Geometry dependence of spin-spin couplings in cyanamide by DFT analysis.
There have been numerous theoretical and experimental investigations examining NMR parameters related to non-amino N-H...N H-bonded moieties in both biological and chemical contexts. In contrast, little information on the geometry dependence of NMR parameters related to the biologically important H-bond donor amino group is available. Herein, the geometric dependencies of the one-bond amino N-H spin-spin coupling constants [(1)J(NH)] in the cyanamide monomer and dimer have been computed with B3LYP and the aug-cc-pVTZ-su0 basis set. In an isolated planar cyanamide molecule, the |(1)J(NH)| couplings were found to increase as the N-H bond lengthened. In contrast, in the planar cyanamide dimer the size of the H-bonded amino N-H coupling (|(1)J(N(d)H(d))|) decreased with increasing N(d)H(d) bond length. The |(1)J(N(d)H(d))| coupling was larger than the |(1)J(N(d)H(free))| coupling for N(d)H(d) distances up to 1.18 A (for a fixed N(d)H(free) distance of 1.006 A). Hence, the decrease of |(1)J(NH)| with increasing N-H distance, as well as the larger value of |(1)J(N(d)H(d))| compared to |(1)J(N(d)H(free))|, were only observed for situations where the amino group is involved in an H-bonding interaction. This is attributed to electron redistribution induced by the presence of the second cyanamide molecule. Similar electron-redistribution effects are thought to be responsible for the observed distance dependence of computed (1)J(NH) couplings of H-bonded amino groups in near-planar G-quartet structures. Here, the |(1)J(NH)| couplings of the amino N-H bonds decreased with increasing N-H bond length whereas the |(1)J(N(d)H(d))| couplings are approximately 7 Hz larger than the |(1)J(N(d)H(free))| couplings, despite the longer N(d)-H(d) bond length.