Theoretical NMR Spectroscopic Analysis of the Intramolecular Proton Transfer Mechanism in ortho-Hydroxyaryl (Un-)Substitued Schiff Bases

NATALIA ZARYCZ Y GUSTAVO AUCAR

JOURNAL OF PHYSICAL CHEMISTRY A

AMER CHEMICAL SOC

Año: 2008 vol. 112 p. 8767 - 8774

1089-5639

Both NMR spectroscopic parameters are calculated as a function of the distance d(N-H) of the O· · ·H· · ·N subsystem of (un- or Z-) substituted ortho-hydroxyaryl Schiff bases, with Z ) 4-OMe and 5-Cl. Typical patterns for NMR J couplings and magnetic shieldings, σ(N) (or the chemical shift δ(N)), are obtained showing that they are reliable sensors from which one can get a deeper insight on the intramolecular proton transfer mechanism. An inflection point is found by representing each NMR spectroscopic parameter as a function of d(N-H) or when the correlation between both parameters is depicted. The analysis of these (cubic) functionsshows whether the proton is bound to the oxygen or to the nitrogen atom or is shared by both atoms. In line with these findings, it is possible to predict the position of the proton in the bridge. These theoretical findings are supported by previous experimental measurements. It is shown that nitrogen chemical shift is quite sensitive to substituent effects though 1J(15NH) is not. This last parameter depends on d(NH). When correlating both spectroscopic parameters, a previous δ(N) vs 1J(15NH) linear dependence is generalized to a cubic dependence which seems to be more reliable. Calculations are based on two state of the art methodologies: DFT-B3LYPand polarization propagators at second order of approach (SOPPA) with large enough basis sets