Spectral calculations spectra

In order to elucidate the interrelation between the energy dependence of the absorption spectrum and the time dependence of the molecular motion it is instructive to investigate the influence of degrading the spectral resolution. The comparison with a measured spectrum often makes it necessary to average the calculated spectrum over an experimental reso-... 

Additional evidence for the effect of polymerization appears in the x-ray photoelectron spectral intensities of sihcates. DVM-Aa calculations on the energies and intensities of spectra by Sasaki and Adachi (1980a,b) satisfactorily reproduce relative intensities in the upper-valence-band region for SO/ [Fig. 5.8(a)] but seriously underestimate the intensity of the 5 i orbital feature of Si02 using a SiO/ cluster model [Fig. 5.8(b)]. This error may be a result of the influence of polymerization in SiOj, although the calculated spectrum is also somewhat different from that observed for olivine in Fig. 5.7. 

As an example of the VCD ability to discriminate absolute configurations, the experimental spectrum of 4-fluorophenyl-3-hydroxymethyl-l-methylpiperidine and the calculated spectra of the (3R,4R)- and (3R,4S)-isomer are presented in Fig. 8.5 [109]. An almost perfect band-to-band correspondence is evident between experimental and calculated spectra of the (3R,4S)-isomer, while the calculated spectrum of the low energy conformer of the (3R,4R)-isomer provides unrealistic VCD pattern with a poor or even no coincidence with the experiment. In the calculated spectrum of the (3R,4S)-isomer, four low-energy conformers were included in the spectral envelope. For comparison, the corresponding calculated and experimental IR absorption spectra are shown. It is evident that the variance with respect to configuration is much more pronounced in VCD spectra While the calculated spectrum for the (3R,4R)-isomer significantly differs from experimental VCD, the unpolarized absorption spectrum of the (3R,4R)-isomer exhibits some common... 

Modem data treatment emphasises a synthetic approach to the interpretation of INS spectra, where the observed spectrum is compared with one obtained from calculations based on a putative ab initio structure and force field for the system. This is a powerful method and certain of success when the nature of the sample is not in question. However, our understanding of the most interesting samples is perhaps more tenuous than we should like, as will be the case for the adsorbed state of molecules on surfaces or for very recently discovered materials. It is when the synthetic approach fails badly (as when the calculated spectrum bears no relationship to that observed) that resort must be had to the analytical techniques of spectral interpretation. 

With slow kinetics of the involved processes or with interfaces where electrode potential modulation might be detrimental because of crystallographic changes in the metal surface, other spectroscopic techniques have to be used. The whole spectrum of interest can be scanned or registered within a few milliseconds with a rapid scan spectrometer or a multichannel (diode array) spectrometer. Repeated acquisition provides the required signal-to-noise ratio. After a potential step, the acquisition is repeated and spectral calculation yields AR/R. This single potential step procedure allows investigation of systems where repeated potential modulation has failed. 

Figure 14 Band deconvolution and component-fitting in the FT-Raman spectrum of a 5 M solution of LiC104 in acrylonitrile [perchlorate Pt(Ai) spectral region]. From top to bottom, experimental spectrum, calculated spectrum, and resolved components. Arrows indicate bands from the solvent. (Reprinted from Ref. 243.)...

The DFT calculation yielded a list of the calculated frequencies, absolute RAIRS intensities, and description of the 70 normal modes possessing frequencies higher than 600 cm [93], Table 3.4.1.6 compares the main bands observed in the experimental RAIR spectrum with the calculated spectrum and the description of the normal modes giving rise to the vibrational bands. The major spectral regions of interest are described in more detail below. 

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