Computational Studies of Ion-Pair Separation of Benzylic Organolithium Compounds in THF: Importance of Explicit and Implicit Solvation

Abstract

Ion-pair separation (IPS) of THF-solvated fluorenyl (1C), diphenylmethyl (2C), and trityl (3C) lithium was studied computationally. Minimum energy equilibrium geometries of explicit bis- and tris-solvated contact ion pairs (CIPs) and tetrakis-solvated separated ion pairs (SIPs) were located at B3LYP/6-31G*. Associative transition structures linking the tris-solvated CIPs and tetrakis-solvated SIPs were also located. Based on MP2/6-31G*//B3LYP/6-31G* energies, the resting states of the CIPs are predicted to be trisolvates. Calculated enthalpies of IPS (ΔHIPS) at 298 K were compared to experimental (UV−vis spectroscopy) solution values reported in the literature. In vacuum, B3LYP/6-31G* ΔHIPS values for 1C·(THF)3−3C·(THF)3 are 6−8 kcal/mol less exothermic than the experimentally determined values in THF solution. Closer examination of the individual steps of ion-pair separation (ionization, solvation, ion-pair recombination), as well as comparison of calculated structures with the published X-ray structures of 1C·(THF)3 and 3S·(THF)4, suggested that in vacuo modeling of the SSIPs was problematic. Incorporation of secondary solvation in the form of Onsager and PCM single-point calculations showed an increase in exothermicity of IPS. Application of a continuum solvation model (Onsager) during optimization at the B3LYP/6-31G* level of theory produced significant changes in the Cα−Li contact distances in the SSIPs, and B3LYP/6-31G* (PCM)//B3LYP/6-31G* (Onsager) energies bring ΔHIPS within 5−6 kcal/mol of experiment. Possible strategies to achieve closer agreement with experiment are discussed