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-ofplane movement of the proton. Colloquially, this proton transfer can be referred to as a "bend-to-break" process.

Ultrafast Proton Transfer Reaction in Phenol–(Ammonia)n Clusters: An Ab-Initio Molecular Dynamics Investigation.

“…The geometries of 126 PhOH-(NH3)n (n=1-10) clusters, calculated using M06-2X/cc-pVTZ method, were reported earlier, 16 and among these 126 structures only 114 structures which show hydrogen bonding between the phenolic OH group to the surrounding ammonia cluster and were considered. 18 These 114 structures were re-optimized using the resolution of identity second-order perturbation theory (RI-MP2) method with cc-pVDZ basis set using ORCA 4.1.2 suite of programs. 20 Following, the electric field along the O-H bond was calculated on the 114 structures using the procedure described elsewhere.…”

“…16 In a recent work our group, with the aid of electric field calculations along the donor O-H of phenol in the phenol-(ammonia)n clusters, using at the M06-2X/cc-pVTZ level of theory, has shown that a critical electric field of a critical field of 236 MV cm -1 is essential to transfer of a proton from phenol to the surrounding ammonia cluster. 18 Interesting, Additionally, six exceptions to this rule were observed, which indicates that the projection of the solvent electric field over the O-H bond is not an unqualified descriptor of the proton transfer reaction. Based on these observations, it was noted that the critical electric field is a necessary, but not sufficient condition for the proton transfer process.…”

“…19 Furthermore, the unique feature of the proton transfer process in the phenol-(ammonia)n clusters is the out-of-plane motion of the proton concurrent with the O-H bond elongation leading to a curvilinear path, which has been referred to as a "Bend-to-Break" process. 18 However, the curvilinear path for the proton transfer process was assessed based on the snapshots that were optimized using electronic structure calculations. In this work, the proton transfer process in the phenol-(ammonia)n clusters is re-evaluated using electric field calculations at the MP2 level theory and Born-Oppenheimer molecular dynamic (BOMD) simulations were carried out to understand the trajectory of the proton motion.…”

Vibrational Stark Fields in Carboxylic Acid Dimers

“…[NH3]n clusters. 29,36 Fig. 5 Plots of pKa of the carboxylic acid against the electric field along the O-H group (A) acetic acid and (B) trifluoroacetic acid.…”

“…However, the ionized and protonated forms of fluoroformic acid have been reported to exist in the gas phase. 36 Moreover, the crystal structure of the anhydrous form of fluoroformic acid has also been reported. 37 Therefore, based on the linear correlation between the pKa and the electric field along the OH group in the carboxylic acid dimer (see Figs 6 and S3) the pKa of fluoroformic acid was estimated to be 0.83 and 1.09 at MP2 and B3LYP-D3 levels of theory, respectively (Table S9, see the SI).…”