N + I rule

One of the main benefits of the paper by Bent and Weinhold is a plausible explanation for the n + i rule which does not, at first sight, seem to suffer from the drawbacks of the explanations of Allen and Knight as well as Ostrovsky. However, the recent explanation by Bent and Weinhold comes at a certain cost as will be explained. 

Schwarz has suggested that the n + ( rule is relatively unimportant. He believes that it is because attention should be directed to the configurations of bonded atoms rather than neutral atoms. As he points out, the configurations of bonded atoms do not follow the n + i rule but rather the simpler rule of increasing values of n. 

We note that the 5d subshcll started before the 4/ subshcll, but only one electron entered that shell before the 4/ subshell started. Indeed, the periodic table predicts this correct configuration for Gd better than the n + I rule or other common memory aids. 

Transition metal atoms form ions by loss of their outermost electrons first (not those governed by the n + i rule). 

The actual configuration has two subshells of enhanced stability (3d and 45) in contrast to one subshell (4 ) of the expected configuration. (There are also some elements whose configurations do not follow the n -I- / rule and which are not enhanced by the added stability of an extra fully filled and half-filled subshells.)... 

I. Answers include the number of signals and (for the spectrum without proton decoupling) the splitting by directly attached hydrogens (N + I rule). Chemical shifts are very approximate (based on Table 10-6). 

N + I rule The rule Ihal A/equivaleni neighbors produce A + I speciral peaks in NMR owing to spin-spin splitting. 

In earlier sections, we have discussed first-order spectra, spectra that can be interpreted by using the n + 1 Rule or a simple graphical analysis (splitting trees). In certain cases, however, neither the n + I Rule nor graphical analysis suffices to explain the splitting patterns, intensities, and numbers of peaks observed. In these last cases, a mathematical analysis must be carried out, usually by computer, to explain the spectrum. Spectra that require such advanced analysis are said to be second-order spectra. 

Below are shown three C4H8CI2 isomers on the left and three sets of H NMR data that one would expect on application of the simple N + I rule on the right. Match the structures to the proper spectral data. (Hint You may find it helpful to sketch the spectra on a piece of scratch paper.)... 

Start by examining the system with two protons, Ha and Hb, on adjacent carbon atoms. Using the n + I Rule, we expect to see each proton resonance as a doublet with components of equal intensity in the H NMR spectrum. In actuality, we see two doublets of equal intensity in this situation only if the difference in chemical shift (Av) between Ha and Hb is large compared to the magnitude of the coupling constant CJab) that links them. Figure 5.35 illustrates this case. 

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