this potential well is described by the diffusion coefficient D. The decay of the autocorrelation functions is then shown to be an infinite sum of exponential terms ... 

Polarization data can help to assign the transition moment direction in an absorption band. 

If computations of orientation and transition moment direction are to be meaningfully correlated with the observations, it is clear that factors of the type just discussed must be carefully taken into consideration. 

In the above instances the qualitative polarization characteristics of a band served to provide information of importance in making assignments. If we make use of the quantitative aspects of the dichroic ratios of absorption bands then we can in addition obtain structural information about the polymer. In particular, it becomes possible to determine in many cases the direction which transition moments make with the fiber axis, which is often significant in establishing structural parameters. It must be noted that transition moment directions do not by themselves always serve to specify the orientation of chemical groups. In cases where... 

It will be apparent from the above discussion that a satisfactory set of assignments has not yet been achieved for the polypropylene spectrum. In most cases this is because the basic nature of the mode is unknown. Studies on partially and fully deuterated polypropylenes would answer these questions, and will clearly have to be undertaken in order for the final results to be more certain. When this has been done it should then be possible to evaluate more meaningfully the significance of the deviations between the observed and calculated I Ie ratios. As we have noted, these ratios are determined by the structure, if we make simple assumptions about the group transition moment directions. The deviations imply that either these assumptions are not valid or that the structure requires... 

Maier and coworkers have shown that it is possible to induce by irridation using the appropriate absorption band, a 1,2-hydrogen shift in silenes and in silylenes29,158. Thus, it is possible to switch photochemically between silylenes and silenes. Michl, West and coworkers have used this approach to isomerize dimethylsilylene 285 and 1-methylsilene 26 several times (equation 70). Due to the clean formation of 285 from the diazido precursor 286153 it was possible to measure the IR transition moment directions for both 26 and 285156. 

From that value a force constant of k = 5.6 mdynA 1 for the Si=C double bond is deduced255. This frequency is clearly higher than the usual range for Si—C stretch vibrations but substantially less than for C=C stretches, both because Si is heavier than C and because the Si=C bond is weaker than the C=C bond. More suitable for the experimental characterization is the vinylic Si—H stretch vibration which gives rise to a medium band at 2239 cm-1 (25) or 2187 cm-1 (2)29, hypsochromically shifted by around 100 cm-1 relative to the Si—H stretch in simple silanes. A detailed analysis of the vibrational spectra of matrix-isolated MeHSi=CH2 26 using polarized IR spectroscopy established IR transition moment directions relative to the tot -transition moment (Si-C axis) in 26156. These data provide detailed information about the vibrational modes and about the structure of 26156. The bathochromic shift of the Si=C stretch in the isomeric 1,3-silabuta-l,3-dienes 289 and 290 by around 70 cm 1 compared with the Si=C stretch in simple silenes (Table 15), was interpreted as an indication of Si=C—C=C and C=Si—C=C 7r-conjugation159. 

Figure 14-1. Structure and atomic numbering schemes of nucleic acid bases and Watson-Crick base pairs. The represents the transition moment direction according to the DeVoe-Tinoco convention [11]...

Table 14-1. Computed and experimental transition energies (AE eV), dipole moments (p Debye), oscillator strengths (f) and transition moment directions (d> °) according to Tinoco-DeVoe convention of hypoxanthine [159]... 

Table 2. Transition moment directions of some of the absorption bands of PBLG in d ee...

The transition moment directions of some of the absorption bands of PBLG in liquid crystalline state have been investigated in a CaF2 cell by means of the infra-red dichroic ratio (56). All the transition moment directions measured at 1 °C are very similar to those observed 32) on mechanically oriented films of PBLG (Table 2), verifying that all the polymer molecules present a parallel or nearly parallel orientation in the molecular cluster and that the main chain and the side chains of the polymer molecule are fixed as in the solid film. No significant difference is observed between the systems of solution testing. The orientation of the solvent molecules (methylene moletules) is detected in an electric field in accordance with the NMR observation (56). 

The absorption spectra of proteins and polypeptides are now much better understood as a result of the vibrational analyses given by Miyazawa (1960, 1962, 1963), but it is still true that structures are more often used to test the interpretation of spectra rather than the reverse. Transition moment directions and coupling effects, however, are now sufficiently well understood for infrared dichroism measurements to provide at least a semiquantitative evaluation of some features of a model. 

The early speculations about transition moment directions in the lower aromatics needed to be tested experimentally. The traditional vapour phase measurements of band lineshapes were not accessible with the spectroscopic resolving power we then had, but came later (sec 4.2). Most absorption systems of aromatics in the UV were in any case diffuse and structureless. The method that could be used was to get the polarization of the light absorbed by the crystal composed of the target molecules. The positions and orientations of the molecules in the laboratory frame were known from X-ray crystal structure analysis. The interpretation of results raised non-trivial theoretical problems of its own. The experiments will be described first. 

Thus, knowledge of the transition moment direction of a phenol band could help in interpreting the fluorescence spectrum of a tyrosine chromophore in a protein in terms of orientation and dynamics. The absorption spectrnm of the first excited state of phenol was observed around 275 nm with a fluorescence peak aronnd 298 nm in water. The tyrosine absorption was reported at 277 nm and the finorescence near 303 nm. Fluorescent efficiency is about 0.21 for both molecules. The fluorescent shift of phenol between protic and aprotic solvents is small, compared to indole, a model for tryptophan-based protein, due to the larger gap between its first and second excited states, which resnlts in negligible coupling . ... 

Since multiple modes occur for a helix, it can be difficult to determine which frequencies should be seen in the spectra. This problem can be resolved for IR-active modes for which the transition moment direction is known, such as amide 1 It is only necessary to sum the transition moments in a translational repeat of the helix, taking account of the appropriate phase relationships between motions in adjacent l,d units. The intensity is proportional to the square of this summed transition moment. 

Thulstrup, E. W., Michl, J., Eggers, J. H., Polarization Spectra in Stretched Polymer Sheets. Physical Significance of the Orientation Factors and Determination of 7t,Jt Transition Moment Directions in Molecules of Low Symmetry, J. Phys. Chem. 1970, 74, 3878 3884. 

---