Study of Electron Delocalization in 1,2-, 1,3-, and 1,4-Azaborines

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Study of Electron Delocalization in 1,2-, 1,3- and 1,4-Azaborines based on the Canonical Molecular Orbital contributions to the Induced Magnetic Field and Polyelectron Population Analysis Anastasios G Papadopoulos, Nickolas D Charistos, Katerina Kyriakidou, and Michael P Sigalas J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b06027 • Publication Date (Web): 08 Sep 2015 Downloaded from http://pubs.acs.org on September 10, 2015

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The Journal of Physical Chemistry

Study of Electron Delocalization in 1,2-, 1,3- and 1,4-Azaborines based on the Canonical Molecular Orbital contributions to the Induced Magnetic Field and Polyelectron Population Analysis Anastasios G. Papadopoulos, Nickolas D. Charistos, Katerina Kyriakidou and Michael P. Sigalas* Aristotle University of Thessaloniki, Department of Chemistry, Laboratory of Applied Quantum Chemistry, Thessaloniki 54 124, Greece.

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Keywords.

Azaborines,

aromaticity,

induced

magnetic

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field,

orbital

contributions,

delocalization, NBO

ABSTRACT. The electron delocalization in 1,2-azaborine, 1,3-azaborine and 1,4-azaborine is studied using Canonical Molecular Orbital contributions to the Induced Magnetic Field (CMOIMF) method and Polyelectron Population Analysis (PEPA). Contour maps of the out-of-plane component of the induced magnetic field (Bzind) of the π-system show that the three azaborines, in contrast to borazine, sustain much of benzene’s π-aromatic character. Among them, 1,3azaborine exhibits the strongest π-delocalization, while 1,4-azaborine the weakest. Contour maps of Bzind for individual π orbitals reveal that the differentiation of the magnetic response among the three isomers originates from the π-HOMO orbitals, whose magnetic response is governed by rotational allowed transitions to unoccupied orbitals. The low symmetry of azaborines enable paratropic response from HOMO to unoccupied orbitals excitations, with their magnitude depending on the shape of interacting orbitals. 1,3-azaborine presents negligible paratropic contributions to Bzind from HOMO to unoccupied orbitals transitions, were 1,2- and 1,4azaborine present substantial paratropic contributions, which lead to reduced diatropic response. Natural Bond Orbital (NBO) analysis employing PEPA shows that only the 1,3-azaborine contains π-electron fully delocalized resonance structures.

1. Introduction Since the investigation of benzene structure by Faraday1, there has been conducted extended research concerning six-membered aromatic species. An interesting case of such species arises from the replacement of one or more non-polar C-C bond by the isoelectronic polar B-N bond in cyclic aromatic hydrocarbons. This change in polarity of the bond alters the electronic properties


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of the molecule, such as conductivity2. Consequently, BN heterocycles have revealed exceptional photophysical properties3, 4, biological activity5, whereas they have been proposed as hydrogen storage materials6. Borazine, isolated by Stock and Pohland7, is an example of six-membered inorganic ring resulting from the full replacement of carbon atoms in benzene ring by boron and nitrogen atoms in adjacent positions. Due to its π electron count (6 π-e), its planar structure and B-N bond equality, borazine has gained the scientists’ interest regarding its possible aromatic character. Several theoretical studies support the non-aromatic character of borazine based on various aromaticity criteria.8, 9 However there are a few studies proposing a weak aromatic character.10-13 Of particular experimental and theoretical interest is the partial replacement of C=C double bonds by BN moieties, respectively, in monocyclic conjugated arenes or in fused rings.3, 4, 14-18 Dewar was the first who studied the synthesis of BN-substituted aromatic compounds such as 9,10-azaboraphenanthrenes19, BN-napthalenes20,

21

or BN-phenalenium ion22. A review by

Bosdet and Piers in 2009, describes thoroughly the studies on such CBN heterocyclic compounds23.

Scheme 1. Structures and atom numbering of 1,2-azaborine (1), 1,3-azaborine (2) and 1,4azaborine (3). The replacement of two carbon atoms in benzene ring by a boron and a nitrogen atom results to an interesting case of isoelectronic to benzene BN heterocyclic compounds consisting from the isomers 1,2-dihydro-1,2-azaborine, 1,3-dihydro-1,3-azaborine and 1,4-dihydro-1,4-azaborine


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(here abbreviated as 1,2-azaborine (1), 1,3-azaborine (2) and 1,4-azaborine (3), respectively), as shown in Scheme 1. In 1 the boron and nitrogen atoms are adjacent and located in “ortho” position, whereas in 2 and 3 are non-adjacent and located in “meta” and “para” position respectively. According to theoretical calculations, the most stable structure, among the three isomers is 1, and the less stable is 224-26. In contrast to thermodynamic stability, the π electron delocalization of the three isomers was found to follow different trend. The most aromatic, according to theoretical studies, appears to be the less stable 1,3-azaborine, 2.17, 24-26 Among the three azaborine’s isomers, the most studied is 1,2-azaborine. Dewar27 and White28 independently, were the first who tried to synthesize and isolate the monocyclic 1,2-azaborine using a desulfurization procedure and a palladium-catalyzed dehydrogenation, respectively. Both had as a result the synthesis of fully substituted 1,2-azaborine. Later, in 1967, Dewar made another attempt using hydroboration29, but it was unsuccessful, as the product wasn’t stable and led to a rigid polymerization. Recently, Liu and coworkers published the isolation and characterization of 1,2-azaborine, using catalytic ring closing metathesis (RCM)30. The product was characterized by 1H NMR and UV/Vis spectroscopy. Recently, the same group suggested a new more economic synthetic route to 1,2-azaborine with fewer steps, and higher yield31. The aromatic character of 1,2-azaborine is a crucial property and has been investigated using a variety of reactivity, energetic, structural and magnetic aromaticity criteria. According to nucleus independent chemical shift (NICS)32 calculations, 1,2-azaborine showed a substantial aromatic character, being less than this of benzene and greater than this of borazine17, 30, 33, 34. The same conclusion results from other aromaticity indices17,

33, 34

, such as para-delocalization index

(PDI)35, harmonic oscillator model of aromaticity (HOMA)36, or the anisotropy of magnetic susceptibility (∆χ)37. Champagne et all. visualized the π and σ current densities, as well as the


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isochemical shielding surfaces (ICSS) of total NICS and its out-of-plane (-σzz) component for 1,2-azaborine34. The π current density of 1,2-azaborine appears stronger in nitrogen atom, while around the boron atom there is a decrease of σ current density. The extend of the shielding isosurfaces in the 1,2-azaborine is smaller than this of benzene and larger than this of borazine. The planarity of 1,2-azaborine’s ring is confirmed by crystallographic analysis38. The shorter BN, B-C and N-C bond lengths compared to unsaturated monocyclic derivatives of 1,2-azaborine serves also as an indication of bond delocalization38. Liu and co-workers39 determined experimentally the resonance stabilization energy (RSE) of 1,2-azaborine equal to 16.6 ± 1.3 kcal/mol. This value is in good agreement with computationally predicted results30, 33 and it’s less than RSE of benzene. Experimentally, the 1,2-azaborine undergoes electrophilic substitution reactions with 3- and 5- favorable ring positions (starting counting from nitrogen)40. This is also supported by electrostatic potential surface (ESP) calculations30. Additionally, it forms pianostool n6-bound complexes with chromioum(0) tricarbonyl (CrCO)3) like benzene30. The other two azaborine isomers, 1,3-azaborine and 1,4-azborine, are less stable and their study is very limited. The presence of 1,4-azaborine appears only in a benzofused polycyclic compounds19-25,

41

. Recently, Braunschweig and co-workers reported the synthesis of a

substituted 1,4-azaborine42 by rhodium catalyzed cycloaddition of tBu-N≡ B-tBu iminoborane and acetylene. The product, 1,4-di-tert-butyl-1,4-azaborine, is very stable with the heteroatoms adopting trigonal planar geometry. Liu and co-workers were the first who synthesized the 1,3azaborine with RCM reaction. X-ray diffraction showed the planarity of the 1,3-azaborine’s ring43, whereas electrostatic potential surfaces showed that the negative charge is concentrated in boron atom, despite its low electronegativity, while the positive charge is localized in nitrogen atom44. This could serve as an indication of strong π delocalization effects. The calculated NICS


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values of 1,3-azaborine are intermediate between benzene and 1,2-azaborine45, whereas the theoretically calculated RSE is approximately 5 kcal/mol less than benzene45. 1,3-azaborine also gives nucleophilic substitution reactions on the boron atom and it undergoes electrophilic aromatic substitution on the adjacent to nitrogen carbon atom on sixth position43, 45. In this study the Canonical Molecular Orbital contributions to the Induced Magnetic Field (CMO-IMF)9 have been calculated and visualized for 1,2-, 1,3- and 1,4-azaborine, in order to investigate their π electron delocalization. Although, visualizations of the total induced magnetic field, as well as its π and σ components, have been extensively applied for the interpretation of aromaticity and antiaromaticity in a wide range of organic and inorganic molecules46-57, CMOIMF9 affords detailed insight of electron delocalization, as it provides visualizations of the magnetic response originating from each occupied molecular orbital individually. We also examine the influence on the magnetic response of the rotational allowed transitions from HOMO to unoccupied orbitals58,

59

, in order to explain the differentiation of electron

delocalization among the molecules under study. Finally we perform NBO-PEPA60-62 calculations, to investigate the resonance structures and bonding nature of these systems. 2. Computational Details Geometry optimizations of 1,2-azaborine (Cs) (1), 1,3-azaborine (Cs) (2) and 1,4-azaborine (C2v) (3) were performed at the B3LYP/6-311++G(d,p) level of theory63, 64 using Gaussian09W65 package. Chemical shielding calculations and its dissection to canonical molecular orbitals contributions were performed at the PW91/TZ2P level66, using the gauge-including atomic orbitals (GIAO) method as implemented by Schreckenbach67-68 in EPR/NMR module of ADF package69.


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The center of each planar molecular ring was placed at the origin of the cartesian axes, with the molecular plane lying on xy plane. Chemical shielding was computed in two-dimensional grids of points ranging from -6Å to +6Å with a step of 0.2Å in each direction, on the molecular plane (xy) and on a plane perpendicular to it (yz). When an external magnetic field, Bext, is applied perpendicular to the molecular plane, the induced magnetic field, Bind, is derived from chemical shielding tensor σ(r), according to the following equation: Biind (r ) = −σ ij (r ) B j

ext

The zx, zy and zz components of the chemical shielding tensor at a specific point are equal to the x, y and z components of the vector of the induced magnetic field (IMF) respectively. Hence, the NICSzz value is equal to the z-component of induced field, Bzind. The results are visualized as contour maps of: a) the z-component of IMF, Bzind, b) the magnitude of IMF, |Bind|, c) the isotropic value of nucleus independent chemical shift, NICSiso, d) the contributions of HOMO to unoccupied orbitals transitions to the z-component of IMF, Bzind, HOMO-unocc and e) as field lines of the IMF, for each valence molecular orbital. Input preparation, output processing and visualization were performed with MIMAF software developed by our group70. Natural Bond Orbital (NBO) analysis was performed using NBO 3.1 software as implemented in Gaussian 09W65 package. The resonance structures and their weights were obtained using PolyElectron Population Analysis (PEPA) software62. For this reason, the geometries, of all molecules were re-optimized at MP2/6-311++G(d,p)71-73 and CCSD/6-311++G(d,p) level using Gaussian 09W package65. These geometries were used to obtain the correlated wave functions. As PEPA62 requires, the initial analyzed Ψ(MO) must have the form of linear combination of MO Slater determinants. This was performed with coupled cluster single-double (CCSD) calculations.


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3. Results and Discussion 3.1 Geometrical Parameters The optimized geometries at the CCSD/6-311++G(d,p) level of the three azaborine isomers are all planar and are shown in Figure 1. In order to estimate the multireference character of the three azaborine’s structures, the T1 diagnostic was performed, showing that the three molecules are closed shell systems (0.014, 0.011 and 0.015 T1 diagnostic values, for 1, 2 and 3 respectively). According to our calculations at the CCSD/6-311++G(d,p) level the most stable isomer is 1,2azaborine, in agreement with previous results24-26. The less stable isomer is the 1,3-azaborine by 32.28 kcal/mol. The 1,4-azaborine is also less stable than 1,2-azaborine by 22.72 kcal/mol. Selected optimized bond lengths at both B3LYP/6-311++G(d,p) and CCSD/6-311++G(d,p) level of theory are shown in Figure 1, whereas, all the computed geometrical parameters are given in Table S2. The computed bond lengths for 1,2-azaborine are in excellent agreement with X-ray crystallographic data30, presenting differences less than ±0.017Å, except in B-C bond where the difference is up to 0.035Å (Table S2).

Figure 1. Geometries, selected optimized geometrical parameters and relative energies of the three isomers of azaborines, at the CCSD/6-311++G(d,p) (first line) and B3LYP/6-311++G(d,p) (in parentheses) level of theory. 3.2 Induced Magnetic Field


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3.2.1 π and σ-subset of molecular orbitals. In this part the total magnetic response to the external magnetic field of π- and σ- subsets of molecular orbitals is discussed. In Figure 2 the field lines and the magnitude of the induced magnetic field of π-, σ- and core electrons of the three azaborines are presented. The field lines of the induced magnetic field of the π-system (Fig. 2A) depict long-range shielding cones perpendicular to the ring plane, colored with blue lines, and short-range deshielding cones outside of the ring, colored with red lines, similar to the induced magnetic field of benzene52, with the hydrogen atoms lying inside the deshielding region in agreement to Pople’s ring current model51,

52

. For the σ-system (Fig. 2B), short-range

deshielding cones are observed inside and outside the ring, while mid-range shielding regions are observed above the ring and along the molecular framework, with the hydrogen atoms lying inside the shielding region. The core electrons (Fig. 2C), exhibit short-range shielding cones perpendicular to the ring plane and short-range deshielding cones outside the ring plane. The maps of the magnitude of the induced magnetic field (Fig. 2D-F) clearly portray the differentiation of the spatial extension of the magnetic response among the π-, σ- and core electrons. The core electrons (Fig. 2F) exhibit a weak and short-range magnetic response firmly encapsulating the molecular ring, which is enchased at heavy atoms’ positions. The σ-system (Fig. 2E) presents a strong but short-range magnetic response denoting the localized nature of σelectrons, while the π-electrons exhibit a strong and long-range magnetic response above the ring (Fig. 2D), indicative of π-electron delocalization.


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Figure 2. Field lines of Bind (A, B, C) and contour maps of the magnitude |Bind| (D, E, F) of π-, σ- and core contributions to the induced magnetic field of 1,2-, 1,3- and 1,4-azaborine on the yz plane. In Figure 3, the isocontour maps of the contributions of π- (Fig. 3A), σ- (Fig. 3B) and core (Fig. 3C) subsets of MOs to the z-component of the induced magnetic field are shown for the three molecules under study, as well as for benzene (5) and borazine (4)9. In the three BN heterocycles, the π-system (Fig. 3A) presents a strong and long range diatropic response inside and above the molecular ring plane, depicted with blue color, and a weak short-range paratropic region outside the ring which covers the hydrogen atoms, depicted with yellow color. In 1,4azaborine (3) the diatropic region above the ring plane is less extended comparing to 1,2- and 1,3-azaborine. The π-magnetic response of the three azaborines is similar to this of benzene but substantially different from this of borazine. Borazine’s π-system presents a weak and less


10

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extended diatropic response with strong diatropic areas localized in the vicinity of nitrogen atoms9.

Figure 3. Contour maps of π- (A), σ- (B) and core (C) contributions to the z-component of the induced magnetic field Bzind (ppm) of 1,2-azaborine (1), 1,3-azaborine (2), 1,4-azaborine (3), Borazine (4) and Benzene (5) on the yz and xy planes. Blue areas represent diatropicity and red paratropicity. The contour maps of the total π contribution to the z-component of the induced magnetic field of 1,2-azaborine can be used to interpret proton’s resonances in 1H NMR experimental spectrum, as measured by Liu and coworkers30. From the rescaled contour map of total π contribution to


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the z component of Bzind on the ring plane for 1, shown in Figure 4, it is clear that the hydrogen atoms lie on the deshielding area, whereas the N-H hydrogen lies in the strongest paratropic region. This is in agreement with the experimental 1H NMR spectrum of 1, in which the N-H proton resonance appears upfield of the other resonances, as shown in Table 1.

Figure 4. Rescaled contour map of π contributions to the z-component of the induced magnetic field, Bzind (ppm), of 1,2-azaborine on the ring plane. Blue areas represent diatropicity and red paratropicity. Table 1. 1H-NMR values of 1,2-azaborine Atoma

Calculatedb

Experimentalc

1

8.48

8.44

2

5.31

4.90

3

7.43

6.92

4

7.90

7.70

5

6.66

6.43

6

7.46

7.40

a

Numbering according to Figure 4. PW91/TZ2P level of theory. cValues taken from reference 30.

b


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The π-aromatic character of the three azaborine isomers can be quantitative depicted by the NICS-SCAN diagrams shown in Chart 1, where the NICSzz values for the total π-system of 1,2-, 1,3-, 1,4-azaborine, benzene and borazine, are presented as a function of the distance above the ring center. The shape of the curves of the three azaborines are similar to benzene’s NICS-SCAN curve and show that their π-magnetic response is smaller than benzene’s and larger than borazine’s magnetic response, in agreement with previous studies17,

26, 33, 34

. They have high

negative values at the ring center, which decline to zero with the distance above the ring. Among the three azaborines, 2 appears to be the most aromatic, while 1 and 3 appear to be less aromatic, presenting almost equal π magnetic response. The πtotal NICSzz values of 1, 2 and 3 are lower than benzene’s and higher than borazine’s values (Table 2).

Chart 1. NICSzz values (ppm) versus the distance (Å) above the ring center for the total π-system of 1,2-, 1,3-, 1,4-azaborine, benzene and borazine. The contour maps of Bzind of each σ MO individually are shown in Figures S1-S3, while their filed lines and their magnitude are shown in Figures S5-S7. The magnetic response of the total σsystem, resulting from the sum of contributions of all σ MOs, is similar for the three molecules under study, as shown in Figure 3B. A strong and short ranged paratropic region is observed


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inside the ring. A strong diatropic region with the same extension perpendicular to the ring appears across the molecular framework and covers all ring atoms. Outside the molecular framework a weak circular and short-ranged paratropic area is observed, which is enforced between hydrogen atoms in the area of C-C and C-N bonds and weakened in the area of C-B bonds. The paratropic region of the σ-system inside the ring is due to the strong and long-range paratropic response of the high-energy σ-orbitals with many nodal planes, HOMO-2 and HOMO-3 (Fig. S1-S3). The σtotal NICSzz values of 1, 2 and 3 vary only 1.3ppm and are similar to benzene’s NICSσ,zz value (Table 2). Finally, the contour maps of the core contributions to the zcomponent of the induced magnetic field (Fig. 3C), reveal a weak and short-range diatropic magnetic response, with the NICScore,zz values being equal in all molecules under study (Table 2). 3.2.2 π orbitals. In this part the magnetic response of each occupied π molecular orbital is discussed for the three molecules under study. Contour maps of the z-component of the induced magnetic field for π3 (Fig. 5A), π2 (Fig. 5B) and π1 (Fig. 5C) orbitals of 1, 2 and 3 are presented in Figure 5. Field lines of the induced magnetic fields and contour maps of their magnitude are presented in Figure S4. Table 2. NICSzz at ring centers, of 1,2-azaborine (1), 1,3- azaborine (2), 1,4-azaborine (3), borazine (4) and benzene (5)a.

Molecule

Total

core

σtotal

πtotal

π1

π2

π3

ΗΟΜΟ →unocb

ΗΟΜΟ →LUMOc

1

-3.9

-10.8

33.1

-26.3

-11.2

-9.9

-5.2

4.0

3.2

2

-7.7

-10.7

34.0

-31.1

-11.0

-10.9

-9.1

0.9

0.9

3

-3.9

-10.7

32.7

-25.8

-12.1

-10.3

-3.5

5.1

4.7d

4

10.3

-10.6

30.1

-9.2

-11.0

0.9

0.9

13.4e

10.5f


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5

-13.7

-10.8

33.3

-36.2

-12.8

-11.7

-11.7

0.0e

0.0f

a

Values are in ppm calculated at the PW91/TZ2P level of theory. bSummation of paramagnetic contributions to NICSzz originating from all HOMO to unoccupied MOs rotational transitions. c Paramagnetic contributions to NICSzz originating from HOMO to LUMO rotational transitions. d HOMO →LUMO+1 excitation. eSum of degenerate π2 and π3 rotational transitions. fSum of degenerate π2 and π3 rotational transitions to both degenerate LUMOs.

The low energy fully delocalized π1 orbital (MO 17 (a") in 1, MO 15 (a") in 2 and MO 16 (b1) in 3) exhibits a similar diatropic magnetic response among the three azaborines (Fig. 5C) with the diatropic region inside the ring being enhanced towards area of the nitrogen atom and weakened towards the area of boron atom. Moderate spatial extension of the diatropic region, perpendicular to the ring is observed, and weak paratropic regions are present outside the ring around the C-N bonds, where the hydrogen atoms are lying. NICSzz values of π1 MOs are similar for the three azaborines, borazine and benzene (Table 2). Concerning the π2 orbital of each molecule, (Fig. 5B) (MO 20 (a") in 1 and 2 and MO 20 (a2) in 3) the magnetic response is also similar among the three azaborines, presenting a diatropic response inside the ring which is weakened towards the boron atom. In 1,2- and 1,3-azaborine the diatropic response is augmented close to the second carbon atom away from boron (C4 and C5 respectively). Spatial extension of the diatropic region, perpendicular to the ring is more extended in 1,3-azaborine (2). Paratropic regions are observed outside the ring in the region of C-C bonds, where the hydrogens of the C-H bonds are lying. NICSzz values of π2 are similar for the three azaborines and benzene (Table 2).


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Figure 5. Contour maps of π molecular orbital contributions to the z component of the induced magnetic field Bzind (ppm) of 1,2-azaborine (1), 1,3-azaborine (2) and 1,4-azaborine (3) on the yz and xy planes. Blue areas represent diatropicity and red paratropicity. Orbitals are numbered with increasing energy. The magnetic response of π3 HOMO orbitals of 1,2-, 1,3- and 1,4-azaborine, is also diatropic and augmented in the area of the C-B bonds, while a weak paratropic region is observed outside the ring in the C-B bonds area, with C-H and B-H hydrogen atoms located inside it (Fig. 5A). However, in contrast to π1 and π2, the π3 orbitals present appreciable differentiation among the three azaborines. The π3 of 1,3-azaborine, MO 21 (a"), exhibits the stronger and more longranged diatropic response inside the ring. In the case of 1,4-azaborine the π3 orbital, MO 21 (b1), presents the weakest and more short-ranged diatropic response, which totally fades away in the vicinity of the C-N bonds inside the ring. In 1,2-azaborine the π3 orbital, MO 21 (a"), exhibits a weak and short-ranged diatropic response, which is intermediate between the magnetic response of π3 in 1,3- and 1,4-azaborine. NICSzz values (Table 2) and NICSzz-SCAN diagrams (Chart S1)


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for the π3-HOMO orbitals of the three molecules show the same trend. As shown in Table 2, 1,3azaborine presents the most diatropic π3-NICSzz value, which is only 2.6 ppm less than benzene, followed by 1,2-azaborine, which is lower by 6.5 ppm, and 1,4-azaborine which is lower by 8.2ppm than benzene. On the other hand, borazine’s HOMOs (π2 and π3) exhibit small paratropic NICSzz values. In contrast, NICSzz values of the summation of π1 and π2 orbitals contributions of the three azaborines are almost equal (-21.1 ppm, -21.9 ppm and -22.4 ppm for 1, 2 and 3, respectively) and the corresponding NICS-SCAN charts (Chart S2) are almost identical. It is necessary to point out that the NICSzz-SCAN diagrams and NICSzz(0) values don’t describe accurately the whole magnetic response in the case of low symmetry heterorings, as the inclusion of heteroatoms induces distortions on the ring currents at heteroatoms’ positions74, and consequently the induced magnetic field is not homogeneous inside the ring. Likewise the diatropic areas of Bzind of π3 inside the azaborines’ rings are not homogenous, but they are shifted towards the B-C area, as these orbitals have major contributions from pz AOs of boron atoms. In conclusion, the differentiation of the π magnetic response among the three azaborines, originates primary from the π3-HOMO orbitals. The other two π orbitals exhibit similar diatropic response. As a result, the 1,3-azaborine exhibits the strongest π-delocalization, while the weakest π-delocalization is displayed in 1,4-azaborine 3.2.3 Contributions from HOMO to unoccupied orbitals transitions. In order to investigate the origin of the most aromatic character of 1,3-azaborine, we analyze the shielding tensor, σ, which, according to GIAO formalism68, 75-78, is a summation of four components: σtotal = σdia + σgauge + σocc-occ + σocc-unocc. Motivated by Steiner’s and Fowler’s79 symmetry criteria for the interpretation of orbital contributions to the current density in aromatic rings, Corminboeuf et. al.58 and Pérez-Juste et. al.59, studied the paramagnetic term, σocc-unocc of shielding tensor, which


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refers to symmetry-allowed rotational transitions from occupied to unoccupied orbitals. They showed that the difference in NICS values between aromatic and antiaromatic compounds primary originates from σocc-unocc term, which refers to rotational transitions from HOMO or HOMO-1 to LUMO or LUMO+1 rotational transitions.

Figure 6. Contour maps of Bzind, HOMO-unocc (ppm) of A) 1,2-azaborine, B) 1,3-azaborine, C) 1,4azaborine, D) borazine, E) benzene and F) cyclobutadiene on the yz and xy planes. The color range in contour maps of benzene and borazine is ±30ppm and corresponds to the sum of both degenerate HOMOs. Blue areas represent diatropicity and red paratropicity. In Figure 6, the contour maps of the contributions to Bzind arising from transitions of HOMO to unoccupied orbitals (Bzind, HOMO-unocc) are presented for the three azaborines (Fig. 6A-C), borazine (Fig. 6D), benzene (Fig. 6E) and cyclobutadiene (Fig. 6F). Benzene (Fig. 6E) does not exhibit any contributions from transitions of HOMO to unoccupied orbitals, while cyclobutadiene (Fig. 6F) exhibits very strong and long range paratropic contributions, which are responsible for its antiaromatic character. On the other hand, borazine (Fig. 6D) presents moderate paratropic and mid-ranged contributions from transitions from each degenerate HOMO, which are responsible for its non-aromatic character. In the case of the three azaborines, the contour maps of Bzind, HOMOunocc

are illustrative of the variation of their π-magnetic response. 1,3-azaborine (Fig. 6B) exhibits


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a trivial response from such transitions, which is slightly paratropic and short ranged in the area of N-C6 inside the ring. On the contrary, 1,2- and 1,4-azaborine (Fig. 6A and Fig. 6C respectively) present substantial and mid-range paratropic contributions, which are augmented towards C4-C5-C6 and C-N-C bonds, respectively. The paratropic response of HOMO transitions is stronger in 1,4-azaborine (Fig. 6C). The paratropic areas of Bzind, HOMO-unocc maps coincide with the diminished diatropic areas of the corresponding Bzind maps of HOMOs (Fig. 5A). Hence, 1,2- and 1,4-azaborine present reduced diatropic response in regard to 1,3azaborine. In Table 2 the contributions to NICSzz of the σocc-unocc term of the total HOMO to unoccupied orbitals transitions (NICSzzHOMO-unocc), as well as HOMO to LUMO and/or LUMO+1 transitions (NICSzzHOMO-LUMO) for the three azaborines (1, 2 and 3), borazine (4) and benzene (5) are presented. The scan curve of the NICSzzHOMO-unocc term with respect to the distance above the ring center for the three azaborines is presented in Chart S3. As expected benzene does not exhibit any contributions from NICSzzHOMO-unocc term, while the non-aromatic borazine presents a paratropic contribution of 6.7 ppm from transitions of each degenerate HOMO to unoccupied orbitals. Among the azaborines, the smallest paratropic contribution of NICSzzHOMO-unocc term is observed in 1,3-azaborine (2) (0.9 ppm) and the highest in 1,4-azaborine (3) (5.1 ppm). 1,2azoborine (2) also exhibits considerable paratropic contributions (4.0 ppm) from NICSzzHOMOunocc

term. In 1,3-azaborine the paratropic contributions of NICSzzHOMO-unocc term vanish rapidly at

1Å above the ring, while in 1,2- and 1,4-azoborine the paratropic contributions are mid-ranged, as they vanish at about 4Å above the ring center (Chart S3). The difference in NICSzz values of HOMO orbitals among the three azaborines (3.9 ppm for 1-2 and 5.6 ppm for 3-2) is mainly due to the difference in contributions of NICSzzHOMO-unocc term (3.1 ppm for 1-2 and 4.2 ppm for 3-2).


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Additionally, as shown in Table 2, the major contributions to the NICSzzHOMO-unoc term arise primary from HOMO→LUMO in 1,2- and 1,3-azaborine (80% and 100% respectively) and from HOMO→LUMO+1 transitions in 1,4-azaborine (92.2%). Consequently, the differentiation of the magnetic response of HOMO orbitals among the three azaborines originate primary from the different paratropic contributions of rotational transitions to LUMO or LUMO+1. The occupied-unoccupied transitions are rotationally allowed when the direct product of the irreducible representations of the interacting orbitals and the rotational axis that represents the external field direction, contains the totally symmetric representation58. The magnitude of the paratropic contributions depends on the spatial overlap between occupied and rotated unoccupied interacting orbitals and their energy difference58, 59. In benzene such transitions are not allowed within D6h point group, while in D4h cyclobutadiene they are allowed and the significant overlap between the interacting orbitals give rise to strong paratropic response58, 59. In borazine with D3h symmetry, HOMO→LUMO transitions are also symmetry allowed (E’’×A’2×E’’= A’1+ A’2+E’), but the spatial overlap between the interacting orbitals is small, given that they have different number of nodal planes, leading to weak paratropic response. Moreover, each HOMO of borazine interacts with both degenerate LUMOs resulting in different paratropic responses (3.8ppm and 1.4ppm) due to different spatial overlaps (see Fig. S14). Concerning the azaborines 1 and 2 within the Cs point group, the transitions of HOMO-1 and HOMO to LUMO and LUMO+1 are rotationally allowed (A’’×A’×A’’=A’). In 1,4-azaborine (3) with C2v symmetry, the HOMO→LUMO transition is not allowed (B1×B2×B1=B2), while the HOMO→LUMO+1 and HOMO-1→LUMO transitions are rotationally allowed (B1×B2×A2=A1) (Fig. S14). As in borazine, the interacting occupied and unoccupied orbitals of azaborines have different number of nodal planes, leading to small spatial overlap, and therefore they present weak paratropic


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contributions. In all cases, HOMO-1→LUMO transitions do not give paratropic contributions due to increased energy difference in regard to HOMO-LUMO transitions, and consequently the HOMO-1 (π2) orbitals exhibit increased diatropic contributions in regard to HOMOs (π3). Concerning the active excitations (HOMO→LUMO in 1 and 2 and HOMO→LUMO+1 in 3) the energy gap is smaller in 2 and larger in 3 (3.41eV, 3.96eV and 4.87eV for 1, 2 and 3 respectively), in contrast to the magnitude of the paratropic response, suggesting that the spatial overlap is the governing factor of the paratropic contributions in azaborines. Hence in 2 the small overlap between HOMO and rotated LUMO leads to weak paratropic response, while in 1 and 3 the greater overlap leads to enhanced paratropic contributions. 3.3 Resonance Structures The weights of electronic resonance structures, as constructed in NBOs, are well determined using the Natural Resonance Theory (NRT)80-82. However, in order to determine the weights of the three azaborine’s molecules, we employed PolyElectron Population Analysis (PEPA)60-62, because NRT doesn’t offer a direct way to visualize the covalent and ionic components of bonds. According to PEPA, the molecular orbitals wave functions, Ψ(MO), are transformed in the basis of natural orbitals as obtained in the framework of NBO methodology. As these natural orbitals span the complete SCF-AO basis set, the initial correlated Ψ(MO), having the form of linear combinations of MO Slater determinants, is transformed (more precisely, is rewritten) in the natural basis without affecting its approximation level. Subsequently, the probabilities (or, in general, the weights) of local valence bond-type (VB-type) resonance structures are calculated in the framework of PEPA using the hole-expansion methodology61.


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Figure 7. The main resonance structures and their weights of the three azaborine’s isomer structures, as created using PolyElectron Population Analysis (PEPA)62 The results of the main resonance structures of the studied molecules are shown in Figure 7. The 1,3-azaborine (2) presents two main fully delocalized resonance structures having the same weights (23.6% and 23.3%). In 1,2-azaborine (1) the main resonance structure has 66.4% weight, which shows that this molecule is more localized than 1,3-azaborine (2). The weight of second resonance structure is quite smaller (8.5%). The same trend is observed in 1,4-azaborine (3). The main resonance structure (45.2%) has two localized double bonds between carbon atoms. On the other hand, the two delocalized resonance structures occur with very small weights (4.1%). According to the resonance structures weights, the 1,3-azaborine appears to be the most delocalized compound, followed by 1,2-azaborine, while 1,4-azaborine appears to be the least delocalized. 4. Conclusions The three azaborine isomers, in contrast to borazine, present a substantial π aromatic character being lower but comparable to benzene. Among them, 1,3-azaborine exhibits the most


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delocalized π orbitals, while 1,4-azaborine the least. Resonance structures obtained by NBO analysis also confirms that the most delocalized isomer is 1,3-azaborine. Differentiation of πdelocalization is portrayed in the magnetic response of π-HOMO orbitals. Maps of the z component of the induced magnetic field of π-HOMO orbitals revealed enhanced diatropic response in 1,3-azaborine and reduced diatropic response in the other two isomers. The low energy π orbitals (π1 and π2) exhibit similar diatropic magnetic response, comparable to benzene’s response, in terms of NICSzz values. In all cases the magnetic response of π orbitals is not homogenous but distorted, following the shape of the corresponding orbitals. Variations in the magnetic response of π-HOMOs are governed by rotational transitions of HOMO to unoccupied orbitals. The symmetry lowering from benzene to lower symmetry in borazine and azaborines activates weak paratropic response from HOMO to LUMO transitions, with their magnitude depending on the spatial overlap and hence the shape of interacting orbitals. Augmented diatropic response of π-HOMO of 1,3-azaborine is attributed to minimal paratropic contributions from HOMO to LUMO rotational transitions. In contrast, 1,2- and 1,4-azaborine exhibit considerable paratropic contributions from π-HOMO to unoccupied orbitals rotational transitions, which lead to weakened diatropic response in regard to 1,3-azaborine. Contour maps of contributions to Bzind from rotational transitions of occupied to unoccupied orbitals can rationalize the differences in π-delocalization of aromatic systems with similar electronic structures. Supporting Information. Relative energies (Table S1), geometrical parameters (Table S2), contour maps of individual σ molecular orbitals contributions to NICSzz (Figures S1-S3), field lines of Bind and contour maps of the magnitude of Bind of individual π molecular orbitals (Figure S4), field lines of Bind and contour maps of the magnitude of Bind of individual σ molecular


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orbitals (Figures S5-S7), contour maps of individual π molecular orbitals contributions to NICSiso (Figure S8), contour maps of individual σ molecular orbitals contributions to NICSiso (Figures S9-S11), contour maps of π-, σ- and core subset of MOs contributions to NICSiso (Figure S12), total magnetic response of 1,2-, 1,3- and 1,4-azaborine (Figure S13). Energy diagram of rotational allowed HOMO-LUMO excitations of 1,2-, 1,3-, 1,4-azaborine and borazine (Figure S14). NICSzz-SCAN diagrams of the π3 (Chart S1), the sum of π1+ π2 orbitals (Chart S2) and HOMO-unocc term (Chart S3) of 1,2-, 1,3- and 1,4-azaborine. Full citation of reference 48. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author Email: [email protected]; Phone: +302310997815; Fax: +2310997738. ACKNOWLEDGMENT This research has been co-financed by the European Union (European Social Fund − ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: Thales. Investing in knowledge society through the European Social Fund. REFERENCES (1) Faraday, M. On New Compounds of Carbon and Hydrogen, and on Certain Other Products Obtained During the Decomposition of Oil by Heat. Philos. Trans. R. Soc. London 1825, 440466. (2) Burdett, J. K. Chemical Bonding in Solids, Oxford University Press, New York, 1995, pp. 71-72.


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Page 25 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3) Liu, Z.; Marder, T. B. B_N versus C_C: How Similar Are They? Angew. Chem. Int. Ed. 2008, 242-244. (4) Jaska, C. A.; Emslie, D. J. H.; Bosdet, M. J. D.; Piers, W. E.; Sorensen, T. S.; Parver, M. Triphenylene Analogues with B2N2C2 Cores: Synthesis, Structure, Redox Behavior, and Photophysical Properties. J. Am. Chem. Soc. 2006, 10885-10896. (5) Baldock, C.; de Boer, Gert-Jan; Rafferty, J. B.; Stuitje, A. R.; Rice D. W. Mechanism of Action of Diazaborines. Biochem. Pharmacol. 1998, 1541-1550. (6) Campell, P. G.; Zakharov, L. N.; Grant, D. J.; Dixon, D. A.; Liu, S.-Y. Hydrogen Storage by Boron-Nitrogen Heterocycles: A Simple Route for Spent Fuel Regeneration. J. Am. Chem. Soc. 2010, 3289-3291. (7) Stock, A.; Pohland, E. Boron Hydride. B2H5 and B5H11. Ber. Dtsch. Chem. Ges. 1926, 22102215. (8) Schleyer, P.; Jiao, H.; Hommes, N.; Malkin, V.; Malkina, O. An Evaluation of the Aromaticity of Inorganic Rings: Refined Evidence from Magnetic Properties. J. Am. Chem. Soc. 1997, 12669–12670. (9) Charistos, N. D.; Papadopoulos, A. G.; Sigalas, M. P. Interpretation of Electron Delocalization in Benzene, Cyclobutadiene, and Borazine Based on Visualization of Individual Molecular Orbital Contributions to the Induced Magnetic Field. J. Phys. Chem. A 2014, 1113−1122.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 45

(10) Fowler, P.; Steiner, E. Ring Currents and Aromaticity of Monocyclic π-Electron Systems C6H6, B3N3H6, B3O3H3, C3N3H3, C5H5-, C7H7+, C3N3F3, C6H3F3, and C6F6. J. Phys. Chem. A 1997, 1409–1413. (11) Madura, I. D.; Krygowski, T. M.; Cyrahski., M. K. Structural Aspects of the Aromaticity of Cyclic π-Electron Systems With BN Bonds. Tetrahedron 1998, 14913-14918. (12) Fernandez, I.; Frenking, G. Direct Estimate of Conjugation and Aromaticity in Cyclic Compounds With the EDA Method. Faraday Discuss. 2007, 403–421. (13) Islas, R.; Chamorro, E.; Robles, J.; Heine, T.; Santos, J.; Merino, G. Borazine: To Be or Not to Be Aromatic. Struct. Chem. 2007, 833–839. (14) Kar, T.; Elmore, D. E.; Scheiner, S. BN-Napthalene and Carbon-Containing Derivatives: an Ab Initio Study. Journal of Molecular Structure (Theochem) 1997, 65-74. (15) Phukan, A. K.; Kalagi, R. P.; Gadre, S. R.; Jemmis E. D. Structure, Reactivity and Aromaticity of Acenes and Their BN Analogues: A Density Functional and Electrostatic Investigation. Inorganic Chemistry 2004, 5824-5832. (16) Bosdet, M. J. D.; Piers, W. E.; Sorensen, T. S.; Parver, M. 10a-Aza-10b-borapyrenes: Heterocyclic Analogues of Pyrene with Internalized BN Moieties. Angew. Chem. Int. Ed. 2007, 4940-4943. (17) Ghosh, D.; Periyasami, G.; Pati, K. S. Density Functional Theoretical Investigation of the Aromatic Nature of BN-Substituted Benzene and Four Ring Polyaromatic Hydrocarbons. Phys. Chem. Chem. Phys. 2011, 20627-20636.


26

Page 27 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Papadopoulos, A. G.; Charistos, N. D.; Sigalas, M. P. Aromaticity variation in BN substituted triphenylene: A theoretical study. AIP Conference Proceedings 2012, 1223-1226. (19) Dewar, M. J. S.; Kubba, V. P. ; Pettit, R. New Heteroaromatic Compounds. Part I. 9-Aza10-boraphenanthrene. J. Chem. Soc. 1958, 3073-3076. (20) Dewar, M. J. S.; Dietz, R. New Heteroaromatic Compounds. Part III. 2,1-Borazaronaphthalene (1,2-dihydro-1-aza-2-boranaphthalene). J. Chem. Soc. 1959, 2728-2730. (21) Dewar, M. J. S.; Gleicher, G. J.; Robinson B. P. Synthesis and Nuclear Magnetic Resonance Spectrum of 10,9-Borazaronaphthalene. J. Am. Chem. Soc. 1964, 5698-5699. (22) Dewar, M. J. S.; Jones, R. New Heteroaromatic Compounds Part XXXI: The 12,11Borazarophenalenium Cation. Tetrahedron Lett. 1968, 2707-2708. (23) Bosdet, M. J. D.; Piers, W. E. B-N as a C-C Substitute in Aromatic Systems. Can. J. Chem. 2009, 8-29. (24) Massey, T. S.; Zoellner, R. W. MNDO Calculations on Borazine Derivatives. The substitution of One [HNBH] Fragment for One [HCCH] Fragment in Benzene to Form the Azaborines and the Nature of the Cyclotrimer of the 1,2-Isomer. Int. J. Quant. Chem. 1991, 787804. (25) Kranz, M.; Clark, T. Azaborines: An Ab Initio Study. J. Org. Chem. 1992, 5492-5500. (26) Baranac-Stojanovic, M. Aromaticity and Stability of Azaborines. Chem. Eur. J. 2014, 16558-16565. (27) Dewar, M. J. S.; Marr, P. A. A Derivative of Borazarene. J. Am. Chem. Soc. 1962, 3782.


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Page 28 of 45

(28) White, D. G. 2-Phenyl-2,1-borazarene and Derivatives of 1,2-Azaboracycloalkanes. J. Am. Chem. Soc. 1963, 3634 – 3636. (29) Davies, K. M.; Dewar, M. J. S.; Rona, P. New Heteroaromatic Compounds. XXVI. Synthesis of Borazarenes. J. Am. Chem. Soc. 1967, 6294-6297. (30) Marwitz, A. J. V.; Matus, M. H.; Zakharov, L. N.; Dixon, D. A.; Liu, S.-Y. A Hybrid Organic/Inorganic Benzene. Angew. Chem. Int. Ed. 2009, 973-977. (31) Abbey, E. R.; Lamm, A. N.; Baggett, A. W.; Zakharov, L. N.; Liu, S.-Y. Protecting Group-Free Synthesis of 1,2-Azaborines: A Simple Approach to the Construction of BNBenzenoids. J. Am. Chem. Soc. 2013, 12908-12913. (32) Schleyer, P. v. R.; Jiao, H.; Eikema Hommes, N. J. R.; Malkin, V. G.; Malkina, O. An Evaluation of the Aromaticity of Inorganic Rings: Refined Evidence from Magnetic Properties. J. Am. Chem. Soc. 1997, 12669-12670. (33) Del Bene, J. A.; Yáñez, M.; Alkorta, I.; Elguero, J. An Ab Initio Study of the Structures and Selected Properties of 1,2-Dihydro-1,2-azaborine and Related Molecules. J. Chem. Theory Comput. 2009, 2239-2247. (34) Carion, R.; Liégeois, V.; Champagne, B.; Bonifazi, D.; Pelloni, S.; Lazzeretti, P. On the Aromatic Character of 1,2-Dihydro-1,2-azaborine According to Magnetic Criteria. J. Phys. Chem. Lett. 2010, 1563-1568. (35) Poater, J.; Fradera, X.; Duran, M.; Solà, M. The Delocalization Index as an Electronic Aromaticity Criterion: Application to a Series of Planar Polycyclic Aromatic Hydrocarbons. Chem. Eur. J. 2003, 400-406.


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Page 29 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) Krygowski, T. M.; Cyranski, M. Separation of the energetic and geometric contributions to the aromaticity. Part IV. A general model for the π-electron systems. Tetrahedron 1996, 10255-10264. (37) Lazzeretti, P. Assessment of Aromaticity via Molecular Response Properties. Phys. Chem. Chem. Phys. 2004, 217–223. (38) Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. Crystal Clear Structural Evidence for Electron Delocalization in 1,2-Dihydro-1,2-azaborines. J. Am. Chem. Soc. 2008, 7250–7252. (39) Campbell, P. G.; Abbey, E. R.; Neiner, D.; Grant, D. J.; Dixon, D. A.; Liu, S.-Y. Resonance Stabilization Energy of 1,2-Azaborines: A Quantitive Experimental Study by Reaction Calorimetry. J. Am. Chem. Soc. 2010, 18048–18050. (40) Pan, J.; Kampf, J. W.; Ashe, A. J., III Electrophilic Aromatic Substitution Reaction of 1,2Dihydro-1,2-azaborines. Org. Lett. 2007, 679-681. (41) Xu, S.; Haeffner, F.; Li, B.; Zakharov, L. N.; Liu, S.-Y. Monobenzofused 1,4-Azaborines: Synthesis, Charactirization, and Discovery of a Unique Coordination Mode. Angew. Chem. Int. Ed. 2014, 6795–6799. (42) Braunschweig, H.; Damme, A.; Jimenez-Hall, J. O. C.; Pfaffinger B.; Radacki, K.; Wolf, J. Metal-Mediated Synthesis of 1,4-Di-tert-butyl-1,4-azaborine. Angew. Chem. Int. Ed. 2012, 10034–10037. (43) Xu, S.; Zakharov, L. N.; Liu, S.-Y. A 1,3-Dihydro-1,3-azaborine Debuts. J. Am. Chem. Soc. 2011, 20152–20155.


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(44) Chrostowska, A.; Xu, S.; Lamm, A. N.; Mazière, A.; Weber, C. D.; Dargelos, A.; Baylère, P.; Graciaa, A.; Liu, S.-Y. UV-Photoelectron Spectroscopy of 1,2- and 1,3-Azaborines: A Combined Experimental and Computational Electronic Structure Analysis. J. Am. Chem. Soc. 2012, 10279–10285. (45) Xu, S.; Mikulas, T. C.; Zakharov, L. N.; Dixon, D. A.; Liu, S.-Y. Boron-Substituted 1,3Dihydro-1,3-azaborine: Synthesis, Structure, and Evaluation of Aromaticity. Angew. Chem. Int. Ed. 2013, 7527–7531. (46) Islas, R.; Heine, T.; Merino, G. The Induced Magnetic Field. Acc. Chem. Res. 2012, 215– 228. (47) Camacho Gonzalez, J.; Morales-Verdejo, C.; Muñoz-Castro, A. Variation of throughSpace Magnetic Response Properties upon the Formation of Cation–π Interactions: A Survey of [Ag(η-CH2 CH2 )3 ]+ via DFT Calculations. New J. Chem. 2015, 4244–4248. (48) Camacho-Gonzalez, J.; Muñoz-Castro, A. Alternation of Aromatic-Nonaromatic Rings in Belt-like Structures. Behavior of [6.8]3cyclacene in Magnetic Fields. Phys. Chem. Chem. Phys. 2015, 17023-17026. (49) Karadakov, P. B.; Horner, K. E. Magnetic Shielding in and around Benzene and Cyclobutadiene: A Source of Information about Aromaticity, Antiaromaticity, and Chemical Bonding. J. Phys. Chem. A 2013, 518–523. (50) Kleinpeter, E.; Klod, S.; Koch, A. Visualization of through Space NMR Shieldings of Aromatic and Anti-Aromatic Molecules and a Simple Means to Compare and Estimate Aromaticity. J. Mol. Struct. THEOCHEM 2007, 45–60.


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(51) Merino, G.; Heine,T.; Seifert, G. The Induced Magnetic Field in Cyclic Molecules. Chem. Eur. J. 2004, 4367-4371. (52) Heine,T.; Islas, R.; Merino, G. σ and π Contributions to the Induced Magnetic Field: Indicators For the Mobility of Electrons in Molecules. J. Comput. Chem. 2007, 302-309. (53) Islas, R.; Heine,T.; Merino, G. Structure And Electron Delocalization in Al42- And Al44-. J. Chem. Theory Comp. 2007, 775-781. (54) Islas, R.; Martínez-Guajardo, G.; Jiménez-Halla, J. O. C.; Solà, M.; Merino, G. Not All That Has a Negative NICS Is Aromatic: The Case of the H-Bonded Cyclic Trimer of HF. J. Chem. Theory Comp. 2010, 1131-1135. (55) Torres, J. J.; Islas, R.; Osorio, E.; Harrison II, J. G.; Tiznado, W.; Merino, G. Is Al2Cl6 Aromatic? Cautions in Superficial NICS Interpretation. J. Phys. Chem. A 2013, 55295533. (56) Jalifea, S.; Audiffreda, M.; Islas, R.; Escalantea,S.; Pane, S.; Chattaraj, P. K.; Merino, G. The inorganic analogues of carbo-benzene. Chem. Phys. Lett. 2014, 209-212. (57) Torres-Vega, J. J.; Vásquez-Espinal, A.; Ruiz, L.; Fernández-Herrera, M. A.; Alvarez-Thon, L.; Merino, G.; Tiznado, W.; Revisiting Aromaticity and Chemical Bonding of Fluorinated Benzene Derivatives, Chemistry Open 2015, 302-309. (58) Corminboeuf, C; King, R. B.; Schleyer, P. v. R. Implications of Molecular Orbital Symmetries and Energies for the Electron Delocalization of Inorganic Clusters. ChemPhysChem 2007, 391-398.


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(59) Pérez-Juste, I., Mandado, M.,; Carballeira, L. Contributions From Orbital–Orbital Interactions to Nucleus-Independent Chemical Shifts and Their Relation With Aromaticity or Antiaromaticity of Conjugated Molecules. Chem. Phys. Let. 2010, 224–229. (60) Karafiloglou, P. A Method to Calculate the Weights of NBO Electronic Structures from Moffit’s Theorem. J. Comput. Chem. 2001, 306-315. (61) Karafiloglou, P. An Efficient Generalized Polyelectron Population Analysis in Orbital Spaces: The Hole-Expansion Methodology. J. Chem. Phys. 2009, 164103. (62) User-friendly computer software for generalized PolyElectron Population Analysis (PEPA) is available from P. Karafiloglou ([email protected]) upon request. (63) Becke A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange J. Chem. Phys. 1993, 5648–5652. (64) Lee C.; Yang W.; Parr R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 785–789. (65) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al., Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT, 2009. (66) Perdew, J. P.; Burke, K.; Wang, Y. Generalized Gradient Approximation for the Exchange-Correlation Hole of a Many-Electron System. Phys. Rev. B 1996, 16533. (67) Schreckenbach, G.; Ziegler, T. Calculation of NMR Shielding Tensors Using GaugeIncluding Atomic Orbitals and Modern Density Functional Theory. J. Phys. Chem. 1995, 606611.


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(68) Schreckenbach, G.,; Ziegler, T. Density Functional Calculations of NMR Chemical Shifts and ESR g-Tensors. Theo. Chem. Acc. 1998, 71–82. (69) Amsterdam Density Functional (ADF), Code, Release 2010, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com (Accessed 2/9/2015). (70) MIMAF (Molecular Induced MAgnetic Fields) software, Charistos, N. D.; Sigalas, M. P. Laboratory of Applied Quantum Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, Greece, 2010. (71) Møller, C. and Plesset, M. S., Note on an Approximation Treatment for Many-Electron Systems. Phys. Rev. 1934, 618-622. (72) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. MP2 Energy Evaluation by Direct Methods. Chemical Physics Letters 1988, 503–506. (73) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. A Direct MP2 Gradient Method. Chem. Phys. Lett. 1990, 275-280. (74) Omelchenko, I. V.; Shishkin, O. V.; Gorb, L.; Leszczynski, J.; Fiase, S. Bultinck, P. Aromaticity in heterocyclic analogues of benzene: comprehensive analysis of structural aspects, electron delocalization and magnetic characteristics. Phys. Chem. Chem. Phys. 2011, 20536– 20548. (75) Wolinski, K.; Hinton, J. F.; Pulay, P. Efficient Implementation of the Gauge-Independent Atomic Orbital Method for NMR Chemical Shift Calculations. J. Am. Chem. Soc. 1990, 82518260.


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(76) Cheesman, R.; Trucks, G. W.; Keith, T. A.; Frisch, M. J. A Comparison of Models for Calculating Nuclear Magnetic Resonance Shielding Tensors. J. Chem. Phys. 1996, 5497-5509. (77) Schreckenbach, G.; Ziegler, T. Density Functional Calculations of NMR Chemical Shifts and ESR g-Tensors. Theo. Chem. Acc. 1998, 71-82. (78) Schreckenbach, G. On the Relation Between a Common Gauge Origin Formulation and the GIAO Formulation of the NMR Shielding Tensor. Theo. Chem. Acc. 2002, 246-253. (79) Steiner, E.; Fowler, P. W. Patterns of Ring Currents in Conjugated Molecules: A FewElectron Model Based on Orbital Contributions. J. Phys. Chem. A 2001, 9553-9562. (80) Glendening. E. D.; Weinhold, F. Natural Resonance Theory: I. General formalism. J. Comput. Chem. 1998, 593-609. (81) Glendening. E. D.; Weinhold, F. Natural Resonance Theory: II. Natural Bond Orbital and Valency. J. Comput. Chem. 1998, 610-627. (82) Glendening. E. D.; Weinhold, F. Natural Resonance Theory: III. Chemical Applications. J. Comput. Chem. 1998, 628-646.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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Figure 1. Geometries, selected optimized geometrical parameters and relative energies of the three isomers of azaborines, at the CCSD/6-311++G(d,p) (first line) and B3LYP/6-311++G(d,p) (in parentheses) level of theory. 82x39mm (300 x 300 DPI)


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Scheme 1. Structures and atom numbering of 1,2-azaborine (1), 1,3-azaborine (2) and 1,4-azaborine (3). 82x22mm (300 x 300 DPI)



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Figure 2. Field lines of Bind (A, B, C) and contour maps of the magnitude |Bind| (D, E, F) of π-, σ- and core contributions to the induced magnetic field of 1,2-, 1,3- and 1,4-azaborine on the yz plane. 168x94mm (300 x 300 DPI)


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Figure 3. Contour maps of π- (A), σ- (B) and core (C) contributions to the z-component of the induced magnetic field Bzind (ppm) of 1,2-azaborine (1), 1,3-azaborine (2), 1,4-azaborine (3), Borazine (4) and Benzene (5) on the yz and xy planes. Blue areas represent diatropicity and red paratropicity. 168x133mm (300 x 300 DPI)



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Figure 4. Rescaled contour map of π contributions to the z-component of the induced magnetic field, Bzind (ppm), of 1,2-azaborine on the ring plane. Blue areas represent diatropicity and red paratropicity. 82x49mm (300 x 300 DPI)


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Chart 1. NICSzz values (ppm) versus the distance (Å) above the ring center for the total π-system of 1,2-, 1,3-, 1,4-azaborine, benzene and borazine. 82x63mm (300 x 300 DPI)



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Figure 5. Contour maps of π molecular orbital contributions to the z component of the induced magnetic field Bzind (ppm) of 1,2-azaborine (1), 1,3-azaborine (2) and 1,4-azaborine (3) on the yz and xy planes. Blue areas represent diatropicity and red paratropicity. Orbitals are numbered with increasing energy. 171x88mm (300 x 300 DPI)


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Figure 6. Contour maps of Bzind, HOMO-unocc (ppm) of A) 1,2-azaborine, B) 1,3-azaborine, C) 1,4azaborine, D) borazine, E) benzene and F) cyclobutadiene on the yz and xy planes. The color range in contour maps of benzene and borazine is ±30ppm and corresponds to the sum of both degenerate HOMOs. Blue areas represent diatropicity and red paratropicity. 171x59mm (300 x 300 DPI)



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Figure 7. The main resonance structures and their weights of the three azaborine’s isomer structures, as created using PolyElectron Population Analysis (PEPA) 82x79mm (300 x 300 DPI)


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160x83mm (300 x 300 DPI)


Study of Electron Delocalization in 1,2-, 1,3-, and 1,4-Azaborines

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