Coupled hydrogen-bonding interactions between β-carboline derivatives and acetic acid

In this work, we have analysed the tendency of two P-carboline derivatives, harmane and norharmane, in the formation of hydrogen bonds. We obtained the H-1 and C-13 NMR spectra of different mixtures of these derivatives with acetic acid (AcOH) in CDCl3. A cyclic 1:3 complex is proposed between harmane and AcOH, while a 1: 2 complex is proposed for norharmane. Chemical shifts at temperatures between 233 and 323 K were measured: lowering the temperature produces the same effect as increasing the amount of AcOH in solution. The C-13 data confirm a delocalisation of the it electron density towards the pyridinic ring that occurs when AcOH is added. Copyright 2007 John Wiley & Sons, Ltd.