Spectral parameters

Specinfo has an additional tool for calculating NMR spectra that is based on the data sets of the compounds contained in the database. This leads to quite reliable calculated spectral parameters for the compound classes which are registered in the database. 

The amplitude of the elastic scattering, Ao(Q), is called the elastic incoherent structure factor (EISF) and is determined experimentally as the ratio of the elastic intensity to the total integrated intensity. The EISF provides information on the geometry of the motions, and the linewidths are related to the time scales (broader lines correspond to shorter times). The Q and ft) dependences of these spectral parameters are commonly fitted to dynamic models for which analytical expressions for Sf (Q, ft)) have been derived, affording diffusion constants, jump lengths, residence times, and so on that characterize the motion described by the models [62]. 

Figure 2 Spectral parameters typically used in band shape analysis of an FTIR spectrum peak position, integrated peak area, and FWHM.

Radical cations can be derived from aromatic hydrocarbons or alkenes by one-electron oxidation. Antimony trichloride and pentachloride are among the chemical oxidants that have been used. Photodissociation or y-radiation can generate radical cations from aromatic hydrocarbons. Most radical cations derived from hydrocarbons have limited stability, but EPR spectral parameters have permitted structural characterization. The radical cations can be generated electrochemically, and some oxidation potentials are included in Table 12.1. The potentials correlate with the HOMO levels of the hydrocarbons. The higher the HOMO, the more easily oxidized is the hydrocarbon. 

The NMR spectral parameters of a number of p3 Tidop3 Timidincs are shown in Table II. 

Fluoride systems containing aluminum trifluoride, A1F3, were investigated in greater detail. Based on Raman spectra, researchers repeatedly demonstrated the presence of two types of complex ions, A1F4" and A1F63 [347 - 350]. The spectral parameters of these complexes are as follows (wave numbers in cm 1) ... 

Fig. 92. Mass spectral parameters pressure (p in torr) and ionic currents (J in relative units) of Li2NbOF5 (A), Na2NbOFs (B) and K2NbOFs (C) versus temperature in °C (after Agulyansky et ah, [383]). Numbers on the curves correspond to ions as shown in Fig. 91.

TABLE 3. Electronic spectral parameters of sulfonyl radicals... 

Table II. MCssbauer Spectral Parameters Obtained from Computer Fitting Spectra of Figure 1...

If all nuclei are assigned and the spectral parameters for the conformational analysis are extracted, a conformation is calculated - usually by distance geometry (DG) or restrained molecular dynamics calculations (rMD). A test for the quality of the conformation, obtained using the experimental restraints, is its stability in a free MD run, i.e. an MD without experimental restraints. In this case, explicit solvents have to be used in the MD calculation. An indication of more than one conformation in fast equilibrium can be found if only parts of the final structure are in agreement with experimental data [3]. Relaxation data and heteronuclear NOEs can also be used to elucidate internal dynamics, but this is beyond the scope of this article. 

Mossbauer spectroscopy is particularly suitable to study ST since (1) the spectral parameters associated with the HS and LS states of iron(II) clearly differ and (2) the time-scale of the technique ( 10 s) allows the detection of the separate spin states in the course of the transition. Typically, Mossbauer spectra of HS iron(II) show relatively high quadrupole splitting (AEq 2-3 mm s ) and isomer shift (3 1 mm s ), while for LS iron(II), these parameters are generally smaller (AEq < 1 mm s 3 < 0.5 mm s ). Among the early applications of Mossbauer spectroscopy to study ST phenomena in iron(II) complexes is the work of Dezsi et al. [7] on [Fe (phen)2(NCS)2] (phen = 1,10-phenanthroline) as a function of temperature (Fig. 8.2). The transition from the HS ( 12) state (quadrupole doublet of outer two lines with AEq 3 mm s ) to the LS CAi) state (quadrupole... 

The growing importance of quantum-chemical calculations is dealt with in a short section, with emphasis on the consideration of relativistic effects, especially in systems containing mercury. These calculations aim at optimization of structures, determination of bond energies, simulation of spectra, and estimation of spectral parameters, independent of but complementary to experiments. 

This benefit comes at a cost, which arises from significantly reduced S/N and some interpretive difficulty as compared to IR. Developments on the latter front are bringing the theoretical prediction capability of VCD for small molecules to a level demonstrably superior to that for ECD (Freedman and Nafie, 1994 Stephens et al., 1994 Stephens and Devlin, 2000), especially for peptide spectra (Kubelka et al., 2002). Most previous protein applications of VCD used empirically based analyses (Keiderling, 1996, 2000). Theoretical methods are limited when applied to large molecules such as proteins however, a hybrid approach using ab initio determination of spectral parameters with modest-sized molecules for transfer to large peptides has made simulation of spectra for large peptides possible (Bour et al., 1997 Kubelka et al., 2002). Theoretical techniques for simulation of small-molecule VCD are the focus of several previous reviews (Stephens and Lowe, 1985 Freedman and Nafie,... 

Smith [83] classified large sets of hydrocarbon oil infrared spectral data by computer into correlation sets for individual classes of compounds. The correlation sets were then used to determine the class to which an unknown compound belongs from its mass spectral parameters. A correlation set is constructed by use of an ion-source summation, in which a low resolution mass spectrum is expressed as a set of numbers representing the contribution to the total ionisation of each of 14 ion series. The technique is particularly valuable in the examination of results from coupled gas chromatography-mass spectrometry of complex organic mixtures. 

The first reliable evidence for the formation of thiabetaines (+)P-C-C-S( ) as true intermediates in thio-Wittig reaction was obtained11-13 only after the synthesis and complete characterization of the corresponding thiabetaines I of the silicon series.14,15 This evidence was based on the resemblance of the spectral parameters of compounds of both series.11... 

The spectral parameters of 15a and other silicon-organophosphorus betaines described henceforth in Section 2.2.2 allowed us to show reliably that the reaction of phosphorus ylides with thiocarbonyl compounds, unlike the classical Wittig reaction, occurs through the intermediate formation of betaines (17)11 (Scheme 9). Erker and coworkers performed a more detailed... 

TABLE 2. Structural (molecular mechanics) and spectral parameters of selected distorted dienes0... 

TABLE 3. ESR spectral parameters for nitroxyl radicals formed from R3 M and nitro compounds... 

TABLE 7. ESR spectral parameters for RaM+ and R.3MMR.3+ radical cations... 

In an effort to understand the mechanisms involved in formation of complex orientational structures of adsorbed molecules and to describe orientational, vibrational, and electronic excitations in systems of this kind, a new approach to solid surface theory has been developed which treats the properties of two-dimensional dipole systems.61,109,121 In adsorbed layers, dipole forces are the main contributors to lateral interactions both of dynamic dipole moments of vibrational or electronic molecular excitations and of static dipole moments (for polar molecules). In the previous chapter, we demonstrated that all the information on lateral interactions within a system is carried by the Fourier components of the dipole-dipole interaction tensors. In this chapter, we consider basic spectral parameters for two-dimensional lattice systems in which the unit cells contain several inequivalent molecules. As seen from Sec. 2.1, such structures are intrinsic in many systems of adsorbed molecules. For the Fourier components in question, the lattice-sublattice relations will be derived which enable, in particular, various parameters of orientational structures on a complex lattice to be expressed in terms of known characteristics of its Bravais sublattices. In the framework of such a treatment, the ground state of the system concerned as well as the infrared-active spectral frequencies of valence dipole vibrations will be elucidated. 

I. UV and IR Spectroscopy of Nitronates Table 3.9 contains selected data from UV and IR spectroscopy of different types of nitronates. It is quite evident that characteristic spectral parameters are virtually independent of the nature of the fragment bound to the oxygen atom of the nitro group. 

These authors noted that if the last group in the Fe(II) coordination sphere is acetonitrile a low-spin complex is obtained [22]. We found [23] that the aqua complex is high-spin (Mossbauer spectral parameters for the sulfate <5=1.15 mm s 1, A q=3.23 mm s 1), whereas the corresponding isothiocyanate complex is a crossover complex [23]. 

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