An intercluster reaction between Au(PET) and Ir(PET) producing the alloy cluster, AuIr(PET) exclusively, is demonstrated where the ligand PET is 2-phenylethanethiol. Typical reactions of this kind between Au(PET) and Ag(SR), and other clusters reported previously, produce mixed cluster products. The cluster composition was confirmed by detailed high-resolution electrospray ionization mass spectrometry (ESI MS) and other spectroscopic techniques. This is the first example of Ir metal incorporation in a monolayer-protected noble metal cluster. The formation of a single product was confirmed by thin layer chromatography (TLC). Density functional theory (DFT) calculations suggest that the most favorable geometry of the AuIr(PET) cluster is one wherein the three Ir atoms are arranged triangularly with one Ir atom at the icosahedral core and the other two on the icosahedral shell. Significant contraction of the metal core was observed due to strong Ir-Ir interactions.
The interaction between graphene-based nanomaterials and modified nucleobases (MBs) is important in the development and design of new biosensors. The adsorption of MBs on the surface of graphene (G), boron-doped graphene (BG), nitrogen-doped graphene (NG), and silicon-doped graphene (SiG) has been investigated using electronic structure calculations and associated analysis methods. It is found from the calculations that the MBs stack with the surface of G, BG, NG, and SiG models and that the π−π stacking interaction plays a dominant role in the stabilization of the intermolecular complexes. The stability of MBs on the surface of SiG is the highest when compared to that of G, BG, and NG models. The highest interaction energies of MBs with SiG is due to the presence of Si•••O(N) and π−π stacking interactions. The theory of atoms in molecules (AIM) analysis indicates that Si•••O(N) interaction has both electrostatic and covalent characters. The calculation of charge transfer by employing the natural bond orbital method showed the donor nature of MBs. It is also found that the variations in the density of states and highest occupied molecular orbital−lowest unoccupied molecular orbital gap of SiG occur upon adsorption of MBs. These results illustrate that SiG can act as a sensor for the detection of MBs.
Experimental evidence for the existence of gas phase isomers in monolayer protected noble metal clusters is presented, taking Ag44(SR)30 (SR = 4-fluorothiophenol, p-mercaptobenzoic acid) and Ag29(BDT)12 (BDT: benzene dithiol) clusters as examples which do not show any isomeric structures in their crystals. Electrospray ionization coupled with ion mobility separation allowed for the identification of multiple isomers of Ag44SR30 cluster in its 3– and 4– charge states, their most abundant gas phase ions. Ag29(BDT)12 showed isomerism in its common 3– charge state. Isomerism is likely to be due to different types of ligand orientations in the staples leading to changes in the overall size and shape of the cluster ions, which was further confirmed by density functional theory calculations on Ag44(FTP)30 4–. No isomers were seen in the ions of the well-known cluster, Au25SR18 (SR = phenylethanethiol, dodecanethiol, and butanethiol).
The interaction of nucleobases (NBs) with the surface of silicon doped graphene (SiGr) and defective silicon doped graphene (dSiGr) has been studied using electronic structure methods. A systematic comparison of the calculated interaction energies (adsorption strength) of NBs with the surface of SiGr and dSiGr with those of pristine graphene (Gr) has also been made. The doping of graphene with silicon increases the adsorption strength of NBs. The introduction of defects in SiGr further enhances the strength of interaction with NBs. The appreciable stability of complexes (SiGr-NBs and dSiGr-NBs) arises due to the partial electrostatic and covalent (Si···O(N)) interaction in addition to π-π stacking. The interaction energy increases with the size of graphene models. The strong interaction between dSiGr-NBs and concomitant charge transfer causes significant changes in the electronic structure of dSiGr in contrast to Gr and SiGr. Further, the calculated optical properties of all the model systems using time dependent density functional theory (TD-DFT) reveal that absorption spectra of SiGr and dSiGr undergo appreciable changes after adsorption of NBs. Thus, the significant variations in the HOMO-LUMO gap and absorption spectra of dSiGr after interaction with the NBs can be exploited for possible applications in the sensing of DNA nucleobases.
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