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Title:Microwave Spectrum, Structure, And Internal Dynamics Of The Pyridine - Acetylene Weakly Bound Complex
Author(s):Mackenzie, Becca
Contributor(s):Leopold, Ken; Legon, Anthony; Jarrett, Emma; Dewberry, Chris
Abstract:A-type rotational spectra of the weakly bound complex formed from pyridine and acetylene are reported. Contrary to expectation based on the symmetric structure of HCCH$\cdot \cdot \cdot$NH$_{3}$, the acetylene moiety in HCCH$\cdot\cdot \cdot$NC$_{5}$H$_{5}$ does not lie along the symmetry axis of the pyridine. Rotational and $^{14}$N hyperfine constants instead indicate that, while the complex is indeed planar with an acetylenic hydrogen directed toward the nitrogen, the HCCH axis forms an angle of ${\sim}23^{\circ}$ with the \textit{C$_{2}$} axis of the pyridine. Spectra of HCCH$\cdot \cdot \cdot$NC$_{5}$H$_{5}$, HCCD$\cdot \cdot \cdot$NC$_{5}$H$_{5}$, DCCH$\cdot \cdot \cdot$NC$_{5}$H$_{5}$, and DCCD$\cdot \cdot \cdot$NC$_{5}$H$_{5}$ are all doubled, revealing the existence of a pair of low energy states. In light of the bent structure, this suggests a tunneling motion through a barrier at the \textit{C$_{2v}$} configuration. Because the splitting persists in the singly deuterated species, we conclude that the motion does not involve interchange of the acetylenic hydrogens. Single $^{13}$C substitution in either the ortho or meta positions of the pyridine eliminates the doubling and gives rise to separate sets of spectra for which the rotational constants are well predicted by a bent geometry. In this case, the two sets correspond to distinct species in which the $^{13}$C is either on the same or the opposite side as the acetylene. This further suggests that the doubling observed with unsubstituted pyridine arises from wagging of the acetylene, as such a tunneling motion is expected to be quenched when the pyridine is rendered asymmetric. The bent structure of the system may arise due to a secondary hydrogen bonding interaction between the ortho hydrogens of the pyridine and the $\pi$ system of the acetylene.
Issue Date:2014-06-19
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:Mackenzie, B.; Leopold, K.; Legon, A.; Jarrett, E.; Dewberry, C. MICROWAVE SPECTRUM, STRUCTURE, AND INTERNAL DYNAMICS OF THE PYRIDINE - ACETYLENE WEAKLY BOUND COMPLEX. Proceedings of the International Symposium on Molecular Spectroscopy, Urbana, IL, June 16-21, 2014. DOI: 10.15278/isms.2014.RB04
Rights Information:Copyright 2014 by the authors. Licensed under a Creative Commons Attribution 4.0 International License.
Date Available in IDEALS:2014-09-17

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