Intramolecular competition between n-pair and π-pair hydrogen bonding: Microwave spectrum and internal dynamics of the pyridine-acetylene hydrogen-bonded complex

Rebecca B. Mackenzie, Christopher T. Dewberry, Emma Coulston, George C. Cole, Anthony C. Legon*, David P. Tew, Kenneth R. Leopold

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

11 Citations (Scopus)

Abstract

a-type rotational spectra of the hydrogen-bonded complex formed from pyridine and acetylene are reported. Rotational and 14N hyperfine constants indicate that the complex is planar with an acetylenic hydrogen directed toward the nitrogen. However, unlike the complexes of pyridine with HCl and HBr, the acetylene moiety in HCCH - NC5H5 does not lie along the symmetry axis of the nitrogen lone pair, but rather, forms an average angle of 46° with the C2 axis of the pyridine. The a-type spectra of HCCH - NC5H5 and DCCD - NC5H5 are doubled, suggesting the existence of a low lying pair of tunneling states. This doubling persists in the spectra of HCCD - NC5H5, DCCH - NC5H5, indicating that the underlying motion does not involve interchange of the two hydrogens of the acetylene. Single 13C substitution in either the ortho- or meta-position of the pyridine eliminates the doubling and gives rise to separate sets of spectra that are well predicted by a bent geometry with the 13C on either the same side ("inner") or the opposite side ("outer") as the acetylene. High level ab initio calculations are presented which indicate a binding energy of 1.2 kcal/mol and a potential energy barrier of 44 cm-1 in the C2v configuration. Taken together, these results reveal a complex with a bent hydrogen bond and large amplitude rocking of the acetylene moiety. It is likely that the bent equilibrium structure arises from a competition between a weak hydrogen bond to the nitrogen (an n-pair hydrogen bond) and a secondary interaction between the ortho-hydrogens of the pyridine and the π electron density of the acetylene.

Original languageEnglish
Article number104309
JournalJournal of Chemical Physics
Volume143
Issue number10
DOIs
Publication statusPublished - 14 Sep 2015

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