spectroscopy and proof of structure: NMR part 2

spin-spin coupling: basics

• H atoms in different environments - chemically inequivalent so magnetically inequivalent

• each H atom has its own unique chemical shift

• magnetic moments of the 2 protons interact - coupling

• 2 peaks separated by specific distance

• coupling transmitted through electrons present in bonds

• more bonds = weaker interaction as information about surrounding chemical environments has to be transmitted through more electrons

• magnetically equivalent nuclei do not exhibit spin-spin coupling

J coupling

• J constant - gives the magnitude of coupling

• measured in Hz - independent to magnefield strength, if given in other units would vary with the spectrometer field

• J more likely to be smaller than the difference in frequency between peaks if a larger operating frequency is used - first order, simpler spectra

• greater chemical shift difference = first order spectrum

• smaller chemical shift difference = second order spectrum, additional lines appear as the signals come closer together so harder to decipher

• arises from very weak interaction between magnetic moments of nuclei via chemical bonding

• J decreases as number of bonds between nuclei increases

coupling patterns

• can use pascal’s triangle to predict splitting and ratio patterns

• splitting patterns depend on number of different combinations of spin states of protons

key features of spin-spin coupling

• J coupling arises from very weak interaction between magnetic moments of nuclei via chemical bonding

• J decreases as number of bonds between nuclei increases

• coupling between equivalent spins does not usually affect spectra - ignore coupling from protons on same carbon atom

• magnitude of coupling depends on geometry/structure

coupling with C=C

• J_cis > J_trans

• vicinal coupling - coupling between H nuclei on adjacent C atoms in a C=C double bond

• 4J coupling negligible apart from in aromatic systems