Theoretical investigation of rare gas hydride cations: HRgN2+ (Rg=He, Ar, Kr, and Xe).

Rare gas containing protonated nitrogen cations, HRgN(2)(+) (Rg=He, Ar, Kr, and Xe), have been predicted using quantum computational methods. HRgN(2)(+) ions exhibit linear structure (C(∞v) symmetry) at the minima and show planar structure (C(s) symmetry) at the transition state. The stability is determined by computing the energy differences between the predicted ions and its various unimolecular dissociation products. Analysis of energy diagram indicates that HXeN(2)(+) is thermodynamically stable with respect to dissociated products while HHeN(2)(+), HArN(2)(+), and HKrN(2)(+) ions are metastable with small barrier heights. Moreover, the computed intrinsic reaction coordinate analysis also confirms that the minima and the 2-body global dissociation products are connected through transition states for the metastable ions. The coupled-cluster theory computed dissociation energies corresponding to the 2-body dissociation (HN(2)(+) + Rg) is -288.4, -98.3, -21.5, and 41.4 kJ mol(-1) for HHeN(2)(+), HArN(2)(+), HKrN(2)(+), and HXeN(2)(+) ions, respectively. The dissociation energies are positive for all the other channels implying that the predicted ions are stable with respect to other 2- and 3-body dissociation channels. Atoms-in-molecules analysis indicates that predicted ions may be best described as HRg(+)N(2). It should be noted that the energetic of HXeN(2)(+) ion is comparable to that of the experimentally observed stable mixed cations, viz. (RgHRg')(+). Therefore, it may be possible to prepare and characterize HXeN(2)(+) ions in an electron bombardment matrix isolation technique.

The van der Waals coefficients between carbon nanostructures and small molecules: A time-dependent density functional theory study.

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.

Theoretical prediction of HRgCO(+) ion (Rg=He, Ne, Ar, Kr, and Xe).

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.

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.

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.

Photoionization of CH(3)I mediated by the C state in the visible and ultraviolet regions.

Three/two-photon resonant multiphoton ionization (MPI) of the CH3I monomer has been studied in the gas phase at 532 and 355 nm using time-of-flight mass spectrometry. Under low laser intensity (approximately 10(9) W/cm2) the mass spectra showed peaks at m/z 15, 127 and 142, corresponding to [CH3]+, [I]+ and [CH3I]+ species, at both these wavelengths. The laser power dependence for [CH3I]+, [I]+ and [CH3]+ ions showed a three-photon dependence at 532 nm. For the same three ions, photoionization studies at 355 nm gave a power dependence of 2. Both these results suggest that a vibronic energy level at approximately 7 eV, lying in the Rydberg C state, acts as a resonant intermediate level in ionization of CH3I. In the case of 355 nm, with increasing intensity additional peaks at m/z 139 and 141 were observed which could be assigned to [CI]+ and [CH2I]+ fragments. In contrast, for high intensity radiation at 532 nm ( approximately 2 x 10(10) W/cm2), only the [CI]+ fragment was observed. At these wavelengths, fragment ions observed in mass spectra mainly arise from photodissociation of the parent ion. Experiments at another wavelength in the visible region (564.2 nm) confirmed the results obtained at 532 nm. In order to assess the role of the A state in these MPI experiments, additional experiments were performed at 266 and 282.1 nm, which access the A state directly via a one-photon transition, and showed absence of a surviving precursor ion. Reaction energies for various possible dissociation channels of CH3I/[CH3I]+/[CH2I]+ were calculated theoretically at the MP2 level using the GAMESS electronic structure program.

Is molecular rotation really influenced by subtle changes in molecular shape?

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.

Antioxidant activity of bakuchiol: experimental evidences and theoretical treatments on the possible involvement of the terpenoid chain.

The protective activity of the plant-derived meroterpene, bakuchiol [1-(4-hydroxyphenyl)-3,7-dimethyl-3-vinyl-1,6-octadiene, 1], against oxidative damages to lipids and proteins has been investigated and rationalized based on the scavenging activity of 1 against various oxidizing radicals (Cl(3)CO(2)(*), linoleic acid peroxyl radicals, LOO(*), DPPH radicals, (*)OH, and glutathiyl radicals). The rate constants of the scavenging reactions, transients formed in these reactions, and their mechanistic pathways have been probed using optical pulse radiolysis technique. Besides 1, its methyl ether derivative 2 also could prevent lipid peroxidation in rat brain homogenate, indicating the probable participation of their terpenoid chains in scavenging LOO(*). This was further corroborated from the pulse radiolytic studies on the reaction between the glutathiyl radicals and the compounds 1 and 2 as well as two other congeners, 3 and 4, which showed transient absorptions at approximately 300 nm attributable to some C-centered allylic radicals. On the basis of the strong signals at approximately 300 nm with 1-3 as compared to compound 4, formation of the allylic radical adjacent to the trisubstituted olefin function in 1-3 was envisaged. This was confirmed by quantum chemical calculations of the relative energies of the probable radical species derivable from 2 using Hartree-Fock and density functional theory along with self-consistent reaction field model. In the case of 1, the allylic radical was found to be transformed into the phenoxyl radical at a later stage. All of these data revealed, for the first time, the importance of the terpenoid moiety of bakuchiol in controlling its antioxidant action via radical scavenging.