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.

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.