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An experimental and computational IR and hybrid DFT-D3 study of the conformations of l-lactic and acrylic acid: new insight into the dehydration mechanism of lactic acid to acrylic acid

Zeinalipour-Yazdi, Constantinos D. and Catlow, C. Richard A. 2019. An experimental and computational IR and hybrid DFT-D3 study of the conformations of l-lactic and acrylic acid: new insight into the dehydration mechanism of lactic acid to acrylic acid. Physical Chemistry Chemical Physics 21 (40) , pp. 22331-22346. 10.1039/C9CP02968K
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Abstract

We have studied using hybrid Density Functional Theory (DFT) with an aug-cc-pVTZ basis set and D3 dispersion corrections the intra-molecular hydrogen bond of L-lactic acid and L-lactic-acid analogs with the hydroxyl group on the alpha carbon atom substituted by α-XH (where X = S, Se, Te) as well as the conformations of acrylic acid. The results show that there are three types of intramolecular hydrogen bonds that can form only when α-OH is present, whereas other less electronegative functional groups such as –SH, –SeH and –TeH do not exhibit the formation of an intramolecular H-bond. We show that the intra-molecular H-bond formed between the alpha-OH hydrogen and the COOH carbonyl oxygen would enhance the rate of nucleophilic substitution of alpha-OH at the K+ sites in the previously suggested dehydration mechanism of L-lactic to acrylic acids. We find that a temperature range between 190 and 210 °C would be optimum to maximise the rate of nucleophilic substitution of the alpha-OH group at the potassium sites during the dehydration mechanism of L-lactic acid to acrylic acid. Additionally, our hybrid-DFT simulation of the infrared spectrum of the various conformers shows that the lowest energy conformer can be identified by a single vibrational band at 3734 cm−1 whereas for the other conformers, this vibrational band is split with Δν that ranges between 6 cm−1 and 176 cm−1. We also find that the various conformers of acrylic acid can be identified by a double peak for the C[double bond, length as m-dash]O and O–H vibrations, which have Δν′ and Δν′′ values of 24 and 42 cm−1, respectively. This computational study is useful for spectroscopic experimental efforts that try to identify the various conformers of L-lactic acid and acrylic acid and to gain mechanistic insight into the dehydration mechanism over K substituted NaY zeolites.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Chemistry
Cardiff Catalysis Institute (CCI)
Publisher: Royal Society of Chemistry
ISSN: 1463-9076
Date of First Compliant Deposit: 5 November 2019
Date of Acceptance: 11 September 2019
Last Modified: 14 Mar 2020 16:56
URI: http://orca.cf.ac.uk/id/eprint/126590

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