Quantum chemical studies on the potentially important imidates Tarek M. El-Gogary 1 Chemistry Department, Faculty of Science, Jazan University, Saudi Arabia article info Article history: Received 18 February 2008 Received in revised form 5 April 2008 Accepted 8 April 2008 Available online 22 April 2008 Keywords: Ab initio Intramolecular hydrogen bonds Transition state Activation energy Imidates abstract The conformational space for the imidates 1–5 was explored with the Monte Carlo conformational search varying all the torsional degrees of freedom. Geometry optimization of the most stable conformers of imi- dates 1–5 were performed at the level B3LYP/6-31+G(d). Computed geometrical parameters were com- pared with the X-ray reported results. Hydrogen bonding, both intra- and intermolecular with methanol as solvent were computed at different computational levels in gas phase and DMSO. Intramo- lecular hydrogen bond strength does not exceed the limit of 8.50 kcal/mol due to geometry restrictions while, on the other hand, intermolecular hydrogen bond strength approaches 30.00 kcal/mol. Transition states of the inter-conversion between the most stable conformers (rotamers) were located and activa- tion barrier of the inter-conversion processes was computed at different levels in gas phase and DMSO. Infrared spectra of these compounds were simulated and compared with experimental results. The car- bonyl of the ester group shows bond stretching vibrational frequency at 1706–1812 cm À1 which agrees well with the experimentally recorded values at 1708–1740 cm À1 . Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Imidates are important compounds that find uses in organic synthesis as building blocks and intermediates in many reactions for example in the Mumm rearrangement and the Overman rear- rangement. These compounds also find use as ligands [1]. The ever-increasing demand for the design and synthesis of small mol- ecule peptidomimetics [2–12] as pharmaceutical probes and drug leads has led to hectic research activities in the area of new drug discovery. 2-(2,2,2-Trichloro-ectimidoyloxymethyl)-acrylic acid al- kyl ester imidate compounds (see Scheme 1) are interesting deriv- atives to be inserted into oligopeptides moieties to induce conformational constrictions. In addition, they can be considered precursors of hydroxy amino acids. In these compounds, the pres- ence of a diversified functionality, such as carbonyl ester and imi- dates, that can undergo inter- or intramolecular hydrogen bonding, makes them interesting for quantum chemical calculations. In this work, the conformational space for molecules 1–5 (Scheme 1) was explored using the Monte Carlo conformational search varying all the torsional degrees of freedom. Geometry optimization of the most stable conformers of imidates 1–5 were performed at B3LYP/6-31+G(d) level. Hydrogen bonding, both intra- and inter- molecular were computed at MP2/6-31+G(d) in gas phase and DMSO. Infrared spectra of these compounds were simulated and compared with experimental results. 2. Computations Calculations on the isolated molecules and molecular com- plexes were performed within GUASSIAN 03 [13] program pack- age. Equilibrium geometries were determined and confirmed by a subsequent calculation of force constants and vibration analysis. Calculations were performed using the closed-shell Hartree–Fock, Becke’s three parameter density functional theory (DFT) [14] in combination with the Lee, Yang and Parr correlation functional [15] (B3LYP) and second-order MØller–Plesset perturbational method (MP2) [16–20]. The basis set used with these methods is the split valence diffuse function 6-31+G * [21,22]. The conforma- tional space for molecules 1–5 was explored using Macromodel package 5.5 [23]. The Monte Carlo conformational search was per- formed using MM2 * force field varying all the torsional degrees of freedom using the solvation model GB/SA [24]. This solvation mod- el treats the solvent (DMSO) as an analytical continuum starting near the van der Waals surface of the solute. The frequencies are consistently overestimated at B3LYP/6-31+(d), which is due to the neglect of correlation energy and presence of anharmonicity in the molecular vibrations. Therefore, frequencies were scaled by a factor 0.98062 [25]. 3. Results and discussion 3.1. Geometry and conformations The conformational space for molecules 1–5 was explored using Macromodel package 5.5 [23]. The most stable conformers (conf1 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.04.011 E-mail address: tarekelgogary@yahoo.com. 1 Present address: Chemistry Department, Faculty of Science (Dumyat), Mansoura University, Dumyat Al-Gideda, Egypt. Journal of Molecular Structure: THEOCHEM 861 (2008) 62–67 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem