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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.

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 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.

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 LiNO3. 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 LiNO3 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 conce...

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 fri...

The orbital momentum profiles and binding energy spectra for the complete valence shell of propane are reported. The experiment has been performed using a high energy resolution (ΔE=0.95 eV FWHM) multichannel (e,2e) electron momentum spectrosocopy spectrometer at an impact energy of 1200 eV plus the binding energy. The measured binding energy spectra are compared and consistent with PES data available in the literature and also with the predictions of Hartree–Fock, Green’s function and MRSD-CI methods. A strong splitting observed in the inner valence energy spectra due to electron correlation and ion relaxation effects is confirmed by MRSD-CI calculations. The experimental momentum profiles have been compared with calculations obtained using the target Hartree–Fock method with a minimum basis set and also a very large basis set. Density functional theory calculations using B3LYP functionals as well as large basis set MRSD-CI calculations are also reported. The agreement between theory and experiment for t...

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...

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.

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.

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.

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.