Interactions between betacarboline and benzenoid pi bases: FTIR evidence for the formation of NH-pi hydrogen bonds

In the presence of benzene, naphthalene, or phenanthrene, the NH stretching band of betacarboline in tetrachloroethane diminishes in intensity while a new red-shifted band grows. These changes are attributed to the formation of molecular association complexes between betacarboline and the benzenoid compounds. Deconvolution of these IR bands allowed us to calculate their as sociation constants, which increase linearly with the number of pi electron s of the benzenoid substrates. The shifts of the associated bands were, how ever, independent of the bases. These results suggest that the complexes ar e stabilized by the hydrogen-bonding interaction between the pyrrolic NH gr oup of betacarboline and the pi-delocalized electrons of the benzenoid subs trates. Low-level semiempirical calculations (AM1/MOPAC) also predict T-sha ped structures as the most stable. On the other hand, experiments using har mane, the 1-methylated betacarboline, proved steric hindrance for naphthale ne and phenanthrene, which should be a consequence of the geometry of the c omplexes.