Theoretical Study of Relationships between Structural, Optical

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Theoretical Study of Relationships between Structural, Optical, Energetic, Magnetic Properties and Reactivity Parameters of Benzidine and its Oxidized Forms Sergey V Bondarchuk, and Boris F Minaev J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp507479p • Publication Date (Web): 04 Sep 2014 Downloaded from http://pubs.acs.org on September 5, 2014

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The Journal of Physical Chemistry

Theoretical Study of Relationships between Structural, Optical, Energetic, Magnetic Properties and Reactivity Parameters of Benzidine and its Oxidized Forms Sergey V. Bondarchuk *a and Boris F. Minaev a,b,c a

Department of Organic Chemistry, Bogdan Khmelnitsky Cherkasy National University, blvd.

Shevchenko 81, 18031 Cherkasy, Ukraine. Fax: (+3) 80472 37-21-42; Tel: (+3) 80472 37-65-76; E-mail: [email protected] b

Department of Physics, Tomsk State University, pr. Lenina 36, 634050 Tomsk, Russian

Federation c

Department of Theoretical Chemistry and Biochemistry, Royal Institute of Technology,

AlbaNova, S-106 91 Stockholm, Sweden

ABSTRACT Structural, topological, optical, energetic, magnetic properties and reactivity parameters of benzidine, its radical cation and dication as well as molecular complexes of the benzidine dication with the F–, Cl–, Br–, I–, NO3–, HSO4– and H2PO4– anions were calculated at

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the B3LYP/6-311++G(2d,2p) level of theory in the CH2Cl2 medium. The CAM-B3LYP functional (as the most reliable one) and the 6-311++G(3df,3pd) basis set were used for the UVVis absorption spectra prediction. The obtained spectral results are in a good agreement with available experimental data. A number of the calculated global and local molecular properties, including several recently developed ones, (in general, more than 20 parameters), namely, λmax, the bond lengths and orders (l and LA,B), adiabatic ionization energy (IEad), global electrophilicity index (ω), condensed electrophilic Fukui functions (f+) and dual descriptor (∆fA), van der Waals molecular volume, nuclear independent chemical shifts (NICS) and QTAIM topological parameters were estimated in the critical points of the C(1)–C(1'), C(2)–C(3) and C(4)–N bonds as well as at the ring critical point. These quantities were found to be in a strong linear dependence (R2 > 0.99 in most cases) with the number of detached electrons (Nel) from the benzidine molecule up to formation of the dication (Nel = 2). On one hand, a position of the longwave absorption band (λCT) corresponding to the anion-to-cation charge transfer in the neutral complexes of the benzidine dication with anions, correlates with the Mulliken electronegativity of the anion (R2 = 0.8646) and its adiabatic ionization energy (R2 = 0.8054). On the other hand, the correlations with the anion charge in the complexes and the anion isotropic polarizability are rather poor (R2 = 0.6392 and 0.3470, respectively). On the ground of the obtained strong relationships, one may recommend the calculated molecular properties as potentially preferable descriptors for the benzidine-based compounds in terms of the QSAR methodology.

KEYWORDS Benzidine-Based Compounds, Electronic Descriptors, Correlation Analysis, DFT, QTAIM 1. INTRODUCTION


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Since discovery of its carcinogenicity in far 1970s, benzidine (Bzd0) became an “outcast” among the potential research objects for more than 30 years. But starting from the mid 2000s, the structural and optical properties of benzidine-based compounds (BBCs) and their oxidized forms, demonstrate a sustainable rising interest of researchers, in particular due to several practical applications.1–7 Despite the fact that the synthesis of Bzd0 was first reported by Zinin in 1845,8 an ambiguous structural nature of this compound was clarified not earlier than in 2006 by Rafilovich and Bernstein.9 They found that Bzd0 exists in four crystal forms corresponding to the P21/n (brown-red), P (brown-orange), P21/c (orange-yellow) and P21/n (brown-red) space groups. Additionally, a considerable attention has been paid to the structural properties of the oxidized forms of Bzd0,7,10–18 as well as of the benzidinium salts with inorganic anions.19–21 In particular, the salts that contain radical cations of benzidine (Bzd•+), 3,3',5,5'-tetramethylbenzidine, 2,2',6,6'tetraisopropylbenzidine, and 4,4'-terphenyldiamine with weakly coordinating anions, like [Al(OC(CF3)3)4]– or SbF6–, have been isolated and their UV-vis, ESR and X-ray spectral data have been recorded.10 Apart from the structural results, there is a huge body of literature, which relates to some extend with the optical (IR, UV-vis) properties of BBCs; we present a reference list, which does not intend to be fully comprehensive.1,2,6,22–34 It is well-known that electrochemical oxidation of BBCs or mixing them with strong oxidants leads to appearance of the color due to formation of the benzidine-based dications (Bzd2+ for benzidine).25,35 The Bzd2+ dication has a planar quinoid-like structure and absorbs always in the visible region. Recently, we have studied theoretically the absorption spectra of BBCs in neutral, radical cation and dication states using CAM-B3LYP/6-311+G(d, p) calculations in the CH2Cl2 medium.7 This functional has been justified as the most reliable one taking into account a careful


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statistical treatment of the calculated and the known experimental data. It was found that our structural and spectral theoretical results are in a good agreement with the previous experimental studies.9,10 To the best of our knowledge, there are no data in the literature containing a complete assignment of the Bzd0, Bzd•+ and Bzd2+ absorption spectra, excluding our recent paper on the latter dication.7 The aim of our previous study7 was to predict position of the λmax for Bzd0, its radical cation (Bzd•+) and dication (Bzd2+) as well as for several benzidine derivatives in order to use them in a recently developed analytical colorimetric determination of strong oxidants, like the ClO–, IO3–, BrO3– anions, etc.6 For example, this reaction can be used as an important analytical tool for express testing of the free chlorine content in wastewater or industrial circulating water due to its cheap and easy application. On the other hand, this analytical method can be applied for the BrO3– anion determination in such food additives as E924a and E924b being potassium and calcium bromates, respectively. The developed protocol is based on anilines oxidation proceeding via an intermediate formation of BBCs due to anilines oxidative coupling reaction (Scheme 1). The subsequent oxidation of neutral BBCs leads to the colored BBCs dications. Note that the aforementioned stepwise reaction can be altered by a direct BBCs oxidation, but this is less preferable method because of safety reasons. It is worthwhile noting that the most plausible resonance structure of the Bzd2+ dication is that illustrated in Scheme 1; this follows from our recent QTAIM topological analysis.36

PLACE FOR SCHEME 1


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An unusual way leading to the Bzd2+ dication includes self-recombination of the triplet state pH2NC6H4+ cations.36 These reactive species exhibit an effective positive charge delocalization and a much lower global electrophilicity;37,38 this allows an effective recombination despite the cationic nature of the reactive species. In contrast to convenient closed-shell carbocations, which have a rather large singlet-triplet gap (e.g., the p-methoxybenzhydryl cation has the B3LYP/631+G(d,p) estimated gap value to be equal to 38.8 kcal mol–1),39 the same value for the Bzd2+ dication is calculated to be only 16.3 kcal mol–1 (at the B3LYP/6-311++G(2d,2p) level of theory). Thus, the triplet state of the latter species can be achievable under the dark conditions and should affect the dication reactivity.40–43 To prevent the Bzd2+ dication degradation, the oxidation of BBCs is usually conducted in the acidic media.6 Thus, the anions, which are present in some excess in the reaction mixture will participate in complexation with the latter dication. This should affect the absorption spectrum of the “pure” Bzd2+ by appearance of the red-shifted charge transfer bands;44,45 therefore, one should take into account the possible ion pairing. In the previous study we have proposed a model involving the two explicit Cl– anions being located perpendicular to the Bzd2+ dication plane (symmetry C2h). This looks like a sign percent “%” (see Figure 2 below).7 The model has been successful to describe the spectral behavior of such systems. The aim of our present work is to study the influence of different counterions, namely the F–, Cl–, Br–, I–, NO3–, HSO4– and H2PO4– anions on the spectral performance of the Bzd2+ dication. To achieve this goal we used a highly accurate functional determined earlier and an extended basis set (see the following Section). In parallel, we have performed a careful analysis of the static global and local properties (structural, topological, optical, energetic, and magnetic ones) as well as some reactivity parameters of the Bzd0, Bzd•+ and Bzd2+ species. It was found that


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there is a strong linear correlation between these values corresponding to the neutral, singly and doubly oxidized forms of benzidine. Obviously, the same correlation should be peculiar for various benzidine derivatives; thus, we propose to use the found quantities as the potential quantitative structure-activity relationship (QSAR) descriptors in the study of toxicity of benzidine-based compounds.

2. COMPUTATIONAL DETAILS Geometry optimization of all studied species has been performed using the density functional theory DFT46/B3LYP47,48 method. Except the iodine atom, the Pople’s split-valence triple-ζ basis set (6-311 G) with addition of both polarization (2d,2p) and diffuse (++) functions49 was applied during the calculations. The iodine atoms were treated with a pseudopotential, namely, the LANL2DZ basis set.50 To justify the obtained geometries as the minima, the vibrational frequency analysis was subsequently performed. All the structures considered were characterized by the absence of the imaginary frequencies. Polar medium simulations were done using the polarizable continuum model (PCM).51 Dichloromethane was chosen as a model solvent because it is the most frequently used one for experimental studies. To define cavities the universal force field (UFF) radii were used.51 The overlap index and a minimum radius of the spheres were specified as 0.8 and 0.5 Å, respectively. Electronic spectra were calculated in terms of Time-Dependent Density Functional Theory (TD DFT). A range-separated hybrid B3LYP functional which is based on the Coulombattenuating method (CAM), namely, CAM-B3LYP,52 was chosen for the UV-vis spectra prediction because it provides the best fit of the calculated spectra with the known experimental data on BBC’s absorption spectra.7,10 This part of the calculations was performed using a much


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more extended basis set, namely, the 6-311++G(3df,3pd), except the iodine atoms. These were still treated using the LANL2DZ pseudopotential. The DFT calculations of the closed-shell species were performed using the spin-restricted Kohn-Sham formalism, while the open-shell species were calculated in terms of the spinunrestricted Kohn-Sham approach. All the SCF procedures as well as TD DFT calculations were done with the Gaussian09 suite of programs.53 Fitting the electronic absorption spectra curves were performed using the Gauss distribution function and a half-width of 3000 cm–1 with the SWizard 5.0 program package.54 The Quantum Theory of Atoms in Molecules (QTAIM) properties were calculated by the AIMQB program within the AIMStudio suite using the Proaim basin integration method.55 All post-SCF analyses were conducted using a recently developed Multiwfn program package.56 Computation of the condensed Fukui functions and the dual descriptor was carried out using the atomic charges determined by the Hirshfeld population analysis.57 This part of calculations was treated using the DFT(BLYP)/TNP method implemented in Materials Studio 5.5 suite of programs.58 Wherein, the polar medium simulation was performed using a conductor-like screening model (COSMO).59 Among the local molecular properties which were included in the regression analysis we have chosen the electron localization function (ELF),60 the ELF value determines how the electron motion is confined to a defined molecular spatial region. Another molecular property is the localized orbital locator (LOL).61 We also estimated local ionization energy ( I ) as an essential topological characteristics of a given molecular site.62 The lower the I (r ) value, the weaker the electrons are bound at the point with coordinates r.62 Very important local molecular property is the information (or Shannon) entropy (S). Being introduced in far 1948 this quantity was recently found to be a useful tool in theoretical chemistry.63 An important property for revealing a weak


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interaction region inside a molecule is sign(λ2)ρ.64 This means the sign of the second largest eigenvalue of the electron density Hessian matrix, λ2, multiplied by the electron density. A commonly used AIM property, namely, Laplacian of the electron density ( ∇ 2 ρ ), was also considered in this work. If ∇ 2 ρ (r ) < 0 , than the electron density is concentrated at the point r. Otherwise, if ∇ 2 ρ (r ) > 0 , then the electron density is depleted from the given molecular site (r).65 Very informative local AIM functions are the Hamiltonian he(r) (or another definition, namely, K(r)) and Lagrangian g(r) kinetic energy densities as well as potential energy density v(r).65 For the bond critical point (3, -1), these can be expressed in terms of the Abramov gradient decomposition.66–68 Recently, the g(r) values were proposed as a measure for estimating the ring strain energy in cyclic molecules.69 Another widely used AIM characteristics is an ellipticity of the electron density (ε).65 This includes λ1 and λ2, which are the smallest and the second smallest eigenvalues of the electron density Hessian matrix. These are always negative when estimating at the bond critical point (BCP) and demonstrate the curvature of electron density perpendicular to the bond. The global molecular properties included adiabatic ionization energies (IEad), which were calculated using eq 1:36–37,39

′ + ZPVE ′) − ( Eelec + ZPVE ) , IEad = U 0′ − U 0 = ( Eelec

(1)

where, U 0 and U 0′ are the total energies of a given molecule and its singly oxidized form at 0 K. These are the sums of the corresponding energies at stationary points on the Born-Oppenheimer potential energy surface (Eelec) and zero-point vibrational correction (ZPVE). Global electrophilicity index (ω) was calculated in terms of the Parr scheme (eq 2):69


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µ2 ω= 2η

(2)

Herein, µ = − ( IEad + EAad ) / 2 µ and η = IEad − EAad , where, IEad is estimated according to eq 1, while EAad is an electron affinity, which is calculated in the similar manner (eq 3):36–37,39

′′ + ZPVE ′′) , EAad = U 0 − U 0′′ = ( Eelec + ZPVE ) − ( Eelec

(3)

where, U 0′′ is the total energy of the singly reduced form of a given molecule at 0 K. The other reactivity parameters are electrophilic Fukui function ( f A+ )70 and the dual descriptor ( ∆f A )71 condensed to the atom A. An interesting geometry-based quantity aimed to measure aromaticity is the Bird index (I).72 The most aromatic system should have the Bird index being close to 100.72 We also calculated the absolute overlap between two orbitals (Si,j). We have calculated the overlap integral between the orbitals with numbers 48 and 49. These correspond to the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals in the Bzd2+ dication. Thus, when reducing the latter species into the Bzd•+ and, finally, the Bzd0 molecule, electrons will occupy the MO number 49 and this should increase the overlapping integral. A recently proposed useful molecular property is the Laplacian bond order (LA,B).73 For atoms A and B this can be simply written as:

LA, B = −10 ×

∫

wA (r ) wB (r )∇ 2 ρ (r )dr ,

(4)

∇ ρ

Theoretical Study of Relationships between Structural, Optical

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