Spectrum prediction additivity rules

The estimation of chemical shifts by examining the spectra of model compounds is not always feasible, and the prediction models fail to distinguish between two or more stereosequences as they cannot always be distinguished on the basis of intensity alone. To overcome these limitations, large numbers of organic compounds have been analyzed by NMR and their chemical shifts have been used to determine a set of empirical correlations that are used to determine the structure based on the polymer s NMR spectrum. The carbon chemical shifts of hydrocarbon-based polymers such as polyethylenes can be predicted by empirical additivity rules such as ... 

The most common approaches to predicting spectra are based on empirical modeling, linear additivity, database retrieval, rule sets, and semiempirical methods. The availability of large spectral libraries has proved to be a valuable resource in these studies. Although ab initio theory relating to spectrum prediction is well advanced, the theoretical equations necessary for application to real-world structures are large and complex... 

Several empirical approaches for NMR spectra prediction are based on the availability of large NMR spectral databases. By using special methods for encoding substructures that correspond to particular parts of the NMR spectrum, the correlation of substructures and partial spectra can be modeled. Substructures can be encoded by using the additive model greatly developed by Pretsch [11] and Clerc [12]. The authors represented skeleton structures and substituents by individual codes and calculation rules. A more general additive model was introduced... 

What conclusions can be drawn from the computations on 1,2-addition The Felkin-Anh rules seem to apply fairly well across a broad spectrum of reactions. Computations clearly indicate that the stereoselectivity is based on a number of competing factors—sterics, orbital interactions, and electrostatic interactions—that can be subtly balanced. Perhaps most critical is that relatively simple computations can offer real predictive power, providing guidance to the synthetic chemist in their pursuit of high enantioselectivity and diastereoselectivity. 

For simple molecules, this approach predicts the number of fundamental vibrations that exist. Use of the dipole moment rule indicates which vibrations are IR-active, but the IR spectrum of a molecule rarely shows the number of absorption bands calculated. Fewer peaks than expected are seen due to IR-inactive vibrations, degenerate vibrations, and very weak vibrations. More often, additional peaks are seen in the spectmm due to overtones and other bands. 

Spectral bands of an aquated lanthanide ion arising from vibronic contributions were reported first by Haas and Stein (1971) in their study of the emission spectrum of aquated Gd. These bands are termed vibronic because they arise from a simultaneous change in the electronic state of the metal ion and the vibrational state of a coordinated ligand. Stavola et al. (1981) noted additional examples of such bands and presented a theoretical model based on the importance of electronic factors for calculating the intensities of lanthanide-ion vibronic transitions. Their theoretical model also predicts selection rules for such transitions. The intensities of observed bands assigned by these workers as being vibronic typically were at least 50 times weaker than the parent purely electronic band. Faulkner and Richardson (1979) have... 

The normal vibration calculations based on a correct structure and correct potential field permit a good correlation to be made between predicted and observed ateorption bands in the FIR spectra of high-crystalline polymers in spite of the disordered regions existing in polymer crystals. The size and defects of these ciystals influence band shape and position because of finite boundary conditions. It also may give rise to additional al orption bands not predicted by the calculation (because the selection rule cannot be applied in this case). The additional bands are observed, indeed, in tlK FIR spectra at the frequencies corresponding to tlK maxima in tte spectrum density of phonon states in the low-frequency regon [19, 23]. 

We shall not be concerned here with the exact treatment of spectra by normal coordinate analysis 24, 29). Instead an attempt will be made to give some semiempirical rules, which will enable us to predict the correct number and types of vibrations in the infrared or Raman spectrum for a given symmetry. If the observed spectrum coincides with theoretical expectations, we can be reasonably sure that we predicted the correct symmetry. However, we cannot be absolutely certain, since one can never be sure that the number of frequencies found is not too low (or more seldom too high) for an assumed symmetry. It is not so much the correct application of the rules, but rather the interpretation of the experimental findings that is decisive. It is, therefore, in most cases involving hydrocarbon complexes, advisable and sometimes necessary to obtain additional evidence from measurements on deuterated compounds, from comparisons of the spectra of similar compounds, or from considerations of chemical facts, in order to make sure that the determination of a symmetry group is well-founded. 

In the previous chapter, vibrational/rotational (i.e. infrared) spectroscopy of diatomic molecules was analyzed. The same analysis is now applied to polyatomic molecules. Polyatomic molecules have more than one bond resulting in additional vibrational degrees of freedom. Rotation of linear polyatomic molecules is mechanically equivalent to that of diatomic molecules however, the rotation of non-linear polyatomic molecules results in more than one degree of rotational freedom. The result of the additional vibrational and rotational degrees of freedom for polyatomic molecules is to complicate the vibrational/rotational spectra of polyatomic molecules relative to spectra of diatomic molecules. Though the spectra of polyatomic molecules are more complicated, many of the same features exist as in the spectra of diatomic molecules. As a result, a similar approach wiU be used in this chapter. The mechanics of a model system will be solved, determine the selection rules, and the features of a spectrum will be predicted. 

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