A study of the solvent dependence of the structures and the vibrational, 1 H and 13 C NMR spectra of L- and DL-mandelic acid and L- and DL-3-phenyllactic acid Hassan M. Badawi ⇑ , Wolfgang Förner, Shaikh A. Ali Department of Chemistry, King Fahd University of Peteroleum & Minerals, Dhahran 31261, Saudi Arabia highlights  The structure of mandelic and 3- phenyllactic acids was optimized at the DFT B3LYP level.  The vibrational wavenumbers and NMR chemical shifts were calculated.  Vibrational and NMR assignments were provided by combining experimental and theoretical data.  The Gibbs free energy changes of solvation were computed. graphical abstract 4000 3000 2000 1000 Wavenumber (cm -1 ) article info Article history: Received 9 February 2015 Received in revised form 18 March 2015 Accepted 24 March 2015 Available online 31 March 2015 Keywords: Structural stability and solvation Vibrational and NMR spectra Assignments L- and DL-mandelic acid L- and DL-3-phenyllactic acid abstract The structural stability of mandelic acid and 3-phenyllactic acid was investigated by Density Functional B3LYP calculations with the 6-311G // basis set. The two acids were predicted to have non-planar forms with cisoid a-hydroxy moiety with respect to the C@O group as their lowest energy structures. From the SCRF = SMD calculations the stability of both acids is predicted to be significantly dependent on the dielectric constant of the solvent. The infrared and Raman spectra of L-mandelic and L-3-phenyllactic acids were compared to the corresponding solid state spectra of the DL-racemic mixtures. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of the mole- cules. The 1 H and 13 C NMR spectra of both acids were interpreted by experimental and DFT calculated chemical shifts of the two acids. The RMSD between experimental and theoretical 1 H and 13 C chemical shifts for mandelic acid is 0.961 and 6.119 ppm, whereas for 3-phenyllactic acid, it is 0.611 and 5.214 ppm, respectively. Ó 2015 Elsevier B.V. All rights reserved. Introduction The interesting biological and structural properties of pheny- lacetic acid [1–5] and its a-hydroxy-derivative mandelic acid [6–11] prompt the investigation of their conformational profile in a recent study [12]. From the calculations at the Density Functional B3LYP/6-311G // level the two acids were predicted to exist predominantly in a non-planar structure as a result of hyper-conjugation effects between the p systems of the carboxylic and of the phenyl groups. It was reported that in the case of the planar structures of man- delic and phenylacetic acids the p system of the carboxylic group and that of the phenyl ring are not quite close enough to lead to http://dx.doi.org/10.1016/j.molstruc.2015.03.050 0022-2860/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +966 3 8604208; fax: +966 3 8604277. E-mail address: hbadawi@kfupm.edu.sa (H.M. Badawi). Journal of Molecular Structure 1093 (2015) 150–161 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc