First-principles calculations of structural, electronic, vibrational, and magnetic properties of C60 and C48N12: A comparative study

Introducing a novel C50N10 azafullerene with chained nitrogen atoms on a buckyball pole: structure, stability, vibration, and electronic properties

CONTEXT

Fullerenes are of high significance due to their unique chemical properties and various applications in technology, particularly materials science, drug delivery, electronics, and nanoelectronics. In the recent years, many attempts have been focused to introduce new heteroatom-doped fullerenes having advanced chemical properties and tunable electronic traits, which make them a potential candidate for application in many branches of sciences. In this study, a novel C₅₀N₁₀ azafullerene with a fascinating structure of chained nitrogen atoms on a buckyball pole, with different electronic and optical properties compared to its other analogs, is introduced and trace of N-N substructures on the surface of C₆₀ fullerene cage is investigated. For this molecule, four structural isomers including 3 structures with chain N atoms on a fullerene buckyball pole (NP isomers) and one isomer with separated N atoms (SN isomer) have been studied. All isomers have been studied with and without symmetry constraints, and the symmetry influence on the structure and stability of each isomer has been investigated. Although the studied NP structures have lower stability than the SN isomer, some reasons (such as their more all-carbon hexagonal rings, breaking some of their N-N bonds for partial opening of the cage and creating bigger rings in order to get rid of the unfavorable strain, as well as decreasing the N-N lone pair repulsions) lead to the acceptable stability of these structures with the bonded N atoms. The results of atomization energy and vibrational frequency calculations indicate that isomers with the bonded N atoms have acceptable stabilities and do not decompose into their constituent components. Investigation on the structural parameters demonstrates important roles of the number of all-carbon hexagonal rings, the number of N-N junction, and the molecule symmetry in the stability of the structures with the bonded N atoms. Study on the electronic and optical properties indicates that the target structures exhibit high electronic polarizability, relatively small HOMO/LUMO gap, high first- and second-order hyperpolarizability, and also large third-order nonlinear optical properties.

METHODS

All calculations have been performed using Gaussian G09 software using density functional theory (DFT) approach. Three-parameter Beck hybrid exchange functional (B3) hybridized with nonlocal correlation functional of Lee, Yang, and Parr (LYP) has been employed as the level of DFT calculations. All optimizations have been performed at double-zeta polarized (DZP) split valence 6-31G(d,p) and also at split valence TZP 6-311G(d,p) basis sets. The global minimum structures have been confirmed by frequency calculations at the same level of optimizations. The natural bond orbital (NBO) analyses, frontier orbital surfaces imaging, atomic charges, and charge transfer analyses have been achieved by GenNBO program package.

Matrix isolation spectroscopy and spectral simulations of isotopically substituted C60 molecules

Isotopically enriched (3.5% ¹³C) and depleted (0.5% ¹³C) fullerene C₆₀ molecules are isolated in parahydrogen (pH₂) solids at cryogenic temperatures and studied by high resolution (0.01-0.1 cm^-1) infrared (IR) absorption measurements. Spectra of natural isotopic abundance (1.1% ¹³C) C₆₀ molecules isolated in solid pH₂, orthodeuterium (oD₂), and Ne matrix hosts serve to identify the relatively minor spectral perturbations due to the trapping environments. Spectral features observed for the four IR-active T_1u modes of threefold degeneracy in I_h symmetry, namely, T_1u(1) at 529.77 cm^-1, T_1u(2) at 578.24 cm^-1, T_1u(3) at 1184.7 cm^-1, and T_1u(4) at 1432 cm^-1, are assigned to the superpositions of matrix perturbed vibrational-mode spectra of a number of ¹³C_n ¹²C_60-n isotopologues. New molecular orbital calculations show the symmetry lowering effects of ¹³C substitution, namely, split vibrational frequencies and modified IR intensities. IR spectral patterns calculated for the 328 distinct isotopomers of ¹³C_n ¹²C_60-n up to n = 3 are used to satisfactorily simulate most of the observed absorption features. For the T_1u(4) mode at 1432 cm^-1, the observed splitting is insensitive to the ¹³C abundance, indicating spectral perturbations due to Fermi resonance. Weak absorption features at 1545 cm^-1 are assigned to a combination of lower frequency modes. We discuss relative and absolute band strengths for the astrophysical application of estimating C₆₀ abundances in planetary nebulae.

Ab Initio Investigation of Nonlinear Mode Coupling in C60

Strong light fields may be used to control molecular structure, thus providing a route to new, light-induced phases of matter. In this context, we present an ab initio molecular dynamics investigation of nonlinear mode coupling in C₆₀ and show that, under suitable conditions, resonant infrared excitation induces significant structural changes in the system. Surprisingly, exciting the highest-frequency infrared mode at field strengths employed in a recent experiment [Nature 2016, 530, 461-464], we observe no significant structural change. However, when targeting the first two infrared modes using stronger fields, all Raman modes gain energy through nonlinear coupling to the infrared modes, leading to large-amplitude vibrational excitations. We find a strong response of the H_g(5) mode, which is symmetric with respect to the equilibrium structure. For sufficiently strong field, it is of the same order as the infrared modes' excitation. This encourages further investigations into the light-induced superconducting properties of alkali-doped fullerenes.

Synthesis and properties of CS x F y thin films deposited by reactive magnetron sputtering in an Ar/SF6 discharge

A theoretical and experimental study on the growth and properties of a ternary carbon-based material, CS _x F _y , synthesized from SF₆ and C as primary precursors is reported. The synthetic growth concept was applied to model the possible species resulting from the fragmentation of SF₆ molecules and the recombination of S-F fragments with atomic C. The possible species were further evaluated for their contribution to the film growth. Corresponding solid CS _x F _y thin films were deposited by reactive direct current magnetron sputtering from a C target in a mixed Ar/SF₆ discharge with different SF₆ partial pressures ([Formula: see text]). Properties of the films were determined by x-ray photoelectron spectroscopy, x-ray reflectivity, and nanoindentation. A reduced mass density in the CS _x F _y films is predicted due to incorporation of precursor species with a more pronounced steric effect, which also agrees with the low density values observed for the films. Increased [Formula: see text] leads to decreasing deposition rate and increasing density, as explained by enhanced fluorination and etching on the deposited surface by a larger concentration of F/F₂ species during the growth, as supported by an increment of the F relative content in the films. Mechanical properties indicating superelasticity were obtained from the film with lowest F content, implying a fullerene-like structure in CS _x F _y compounds.

High-resolution photoelectron imaging of cold C₆₀⁻ anions and accurate determination of the electron affinity of C₆₀

High-resolution photoelectron imaging and spectroscopy of cold C₆₀⁻ anions are reported using a newly built photoelectron imaging apparatus coupled with an electrospray ionization source and a temperature-controlled cryogenic ion trap. Vibrationally resolved photoelectron spectra are obtained for the detachment transition from the ground state of C₆₀⁻ to that of C60 at various detachment wavelengths from 354.84 nm to 461.35 nm. The electron affinity of C60 is accurately measured to be 2.6835 ± 0.0006 eV. Numerous unexpected vibrational excitations are observed in the photoelectron spectra due to the Jahn-Teller effect in C₆₀⁻ and Hertzberg-Teller vibronic coupling in both C₆₀⁻ and C60. Both the relative intensities of vibrational peaks and their photoelectron angular distributions provide evidence for the vibronic couplings. The observed p-wave-like behavior in the angular distribution of the 0₀⁰ transition suggests that the electron is detached from an s-type orbital.

Azafullerene-like nanosized clusters

Carbon nitride materials have extraordinary potential in various applications, including catalysts, filled-particles, and superhard materials. Carbon nitride nanoclusters have been prepared under mild solvothermal conditions by a reaction between 1,3,5-trichlotriazine and sodium azide in toluene. The bulk material formed has a C(3)N(4) composition and consists of spheres with diameters ranging from approximately 1 nm to 4 mum. Nanometer-sized clusters of C(3)N(4) stoichiometry have been isolated on surfaces by sublimation or simple physicochemical methods. The clusters have then been characterized by atomic force microscopy and X-ray photoelectron spectroscopy. The laser desorption ionization mass spectra show peaks assignable to the C(12)N(16), C(21)N(28), and C(33)N(44) molecules which could correspond to cage structures with 4, 7, and 11 units of the C(3)N(4) subunit, respectively. The structure and stability of these new nitrogen-rich carbon nitride nanocages has been investigated using density functional theory calculations.

Density Functional Study of Chemical Stability and Nitrogen Encapsulation of C48N12 and C58N12

On the basis of calculations using density functional theory, we show that C58N12, just as C48N12, can be a stable N-dopant of C70. By considering many different isomers of the product, we find that the chemical stability of C48N12 and C58N12, with respect to oxygenation, is not significantly different from that of C70, thereby indicating that the N-dopant would not easily be oxygenated in air under normal conditions. In both C48N12O and C58N12O, many different isomers are expected, in which oxygenation occurs at different C-N bonds as well as at C-C bonds, among which specific C-N bonds are the most amenable to the reaction. Investigation of their hydrogenations shows that C48N12 is slightly more easily hydrogenated than C60, while C58N12 is less easily hydrogenated. In addition, we expect a regiospecificity in the hydrogenated products of C58N12, which prefers to react at equatorial sites, while C70 prefers reaction at polar sites. Meanwhile, comparison of the encapsulation energy of a nitrogen atom (=N en) in C60, C48N12, C70, and C58N12 shows that the N-doped fullerenes, particularly C58N12, can encase the atom much better than the undoped ones, allowing us to expect the existence of N@C48N12 and N@C58N12. Spin multiplicities are doublet for most of their stable structures. These observations correlate with the formation of N en-C bonds, which are not found in N@C60 and N@C70. Various isomers of the N-encapsulating fullerenes were identified. The relative stability of these isomers heavily depends on the number of substitutional nitrogen atoms around N en-C bonds.

Nitrogen violation of the isolated pentagon rule

We examine a range of fullerene isomers with and without the presence of nitrogen using density-functional-based calculations. We show that nitrogen stabilizes those with neighboring pentagons, allowing sp(2) bonded carbon networks to overcome the conventional rule that paired pentagons are not allowed through energetic constraints. This implies a new family of azafullerenes containing nitrogen-stabilized pentagon pairs, many of which will be considerably smaller than C(60). We discuss the relevance for nitrogen-doped nanotube structures and the topological importance for nanotube distortion and closure.

Polarizability and optical rotation calculated from the approximate coupled cluster singles and doubles CC2 linear response theory using Cholesky decompositions

A new implementation of the approximate coupled cluster singles and doubles CC2 linear response model using Cholesky decomposition of the two-electron integrals is presented. Significantly reducing storage demands and computational effort without sacrificing accuracy compared to the conventional model, the algorithm is well suited for large-scale applications. Extensive basis set convergence studies are presented for the static and frequency-dependent electric dipole polarizability of benzene and C60, and for the optical rotation of CNOFH2 and (-)-trans-cyclooctene (TCO). The origin-dependence of the optical rotation is calculated and shown to persist for CC2 even at basis set convergence.

Excitations, optical absorption spectra, and optical excitonic gaps of heterofullerenes. I. C60, C59N+, and C48N12: Theory and experiment

Low-energy excitations and optical absorption spectrum of C(60) are computed by using time-dependent (TD) Hartree-Fock, TD-density functional theory (TD-DFT), TD DFT-based tight-binding (TD-DFT-TB), and a semiempirical Zerner intermediate neglect of diatomic differential overlap method. A detailed comparison of experiment and theory for the excitation energies, optical gap, and absorption spectrum of C(60) is presented. It is found that electron correlations and correlation of excitations play important roles in accurately assigning the spectral features of C(60), and that the TD-DFT method with nonhybrid functionals or a local spin density approximation leads to more accurate excitation energies than with hybrid functionals. The level of agreement between theory and experiment for C(60) justifies similar calculations of the excitations and optical absorption spectrum of a monomeric azafullerene cation C(59)N(+), to serve as a spectroscopy reference for the characterization of carborane anion salts. Although it is an isoelectronic analogue to C(60), C(59)N(+) exhibits distinguishing spectral features different from C(60): (1) the first singlet is dipole-allowed and the optical gap is redshifted by 1.44 eV; (2) several weaker absorption maxima occur in the visible region; (3) the transient triplet-triplet absorption at 1.60 eV (775 nm) is much broader and the decay of the triplet state is much faster. The calculated spectra of C(59)N(+) characterize and explain well the measured ultraviolet-visible (UV-vis) and transient absorption spectra of the carborane anion salt [C(59)N][Ag(CB(11)H(6)Cl(6))(2)] [Kim et al., J. Am. Chem. Soc. 125, 4024 (2003)]. For the most stable isomer of C(48)N(12), we predict that the first singlet is dipole-allowed, the optical gap is redshifted by 1.22 eV relative to that of C(60), and optical absorption maxima occur at 585, 528, 443, 363, 340, 314, and 303 nm. We point out that the characterization of the UV-vis and transient absorption spectra of C(48)N(12) isomers is helpful in distinguishing the isomer structures required for applications in molecular electronics. For C(59)N(+) and C(48)N(12) as well as C(60), TD-DFT-TB yields reasonable agreement with TD-DFT calculations at a highly reduced cost. Our study suggests that C(60), C(59)N(+), and C(48)N(12), which differ in their optical gaps, have potential applications in polymer science, biology, and medicine as single-molecule fluorescent probes, in photovoltaics as the n-type emitter and/or p-type base of a p-n junction solar cell, and in nanoelectronics as fluorescence-based sensors and switches.