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

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

Protonated 2-Methyl-1,2-epoxypropane: A Challenging Problem for Density Functional Theory

Protonated epoxides feature prominently in organic chemistry as reactive intermediates. Herein, we describe 10 protonated epoxides using B3LYP, MP2, and CCSD/6-311++G** calculations. Relative to CCSD, B3LYP consistently overestimates the C2−O bond length. Protonated 2-methyl-1,2-epoxypropane is the most problematic species studied, where B3LYP overestimates the C2−O bond length by 0.191 Å. Seventeen other density functional methods were applied to this protonated epoxide; on average, they overestimated the CCSD bond length by 0.2 Å. We present a range of data that suggest the difficulty for DFT methods in modeling the structure of the titled protonated epoxide lies in the extremely weak C2−O bond, which is reflected in the highly asymmetric charge distribution between the two ring carbons. Protonated epoxides featuring more symmetrical charge distribution and cyclic homologues featuring less ring strain are treated with greater accuracy by B3LYP. Finally, MP2 performed very well against CCSD, deviating in the C2−O bond length at most by 0.009 Å; it is, therefore, recommended when computational resources prove insufficient for coupled cluster methods