On the Importance of the Clar Structure of Borazine Revealed by the π-Contribution to the Electron Localization Function Speaker: Jingjing Wu Advisor: Jun Zhu 2013/11/29 1

Conclusion Results and Discussion Computed Details Introduction Main Contents 2

Introduction 3 1. Clar structure (1)Two circles must not be drawn in neighbouring hexagons. (2)The circles must be arranged in a way so that a Kekulé structure can be written for the rest of the molecule. (3)The formula must contain the maximum number of circles. 1.Chengde Lin, Gui qing Fan, Journal of Xiamen University 1998, 37, Ivan Gutman, Match. 1985, 17, Jun Zhu, Christian Dahlstrand, Joshua R. Smith, Sébastien Villaume, and Henrik Ottosson. Symmetry 2010, 2, 1653.

Introduction Figure 1. The electronic structure of benzopyrene is represented by the three Clar formulas C1, C2, C3. The formulas C4, C5 and C6 are incorrect because they violate rules 1, 2 and 3, respectively. 4

Introduction 2. In recent years, polybenzenoid hydrocarbons (PBHs) have received growing attention as a result of their increased number of applications in electronics and materials sciences. The degree of π-electron (de)localization and aromaticity of a series of PBHs has been analyzed through the  - contribution to the electron localization function (ELF  ). As measures of  -electron (de)localization, and thus, as aromaticity indicators, we now apply: (i) the bifurcation values of the ELF  basins (BV(ELF  )) at the individual CC bonds, (ii) the range in the bifurcation values of the ELF  basins of a ring [  BV(ELF  )]. The character of the individual  -bonds are important, and we classify a CC  -bond as delocalized, if its BV(ELF  ) is in the 0.65–0.96 range. Only if all CC  -bonds of a hexagon are of delocalized nature do we categorize it as an aromatic  -sextet. If the BV(ELF  ) of an ELF  bifurcation along a CC bond is larger than 0.96 or smaller than 0.65 we categorize this bond as a double or single bond, respectively, and we do not allow any such bonds in an aromatic  -sextet. 5

Introduction 6 D2h Cs C2h

Introduction 7 C2v Cs

Figure 2. ELF  bifurcation values (BV(ELF  )), the BV(ELF  ) ranges of each ring (ΔBV(ELF  )) (in red) of 1– 6, the relative energies (kcal/mol), and the bifurcation point of 6 at which the ELF  separates into five 6  -electron basins (BV(ELF  ) = 0.449) at B3LYP/6-311G (d, p) level.  bonds with BV(ELF  )’s in either of the three intervals BV(ELF  ) < 0.65, 0.65 ≤ BV(ELF  ) < 0.96, and 0.96 ≤ BV(ELF  ), are denoted as single, single plus dashed, and double bonds, respectively. Introduction D2h 0.445

Computed Details Package: Gaussian 09DFT method: B3LYP/M062XBasis set: 6-311G** 9

Results and Discussion While the chemistry of benzene and its compounds are well-developed, the corresponding B-N analogues are relatively unknown, especially the aromaticity revealed by the  -contribution to the electron localization function. 10

Results and Discussion 11

Results and Discussion Figure 3. ELF  bifurcation values (BV(ELF  )), the BV(ELF  ) ranges of each ring(  BV(ELF  )) (in the center of ring) of 1 – 6, the relative Gibbs free energies (kcal mol -1 ) compared with 6 and the point group of every structure. Optimized 1-6 at the B3LYP/6- 311G (d, p) level. 12

Results and Discussion 13

Results and Discussion Figure 4. The structures are optimized at B3LYP and M062X level, respectively. The relative Gibbs free energies are given in kcal/mol. The numbers above the arrows are optimized at B3LYP, while the belows are optimized at M062X level. 14

Results and Discussion Figure 5. Optimized (B3LYP/6-311G**) structures for benzene and BN analogue. ELF  bifurcation values (BV(ELF  )), the BV(ELF  ) ranges of each ring (  BV(ELF  )) (in the center of ring),  -bonds with BV(ELF  )’s in either of the three intervals BV(ELF  ) < 0.65, 0.65 ≤ BV(ELF  ) < 0.96, and 0.96 ≤ BV(ELF  ), are denoted as single, CC  -bond, and double bonds, respectively. 15

Results and Discussion Figure 6. Optimized (B3LYP/6-311G**) structures for the benzene and BN analogue isomerization stabilization energy (ISE) and NICS values for the individual rings were given in the circle. The lower number and the upper number indicate the NICS values in the plane and 1 Å above the plane of the ring, respectively. 16

Results and Discussion 17

Results and Discussion Figure 4. Optimized (B3LYP/6-311G**) structures for the BN analogues (1-6). NICS values for the individual rings were given in the circle. The upper numbers and the lower numbers indicate the NICS values 1 Å above the plane of the ring and in the plane, respectively. 18

Results and Discussion 19

Results and Discussion Figure 5. Optimized (B3LYP/6-311G**) structures for different BN analogues, containing 3 to 7 BN acenes, respectively. 20

Conclusion 1. More circles containing, more stable the structure is. 2. ELF  indicates that borazines almost have no aromaticity. Due to the large difference of electronegativity between boron (2.0) and nitrogen (3.0), the  electron delocalization may be expected to be reduced in borazines as compared to benzene With the ring numbers increasing, the relative Gibbs free energy difference between the two isomers are also increasing, but it is not obvious.

Thanks For Your Attention ! 22