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It is well known that a nuclear reactor generates various fission products including radioactive fission gases made of isotopes of Xe and Kr. The separation of Xe and Kr isotopes and their entrapme...

Motivated by the very recent investigations of neutral noble gas compounds in the open-shell configuration, we explored a new series of noble gas hydrides in the triplet state. The possible existence of noble gas-inserted ketenyl cations, HNgCCO+ (Ng = He, Ne, Ar, Kr, and Xe), in their triplet electronic state has been predicted by various ab initio quantum chemical techniques. Density functional theory (DFT), second-order Moller–Plesset perturbation theory (MP2), and coupled-cluster theory (CCSD(T)) based methods have been employed to investigate the structures, energetics, harmonic vibrational frequencies, and charge distribution analysis of these ions. The aforementioned ions have been found to be thermodynamically stable with respect to all plausible 2-body and 3-body dissociation channels, except the 2-body dissociation pathway leading to the formation of global minima products (Ng + HCCO+). Nevertheless, each of the predicted HNgCCO+ ions is connected to the global minima products through a transition state with a finite barrier height on the potential energy surface, which confirms the kinetic stability of the metastable species. Detailed analysis of the optimized structural parameters, energetics, and harmonic vibrational frequencies of the predicted species clearly indicated that a strong covalent bond exists between H and Ng atoms, while a comparatively weak interaction is found between Ng and C atoms. Moreover, charge distribution and atoms-in-molecules (AIM) analysis strongly concurred with the above inferences and also suggested that the predicted metastable ions should exist essentially in the form of [HNg]+[CCO] complex. These results ultimately indicate that these predicted species may be prepared and characterized by suitable experimental technique(s) under a cryogenic environment.

A new series of neutral noble gas inserted compounds involving halocarbenes, mainly, FNgCX (Ng = Kr, and Xe; X = F, Cl, Br, and I) has been predicted through various ab initio quantum chemical techniques such as MP2, DFT, CCSD(T) and MRCI. The structure, stabilities, charge distribution, harmonic vibrational frequencies and topological properties of these compounds have been investigated. It is found that the predicted species are energetically stable with respect to all the plausible 2-body and 3-body dissociation pathways, with the exception of the 2-body channel that leads to the global minimum products (FCX + Ng). Despite this, existence of finite barrier heights indicates that these compounds are kinetically stable with respect to global minimum products. The computational results indicate that it might be possible to prepare and characterize the most stable singlet state of FNgCX molecules under cryogenic conditions through suitable experimental technique(s).

In accordance with the well established gold-hydrogen analogy, a hydrogen atom mimics the properties of a gold atom in gold clusters. In a recent study it has been demonstrated that the properties of a hydrogen atom doped small gold cluster (Au7H) are not in conformity with the aforementioned analogy. In this paper we study the properties of the Au7H cluster exhaustively to re-examine the validity of the gold-hydrogen analogy in the context of adsorption of CO and O2 molecules on pristine gold and hydrogen atom doped gold clusters. For this purpose we first determine the most stable structure of the Au7H cluster by using an ab initio density functional theory based method with generalized gradient approximation (GGA) and Meta-GGA exchange-correlation functionals. We carry out geometry optimization by considering various planar and three-dimensional isomers of the Au7H cluster as initial geometries. We find that the lowest energy structure of Au7H is a planar one with C2 v symmetry, and it is very close to the structure of the Au8 cluster with D4 h symmetry. Furthermore, to examine the validity of the gold-hydrogen analogy we carry out a detailed investigation of the adsorption of CO and O2 molecules on the most stable as well as various other low energy isomers of the Au7H cluster. We find that the adsorption energies and the extent of activation of CO and O2 molecules on the most stable planar isomer of Au7H are almost the same as those on the parent Au8 cluster with D4 h symmetry proving the validity of the gold-hydrogen analogy. On the other hand, for the high energy three-dimensional isomers of the Au7H cluster obtained from the pristine Au8 cluster with T d symmetry, we find a significant enhancement in adsorption energy as well as the extent of activation of CO and O2 molecules as compared to those for the corresponding pristine cluster. Therefore, the high reactivity of the 3D isomer of the Au7H cluster may be attributed to its existence in a state which is higher in energy than its most stable planar isomer.

Behavior of gold as hydrogen in certain gold compounds and a very recent experimental report on the noble gas–noble metal interaction in Ar complexes of mixed Au–Ag trimers have motivated us to investigate the effect of hydrogen doping on the Ng–Au (Ng = Ar, Kr, and Xe) bonding through various ab initio based techniques. The calculated results show considerable strengthening of the Ng–Au bond in terms of bond length, bond energy, stretching vibrational frequency, and force constant. Particularly, an exceptional enhancement of Ar–Au bonding strength has been observed in ArAuH2+ species as compared to that in ArAu3+ system, as revealed from the CCSD(T) calculated Ar–Au bond energy value of 32 and 72 kJ mol–1 for ArAu3+ and ArAuH2+, respectively. In the calculated IR spectra, the Ar–Au stretching frequency is blue-shifted by 65% in going from ArAu3+ to ArAuH2+ species. Similar trends have been obtained in the case of all Ar, Kr, and Xe complexes with Ag and Cu trimers. Among all the NgM3–kHk+ complexes (wher...

In this work we have predicted novel rare gas containing cationic molecules, XRgCO(+) (X = F, Cl and Rg = Ar, Kr, Xe) using ab initio quantum chemical methods. Detail structural, stability, vibrational frequency, and charge distribution values are reported using density functional theory, second-order Møller-Plesset perturbation theory, and coupled-cluster theory based methods. These ions are found to be metastable in nature and exhibit a linear geometry with C∞v symmetry in their minima energy structures, and the nonlinear transition state geometries are associated with Cs symmetry. Except for the two-body dissociation channel (Rg + XCO(+)), these ions are stable with respect to all other dissociation channels. However, the connecting transition states between the above-mentioned two-body dissociation channel products and the predicted ions are associated with sufficient energy barriers, which restricts the metastable species to transform into the global minimum products. Thus, it may be possible to detect and characterize these metastable ions using an electron bombardment technique under cryogenic conditions.

A novel class of interesting insertion compounds obtained through the insertion of a noble gas atom into the heavier pnictides have been explored by various ab initio quantum chemical techniques. Recently, the first neutral noble gas insertion compounds, FXeY (Y = P, N), were theoretically predicted to be stable; the triplet state was found to be the most stable state, with a high triplet-singlet energy gap, by our group. In this study, we investigated another noble gas inserted compound, FNgY (Ng = Kr and Xe; Y = As, Sb and Bi), with a triplet ground state. Density functional theory (DFT), second order Møller-Plesset perturbation theory (MP2), coupled-cluster theory (CCSD(T)) and multi-reference configuration interaction (MRCI) based techniques have been utilized to investigate the structures, stabilities, harmonic vibrational frequencies, charge distributions and topological properties of these compounds. These predicted species, FNgY (Ng = Kr and Xe; Y = As, Sb and Bi) are found to be energetically stable with respect to all the probable 2-body and 3-body dissociation pathways, except for the 2-body channel leading to the global minimum products (FY + Ng). Nevertheless, the finite barrier height corresponding to the saddle points of the compounds connected to their respective global minima products indicates that these compounds are kinetically stable. The structural parameters, energetics, and charge distribution results as well as atoms-in-molecules (AIM) analysis suggest that these predicted molecules can be best represented as F(-)[(3)NgY](+). Thus, all the aforementioned computed results clearly indicate that it may be possible to experimentally prepare the most stable triplet state of FNgY molecules under cryogenic conditions through a matrix isolation technique.

We employ an ab initio scalar relativistic density functional theory based method to calculate the ground state structures and the electronic properties for Au19X clusters, X being the alkali metal atoms, Li, Na, K, Rb, and Cs as well as the coinage metal atoms, Ag and Cu. The tetrahedral Au20 clusters have been doped exohedrally with these atoms at three different types of unique sites where the dopant atom substitutes one gold atom from (i) the vertex, (ii) the surface, and (iii) the edge sites. In addition to the structures based on tetrahedral Au20, we also consider cage-like structures for Au19X clusters with the dopant atom located at an endohedral position. We first optimize the geometries of these clusters and then we carry out vibrational analysis on these optimized structures of the substituted Au20 clusters in order to check the stability of the final optimized structures. Further, using the optimized geometries of these doped clusters, we calculate the binding energy, interaction energy of the...

We have studied the properties of various isomers of potassium clusters containing even number of atoms ranging from 2 to 20 at the ab initio level. The geometry optimization calculations of the isomers of each cluster are performed by using all-electron density functional theory with gradient corrected exchange-correlation functional. Using the optimized geometries of different isomers we investigate the evolution of binding energy, ionization potential, and static polarizability with the increasing size of the clusters. The polarizabilities are calculated by employing Möller-Plesset perturbation theory and time-dependent density functional theory. The polarizabilities of dimer and tetramer are also calculated by employing large basis set coupled cluster theory with single and double excitations and perturbative triple excitations. The time-dependent density functional theory calculations of polarizabilities are carried out with two different exchange-correlation potentials: (i) an asymptotically correct model potential and (ii) within the local density approximation. A systematic comparison with the other available theoretical and experimental data for various properties of small potassium clusters mentioned above has been performed. These comparisons reveal that both the binding energy and the ionization potential obtained with gradient-corrected potential match quite well with the already published data. Similarly, the polarizabilities obtained with Möller-Plesset perturbation theory and with model potential are quite close to each other and also close to experimental data.

A number of heterocycle-based aromatic and quinonoid molecular systems have been considered for the theoretical study of their electric response properties. The nonlinear optical (NLO) parameters have been calculated by using the ab initio MO and DFT methods. An approximate scheme for calculating the first hyperpolarizability (beta) and second hyperpolarizability (gamma) in the framework of the sum-over-state (SOS) method have been proposed by exploiting the generalized Thomas-Kuhn sum rule (TK-SR). The NLO properties in the present scheme can be evaluated solely from the ground-state dipole moment (mu) and linear polarizability (alpha) and have been found to correlate fairly with the ab initio calculated values. The approximate scheme can be reasonably used to explain the wider range of variation of higher-order polarizabilities in terms of the above quantities. The position of the N atom in the thiazole ring at the ortho position (versus meta position) to the acceptor increases beta and decreases gamma for aromatic compounds, while the reverse trend is found with quinonoid compounds. In the case of the pyridine ring, the shifting of the N atom toward the acceptor enhances gamma, with insignificant variation of beta predicted for both the aromatic and quinonoid molecules. The negative contribution of the cubic polarizability of the quinonoid species increases linearly with alpha(2)/mean transition energy (Delta E).

The existence of noble gas containing protonated silicon monoxide complexes have been predicted theoretically through ab initio quantum chemical methods. The predicted HNgOSi(+) ions are obtained by insertion of a noble gas atom (Ng = He, Ne, Ar, Kr, and Xe) between the H and O atoms in SiOH(+) ion. The structural parameters, energetics, harmonic vibrational frequencies, and charge distributions have been analyzed by optimizing the minima and the transition state structures using second-order Møller-Plesset perturbation theory (MP2), density functional theory (DFT), and coupled-cluster theory (CCSD(T)) based techniques. The predicted HNgOSi(+) ions are found to be stable with respect to all possible 2-body and 3-body dissociation channels, except the dissociation path leading to the respective global minimum products. However, these ions are found to be kinetically stable with respect to the global minimum dissociation process as revealed from the finite barrier heights, which in turn can prevent the transformation of these metastable species to the global minimum products. Furthermore, the computed bond lengths, vibrational frequencies, and force constant values suggest that a strong covalent bond exists between the H and Ng atoms in HNgOSi(+) ions while the Ng and O atoms share a strong van der Waals kind of interaction. Charge distributions and bonding analysis indicate that HNgOSi(+) ions can be best represented as strong complexes between the [HNg](+) ions and OSi molecule. All the computational results suggest that the predicted species, HNgOSi(+), may be prepared and characterized by suitable experimental technique at cryogenic temperature.

Ab initio and density functional theoretical studies on hydrogen-bonded complexes of azabenzenes with water, acetamide, and thioacetamide have been carried out to explore the controversy involved in the relative order of their stability in a systematic way. The interaction energies of these complexes have been analyzed using the Morokuma energy decomposition method, and the nature of the various hydrogen bonds formed has been investigated through topological aspects using Bader's atom in a molecule (AIM) theory. Morokuma energy decomposition analysis reveals that the major contributions to the energetics are from the polarization (PL) and charge transfer (CT) energies. From the calculated topological results, excellent linear correlation is shown to exist between the hydrogen-bond length, electron density [rho(r)], and its Laplacian [nabla(2)rho(r)] at the bond critical points for all the complexes considered.

The various possible structures of thiocyanate dimer radical anion, ( SCN ) 2 · − , have been optimized using Hartree–Fock (HF), density functional theory with B3LYP functional (DFT) and second order Moller–Plesset perturbation (MP2) methods. The most stable structure has been found to be the one where the two SCN subsystems are mutually out-of-plane with respect to each other involving a weak bonding between sulfur–sulfur atoms. The sulfur–sulfur bond length and dihedral angle have been calculated to be larger as compared to that of thiocyanogen, (SCN)2. Binding energy for the global minimum of ( SCN ) 2 · − has been obtained as 53.7, 129.3 and 139.0 kJ/mol with the aid of HF, DFT and MP2 methods, respectively, suggesting a strong electron correlation effect. Similar to other dimer radical ion systems, the agreement in binding energy between DFT and MP2 results is quite satisfactory for ( SCN ) 2 · − . However, the DFT calculated S⋯S bond distance is highly overestimated as compared to the corresponding MP2 value. Energy and geometry of these optimized structures have been discussed and compared with similar systems.

The polarizability of lithium clusters was studied using ab initio as well as Density Functional Theory (DFT)-based methods. The electron correlation was found not to play any major role in the evaluation of polarizability of the lithium clusters, contrary to the case of sodium and other types of clusters. The theoretical methods used in the present study are not able to reproduce the unusual features in the size evolution of the polarizability observed experimentally. An interesting feature is that the polarizability of lithium clusters calculated by various methods at 0K temperature is found to be higher than the experimental results measured at higher temperature, which is quite unusual. The significance of these issues is highlighted and the need of highly accurate DFT exchange-correlation functionals and ab initio methods in the study of the electronic properties of lithium clusters is emphasized. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Radiolytic reduction of a substituted 5,8-naptha dione (THMND), synthesized in our laboratory, has been investigated by pulse radiolysis and steady-state γ-radiolysis in pure aqueous, aqueous-formate and in aqueous-2-propanol-acetone mixed solvent systems. The rate constants of formation of the semi-dione radicals were approx. 109 dm3 mol-1 s-1 in aqueous-formate and aqueous-2-propanol-acetone mixed solvent. The semi-dione radicals decay by second order kinetics with rate constants (2k) of about 109 and 108 dm3 mol-1 s-1 in the above two solvents, respectively. The pK a value of the radical was found to be 5.0 in aqueous-formate solution and 5.8 in the aqueous-2-propanol-acetone mixed solvent. The one-electron reduction potential (E 1) value at pH 7, determined from the pulse-radiolysis experiment, was found to be –420 ± 20 mVvs. NHE at 298 K and was independent of solvent. Ab initio calculations on its one-electron reduction reaction suggest the formation of a radical, which is different from a semiquinone where the electron density is delocalised over the two oxygen atoms. Experimental absorption maxima of the radical in aqueous solution also agree very well with the ab initio calculated values. Steady-state γ-radiolysis of THMND produces the corresponding two-electron reduced species.

The properties of the alkali metal clusters have been investigated by using ab initio electronic structure calculations, with special reference to their structural evolution and the size dependence of several reactivity descriptors, such as ionization potential, electron affinity, polarizability, chemical potential, hardness, softness, etc. Also shown is a good inverse correlation between the dipole polarizability and the ionization potential of the neutral clusters. Similarly, the softness parameter has also been shown to correlate strongly with the dipole polarizability of the clusters. The present work thus will have some important implications in the calculation of polarizability of metal clusters in terms of the corresponding ionization potentials directly.

Rotational dynamics of two structurally similar nondipolar probes; 2,5-dimethyl-1,4-dioxo-3,6diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) has been investigated in ethanol (EtOH) and 2,2,2-trifluoroethanol (TFE) in the temperature range 243–298 K in an attempt to understand how the strength of the solute–solvent hydrogen bond impedes molecular rotation. It has been observed that the reorientation times of DPP are slower compared to DMDPP by about a factor of 2 in EtOH and this factor is only 1.3–1.4 in TFE. Another interesting observation is that the viscosity normalized reorientation times of DPP at a given temperature are almost identical in EtOH and TFE, whereas those of DMDPP are slower by a factor of 1.5 in TFE compared to EtOH. These observations have been rationalized on the basis of hydrogen bond donating and hydrogen bond accepting abilities of the respective solute and the solvent. Further evidence for such a rationale has been provided with the aid of ab initio molecular orbital methods.

Ab initio and density functional theoretical calculations have been performed to quantify the hydrogen-bonding interactions for selected supermolecular systems, experimental investigations on which have been reported very recently (Angew. Chem., Int. Ed. 2001, 40, 3240). An analysis and rationalization of the nature of pairwise interactions in different hydrogen bonds involved in these ternary supermolecular systems is presented that uses the frameworks of Morokuma energy decomposition as well as Bader's topological theory of atoms in molecules involving the electron density ρ(r), its Laplacian ∇2ρ(r), and also other related quantities at the bond critical points. The pKa values of the aromatic acids, which have been used earlier to rationalize the specific intermolecular interactions between aromatic acids (hydrogen-bond donor) and isonicotinamide (hydrogen-bond acceptor as well as donor), are, however, found not to show any regular trend with the calculated binary interaction energy values or the electr...

The mechanistic details of a H atom (H• radical) reaction with the methyl viologen (Paraquat) dication, MV2+, in aqueous solutions is presented, based on pulse radiolysis measurements in acid (HClO4) solution and supported by ab initio molecular orbital calculations. H atom-initiated reduction follows three different paths. About 76% of reactions follow the H atom addition to one of the ring C atom positioned meta to a N atom. The resulting cyclohexadienyl-type radical (MVHC•2+) shows an absorption peak at 350 nm (fwhm = 40 nm, e = 4470 ± 300 M-1 cm-1), and its second-order decay rate (2k350nm = 2.6 ± 1.1 × 109 M-1 s-1) suggests a radical−radical disproportionation reaction. About 10% of H atom reactions proceed with abstraction of one H atom from a >N−CH3 group, resulting in the formation of a >N−CH2• radical. Subsequently, its transformation into the N-hydroxycyclohexadienyl-type radical (MVOHN•2+) in the presence of water is indicated. The MVOHN•2+ radical is exclusively generated during the •OH radica...

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.

High level ab initio calculations using a complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate perturbation theory (MCQDPT2) methods as well as density functional theory (DFT)-based calculations with different exchange−correlation energy density functionals have been performed for predicting the relative stability of the proton-transferred vs hemi-bonded isomers of (H2O)2+ and (H2S)2+ species. For (H2O)2+, DFT calculation using conventional exchange−correlation functionals predicts the hemi-bonded structure to be the ground state while use of full or half Hartree−Fock exchange and local correlation predicts a higher stability of the proton-transferred structure in agreement with ab initio results. For the (H2S)2+ system, all of the methods lead to the prediction of lower energy for the hemi-bonded isomer. No regular trend of the exchange−correlation energy component with the total energy difference is however observed. Dynamical electron correlation effect incorpo...

The first nuclear hyperfine measurements of 17O (I=5/2) have been made for Sc17O, Y17O and La17O in their X 2Σ ground electronic states. These metal oxide radicals were generated by the pulsed-laser vaporization of the metals in the presence of 16O2/17O2 and trapped in neon and argon matrices for electron spin resonance investigations. The fully resolved A tensors of the metal and 17O were compared with ab initio theoretical calculations—a comparison previously reported only for the ScO radical. The computational methods employed were unrestricted Hartree–Fock, density functional theory (DFT), and restricted open-shell Hartree–Fock. Having the metal and 17O hyperfine interactions available has permitted a more thorough description of the electronic structure and charge distribution in these metal oxide molecules. An electronic structure comparison with the AlO, GaO, and InO radicals has also been made. Reasonably good agreement between the observed and calculated values of Aiso and Adip were achieved with...

Complete active space self-consistent field (CASSCF) and multi-reference singles and doubles configuration interaction (MRSDCI) theoretical calculations of the energy and wavefunctions for the low lying states of AlSi— and AlSi with various internuclear distances are used to interpret the recently reported experimental photoelectron spectrum of AlSi—. On the basis of the MRSDCI calculations it is found that the lowest 3II and 3Σ— states of AlSi— lie very close in energy to one another, with the 3Σ— state being the more stable. The ground state of AlSi is determined to be 4Σ—. In terms of peak positions and intensities, semiquantitative agreement is observed between the experimental and the theoretically derived spectra. New assignments have been made for some of the previously assigned photoelectron peaks on the basis of these MRSDCI results.

The existence of rare-gas-containing hydride ions of boron (HRgBF(+)) has been predicted by using ab initio quantum chemical methods. The HRgBF(+) ions are obtained by inserting a rare gas (Rg) atom in between the H and B atoms of a HBF(+) ion, and the geometries are optimized for minima as well as transition states using second-order Møller-Plesset perturbation theory (MP2), density functional theory (DFT), and coupled-cluster theory (CCSD(T)) based techniques. The predicted HRgBF(+) ions are found to be metastable, and they exhibit a linear structure at the minima and a nonlinear planar structure at the transition state, corresponding to C∞v and Cs symmetries, respectively. All of the predicted HRgBF(+) ions show negative binding energies with respect to the two-body dissociation channel, leading to global minima (HBF(+) + Rg) on the singlet potential energy surface. In contrast, the dissociation energies corresponding to another two-body dissociation channel leading to HRg(+) + BF and two three-body dissociation channels corresponding to the dissociation into H + Rg + BF(+) and H(+) + Rg + BF show very high positive energies. Apart from positive dissociation energies, the predicted ions show finite barrier heights corresponding to the transition states involving a H-Rg-B bending mode, leading to the global minima products (HBF(+) + Rg). The finite barrier heights in turn would prevent the metastable HRgBF(+) species from transforming to global minima products. Structure, harmonic vibrational frequencies, stability, and Mulliken and natural bonding orbital (NBO) charge distribution values for all of the species are reported using the MP2 and DFT methods. Furthermore, the intrinsic reaction coordinate analysis confirms that the metastable minimum-energy structure and the global minimum products are connected through the corresponding transition state for each of the species on the respective singlet potential energy surface. Atoms-in-molecules (AIM) analysis indicates that the HRgBF(+) ions are best described as HRg(+)BF and are analogous to the isoelectronic HRgCO(+) and HRgN2(+) ions. The energetic along with charge redistribution and spectroscopic data strongly support the possible existence of HRgBF(+) ions. Hence, it might be possible to generate HRgBF(+) ions in the DC discharge plasma of a BF3/H2/Rg mixture at low temperature, and the predicted ions may be characterized using the magnetic field modulated infrared laser spectroscopic technique, which has been used earlier to characterize HBF(+) ions.

Several diglycolamide-functionalized calix[4]arenes (DGA–Calix) were evaluated for actinide extraction from acidic feeds. The ligands with four diglycolamide (DGA) pendent arms are significantly more effective extractants than those with two DGA pendent arms. The ligands have a preference for the extraction of Eu3+, a representative trivalent lanthanide ion, as compared to Am3+, a commonly encountered trivalent actinide ion. The role of organic diluents on the metal ion extraction was investigated and the results were compared with the widely studied DGA-based extractant TODGA (N,N,N′,N′-tetra-n-octyl diglycolamide). Time resolved laser fluorescence spectroscopy (TRLFS) studies showed a strong complexation with no inner-sphere water molecules in the Eu(III)–DGA–Calix complexes and the complex formation constants (log β) were calculated. Ab initio density functional calculations were carried out to explain the higher stability of the Eu-complex of the DGA–Calix ligand with four pendent arms as compared to the one with two pendent arms.

The properties of methanol clusters [(CH(3)OH)(n), n = 1-12] have been studied by using ab initio electronic structure calculations with reference to the aggregation number dependence of several reactivity descriptors, such as ionization potential, electron affinity, polarizability, hardness, and binding energy. A good correlation between the dipole polarizability and the ionization potential of these hydrogen-bonded molecular clusters is shown to exist. The softness parameter has also been shown to correlate strongly with the dipole polarizability of these molecular clusters. Similar good correlations are also demonstrated to exist for water clusters [(H(2)O)(n), n = 1-20]. This work can thus be useful for calculating the polarizability of larger methanol or water clusters in terms of the corresponding ionization potential.

An ab initio investigation of the polarizabilities of the first two excited states of methanol clusters [(CH(3)OH)(n), n = 1-12] has been carried out employing time dependent density functional theory with B3LYP exchange correlation functional using 6-311++G(d,p) and Sadlej basis sets. Good linear correlations have been demonstrated for the first and second excited state polarizabilities as a function of the aggregation number (n) of the methanol clusters. On the contrary, for water clusters the variations of excited state polarizabilities with cluster size are found to be non-monotonic. The variations of the excitation energies and oscillator strengths with n for the first three excited states of methanol clusters and water clusters are also reported.

We propose a new approach to the calculation of partial atomic charges in polyatomic molecules. The method involves a locally groupwise electronegativity equalisation procedure and is able to incorporate the effect of connectivity of atoms in the molecule. The calculated charges for a number of isolated molecules and interacting molecular systems involving intermolecular charge transfer show good agreement with ab initio results. The “pile-up” and “spill-over” effects in donor-acceptor interaction is correctly predicted. The same approach has been used for the calculation of other reactivity indices such as the Fukui function and self or mutual softness, for which the numerical results are reported.

We employ all-electron ab initio time-dependent density functional theory based method to calculate the long-range dipole-dipole dispersion coefficient, namely, the van der Waals (vdW) coefficient (C(6)) between fullerenes and finite-length carbon nanotubes as well as between these structures and different small molecules. Our aim is to accurately estimate the strength of the long-range vdW interaction in terms of the C(6) coefficients between these systems and also compare these values as a function of shape and size. The dispersion coefficients are obtained via Casimir-Polder relation. The calculations are carried out with the asymptotically correct exchange-correlation potential-the statistical average of orbital potential. It is observed from our calculations that the C(6) coefficients of the carbon nanotubes increase nonlinearly with length, which implies a much stronger vdW interaction between the longer carbon nanostructures compared with the shorter ones. Additionally, it is found that the values of C(6) and polarizability are about 40%-50% lower for the carbon cages when compared with the results corresponding to the quasi-one-dimensional nanotubes with equivalent number of atoms. From our calculations of the vdW coefficients between the small molecules and the carbon nanostructures, it is observed that for H(2), the C(6) value is much larger compared with that of He. It is found that the rare gas atoms have very low values of vdW coefficient with the carbon nanostructures. In contrast, it is found that other gas molecules, including the ones that are environmentally important, possess much higher C(6) values. Carbon tetrachloride as well as chlorine molecule show very high C(6) values with themselves as well as with the carbon nanostructures. This is due to the presence of the weakly bound seven electrons in the valence state for the halogen atoms, which makes these compounds much more polarizable compared with the others.

We have studied the static dipole polarizability of stoichiometric gallium phosphide clusters (Ga(n)P(n) with n=2-5) by employing various ab initio wave function based methods as well as density functional theory/time dependent density functional theory (DFT/TDDFT). The calculation of polarizability within DFT/TDDFT has been carried out by employing different exchange-correlation functionals, ranging from simple local density approximation to an asymptotically correct model potential-statistical average of orbital potential (SAOP) in order to study their influences. The values obtained by using the model potential-SAOP are lower than those obtained by local density approximation and generalized gradient approximation. A systematic analysis of our results obtained using the DFT/TDDFT with several exchange-correlation functionals shows that the values of polarizability obtained within generalized gradient approximation by using Perdew-Burke-Ernzerhof exchange with Lee-Yang-Parr correlation functional and Perdew-Burke-Ernzerhof exchange-correlation functionals are the closest to the corresponding results from Møller-Plesset perturbation theory. We have found that the value of average static dipole polarizability per atom reaches the bulk limit from the above as the size of the clusters increases.

We discuss two methods within the realm of time‐dependent density functional theory to calculate frequency dependent polarizabilty of many‐electron systems and use them to obtain van der Waals coefficient C6 between two such systems. One method is based on time dependent Kohn‐Sham formalism and the other one is hydrodynamic approach based in terms of particle‐ and current‐density. We apply these methods to calculate C6 of inert gas atoms and alkali metal clusters within the jellium model and at the ab initio level as well by taking discrete ionic positions into account. The accuracy of the jellium model results are assessed against more accurate ab initio data.

Ab initio quantum chemical methods have been employed to investigate the structure, stability, charge redistribution, and harmonic vibrational frequencies of rare gas (Rg=He, Ne, Ar, Kr, and Xe) containing HRgCO(+) ion. The Rg atoms are inserted in between the H and C atoms of HCO(+) ion and the geometries are optimized for minima as well as transition state using second order Moller-Plesset perturbation theory, density functional theory, and coupled-cluster theory [CCSD(T)] methods. The HRgCO(+) ions are found to be metastable and exhibit a linear structure at the minima position and show a nonlinear structure at the transition state. The predicted ion is unstable with respect to the two-body dissociation channel leading to the global minima (HCO(+)+Rg) on the singlet potential surface. The binding energies corresponding to this channel are -406.4, -669.3, -192.3, -115.4, and -52.2 kJ mol(-1) for HHeCO(+), HNeCO(+), HArCO(+), HKrCO(+), and HXeCO(+) ions, respectively, at CCSD(T) method. However, with respect to other two-body dissociation channel, HRg(+)+CO, the ions are found to be stable and have positive energies except for HNeCO(+) at the same level of theory. The computed binding energies for this channel are 15.0, 28.8, 29.5, and 29.1 kJ mol(-1) for HHeCO(+), HArCO(+), HKrCO(+), and HXeCO(+) ions, respectively. Very high positive three-body dissociation energies are found for H+Rg+CO(+) and H(+)+Rg+CO dissociation channels. It indicates the existence of a very strong bonding between Rg and H atoms in HRgCO(+) ions. The predicted ions dissociate into global minima, HCO(+)+Rg, via a transition state involving H-Rg-C bending mode. The barrier heights for the transition states are 22.7, 10.1, 13.1, and 15.0 kJ mol(-1) for He, Ar, Kr, and Xe containing ions, respectively. The computed two-body dissociation energies are comparable to that of the experimentally observed mixed cations such as ArHKr(+), ArHXe(+), and KrHXe(+) in an electron bombardment matrix isolation technique. Thus HRgCO(+) cations may also be possible to prepare and characterize similar to the mixed cations (RgHRg('))(+) in low temperature matrix isolation technique.

Transport of Am(III) was studied through the composite polymer membrane of Cyanex-301 [bis(2,4,4-trimethylpentyl)dithiophosphinic acid] and tri-n-butylphosphate (TBP). Depending on the pH of the strip solution containing alpha-hydroxyisobutyric acid (AHIBA), the transport behaviour of Am(III) was changed significantly. After approximately 70% of the Am(III) transported to the strip side, interestingly, back transport of Am(III) was observed at a pH of 3.5. The back transport phenomenon was not so significant at pH 1 and 5.7. The back transport of Am(III) was attributed to the transport of AHIBA from strip to the feed side due to its interaction with TBP in the membrane and the attainment of Donnan equilibrium because of the presence of Na(+) in the feed as the driving ion. The experimental observations were rationalized using the hydrogen bonding interaction energies obtained through ab initio molecular orbital and DFT calculations.

Ab initio molecular orbital calculations have been employed to characterize the structure and bonding of the (HO2-H2O)+ radical cation system. Geometry optimization of this system was carried out using unrestricted density functional theory in conjunction with the BHHLYP functional and 6-311++G(2df,2p) as well as 6-311++G(3df,3p) basis sets, the second-order Møller-Plesset perturbation (MP2) method with the 6-311++G(3df,3p) basis set, and the couple cluster (CCSD) method with the aug-cc-pVTZ basis set. The effect of spin multiplicity on the stability of the (HO2-H2O)+ system has been studied and also compared with that of oxygen. The calculated results suggest a proton-transferred hydrogen bond between HO2 and H2O in H3O3+ wherein a proton is partially transferred to H2O producing the O2...H3O+ structure. The basis set superposition error and zero-point energy corrected results indicate that the H3O3+ system is energetically more stable in the triplet state; however, the singlet state of H3O3+ is more stable with respect to its dissociation into H3O+ and singlet O2. Since the resulting proton-transferred hydrogen-bonded complex (O2...H3O+) consists of weakly bound molecular oxygen, it might have important implications in various chemical processes and aquatic life systems.

In this paper we employ all-electron \textit{ab-initio} time-dependent density functional theory based method to calculate the long range dipole-dipole dispersion coefficient (van der Waals coefficient) $C_{6}$ of sodium atom clusters containing even number of atoms ranging from 2 to 20 atoms. The dispersion coefficients are obtained via Casimir-Polder relation. The calculations are carried out with two different exchange-correlation potentials: (i) the asymptotically correct statistical average of orbital potential (SAOP) and (ii) Vosko-Wilk-Nusair representation of exchange-correlation potential within local density approximation. A comparison with the other theoretical results has been performed. We also present the results for the static polarizabilities of sodium clusters and also compare them with other theoretical and experimental results. These comparisons reveal that the SAOP results for C_{6} and static polarizability are quite accurate and very close to the experimental results. We examine the relationship between volume of the cluster and van der Waals coefficient and find that to a very high degree of correlation C_{6} scales as square of the volume. We also present the results for van der Waals coefficient corresponding to cluster-Ar atom and cluster-N_{2} molecule interactions., 22 pages including 6 figures. To be published in Journal of Chemical Physics

Ab initio molecular orbital and hybrid density functional methods have been employed to characterize the structure and bonding of (H2O-H2S)+, an asymmetrical dimer radical cation system. A comparison has been made between the two-center three-electron (2c-3e) hemi-bonded system and the proton-transferred hydrogen-bonded systems of (H2O-H2S)+. Geometry optimization of these systems was carried out using unrestricted Hartree Fock (HF), density functional theory with different functionals, and second-order Møller-Plesset perturbation (MP2) methods with 6-311++G(d,p) basis set. Hessian calculations have been done at the same level to check the nature of the equilibrium geometry. Energy data were further improved by calculating basis set superposition error for the structures optimized through MP2/6-311++G(d,p) calculations. The calculated results show that the dimer radical cation structure with H2O as proton acceptor is more stable than those structures in which H2O acts as a proton donor or the 2c-3e hemi-bonded (H2O thereforeSH2)+ system. This stability trend has been further confirmed by more accurate G3, G3B3, and CCSD(T) methods. On the basis of the present calculated results, the structure of H4OS+ can best be described as a hydrogen-bonded complex of H3O+ and SH with H2O as a proton acceptor. It is in contrast to the structure of neutral (H2O...H2S) dimer where H2O acts as a proton donor. The present work has been able to resolve the ambiguity in the nature of bonding between H2O and H2S in (H2O-H2S)+ asymmetrical dimer radical cation.

The stability of closed shell bimetallic magic clusters MAu(4) (M=Ti and Zr) is investigated theoretically through ab initio molecular orbital calculations. Both these clusters have tetrahedral structures and are found to be associated with large values of the ionization potential, HOMO-LUMO gap as well as the binding energies, which are characteristic of the magic clusters. However, the cluster-cluster interaction energy corresponding to a dimer formation is found to be unusually high ( approximately 5-7 eV) in contradiction to the usual properties of a magic cluster and is attributed to a 3-center-2-electron M-Au-M type bridge bonding as well as aurophilic attraction. Gross geometrical features of the individual clusters are, however, mostly retained in the dimer, thus satisfying the basic requirements for the cluster-assembled materials. This work would have important implications in the design of novel cluster-based nanomaterials for various nanoscale applications.

A possibility of existence of new species through insertion of a rare gas atom in hydronium ion resulting into HRgOH2(+) cation (Rg = He, Ar, Kr, and Xe) has been explored by using various ab initio quantum chemical techniques. Structure, harmonic vibrational frequencies, stability, and charge distribution of HRgOH2(+) species as obtained using density functional theory, second order Møller-Plesset perturbation theory, and coupled-cluster theory based methods are reported in this work. All the calculated results suggest that the HRgOH2(+) species are stable enough with respect to all the dissociation channels, except the 2-body dissociation path (H3O(+) + Rg). Nevertheless, this 2-body dissociation channel connected through the relevant transition state is associated with a finite barrier, which in turn would prevent the metastable species in transforming to global minimum products. The calculated values of topological properties within the framework of quantum theory of atoms-in-molecules are found to be consistent with the bond length values. Structural and energetic parameters clearly suggest that it might be possible to prepare and characterize the HRgOH2(+) species (except HHeOH2(+)) using electron bombardment matrix isolation technique in a way similar to that of the preparation of (Rg2H)(+) or mixed (RgHRg('))(+) cations.

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.

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. Bader's [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.

In this Communication we have reported the prediction of a new class of compounds, AuBX (with X = F,Cl,Br), using the results obtained from ab initio quantum-chemical calculations. We have compared their electronic structures, bonding, and stability with that of the recently discovered HBX radicals and demonstrated an excellent one-to-one quantitative correspondence between the structures, nature of bonding, and stability of AuBX radicals with the corresponding HBX radicals, which is of considerable significance. Comparison has also been made with the radicals containing other coinage metal atoms, viz., CuBX and AgBX. Structurally they are found to be quite similar to the HBX radicals. However, the stability in terms of some of the bond dissociation energy values differs considerably from the corresponding values in HBX or AuBX species. This feature is attributed to the unusually high relativistic effects in gold. The present results suggest that AuBX radicals are stable enough to be prepared experimentally in analogy with the experimentally observed HBX radicals. The gold-hydrogen analogy demonstrated here quantitatively would motivate further research to predict gold analogs of novel hydride species and vice versa.

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 Bader's 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.