The effects of chemical or dynamic exchange on NMR spectra are very well known. Exchange is often studied by observing line shape changes as a function of temperature, by 2d EXSY, inversion transfer or saturation transfer methods. Effects due to exchange can also be observed in 1H - 13C HSQC spectra. The HSQC method works by transferring 1H magnetization to 13C magnetization via an INEPT transfer through the one-bond J coupling across the 1H - 13C chemical bond. The 13C magnetization evolves during the incremented delay, t1, of the 2D pulse sequence according to its chemical shift. The 13C magnetization is then transferred back to 1H magnetization where is observed during t2. HSQC spectra thus exhibit cross peaks between 1H resonances and the resonances of their attached carbons. If there is exchange between nonequivalent carbon sites during t1, some 1H resonances may appear to be correlated to two carbon resonances. An example of this is shown in the figure below.
NMR time scale to produce resolved resonances yet fast enough to cause significant line broadening. For each of the two aromatic protons, the HSQC spectrum shows correlations to both C3 and C5; a strong correlation to the carbon to which it is chemically bonded and a weaker correlation to the carbon site in exchange with its attached carbon.