Influence of a Polar Near-Neighbor on Incipient Proton Transfer in a Strongly Hydrogen Bonded Complex

Rotational spectroscopy and ab initio calculations have been used to characterize the complexes H₃N-HF and H₃N-HF-HF in the gas phase. H₃N-HF is a C_3v symmetric, hydrogen bonded system with an NF distance of 2.640(21) Å and an N··· H hydrogen bond length of 1.693(42) Å. The H₃N-HF-HF complex, on the other hand, forms a six-membered HN-HF-HF ring, in which both the linear hydrogen bond in the H₃N-HF moiety and the F-H-F angle of (HF) ₂ are perturbed relative to those in the corresponding dimers. The N···F and F···F distances in the trimer are 2.4509(74) Å and 2.651(11) Å, respectively. The N···H hydrogen bond length in H₃N-HF-HF is 1.488(12) Å, a value which is 0.205(54) Å shorter than that in H₃N-HF. Similarly, the F···F distance, 2.651(11) Å, is 0.13(2) Å shorter than that in (HF)₂. Counterpoise-corrected geometry optimizations are presented, which are in good agreement with the experimental structures for both the dimer and trimer, and further characterize small, but significant, changes in the NH₃ and HF subunits upon complexation. Analysis of internal rotation in the spectrum of H₃N-HF-HF gives the potential barrier for internal rotation of the NH₃ unit, V₃, to be 118(2) cm^-1. Ab initio calculations reproduce this number to within 10% if the monomer units and the molecular frame are allowed to fully relax as the internal rotation takes place. The binding energies of H₃N-HF and H₃N-HF-HF, calculated at the MP2/aug-cc-pVTZ level and corrected for basis set superposition error are 12.3 and 22.0 kcal/mol, respectively. Additional energy calculations have been performed to explore the lowest frequency vibration of H ₃N-HF-HF, a ring-opening motion that increases the NFF angle. The addition of one HF molecule to H₃N-HF represents the first step of microsolvation of a hydrogen bonded complex and the results of this study demonstrate that a single, polar near-neighbor has a significant influence on the extent of proton transfer across the hydrogen bond. As measured using the proton-transfer parameter ρ_PT, previously defined by Kurnig and Scheiner [Int. J. Quantum Chem., Quantum Biol. Symp. 1987, 14, 47], the degree of proton transfer in H₃N-HF-HF is greater than that in either (CH₃)₃N-HF or H₃N-HCl but less than that in (CH₃)₃N-HCl.