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Formation constants (K eq ) have been measured using 1 H NMR for H‐bond complexes with HMPA in CCl 4 of 35 aromatic compounds variously substituted with cyano, nitro, and trifluoromethyl groups; several compounds contained F and Cl. The three strongly polar groups enhance H‐bonding significantly, usually in the ord er NO 2 > CN > CF 3; all are superior to Cl and F. 1,3,5‐Trinitrobenzene fails to H‐bond at all ; however, TNT, its tert‐butyl analog, and trinitro‐m‐xylene show significant Keq values. Coplanarity of nitro groups with the ring blocks approach of HMPA, probably via intramolecular H‐bonds. The buttressing effect is evident in some crowded compounds.

Formation constants in C–H hydrogen bonding. 4. Effects of cyano, nitro, and trifluoromethyl substituents in aromatic compounds