Abstract
In the present study, the performance of a set of density functionals: BP86, PBE, OLYP, BEEF, PBEpow, TPSS, SCAN, PBEGXPBE, M06L, MN15L, B3LYP, PBE0, mPW1PW, B97, BHandHLYP, mPW1PW, B98, TPSS0, PBE1KCIS, SCAN0, M06, M06-2X, MN15, CAM-B3LYP, ωB97x, B2PLYP, and the B3LYP/N07D and PBE/N07D schemes in the calculation of the 14N anisotropic hyperfine coupling (HFC) constants of a set of 23 nitroxide radicals is evaluated. The results are compared with those obtained with the DLPNO-CCSD method and experimental HFC values. Harmonic contribution to the 14N HFC vibrational correction was calculated at the revPBE0/def2-TZVPP level and included in the evaluation. With the vibrational correction, the DLPNO-CCSD method yielded HFC values in good agreement with the experiment (mean absolute deviation (MAD) = 0.3G for the dipole-dipole contribution and MAD = 0.8G for the contact coupling contribution). The best DFT results are obtained using the M06 functional with MAD = 0.2G for the dipole-dipole contribution and MAD = 0.7G for the contact coupling contribution. In general, vibrational correction significantly improved most DFT functionals' performance but did not change its overall ranking.
Highlights
Density functional theory (DFT) is the most used method in quantum chemical modeling as it affords the best balance of cost and accuracy for the time being
One can extract some information on spin density distribution in the radical from hyperfine coupling (HFC) directly measured by Electron Paramagnetic Resonance (EPR) spectroscopy and use it in DFT functionals performance evaluation [5,6,7]
It is interesting to note that the B3LYP/N07D functional/basis combination, which is frequently used for routine EPR calculations due to its low computational cost and accuracy, strongly suffers from the inclusion of the vibrational correction as its aiso mean absolute deviation from the experiment doubles (1.0 G without correction and 2.0 G with correction)
Summary
Density functional theory (DFT) is the most used method in quantum chemical modeling as it affords the best balance of cost and accuracy for the time being. The former requires taking into account solvation and vibrational averaging effects and calculating accurate geometry[16] The latter way, which requires, for example, coupled-cluster calculation of the reference values, is preferred, especially if the test set consists of small species, treated using accurate ab initio methods. 14N hyperfine couplings of nitroxide radicals were calculated using the domain-based local pair-natural orbital coupled-cluster with singles and doubles (DLPNO-CCSD) method [17, 60, 61] with DLPNO-HFC1 set of the thresholds and quasi-restricted orbitals (QRO) from unrestricted B3LYP/COSMO calculation as a reference. The labels used for nitroxide radicals by Lebedev et al were adopted for use in the present paper
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