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

Published on 2020-06-30T11:33:51Z (GMT) by
Matrix isolation studies in solid argon and neon at 4.2 K reveal that iodoacetic acid initially only exists as its ground state (<i>c</i>,<i>x</i>) 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<sup>–1</sup> less stable (<i>c</i>,<i>c</i>) 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 (<i>c</i>,<i>c</i>) to (<i>c</i>,<i>x</i>) and leads to an estimated upper limit of 0.38 kcal mol<sup>–1</sup> for the associated torsional barrier. While wave function methods including completely uncorrelated Hartree–Fock theory have no problem to identify (<i>c</i>,<i>c</i>) as a proper minimum, many popular density functionals fail to describe the C–C torsional potential in <i>cis</i>-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 (<i>c</i>,<i>c</i>) 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.

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Wagner, J. Philipp (2020): Difficulties of Popular Density Functionals to Describe

the Conformational Isomerism in Iodoacetic Acid. ACS Publications. Collection. https://doi.org/10.1021/acs.jpca.0c03322