Hydrogen-bond couplings, variation

Figure 2. Variation of G with R for P strands (0) and a helices ( ) with three different treatments of hydrogen-bond couplings in the helices. (Reproduced with permission from r erence 5. Copyright 1996.)...

Since the immediate electronic environment of the deuterium nucleus in most compounds is relatively simple, deuterium quadrupole coupling constants show very systematic variations with molecular structure. In covalent compounds, except those with hydrogen bonds or multicentered bonds (as in many boron deuterides), deuterium is bonded to only one atom. The bonding electrons have essentially cylindrical symmetry, and the only asymmetry in the electronic environment of deuterium comes from indirectly bonded atoms. Therefore, in many cases, 17 is very small 0.1). 

The variations of /uc- h d 7ih-ih acetone and dimethyl sul-phoxide in various solvents show an almost linear relationship with the carbonyl-and chemical shifts/ " Hydrogen bonding with the solvent, or the presence of large solvent dipoles, may increase the relative importance of polar resonance structures of the carbonyl or sulphoxide group (33), and the electron withdrawl experienced by the methyl group will result in increased values of C—H couplings. 

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