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Title:The Effects Of Proton Tunnelling, 14n Quadrupole Coupling, And Methyl Internal Rotations Of Planar Secondary Amines
Author(s):Koziol, Kenneth J
Contributor(s):Nguyen, Ha Vinh Lam; Stahl, Wolfgang
Subject(s):Mini-symposium: Large Amplitude Motions
Abstract:The rotational spectra of three planar secondary amines diethylamine\footnote{H. V. L. Nguyen and W Stahl, \textit{J. Chem. Phys.} \textbf{135}, 024310 (2011)}, ethylmethylamine\footnote{K. J. Koziol, W. Stahl, H. V. L. Nguyen, \textit{J. Chem. Phys} \textbf{153}, 184308 (2020)} and dimethylamine\footnote{J. E. Wollrab and V. W. Laurie, \textit{J. Chem. Phys.} \textbf{48}, 5058 (1968)} have been investigated. The hydrogen atom of the amino group performs an inversion tunnelling motion, causing splittings of all \textit{c}-type transitions into doublets. The respective values of the splittings are 1521.54134(40) MHz$^b$ 1980.9176(62) MHz$^c$ and 2646.0 MHz$^d$. The first microwave spectra of dimethylamine and ethylmethylamine have been recorded in 1967 and 1973\footnote{R. E. Penn and J. E. Boggs, \textit{J. Mol. Spectrosc.} \textbf{47}, 340 (1973)}, respectively, using Stark modulated spectrometers with a resolution of 25 kHz. The $^{14}$N quadrupole hyperfine structure was reported to be resolved only for some transitions and the effects of internal rotation have not been mentioned. In the present work, the spectra of \textit{N}-ethylmethylamine and dimethylamine were remeasured using two molecular jet Fourier transform microwave spectrometers operating from 2 to 40 GHz. The proton tunnelling was analysed, taking into account Coriolis interactions which cause splittings in the order of about 0.2 to 1.2 MHz for all \textit{b}-type transitions. Furthermore, hyperfine splittings due to the quadrupole coupling of the $^{14}$N nucleus were fully resolved. The quadrupole coupling constants $\chi_{aa}$ and $\chi_{bb} - \chi_{cc}$ were determined with high accuracy and calculated well using Bailey's method\footnote{W. C. Bailey, \textit{Chem. Phys.} \textbf{252}, 57, (2000)}. Due to the difference in symmetry between ethylmethylamine and dimethylamine, quartets and triplets with splittings in the order of 150 kHz and 200 kHz, respectively, arising from internal rotations of two methyl groups, could be resolved. We observed that large amplitude motions such as internal rotation and/or proton tunnelling influence(s) the hyperfine patterns. Quantum chemical calculations carried out to support the experiment.
Issue Date:2021-06-23
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper / Presentation
Date Available in IDEALS:2021-09-24

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