Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia.
J Phys Chem A. 2010 Nov 18;114(45):12083-7. doi: 10.1021/jp105449w. Epub 2010 Oct 22.
The complete (14)N nuclear quadrupole resonance (NQR) spectra have been measured in the two polymorphic crystalline phases of the molecular complex isonicotinamide-oxalic acid (2:1) by nuclear quadrupole double resonance. The observed NQR frequencies, quadrupole coupling constants, and asymmetry parameters (η) have been assigned to the two nitrogen positions (ring and amide) in a molecule on the basis of the intensity and multiplicity of the double resonance signals. The NQR data for the ring nitrogen in both polymorphic phases deviate from the correlation relations observed in substituted pyridines. This deviation is analyzed in a model, where it is assumed that an additional electric charge on the nitrogen atom changes the NQR parameters. The model suggests that this additional electric charge is negative so that the N···H-O hydrogen bond seem to be partially ionic, of the type N(-)···H-O.
已通过核四极矩双共振测量了异烟酰胺-草酸（2:1）分子配合物的两种晶态多晶型物的完整 (14)N 核四极矩共振 (NQR) 谱。根据双共振信号的强度和多重性，将观察到的 NQR 频率、核四极矩耦合常数和不对称参数 (η) 分配给分子中的两个氮位置（环和酰胺）。两种多晶型相中环氮的 NQR 数据偏离了在取代吡啶中观察到的相关关系。在一个模型中分析了这种偏差，其中假设氮原子上的附加电荷会改变 NQR 参数。该模型表明，这种附加电荷为负，因此 N···H-O 氢键似乎部分为离子键，即 N(-)···H-O 型。
