The Substituent Effect of 1‐Arylazonaphthen‐2‐ols on Azo‐hydrazone Tautomerization According to NMR Analysis

Abstract

The substituent effect on azo‐hydrazone tautomerization of 1‐arylazonaphthen‐ols is studied by means of NMR analysis. Among the 13C chemical shifts, the C(2) of this series compound is the most sensitive to the variation in the nature of substituent on the phenyl ring. Therefore, the variation in the chemical shifts of C(2) is used to probe the substituent effect by using the substituent chemical shifts and free energy vs. Hammett’s constant (χρ+). Both methods give a negative correlation slope, indicating the electron‐with‐ drawing groups favor the hydrazone tautomer form. The effect on the chemical shifts of C(2) of compound 8 in ten solvents can be classified as the solvent with a proton‐donor, proton‐acceptor and arenes system. The substituent with electron‐donating character is more sensitive to the nature of solvent and it favors the hydrazone form. Free energy obtained from the dynamic NMR technique indicates the tautomerization favors the hydrazone‐form for the substituent with electron‐withdrawing character.

Substituent and Solvent Polarity on the Spectroscopic Properties in Azo Derivatives of 2-Hydroxynaphthalene and Their Difluoroboranes Complexes

“…The analysis of all values indicated that the presence of chemical groups with higher electron-donating character caused a decrease of energy differences between compared tautomeric forms. This supports previous reports [ 18 , 19 , 20 , 21 , 22 ].…”

“…The analysis of all values indicated that the presence of chemical groups with higher electron-donating character caused a decrease of energy differences between compared tautomeric forms. This supports previous reports [18][19][20][21][22]. The characteristics of frontier molecular orbital properties including energy levels of HOMO, LUMO, and other descriptors [66] gave insight into spectroscopic properties of considered dyes.…”

“…Additionally, 11 B and 19 F NMR spectra were recorded for complexes ( 5–8 ) (see details in the Materials and Methods section). As previously reported [ 18 , 19 , 20 , 21 , 22 ], the compounds showed in Scheme 1 were present in solution as a proton transfer equilibrium. In all cases, their 1 H NMR spectra showed a signal with chemical shift in the range of 15.66–16.62 ppm, corresponding to the O–H ... N proton involved in the relevant intramolecular hydrogen bond.…”

“…Additionally, 11 B and 19 F NMR spectra were recorded for complexes (5)(6)(7)(8) (see details in the Materials and Methods section). As previously reported [18][19][20][21][22], the compounds showed in Scheme 1 were present in solution as a proton transfer equilibrium. In all cases, their 1…”

“…This tautomerization is quite interesting from a theoretical standpoint because the two tautomers have different technical properties. In azonaphthols, the equilibrium depends on temperature, the solvent, and the substitution pattern [ 18 , 19 , 20 , 21 , 22 ].…”