Theoretical Study of the Interaction of Molecular Oxygen with Copper Clusters

Theoretical studies of phosphorene as a drug delivery nanocarrier for fluorouracil

The interactions between phosphorene nanosheets (PNSs) and 5-fluorouracil (FLU) were explored using the density functional theory (DFT) method and molecular dynamics (MD) simulations. DFT calculations were performed utilizing M06-2X functional and the 6-31G(d,p) basis set in both gas and solvent phases. Results showed that the FLU molecule is adsorbed horizontally on the PNS surface with an adsorption energy (Eads) of -18.64 kcal mol-1. The energy gap (Eg) between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) of PNS remains constant after the adsorption process. The adsorption behavior of PNS is not affected by carbon and nitrogen doping. The dynamical behavior of PNS-FLU was studied at T = 298, 310, and 326 K reminiscent of room temperature, body temperature, and temperature of the tumor after exposure to 808 nm laser radiation, respectively. The D value decreases significantly after the equilibration of all systems so that the equilibrated value of D is about 1.1 × 10-6, 4.0 × 10-8, and 5.0 × 10-9 cm2 s-1 at T = 298, 310, and 326 K, respectively. About 60 FLU molecules can be adsorbed on both sides of each PNS, indicating its high loading capacity. PMF calculations demonstrated that the release of FLU from PNS is not spontaneous, which is favorable from a sustained drug delivery point of view.

Dissociative chemisorption of O2 on Agn and Agn−1Ir (n = 3–26) clusters: a first-principle study

Lower dissociation barriers and higher reaction rates of O₂ on doped Ag_n−1Ir clusters, and a gradually weakened dopant effect.

Orbital-Weighted Dual Descriptor for the Study of Local Reactivity of Systems with (Quasi-) Degenerate States

An alternative response function, based on the dual descriptor in terms of Koopmans' approximation, is hereby proposed for the description of chemical reactivity in systems with (quasi-) degenerate frontier molecular orbitals. This descriptor is constructed from Fukui functions that include contributions from different orbitals, i.e., orbital-weighted Fukui functions. The methodology is applied to three case studies: the first case consists of a series of benchmark organic and inorganic molecules from which the dual descriptor, based only on frontier orbitals, is not appropriate to describe their reactivity. The second case deals with the proper description of chemical reactivity in Diels-Alder reactions between fullerene C₆₀ and cyclopentadiene (CP), revealing the importance of considering secondary orbital interactions for an adequate regioselectivity description. The third, and last case, consists of a series of polycyclic aromatic hydrocarbons (PAHs) possessing molecular orbital degeneracy. By means of analyzing of this descriptor, an alternative approach to the description of aromaticity is proposed. In all cases, the proposed index called "orbital-weighted dual descriptor" has proven to accurately describe the chemical reactivity and aromaticity of the studied systems.

Effect of atomicity on the oxidation of cationic copper clusters studied using thermal desorption spectrometry

The resistivity to oxidation of small copper clusters, Cun+ (n ≤ 5), in the gas phase with a precise atomicity at the molecular level was investigated using a combination of thermal desorption spectrometry and mass spectrometry. Oxide clusters, CunOm+, with more O atoms than those present with a stoichiometry of n : m = 1 : 1 were produced at room temperature in the presence of O2, and the weakly bound excess oxygen atoms involved in the clusters were removed by post heating. Non-oxidized Cu2+ and Cu3+ clusters were formed in the range of 323-923 K, whereas partially oxidized clusters, Cu4O2+ and Cu5O2+, were generated for n = 4 and 5. Considering the fact that CunOm+ (m = n/2 + 1) tends to be generated for n ≥ 6, the small copper clusters were concluded to be resistive to oxidation. The possible reaction paths for the oxidation of Cu2+ and Cu4+ clusters were obtained by density functional calculations, which were consistent with the experimental findings. The oxidation states of the Cu atoms in the clusters were discussed based on the natural charges of the atoms.

Varying oxygen coverage on Cu55 and its effect on CO oxidation

Adsorption of molecular oxygen on a Cu₅₅ cluster and the resulting oxidation effects have been investigated by spin-polarized density functional theory (DFT). The optimal structure for each Cu₅₅O_2N (N = 1-20) complex has been obtained via a sequential addition of O₂ and systematic screening of the preferable adsorption sites. Upon structural optimization, several O₂ molecules dissociate readily on Cu₅₅ at different oxygen coverages, and further DFT molecular dynamics simulations at 300 K confirm the instability (small dissociation barrier) of the remaining O₂ and a spontaneous movement of some oxygen atoms from the surface sites towards the cluster interior. The Cu₅₅ cluster and its oxidized derivatives have been placed on a γ-Al₂O₃(100) surface to study the cluster-support interaction, and furthermore, CO oxidation reactions on both Cu₅₅(O)_2N and Cu₅₅(O)_2N/γ-Al₂O₃(100) have been studied as a function of oxygen coverage. The CO oxidation reaction barrier is rather insensitive to the oxygen coverage regardless of the support, indicating a small increase in activity with the number of surface oxygen atoms.

Structures of Cu n+ ( n = 3-10) Clusters Obtained by Infrared Action Spectroscopy

Coinage metal clusters are of great importance for a wide range of scientific fields, ranging from microscopy to catalysis. Despite their clear fundamental and technological importance, the experimental structural determination of copper clusters has attracted little attention. We fill this gap by elucidating the structure of cationic copper clusters through infrared (IR) photodissociation spectroscopy of Cu _n⁺-Ar _m complexes. Structures of Cu _n⁺ ( n = 3-10) are unambiguously assigned based on the comparison of experimental IR spectra in the 70-280 cm^-1 spectral range with spectra calculated using density functional theory. Whereas Cu₃⁺ and Cu₄⁺ are planar, starting from n = 5, Cu _n⁺ clusters adopt 3D structures. Each successive cluster size is composed of its predecessor with a single atom adsorbed onto the face, giving evidence of a stepwise growth.

Nanostructured copper/copper oxide hybrids: combined experimental and theoretical studies

The inhomogeneous distribution of Al and Cu atoms in sintered precursors makes the released Cu atoms susceptible to oxidation during dealloying.

CO oxidation catalyzed by neutral and anionic Cu20 clusters: relationship between charge and activity

Reactions of CO and O2 on neutral and anionic Cu20 clusters have been investigated by spin-polarized density functional theory. Three reaction mechanisms of CO oxidation are explored: reactions with atomic oxygen (dissociated O2) as well as reactions with molecular oxygen, including Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. The adsorption energies, reaction pathways, and reaction barriers for CO oxidation are calculated systematically. The anionic Cu20(-) cluster can adsorb CO and O2 more strongly than the neutral counterpart due to the superatomic shell closing of 20 valence electrons which leaves one electron above the band gap. The activation of O2 molecule upon adsorption is crucial to determine the rate of CO oxidation. The CO oxidation proceeds efficiently on both Cu20 and Cu20(-) clusters, when O2 is pre-adsorbed dissociatively. The ER mechanism has a lower reaction barrier than the LH mechanism on the neutral Cu20. In general, CO oxidation occurs more readily on the anionic Cu20(-) (effective reaction barriers 0.1-0.3 eV) than on the neutral Cu20 cluster (0.3-0.5 eV). Moreover, Cu20(-) exhibits enhanced binding for CO2. From the analysis of the reverse direction of CO oxidation, it is observed that the transition of CO2 to CO + O can occur on the Cu20(-) cluster, which demonstrates that Cu clusters may serve as good catalyst for CO2 chemistry.

Density functional study of structural and electronic properties of small binary Be(n)Cu(m) (n + m = 2~7) clusters

The geometrical structures, electronic properties and relative stabilities of small bimetallic Be n Cu m (n + m = 2-7) clusters have been systematically investigated by using a density functional method at the B3PW91 level. In the most stable structures of Be n Cu m , the Be atoms tend to gather together and construct similar configurations to those of pure Be n clusters. Meanwhile, there is a tendency for Cu atoms to segregate toward the Be n cluster surface. The successive binding energies, cohesive energies, second difference of energies, the highest occupied-lowest unoccupied molecular orbital energy gaps and chemical hardness of Be n Cu m are also investigated. All of them demonstrate that the clusters with even number of copper atoms present relatively higher stabilities. The natural population analyses on the Be n Cu m clusters reveal that, the charge transfers from Be to Cu when the average coordination numbers (Nc) of Be atom is less than 3, whereas the charge-transferring direction reverses when Nc(Be) increases.

Why is quercetin a better antioxidant than taxifolin? Theoretical study of mechanisms involving activated forms

The stronger antioxidant capacity of the flavonoid quercetin (Q) compared with taxifolin (dihydroquercetin, T) has been the subject of previous experimental and theoretical studies. Theoretical work has focused on the analysis of hydrogen bond dissociation energies (BDE) of the OH phenolic groups, but consider mechanisms that only involve the transfer of one hydrogen atom. In the present work we consider other mechanisms involving a second hydrogen transfer in reactions with free radicals. The relative stability of the radicals formed after the first hydrogen transfer reaction is considered in discussing the antioxidant activity of Q and T. In terms of global and local theoretical reactivity descriptors, we propose that the radical arising from Q should be more persistent in the environment and with the capability to react with a second radical by hydrogen transfer, proton transfer and electron transfer mechanisms. These mechanisms could be responsible of the stronger antioxidant capacity of Q.

Structures and Bonding Nature of Small Monoligated Copper Clusters (HCN−Cun, n = 1−3) through High-Resolution Infrared Spectroscopy and Theory

The structures, C-H stretching frequencies, and dipole moments of HCN-Cun (n = 1-3) clusters are determined through high-resolution infrared spectroscopy. The complexes are formed and probed within superfluid helium droplets, whereby the helium droplet beam is passed over a resistively heated crucible containing copper shot and then through a gas HCN pickup cell. All complexes are found to be bound to the nitrogen end of the HCN molecule and on the "atop site" of the copper cluster. Through the experimental C-H vibrational shifts of HCN-Cun and ab initio calculations, it was found that the HCN-metal interaction changes from a strong van der Waals bond in n = 1 to a partially covalent bond in HCN-Cu3. Comparisons with existing infrared data on copper surfaces show that the HCN-Cun bond must begin to weaken at very large copper cluster sizes, eventually returning to a van der Waals bond in the bulk copper surface case.

Effect of Ni and Pd on the Geometry, Electronic Properties, and Active Sites of Copper Clusters

The geometry, electronic properties, and active sites of copper clusters doped with Ni or Pd atoms, Cu(n)()(-)(1)M (n = 2-6; M = Ni, Pd) have been investigated using first-principles methods. Planar structures are energetically favorable in Cu(n)()(-)(1)Ni (n = 2-6). However, for Pd-doped clusters, three-dimensional structures are competitive in energy, and for n = 6, the most stable structure is not planar. Several properties of doped copper clusters present odd-even oscillations as the number of copper atoms grow. The different atomic ground-state configuration of Ni and Pd determines the bonding and electronic properties of doped copper clusters. The interaction between impurities and copper atoms can modify the chemical hardness and active sites of doped copper clusters markedly inducing directionality in the reactivity. This effect is relevant to the behavior of catalysts as well as in the growth of metallic films.

Molecular Structure and Bonding of Copper Cluster Monocarbonyls CunCO (n = 1−9)

In this work we analyze CO binding on small neutral copper clusters, Cun (n = 1-9). Molecular structures and reactivity descriptors of copper clusters are computed and discussed. The results show that the condensed Fukui functions and the frontier molecular orbital theory are useful tools to predict the selectivity of CO adsorption on these small clusters. To get further insight into the CO binding to copper clusters, an energy decomposition analysis of the CO binding energy is performed. The Cs symmetry of the formed CunCO clusters (n = 1-8) allows the separation between the orbital interaction terms corresponding to donation and back-donation. It is found that, energetically, the donation is twice as important as back-donation.