Protonated 2-methyl-1,2-epoxypropane: A challenging problem for density functional theory

[GRAPHICS] 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 angstrom. Seventeen other density functional methods were applied to this protonated epoxide; on average, they overestimated the CCSD bond length by 0.2 angstrom. 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 angstrom; it is, therefore, recommended when computational resources prove insufficient for coupled cluster methods.