Observation, qualitative versus

For speetroseopie analysis (e.g. XRF, XRD, IR, Raman, Mossbauer etc.), a plot of the observed intensity versus the eorresponding wavelength or frequeney (or some other related parameter) is ealled the spectrum of that particular analytical method. The spectrum or the data obtained from such experiments contain information about nature of the interactions, atomic and molecular energy levels, chemical bonds, crystallographic information and other related processes. When only the item of interest is identified, it is called qualitative analysis and when the amoimt present is estimated, it is known as quantitative analysis. The effect of heat on a sample is reflected through its variation of thermodynamic properties. Such studies are done by thermal analysis. 

We started this review with a brief discussion of some apparent dilemmas that we face in chemistry qualitative versus quantitative approaches, observables versus non-observables, structural criteria versus properties as criteria for characterization of aromaticity, chemical graph theory versus quantum chemistry, Clar 6/j rule versus Huckel An+ 2 rule, and hydrocarbons versus heteroatomic systems. As we have seen, most of the mentioned dilemmas are man-made and reflect inbred biases of different circles of chemists. It is not uncommon to come across critics with strong opinions and weak arguments, and it would be a waste of time to try to point out to them the other side of the coin . Max Planck apparently experienced difficult times before his quantum constant was accepted, as is reflected in the following quotations ... 

When this stereoelectronic requirement is combined with a calculation of the steric and angle strain imposed on the transition state, as determined by MM-type calculations, preferences for the exo versus endo modes of cyclization are predicted to be as summarized in Table 12.3. The observed results show the expected qualitative trend. The observed preferences for ring formation are 5 > 6, 6 > 7, and 8 > 7, in agreement with the calculated preferences. The relationship only holds for terminal double bonds. An additional alkyl substituent at either end of the double bond reduces the relative reactivity as a result of a steric effect. 

The rationale for the predominance of linear cyclization products versus angular cyclization products has been accepted as qualitative." The mechanism of the Combes reaction has been argued. It was initially proposed that cyclization to linear products was due to initial protonation of a more reactive site on the aromatic ring (1-position of 13 corrresponding to the 10-position of 15) thus, blocking cyclization to angular products. Bom showed this not to be the case for the cyclization of 2-naphthyl amino-2-penten-4-one. No 10-deutero material was observed. 

The observed catalytic effect of the crown ether appears to be dependent on the nucleophile employed in both polymerization and corresponding model reactions. Not surprisingly, it appears that the stronger the nucleophile employed, the smaller the catalytic influence of the crown ether. For example, with potassium thiophen-oxide yields of polymer or model products were almost quantitative with or without catalyst. By contrast, the reaction of PFB with potassium phthalimide, a considerably weaker nucleophile, affords 6 in 50% with catalyst and in 2-3% without catalyst under identical conditions. However, it may be that this qualitative difference in rates is, in fact, an artifact of different solubilities of the crown complexed nucleophiles in the organic liquid phase. A careful kinetic study of nucleophilicity in catalyzed versus non-catalyzed reactions study is presently underway. 

Solutions of Eqns. 32 through 35 on an analog computer are shown in Fig. 6 for the case of polymerization in a closed system. The plots of rp(t) and p(t) are seen to be in qualitative agreement with the experimental observations shown in Fig. 7. The model of polymerization kinetics was also found to provide curves of rp versus discharge current density and monomer flow rate which were consistent qualitatively with the experimentally observed results. 

The results of Shephard and Rooney support these observations. The activation energy for n-propylbenzene over platinum-on-alumina catalyst is twice that of the cyclization of 1-methyl-2-ethylbenzene (11.6 versus 5.8 kcal/mol Fig. 2). However, these last values are based on pulse-reactor results therefore, they should be used only for qualitative comparisons. 

Figures 5 and 6 show the effect of temperature on the removal of solid C20 (melting point = 37 °C) by C E04. These plots of normalized intensity of the v CH2 band versus time were obtained from two series of experiments, in which the initial layer thickness was varied somewhat. As discussed above, these plots must be regarded as qualitative descriptors of the removal process, due to the optical complexity of the interface. The removal process may involve not only solubilization, but also a surfactant - induced displacement of C q crystallites from the IRE surface, which cannot be treated as a gradual thinning of the C20 layer. Repeated experiments on the effect of temperature on removal rate indicate that if the conditions of layer preparation (hydrocarbon concentration in hexane, speed of withdrawal from the solution) are held constant, then reproducible band intensities of the initial layers are obtained. The shape of the removed plots (Figures 5 and 6) are affected by the initial layer thicknesses. More rapid removal was usually observed for thinner layers of smaller initial Qq band intensity.

For example, the faster rate of reaction from of cyclobutanones relative to cyclopentanones can be qualitatively understood on the basis of greater relief of strain in for the four-membered ring versus the five membered ring. Furthermore, since the 1,4-biradical is produced cis and in the singlet state, all of the observed reactions can occur with relatively minor nuclear reorganization and with no prohibitions. 

Linear potential sweep and cyclic voltammetry are at their best for qualitative studies of the reactions occurring in a certain range of potential. In Fig. 5L, for example, we see the cyclic voltammogram obtain on a mercury-drop electrode in a solution of p-nitrosophenol in acetate buffer. Starting at a potential of 0.3 V versus SCE, and sweeping in the cathodic direction, one observes the first reduction peak at about - 0.1 V. This potential corresponds to the reduction of... 

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