The shielding constants and scalar couplings in N-H center dot center dot center dot O=C and N-H center dot center dot center dot N=C hydrogen bondedsystems: An ab initio MO study

The scalar spin-spin coupling constants, both intra- and intermolecular, we re evaluated for the models of biological systems: formamide-formamide (Fa- Fa) and formamide-formamidine (Fa-Fi) dimers using the MCSCF method. Additi onally, 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 th e hydrogen bond formation. The hydrogen-bond transmitted coupling constants (1h)J(NH) and (2h)J(NN) calculated for the Fa-Fi dimer are in agreement wi th recent experimental results for nucleic acids. The short-distance hydrog en-bond-transmitted couplings (e.g., (1h)J(NH) and (2h)J(NN)) decrease fast with increase in the hydrogen bond length, in contrast to the long-distanc e proton-proton intermolecular couplings. The changes in the shieldings of the protons engaged in the hydrogen bond formation and proton accepters are found to decrease fast with increase in the hydrogen bond length, in accor dance with the previous results.