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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 (C6) 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 C6 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 C6 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 C6 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 H2, the C6 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 C6 values. Carbon tetrachloride as well as chlorine molecule show very high C6 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. [ABSTRACT FROM AUTHOR]

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 Mo\ller–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. [ABSTRACT FROM AUTHOR]

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 Mo\ller–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[Single_Bond]Kr[Single_Bond]Xe, respectively. The M[Single_Bond]Rg bond energies of the FMRgF species is significantly higher than the corresponding bond energies of the M+[Single_Bond]Rg species (∼53 and ∼15 kJ mol-1 for Be+[Single_Bond]Ar and Mg+[Single_Bond]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[Single_Bond]Rg bonding. Thus, it may be possible to prepare and to characterize these species using low temperature matrix isolation technique. [ABSTRACT FROM AUTHOR]

The structure, stability, charge redistribution, and harmonic vibrational frequencies of rare gas inserted group III-B fluorides with the general formula F–Rg–MF2 (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 MF3 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–BF2 and F–Rg–AlF2 species. The binding energies corresponding to the lowest energy fragmentation products MF3+Rg (two-body dissociation) have been computed to be -670.4, -598.8, -530.7, -617.0, -562.1, and -494.0 kJ/mol for F–Ar–BF2, F–Kr–BF2, F–Xe–BF2, F–Ar–AlF2, F–Kr–AlF2, and F–Xe–AlF2 species, respectively. The dissociation energies corresponding to MF2+Rg+F fragments (three-body dissociation) are found to be positive with respect to F–Rg–MF2 species, and the computed values are 56.3, 127.8, and 196.0 kJ/mol for F–Ar–BF2, F–Kr–BF2, and F–Xe–BF2 species, respectively. The corresponding values for F–Ar–AlF2, F–Kr–AlF2, and F–Xe–AlF2 species are also found to be positive. The decomposition of F–Rg–MF2 species into the MF3+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 kJ/mol for F–Ar–BF2, F–Kr–BF2, F–Xe–BF2, F–Ar–AlF2, F–Kr–AlF2, and F–Xe–AlF2 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. [ABSTRACT FROM AUTHOR]

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 kJ/mol 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 kJ/mol 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 kJ/mol 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 kJ/mol 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. [ABSTRACT FROM AUTHOR]

In an attempt to seek out whether the reorientation time of a solute molecule is influenced by marginal changes to its shape, rotational relaxation of four coumarin solutes that are almost identical in size but subtly distinct in shape has been investigated in a viscous nonpolar solvent as a function of temperature. It has been observed that the reorientation times of the four coumarins differ significantly from one another. The four solutes have been treated as asymmetric ellipsoids and Stokes-Einstein-Debye hydrodynamic theory has been employed to calculate the shape factors and boundary condition parameters. The measured reorientation times when normalized by respective shape factors and boundary condition parameters can be scaled on a common curve, which is an indication that ellipsoid based hydrodynamic theory is adequate to model the reorientation times even when the differences in the shapes of the solute molecules are minimal. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

As an exercise in our ongoing efforts to understand the solute–solvent frictional coupling, hydrogen bonding interactions between the probe 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole and associative solvents such as n-alcohols, formamide and N-methylformamide are modeled as dielectric friction using Alavi–Waldeck (AW) extended charge distribution theory. The mechanical friction experienced by the probe is calculated using Stokes–Einstein–Debye hydrodynamic theory with slip boundary condition and also from the measured reorientation times of another structurally similar, but nonhydrogen bonding solute molecule, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole. When the size of the solvent domains is used as an adjustable parameter in the AW analysis, a good agreement between the experimentally measured reorientation times and the calculated ones is obtained. The limitations of the applicability of the AW model to associative solvents are discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In a bid to explore how the presence of electrolyte ions influence the friction experienced by hydrogen bonding and nonhydrogen bonding solute molecules, rotational dynamics of two structurally similar nondipolar probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been investigated in dimethylsulfoxide (DMSO) at several concentrations of LiNO[sub 3]. The reorientation times of DMDPP, which does not strongly interact with the solvent, follow solution viscosity and dielectric parameters as the electrolyte concentration is increased. However, for DPP, which forms hydrogen bonds with DMSO, there is a 30% decrease in the viscosity-normalized reorientation times upon the addition of 2M LiNO[sub 3] due to the presence of electrolyte ions that shield the hydrogen-bonding interactions between the solute and the solvent. However, the reorientation times correlate well with the solution dielectric parameters with an increase in the electrolyte concentration as in the case of DMDPP. An attempt has been made to model the specific interactions between DPP and DMSO as dielectric friction using the extended charge distribution model of Alavi and Waldeck since both are electrostatic in nature. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Temperature dependent rotational relaxation study of neutral and cationic forms of neutral red has been carried out in dimethylsulfoxide (DMSO) in an attempt to find out how the charge on the solute influences its dynamics. Experimental results indicate that the cationic form rotates marginally slower (less than 20%) than the neutral form. The mechanical friction has been modeled using the Stokes–Einstein–Debye hydrodynamic theory with slip boundary condition and the dielectric friction using the extended charge distribution model of Alavi–Waldeck. The marginally slower reorientation times of the cationic form has been ascribed to the effect of dielectric friction. Alternatively, it has also been explained by invoking the concept of solute–solvent hydrogen bonding due to the presence of an additional hydrogen bonding site on the cation in the form of a hydrogen atom attached to the ring nitrogen. This result is different from that of the others in literature where cationic probes experience a lot more friction than their neutral counterparts in DMSO, the reason being that the cationic and neutral solutes used in almost all the previous studies possess different functional groups. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The rotational diffusion of two structurally similar coumarins, coumarin 343 (C343) and coumarin 334 (C334) was studied in dimethyl sulfoxide (DMSO) at several concentrations of LiNO3. The addition of electrolyte ions shifts the equilibrium toward the anionic form of C434, which in turn forms ion pairs with Li(super +), and the large increase in viscosity normalized reorientation times values is due to the association of DMSO solvent molecule with these ion pairs.

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 pKa 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 (E1) value at pH 7, determined from the pulse-radiolysis experiment, was found to be –420 ± 20 mV vs. 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. [ABSTRACT FROM AUTHOR]