Difficulties of Popular Density Functionals to Describe the Conformational Isomerism in Iodoacetic Acid

2020-06-30T11:33:51Z (GMT) by J. Philipp Wagner
Matrix isolation studies in solid argon and neon at 4.2 K reveal that iodoacetic acid initially only exists as its ground state (c,x) conformer with an almost perpendicular I–C–CO dihedral angle, but UV irradiation in the 240–255 nm range leads to population of the 0.8 kcal mol–1 less stable (c,c) isomer. The latter structure exhibits a close 3.23 Å contact of the iodine and carbonyl oxygen atoms decidedly below the sum of their van der Waals radii (3.50 Å). Increasing the matrix temperature by only a few Kelvin triggers the thermal back reaction of (c,c) to (c,x) and leads to an estimated upper limit of 0.38 kcal mol–1 for the associated torsional barrier. While wave function methods including completely uncorrelated Hartree–Fock theory have no problem to identify (c,c) as a proper minimum, many popular density functionals fail to describe the C–C torsional potential in cis-iodoacetic acid qualitatively correct. We assessed the performance of 12 density functionals of different levels of sophistication, namely, the BLYP, PBE, TPSS, B3LYP, BHandHLYP, PBE0, M06-2X, CAM-B3LYP, ωB97X-D3, B2-PLYP, B2GP-PLYP, and DSD-PBEP86 methods, against accurate extrapolated CCSD­(T)/CBS­(T–Q)//MP2/def2-TZVPP energies and found that almost all of them yield acceptable relative energies. Still, even some of the best performers fail to find a reasonably deep minimum in the region of the (c,c) conformer, and addition of the empirical D3-dispersion correction does not remedy the qualitative shortcoming. Instead, inclusion of a sufficient amount of (long-range) exact exchange and likely a proper treatment of medium-range correlation effects all along the torsional coordinate play an important role in the proper description of the sub-van der Waals iodine–oxygen contact. More modern, recommended functionals do not suffer from the described shortcoming.