Density Functional Study of 2-[(R-Phenyl)amine]-1,4-naphthalenediones.

The molecular and electronic structures of a series of 2-[(R-phenyl)amine]-1,4-naphthalenediones (R = m-Me, p-Me, m-Et, p-CF3, p-Hex, p-Et, m-F, m-Cl, p-OMe, p-COMe, p-Bu, m-COOH, p-Cl, p-COOH, p-Br, m-NO2, m-CN, and p-NO2) and their anions are investigated in the framework of density functional theory. The calculations are of all-electron type using a double zeta valence polarization basis set optimized for density functional theory methods. The theoretical study shows that all compounds are nonplanar. The nonplanarity can be rationalized in terms of occupied π orbitals. A linear correlation between the measured half-wave potentials and the calculated gas-phase electron affinities is found. It holds for local as well as generalized gradient corrected functionals. Structural parameters, harmonic vibrational frequencies, and adiabatic and vertical electron affinities as well as orbital and spin density plots of the studied compounds are presented.

Relationship between molecular structure and electron targets in the electroreduction of benzocarbazolediones and anilinenaphthoquinones. Experimental and theoretical study.

We report the synthesis and voltamperometric reduction of 5H-benzo[b]carbazole-6,11-dione (BCD) and its 2-R-substituted derivatives (R = -OMe, -Me, -COMe, -CF(3)). The electrochemical behavior of BCDs was compared to that of the 2-[(R-phenyl)amine]-1,4-naphthalenediones (PANs) previously studied. Like PANs, BCDs exhibit two reduction waves in acetonitrile. The first reduction step for the BCDs represents formation of the radical anion, and the half-wave potential (E(1/2)) values for this step are less negative than for that of the PANs. The second reduction wave, corresponding to the formation of dianion hydroquinone, has E(1/2) values that shift to more negative potentials. A good linear Hammett-Zuman (E(1/2) vs sigma(p)) relationship, similar to that for the PAN series, was also obtained for the BCDs. However, unlike the PANs, in the BCDs, the first reduction wave was more susceptible to the effect of the substituent groups than was the second wave, suggesting that the ordering of the two successive one-electron reductions in BCDs is opposite that in PANs. This is explained by the fact that the electron delocalizations in the two systems are different; in the case of BCDs there is an extra aromatic indole ring, which resists loss of its aromatic character. The electronic structures of BCD compounds were, therefore, investigated within the framework of the density functional theory, using the B3LYP hybrid functional with a double zeta split valence basis set. Our theoretical calculations show that the O(1).H-N hydrogen bond, analogous to that previously described for the PAN series, is not observed in the BCDs. Laplacians of the critical points (nabla(2)rho) and the natural charges for the C-O bonds indicate that the first reduction wave for the BCDs corresponds to the C(4)-O(2) carbonyl, while in the PAN series the first one-electron transfer occurred at the C(1)-O(1) carbonyl. Natural bond orbital analysis showed that, in all the BCDs, the lowest unoccupied molecular orbital (LUMO) is located at C(4), whereas for the PANs, the LUMO is found at C(1). The good correlation between the LUMO energy values and the E(1/2) potentials (wave I) established that the first one-electron addition takes place at the LUMO. Analysis of the molecular geometry confirmed that, in both series of compounds, the effect of the substituent groups is mainly on the C(4)-O(2) carbonyl. These results explain the fact that reduction of the C(4)-O(2) carbonyl (voltammetric wave II in the PANs and voltammetric wave I in the BCDs) is more susceptible to the effect of the substituent groups than is reduction of the C(1)-O(1) carbonyl (wave I in the PANs and wave II in the BCDs).

Molecular structure of substituted phenylamine alpha-OMe- and alpha-OH-p-benzoquinone derivatives. Synthesis and correlation of spectroscopic, electrochemical, and theoretical parameters.

Thirteen C(6) para-substituted anilinebenzoquinones derived from perezone (PZ) (2-(1,5-dimethyl-4-hexenyl)-3-hydroxy-5-methyl-1,4-benzoquinone) were prepared to analyze the effect of the substituents on quinone electronic properties. The effect of a hydrogen bond between the alpha-hydroxy and carbonyl C(4)-O(4) groups was determined in perezone derivatives by substituting electron-donor and electron-acceptor groups such as -OMe, -Me, -Br, and -CN and comparing the -OH (APZs) and -OMe (APZms) derivatives. Reduction potentials of these compounds were measured using cyclic voltammetry in anhydrous acetonitrile. The typical behavior of quinones, with or without alpha-phenolic protons, in an aprotic medium was not observed for APZs due to the presence of coupled, self-protonation reactions. The self-protonation process gives rise to an initial wave, corresponding to the irreversible reduction reaction of quinone (HQ) to hydroquinone (HQH(2)), and to a second electron transfer, attributed to the reversible reduction of perezonate (Q(-)) formed during the self-protonation process. This reaction is favored by the acidity of the alpha-OH located at the quinone ring. To control the coupled chemical reaction, we considered both methylation of the -OH group (APZms) and addition of a strong base, tetramethylammonium phenolate (Me(4)N(+)C(6)H(5)O(-)), to completely deprotonate the APZs. Methylation led to recovery of reversible, bi-electronic behavior (Q/Q(*)(-) and Q(*)(-)/Q(2)(-)), indicating the nonacidic properties of the NH group. The addition of a strong base resulted in reduction of perezonate (Q(-)) obtained from the acid-base reaction of APZs with Me(4)N(+)C(6)H(5)O(-) to produce the dianion radical (Q(*)(2)(-)). Although the nitrogen atom interferes with direct conjugation between both rings by binding the quinone with the para-substituted ring, the UV-vis spectra of these compounds showed the existence of intramolecular electronic transfer from the respective aniline to the quinone moiety. (13)C NMR chemical shifts of the quinone atoms provided additional evidence for this electron transfer. These findings were also supported by linear variation in cathodic peak potentials (E(pc)) vs Hammett sigma(p) constants associated with the different electrochemical transformations: Q/Q(*)(-), Q(*)(-)/Q(2)(-) for APZms or HQ/HQH(2) and Q(-)/Q(*)(2)(-) for APZs. The electronic properties of model anilinebenzoquinones were determined at a B3LYP/6-31G(d,p) level of theory within the framework of the density functional theory. Our theoretical calculations predicted that all the compounds are floppy molecules with a low rotational C-N barrier, in which the degree of conjugation of the lone nitrogen pair with the quinone system depends on the magnitude of the electronic effect of the substituents of the aniline ring. Natural charges show that C(1) is more positive than C(4) although the LUMO orbital is located at C(4). Hence, if the natural charge distribution in the molecule controls the first electron addition, this should occur at carbon atom C(1). If the process is controlled by the LUMO orbitals, however, electron addition would first occur at C(4). For the APZms series susceptibility of the first reduction wave to the substitution effect (rho(pi) = 147 mV) is lower than that of the second reduction wave (rho(pi) = 156 mV). Thus, the first, one-electron transfer in the quinone system is controlled by the natural charge distribution of the molecule and therefore takes place at C(1).

An Experimental and Theoretical Study of the Substituent Effects on the Redox Properties of 2-[(R-phenyl)amine]-1,4-naphthalenediones in Acetonitrile.

We synthesized and analyzed 19 compounds of 3'- (meta-) and 4'- (para-) substituted 2-[(R-phenyl)amine]-1,4-naphthalenediones (PANs) R = p-MeO, p-Me, p-Bu, p-Hex, p-Et, m-Me, m-Et, H, p-Cl, p-Br, m-F, m-Cl, p-COCH(3), m-CN, m-NO(2), m-COOH, and p-COOH. Despite the fact that the nitrogen atom, which binds the quinone with the meta- and para-substituted ring, interferes with the direct conjugation between both rings, the UV-vis spectra of these compounds show the existence of an intramolecular electronic transfer from the respective aniline to the p-naphthoquinone moiety. In accordance with this donor-acceptor character, the cyclic voltammograms of these compounds exhibit two, one-electron reduction waves corresponding to the formation of radical-anion and dianion, where the half-wave potential values vary linearly with the Hammett constants (sigma(x)). The analysis of the different voltammetric parameters (e.g., voltammetric function, anodic/cathodic peak currents ratio, and the separation between the anodic and cathodic potential peaks) show that with the exception of the carboxylic PAN derivatives, all compounds present the same reduction pathway. We investigated the molecular and electronic structures of these compounds using the semiempirical PM3 method and, within the framework of the Density Functional Theory, using the Becke 3LYP hybrid functional with a double zeta split valence basis set. Our theoretical calculations predict that, with the exception of the p-nitro compound, all the compounds are planar molecules where the conjugation degree of the nitrogen lone pair with the quinone system depends on the position and magnitude of the electronic effect of the substituent in the aniline ring. The Laplacians of the critical points (nabla(2)rho), for the C-O bonds, show that the first reduction wave corresponds to the carbonyl group in alpha-position to the aniline, and that the second one-electron transfer is due to the C(4)-O(2) carbonyl reduction. Thus, the higher reaction constant value (rho) obtained for the second one-electron transfer is due to the fact that the displacement of the nonshared electrons of the amino nitrogen merely modifies the electron density of C(4)-O(2) bond. The positive correlation between the LUMO energy values calculated for these compounds and the E(1/2) potentials corresponding to the C(1)-O(1) carbonyl reduction show that the electron addition takes place at the lowest unoccupied molecular orbital, supporting the fact that this wave is also prone to the substituent effect.