Abstract
The spin–spin coupling constants in ethane, methylamine, and methanol have been calculated using density-functional theory (DFT), coupled-cluster singlesand-doubles (CCSD) theory, and multiconfigurational self-consistent field (MCSCF) theory so as to benchmark the performance of DFT against high-level ab initio methods and experimental data. For each molecule, the Karplus curve has been evaluated at the three computational levels. The comparisons with ab initio methods indicate that DFT reproduces the 1J(CH), 1J(CC), and 1J(NH) one-bond couplings well but is less accurate for 1J(CN), 1J(OH), and 1J(CO). While DFT performs well for the geminal couplings 2J(HH) and 2J(CH), it tends to overestimate the vicinal 3J(HH) couplings slightly although it is sufficiently accurate for most purposes.
Highlights
The indirect nuclear spin–spin coupling constants are, along with the nuclear shielding constants, the most important parameters of NMR spectra
Before density-functional theory (DFT) is applied to the calculation of unknown coupling constants, a careful comparison of the spin–spin coupling constants obtained for some simple molecules by DFT and by wave-function methods such as the coupled-cluster singles-and-doubles (CCSD) and multiconfigurational self-consistent field (MCSCF) theories is required
Such contributions may be substantial for spin–spin coupling constants, which are sensitive to small changes in the geometry [21, 22, 34, 35]
Summary
The indirect nuclear spin–spin coupling constants are, along with the nuclear shielding constants, the most important parameters of NMR spectra. All the important coupling constants of organic chemistry are present in CH3CH3, CH3NH2 and CH3OH, making these molecules suitable for testing DFT against high-level wave-function methods. Unlike the original implementations of Malkina, Salahub and Malkin [11, 12] and of Dickson and Ziegler [15], in which only the local-density approximation (LDA) and the generalized-gradient approximation (GGA) can be used, the implementation of Helgaker and coworkers allows for the use of the exact Hartree–Fock exchange The latter is an important point since the use of hybrid functionals such as B3LYP improves considerably the calculated spin–spin coupling constants [14]. We do not claim that the presented MCSCF and CCSD spin–spin couplings are close to the true nonrelativistic results, they are representative of the best spin–spin calculations that can nowadays be carried out for molecules of this size
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