Ma. Munoz et al., Interactions between betacarboline and benzenoid pi bases: FTIR evidence for the formation of NH-pi hydrogen bonds, J PHYS CH B, 103(41), 1999, pp. 8794-8798
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.