Table 1. Stability Constants for 1 with neutral substrate added K (M^-1)^a

Neutral guest	Stability constant (M^-1)
methanol	12.7 ± 1.0
ethanol	10.7 ± 0.7
benzyl alcohol	9.7 ± 0.7
isopropyl alcohol	7.0 ± 0.4
sec-butanol	6.2 ± 0.4
N-formylglycine ethyl ester	13.3 ± 1.0
N,N-dimethylformamide	11.3 ± 0.8
N,N-dimethylacetamide	9.0 ± 0.9
1,1,3,3-tetramethylurea	2.2 ± 0.1
dimethyl sulfoxide	16.2 ± 1.1
1,2-dimethylimidazole	5.4 ± 0.3
acetone	2.2 ± 0.2
nitromethane^b	-

^aIn benzene-d₆ at 298 K. For each titration, the concentration of 1 was held constant (at ca. 4 x 10^-3 M) as aliquots of the substrate in benzene-d₆ (ca. 1 M) were added. ^b In this instance, the induced shifts in the NH proton(s) of 1 were too small (< 0.15 ppm) to afford a reliable K_a value.