Novel ultra-hard hexacarbon allotropes from first principles

Novel ultra-hard hexacarbon C6 allotropes are proposed based on crystal chemistry rationale and geometry optimization onto ground state structures. Similar to diamond, the orthorhombic, tetragonal and trigonal C6 are cohesive networks of C4 tetrahedra illustrated by charge density projections exhibiting sp3-like carbon hybridization. All three allotropes are identified as mechanically (elastic constants) and dynamically (phonons) stable. The electronic band structures are characteristic of insulators with large band gaps of 4 to 5 eV, like diamond. From three different models evaluating Vickers hardness HV, all new carbon allotropes are identified as ultra-hard.

Ground-State Structures of Hydrated Calcium Ion Clusters From Comprehensive Genetic Algorithm Search

We searched the lowest-energy structures of hydrated calcium ion clusters Ca2+(H2O)n (n = 10–18) in the whole potential energy surface by the comprehensive genetic algorithm (CGA). The lowest-energy structures of Ca2+(H2O)10–12 clusters show that Ca2+ is always surrounded by six H2O molecules in the first shell. The number of first-shell water molecules changes from six to eight at n = 12. In the range of n = 12–18, the number of first-shell water molecules fluctuates between seven and eight, meaning that the cluster could pack the water molecules in the outer shell even though the inner shell is not full. Meanwhile, the number of water molecules in the second shell and the total hydrogen bonds increase with an increase in the cluster size. The distance between Ca2+ and the adjacent water molecules increases, while the average adjacent O-O distance decreases as the cluster size increases, indicating that the interaction between Ca2+ and the adjacent water molecules becomes weaker and the interaction between water molecules becomes stronger. The interaction energy and natural bond orbital results show that the interaction between Ca2+ and the water molecules is mainly derived from the interaction between Ca2+ and the adjacent water molecules. The charge transfer from the lone pair electron orbital of adjacent oxygen atoms to the empty orbital of Ca2+ plays a leading role in the interaction between Ca2+ and water molecules.

Thionitroxyl Radical (H2NS) Isomers: Structures, Vibrational Spectroscopy, Electronic States and Photochemistry

The thionitroxyl radical (H2NS) isomers are characterized using advanced ab initio methodologies. Computations are done using standard and explicitly correlated coupled cluster, CASSCF and MRCI approaches in conjunction with large basis sets, extrapolated to the complete basis set (CBS) limit. The lowest electronic states of different isomers are mapped along the stretching coordinates, thereby confirming the existence of the four already known ground state structures, namely H2NS, H2SN, cis-HNSH and trans-HNSH. Also, it is shown that only the lowest electronic excited states are stable, whereas the upper electronic states may undergo unimolecular decomposition processes forming H + HNS/HSN or the HN + SH or N + H2S or S + NH2 fragments. These data allow an assignment of the deep blue glow observed after reactions between “active nitrogen” and H2S at the beginning of the XXth century. For stable species, a set of accurate structural and spectroscopic parameters are provided. Since small nitrogen-sulfur molecular species are of astrophysical relevance, this work may help for identifying the thionitroxyl radical isomers in astrophysical media and in the laboratory.

Spontaneous dimensional reduction and novel ground state degeneracy in a simple chain model

Chain molecules play a key role in the polymer field and in living cells.Our focus is on a new homopolymer model of a linear chain molecule subject to an attractive self-interaction promoting compactness. We analyze the model using simple analytic arguments complemented by extensive computer simulations. We find several striking results: there is a first order transition from a high temperature random coil phase to a highly unusual low temperature phase; the modular ground states exhibit significant degeneracy; the ground state structures exhibit spontaneous dimensional reduction and have a two-layer structure; and the ground states are assembled from secondary motifs of helices and strands connected by tight loops. We discuss the similarities and notable differences between the ground state structures (we call these PoSSuM - Planar Structures with Secondary Motifs) in the novel phase and protein native state structures.

A Computational Study of the Relative Aromaticity of Pyrrole, Furan, Thiophene and Selenophene, and Their Diels-Alder Stereoselectivity

The collinearity of terminal <i>p</i> orbitals of a diene with that of a dienophile is required for an effective overlap to result in s bond formation during the Diels-Alder reaction. The ease of the DA reaction of a cyclic diene with a given dienophile, therefore, must also depend on the distance between the termini of the diene. A distance larger than the unsaturated bond of the dienophile is expected to raise the energy of activation. This scenario has been amply demonstrated from the study of reactions of several dienes, some designed to serve the purpose, with different dienophiles. The five-ring heterocycles pyrrole, furan, thiophene and selenophene possess varying aromatic character for the varied resonance participation of the heteroatom lone pair with ring p bonds. The aromaticity decreases in the same order due to: (a) the increasing s_C-X (X = heteroatom) bond length lifts the bond uniformity required for ring current, hence aromaticity, such as in benzene and (b) size-mismatch of the interacting lone pair orbital and the ring <i>p</i> orbitals, especially in thiophene and selenophene, both allowing poor overlap in the ground state structures. It is demonstrated that increase alone in the activation energies of the DA reactions of pyrrole, furan, thiophene and selenophene cannot be considered a measure of relative aromaticity as often done and even theoretically attempted in many ways to prove just that. The separation of the termini of the diene has a much larger role in the determination of activation energy, especially in thiophene and selenophene, than their aromaticity profile. There cannot be a measure better than the relative intensity of heteroatom lone pair overlap with ring p bonds, giving rise to a six-electron like system in following Hückel’s 4n+2 rule, to assess the relative aromaticity.

A Computational Study of the Relative Aromaticity of Pyrrole, Furan, Thiophene and Selenophene, and Their Diels-Alder Stereoselectivity

The collinearity of terminal <i>p</i> orbitals of a diene with that of a dienophile is required for an effective overlap to result in s bond formation during the Diels-Alder reaction. The ease of the DA reaction of a cyclic diene with a given dienophile, therefore, must also depend on the distance between the termini of the diene. A distance larger than the unsaturated bond of the dienophile is expected to raise the energy of activation. This scenario has been amply demonstrated from the study of reactions of several dienes, some designed to serve the purpose, with different dienophiles. The five-ring heterocycles pyrrole, furan, thiophene and selenophene possess varying aromatic character for the varied resonance participation of the heteroatom lone pair with ring p bonds. The aromaticity decreases in the same order due to: (a) the increasing s_C-X (X = heteroatom) bond length lifts the bond uniformity required for ring current, hence aromaticity, such as in benzene and (b) size-mismatch of the interacting lone pair orbital and the ring <i>p</i> orbitals, especially in thiophene and selenophene, both allowing poor overlap in the ground state structures. It is demonstrated that increase alone in the activation energies of the DA reactions of pyrrole, furan, thiophene and selenophene cannot be considered a measure of relative aromaticity as often done and even theoretically attempted in many ways to prove just that. The separation of the termini of the diene has a much larger role in the determination of activation energy, especially in thiophene and selenophene, than their aromaticity profile. There cannot be a measure better than the relative intensity of heteroatom lone pair overlap with ring p bonds, giving rise to a six-electron like system in following Hückel’s 4n+2 rule, to assess the relative aromaticity.