Isomers of C24. Density functional studies including gradient corrections

Thermal decomposition of FC(O)OCH3 and FC(O)OCH2CH3

The thermal decomposition of methyl and ethyl formates has been extensively studied due to their importance in the oxidation of several fuels, pesticidal properties and their presence in interstellar space. We hitherto present the study of the thermal decomposition of methyl and ethyl fluoroformates, which could help in the elucidation of the reaction mechanisms. The reaction mechanisms were studied using FTIR spectroscopy in the temperature range of 453-733 K in the presence of different pressures of N2 as bath gas. For FC(O)OCH3 two different channels were observed; the unimolecular decomposition which is favored at higher temperatures and has a rate constant kFC(O)OCH3 = (5.3 ± 0.5) × 1015 exp[-(246 ± 10 kJ mol-1/RT)] (in units of s-1) and a bimolecular channel with a rate constant kFC(O)OCH3 = (1.6 ± 0.5) × 1011 exp[-(148 ± 10 kJ mol-1/RT)] (in units of s-1 (mol L)-1). However for ethyl formate, only direct elimination of CO2, HF and ethylene operates. The rate constants of the homogeneous first-order process fit the Arrhenius equation kFC(O)OCH2CH3 = (2.06 ± 0.09) × 1013 exp[-(169 ± 6 kJ mol-1/RT)] (in units of s-1). The difference between the mechanisms of the two fluoroformates relies on the stabilization of a six-centered transition state that only exists for ethyl formate. First principles calculations for the different channels were carried out to understand the dynamics of the decomposition.

The effect of C-vacancy on hydrogen storage and characterization of H2 modes on Ti functionalized C60 fullerene a first principles study

Density functional theory calculations were performed to examine the effect of a C vacancy on the physisorption of H(2) onto Ti-functionalized C(60) fullerene when H(2) is oriented along the x-, y-, and z-axes of the fullerene. The effect of the C vacancy on the physisorption modes of H(2) was investigated as a function of H(2) binding energy within the energy window (-0.2 to -0.6 eV) targeted by the Department of Energy (DOE), and as functions of a variety of other physicochemical properties. The results indicate that the preferential orientations of H(2) in the defect-free (i.e., no C vacancy) C(60)TiH(2) complex are along the x- and y-axes of C(60) (with adsorption energies of -0.23 and -0.21 eV, respectively), making these orientations the most suitable ones for hydrogen storage, in contrast to the results obtained for defect-containing fullerenes. The defect-containing (i.e., containing a C vacancy) C(59)TiH(2) complex do not exhibit adsorption energies within the targeted energy range. Charge transfer occurs from Ti 3d to C 2p of the fullerene. The binding of H(2) is dominated by the pairwise support-metal interaction energy E(i)(Cn...Ti), and the role of the fullerene is not restricted to supporting the metal. The C vacancy enhances the adsorption energy of Ti, in contrast to that of H(2). A significant reduction in the energy gap of the pristine C(60) fullerene is observed when TiH(2) is adsorbed by it. While the C( n ) fullerene readily participates in nucleophilic processes, the adjacent TiH(2) fragment is available for electrophilic processes.

Cation-π interaction of alkali metal ions with C24 fullerene: a DFT study

Using first principle calculations, we investigated cation-π interactions between alkali cations (Li(+), Na(+), and K(+)) and pristine C(24) or doped fullerenes of BC(23), and NC(23). The most suitable adsorption site is found to be atop the center of a six-membered ring of the exterior surface of C(24) molecule. Interaction energies of these cations decreased in the order: Li(+) > Na(+) > K(+), with values of -31.82, -22.36, and -15.68 kcal mol(-1), respectively. It was shown that the interaction energies are increased and decreased by impurity doping of B and N atoms in adjacent wall of adsorption site, depending on electron donating or receptivity of the doping atoms.

DFT calculations on the structural stability and infrared spectroscopy of endohedral metallofullerenes

Endohedral metallofullerenes M@C(24) (M = Li(0/+), Na(0/+), K(0/+), Be(0/2+), Mg(0/2+) and Ca(0/2+)) with different spin configurations have been systematically investigated using the hybrid DFT-B3PW91 functional in conjunction with 6-31 G(d) basis sets. Our theoretical studies show that Li@C(24), Be@C(24), Be(2+)@C(24), and Mg(2+)@C(24) are energetically favorable. In these endohedral metallofullerenes, only the encapsulated Be and Ca atoms can donate the electrons to the cage. With exception of Be(2+)@C(24), the energy gaps of other charged compounds are larger than that of corresponding neutral compounds. We also find that some endohedral metallofullerenes have high energy gaps, but they are unlikely to show high thermodynamic stability. Additionally, the vibrational frequencies and active infrared intensities are also used as evidence to identify these endohedral metallofullerenes.

Ab initio calculation of carbon clusters. II. Relative stabilities of fullerene and nonfullerene C24

Chemical stabilities of six low-energy isomers of C24 derived from global-minimum search are investigated. The six isomers include one classical fullerene (isomer 1) whose cage is composed of only five- and six-membered rings (56-MRs), three nonclassical fullerene structures whose cages contain at least one four-membered ring (4-MR), one plate, and one monocyclic ring. Chemical and electronic properties of the six C24 isomers are calculated based on a density-functional theory method (hybrid PBE1PBE functional and cc-pVTZ basis set). The properties include the nucleus-independent chemical shifts (NICS), singlet-triplet splitting, electron affinity, ionization potential, and gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) gap. The calculation suggests that the neutral isomer 2, a nonclassical fullerene with two 4-MRs, may be more chemically stable than the classical fullerene (isomer 1). Analyses of molecular orbital NICS show that the incorporations of 4-MRs into the cage considerably reduce paratropic contributions from HOMO, HOMO-1, and HOMO-2, which are mainly responsible for the sign change in NICS from positive for isomer 1 (42) to negative (-19) for isomer 2, although C24 clusters satisfy neither 4N+2 nor 2(N+1)2 aromaticity rule. Anion photoelectron spectra of four cage isomers, one plate, one monocyclic ring, and one tadpole isomer, as well as three bicyclic ring isomers are calculated. The simulated photoelectron spectra of mono- and bicyclic rings (with C1 symmetry) appear to match the measured HOMO-LUMO gap (between the first and second band in the experimental spectra) [S. Yang et al., Chem. Phys. Lett. 144, 431 (1988)]. Nevertheless, the nonclassical fullerene isomers 3 and 4 apparently also match the measured vertical detachment energy (2.90 eV) reasonably well. These results suggest possible coexistence of nonclassical fullerene isomers with the mono- and bicyclic ring isomers of C24(-) under the experimental conditions.

A Family of Stable Silica Fullerenes with Fully Coordinated Structures

Theoretical electronic structure techniques have become an indispensable and powerful tool for predicting molecular properties and designing new materials. The discovery of C(60) opened a challenging field in nanoscale materials science, and since then people have been looking for its inorganic analogues. On the basis of the B3LYP/6-31G(d) calculations, here we provide theoretical evidence for a family of stable silica fullerenes with fully coordinated structures, which exhibit highly structural and energetic stabilities, very large energy gaps, and extremely good resistibilities to breakdown of the insulating capability in an applied electric field. Our calculations indicate that the discrete silica fullerenes are a possible polymorph of silica and can be synthesized under some conditions. They are expected to find novel applications in silica-based molecular devices. The present results may provide an aid in the experimental design for controllably producing desired silica clusters.

Synthesis and characterization of trifluoromethyl fluoroformyl peroxycarbonate, CF3OC(O)OOC(O)F

The synthesis of CF(3)OC(O)OOC(O)F is accomplished by the photolysis of a mixture of (CF(3)CO)(2)O, FC(O)C(O)F, CO, and O(2) at -15 degrees C using a low-pressure mercury lamp. The new peroxide is obtained in pure form in low yield after repeated trap-to-trap condensation and is characterized by NMR, IR, Raman, and UV spectroscopy. Geometrical parameters were studied by ab initio methods [B3LYP/6-311+G(d)]. At room temperature, CF(3)OC(O)OOC(O)F is stable for many days in the liquid or gaseous state. The melting point is -87 degrees C, and the boiling point is extrapolated to 45 degrees C from the vapor pressure curve log p = 8.384 - 1715/T (p/mbar, T/K). A possible mechanism for the formation of CF(3)OC(O)OOC(O)F is discussed, and its properties are compared with those of related compounds.

Computational Comparison of S(N)()2 Substitution Reactions of CHX(*-)() and CH(2)()X(-)() with CH(3)()X (X = Cl, Br). Do Open-Shell and Closed-Shell Anions React Differently?

The S(N)2 displacement reaction of the radical anions (CHCl(*-) and CHBr(*-)) and the closed-shell anions (CH(2)Cl(-) and CH(2)Br(-)) with CH(3)Cl and CH(3)Br were studied with density functional theory. It was determined that the anions CH(2)Cl(-) and CH(2)Br(-) were more reactive than the radical anions CHCl(*-) and CHBr(*-), in agreement with experiment. The degree of charge transfer in the reactant complex is the best indicator of the reactivity trend. It was found that two reactions (CH(2)Cl(-) with CH(3)Cl and CH(3)Br) proceeded without an activation barrier at the B3LYP/6-31+G(d) level. The backside transition states (leading to inversion of configuration at the methyl group) were 20-30 kcal/mol lower in energy than the frontside transition states (leading to retention of configuration). Interestingly, it was found that the mechanism of backside and frontside attack by radical anions was different. In backside attack, the radical anion leads with the lone pair directed toward the methyl group to form an incipient two-center, two-electron bond. In frontside attack, the radical anion leads with the unpaired electron directed toward the methyl group to form an incipient two-center, one-electron bond.