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

Sherri W. Hunt, Kelly J. Higgins, Matthew B. Craddock, Carolyn S. Brauer, Kenneth R. Leopold

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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 C3v 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 ρPT, 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.

Original languageEnglish (US)
Pages (from-to)13850-13860
Number of pages11
JournalJournal of the American Chemical Society
Issue number45
StatePublished - Nov 12 2003

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