Spectral information, numerical methods

Tables 6.3-6.5 record data developed to undertake structural analysis in systems possessing chromophores that are conjugated or otherwise interact with each other. Chromophores within a molecule interact when linked directly to each other or when they are forced into proximity owing to structural constraints. Certain combinations of functional groups comprise chromophoric systems that exhibit characteristic absorption bands. In the era when UV-VIS was one of the principal spectral methods available to the organic chemist, sets of empirical rules were developed to extract as much information as possible from the spectra. The correlations referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 6.3) and enones and dienones (Table 6.4) to be predicted. When this method is applied, wavelength increments correlated to structural features are added to the respective base values (absorption wavelength of parent compound). The data refer to spectra determined in methanol or ethanol. When other solvents are used, a numerical correction must be applied. These corrections are recorded in Table 6.5.

Dynamic analysis of photochromic systems under continuous irradiation represents a powerful method of investigation of the reaction mechanisms. The characteristic kinetic and spectral parameters such as the quantum yields of the photochemical steps and the molar extinction coefficients of the transient species can be derived using this method. The essence of the method is the inverse treatment based on numerical simulation and fitting of the plots (Abs versus t) obtained under continuous irradiation. This also exploits the information contained in the irradiation kinetics. In order to extract one or more of the relevant parameters of a given process, specially designed experiments need to be carried out in which the effect of the process under consideration is conspicuous. 

Dynamic Response Functions. - The perturbation series formula or spectral representation of the response functions can be used only in connection with theories that incorporate experimental information relating to the excited states. Semi-empirical quantum chemical methods adapted for calculations of electronic excitation energies provide the basis for attempts at direct implementation of the sum over states (SOS) approach. There are numerous variants using the PPP,50,51 CNDO(S),52-55 INDO(S)56,57 and ZINDO58 levels of approximation. Extensive lists of publications will be found, for example, in references 5 and 34. The method has been much used in surveying the first hyperpolarizabilities of prospective optoelectronically applicable molecules, but is not a realistic starting point for quantitative calculation in un-parametrized calculations. 

Once a reflectance curve is obtained spectrophotometrically, the measured information is mathematically transformed according to standard conventions, into the numbers used to describe the color of the sample. In practice, a standard observer and standard illuminant are selected based on test methods defined by the Commission International de I Eclairage (CIE). Calculation procedures involve the numerical integration of the product of the spectral power distribution S(l) of the light source and the reflectance factor R(l). Reflectance factors represent the percentage of light reflected by the sample at each wavelength. 

The purpose of a chemical expert system is to provide fast, easy, efficient and effective access to chemical information and knowledge in a specific domain of expertise, via computer-representation of integrated reference data, theoretical and empirical knowledge (e.g., in the form of rules ), system models and reasoning mechanisms. An expert system should not only contain (integrated) relevant hard facts (e.g., numerical correlations and statistics) but also the expertise ( soft knowledge ) of experienced specialists in the field. For structure elucidation of organic molecules various spectrometric methods are available. In many cases different methods such as mass, infra-red, NMR and ultra-violet spectroscopy provide complementary structural information. This is one of the basic elements of EXSPEC, an expert system for computer-aided interpretation of combined spectral data. ... 

For the VCD and ROA a relatively large number of IR/Raman bands are accessible in the available spectral region. Normal vibrations are only very weakly coupled and thus, the information of the CD is localized to the surroundings of the normal coordinate. Because good empirical rules are not available, the quantum mechanical numerical computations are not an additional but, in general, the only information for the assignment of the absolute configuration. Restriction of the method lies within the scope of the quality of the calculation of the VCD/ROA spectra. 

The desired spectroscopic information is obtained by numerical Fourier transformation of the experimental points of Fig.6. The result is shown in Fig. 7a where the computed spectral intensity distribution is plotted. A number of spectral lines is readily seen. Fig. 7a represents experimental information without use of the available spectroscopic data of CH. For improved frequency resolution we also applied the maximum entropy method (MEM). The spectrum obtained for this nonlinear transformation is depicted in Fig. 7b. 

Of all the numerous spectroscopic methods to probe molecular conformation (including NMR, circular dichroism, Raman spectroscopy, two-dimensimial IR spectroscopy, etc.), UV or IR action spectroscopy is most suited for the application to gas-phase molecules, either neutral or charged. Optical spectroscopy in the microwave, infrared or UVA is spectral range can provide detailed structural information, especially in combination with molecular beam techniques or other cryogenic methods (low internal temperatures), resulting in highly resolved spectroscopic signatures [73]. 

---