Complex Splitting Patterns

Splitting due to geminal coupling is seen only in CH2 groups and only when the two protons have different chemical shifts. All three protons of a methyl (CHj) group are equivalent and caimot split one another s signal. 

The 300-MHz H NMR spectrum of 2,3,4-trichloroanisole, showing the splitting of the ring protons into a pair of doublets that lean toward each other. 

The protons in 1-chloro-l-cyanoethene are diastereotopic (Section 13.6). They are nonequivalent and have different chemical shifts. Remember, splitting can only occur between protons that have different chemical shifts. 

Hb protons, each of these lines is further split into n + 1 peaks by nH protons, and each of these into n + I lines by n H protons, and so on. Bear in mind toat because of overlapping peaks, the number of lines actually observed can be less than that expected on the basis of the splitting rule. 

The methylene protons and H of (S)-l,2-diphenyl-2-propanol are diastereotopic and appear as a pair of doublets in the 300-MHz H NMR spectrum. 

PROBLEM 13.11 Describe the splitting pattern expected for the proton at 

SAMPLE SOLUTION (a) The signal of the proton at C-2 is split into a doublet by coupling to the proton cis to it on the double bond, and each line of this doublet is split into a triplet by the two protons of the CH2CI group. 

The a + 1 rule should be amended to read When a proton H is coupled to H, He, H(i, etc., and Jac he original signal for H is split into n + peaks 

This proton splits — signal for proton at C-2 into a doublet. 

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Si(2) resonances are split into doublets by 1J(Si(2)H(b . The complex splitting patterns of the single resonance lines arise from long-range 29Si- H- couplings. 

The signal at 4.45 is proper for the benzyl group, but the splitting pattern is problematic until it is recognized that because there is a chiral center at C-2, the benzyl protons are diastereotopic and thus nonequivalent. They are part of an ABX spin system and thus give the complex splitting pattern seen—actually a two-proton multiplet that looks like a doublet or a very close AB quartet. 

Complex splitting patterns can often be simplified by replacing a hydrogen with deuterium. Deuterium is invisible in the proton NMR, so the resulting spectrum shows the loss of a signal and simplified signals from the adjacent hydrogens. 

The inaccessibility of sulfur by common spectroscopic methods requires in most cases X-ray structure determinations in order to establish the structure of [M(S )] complexes unambiguously. Practically all complexes that are mentioned in this chapter have been characterized by X-ray crystallography. The IR spectra usually are suited only to serve as fingerprints. The same holds true for the complex splitting patterns observed in 1H NMR spectra. 

Due to the large number of hydrogen atoms in the molecules rather complex splitting patterns are obtained because of Si H couplings, which allow an unequivocal structure assignment in each case. As an illustrative example the coupled Si NMR spectrum of l,3-bis(triphenylsilylthio)trisilane is shown in Fig. 1. 

ESR hyperfine splittings (as the coupling patterns are known) can give quite a lot of information about a radical. Eor example, here is the hyperfine splitting pattern of the cyclohepta-trienyl radical. The electron evidently sees all seven protons around the ring as equivalent, and must therefore be fully delocalized. A localized radical would see several different types of proton, resulting in a much more complex splitting pattern. 

The benzylic protons on carbon atoms adjacent to an aromatic ring are also deshielded by the ring but to a lesser extent than the ring protons. The characteristic absorbance values for benzylic protons are in the 2.2-2.8 ppm range. An example is ethylbenzene, CeHs—CH2—CH3, whose spectrum is shown in Fig. 3.32. The compound is a monosubstituted aromatic ring, so the ring protons at 7.2 ppm show the same complex splitting pattern we saw in toluene. The benzylic protons are the methylene protons, on... 

Spin-Spin Splitting in NMR Spectroscopy 555 Splitting Patterns The Ethyl Group 557 Splitting Patterns The Isopropyl Group 559 Splitting Patterns Pairs of Doublets 559 Complex Splitting Patterns 561 NMR Spectra of Alcohols 563... 

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