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A: Molecular Structure, Quantum Chemistry, and General Theory

The Evolution of Electronic and Magnetic Properties of the Chain and Sheet Assemblies Based on Planar Tetracoordinate Carbon CAl(CH) 2

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Afshan Mohajeri, and Azadeh Yeganeh Jabri J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01599 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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The Journal of Physical Chemistry

The Evolution of Electronic and Magnetic Properties of the Chain and Sheet Assemblies Based on Planar Tetracoordinate Carbon C2Al4(CH3)8 Afshan Mohajeri* and Azadeh Yeganeh Jabri

Department of Chemistry, College of Sciences, Shiraz University, Shiraz 7194684795, Iran

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Abstract The new planar tetracoordinate carbon (ptC) compounds have received significant research attention in recent years. The present study is devoted to investigating the structural, electronic, and magnetic features of one-dimensional chains and two-dimensional sheets composed of C2Al4(CH3)8 building blocks. All possible condensations are studied and the stabilities of different ptC assemblies have been compared. Several properties such as energy gap, dipole polarizability, electronic excitation energies, and nucleus chemical shift are computed for chains up to 7 and sheets up to 16 units. A systematic analysis has been performed to assess the impact of condensation pattern and number of units on the calculated properties. Topological analysis of density and electron localization functions reveals that AlC bonds in the considered ptCs have mixed covalent/ionic character with larger ionic contribution. It is found that the electronic spectra of the condensed ptCs exhibit red shift toward larger wavelengths when compared to the C2Al4(CH3)8 building block. The amount of red shift enhances with increasing number of units. We show that the stability trend, predicted by electronic and magnetic descriptors, are in qualitative agreement with the thermodynamic stability obtained through Gibbs free energy change of condensation reaction.


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1. INTRODUCTION The planer tetracoordinate carbon (ptC), unfamiliar carbon body with four ligands in the same plane, has attracted widespread interest in experimental and theoretical studies.1-3 Advances in the ptC chemistry encouraged researchers to design solid state or nanoscale materials based on nonclassical carbon bonding. Both electronic and mechanical effects stabilize these unusual bonding arrangements. One of the strategies suggested for stabilizing a ptC is based on the replacement of the hydrogens in planar methane by ligands that facilitate electron transfer to the electron-deficient carbon such as σ-donors or by incorporating the lone pair into the π-delocalized system.4 In this context, various ptC species or molecules have been realized experimentally or designed theoretically, such as CB4+,5 CCu4+,6 CE4-(E= Al, Ga, In),7 CAl4Be and CAl3Be2-.8 Individual ptC can also serve as building blocks for larger structures or even bulk solids. Incorporating ptC units into molecular assemblies could bridge the gap from isolated clusters to potential macroscopic solid materials. These novel materials might exhibit unique mechanical, optical, electronic, magnetic, or catalytic properties.9 In this context, growing attention has been paid to the large molecular structures with multiple ptC blocks. Examples include salt-like three-dimensional (3D) solids consisting of anionic ptC such as CB6-, CAl2Si2, CAl4-, CAl3Si, CAl42- and periodic one-dimensional (1D) and two-dimensional (2D) structures such as nanoribbons, nanosheets and nanotubes. Among ptCs, carbon–aluminum binary clusters have attracted attention because of their nonclassical structures and exotic chemical bonds.10,11 In particular, the experimentally characterized CAl42- and C2Al4 units have the potential to construct large ptC species.12 Geske and Boldyrev designed the salt-like solid using CAl42- ptC and Na as counterionion.13 In 2007, the Wu and Wang group designed the nanoribbons and nanotubes based on the CM4H4 (M = Ni, Pd and Pt) ptC units.14 They also carried out density functional theory



(DFT) calculations to search for the neutral singlet C2E4 (E=Al, Ga, In, and Tl) global minima with double planar ptCs.15 Afterward, they showed that C2Al4E8 (E = CH3, NH2, and OH) can be condensed to 1D molecular chains containing planar C2Al4 units by eliminating CH4, NH3, and H2O molecules.16 Further, they also reported linear, flat and tubular molecular structures constructed by C2Al4 units.17 In addition to the carbon-aluminum ptC, many other planar carbon compounds have been theoretically and experimentally designed.18-21 Pancharatna et al. constructed the solid by using the all-carbon ptC block C52-.22 Cui et al. reported the lowest energy isomers of CB4 and its cation CB4+.23 Also, it has been shown that boron-rich 2D boron-carbon nanostructures have ptC motif with C2v symmetry.24 Despite numerous studies on computational designing the ptC species, investigations concerning the evolution of the electronic and magnetic properties of compounds with multiple blocks of ptC are still missing. Moreover, the structure of nano-sized molecules is dependent on the type of ptC and the way that the ptC building blocks are assembled. In this context, understanding the structural evolution of different ptC assemblies is also of great importance. This work presents a systematic study on the evolution of structural, electronic, and magnetic characteristics of linear and flat molecular assemblies based on C2Al4 core. All possible linking patterns are considered for the condensation of [C2Al4(CH3)8]n units with n up to 7 and 16 for 1D chains and 2D sheets, respectively. For the considered ptC compounds, we explore the size- and the shape-dependence of different properties, namely, the energy gap between highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMOLUMO), the dipole polarizability, the electronic excitation energy, and the nucleus chemical shift. Moreover, we have also analyzed the bonding pattern in linear and planar ptCs of different sizes. Our results would provide important knowledge for the potential technological applications like fabrication of these materials with desired properties.


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2. COMPUTATIONAL DETAILS The geometry optimization and electronic structure calculations were performed at the B3LYP level of DFT with 6-31G(d) basis set as implemented in Gaussian 09 program of package.25 The reliability of B3LYP functional for computational study of ptC compounds has been verified in earlier works.15-17 Moreover, it has been shown that 6-31G(d) basis set can predict geometries and condensation energies close to larger basis set (6-311++G(d,p)).17 In order to explore the evolution of electronic properties we start from C2Al4(CH3)8 building block and systematically increase the length of chains and the size of planar ptCs via consecutive condensations. For all systems under our consideration, including C2Al4(CH3)8 building block, 1D chains, and 2D-sheets the harmonic vibrational frequencies were calculated to affirm that the obtained structures are true minima. The bonding analysis were performed with the help of AOMix program.26 The topological analysis of the electron density and electron localization function (ELF) were carried out by using MultiWFN analyzer.27 Single-point time dependent DFT (TDDFT) calculations were also performed on the optimized geometries of considered ptCs. The dominant molecular orbital transitions along with their oscillator strengths and excitation wavelengths have been determined. Further, we use nucleus-independent chemical shift (NICS) as a magnetic index to gauge the aromatic nature of these ptC compounds.28

3- RESULTS AND DISCUSSION The C2Al4(CH3)8 unit has two types of aluminum atoms which are termed AlT (terminal Al) and AlB (bridge Al). A shown in Figure 1, C2Al4(CH3)8 units can be connected together in two ways. The first is vertex-to-vertex condensation, which eliminates two methane molecules and results in three types of 1D chains; TT, BB, and TB. In TT or BB the condensation occurs between two AlT or between two AlB atoms while in TB the attachments 5 ACS Paragon Plus Environment


of two units occurs between AlT of one unit and AlB of the other unit. The second type of linking pattern, face-to-face condensation, results the FF molecular chain through eliminating four methane molecules. Both types of condensations can be extended to more than two building units by consecutive condensations. It is also possible to make 2D sheets via condensations between chains. Figure 1 illustrates two typical condensations termed TTBB and TBTB, which combines the products of TT and TB condensation, respectively. Depending on the number of C2Al4(CH3)8 units along the two orthogonal directions, sheets with different sizes can be formed. For the convenience of description of various assemblies, we use the terms ‘XX-Sn’ for 1D chains and XXYY-Sn for 2D sheets. XX and YY denote the pattern of assembly which could be TT, BB, TB, or FF and ‘S’ refers to the shape of the assembly which could be ‘C’ or ‘P’ for chain or planar sheet, and n is the number of C2Al4(CH3)8 units in the assembly. For instance, TB-C4 represents a chain formed by condensation of four C2Al4(CH3)8 units via TB pattern and TTBB-P6 names a 2×3 sheet consisting of six C2Al4(CH3)8 units condensed via TT pattern. Before analyzing the electronic and magnetic properties, it is interesting to compare the thermodynamic stability of various assemblies. Figure 1 includes the Gibbs free energy change (∆G) of the condensation reaction. The negative ∆G for FF-C2 indicates the thermodynamic favorableness of face-to-face condensation compared to vertex-to-vertex condensations and implies the promise for the experimental synthesize. To explore the size dependence of the calculated properties we start from dimer and by consecutive condensations increase the number of units (n) up to 7 and 16 units for 1D chains and 2D sheets, respectively. 3-1 Structure and bonding features The optimized structures for all studied compounds are displayed in Figure S1 of the Supporting Information and the largest chains or sheets in each series are presented in Figure 6 ACS Paragon Plus Environment

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2 as illustrative examples. They are all energy minima without imaginary frequency. However, the lowest vibrational frequencies are relatively small (2-23 cm-1) which indicates the flexibility of large assemblies. Previous studies suggest that the Al-C bonds possess obvious character of covalent bonding.29 However, the difference in electronegativities of C and Al atoms is quite large. The question is that the Al-C bonds in theses ptC compounds are ionic or polarized covalent? Accordingly, we are aiming to shed light onto the structure and bonding features of different ptC assemblies based on the topological analysis of the electron density and ELF, together with the three-center bond order (TCBO) indices. Table 1 contains some topological properties for different series of considered chains and sheets. Note that for each quantity we have reported the average value. As shown in Table 1, the TCBO indices for C-AlB-C and AlB-C-AlB are less than 0.1 and that of for AlT-C-AlB is around 0.1 suggesting that the multi-center orbital interactions has not significant role in the formation of different series of considered assemblies. Moreover, TCBO of C2Al4(CH3)8 unit is similar to those in 1D chains and 2D sheets indicating that bonding character in the C2Al4 moieties does not change significantly when the new units are added. Table 1 shows that the electron density, (), at the bond critical point (BCP) for AlB-C and AlT-C bonds are in the range of 0.0527- 0.0560 au and 0.0740-0.0760 au, respectively. Irrespective of the type of condensation, for all series, the Laplacians of electron densities ( ∇ ()) are positive implying depletion of charge density as in ionic bond. We also derived the local energy density (()) which is defined as () = () + (). The G(r) and V(r) correspond to a local kinetic and potential energy density, respectively.30,31 The sign of () determines whether a charge accumulation at a point r is stabilizing or destabilizing. Negative () at a BCP is associated with the concentration of charge between the nuclei as found in covalent bonds whereas positive () is an indicator of electrostatic dominant interaction as in ionic bonds. Our analysis reveals that, for all series of ptCs investigated here, () values



are negative for both AlB-C and AlT-C bonds. The positive Laplacian and the negative energy density are clear evidence that the AlB-C and AlT-C bonds are transit closed-shell with partly electrostatic and partially covalent character. To prove this, we have further computed the ELF at the BCPs of AlB-C and AlT-C bonds. The concept of ELF provides important information about the electronic structure and chemical bonding properties.32,33 According to its definition, ELF is a dimensionless quantity that varies between 0 and 1. The high value of ELF at certain point shows high degree of localization implying the existence of covalent bond, lone pair or core electrons. On the contrary, in the case of an ionic bond, the ELF value at the interstitial positions of the two atoms is very low. Table 1 shows that in C2Al4(CH3)8 unit and all series of condensed ptCs, the ELF at the BCP of AlB-C and AlT-C bonds are quite small conforming the delocalized feature and ionic character. Summing it up, the signature of Al-C bonds in the considered ptCs is a combination of small (), positive ∇ (), small negative (), and ELF values around 0.1 featuring mixed covalent/ionic character with larger ionic contribution.

3-2 Electronic properties Apart from bonding analysis, it is also of interest to investigate the evolution of electronic properties with the size and the pattern of ptC assemblies. The energy gap is the quantity that can reflect the ptC chemical stability via the energy cost of an electron excitation from HOMO to LUMO. Higher energy gap implies less chemical reactivity. The calculated HOMO-LUMO gap for the isolated. Figure 3 plots the variation of energy gap for the considered 1D chains and 2D sheets as a function of their size. It is observed that, regardless of the type of assembly, the condensation leads to a decrease in the energy gap. Hence, the energy gaps of the condensed ptCs are less that of C2Al4(CH3)8 unit (4.56 eV). However, the amount of reduction depends on the linking pattern and energy gaps follow the trend; FF

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