Bend-to-Break: Curvilinear Proton Transfer in Phenol-Ammonia Clusters

Abstract

The electric field experienced by the OH group of phenol embedded in the cluster of ammonia molecules depends on the relative orientation of the ammonia molecules, and a critical field of 236 MV cm–1 is essential for the transfer of a proton from phenol to the surrounding ammonia cluster. However, exceptions to this rule were observed, which indicates that the projection of the solvent electric field over the O–H bond is not a definite descriptor of the proton transfer reaction. Therefore, a critical electric field is necessary, but it is not a sufficient condition for the proton abstraction. This, in combination with an adequate solvation of the acceptor ammonia molecule in a triple donor motif that energetically favors the proton transfer process, constitutes necessary and sufficient conditions for the spontaneous proton abstraction. The proton transfer process in phenol-(ammonia)n clusters is statistically favored to occur away from the plane of the phenyl ring and follows a curvilinear path which includes the O–H bond elongation and out-of-plane movement of the proton. Colloquially, this proton transfer can be referred to as a “bend-to-break” process.