How Amino and Nitro Substituents Direct Electrophilic Aromatic

Volume 18 Number 17 7 May 2016 Pages 11561–12378 PCCP

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Themed issue: Electron delocalization and aromaticity: 150 years of the Kekulé benzene structure

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PAPER C. Fonseca Guerra et al. How amino and nitro substituents direct electrophilic aromatic substitution in benzene: an explanation with Kohn–Sham molecular orbital theory and Voronoi deformation density analysis PCCP

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How amino and nitro substituents direct electrophilic aromatic substitution in benzene: Cite this: Phys. Chem. Chem. Phys., 2016, 18, 11624 an explanation with Kohn–Sham molecular orbital theory and Voronoi deformation density analysis†

O. A. Stasyuk,a H. Szatylowicz,a T. M. Krygowskib and C. Fonseca Guerra*c

The substituent eﬀect of the amino and nitro groups on the electronic system of benzene has been investigated quantum chemically using quantitative Kohn–Sham molecular orbital theory and a corresponding energy decomposition analysis (EDA). The directionality of electrophilic substitution in aniline can

accurately be explained with the amount of contribution of the 2pz orbitals on the unsubstituted carbon atoms to the highest occupied p orbital. For nitrobenzene, the molecular p orbitals cannot explain the regioselectivity of electrophilic substitution as there are two almost degenerate p orbitals with nearly the

Creative Commons Attribution 3.0 Unported Licence. same 2pz contributions on the unsubstituted carbon atoms. The Voronoi deformation density analysis has been applied to aniline and nitrobenzene to obtain an insight into the charge rearrangements due to the substituent. This analysis method identified the orbitals involved in the C–N bond formation of the p

Received 4th December 2015, system as the cause for the p charge accumulation at the ortho and para positions in the case of the NH2 Accepted 14th January 2016 group and the largest charge depletion at these same positions for the NO2 substituent. Furthermore, we

DOI: 10.1039/c5cp07483e showed that it is the repulsive interaction between the pHOMO of the phenyl radical and the pHOMO of the

NH2 radical that is responsible for pushing up the pHOMO of aniline and therefore activating this p orbital of www.rsc.org/pccp the phenyl ring towards electrophilic substitution. This article is licensed under a Introduction into the field/inductive eﬀect and the resonance (or mesomeric) eﬀect.1 The main problem is to determine individual contributions 2

Open Access Article. Published on 15 January 2016. Downloaded 10/7/2021 5:25:04 AM. In modern organic chemistry textbooks an explanation of the of each effect for a particular substituent. When the resonance and chemical reactivity of the benzene ring in electrophilic aromatic field effects act in the same direction, their separation becomes substitution is directly connected with electron-donating or even more difficult. Undoubtedly, the classical Hammett approach3 electron-withdrawing influence of substituents on the electronic (proposed in 1937) expressed by the s substituent constants has system. The explanation is not relying on modern quantitative been the first attempt of a quantitative description of the computational techniques, but quite often on the concepts of substituent effects. Also this approach, based on experimental Lewis from 1916,1e where the electrons are thought to be at an data on benzoic acid derivatives, is still widely used in chemistry. intermediate state between different resonance structures (see To date, different quantitative characteristics are also used to Scheme 1).1 This, however, does not describe what happens estimate contributions of inductive and resonance effects but quantum chemically in the electronic system. The effect of the they are not general and usually depend on the kind of studied substituent on the reactivity of the benzene ring can be divided systems.4 It is known that the inductive effect operates through s-bonds, while the resonance effect is transmitted through p-electrons. Lewis structures, which can be seen as a qualitative a Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, application of the valence bond theory, are used to explain Warsaw 00-664, Poland b Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, activating or deactivating substituent properties and also to Poland predict a favored product of the electrophilic attack. A more c Department of Theoretical Chemistry and Amsterdam Center for Multiscale rigorous way to describe a relationship between molecular Modeling, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, structure and electron density is the analysis using qualitative The Netherlands. E-mail: [email protected] molecular orbitals.1a,d,5 † Electronic supplementary information (ESI) available: MO diagrams for nitrobenzene and aniline in a perpendicular orientation; interaction energies and In this work, we would like to study the substituent effects of rotational barriers for substituted benzenes; and Cartesian coordinates of nitro- NH2 and NO2 groups on benzene using Kohn–Sham density benzene and aniline at the BLYP/TZ2P level. See DOI: 10.1039/c5cp07483e functional theory. As is known, these two substituents have

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Scheme 1 Resonance structures of aniline and nitrobenzene.

completely opposite electronic properties and therefore, opposite substitution and the change in reactivity of the benzene ring 1 effects on the electrophilic substitution reactions of benzene. caused by the NH2 or NO2 substituents. The amino group is activating and ortho-andpara-directing, whereas the nitro substituent is known to be deactivating and meta-directing. This difference in reactivity can be made plausible Methodology by the Lewis structures of aniline and nitrobenzene (Scheme 1), but General procedure it does not give an explanation of the influence of the substituent All calculations were carried out using the Amsterdam Density s p 12 on the molecular and orbitals located on the phenyl ring. The Functional (ADF) program. Geometries and energies were calcu-

Creative Commons Attribution 3.0 Unported Licence. resonance structures suggest that the amino group increases the lated using the generalized gradient approximation (GGA) with the electron density only at ortho-andpara-positions of the ring, BLYP functional.12c,d The MOs were expanded in uncontracted sets making them more preferred to the electrophilic attack. In turn, of Slater type orbitals (STOs) containing diffuse functions with two the nitro group, according to the resonance structures, promotes sets of polarization functions (TZ2P).12e The1scoreshellsofcarbon, p the formation of a meta-product by decreasing the -electron nitrogen and oxygen were treated by the frozen-core approximation. density at the ortho and para positions (Scheme 1). The field effect Geometries were optimized in the gas phase assuming C2v symmetry was deduced to be also important for the nitro substituent and to 1a,6a 6b–d (the difference in energy of the amino group between planar and explain its deactivating properties. In previous studies, some pyramidal geometries is only 0.3 kcal mol1, see also ref. 13). attempts were done to separate the total substituent effect of the This article is licensed under a nitro group into s-andp-contributions. However, the outcome was Energy decomposition analysis not conclusive, in particular about an importance of the resonance To analyze the effect of the substituent on the benzene ring, we effect, that is the directing effect, in nitrobenzene. defined two fragments consisting of two radicals with opposite

Open Access Article. Published on 15 January 2016. Downloaded 10/7/2021 5:25:04 AM. In this study, the resonance and inductive eﬀects of the spins, which are combined into aniline or nitrobenzene (see substituents on the benzene ring are analyzed and separated Scheme 2). The fragments are treated in the KS open-shell using Kohn–Sham Molecular Orbital (KS-MO) theory and the restricted formalism as proposed in ref. 7a. accompanying Energy Decomposition Analysis7 (EDA). The The interaction energy is examined in the substituted ben- quantitative KS-MO model together with the EDA have proven zene in the framework of the KS-MO model using a quantitative to be able to elucidate the nature of different types of bonds: energy decomposition analysis (EDA)7a into electrostatic inter- the nature of resonance-assisted hydrogen bonding8 and halo- actions, Pauli repulsive orbital interactions and attractive orbital gen bonding9 has been defined. Moreover, the KS-MO model interactions: was successfully used for explaining the organic reaction 10 mechanism, the concept of aromaticity and other properties DEint = DVelstat + DEPauli + DEoi (1) of chemical compounds.11 This method uses symmetry which D allows separating the interactions into s- and p-electronic The term Velstat corresponds to the classical electrostatic interaction systems of the benzene ring and the substituents. Previously, between the unperturbed charge distributions of the distorted Fernandez et al.11d found a correlation between the Hammett fragments and is usually attractive. The Pauli repulsion energy, D constants and the p electronic energy term. We will also use the EPauli, comprises the destabilizing interactions between the Voronoi Deformation Density (VDD) charge analysis, which occupied orbitals and is responsible for the steric repulsion. D enables us to track the reorganization in the s and p electron The orbital interaction energy, Eoi, represents the donor–acceptor densities of the phenyl ring due to the substituent and to determine the resonance and inductive effects on the ring. Based on frontier molecular orbital theory to explain chemical reactivity, our quantitative KS-MO results together with the VDD charges clarify the orientation of the electrophilic aromatic Scheme 2 Fragment decomposition of substituted benzenes (X = O, H).

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interactions between the occupied molecular orbitals on one This functionality is extended by the decomposition into s

fragment with the unoccupied molecular orbitals of the other and p components (for the planar, Cs symmetric molecules): fragment, the electron-pair bond formation, as well as the mixing DQs=p of occupied and virtual orbitals within the same fragment. A ð The orbital interaction energy can be further decomposed s=p s=p s=p ¼ r ðrÞr ðrÞr ðrÞ dr into the contributions from each irreducible representation G PhNX2 Ph NX2 Voronoi cell of A in PhNX2 7 of the interacting system (eqn (2)): (5)

DEoi = DEs + DEp (2) For the p system, this functionality is additionally extended by the decomposition of the electronic redistribution per atom Both nitrobenzene and aniline have C symmetry, which 2v DQ into a component associated with the Pauli repulsion decomposes DE into DE + DE and DE into DE + DE A s A1 B2 p A2 B1 DE and a component associated with the bonding orbital components. Pauli interactions DEoi.

p p p Analysis of the charge distribution DQA = DQA,Pauli + DQA,oi (6) 14 The Voronoi Deformation Density (VDD) atomic charges for This p charge decomposition constitutes a bond analysis tool aniline and nitrobenzene have also been computed. The VDD charge that mirrors the p term of the DEPauli and DEoi terms occurring QA is computed as the (numerical) integral of the deformation in the bond energy decomposition of eqn (1) and (2) (note that density Dr(r) associated with the formation of the molecule from DVelstat is not associated with any charge redistribution). This its atoms in the volume of the Voronoi cell of atom A (eqn (3)). The can be obtained by writing the deformation density of p Voronoi cell of an atom A is defined as the compartment of space electronic systems as follows: bound by the bond midplanes on and perpendicular to all bond axes p p p between the nucleus A and its neighboring nuclei. Dr (r)=DrPauli(r)+Droi(r) (7) Creative Commons Attribution 3.0 Unported Licence. ! The change in p atomic charge caused by Pauli repulsion ð X between the monomers in the complex is defined by eqn (8), QA ¼ rðrÞ rBðrÞ dr (3) Voronoi cell of A B and the corresponding change caused by charge transfer and polarization is given by eqn (9).

p Here, r(r) is the electron density of the molecule and the sum DQA;Pauli over rB(r) is the superposition of atomic densities rB of a ð 0;p p p fictitious promolecule without chemical interactions that is ¼ r ðrÞr ðrÞr ðrÞ dr PhNX2 Ph NX2 Voronoi cell of A in PhNX2

This article is licensed under a associated with the situation in which all atoms are neutral.

QA directly monitors how much charge flows, due to chemical (8) interactions, out of (QA 4 0) or into (QA o 0) the Voronoi cell of ð atom A, that is, the region of space that is closer to nucleus A DQp ¼ rp ðrÞr0;p ðrÞ dr A;oi PhNX2 PhNX Open Access Article. Published on 15 January 2016. Downloaded 10/7/2021 5:25:04 AM. 2 than to any other nucleus. Voronoi cell of A in PhNX2 The VDD method also allows us to analyze the electronic (9) redistributions within two polyatomic fragments when a chemical 0 0 The r densityisobtainedfromC ¼ NA^ CPh CNX bond is formed between these two molecular fragments. Previously, PhNX2 2 ˆ we have shown that this method is most informative to analyze through explicit anti-symmetrization (A operator) and renormaliza- resonance-assisted hydrogen bonding appropriately as it allows tion (N constant) of the product of monomer wave functions when the unperturbed monomer densities r + r are superimposed. decomposition of the total changes into s and p electronic Ph NX2 p 8a,b The p density r is obtained as the sum of orbital densities of the rearangements. The change in VDD atomic charges DQA is 00 defined by eqn (4), which relates this quantity directly to the occupied molecular orbitals belonging to the A irreducible repre- deformation density r (r) r (r) r (r) associated sentation (in Cs symmetry). As the molecular symmetry of aniline Ph–NX2 Ph NX2 with the formation of the overall molecule from the two and nitrobenzene is C2v, thus of higher symmetry than Cs,the 8a,b,14 changes in the atomic charge can be split further into A2 and B1. fragments Ph and NX2 (with X = H or O). ð DQp = DQA2 + DQB1 (10) A,Pauli A,Pauli A,Pauli DQA ¼ rPh NX ðrÞrPh ðrÞrNX ðrÞ dr p A2 B1 2 2 DQA,oi = DQA,oi + DQA,oi (11) Voronoi cell of A in PhNX2 (4) Results and discussion DQA monitors how much charge flows out of (DQA 4 0) or into Energy decomposition analysis and bonding mechanism (DQA o 0) the Voronoi cell of atom A as a result of the chemical bond formation between fragments Ph and NX2 in the sub- The energy decomposition analysis and the molecular orbital stituted benzene. diagrams of the bonding mechanism in nitrobenzene and

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1 Table 1 Energy decomposition analysis (in kcal mol ) of the C–N bond for substituted benzenes from Ph and NX2 (with X = H and O)

a a NX2 d(CN)/Å DVelstat DEPauli DEs DEA1 DEB2 DEp DEA2 DEB1 DEoi DEint NH2 0 1.389 160.0 288.5 247.0 239.5 7.5 25.1 0.2 24.9 272.1 143.6 NH2 90 1.389 165.5 292.3 249.5 238.0 11.5 11.0 0.1 9.2 260.5 133.7 NH2 0 1.492 124.2 211.0 208.9 204.1 4.8 17.3 0.1 17.2 226.2 139.4

NO2 0 1.492 161.2 330.0 224.4 217.7 6.7 18.5 0.7 17.8 242.9 74.1 NO2 90 1.492 159.5 330.3 229.6 219.9 9.7 11.0 0.7 9.1 239.6 68.8 NO2 0 1.389 211.6 445.0 276.0 265.9 10.1 27.9 1.0 26.9 303.9 70.5

a DEs = DEA1 + DEB2 and DEp = DEA2 + DEB1.

aniline between Ph and NX2, obtained at the KS-DFT BLYP/ TZ2P level of theory, are displayed in Table 1 and Fig. 1 and 2 (only the most important interactions are represented). The energy decomposition analysis of the electron-pair bond formation between Ph and the substituent NX2 shows that interaction energy for the formation of nitrobenzene is about half of the formation of aniline (74.1 kcal mol1 and 143.6 kcal mol1, respectively) and the C–N bond is also shorter in aniline than in nitrobenzene (1.389 Å and 1.492 Å, respectively). The longer C–N distance in nitrobenzene is caused by the larger Pauli repulsion 1 (330.0 kcal mol for Ph–NO2 and 288.5 for Ph–NH2). Compression

Creative Commons Attribution 3.0 Unported Licence. of the C–N distance in nitrobenzene to the distance in aniline makes the Pauli repulsion go up from 330.0 kcal mol1 to 445.0 kcal mol1, whereas in aniline the Pauli repulsion is only 288.5 kcal mol1 at This article is licensed under a Open Access Article. Published on 15 January 2016. Downloaded 10/7/2021 5:25:04 AM. Fig. 2 MO diagrams for the C–N bond formation from Ph and NO2 into nitrobenzene with the electron-pair formation in the s system (only A1 orbitals) at the top and the donor–acceptor interactions in the p system (only B1 orbitals) at the bottom. The energies (eV) of the molecular or fragment orbitals are given in black and the gross electron population of

the fragment orbitals in red. (The sHOMO on both fragments are of B2 orbitals, which do not overlap.)

1.389 Å. The shortening of the C–N distance in nitrobenzene results in an increase of 115.0 kcal of the Pauli repulsion and only in an extra attraction of 111.4 kcal mol1 of the bonding components (electrostatic and orbital interaction). The s MO diagram shows the repulsive interaction between the occupied sHOMO1 of Ph and sHOMO1 of NO2, which is absent in the s MO diagram of aniline (see Fig. 1 and 2). The largest bonding contribution to the interaction energy is the A1 component of the orbital interaction (that is the formation of the electron-pair 1 Fig. 1 MO diagrams for the C–N bond formation from Ph and NH2 into bond), which amounts to 239.5 kcal mol for aniline and aniline with the electron-pair formation in the s system (only A1 orbitals) at 217.7 kcal mol1 for nitrobenzene. The electrostatic inter- the top and the donor–acceptor interactions in the p system (only B1 1 orbitals) at the bottom. The energies (eV) of the molecular or fragment action is of similar strength for both systems: 160.0 kcal mol 1 orbitals are given in black and the gross electron population of the for aniline and 161.2 kcal mol for nitrobenzene. Note that the fragment orbitals in red. DEA2 and DEB2 only account for polarization within each fragment

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as the A2 and B2 orbitals have no amplitude on the atoms the directionality of the electrophilic attack. Recently, Fievez et al.10c

located on the C2 rotation axis and therefore do not overlap. showed that the orbital interaction is the driving force towards The stabilization caused by the donor–acceptor interactions selectivity in electrophilic aromatic substitution for activating

in the B1 irreducible representation (that is in the p electronic substituents, but it is not the main factor for the NO2 substi- system) is larger for aniline (24.9 kcal mol1) than for tuent. They studied the interaction energy and its components nitrobenzene (17.8 kcal mol1). Compression of the C–N (eqn (1)) between monosubstituted benzene derivatives and a

bond in Ph–NO2 to the distance of Ph–NH2 enlarges the DEB1 model electrophile at the onset of the reaction in a plane parallel to 26.9 kcal mol1. to the molecular plane.

For both substituents, NH2 and NO2, the formation of the Here, we would like to analyze the molecular orbitals of the

electron-pair bond in the s electronic system causes the singly substituted benzenes in terms of the contribution of the 2pz occupied molecular orbital (SOMO) of the phenyl radical to orbitals on the carbon, nitrogen and oxygen atoms to the donate 0.2 electrons to the SOMO of the substituent accounting molecular p orbitals, allowing us to explain the directionality for the inductive eﬀect of the substituent. In the case of from the molecular orbital perspective (see Fig. 3). The high

nitrobenzene, the bond formation is accompanied by the Pauli contribution of a 2pz orbital on a certain carbon atom to the repulsive interaction between occupied orbitals on the Ph and pHOMO of the substituted benzene will direct the electrophilic NO2 fragments (see Fig. 2). attack towards that atom because of the larger overlap between

The donor–acceptor interactions in the p electronic system the accepting orbital of the electrophile and the pHOMO of the of aniline and nitrobenzene have an opposite effect on the substituted benzene. The most favorable interaction occurs at

phenyl radical: NH2 is electron donating and NO2 is electron- the atom with the highest 2pz contribution to the pHOMO. withdrawing. The MO diagrams illustrate this clearly. For aniline, Aniline has only one highest occupied p orbital at 4.6 eV

the p-orbital of the NH2 group donates 0.17 electrons to the with the largest gross Mulliken contributions on the meta (14%) pLUMO+1 orbital of the benzene ring (see Fig. 1). This counteracts and the para (22%) positions. The electrophilic attack will

Creative Commons Attribution 3.0 Unported Licence. the charge-flow in the s system. There is also Pauli repulsion therefore occur at these positions. Nitrobenzene has two almost between the pHOMO on Ph and NH2, which pushes up the energy degenerate highest occupied p orbitals at 6.9 eV and 7.0 eV.

of the pHOMO of aniline to 4.6 eV and accounts for the activation The pHOMO directs the electrophilic attack towards ortho and

towards electrophilic substitution and not the donor–acceptor meta positions with 2pz contributions of 23% on these two

interactions. This mechanism has also been encountered in the positions, and the pHOMO1 towards the para position with a 2pz study of the substituent effects on the optical properties of contribution of 31% on the para (contributions on the ortho naphthalene diimides.11c and meta positions are small, 5% and 8% respectively). Thus, In the case of nitrobenzene, the electron transfer goes in the p orbitals of nitrobenzene are deactivated (because they are

the opposite direction. The pLUMO of the NO2 group accepts low-lying) and the experimentally observed meta directing eﬀect This article is licensed under a 0.11 electrons from the p-system of the benzene ring increasing of the nitro substituent for electrophilic aromatic substitution

the deactivation for electrophilic substitution (see Fig. 2). The cannot be explained from the character of the pHOMO and

pHOMO of nitrobenzene lies at 7.0 eV, which is 2.4 eV lower pHOMO1.

Open Access Article. Published on 15 January 2016. Downloaded 10/7/2021 5:25:04 AM. than the same orbital in aniline and explains the lower reactivity of nitrobenzene. VDD charge analysis Another way to investigate the effect of the substituents is to Complementary to the KS-MO analysis and the EDA, we have rotate the substituents by 90 degrees, that is, perpendicular to also investigated the VDD atomic charges (eqn (3)) and charge the plane of the benzene ring and analyze the capability of the rearrangements (eqn (4)) on the phenyl ring due to the sub- substituents to participate in the p-electron delocalization of stituent effects to predict the regioselectivity of electrophilic the ring. The donor–acceptor interactions are either lost as in substitution and explain the reactivity of the aromatic ring. the nitrobenzene or become negligible as in aniline (the amino Fig. 4 displays the VDD atomic charges (eqn (3)) for benzene, group donates only 0.03 electrons, see Fig. S1 and S2, ESI†). nitrobenzene and aniline, which are the differences between

Rotation of the substituent NX2 (the rotation barriers are given in the final density and the density of the promolecule (the sum Table S1, ESI†) switches off the donor–acceptor interactions in over spherical atomic densities). The comparison of the VDD the p electronic system, however the energetic loss is quite small atomic charges shows that aniline has a more negative charge 1 in both cases. For nitrobenzene, DEp goes from 18.5 kcal mol than benzene at the ortho and para positions, 78 and to 11.0 kcal mol1 and for aniline from 25.1 kcal mol1 to 74 milli-electrons, respectively, compared to only 48 milli- 11.0 kcal mol1.Clearly,thep orbitals of the phenyl ring can electrons in benzene. This is in line with the large amplitudes

also interact with the perpendicular substituent and the p of the pHOMO on the ortho and para carbon atoms. Nitrobenzene delocalization is therefore not switched off completely (see has at all positions a less negative charge than benzene, which Fig. S1 and S2 in the ESI,† for the MO diagrams for the is in line with the deactivating properties of the nitro group. perpendicular conformation). The meta position in nitrobenzene is slightly more negative After this explanation of the activating and deactivating (41 milli-electrons) than the ortho and para positions

eﬀects of the NH2 and NO2 substituents on the benzene ring (34 and 28 milli-electrons). Note that the diﬀerences in based on the MO diagrams, we would also like to understand aniline are larger than in nitrobenzene.

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Paper PCCP Creative Commons Attribution 3.0 Unported Licence. This article is licensed under a Open Access Article. Published on 15 January 2016. Downloaded 10/7/2021 5:25:04 AM. Fig. 3 The p orbitals of aniline (left) and nitrobenzene (right) in A2 and B1 representation. The energies of the molecular orbitals are given in eV (black)

and the gross Mulliken 2pz contributions to MO are given in white on the atom (in percentages).

Fig. 4 VDD atomic charges (milli-electrons) of benzene, aniline and nitrobenzene (eqn (3)).

The VDD charge rearrangements, as defined in eqn (4), decomposition of the total changes into the components of allows analysis of the charge redistributions within two poly- diﬀerent irreducible representations. In Fig. 5, the VDD charge atomic fragments when a chemical bond is formed between rearrangements (DQ) due to the formation of the C–N bond in these two molecular fragments. Furthermore, it allows aniline and nitrobenzene are given. The bond formation causes

PCCP Paper Creative Commons Attribution 3.0 Unported Licence. This article is licensed under a Fig. 5 VDD electronic redistribution (DQ) and the partitioning in the DQs and DQp components caused by the formation of the C–N bond in aniline (left) and nitrobenzene (right) from the fragments Ph and NH2 and NO2 respectively. Open Access Article. Published on 15 January 2016. Downloaded 10/7/2021 5:25:04 AM.

the phenyl ring of aniline to be activated with the largest which factor is more important, orbital interactions or charge electronic charge accumulations on the ortho and para posi- redistribution, an additional investigation of the transi- tions, and the phenyl ring of nitrobenzene to be deactivated tion state of an electrophile with the substituted benzene is with the smallest electronic charge depletions at the meta necessary. positions. Decomposition into s and p charge rearrangements, Moreover, the VDD charge rearrangements allow for decom- shown in Fig. 5, reveals that the p electronic system, and thus position into the charge rearrangements caused by the Pauli the p bond formation, is responsible for the regioselectivity of repulsion and orbital interactions. As aniline and nitrobenzene

electrophilic substitution, which is in line with previous are C2v symmetric molecules, we decomposed the p charge research.6c Negative numbers on the atoms mean an accumula- rearrangements into the A2 and B1 contributions. Only the B1 tion of p-electron density on those atoms and positive numbers charge rearrangements are caused by the C–N bond formation, mean a depletion of p-electron density at atoms. The p C–N whereas the A2 charge rearrangements are only due to polar- bond formation in aniline causes DQp to become more negative ization (no A2 orbital has amplitude on the atoms lying on the

at the ortho and para positions (50 and 38 milli-electrons) C2 symmetry axis). Fig. 6 and 7 clearly show that the direction- experimentally leading to a mixture of ortho and para products, ality of the electrophilic substitution is caused by the B1 charge B1 and in nitrobenzene to become more positive at all positions, rearrangements in the orbital interactions, DQoi . The largest B1 but less positive at the meta position (only 13 millielectrons are charge accumulations of DQoi are at the ortho and para posi- B1 lost at the meta position), thus the nitro group is a deactivating tions for aniline and the largest charge depletions of DQoi are meta-directing substituent.15,16 The VDD electronic rearrange- at the ortho and para positions for nitrobenzene. For ments are in line with the experimentally observed regio- aniline, this finding confirms our results from the molecular selectivity of electrophilic aromatic substitution. To determine orbital analysis, whereas for nitrobenzene the VDD charge

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Fig. 6 VDD electronic redistribution (DQ) in the A2, B1 irreducible representations of the p system due to Pauli repulsion and the orbital interaction for aniline caused by the formation of the C–N bond from the fragments Ph and NH2. Creative Commons Attribution 3.0 Unported Licence. This article is licensed under a Open Access Article. Published on 15 January 2016. Downloaded 10/7/2021 5:25:04 AM.

Fig. 7 VDD electronic redistribution (DQ) in the A2, B1 irreducible representations of the p system due to Pauli repulsion and the orbital interaction for nitrobenzene caused by the formation of the C–N bond from the fragments Ph and NO2.

rearrangements are essential to pinpoint the orbitals respon- system of benzene caused by the formation of the C–N bond. sible for the directionality, namely the B1 orbitals are respon- The computations revealed that the C–N bond in aniline is sible for the C–N bond formation. stronger and shorter than in nitrobenzene. With our quantitative Kohn–Sham molecular orbital (KS-MO) model and the corresponding energy decomposition analysis (EDA) we Conclusions explained that the NH2 group can approach the phenyl ring closer than the NO2 group because there is less Pauli repulsion

In this paper, we studied using density functional theory the between the hydrogen atoms of NH2 and the hydrogen atoms

substituent eﬀect of NH2 and NO2 groups on the electronic of the phenyl radical at the ortho positions, whereas these

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