The mass-resolved spectrum indicates that only one conformation contributes to sharp peaks observed in the S-1 <-- S-0 resonance two-photon ionization (R2PI) spectrum. However, geometry optimizations at the MP2/cc-pVDZ level suggest that two conformational isomers are stable: a head-to-head conformation with a single NH . . .N hydrogen bond and a head-to-tail conformation with double NH2. . . pi hydrogen bonds. The calculations show that the head-to-tail conformation is more stable by 1.18 kcal mol(-1).