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Barriers to internal rotation in methylimidazole isomers determined by rotational spectroscopy

Lookup NU author(s): Eva Gougoula, Dr Chris Medcraft, Juliane Heitkamper, Dr Nick WalkerORCiD



This is the final published version of an article that has been published in its final definitive form by AIP Publishing, 2019.

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© 2019 Author(s).The rotational spectra of N-, 2-, 4-, and 5-methylimidazole are reported and analyzed. Liquid N-methylimidazole was vaporized from a reservoir, and each of 2-, 4-, and 5-methylimidazole was laser-vaporized from a solid target prior to mixing with argon buffer gas and undergoing supersonic expansion from a pulsed nozzle. The spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy in the 7.0-18.5 GHz frequency range. Rotational constants, A0, B0, and C0, centrifugal distortion constants, DJ, DJK, DK, d1, and d2, and nuclear quadrupole coupling constants of nitrogen atoms, χaa(N1), χbb(N1) - χcc(N1), χaa(N3), and χbb(N3) - χcc(N3), are determined from experimentally measured transition frequencies. Data recorded for isotopologues containing 13C or 15N are used to determine the rs coordinates of all heavy atoms in N-, 2-, and 4-methylimidazole. The results allow fitting of parameters in the Hamiltonian that describes internal rotation of the CH3 group about its local C3 axis. The V3 terms in the periodic potential energy functions that describe the internal rotation in N-, 2-, 4-, and 5-methylimidazole are determined to be 185.104(11), 122.7529(38), 317.20(14), and 386.001(19) cm-1, respectively, by the internal axis method. The experiments are supported by density functional theory calculations. Observed variations in barrier height are explained with reference to the symmetry of overlap between a π-like orbital on the CH3 group and π-orbitals on the aromatic ring.

Publication metadata

Author(s): Gougoula E, Medcraft C, Heitkamper J, Walker NR

Publication type: Article

Publication status: Published

Journal: Journal of Chemical Physics

Year: 2019

Volume: 151

Issue: 14

Online publication date: 08/10/2019

Acceptance date: 01/09/2019

Date deposited: 09/10/2019

ISSN (print): 0021-9606

ISSN (electronic): 1089-7690

Publisher: AIP Publishing


DOI: 10.1063/1.5119997


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