The refined 1H NMR method (AUS concept, CCl4) provided association constants K and approximate complex shifts IK for complexes of nitroaromatics A with aromatic hydrocarbons D. 5-Nitrofurfural (1) and benzene (B) or toluene (T) form syn-1B or syn-1T, respectively; a proton dependence of K indicates about 10% anti complexes. The dimer T2 of T additionally yields some 1T2 since IK values are 29-34% higher than for 1B. With naphthalene (N) or phenanthrene all protons of 1 provide the same K. Picrylacetone (2) and B form 2B and at least one 2BB adduct; published parameters of 2B and 2BB are inconsistent. 2 and N or M (1,3,5-trimethylbenzene) form only 1:1 complexes. All results are in accord with CH2COMe standing perpendicular to picryl. 2,4-Dinitrophenylacetone (3) in a rather flat conformation coordinates with a single stacking N centred over C-6; 3 yields two isomeric stacking complexes 3B and at least one edge-on 3B complex. 2-Chloro-5-nitropyridine (5) forms stacking complexes under repulsion of D by the ring N. In contrast to 1 and 4-nitrobenzaldehyde (6) 1,4-dinitrobenzene (7) and T form only a 1:1 complex that is stabilized by dipole-dipole attraction. Equal and unequal shielding of proton pairs (ortho or meta) in complexes of type 6B is discussed.
1H-NMR studies of the structure and stability of the bovine pancreatic secretory trypsin inhibitor.
