Chemically reactive species remain alive inside carbon nanotubes: a density functional theory study

Theoretical understanding of stability of mechanically interlocked carbon nanotubes and their precursors

Dispersion-corrected DFT calculations were performed on (a,a) nanotubes (a = 5-10) attached by a U-shaped functional group consisting of p-xylene-linked double 9,10-di(1,3-dithiol-2-ylidene)-9,10-dihydro anthracene terminated by C_nH_2n chains (n = 6, 8, and 9), and their ring-closing macrocycles containing tubes. The reactant precursors and macrocycles are denoted by UP-n-(a,a) and (a,a)@Cycle-n, respectively. We found that UP-n-(a,a) are energetically preferable relative to the dissociation limit toward a U-shaped functional group (UP-n) and a tube (initial state) due to the attractive CH-π and π-π interactions. The attractive interactions are enhanced by increasing the tube diameters and C_nH_2n chain lengths because UP-n structures can be easily adjusted to interact with the tubes. The stability of (a,a)@Cycle-n and related (a,b)@Cycle-n is sensitive to tube diameters due to the restriction of ring structures. When diameter differences between a Cycle-n and a tube (D-d) are larger than 5 Å, (a,a)@Cycle-n plus C₂H₄ are energetically preferable relative to the initial state. However, the (a,a)@Cycle-n plus C₂H₄ byproduct is always energetically unstable relative to UP-n-(a,a). The DFT calculations found that the energy differences were low at D-d values ranging from 7 to 8 Å, explaining the tube-diameter-selective formation of the mechanically-interlocked tubes, observed experimentally.

Importance of the alignment of polar π conjugated molecules inside carbon nanotubes in determining second-order non-linear optical properties

Dispersion-corrected DFT calculations found energetically preferred alignments of certain p,p′-dimethylaminonitrostilbene (DANS) molecules inside an carbon nanotube, and their importance in determining second-order non-linear optical properties.

Hydrogen bonding inside and outside carbon nanotubes: HF dimer as a case study

In this theoretical work we analyze the noncovalent interactions of molecular complexes formed between the hydrogen bonded HF dimer and single-walled carbon nanotubes (SWCNTs) of different diameters. In particular, the interaction energies of: (i) spatially confined hydrogen fluoride molecules and (ii) HF dimer and the exterior or interior of SWCNTs are investigated. The computations are carried out in a supermolecular manner using the M06-2X exchange-correlation functional. In order to establish the influence of mutual orientation of the hydrogen fluoride dimer and molecular carbon cages on the analyzed energetic parameters energy scans are performed. Furthermore, changes in the charge distribution of the investigated endo- and exohedral complexes are studied employing the Natural Bond Orbital analysis. Among others, the position of the HF dimer with respect to the carbon cages proves to have a significant influence on the analyzed quantities. The results of our study also indicate that the HF dimer interacts stronger with the interior rather than the exterior of SWCNTs. Moreover, a substantial enhancement of the basis set superposition error is disclosed.

Roles of water molecules in trapping carbon dioxide molecules inside the interlayer space of graphene oxides

Density functional theory (DFT) calculations were employed to investigate the energetics of carbon dioxide migration within hydrated or anhydrous graphene oxides (GOs). When anhydrous GO structures contain a carbon dioxide molecule, the carbon dioxide interacts repulsively with the GO layers to increase the interlayer spacing. The repulsive electrostatic interactions are reduced by the insertion of water molecules into CO2-containing GO structures due to the occurrence of attractive water-layer interactions through hydrogen bonding. Consequently, the interlayer spacings in CO2-containing hydrated structures are shortened compared with those in the anhydrous structures. The results indicate that the intercalated water molecules have the ability to connect the GO layers in the presence of carbon dioxide. Furthermore, the DFT calculations indicated that the GO interlayer spacings, which are influenced by the intercalation of water molecules, control carbon dioxide migration within the GO layers. The importance of the interlayer spacings on the migration of carbon dioxide arises from the occurrence of repulsive interactions between CO2 and oxygen-containing groups attached on the graphene sheets. When the GO interlayer spacings are short due to the presence of intercalated water molecules, the repulsive interactions between carbon dioxide and the GO layers are strong enough to prevent CO2 from migrating from its original position. Such repulsive interactions do not occur during the migration of CO2 within anhydrous GO structures because of the relatively longer interlayer spacing. Accordingly, CO2 migrates within anhydrous GO with a less significant barrier, indicating that carbon dioxide molecules are easily released from the GO.

Key factors in determining the arrangement of π-conjugated oligomers inside carbon nanotubes

Dispersion corrected DFT calculations found different arrangements of π-conjugated oligomers inside a carbon nanotube, dependent on the type of oligomer, which are responsible for determining the oligomers’ electronic properties.

Encapsulation and convex-face thiozonolysis of triatomic sulfur (S(3)) with carbon nanotubes

Nanotubes are a class of host cavities increasingly used to encapsulate unstable molecules, yet none have been exploited to host reactive sulfur species, such as thiozone (S(3)). In this paper, density functional theory and (ONIOM) calculations were used to compute single-walled carbon nanotube (SWNT)-thiozone combinations for the inclusion of S(3) into the hollow nanotube space and to rationalize when 1,2,3-thiozonide formation can take place. Nanotube diameter selectivity for the isomerization of the C(2v) form of S(3) to the D(3h) form proved to be elusive. Acyclic C(2v) S(3) was ~6 kcal/mol more stable than cyclic D(3h) S(3) whether it was free or encapsulated within an SWNT. 1,2,3-Thiozonide formation took place on the convex side of nanotubes of low tube radii, such as the armchair (4,4) and (5,5) SWNTs. In terms of the reaction mode of C(2v) S(3), the 1,3-dipolar addition reaction was preferred compared with the [2 + 2] cycloaddition and chelotrope paths.

Behavior of Ag3 clusters inside a nanometer-sized space of ZSM-5 zeolite

We found from DFT calculations that Ag-Ag orbital interactions as well as Ag-O electrostatic interactions determine the structures of three silver cations inside a nanometer-sized cavity of ZSM-5 (Ag(3)-ZSM-5) in lower and higher spin states. Both interactions strongly depend on the number of Al atoms substituted for Si atoms on the ZSM-5 framework (ZSM-5(Al(n))), where n ranges from 1 to 3. In smaller n, stronger Ag-Ag orbital interactions and weaker Ag-O electrostatic interactions operate. Accordingly, there are significant dependencies of the structures of three silver cations on the number of Al atoms. In lower spin states of Ag(3)-ZSM-5(Al(1)) and Ag(3)-ZSM-5(Al(2)), D(3h)-like triangle clusters are contained inside ZSM-5 whereas their higher spin states have triangle clusters distorted significantly from the D(3h) structure. In lower spin states, the totally symmetric orbital consisting of 5s(Ag) orbitals is responsible for cluster formation, whereas in higher spin states occupation of a 5s(Ag)-based orbital with one node results in significant distortion of the triangle clusters. The distortion can be partially understood by analogies to Jahn-Teller distortion of the bare D(3h) Ag(3)(+) cluster in the triplet spin state. When n is 3, we found that three silver cations are isolated in a lower spin state and that a linear cluster consisting of two silver cations is formed in a higher spin state. Thus, we demonstrate from DFT calculations that the number of Al atoms can control the properties of three silver cations inside a ZSM-5 cavity. Since the structural and electronic features of the enclosed silver clusters can link to their catalytic properties, the DFT findings can help us to understand the catalytic activity of Ag-ZSM-5.