A theoretical study of protonation and tautomerization of N-substituted aminoazobenzenes

Abstract

Semiempirical AM1 and ab initio SCF STO-3G calculations with full geometry optimization were performed on aminoazobenzene (AAB) and its N-methyl (MAAB), N,N-dimethyl (DMAAB), and Nphenyl (PhAAB) derivatives, as well as their azonium and ammonium conjugated acids. AM1 calculations were also performed on hydrated cationic acids, in order to estimate the effect of amphiprotic solvents on tautomerization. In all the cases studied but DMAAB the AM1 and STO-3G proton affinity of the azo nitrogen was definitely higher than that of the amino nitrogen. For the amino nitrogen the calculated proton affinity was found to increase in the series AAB < MAAB < PhAAB < DMAAB. The calculated proton affinity of the azo nitrogen increased in the same order with the exception of the STO-3G results of PhAAB. The tautomerization energy/ enthalpy (i.e. difference of the gas-phase proton affinity of the azo and the amino nitrogen) was found to increase in the series DMAAB < MAAB < AAB, the position of PhAAB in the series depending on whether the AM1 or STO-3G method was used. These results contradict the experimental data regarding aminoazobenzene protonation and tautomerization constants determined in various solvents, which indicates a strong effect of solvation on the protonation and tautomerization equilibria of aminoazobenzenes. AM1 calculations on hydrated cationic acids showed that solvation effects can be satisfactorily accounted for by enthalpy contributions in the case of tautomerization, because the order of tautomerization constants determined in methanol and dioxane-water mixture generally conforms with the order of tautomerization enthalpies with hydration included. However, the estimated proton affinities in water are still ranked in the same order as in vacuo which may indicate that entropy contributions play a much greater role in the case of protonation than in the case of tautomerization phenomena.Aminoazobenzenes have three potential centres of protonation: the amino (NY), and two azo (N" and NB) nitrogen atoms (Scheme 1). In our previous papers'.' we investigated, by means of UV-spectroscopy, the proton-exchange phenomena of N-substituted aminoazobenzenes in various solvents, in order to evaluate the relative proton affinity of the possible protonation centres.163, 149.

Photodissociation UV–Vis Spectra of Cold Protonated Azobenzene and 4-(Dimethylamino)azobenzene and Their Benzenediazonium Cation Fragment

“…The preference of structure I is consistent with previous 13 C and 15 N NMR measurements combined with MINDO calculations 35 done on 4-aminoazobenzene and is in accordance with the higher proton affinity computed for the dmaAB azo nitrogen using semiempirical AM1 and ab initio STO-3G studies. 36 By considering the low temperatures achieved in our experiment and taking into account the larger stability of the azonium structures derived from the calculations, it is possible to assume that the ammonium tautomer will likely not be present in the trap. Consequently, we have assigned the entire spectrum shown in Figure 2b to the azonium form of trans-dmaABH + , without the possibility to distinguish between isomers I and II.…”

“…The calculated vertical transition energies of the S i=13 excited states,and the corresponding wavelengths and oscillator strengths are given in The band assignment presented in Table 1 allowed consistent with previous 13 C-and 15 N-NMR measurements combined with MINDO calculations 35 done on 4-aminoazobenzene, and is in accordance with the higher proton affinity computed for the dmaAB azo nitrogen using semiempirical AM1 and ab initioSTO-3G studies. 36 By considering the low temperatures achieved in our experiment and taking into account the larger stability of the azonium structures derived from the calculations, it is possible to assume that the ammonium tautomer will likely not be present in the trap.…”

Rearrangements of Hydrazo, Azoxy and Azo Compounds: Kinetic, Product and Isotope Studies

“…The equilibria between the 4-NH 2 , 4-NHMe and 4-NMe 2 azobenzene derivatives and carboxylic acids have been studied 122 . AM1 and ab initio calculations have been performed on these species plus the 4-NHPh derivative, and all of their possible azonium and ammonium conjugate acids 123 . Unfortunately the results were not in very good agreement with the experimental data 123 .…”

Water-Soluble Colorimetric Amino[bis(ethanesulfonate)] Azobenzene pH Indicators: A UV–Vis Absorption, DFT, and 1H–15N NMR Spectroscopy Study

“…The motivation of this present study was to investigate the mechanism of protonation of these remarkable amino­[ bis (ethanesulfonate)] azobenzene pH indicators in water. Aminoazobenzenes have been the subject of numerous studies by UV–vis absorption, 1 H NMR, and Raman spectroscopy over the past decades. − , A common challenge has been to determine which of the two nitrogen atoms is preferentially protonated: the p -hybridized atom (Nγ) or the s -hybridized atom (Nβ) (Scheme ). Pioneering work relied solely on UV–vis absorption spectroscopy for determining the tautomeric equilibrium. − More recent studies have relied on other spectroscopy techniques such as 1 H NMR and Raman, the majority of which were performed in organic solvents.…”

“…Aminoazobenzenes have been the subject of numerous studies by UV−vis absorption, 1 H NMR, and Raman spectroscopy over the past decades. [27][28][29][30]32 A common challenge has been to determine which of the two nitrogen atoms is preferentially protonated: the p-hybridized atom (Nγ) or the s-hybridized atom (Nβ) (Scheme 1). Pioneering work relied solely on UV− vis absorption spectroscopy for determining the tautomeric equilibrium.…”