title: Hydrogen bonds in cocrystals and salts of 2-amino-4,6-dimethylpyrimidine and carboxylic acids studied by nuclear quadrupole resonance.
abstract: (14)N and (17)O nuclear quadrupole resonance frequencies have been measured in 1:1 cocrystals and salts of 2-amino-4,6-dimethylpyrimidine and several carboxylic acids. A systematic decrease of the (17)O quadrupole coupling constant on increasing strength of the hydrogen bond is observed in cocrystals bound by O-H···N hydrogen bonds. The O-H distances deduced from the line widths of the (17)O NQR lines show that the hydrogen atom is in a hydrogen bond formed by a carboxylic groups for about 0.01 nm displaced from the oxygen atom toward the center of the hydrogen bond. In the O-H···N hydrogen bond formed by the hydroxyl group, which is only slightly longer than the hydrogen bonds formed by the carboxyl group, the hydrogen atom is much less displaced. A linear relation between the (14)N quadrupole coupling constant and the sum of the inverse third powers of the H···A (A = O or N) distances is deduced for the amino group. A linear correlation of the principal values of the (14)N quadrupole coupling tensor in -NH2, as observed in the solid phase and in the gas phase, is analyzed in a simple model assuming a displacement of the electron charge in the N-H σ bond and simultaneous deformation of the nitrogen lone pair electron orbital. At the ring nitrogen position, hydrogen bonding and proton transfer produce a large decrease of the (14)N quadrupole coupling constant. A linear correlation of the principal values of the (14)N quadrupole coupling tensor is observed in cocrystals and salts of 2-amino-4,6-dimethylpyrimidine. This correlation differs from the correlation observed in substituted pyrimidine, where the hydrogen atoms are replaced by other atoms or functional groups. The difference is analyzed in a model, which assumes that the hydrogen bonding and substituents affect the nitrogen lone pair and π electron orbitals. The analysis shows that the two effects are nearly independent.
authors.affiliation: Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. janez.seliger@fmf.uni-lj.si
authors.forename: Janez
authors.initials: J
authors.lastname: Seliger
authors.forename: Veselko
authors.initials: V
authors.lastname: Žagar
mesh_terms: D002264:Carboxylic Acids / Q000737:chemistry*; D003460:Crystallization; D006860:Hydrogen Bonding; D009584:Nitrogen / Q000737:chemistry; D010103:Oxygen Isotopes / Q000737:chemistry; D011743:Pyrimidines / Q000737:chemistry*; D012492:Salts / Q000737:chemistry
chemical_list: C464526:2-amino-4,6-dimethyl pyrimidine; D002264:Carboxylic Acids; D010103:Oxygen Isotopes; D011743:Pyrimidines; D012492:Salts; D009584:Nitrogen