Abhijit Nag

Fullerene-Functionalized Monolayer-Protected Silver Clusters: [Ag29(BDT)12(C60)n]3– (n = 1–9)

We report the formation of supramolecular adducts between monolayer-protected noble metal nanoclusters and fullerenes, specifically focusing on a well-known silver cluster, [Ag29(BDT)12]3-, where BDT is 1,3-benzenedithiol. We demonstrate that C60 molecules link with the cluster at specific locations and protect the fragile cluster core, enhancing the stability of the cluster. A combination of studies including UV-vis, high-resolution electrospray ionization mass spectrometry, collision-induced dissociation, and nuclear magnetic resonance spectroscopy revealed structural details of the fullerene-functionalized clusters, [Ag29(BDT)12(C60) n]3- ( n = 1-9). Density functional theory (DFT) calculations and molecular docking simulations affirm compatibility between the cluster and C60, resulting in its attachment at specific positions on the surface of the cluster, stabilized mainly by π-π and van der Waals interactions. The structures have also been confirmed from ion mobility mass spectrometry by comparing the experimental collision cross sections (CCSs) with the theoretical CCSs of the DFT-optimized structures. The gradual evolution of the structures with an increase in the number of fullerene attachments to the cluster has been investigated. Whereas the structure for n = 4 is tetrahedral, that of n = 8 is a distorted cube with a cluster at the center and fullerenes at the vertices. Another fullerene, C70, also exhibited similar behavior. Modified clusters are expected to show interesting properties.

Sequential Dihydrogen Desorption from Hydride Protected Atomically Precise Silver Clusters and the Formation of Naked Clusters in the Gas Phase

We report the formation of naked cluster ions of silver of specific nuclearities, uncontaminated by other cluster ions, derived from monolayer-protected clusters. The hydride and phosphine co-protected cluster, [Ag18(TPP)10H16]2+ (TPP, triphenylphosphine), upon activation produces the naked cluster ion, Ag17+, exclusively. The number of metal atoms present in the naked cluster is almost the same as that in the parent material. Two more naked cluster ions, Ag21+ and Ag19+, were also formed starting from two other protected clusters, [Ag25(DPPE)8H22]3+ and [Ag22(DPPE)8H19]3+, respectively (DPPE, 1,2-bis(diphenylphosphino)ethane). By systematic fragmentation, naked clusters of varying nuclei are produced from Ag17+ to Ag1+ selectively, with systematic absence of Ag10+, Ag6+, and Ag4+. A seemingly odd number of cluster ions are preferred due to the stability of the closed electronic shells. Sequential desorption of dihydrogen occurs from the cluster ion, Ag17H14+, during the formation of Agn+. A comparison of the pathways in the formation of similar naked cluster ions starting from two differently ligated clusters has been presented. This approach developed bridges the usually distinct fields of gas-phase metal cluster chemistry and solution-phase metal cluster chemistry. We hope that our findings will enrich nanoscience and nanotechnology beyond the field of clusters.

Isomerism in Supramolecular Adducts of Atomically Precise Nanoparticles

We present isomerism in a few supramolecular adducts of atomically precise nanoparticles, [Ag29(BDT)12∩(CD) n]3- ( n = 1-6), abbreviated as I where BDT and CD are 1,3-benzenedithiol and cyclodextrins (α, β and γ), respectively; ∩ symbolizes an inclusion complex. The different host-guest complexes of I were characterized in the solution state as well as in the gas phase. The CDs (α, β and γ) encapsulate a pair of BDT ligands protecting the Ag29 core. This unique geometry of the supramolecular adducts makes the system similar to octahedral complexes of transition metals, which manifest various isomers. These isomers of I ( n = 2-4) were separated by ion mobility mass spectrometry (IM MS). We proposed structures of all the inclusion complexes with the help of IM MS measurements and molecular docking, density functional theory (DFT), and collision cross section (CCS) calculations.

Monolayer-Protected Noble-Metal Clusters as Potential Standards for Negative-Ion Mass Spectrometry

A detailed mass-spectrometric study of atomically precise monolayer-protected clusters revealed the potential application of such materials as mass-spectrometric standards, mostly in negative-ion mode and in the high-mass range. To date, very few molecules are known that can be efficiently ionized and detected at lower concentrations as negative ions with high signal intensities beyond m/ z 3000. Noble-metal clusters are molecules with definite masses, sizes, and shapes, which makes them excellent candidates to choose as standards over conventional low-molecular-weight polymers or clusters of ionic salts. They may be used as calibrants in all possible modes, including tandem mass spectrometry and ion mobility. With the advancement in materials science, more and more molecules are being added to the list that are inherently negatively charged in solution and can be examined by mass spectrometry. In this report, we demonstrate the use of three such model cluster systems for their potential to calibrate mass spectrometers in negative-ion mode. This idea can be extended to many other clusters known so far to achieve calibration in extended mass ranges.

Bent Keto Form of Curcumin, Preferential Stabilization of Enol by Piperine, and Isomers of Curcumin∩Cyclodextrin Complexes: Insights from Ion Mobility Mass Spectrometry

A detailed examination of collision cross sections (CCSs) coupled with computational methods has revealed new insights into some of the key questions centered around curcumin, one of the most intensively studied natural therapeutic agents. In this study, we have distinguished the structures and conformers of the well-known enol and the far more elusive keto form of curcumin by using ion mobility mass spectrometry (IM MS). The values of the theoretically predicted isomers were compared with the experimental CCS values to confirm their structures. We have identified a bent structure for the keto form and the degree of bending was estimated. Using IM MS, we have also shown that ESI MS reflects the solution phase structures and their relative populations, in this case. Piperine, a naturally occurring heterocyclic compound, is known to increase the bioavailability of curcumin. However, it is still not clearly understood which tautomeric form of curcumin is better stabilized by it. We have identified preferential stabilization of the enol form in the presence of piperine using IM MS. Cyclodextrins (CDs) are used as well-known carriers in the pharmaceutical industry for increasing the stability, solubility, bioavailability, and tolerability of curcumin. However, the crystal structures of supramolecular complexes of curcumin∩CD are unknown. We have determined the structures of different isomers of curcumin∩CD (α- and β-CD) complexes by comparing the CCSs of theoretically predicted structures with the experimentally obtained CCSs, which will further help in understanding the specific role of the structures involved in different biological activities.