ABSTRACT
Carbon has many allotropes possessing unique properties. In this work, we predicted an orthogonal carbon crystal, named ort-C24, with dynamic, mechanical and thermodynamic stability. Studies indicate that it is a topological semimetal having both nodal rings and nodal lines in its Brillouin zone. Ab initio molecular dynamics simulations reveal that it is a rare material having a negative thermal expansion coefficient along the a axis. It also has negative compressibility along the same axis under hydrostatic pressure. Its b axis can bear an astonishing strain of 115% even if the dynamical stability is considered. Tensioning along different axes can either change it into a metal or alter the nodal ring into nodal lines or only modify the shape of the nodal ring, together with the variation of the number of Dirac cones. Theoretically, temperature has a limited influence on its electronic topological properties while a hydrostatic pressure of 5 GPa can alter it noticeably. The simulated X-ray diffraction peaks indicate the possible existence of ort-C24 in carbon soot. These adjustable electronic topological properties may provide us with an interesting platform for studying such topological semimetals.
ABSTRACT
The structures of cationic water clusters (H2O)8+ have been globally explored by the particle swarm optimization method in combination with quantum chemical calculations. Geometry optimization and vibrational analysis for the 15 most interesting clusters were computed at the MP2/aug-cc-pVDZ level and infrared spectrum calculation at MPW1K/6-311++G** level. Special attention was paid to the relationships between their configurations and energies. Both MP2 and B3LYP-D3 calculations revealed that the cage-like structure is the most stable, which is different from a five-membered ring lowest energy structure but agrees well with a cage-like structure in the literature. Furthermore, our obtained cage-like structure is more stable by 0.87 and 1.23 kcal/mol than the previously reported structures at MP2 and B3LYP-D3 levels, respectively. Interestingly, on the basis of their relative Gibbs free energies and the temperature dependence of populations, the cage-like structure predominates only at very low temperatures, and the most dominating species transforms into a newfound four-membered ring structure from 100 to 400 K, which can contribute greatly to the experimental infrared spectrum. By topological analysis and reduced density gradient analysis, we also investigated the structural characteristics and bonding strengths of these water cluster radical cations.
ABSTRACT
The particle swarm optimization method in conjunction with density functional calculations is used to search the lower energy structures for the cationic water clusters (H2O)5(+). Geometry optimization, vibrational analysis, and infrared spectrum calculation are performed for the most interesting clusters at the MP2/aug-cc-pVDZ level. The relationships between their structural arrangements and their energies are discussed. According to their relative Gibbs free energies, their energy order is determined and four lowest energy isomers are found to have a relative population surpassing 1% below 350 K. Studies reveal that, among these four isomers, one new cluster found here also contributes a lot to the experimental infrared spectrum. Based on topological analysis and reduced density gradient analysis, some meaningful points are found by studying the structural characteristics and the bonding strengths of these cationic water clusters: in the first solvation shell, the central H3O(+) motifs may have a stronger interaction with the OH radical than with the water molecules. The interaction in the second solvation shell may also be stronger than that in the first solvation shell, which is opposite to our intuition.
