We have looked at coupling between hydrogen atoms and you may have wondered why we have ignored coupling between other NMR active nuclei. Why does 13C not cause similar couplings? In this section we are going to consider not only couplings between the same kind of nuclei, such as two protons, called homonuclear coupling, but also coupling between different nuclei, such as a proton and a fluorine atom or 13C and 31P, called heteronuclear coupling. Two nuclei are particularly important, 19F and 31P, since many organic compounds contain these elements and both are at essentially 100% natural abundance and have spin I = 1/2. We shall start with organic compounds that have just one of these nuclei and see what hap pens to both the 1H and the 13C spectra. In fact, it is easy to find a 19F or a 31P atom in a molecule because these elements couple to all nearby carbon and hydrogen atoms. Since they can be directly bonded to either, ¹J coupling constants such as 1JCF or 1JPH become possible, as well as the more ‘normal’ couplings such as ²JCF or ³JPH, and these ¹J coupling constants can be enormous. We shall start with a simple phosphorus compound, the dimethyl ester of phosphorous acid (H₃PO₃). There is an uncertainty about the structure of both the acid and its esters. They could exist as P(III) compounds with a lone pair of electrons on phosphorus, or as P(V) compounds with a P=O double bond.

In fact, dimethyl phosphite has a ¹H doublet with the amazing coupling constant of 693 Hz: on a 250 MHz machine the two lines are over 2 ppm apart and it is easy to miss that they are two halves of the same doublet. This can only be a 1JPH as it is so enormous. The compound has to have a P–H bond and the P(V) structure is correct. The coupling to the protons of the methyl group is much smaller but still large for a three-bond coupling (3JPC of 18 Hz). Next, turning aldehydes and ketones to alkenes. It has a ²JPH of 18 Hz. There is no doubt about this structure—it is just an illustration of coupling to phosphorus. There is coupling to phosphorus in the carbon spectrum too: the methyl group appears at δC 10.6 ppm with a ¹JPC of 57 Hz, somewhat smaller than typical 1JPH. We haven’t yet talked about couplings to 13C: we shall now do so.

مواضيع ذات صلة

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Electrophilic additions to alkenes can be stereospecific

The regioselectivity of epoxide opening can depend on the conditions

Unsymmetrical bromonium ions open regioselectively

Electrophilic addition to unsymmetrical alkenes is regioselective

Alkenes react with bromine

Effects of substituents on CH2 groups

Effects of functional groups

Identifying compounds from nature