Temperature-induced intramolecular proton transfer in a novel polymorph of 2-(2'-hydroxyphenyl)benzimidazole crystal

Ohara, Takashi   ; Nakao, Akiko*; Munakata, Koji*; Moyoshi, Taketo*; Hanashima, Takayasu*; Kiyanagi, Ryoji  ; Hosoya, Takaaki*; Hara, Yoshiaki*

2-(2'-hydroxyphenyl)benzimidazole (HPBI) has an intramolecular O-H...N hydrogen bond and shows photo-induced excited-state intramolecular proton transfer, from the enol form to the keto form. Recently, we found that a novel polymorph (gamma-form) of HPBI crystal. We carried out single-crystal neutron diffraction measurements of the gamma-form at 90K and 298K by using SENJU single-crystal diffractometer at J-PARC MLF to observe the temperature-induced proton transfer in the intramolecular hydrogen bond from the enol-form (O-H...N) to the keto-form (O...H-N). In the result, negative density of atomic scattering length which comes from the proton of the keto-form was observed at 298K even though no keto-form was observed at 90K. In the gamma-form, HPBI molecule forms stronger intermolecular N-H...O hydrogen bond than that in the known polymorph. This hydrogen-bond possibly contributes to lowering the potential barrier between the enol and keto-forms.