A series of sterically hindered o‐hydroxy aromatic ketones were synthesized, including benzene, naphthalene, phenanthrene and pyrene derivatives. Deuterium isotope effects on the 13C chemical shifts of 2‐hydroxy‐1‐acenaphthone and other sterically hindered, intramolcularly hydrogen‐bonded aromatic ketones (OH exchanged) are shown to be unusual. The two‐bond isotope effects are very large. Likewise are the istope effects on CO, C1, C3 and C4 carbon resonances and some show unusual signs. These unusual effects are explained by a higher degree of twist in the deuterio than the protio compound. Steric isotope effects are also observed on OH chemical shifts of sterically hindered o‐hydroxy acetyl aromatic compounds deuteriated at the methyl group. These isotope effects show non‐additivity. For one‐bond isotope effects, 1Δ13C(18O), hydrogen bonding leads to a decrease, whereas twisting of the carbonyl group leads to an increase. Two hydrogen bonds to the same acceptor has a reduced cumulative effect. Data for sterically hindered, hydrogen‐bonded compounds are found to fall outside the correlation between δ(17O) and 1Δ13C(18O).
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Hansen et al. (1994) studied this question.