Efficient evaluation of poly-electron populations in natural bond orbital analysis

An Intramolecular-Lock Facilitates Planar Tricyclic Fused Energetic Compounds

Exploiting novel fused cyclic frameworks through simple and efficient methods has provided a blueprint for developing advanced explosives. In this study, six new [5,6,5]-tricyclic fused energetic compounds (I-VI) were synthesized through an intramolecular cyclization strategy involving a C-NH2 directed cyclization reaction. The work not only boosts the development of fused cyclic energetic compounds but also highlights their potential applications as secondary or heat-resistant explosives.

QM Calculations Revealed that Outer-Sphere Electron Transfer Boosted O-O Bond Cleavage in the Multiheme-Dependent Cytochrome bd Oxygen Reductase

The cytochrome bd oxygen reductase catalyzes the four-electron reduction of dioxygen to two water molecules. The structure of this enzyme reveals three heme molecules in the active site, which differs from that of heme-copper cytochrome c oxidase. The quantum chemical cluster approach was used to uncover the reaction mechanism of this intriguing metalloenzyme. The calculations suggested that a proton-coupled electron transfer reduction occurs first to generate a ferrous heme b₅₉₅. This is followed by the dioxygen binding at the heme d center coupled with an outer-sphere electron transfer from the ferrous heme b₅₉₅ to the dioxygen moiety, affording a ferric ion superoxide intermediate. A second proton-coupled electron transfer produces a heme d ferric hydroperoxide, which undergoes efficient O-O bond cleavage facilitated by an outer-sphere electron transfer from the ferrous heme b₅₉₅ to the O-O σ* orbital and an inner-sphere proton transfer from the heme d hydroxyl group to the leaving hydroxide. The synergistic benefits of the two types of hemes rationalize the highly efficient oxygen reduction repertoire for the multi-heme-dependent cytochrome bd oxygen reductase family.

Adsorption of Industrial Gases (CH4, CO2, and CO) on Olympicene: A DFT and CCSD(T) Investigation

Olympicene C₁₉H₁₂, an organic semiconductor, is investigated as an adsorption material for toxic industrial gas molecules such as CH₄, CO₂, and CO. A deep insight of complexation of CH₄, CO₂, and CO with olympicene (analyte@OLY) was obtained by interaction energy, symmetry-adopted perturbation theory (SAPT2+), quantum theory of atoms in molecules (QTAIM), density of states (DOS), noncovalent interaction (NCI), and frontier molecular orbital and natural bond orbital analysis. Domain-based local pair natural orbital coupled cluster theory single-point energy calculations were performed using the cc-pVTZ basis set in combination with corresponding auxiliary cc-pVTZ/JK and cc-pVTZ/C basis sets. For all property calculations of doped olympicene complexes, the ωB97M-V functional was employed. The stability trend for interaction energies is CO₂@OLY > CH₄@OLY > CO@OLY. QTAIM and NCI analysis confirmed the presence of NCIs, where the dispersion factor (in CH₄@OLY) has the highest contribution, as revealed from SAPT2+. The chemical sensitivity of the system was evidenced by the origination of new energy states in DOS spectra. The recovery time for the analyte@OLY complex was calculated at 300 K, and an excellent recovery response was observed. All results evidently indicated weak interactions of the olympicene surface with CH₄, CO₂, and CO.

NBO 7.0: New vistas in localized and delocalized chemical bonding theory

We briefly outline some leading features of the newest version, NBO 7.0, of the natural bond orbital (NBO) wavefunction analysis program. Major extensions include: (1) a new NPEPA module implementing Karafiloglou's "polyelectron population analysis" in the NBO framework; (2) new RDM2 program infrastructure for describing electron correlation effects based on full evaluation of the second-order reduced density matrix; (3) improved convex-solver implementation of natural resonance theory (NRT), allowing a greatly expanded range of applications and associated "resonance NBO" (RNBO) visualization of chemical reactivity; (4) a variety of other improvements in well-established NBO algorithms. We also provide brief introduction to the new NBOPro@Jmol utility program, a plugin to the Jmol chemical structure viewer that serves as a convenient tool to provide on-demand NBO descriptors or orbital visualizations for a broad variety of chemical inquiries in research or classroom applications. © 2019 Wiley Periodicals, Inc.

Analysis of pseudo jahn-teller distortion based on natural bond orbital theory: Case study for silicene

Ground state (GS) instability of nondegenerate molecules in high symmetric structures is understood through Pseudo Jahn-Teller mixing of the electronic states through the vibronic coupling. The general approach involves setting up of a Pseudo Jahn-Teller (PJT) problem wherein one or more symmetry allowed excited states couple to the GS to create vibrational instability along a normal mode. This faces two major complications namely (1) estimating the adiabatic potential energy surfaces for the excited states which are often difficult to describe in case the excited states have charge-transfer or multi-excitonic (ME) character and (2) finding out how many such excited states (all satisfying the symmetry requirements for vibronic coupling) of increasing energies need to be coupled with the GS for a particular PJT problem. An analogous alternative approach presented here for the well-known case of symmetry breaking of planar (D_6h ) hexasilabenzene (Si₆ H₆ ) to the buckled (D_3d ) structure involves identifying the second-order donor-acceptor, hyperconjugative interactions (E² _i → j ) that stabilize the distorted structure. Following the recent work of Nori-Shargh and Weinhold, one observes that the orbitals involved in the vibronic coupling between the S₀ /S_n states and those for the donor (filled)-acceptor (empty) interactions are identical. In fact, deletion of any particular pair of E² _i → j interaction creates vibrational instability in the buckled structure and as a corollary, deleting it for the planar structure removes its instability. The one-to-one correlation between the natural bond orbital theory and PJT theory assists in an intuitive identification of the relevant (few) excited states from a manifold of computed ones that cause symmetry breaking by vibronic coupling. © 2019 Wiley Periodicals, Inc.