First order analysis

Work on indazoles has the drawback of having to employ computer procedures to analyze the ABCDX system of C-unsubstituted indazoles. The first five examples in Table 9 correspond to LAOCOON-analyzed spectra and the last two to simpler systems for which the first-order analysis gives acceptable chemical shifts. 

Proton and carbon spectra of 3,3,3-trifluoropropene are provided in Figs. 5.12 and 5.13 as specific examples of such spectra. The proton spectrum is more complicated than one would have expected based on a first-order analysis. However, a fluorine-decoupled spectrum becomes first order, as was depicted and discussed in Chapter 2, Section 2.3.5, Figs. 2.9 and 2.10. 

Pyridine-d5 was found to show the most favorable solvent-induced changes of the chemical shifts for the three compounds in so far as a first-order analysis of the 60-MHz spectra was possible, the observed splittings gave no indication for solvent-induced, conformational changes. 

The results obtained with PhViE in the extra dry PhN02 are listed in Table 3 and are plotted in Figures 4-6. All the reaction curves were sigmoid and kx was evaluated by the usual first-order analysis of the reaction curves beyond the inflection point (line A in Figure 4) and also from the relation... 

The validity of a first order analysis can be verified by calculating the ratio Av/J for each pair of nuclei and establishing that it is greater than 3. 

Each ratio is greater than 3 so a first order analysis is justified and the 100 MHz spectrum of the aliphatic protons of styrene oxide is indeed a first order spectmm and could be labelled as an AMX spin system. 

The material is normally utilized directly without further purification. If the solid is very gray, it may be recrystallized. For the recrystallization the salt is dissolved in hot 95% ethanol (approximately 350 ml. per 100 g. of salt) containing decolorizing carbon, filtered rapidly, and the clear supernatant liquid is allowed to cool in a freezer (—20°). In this way, white crystals, m.p. 106-107°, may be obtained with nearly quantitative recovery. The checkers obtained the purified material of m.p. 108-109° with 95% recovery and used this material for the next step. The purified material has the following spectral data ultraviolet (95% ethanol) nm. max (e) 236 shoulder (13,200), 262 (2200), 268 (2680), 275 (1010) infrared (Nujol) cm-1 3090 weak, 3060 weak (aromatic CH), 1580 weak (C=C) proton magnetic resonance (CDCla), first-order analysis, 6 in p.p.m. 7.5-8.2 (multiplet 10H, Ha), 4.3 (poorly resolved triplet 2H, Hb), 3.75 (triplet 2H, Hd), 2-2.5 (multiplet 2H, He) coupling constant J in Hz. Jbc — 8, Jcd = 6.5. 

By this method, assignments of the signals are, in most cases, readily made by employing first-order analysis with the aid of a decoupling technique, the effectiveness of which is exemplified by the various spectra (see Fig. 3) of D-glucopyranose85 134. 

Consider acetaldehyde, CH3CHO. Figure 8.3 shows a for the methyl and CHO protons to differ substantially, so that Vq ax Jax- The low barrier to internal rotation causes condition (1) to be satisfied. Hence the first-order analysis of the preceding paragraphs is applicable. We have an A3X case and the spectrum consists of a doublet (from the methyl protons) whose lines are of equal intensity and a quartet (from the CHO proton) whose lines have the intensity ratios 1 3 3 1 the doublet and quartet are well separated and show the same splitting (Fig. 8.9). 

The simplified analysis is valid only when conditions (1) and (2) are met. Otherwise, the line intensities deviate considerably from the binomial pattern, the line frequencies differ considerably from those of the first-order analysis, and the first-order analysis may even predict the wrong number of NMR lines. (Even for ethanol, which was discussed above as an A3M2X system, a good NMR spectrometer will show that there are small but significant deviations from a first-order spectrum.)... 

For an unpaired electron interacting with both a set of m equivalent protons and a set of n equivalent protons, the first-order analysis is carried out in two stages, like the treatment in Section 8.5 of the CH2 portion of... 

Calculate the percent error in 512 when a first-order analysis is applied to an AB spectrum when v08n/Jn has each of the following values20 (a) 2 (b) 4 (c) 7 (d) 10. 

A number of chemical shifts reported in the literature by organic chemists are inaccurate due to unjustified use of a first-order analysis... 

More complex spin-spin interactions. As the chemical shift difference becomes similar to the value of the coupling constant, the first-order analysis breaks down completely. It is then not possible to measure either the chemical shift or the coupling constants directly from the spectrum and resort must be made to more rigorous analytical procedures which are beyond the scope of this book. However, there are available a variety of computer programs specifically designed to analyse complex n.m.r. spectra.3 1... 

The chemist can adopt a variety of procedures to simplify complex spectra to make them more amenable to first-order analysis or analysis by inspection. A simple example is deuterium exchange which was described in the previous section. Acidic protons attached to carbon may also be exchanged under basic conditions. 

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