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Difficulties of Popular Density Functionals to Describe the Conformational Isomerism in Iodoacetic Acid
journal contribution
posted on 2020-06-30, 11:33 authored by J. Philipp WagnerMatrix
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–CO 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.
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WaalMatrix isolation studiesBLYPmolconformerTPSSshortcomingPopular Density Functionalslatter structure exhibitsUVkcal2-PLYPCCSD97XD 3-dispersion correction2GPderiodoacetic acidiodinemedium-range correlation effectsB 3LYP BHandHLYPCAM-Bwave function methodsDSD-PBEP 86 methods12 density functionals06-2Xtorsional3.23 Å contactcarbonyl oxygen atoms
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