¹³C NMR spectroscopic and AM1 study of the intramolecular proton transfer in anils of salicylaldehyde and 2-hydroxynaphthalene-1-carbaldehyde

Deuterium isotope effects on 13C NMR chemical shifts of some α‐(2‐hydroxyaryl)‐N‐phenylnitrones (Schiff base N‐oxides)

“…We have undertaken a study of the intramolecular hydrogen bond in some N-(R-salicylidene)aniline N-oxides, where R D H (1), 3,5-di-Cl (2) and 3-OCH 3 (3), and N-(2-hydroxynaphthylidene)aniline N-oxide (4) by measuring the deuterium isotope effect on the 13 C NMR chemical shifts.…”

“…9 An exception is sterically hindered compounds, in which an unusually large two-bond isotope effect 2 C-2(D) was observed. 10,11 If the proton transfer takes place, the deuterium isotope effect on 13 C-2 chemical shifts depends on temperature and the position of the equilibrium. We measured the two-bond and long-range deuterium isotope effects on the 13 C chemical shifts for Schiff base Noxides 1-4 and for the parent Schiff bases 1a-4a.…”

“…It is known from spectroscopic studies 12,13 and deuterium isotope effects on 13 C NMR chemical shifts 14 that in N-(salicylidene)anilines the proton is localized on the phenolic oxygen atom, whereas in derivatives of 2-hydroxynaphthaldehyde (4a) a proton transfer equilibrium exists. 13,14 The values of 2 C-2(D) and 4 C-(D) for the Schiff base N-oxides 1-4 are unexpectedly large and at the same time do not change with decrease in temperature. A deuterium isotope effect on H-˛has also been observed (Table 3).…”

“…In this and some other Mannich base N-oxides in solution, a single-minimum intramolecular hydrogen bond is present. 6,7 In order to verify the existence of a proton transfer equilibrium in the Schiff base N-oxides N-(salicylidene)aniline N-oxide (1), N- (3,5-dichlorosalicylidene)aniline N-oxide (2), N- (3-methoxysalicylidene)aniline N-oxide (3) and N-(2-hydroxynaphthylidene)aniline N-oxide (4) in solution, we have reinvestigated their 1 H and 13 C NMR spectra (Table 2) and undertaken an investigation of the deuterium isotope effect on the 13 C chemical shifts. Measurement of the deuterium isotope effect is known to be a very good tool in studying intramolecular hydrogen bond and proton transfer equilibria.…”

Photochrome That Was Not: 2-Hydroxynaphtylidene-(8-aminoquinoline)

“…9 The stability of the enol form that is found in Schiff bases that are derivatives of salicylaldehyde, is much less pronounced in those bases that are derivatives of 2-hydroxynaphthaldehyde. 10,11 Amore extended delocalization of the π-electron system on the naphthalene ring tends to favor the keto tautomer. In particular, 2-hydroxynaphtylidene-(8-aminoquinoline) (HNAQ) (Scheme 1) shows unusual stability of the keto form, such that no enol form was detected by 15 N-NMR study of HNAQ in solid phase.…”