The Structure of Lithiated Aminonitriles in Solution: Interpretation of their Experimental IR Spectra by Means of ab initio Calculations

Abstract

Abstract Quantum chemical calculations of relative energies and vibrational frequencies have been performed in order to elucidate the structure of metalated aminonitriles (1R2RN)3RC2NLi formed upon lithiation of (1R2RN)3RCH-CN in tetrahydrofuran at -78 °C. For the sake of feasibility the calculations have been performed on model compounds where the heavy substituents 1R, 2R and 3R and in some cases even the complete amino group (1R2RN) and 3R have been replaced by hydrogen atoms. In the gasphase a lithium-bridged (nonclassical) monomer, whose lithium atom is within bonding distance to the atoms of the cyano group and the adjacent carbon atom, was found to be about 9 kcal/mol lower in energy than the most stable isomer of the corresponding nitrogen metalated ketenimine with amost linear CNLi moiety. However, the relative stability of the two structures will most likely be reversed under the influence of a solvent (modeled by H20). Dimerization of H2C2NLi, yielding a product of D2h symmetry with a four-membered (LiN)2 ring, turned out to be exothermic by about 41 kcal/mol in the gasphase and 15 kcal/mol when the model solvent is present. While in the case of the lithiated amino ketenimine with almost linear CNLi segment the structure obtained with the semiempirical MNDO method widely parallels the ab initio results, the semiempirical method seems to perform less reliably for the nonclassical monomer. Formation of the trapping products obtained with methyl iodide and acetyl chloride is explained in terms of orbital- and charge control, respectively. According to this analysis, the results of trapping experiments are compatible with presence of a dimer, of a monomer with almost linear CNLi moiety, or of a monomer with lithium in a bridging position. However, comparison of observed and computed vibrational frequencies and of the calculated energies of classical and nonclassical structures led to the conclusion that the observed IR spectra are not due to nonclassical molecules with lithium in a bridging position. Taking into account the results of recent cryoscopic measurements we assign the characteristic absorptions in the experimental IR spectra to keteniminelike monomers and their dimers