Table The stability constants (M -1 ) for the complexation of receptors 1-10 and Cl -(as TBA salt) in DMSO-d6/0.5% D2O at 298K. A potential explanation the contrasting shifts of the acridinone NH proton resonances of the urea and thiourea compounds upon addition of chloride could be the tautomerism of the acridinone group. Previously, we elucidated the solid-state structure of the chloride complex of compound 1 and found it crystallised as the hydroxy tautomer (CCDC 629282). A chloride anion was bound by the protons of the two urea groups, and a second chloride was stabilised by a hydrogenbonding interaction with the hydroxy O-H donor group. After examination of the 13 C-NMR spectra of compounds 1-10, the formation of the alternate tautomer could be discounted. Each 13 C-NMR spectrum contains a resonance between 176.8-177.4 ppm, which is a fingerprint resonance for the C9 carbon of acridinone in DMSO-d6. This characteristic resonance is present in numerous acridinone systems reported in the literature, including in systems locked in the acridinone form via N-substitution. In addition, physicochemical studies and theoretical calculations on similar acridinone molecules have confirmed the hydroxyacridine tautomer is not present at equilibrium in DMSO solutions. A permanent tautomeric switch from the acridinone to the hydroxyacridine form of the system would result in a significant change in the C9 carbon environment, and the 13 C-NMR resonance for this carbon in acridine systems is more typically found in the range 140-160 ppm. Furthermore, the occurrence of tautomerism over the course of the 1 H-NMR titrations is unlikely as the signal attributed to the acridinone NH proton resonance remains sharp and can be followed continuously for each titration. However, the appearance of new resonances upon addition of multiple (up to 300) equivalents of chloride during the 1 H-NMR titrations of compounds 1-5 is evidence in support of tautomerism of the chloride complex that is slow on the NMR timescale. Specifically, the appearance of a new peak at 11.56 ppm in the spectrum of 10 after the addition of 300 equivs. of TBACl (Figure ) may be attributed to the OH proton of the hydroxyacridine species.