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Mackenzie RB, Dewberry CT, Coulston E, et al. Intramolecular competition between n-pair and π-pair hydrogen bonding: Microwave spectrum and internal dynamics of the pyridine-acetylene hydrogen-bonded complex. J Chem Phys. 2015;143(10):104309doi: 10.1063/1.4929997.
Mackenzie, R. B., Dewberry, C. T., Coulston, E., Cole, G. C., Legon, A. C., Tew, D. P., & Leopold, K. R. (2015). Intramolecular competition between n-pair and π-pair hydrogen bonding: Microwave spectrum and internal dynamics of the pyridine-acetylene hydrogen-bonded complex. The Journal of chemical physics, 143(10), 104309. https://doi.org/10.1063/1.4929997
Mackenzie, Rebecca B, et al. "Intramolecular competition between n-pair and π-pair hydrogen bonding: Microwave spectrum and internal dynamics of the pyridine-acetylene hydrogen-bonded complex." The Journal of chemical physics vol. 143,10 (2015): 104309. doi: https://doi.org/10.1063/1.4929997
Mackenzie RB, Dewberry CT, Coulston E, Cole GC, Legon AC, Tew DP, Leopold KR. Intramolecular competition between n-pair and π-pair hydrogen bonding: Microwave spectrum and internal dynamics of the pyridine-acetylene hydrogen-bonded complex. J Chem Phys. 2015 Sep 14;143(10):104309. doi: 10.1063/1.4929997. PMID: 26374037.
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J Chem Phys. 2015 Sep 14;143(10):104309. doi: 10.1063/1.4929997.

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

The Journal of chemical physics

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

Affiliations

  1. Department of Chemistry, University of Minnesota, 207 Pleasant St., SE, Minneapolis, Minnesota 55455, USA.
  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.

PMID: 26374037 DOI: 10.1063/1.4929997

Abstract

a-type rotational spectra of the hydrogen-bonded complex formed from pyridine and acetylene are reported. Rotational and (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-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 (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 (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 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.

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