Solid State NMR Spectroscopy as a Precise Tool for Assigning the Tautomeric Form and Proton Position in the Intramolecular Bridges of <i>o</i>-Hydroxy Schiff Bases

Two analogous Schiff bases, (<i>S</i>,<i>E</i>)-2-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylimino)methyl)phenol (<b>1</b>) and (<i>S</i>,<i>Z</i>)-2-hydroxy-6-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylamino)methylene)cyclohexa-2,4-dienone (<b>2</b>), exist in the solid state as phenol-imine and keto-amine tautomers, respectively. Their crystal structures were solved using the X-ray diffraction method. Sample <b>1</b> forms orthorhombic crystals of space group <i>P</i>2(1)2(1)2(1), while <b>2</b> forms monoclinic crystals of space group <i>P</i>2(1). In each sample, one molecule is in the asymmetric unit of the crystal structure. One-dimensional and two-dimensional solid state NMR techniques were used for structure assignment and for inspection of the <sup>13</sup>C and <sup>15</sup>N δ<sub><i>ii</i></sub> of the chemical shift tensor (CST) values. NMR study indicates that the span (Ω = δ<sub>11</sub> − δ<sub>33</sub>) and the skew (κ = 3(δ<sub>22</sub> − δ<sub>iso</sub>/Ω) are extremely sensitive to change in the tautomeric form of the Schiff bases. Theoretical calculations of NMR shielding parameters for <b>1</b> and <b>2</b> and a model compound with reduced aliphatic residue were performed using the GIAO method with B3LYP functional and 6-311++g(d,p) basis sets. From comparative analysis of the experimental and theoretical parameters, it was concluded that the position of hydrogen in the intramolecular bridge has tremendous influence on <sup>13</sup>C and <sup>15</sup>N CST parameters. Inspection of Ω and κ parameters allowed for the establishment of the nature of the hydrogen bonding and the assignment of the equilibrium proton position in the intramolecular bridges in the solid state.