Subhajit Mandal

Cucurbit[6]uril: A Possible Host for Noble Gas Atoms.

Density functional and ab initio molecular dynamics studies are carried out to investigate the stability of noble gas encapsulated cucurbit[6]uril (CB[6]) systems. Interaction energy, dissociation energy and dissociation enthalpy are calculated to understand the efficacy of CB[6] in encapsulating noble gas atoms. CB[6] could encapsulate up to three Ne atoms having dissociation energy (zero-point energy corrected) in the range of 3.4-4.1 kcal/mol, whereas due to larger size, only one Ar or Kr atom encapsulated analogues would be viable. The dissociation energy value for the second Ar atom is only 1.0 kcal/mol. On the other hand, the same for the second Kr is -0.5 kcal/mol, implying the instability of the system. The noble gas dissociation processes are endothermic in nature, which increases gradually along Ne to Kr. Kr encapsulated analogue is found to be viable at room temperature. However, low temperature is needed for Ne and Ar encapsulated analogues. The temperature-pressure phase diagram highlights the region in which association and dissociation processes of Kr@CB[6] would be favorable. At ambient temperature and pressure, CB[6] may be used as an effective noble gas carrier. Wiberg bond indices, noncovalent interaction indices, electron density, and energy decomposition analyses are used to explore the nature of interaction between noble gas atoms and CB[6]. Dispersion interaction is found to be the most important term in the attraction energy. Ne and Ar atoms in one Ng entrapped analogue are found to stay inside the cavity of CB[6] throughout the simulation at 298 K. However, during simulation Ng2 units in Ng2@CB[6] flip toward the open faces of CB[6]. After 1 ps, one Ne atom of Ne3@CB[6] almost reaches the open face keeping other two Ne atoms inside. At lower temperature (77 K), all the Ng atoms in Ngn@CB[6] remain well inside the cavity of CB[6] throughout the simulation time (1 ps).

Pyrazine Functionalized Ag(I) and Au(I)-NHC Complexes are Potential Antibacterial Agents

Antimicrobial resistance is an ever-increasing problem throughout the world and has already reached severe proportions. Bacteria can develop ways to render traditional antibiotics ineffective, raising a crucial need to find new antimicrobials with novel mode of action. We demonstrate here a novel class of pyrazine functionalized Ag(I) and Au(I)-NHC complexes as antibacterial agents against human pathogens that are resistant to several antibiotics. Complete synthetic and structural studies of Au(I) and Ag(I) complexes of 2-(1-methylimidazolium) pyrimidinechloride (L-1), 2,6-bis(1-methylimidazol)pyrazinechloride (L-2) and 2,6-bis(1-methyl imidazol) pyrazinehexa-fluorophosphate (L-3) are reported herein. Chloro[2,6-bis(1-methyl imidazol)pyrazine]gold(I), 2b and chloro [2,6-bis(1-methyl imidazol)pyrazine]silver(I), 2a complexes are found to have more potent antimicrobial activity than other synthesized compounds and several conventionally used antibiotics. Complexes 2b and 2a also inhibit the biofilm formation by Gram-positive bacteria, Streptococcus mutans and Gram-negative bacteria, Escherichia coli, causing drastic damage to the bacterial cell wall and increasing membrane permeability. Complexes 2b and 2a strongly binds to both Lys and Dap-Type peptidoglycan layers, which may be the reason for damage to the bacterial cell wall. Theoretical studies of all the complexes reveal that 2b and 2a are more reactive than other complexes, and this may be the cause of differences in antibacterial activity. These findings will pave the way towards developing a new class of antibiotics against different groups of conventional antibiotic-resistant bacteria.

Noble gas supported B3+ cluster: formation of strong covalent noble gas–boron bonds

The stability of noble gas (Ng) bound B3+ clusters is assessed via an in silico study, highlighting their structure and the nature of the Ng–B bonds. Ar to Rn atoms are found to form exceptionally strong bonds with B3+ having each Ng–B bond dissociation energy in the range of 15.1–34.8 kcal mol−1 in B3Ng3+ complexes with a gradual increase in moving from Ar to Rn. The computed thermochemical parameters like enthalpy and free energy changes for the Ng dissociation processes from B3Ng3+ also support the stability of Ar to Rn analogues for which the corresponding dissociation processes are endergonic in nature even at room temperature. The covalent nature of the Ng–B bonds is indicated by the localized natural Ng–B bond orbitals and high Wiberg bond indices (0.57–0.78) for Ng–B bonds. Electron density analysis also supports the covalency of these Ng–B bonds where the electron density is accumulated in between Ng and B centres. The orbital interaction energy is the main contributor (ca. 63.0–64.4%) of the total attraction energy in Ng–B bonds. Furthermore, the Ng–B bonding can be explained in terms of a donor–acceptor model where the Ng (HOMO) → B3Ng2+ (LUMO) σ-donation has the major contribution.

Structure-selectivity relationship in ruthenium-catalyzed regio- and stereoselective addition of alkynes to pyrazoles: an experimental and theoretical investigation.

Ruthenium(II) complexes, [Ru(dppe)(PPh3)(CH(3)CN)(2)Cl][BPh4] {dppe = diphenylphosphinoethane} (1) and [Ru(dppp)2(CH(3)CN)Cl][BPh4] (2){dppp = diphenylphosphinopropane}, are efficient catalysts for vinylation of pyrazoles by alkynes. While the 1-catalyzed reaction is trans-selective, the corresponding 2-catalyzed reaction is cis-selective. The experimental results have been rationalized by density functional theory calculations.