Intensity of spectra

Fig. 4. Integrated intensity of spectra for a methyl rotation with the three site jump as a function of jump rate k.

The intensity of resonances may be increased by up to 200% when iR nuclei which are directly bonded to the carbon atom are irradiated. This effect is very important in increasing the intensity of spectra when they are proton-decoupled. The efficiency of the proton/carbon NOE varies from carbon to carbon and this is a factor that contributes to the generally non-quantitative nature of NMR. While the intensity of protonated carbon atoms can be increased significantly by NOE, non-protonated carbons (quaternary carbon atoms) receive little NOE and are usually the weakest signals in a NMR spectmm. 

Figure 6 Emission spectra of /ac-ClRe(CO)3(4,7-Ph2-phen) in the cycloaliphatic epoxy (ERL-4221)/anhydride resin at 293 K (a) before curing, (b) partially cured by heating at 393 K for 30 min, and (c) cured by heating at 393 K for 60 min. Excitation wavelength is 400 nm. Intensities of spectra (a) and (b) have been scaled xlO and x5, respectively. (From Ref. 80.)...

Fig. 22. Effects of oxygenation on the linewidth of hyperfine-shifted proton resonances of 12% Hb A in 0.1 Af Bis-Tris in D2O at pH 6.6 and 27°C. (A) Fully deoxy sample (-—) (B) 23.5% oxygenation (—) (C) 31.6% oxygenation (—). The intensities of spectra B and C have been prescaled to give the same intensity as spectrum A. [From Viggiano et al. (1979)].

Bour and Keiderling (1993) report ah initio simulation of the vibrational circular dichroism of coupled peptides in the amide I and II region. Using the MFP model and the 4-3IG basis set they were able to reproduce the VCD sign pattern and the relative intensities of spectra of proteins in the a-helical, /)-sheet 3io-helical and polyproline II conformations. 

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. 

Figure 19 Powder XRD spectra of metal complex (A) and silica-supported samples at metal complex loadings of (B) 285, (C) 221, (D) 166 mmol metal complex/g silica, and (E) silica. The intensities of spectra A B are a factor of 10 greater than the intensities of spectra C-E...

Reduction had to be made cautiously as Cu can be readily reduced by hydrogen above 573 K. This treatment at 373 K resulted in an increase in the symmetry of copper ions. The signal intensities of spectra CuLR and CuSR are lower due to the fact that Cu and Cu are ESR silent species. By reduction water molecules can be formed, which can be coordinated to the remaining Cu ions forming octahedral complexes, different from that of the hydrated samples. 

In spectra-structure correlation problems, the spectral representations, be it for IR, for mass, or for any other spectroscopy, are usually input as sets of equidistant intensities with significantly high resolution. Such inputs require hundreds if not thousands of units on input. Additionally, the intensities of spectra are given as real values and mostly large data bases of spectra have to be taken into account if reliable predictions are to be expected (see Spectroscopic Databases). Therefore, spectra-structure correlation problems have to be implemented on larger and faster work-stations (or mainframes) rather than on personal computers. ... 

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is described in section Bl.25,2.1. The most connnonly employed x-rays are the Mg Ka (1253.6 eV) and the A1 Ka (1486.6 eV) lines, which are produced from a standard x-ray tube. Peaks are seen in XPS spectra that correspond to the bound core-level electrons in the material. The intensity of each peak is proportional to the abundance of the emitting atoms in the near-surface region, while the precise binding energy of each peak depends on the chemical oxidation state and local enviromnent of the emitting atoms. The Perkin-Elmer XPS handbook contains sample spectra of each element and bindmg energies for certain compounds [58]. 

The interpretation of emission spectra is somewhat different but similar to that of absorption spectra. The intensity observed m a typical emission spectrum is a complicated fiinction of the excitation conditions which detennine the number of excited states produced, quenching processes which compete with emission, and the efficiency of the detection system. The quantities of theoretical interest which replace the integrated intensity of absorption spectroscopy are the rate constant for spontaneous emission and the related excited-state lifetime. 

Albrecht M G and Greighton J A 1977 Anomalously intense Raman spectra of pyridine at a silver electrode J. Am. Chem. Soc. 99 5215-17... 

The polarization dependence of the photon absorbance in metal surface systems also brings about the so-called surface selection rule, which states that only vibrational modes with dynamic moments having components perpendicular to the surface plane can be detected by RAIRS [22, 23 and 24]. This rule may in some instances limit the usefidness of the reflection tecluiique for adsorbate identification because of the reduction in the number of modes visible in the IR spectra, but more often becomes an advantage thanks to the simplification of the data. Furthenuore, the relative intensities of different vibrational modes can be used to estimate the orientation of the surface moieties. This has been particularly useful in the study of self-... 

Figure Bl.22.6. Raman spectra in the C-H stretching region from 2-butanol (left frame) and 2-butanethiol (right), each either as bulk liquid (top traces) or adsorbed on a rough silver electrode surface (bottom). An analysis of the relative intensities of the different vibrational modes led to tire proposed adsorption structures depicted in the corresponding panels [53], This example illustrates the usefiilness of Raman spectroscopy for the detennination of adsorption geometries, but also points to its main limitation, namely the need to use rough silver surfaces to achieve adequate signal-to-noise levels.

Figure Bl.25.5. (a) XPS spectra at take-off angles of 0° and 60° as measured from the surface nonnal from a silicon crystal with a thin layer of Si02 on top. The relative intensity of the oxide signal increases significantly at higher take-off angles, illustrating that the surface sensitivity of XPS increases, (b) Plot of...

Multivariate data analysis usually starts with generating a set of spectra and the corresponding chemical structures as a result of a spectrum similarity search in a spectrum database. The peak data are transformed into a set of spectral features and the chemical structures are encoded into molecular descriptors [80]. A spectral feature is a property that can be automatically computed from a mass spectrum. Typical spectral features are the peak intensity at a particular mass/charge value, or logarithmic intensity ratios. The goal of transformation of peak data into spectral features is to obtain descriptors of spectral properties that are more suitable than the original peak list data. 

In an emission spectrum a fixed wavelength is used to excite the molecules, and the intensity of emitted radiation is monitored as a function of wavelength. Although a molecule has only a single excitation spectrum, it has two emission spectra, one for fluorescence and one for phosphorescence. The corresponding emission spectra for the hypothetical system in Figure 10.43 are shown in Figure 10.44. 

The ubiquitous use of the word Tine to describe an experimentally observed transition goes back to the early days of observations of visible spectra with spectroscopes in which the lines observed in, say, the spectmm of a sodium flame are images, formed at various wavelengths, of the entrance slit. Although, nowadays, observations tend to be in the form of a plot of some measure of the intensity of the transition against wavelength, frequency or wavenumber, we still refer to peaks in such a spectmm as lines. 

In electronic spectra there is no restriction on the values that Au can take but, as we shall see in Section 1.2.53, the Franck-Condon principle imposes limitations on the intensities of the transitions. 

It might be thought that the small number of molecules in a typical supersonic jet or beam would seriously limit the sensitivity of observation of the spectra. Flowever, the severe rotational cooling which may be produced results in a collapsing of the overall intensity of a band into many fewer rotational transitions. Vibrational cooling, which greatly increases the population of the zero-point level, concentrates the intensity in few vibrational transitions, and these two effects tend to compensate for the small number of molecules. 

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