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

a-type rotational spectra of the hydrogen-bonded complex formed from pyridine and acetylene are reported. Rotational and {sup 14}N 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—NC{sub 5}H{sub 5} does not lie along the symmetry axis of the nitrogen lone pair, but rather, forms an average angle of 46° with the C{sub 2} axis of the pyridine. The a-type spectra of HCCH—NC{sub 5}H{sub 5} and DCCD—NC{sub 5}H{sub 5} are doubled, suggesting the existence of a low lying pair of tunneling states. This doubling persists in the spectra of HCCD—NC{sub 5}H{sub 5}, DCCH—NC{sub 5}H{sub 5}, indicating that the underlying motion does not involve interchange of the two hydrogens of the acetylene. Single {sup 13}C 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 {sup 13}C 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 potentialmore » energy barrier of 44 cm{sup −1} in the C{sub 2v} 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.« less
Authors:
; ;  [1] ; ;  [2] ; ;  [3]
  1. Department of Chemistry, University of Minnesota, 207 Pleasant St., SE, Minneapolis, Minnesota 55455 (United States)
  2. Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD (United Kingdom)
  3. Department of Chemistry, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS (United Kingdom)
Publication Date:
OSTI Identifier:
22489571
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 143; Journal Issue: 10; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ACETYLENE; BONDING; CARBON 13; ELECTRON DENSITY; HYDROGEN; INTERACTIONS; MICROWAVE SPECTRA; NITROGEN; PYRIDINE; TUNNEL EFFECT