Experimental and theoretical spin-spin coupling constants for [15N] formamide

The 15 spin-spin coupling constants (SSCCs) of formamide have been calculated using a new method that combines finite perturbation theory (FPT) and density functional theory (DFT). To test the reliability of this method, different hybrid density functionals and basis sets were used to calculate the Fermi contact (FC) term which was then used to calculate the SSCCs. All three of the hybrid density functionals (B3LYP, B1LYP, and MPW1PW91) gave very similar results; however, the SSCCs changed markedly with basis set. For the standard basis sets, (3-21G, 6-31G, 6-31+G* and 6-311++G**) the agreement with experiment is improved as the core orbital description is improved, but the basis set convergence is slow. To improve the basis set convergence, new basis sets were constructed with decontracted s functions. These basis sets showed much faster convergence for the FC term. To test the accuracy of the DFT/FPT method, four experimental values for the SSCCs of formamide were measured in carbon tetrachloride to closely approximate the monomer environment. Comparing this experimental value with the SSCC calculated at the largest basis set (6-311++G**) resulted in a mean absolute deviation of 2.6 Hz for B3LYP, 0.8 Hz for B1LYP, and 4.4 Hz for MPW1PW91.