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\title{INTERNAL ROTATION IN 4-FLUOROACETOPHENONE AND METHYL 4-FLUOROBENZOATE\\
AND THEIR MONO-HYDRATES}

\author{\underline{\FirstName X. \LastName WANG},
\FirstName J.-U. \LastName GRABOW}
\address{Institut f\"ur Physikalische Chemie und Elektrochemie, Leibniz Universit\"at Hannover, 30167 Hannover, Germany }
\author{{\FirstName J. \LastName LI},
\FirstName Y. \LastName ZHENG,
{\FirstName Q. \LastName GOU}}
\address{School of Chemistry and Chemical Engineering, Chongqing University, Daxuecheng South Rd. 55, 401331, Chongqing}

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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
Aug. 28 - Sept. 1, 2023}}}
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The rotational spectra of 4-Fluoroactophenone and Methyl 4-fluorbenzoate have been investigated by rotational spectroscopy, using a Fourier transform microwave spectrometer operated in the frequency ranges 2-20 GHz\footnote{\href{https://doi.org/10.1063/1.4994865}{doi:10.1063/1.4994865}, J. Chen, Y. Zheng, J. Wang, G, Feng, Z. Xia and Q. Gou, \textit{J. Chem. Phys.}, \textbf{147}, 094301, (2017).} and 2-26.5 GHz\footnote{\href{https://doi.org/10.1016/j.jms.2012.07.006}{doi:10.1016/j.jms.2012.07.006}, M. K. Jahn, D. A. Dewald, D. Wachsmuth, J.-U. Grabow and S. C. Mehrotra, \textit{J. Mol. Spectrosc.}, \textbf{280}, 54-60, (2012).}, respectively. Quantum chemical calculations were employed to perform conformational analyses, obtaining only one stable conformer at the MP2/6-311+G(d,p) level for both molecules. Arising from the different CH\textsubscript{3}O- and CH\textsubscript{3}- groups of 4-Fluoroactophenone and Methyl 4-fluorbenzoate, the changes in the structure and the \textit{V}\textsubscript{3} barrier of the methyl internal rotation were analyzed. Furthermore, the non-covalent interactions in 4-Fluoroactophenone-H\textsubscript{2}O and Methyl 4-fluorbenzoate-H\textsubscript{2}O were visualized using Bader's quantum theory of atoms-in-molecules\footnote{\href{https://doi.org/10.1021/cr00005a013}{doi:10.1021/cr00005a013}, R. F. W. Bader, \textit{Chem. Rev.}, \textbf{91}, 893-928, (1991).} and Johnson's non-covalent interaction analysis\footnote{\href{https://doi.org/10.1021/ja100936w}{doi:10.1021/ja100936w}, E. R. Johnson, S. Keinan, P. Mori-S\'{a}nchez, J. Contreras-Garc\'{i}a, A. J. Cohen and W. Yang, \textit{J. Am. Chem. Soc.}, \textbf{132}, 6498-6506, (2010).}. The rotational spectrum of 4-Fluoroactophenone-H\textsubscript{2}O has already been analyzed. Interestingly, the same non-covalent interaction binding sites are also present in Methyl 4-fluorbenzoate-H\textsubscript{2}O, but the preference in the binding sites for the global minimum reverses between the two most stable isomers of 4-Fluoroactophenone-H\textsubscript{2}O and Methyl 4-fluorbenzoate-H\textsubscript{2}O at the MP2/6-311++G(d,p) level. The molecular electrostatic potential\footnote{\href{https://doi.org/10.1039/D1CP02805G}{doi:10.1039/D1CP02805G}, J. Zhang and T. Lu, \textit{Phys. Chem. Chem. Phys.}, \textbf{23}, 20323-20328, (2021).} of 4-Fluoroactophenone and Methyl 4-fluorbenzoate was mapped to visualize the different effect of the CH\textsubscript{3}O- and CH\textsubscript{3}- groups on the electron densities, which explains why the preference for the global minimum is predicted to be opposite.

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