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Geometry Dependence of Spin–Spin Couplings in Cyanamide by DFT Analysis
Author(s) -
van Mourik Tanja,
Dingley Andrew J.
Publication year - 2007
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200600489
Subject(s) - cyanamide , chemistry , dimer , bond length , crystallography , molecule , coupling constant , chemical shift , computational chemistry , stereochemistry , geometry , crystal structure , organic chemistry , physics , particle physics , mathematics
There have been numerous theoretical and experimental investigations examining NMR parameters related to non‐amino NH⋅⋅⋅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 NH 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 NH bond lengthened. In contrast, in the planar cyanamide dimer the size of the H‐bonded amino NH 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 Å (for a fixed N d H free distance of 1.006 Å). Hence, the decrease of | 1 J (NH)| with increasing NH 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 NH bonds decreased with increasing NH 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.