STRUCTURES OF TRIS(DONOR)-TRIS(ACCEPTOR)-SUBSTITUTED BENZENES .1. STERIC, POLAR AND HYDROGEN-BONDING EFFECTS IN TRIAMINOTRINITROBENZENES

The X-ray analyses of three 1,3,5-tris(alkylamino)-2,4,6-trinitrobenzenes (alkyl = iPr, neopentyl, tBu; 1 d - f) show their benzene rings to be highly distorted towards boat (1 d . iPrNH2) and twist-boat (1 e,f) forms. The patterns found for intramolecular donor-acceptor interaction and hydrogen bonding depend on the conformation of the benzene core: boat structures have the ''stem'' amino group strongly donating to the ''bow'' nitro group in the para position and two stronger hydrogen bonds between the substituents on the ''side'' of the boat, whereas twist-boat structures have two amino groups strongly interacting with the nitro group in their common ortho position to which there are also two strong hydrogen bonds. 1 d-f undergo conformational exchange on the C-13-NMR time scale at room temperature. Cooling to 220 K allows detection of all carbon signals and shows C1 symmetry to be present. AM1 calculations reproduce the distortions of 1 d - f. Computations on several 2,4,6-tridonor-substituted 1,3,5-trinitrobenzenes indicate that the pronounced deviations from planarity in 1 a - f originate largely from electronic effects.