Rotational constants from optical spectra

The electron spin-spin constant kv is not determined directly through the radiofrequency spectrum, but nevertheless plays a very important role, as does also the rotational constant Bv. In both cases the required values were taken from the optical electronic spectrum. 

One of the most interesting and important results of the study was to show how the molecular constants change as the vibrational quantum number v increases. This behaviour is presented in table 8.10. The electron spin-spin and rotational constant values came, initially, from the analysis of the optical electronic spectrum [47], although the values of the spin-spin constants for different vibrational levels were refined by the analysis of the radiofrequency spectrum. The nuclear hyperfine parameters are obtained solely from the magnetic resonance experiments. We will discuss the significance of these constants in the following subsection. 

In the second method, the ground-state rotational constants obtained from the analysis of an optical spectrum are compared with the constants obtained from a microwave investigation. Some values for cw-glyoxal are given in Table 4. Again it is found that it is necessary to multiply the statistical standard deviations of the constants by factors from 3 to 6 to obtain realistic estimates of the accuracy. The correlation matrix obtained from the analysis of the optical spectrum is given in Table 5. 

Many early infrared and Raman papers have reported studies on polar molecules which subsequently have been reexamined in the microwave region. In most of these cases, the microwave woik is clearly superior and the infrared results have not been included in these tables. In some cases, however, the addition of even relatively low precision optical data, when combined with microwave data, will lead to improved structural estimates. For example, frequently the Aq (or Co) rotational constant of a symmetric top can be obtained either from pertutbation-induced transitions in the infrared spectrum or from suitable combinations of transitions in a fundamental band, a combination band and a hot band, or else by the analysis of a perpendicular band in the Raman spectrum. It is not possible to obtain this rotational constant in the pure rotational spectrum of a symmetric top molecule, and therefore combining the optical and microwave data leads to much improvement in determining the positions of the off-axis atoms of such molecules. 

Fig. 2c. It can be seen that at 530 nm, the fluorescence decays mono-exponentially with the fluorescence lifetime of 3.24 ns. The rise of the emission seen below 50 ps in the corresponding FlUp data is obviously not resolved here. In contrast, the TCSP data at 450 nm is described by a triple-exponential decay whose dominant component has a correlation time well below the time resolution. This component is obviously equivalent to the fluorescence decay observed in the FlUp experiment. A minor contribution has a correlation time of about 3.2 ns and reflects again the fluorescence lifetime that was also detected at 530 nm. The most characteristic component at 450 nm however has a time constant of about 300 ps. It is important to emphasize that this 300 ps decay does not have a rising counterpart when emission near the maximum of the stationary fluorescence spectrum is recorded. In other words, the above mentioned mirror image correspondence of the fluorescence dynamics between 450 nm and 530 nm holds only on time scales shorter than 20 ps. Finally, in contrast to picosecond time scales, the anisotropy deduced from the TCSPC data displays a pronounced decay. This decay is reminiscent of the rotational diffusion of the entire protein indicating that the optical chromophore is rigidly embedded in the core of the 6-barrel protein.

The confonnational mobility of a-cyclodextrin derivative 24 and some related compounds in solvents such as benzene, dichloromethane, and chloroform has been thoroughly investigated by dynamic n.m.r. spectroscopy, and the H-n.m.r. chemical shifts and coupling constants for a- and P-cyclodextiin and their permethyl ethers in neutral aqueous media have been assigned to obtain accurate data, the experimental spectra were analysed with the Raccoon spin simulation program. The structure of the copolymer obtained by treatment of l,4-anhydro-2,3-0-benzylidene-a-D-ribopyranose and l,4-anhydro-2,3-di-0-benzyl-a-D-ribopyranose with SbCls (see Chapter 5) has been determined from its C-n.m.r. spectrum and its optical rotation. ... 

The rotatory dispersion of amino acids in the visible range of the spectrum has been studied by several workers (Karrer and Kaase, 1919 Waser, 1923 Pertzoff, 1927). Recently, Patterson and Erode (1943) have published detailed results with thirteen a-amino acids. With most compounds studied, the dispersion is normal, i.e., the specific rotation [a] at a particular wave length can be expressed by a one-term Drude equation which has the form a = a/(X — Xo ), where a and Xo are constants. It is likely that, if measurements were extended to the far ultraviolet, amino acids, like many other optically active compounds, would exhibit anomalous dispersion, especially in those parts of the spectrum in which absorption of amino and carboxyl groups becomes significant. It appears, especially from the work of Patterson and Erode (1943) that the l forms of amino acids fall into three classes Group I consists of those amino acids which have a normal and positive dispersion for this class, to which most purely aliphatic amino acids belong, Xo has... 

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