Rapid NMR screening of chloride receptors: uncovering catechol as a useful anion binding motifElectronic supplemenary information (ESI) available: calibration graphs for the binding process between receptor 1 and chloride, and a worked example illustrating the use of this calibrated competitive method to determine the binding constant between receptor 15 and chloride. See http://www.rsc.org/suppdata/ob/b3/b310455a/