Tapan K. Ghanty

How strong is the interaction between a noble gas atom and a noble metal atom in the insertion compounds MNgF (M=Cu and Ag, and Ng=Ar, Kr, and Xe)?

Ab initio molecular orbital calculations have been carried out to investigate the structure and the stability of noble gas insertion compounds of the type MNgF (M=Cu and Ag, and Ng=Ar, Kr, and Xe) through second order Moller-Plesset perturbation method. All the species are found to have a linear structure with a noble gas-noble metal bond, the distance of which is closer to the respective covalent bond length in comparison with the relevant van der Waals limit. The dissociation energies corresponding to the lowest energy fragmentation products, MF+Ng, have been found to be in the range of -231 to -398 kJ/mol. The respective barrier heights pertinent to the bent transition states (M-Ng-F bending mode) are quite high for the CuXeF and AgXeF species, although for the Ar and Kr containing species the same are rather low. Nevertheless the M-Ng bond length in MNgF compounds reported here is the smallest M-Ng bond ever predicted through any experimental or theoretical investigation, indicating strongest M-Ng interaction. All these species (except AgArF) are found to be metastable in their respective potential energy surface, and the dissociation energies corresponding to the M+Ng+F fragments have been calculated to be 30.1-155.3 kJ/mol. Indeed, in the present work we have demonstrated that the noble metal-noble gas interaction strength in MNgF species (with M=Cu and Ag, and Ng=Kr and Xe) is much stronger than that in NgMF systems. Baders [Atoms in molecules-A Quantum Theory (Oxford University Press, Oxford, 1990)] topological theory of atoms in molecules (AIM) has been employed to explore the nature of interactions involved in these systems. Geometric as well as energetic considerations along with AIM results suggest a partial covalent nature of M-Ng bonds in these systems. The present results strengthen our earlier work and further support the proposition on the possibility of experimental identification of this new class of insertion compounds of noble gas atoms containing noble gas-noble metal bond.

Insertion of noble-gas atom (Kr and Xe) into noble-metal molecules (AuF and AuOH): Are they stable?

The structure and the stability of a new class of insertion compounds of noble-gas atoms of the type AuNgX (Ng=Kr, Xe and X=F, OH) have been investigated theoretically through ab initio molecular-orbital calculations. All the species are found to have a linear structure with a noble-gas-noble-metal bond, the distance of which is comparable to covalent bond length except the AuKrOH system, for which it lies in between the covalent and van der Waals limits. The dissociation energies corresponding to the lowest-energy fragmentation products, AuX+Ng have been computed to be -166.2, -276.0, -194.4, and -257.6 kJ/mol for AuXeF, AuKrF, AuXeOH, and AuKrOH, respectively, at the MP2 level of theory. The respective barrier heights corresponding to the bent transition states (Au-Ng-X bending mode) have been calculated to be 119.1, 74.9, 160.7, and 141.6 kJ/mol. However, three of these species are found to be metastable in their respective potential-energy surface, and the dissociation energies corresponding to the Au+Ng+X fragments have been calculated to be 112.9, 3.0, and 18.7 kJ/mol for AuXeF, AuKrF, and AuXeOH, respectively, at the same level of theory. An analysis of the nature of interactions involved in the Au-Ng-X systems has been performed using Baders topological theory of atoms-in-molecules (AIM). Geometric as well as energetic considerations along with AIM results suggest a partial covalent nature of Au-Ng bonds in these systems. This work might have important implications in the preparation of a new class of insertion compounds of noble-gas atoms containing noble-gas-noble-metal bond.

Static dipole polarizability and binding energy of sodium clusters Nan (n=1–10): A critical assessment of all-electron based post Hartree–Fock and density functional methods

A systematic all electron post Hartree-Fock as well as density functional theory (DFT) based calculations for the polarizability and binding energy of sodium metal clusters have been performed and an in-depth analysis of the discrepancy between the experimental and theoretical results is presented. A systematic investigation for the assessment of different DFT exchange-correlation functionals in predicting the polarizability values has also been reported. All the pure DFT functionals have been found to considerably underestimate the calculated polarizability values as compared to the MP2 results. DFT calculations using the full Hartree-Fock exchange along with one-parameter progressive correlation functional have, however, been shown to yield results in good agreement with the MP2 and experimental results. The possible sources of error present in the experimental measurements as well as in the different theoretical methods have also been analyzed. One of the most important conclusions of the present study is that the effect of electron correlation plays a significant role in determining the polarizability of the clusters and the MP2 method can be considered to be one of the most reliable methods for their prediction. It has also been noted that the polarizability value of the lower member clusters (Na2 and Na4) calculated by highly sophisticated methods such as, CCSD and CCSD(T) are found to be very close to the corresponding MP2 values. The polarizability and the binding energy of the clusters are found to be inversely related to each other and their correlation is rationalized by invoking the minimum polarizability principle. A good linear correlation between the polarizability and volume of the cluster has also been found to exist.

Selective Americium(III) Complexation by Dithiophosphinates: A Density Functional Theoretical Validation for Covalent Interactions Responsible for Unusual Separation Behavior from Trivalent Lanthanides

The separation of trivalent actinides and lanthanides is a challenging task for chemists because of their similar charge and chemical behavior. Soft donor ligands like Cyanex-301 were found to be selective for the trivalent actinides over the lanthanides. Formation of different extractable species for Am(3+) and various lanthanides (viz. La(3+), Eu(3+), and Lu(3+)) was explained on the basis of their relative stabilities as compared to their corresponding trinitrato complexes calculated using the density functional method. Further, the metal-ligand complexation energy was segregated into electrostatic, Pauli repulsion, and orbital interaction components. Higher covalence in the M-S bond in the dithiophosphinate complexes as compared to the M-O bond in the nitrate complexes was reflected in the higher orbital and lower electrostatic interactions for the complexes with increasing number of dithiophosphinate ligands. Higher affinity of the dithiophosphinate ligands for Am(3+) over Eu(3+) was corroborated with higher covalence in the Am-S bond as compared to the Eu-S bond, which was reflected in shorter bond length in the case of the former and higher ligand to metal charge transfer in Am(III)-dithiophosphinate complexes. The results were found to be consistent in gas phase density functional theory (DFT) calculations using different GGA functional. More negative complexation energies in the case of Eu(3+) complexes of Me(2)PS(2)(-) as compared to the corresponding Am(3+) complexes in spite of marginally higher covalence in the Am-S bond as compared to the Eu-S bond might be due to higher ionic interaction in the Eu(3+) complexes in the gas phase calculations. The higher covalence in the Am-S bond obtained from the gas phase studies of their geometries and electronic structures solely cannot explain the selectivity of the dithiophosphinate ligands for Am(3+) over Eu(3+). Presence of solvent may also play an important role to control the selectivity as observed from higher complexation energies for Am(3+) in the presence of solvent. Thus, the theoretical results were able to explain the experimentally observed trends in the metal-ligand complexation affinity.

Theoretical prediction of noble gas inserted thioformyl cations: HNgCS⁺ (Ng = He, Ne, Ar, Kr, and Xe).

The existence of new interesting insertion compounds, HNgCS(+) (Ng = He-Xe), have been predicted theoretically through insertion of a noble gas atom into the thioformyl cation, HCS(+). Second-order Møller-Plesset perturbation theory (MP2), density functional theory (DFT), and coupled-cluster theory (CCSD(T)) based techniques have been used to explore the structure, energetics, charge distribution, and harmonic vibrational frequencies of these compounds. These predicted ions are found to be energetically stable with respect to all the possible 2-body and 3-body dissociation pathways, except the 2-body channel leading to the global minimum products (HCS(+) + Ng). Nevertheless, all these ions are found to be kinetically stable with a finite barrier height corresponding to their transition states, which are connected to their respective global minima products. The results obtained from charge distribution as well as atoms in molecules (AIM) analysis suggest that these ions can be best described as [HNg](+)CS. Strong covalent character in the H-Ng bond is supported by the high positive energy value corresponding to the 3-body dissociation pathways. Thus, it might be possible to prepare the HNgCS(+) ions in a glow discharge containing H2S, CO, and noble gas under cryogenic conditions through matrix isolation technique.

Structure and stability of xenon insertion compounds of hypohalous acids, HXeOX [X=F, Cl, and Br]: an ab initio investigation.

The structure and stability of xenon-inserted hypohalous acids HXeOX (X=F, Cl, and Br) have been investigated theoretically using ab initio molecular orbital calculations. All these molecules are found to consist of a nearly linear HXeO moiety and a bend XeOX fragment. Geometrical parameters of HXeOX are comparable with that of experimentally observed HXeOH species. The dissociation energies corresponding to the lowest-energy fragmentation products, HOX+Xe have been computed to be -398.1, -385.5, and -386.7 kJmol for HXeOF, HXeOCl, and HXeOBr, respectively, at the MP2 level of theory. The respective barrier heights corresponding to the bent transition states (H-Xe-O bending mode) have been calculated to be 138.1, 138.4, and 138.2 kJmol with respect to HXeOX minimum. These species are found to be metastable in their respective potential-energy surface, and the dissociation energies corresponding to the H+Xe+OX products are found to be 56.8, 66.0, and 80.8 kJmol for HXeOF, HXeOCl, and HXeOBr, respectively. The energies corresponding to the H+Xe+O+X dissociation channel have been computed to be 272.0, 309.3, and 299.7 kJmol for HXeOF, HXeOCl, and HXeOBr, respectively, at the same level of theory. Energetics as well as geometrical considerations suggests that it may be possible to prepare these species experimentally similar to that of HXeOH species at low-temperature laser photolysis experiments.

Polarizability of water clusters: An ab initio investigation

Ab initio calculations have been performed to obtain the polarizability of water clusters (H2O)n up to n=20. It is shown that a simple linear relation involving the aggregation number fits the polarizability results extremely well (with correlation coefficient >0.999) indicating a near additive nature of this quantity for weakly bonded molecular clusters. Calculated dynamic polarizabilities are also shown to follow the same trend. The effect of electron correlation on the static polarizability of water clusters has also been investigated.

Significant increase in the stability of rare gas hydrides on insertion of beryllium atom.

Chemical binding between a rare gas atom with other elements leading to the formation of stable chemical compounds has received considerable attention in recent years. With an intention to predict highly stable novel rare gas compounds, the process of insertion of beryllium atom into rare gas hydrides (HRgF with Rg=Ar, Kr, and Xe) has been investigated, which leads to the prediction of HBeRgF species. The structures, energetic, and charge distributions have been obtained using MP2, density functional theory, and CCSD(T) methods. Analogous to the well-known rare gas hydrides, HBeRgF species are found to be metastable in nature; however, the stabilization energy of the newly predicted species has been calculated to be significantly higher than that of HRgF species. Particularly, for HBeArF molecule, it has been found to be an order of magnitude higher. Strong chemical binding between beryllium and rare gas atom has also been found in the HBeArF, HBeKrF, and HBXeF molecules. In fact, the basis set superposition error and zero-point energy corrected Be-Ar bond energy calculated using CCSD(T) method has been found to be 112 kJ/mol, which is the highest bond energy ever achieved for a bond involving an argon atom in any chemically bound neutral species. Vibrational analysis reveals a large blueshift (approximately 200 cm(-1)) of the H-Be stretching frequency in HBeRgF with respect to that in BeH and HBeF species. This feature may be used to characterize these species after their preparation by the laser ablation of Be metal along with the photolysis of HF precursor in a suitable rare gas matrix. An analysis of the nature of interactions involved in the present systems has been performed using theory of atoms in molecules (AIM). Geometric as well as energetic considerations along with the AIM results suggest a substantial covalent nature of Be-Rg bond in these systems. Thus, insertion of a suitable metal atom into rare gas hydrides is a promising way to energetically stabilize the HRgX species, which eventually leads to the formation of a new class of insertion compounds, viz., rare gas metallohydrides.

Prediction of metastable metal-rare gas fluorides: FMRgF (M=Be and Mg; Rg=Ar, Kr and Xe).

The structure, stability, charge redistribution, bonding, and harmonic vibrational frequencies of rare gas containing group II-A fluorides with the general formula FMRgF (where M=Be and Mg; Rg=Ar, Kr, and Xe) have been investigated using second order Møller-Plesset perturbation theory, density functional theory, and coupled cluster theory [CCSD(T)] methods. The species, FMRgF show a quasilinear structure at the minima and a bent structure at the transition state. The predicted species are unstable with respect to the two-body dissociation channel, leading to the global minima (MF2+Rg) on the singlet potential energy surface. However, with respect to other two-body dissociation channel (FM+RgF), they are found to be stable and have high positive energies on the same surface. The computed binding energy for the two-body dissociation channels are 94.0, 164.7, and 199.7 kJ mol(-1) for FBeArF, FBeKrF, FBeXeF, respectively, at CCSD(T) method. The corresponding energy values are 83.4, 130.7, and 180.1 kJ mol(-1) for FMgArF, FMgKrF, and FMgXeF, respectively, at the same level of theory. With respect to the three-body dissociation (FM+Rg+F) channel as well as dissociation into atomic constituent, they are also found to be stable and have high positive energies. The dissociation of the predicted species typically proceeds via MRgF bending mode at the transition state. The computed barrier heights for the transition states are 11.4, 32.2, and 57.6 kJ mol(-1) for FBeArF, FBeKrF, and FBeXeF, respectively, at the CCSD(T) method. The corresponding barrier heights for the Mg containing species are 2.1, 9.2, and 32.1 kJ mol(-1) along the series Ar--Kr--Xe, respectively. The M--Rg bond energies of the FMRgF species is significantly higher than the corresponding bond energies of the M+--Rg species ( approximately 53 and approximately 15 kJ mol(-1) for Be+--Ar and Mg+--Ar, respectively). The computed energy diagram as well as the geometrical parameters along with the AIM results suggest that the species are metastable with partial covalent character in the M--Rg bonding. Thus, it may be possible to prepare and to characterize these species using low temperature matrix isolation technique.

Insertion of rare gas atoms into BF3 and AlF3 molecules: An ab initio investigation

The structure, stability, charge redistribution, and harmonic vibrational frequencies of rare gas inserted group III-B fluorides with the general formula F-Rg-MF(2) (where M=B and Al; Rg=Ar, Kr, and Xe) have been investigated using ab initio quantum chemical methods. The Rg atom is inserted in one of the M-F bond of MF(3) molecules, and the geometries are optimized for ground as well as transition states using the MP2 method. It has been found that Rg inserted F-Rg-M portion is linear in both F-Rg-BF(2) and F-Rg-AlF(2) species. The binding energies corresponding to the lowest energy fragmentation products MF(3)+Rg (two-body dissociation) have been computed to be -670.4, -598.8, -530.7, -617.0, -562.1, and -494.0 kJmol for F-Ar-BF(2), F-Kr-BF(2), F-Xe-BF(2), F-Ar-AlF(2), F-Kr-AlF(2), and F-Xe-AlF(2) species, respectively. The dissociation energies corresponding to MF(2)+Rg+F fragments (three-body dissociation) are found to be positive with respect to F-Rg-MF(2) species, and the computed values are 56.3, 127.8, and 196.0 kJmol for F-Ar-BF(2), F-Kr-BF(2), and F-Xe-BF(2) species, respectively. The corresponding values for F-Ar-AlF(2), F-Kr-AlF(2), and F-Xe-AlF(2) species are also found to be positive. The decomposition of F-Rg-MF(2) species into the MF(3)+Rg (two-body dissociation) channel typically proceeds via a transition state involving F-Rg-M out-of-plane bending mode. The transition state barrier heights are 35.5, 62.7, 89.8, 22.0, 45.6, and 75.3 kJmol for F-Ar-BF(2), F-Kr-BF(2), F-Xe-BF(2), F-Ar-AlF(2), F-Kr-AlF(2), and F-Xe-AlF(2) species, respectively. The calculated geometrical parameters and the energy values suggest that these species are metastable and may be prepared and characterized using low temperature matrix isolation techniques, and are possibly the next new candidates for gas phase or matrix experiments.