These assumptions allow the 13 C spin system of each O-isotopologues to be classified as either a magnetically equivalent A 2 spin system, or a near-equivalent AB spin system. Each A 2 spin systems gives rise to a single C peak, while in general, each near-equivalent AB system gives rise to a four-peak subspectrum with two strong central peaks and two weak outer peaks. However, under the conditions that apply here, the outer peaks have small intensity, and the two inner peaks are unresolved and coalesce to a single peak at the mean of the two 13 C chemical shifts. The experimentally observed spectral pattern may therefore be understood as a superposition of these single C resonances (see figure ). The observed mean chemical shifts of the different peaks are thus given by