Structural, electronic, and magnetic properties of heterofullerene C48B12

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Abstract

Bonding, electric (hyper)polarizability, vibrational, and magnetic properties of heterofullerene C48B12 are studied by first-principles calculations. Infrared- and Raman-active vibrational frequencies of C48B12 are assigned. Eight 13C and two 11B nuclear magnetic resonance (NMR) spectral signals of C48B12 are characterized. The average second hyperpolarizability of C48B12 is about 180% larger than that of C60. Our results suggest that C48B12 is a candidate for photonic and optical limiting applications because of the enhanced third-order optical non-linearities.

Introduction

In 1985, Kroto et al. [1] proposed the existence of C60 clusters in their graphite laser vaporization experiment. This proposal was subsequently confirmed in 1990 when Krätschmer et al. [2] reported a method for the mass production of C60 in a carbon arc along with infrared (IR) spectroscopic evidence for the C60 carbon structure. These pioneering works have stimulated extensive research into fullerenes [3], [4], a new form of pure carbon, where an even number of three-coordinated sp2 carbon atoms arrange themselves into 12 pentagonal faces and any number (>1) of hexagonal faces. These carbon-cage molecules can crystallize into a variety of three-dimensional structures [2] and be doped in several different ways [4]: endohedral doping, where the dopant is inside the fullerene cage; substitutional doping, where the dopant is on the fullerene cage; and exohedral doping, where the dopant is outside or between fullerene cages. It has been shown that doped fullerenes have remarkable structural, electronic, optical, and magnetic properties [4], [5], [6].

In 1995, the heterofullerene C59N was formed efficiently in the gas phase during fast atom bombardment mass spectroscopy of a cluster-opened N-methoxyethoxy methyl ketolactam [7]. The isolation and characterization of biazafullerenyl has opened a viable route for the preparation of C59N and other heterofullerenes in solution, leading to a number of detailed theoretical and experimental studies of C59N and heterofullerenes [4], [5], [6]. In 1991, the Smalley group [8] successfully synthesized boron-substituted fullerenes C60  nBn (1⩽n⩽6). Very recently, Hultman et al. [9] have successfully synthesized aza-fullerenes C60  nNn, formed by substituting carbon atoms in C60 with more than one nitrogen atom, and the existence of a stable C48N12 aza-fullerene [9], [10], [11], [12] was revealed. Stimulated by the high stability of C48N12, we have recently predicted that C48B12[13] is also a stable heterofullerene and can be a promising component for molecular rectifiers, nanotube-based transistors, and p–n junctions.

In this Letter, we further study the bonding, Mulliken charges, electric (hyper)polarizability, vibrational, and magnetic properties of C48B12. We characterize 13C and 11B NMR spectral lines of C48B12 and show how the boron-substitutional doping modifies the IR and Raman spectra of the pristine C60. We also find that C48B12 exhibits enhanced second hyperpolarizability (enhanced third-order optical non-linearity) and can compete with C60 and aza-fullerene C48N12 as a candidate for photonic and optical limiting applications (for example, data processing, eye and sensor protection, all-optical switching, and optical limiting) [6].

Section snippets

Bonding and Mulliken charge

The geometry of C48B12, shown in Fig. 1, was fully optimized by using the Gaussian 98 program1[14]. We have used the B3LYP [15] hybrid density functional theory (DFT) method, which includes a mixture of Hartree–Fock (exact) exchange, Slater local exchange [16], Becke 88 non-local exchange [17], the VWN III local exchange-correlation functional [18] and the LYP correlation functional [19], and a 6-31G(d) basis set.

Electric (hyper)polarizability

The static dipole polarizability (SDP) for heterofullerene C48B12 is presented in Table 3. The B3LYP results were obtained by using the Gaussian 98 program1[14], while the local density approximation (LDA) results were calculated by using the Amsterdam Density Functional (ADF) program1[22], [23]. The SDPs for C48N12 and C60 listed in Table 3 are taken from our recent work [24]. For the B3LYP calculations, we use the valence-split basis set 6-31G(d) including the polarization functions for boron

IR and Raman spectra

Using the Gaussian 98 program1[14], we first optimize the geometry of C48B12 and C60 with the B3LYP method and 3-21G basis set. Then, we calculate the vibrational frequencies of C48B12 and C60 with the same method and basis set. Our results for C60 are in agreement with experiment [30], [31]. C60 has totally 46 vibrational modes [4]. Since C48B12 has lower symmetry (Ci) than C60, we find 174 independent vibrational modes for C48B12: 87 non-degenerate IR-active modes with au symmetry and 87

Second-order magnetic response

There are a number of theoretical methods for calculating the second-order magnetic response properties of molecules. In this Letter, we use both the gauge-including-atomic-orbital (GIAO) method and the continuous-set-of-gauge-transformation (CSGT) procedure [33], which is implemented in the Gaussian 98 program1[14], to predict the NMR shielding tensors σ of C48B12. In high-resolution NMR, the isotropic part σiso of σ is measured by taking the average of σ with respect to the orientation to the

Summary

In summary, we have performed first-principles calculations of bonding, Mulliken charges, dipole polarizability, hyperpolarizability, vibrational frequencies, IR intensities, Raman scattering activities, and second-order magnetic response properties of heterofullerene C48B12. Eighty-seven independent IR-active and 87 independent Raman-active vibrational modes for C48B12 are assigned. Eight 13C and two 11B NMR spectral lines for C48B12 are characterized. Compared to C60 and C48N12, C48B12

Acknowledgements

We thank Dr. Denis A. Lehane and Dr. Hartmut Schmider for their technical help. One of us (R.H.X.) thanks the HPCVL at Queen’s University for the use of its parallel supercomputing facilities. L.J. gratefully acknowledges the Danish Research Training Council for financial support. V.H.S. acknowledges support from the Natural Science and Engineering Research Council of Canada (NSERC).

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