The Shielding Constants and Scalar Couplings in N−H···OC and N−H···NC Hydrogen Bonded Systems: An ab Initio MO Study

The scalar spin−spin coupling constants, both intra- and intermolecular, were evaluated for the models of biological systems: formamide−formamide (Fa−Fa) and formamide−formamidine (Fa−Fi) dimers using the MCSCF method. Additionally, the shielding constants were calculated at the MCSCF and MP2 levels. 1 J(NH) and 2 J(NH) couplings are the most significantly affected by the hydrogen bond formation. The hydrogen-bond transmitted coupling constants 1 h J(NH) and 2 h J(NN) calculated for the Fa−Fi dimer are in agreement with recent experimental results for nucleic acids. The short-distance hydrogen-bond-transmitted couplings (e.g., 1 h J(NH) and 2 h J(NN)) decrease fast with increase in the hydrogen bond length, in contrast to the long-distance proton−proton intermolecular couplings. The changes in the shieldings of the protons engaged in the hydrogen bond formation and proton acceptors are found to decrease fast with increase in the hydrogen bond length, in accordance with the previous results.