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Introduction of Carbon Nanotubes
Published in Abhay Kumar Singh, Tien-Chien Jen, Chalcogenide, 2021
Abhay Kumar Singh, Tien-Chien Jen
The third and last possibility of hybridization, in which mixing one 2s orbital and one 2p orbital atomic orbitals with four can lead to the formation of two sp hybrid orbitals, where each filled orbital can have one electron [Fig. 3.1(c)]. Their geometry could be in a linear form with the sp orbitals angle of 180°. And the remaining two p-type orbitals cannot mix perpendicularly to each other. Under such a configuration the two sp hybrid orbitals can form σ bonds with the two nearest neighbors, while the side-by-side overlap of the two unmixed pure p orbitals can form π bonds between the carbon atoms. Such configurations are accounted for the carbon–carbon triple bond formation (one σ bond and two π bonds). The most typical example is acetylene (H−C≡C−H) that the linear molecule can satisfy this specific bonding condition. The carbon based materials also have the ability to form one-dimensional chains, typically known as carbynes. They are traditionally classified as cumulene that have monoatomic chains with double bonds, =C=C=. A similar form of polyyne are those that have dimerized chains with alternating single and triple bonds, −C≡C−. Although sp2 and sp3 carbon-based materials structures have been extensively synthesized and characterized, but precise synthesis of the carbynes is still challenging due to their high reactivity of chain ends as well as a strong tendency to form interchain crosslinking [3]. Cataldo was the first to synthesize the linear carbon chains up to a few tens of atoms adopting the chemical route, the chain ends stabilization with the nonreactive terminal groups were provided by Kavan and Lagow et al. [4–6]. Though, all the demonstrated systems consisted of a mixture of carbons and other chemical elements. Moreover, a pure carbon environment was also achieved using the supersonic cluster beam deposition as well as the electronic irradiation of a single graphite basal plane (graphene) inside a transmission electron microscope [7–11].
DFT approach on stability and conductance of nine different polyyne and cumulene molecules
Published in Molecular Physics, 2020
AbhayRam Balakrishnan, R. Shankar, S. Vijayakumar
In this study, both polyyne molecules and cumulene molecules are considered. The crystallographic structural data based on X-ray crystallography was available for Mes[n] (n = 6, 8 and 10) and Ph[n] (n = 4, 8, 10 and 12) chains [1,3]. Mes[n] chains are cumulenes, whereas Ph[n] chains are polyynes. Figures 2 and 3 show a comparison of the experimental and theoretical bond lengths for Mes[n] and Ph[n] chains respectively. It can be clearly seen that almost all the values agreed well to that of the experimental ones and the deviation is found very small. This shows that the level of theory used in this study is accurate enough in calculating the structural properties of those molecules. The small error may be due to the fact that the experimental values are taken for molecules in crystal form whereas theoretical values are for isolated molecules. The values of closely packed crystal structures may vary slightly from that of isolated structure. This may be the reason for the small deviation in the structural values. Still the error is only negligible value and the level of theory correctly predicts the structure for these molecules and we can assume that the same theoretical calculations are accurate for similar types of molecules.
Tunneling rules for electronic transport in 1-D systems
Published in Molecular Physics, 2021
C.A.B. da Silva, K.R. Nisioka, M. Moura-Moreira, R.F. Macedo, J. Del Nero
In the simulation, the bond length is not forced in polyyne or cumulene. As a result, we obtained the polyyne situation (–C≡C–) [34], which has the diameter (width) of 1C atom (≈1.34 Å) and presents 16C atoms (4C left lead – 4C molecule – dn – 4C molecule – 4C right lead) for the molecular device. The electronic transport occurs by tunnel effect or quantum tunneling. The calculations start from a distance (dn) of 4 Å between the polyynes varying each system in 0.1 Å up to a distance (dn) of 2.1 Å.
Infrared photodissociation spectroscopic and theoretical study of H n C4O+ (n = 1, 2) cation clusters in the gas phase
Published in Molecular Physics, 2021
Wei Li, Jiaye Jin, Xiaonan Wu, Xunlei Ding, Guanjun Wang
The delocalised electron number of the HnC4O+ (n = 1, 2) (Ntotal, in e) and each of atom (N, in e), the proportion (N/Ntotal) are calculated to further analyse the delocalisation of electrons and the results are shown in Table 2. The analysis of AdNDP shows that HC4O+ has ten delocalised electrons to form four nc–2e bonds and two nc–1e bonds in C4O chains. The 1C, 2C, 3C, 4C and 5O of HC4O+ contribute 1.5, 2.0, 2.0, 1.6 and 2.9 e to these delocalised nc–2e bonds and the proportion are 15.0%, 20.0%, 20.0%, 16.0% and 29.0%. Each of carbon atoms has a similar contribution to the nc–2e bonds. The electrons are completely delocalised on the C4O chain, resulting in cumulene-like carbon chain structure of HC4O+. As for H2C4O+, which has nine delocalised electrons to form nc–2e bonds in C4O chains. The 1C, 2C, 3C, 4C and 5O of H2C4O+ contribute 0.8, 1.7, 2.1, 1.5 and 2.9 e with the proportion of 8.9%, 18.9%, 23.3%, 16.7%, 32.2% to these delocalised nc–2e bonds. The contribution of carbon atom to the nc–2e bonds is 3C > 2C > 4C ≫ 1C. The carbon atom bonded to the two hydrogen atoms will form three two-centre-two-electron (2c–2e) localised bonds, resulting in polyyne-like 2C–3C–4C carbon chain structure. These results are based on sp and sp2 hybridisation of the CH and CH2 groups which are consistent with previous understanding. So, the number of hydrogen atoms changes the type of terminal CH to CH2 group, causing a remaining polyyne-like C3 chain instead of cumulene-like chain found in HCnO+ cations derivatives.