A Broadband Rotational Spectroscopic Study Of Diethyl Phthalate

Diethyl phthalate (DEPh), a diethyl ester of phthalic acid, is commonly used as a plasticizer in products such as plastic packaging films, toiletries, and plastic bags.$^{1}$ The two ester groups can rotate about the respective single C – C bonds, making the conformational landscape of DEPh quite complex, and further investigation may improve our knowledge about the conformational landscape and dynamics of DEPh. Additionally, the close proximity of the two substituent groups and the resulting non-covalent interactions that will occur between them will ultimately dictate the monomeric structure. A thorough experimental investigation and assignment of the DEPh conformers may not only provide valuable insights into non-covalent intramolecular interactions, but also valuable conformational benchmark data for theory. We use rotational, microwave, spectroscopy to experimentally probe the conformational diversity of DEPh, as it has been shown before to be an excellent tool to investigate conformationally flexible molecules.$^{2}$ A broadband rotational spectrum was recorded in the 2-6 GHz region using a chirped pulse Fourier transform microwave spectrometer (CP-FTMW). With the help of the Conformer-Rotamer Ensemble Sampling Tool (CREST)$^{3}$ and DFT calculations, we identified 16 unique theoretical conformers. Using these results as an assignment aid, a total of 7 conformers were identified in our rotational spectrum. Non-covalent interactions (NCI)$^{4}$ analyses were carried out to examine the intermolecular interactions present within the DEPh conformers. 1. J. Sekizawa, S. Dobson, R. J. Touch III, DIETHYL PHTHALATE, CICAD. 2003;2. F. Xie, N. A. Seifert, M. Heger, J. Thomas, W. Jäger, Y. Xu, Phys. Chem. Chem. Phys. 2019. 21, 15408-16.; 3. P. Pracht, F. Bohle, S. Grimme, Phys. Chem. Chem. Phys. 2020., 22, 7169-7192; 4. E. Johnson, S. Keinan, P. Mori-Sánchez and J. Contreras-García, J. Am. Chem. Soc., 2010, 2010, 132.