Quantum chemical studies on beryllium hydride oligomers

A Holistic View of the Interactions between Electron-Deficient Systems: Clustering of Beryllium and Magnesium Hydrides and Halides

In the search for common bonding patterns in pure and mixed clusters of beryllium and magnesium derivatives, the most stable dimers and trimers involving BeX2 and MgX2 (X = H, F, Cl) have been studied in the gas phase using B3LYP and M06-2X DFT methods and the G4 ab initio composite procedure. To obtain some insight into their structure, stability, and bonding characteristics, we have used two different energy decomposition formalisms, namely MBIE and LMO-EDA, in parallel with the analysis of the electron density with the help of QTAIM, ELF, NCIPLOT, and AdNDP approaches. Some interesting differences are already observed in the dimers, where the stability sequence observed for the hydrides differs entirely from that of the fluorides and chlorides. Trimers also show some peculiarities associated with the presence of compact trigonal cyclic structures that compete in stability with the more conventional hexagonal and linear forms. As observed for dimers, the stability of the trimers changes significantly from hydrides to fluorides or chlorides. Although some of these clusters were previously explored in the literature, the novelty of this work is to provide a holistic approach to the entire series of compounds by using chemical bonding tools, allowing us to understand the stability trends in detail and providing insights for a significant number of new, unexplored structures.

Computational and spectroscopic studies of biologically active coumarin-based fluorophores

We used density functional theory (DFT) calculations to examine the various molecular properties of two coumarin derivatives, namely 4-(5-amino-[1,3,4]thiadiazol-2-ylsulfanylmethyl)-7-methyl-chromen-2-one and 4-(5-amino-[1,3,4]thiadiazol-2-ylsulfanylmethyl)-7-methoxy-chromen-2-one at different levels of theory and basis sets. The calculated highest occupied molecular orbital and lowest unoccupied molecular orbital energies revealed that the investigated molecules were chemically active with a tendency for molecular interactions. The theoretical vibrational frequencies of these molecules were found to be consistent with the experimentally obtained frequencies. Moreover, solvatochromic measurements indicated no significant change in absorption spectral peak by varying the polarity of solvent. Under the same conditions we found that there was a red shift of 39 nm in the fluorescence spectral peak with increase in solvent polarity. The solvatochromic data were used to estimate excited dipole moments and the change in dipole moment was interpreted based on resonance structure of molecules.

Global minimum beryllium hydride sheet with novel negative Poisson's ratio: first-principles calculations

As one of the most prominent metal-hydrides, beryllium hydride has received much attention over the past several decades, since 1978, and is considered as an important hydrogen storage material. By reducing the dimensionality from 3 to 2, the beryllium hydride monolayer is isoelectronic with graphene; thus the existence of its two-dimensional (2D) form is theoretically feasible and experimentally expected. However, little is known about its 2D form. In this work, by a global minimum search with the particle swarm optimization method via density functional theory computations, we predicted two new stable structures for the beryllium hydride sheets, named α-BeH2 and β-BeH2 monolayers. Both structures have more favorable thermodynamic stability than the recently reported planar square form (Nanoscale, 2017, 9, 8740), due to the forming of multicenter delocalized Be-H bonds. Utilizing the recently developed SSAdNDP method, we revealed that three-center-two-electron (3c-2e) delocalized Be-H bonds are formed in the α-BeH2 monolayer, while for the β-BeH2 monolayer, novel four-center-two-electron (4c-2e) delocalized bonds are observed in the 2D system for the first time. These unique multicenter chemical bonds endow both α- and β-BeH2 with high structural stabilities, which are further confirmed by the absence of imaginary modes in their phonon spectra, the favorable formation energies comparable to bulk and cluster beryllium hydride, and the high mechanical strength. These results indicate the potential for experimental synthesis. Furthermore, both α- and β-BeH2 are wide-bandgap semiconductors, in which the α-BeH2 has unusual mechanical properties with a negative Poisson's ratio of -0.19. If synthesized, it would attract interest both in experiment and theory, and be a new member of the 2D family isoelectronic with graphene.

Fully Hydrogenated Beryllium Nanoclusters

We present the ground state and energetically low structures of BenH2n nanoclusters as predicted using density functional theory (DFT) and employing the M06 meta-hybrid exchange-correlation functional. Results using the M06 functional are benchmarked against high accuracy coupled-cluster CCSD(T) and found to be in excellent agreement. For small values of n, the linear or polymeric form is the lowest energy geometry, while for sizes larger, n > 9 ring type and link type structures are the energetically lowest configurations. This trend has also been observed through ab initio molecular dynamics (AIMD) simulations at finite temperatures. In addition to the binding energies of the structures we report on polymerization energies, Be-H bond energies with respect to coordination details, hydrogen desorption energies of saturated and oversaturated species, as well as computed infrared spectra of all the ground state and energetically low lying structures presented. Furthermore, we find that the saturated polymeric forms of the nanoclusters cannot retain molecular hydrogen, in contrast to what is expected when zero point energy corrections are not taken into account.

Ab initio theoretical investigation of beryllium and beryllium hydride nanoparticles and nanocrystals with implications for the corresponding infinite systems

Using judicially chosen DFT calculations for Be_n and Be_nH_x nanoparticles we predict correctly the n → ∞ behavior for crystals and polymers.