Proton-transfer reaction in isolated and water-complexed 8-hydroxyimidazo[1,2-a]pyridine in the S-0 and S-1 electronic states. A theoretical study

The intramolecular proton-transfer reaction of 8-hydroxyimidazo[1,2-a]pyrid ine (HIP) is studied in both the ground (S-0) and first singlet excited (S- 1) electronic states. Ab initio calculations at the HF/MP2 and CIS/MP2 leve ls, respectively for S-0 and S-1, reveal the existence of two tautomers. In gas phase the enol (E) form is the more stable one in S-0, whereas in S-1 the zwitterionic (Z) tautomer becomes the lowest in energy. The energy barr ier for the proton-transfer process is quite high in both electronic states . Introduction of one water molecule in the system greatly changes the pict ure of the whole process. A 1:1 complex involving two hydrogen bonds betwee n HIP and water is formed. The H-bond bridge greatly facilitates the proton transfer in both S-0 and S-1 states. Additionally, for the two considered electronic states the Z structure is more stabilized than the E one. The tr ends of lowering the energy barrier and stabilizing the Z structure with re spect to E are more prominent when the bulk effect of the solvent is introd uced through a continuum (i.e. cavity) model. The energy profiles obtained for both S-0 and S-1 allow prediction of the electronic spectra of HIP in d ifferent media.