Structure and Vibrational Spectroscopy of C82 Fullerenol Valent Isomers: An Experimental and Theoretical Joint Study

Abstract

Gd@C₈₂O_xH_y endohedral complexes for advanced biomedical applications (computer tomography, cancer treatment, etc.) were synthesized using high-frequency arc plasma discharge through a mixture of graphite and Gd₂O₃ oxide. The Gd@C₈₂ endohedral complex was isolated by high-efficiency liquid chromatography and consequently oxidized with the formation of a family of Gd endohedral fullerenols with gross formula Gd@C₈₂O₈(OH)₂₀. Fourier-transformed infrared (FTIR) spectroscopy was used to study the structure and spectroscopic properties of the complexes in combination with the DFTB3 electronic structure calculations and infrared spectra simulations. It was shown that the main IR spectral features are formed by a fullerenole C₈₂ cage that allows one to consider the force constants at the DFTB3 level of theory without consideration of gadolinium endohedral ions inside the carbon cage. Based on the comparison of experimental FTIR and theoretical DFTB3 IR spectra, it was found that oxidation of the C₈₂ cage causes the formation of Gd@C₈₂O₂₈H₂₀, with a breakdown of the integrity of the parent C₈₂ cage with the formation of pores between neighboring carbonyl and carboxyl groups. The Gd@C₈₂O₆(OOH)₂(OH)₁₈ endohedral complex with epoxy, carbonyl and carboxyl groups was considered the most reliable fullerenole structural model.

Keywords: C82; DFTB3 electronic structure calculations; Gd endohedral complexes; IR spectra; biomedical applications; fullerenols.

Figure 1

Experimental FTIR spectrum of Gd@C₈₂O_xH_y and theoretical spectra of C₈₂O_xH_y possible isomers. Top: Experimental FTIR spectrum of Gd@C₈₂O_xH_y in the KBr matrix (black curve). Bottom: Theoretical IR spectra of free–standing C₈₂O₃(OH)₂₄ (green curve). Middle: Theoretical IR spectrum of I1 isomer (C₈₂O₈(OH)₂₀, blue curve), and theoretical IR spectrum of I2 isomer (C₈₂O₆(OOH)₂(OH)₁₈, red curve). Theoretical IR spectra calculated in the gas phase at the DFTB3 level of theory.

Figure 2

Some structural models of possible C₈₂O_xH_y complexes. (A) Gd@C₈₂; (B) Gd@C₈₂O₃(OH)₂₄. (C,F,G) Initial structural models of C₈₂O₃(OH)₂₄, where I1 is C₈₂O₈(OH)₂₀ and I2 is C₈₂O₆(OOH)₂(OH)₁₈, valent isomers of the C₈₂O₂₈H₂₀ complex, respectively. (D,H,I) Equilibrium geometries of C₈₂O₃(OH)₂₄, I1 and I2 isomers of the C₈₂O₂₈H₂₀ complex, respectively (atomic coordinates of I1 and I2 isomers can be found in the Supplementary Materials Section). Carbon, gadolinium, oxygen and hydrogen atoms are depicted in black, gray, red and white, respectively.