Spontaneous Formation of Star-Shaped Surface Patterns in a Driven Bose-Einstein Condensate

We observe experimentally the spontaneous formation of star-shaped surface patterns in driven Bose-Einstein condensates. Two-dimensional star-shaped patterns with l-fold symmetry, ranging from quadrupole (l=2) to heptagon modes (l=7), are parametrically excited by modulating the scattering length near the Feshbach resonance. An effective Mathieu equation and Floquet analysis are utilized, relating the instability conditions to the dispersion of the surface modes in a trapped superfluid. Identifying the resonant frequencies of the patterns, we precisely measure the dispersion relation of the collective excitations. The oscillation amplitude of the surface excitations increases exponentially during the modulation. We find that only the l=6 mode is unstable due to its emergent coupling with the dipole motion of the cloud. Our experimental results are in excellent agreement with the mean-field framework. Our work opens a new pathway for generating higher-lying collective excitations with applications, such as the probing of exotic properties of quantum fluids and providing a generation mechanism of quantum turbulence.

Tailoring of spectral response and intramolecular charge transfer in β-enaminones through band gap tuning: synthesis, spectroscopy and quantum chemical studies

In this paper, we investigate the synthetic tailoring of the spectral response and intramolecular charge transfer (ICT) of β-enaminones through bandgap modulation.

Supramolecular Macrocyclic Pd(II) and Pt(II) Squares and Rectangles with Aryldithiolate Ligands and their Excellent Catalytic Activity in Suzuki C-C Coupling Reaction

Addition of 1,4-benezenedithiol and 4,4'-biphenyldithiol to M(OTf)2 (M = cis-[Pt(PEt3)2](2+) or cis-[Pd(dppe)](2+)) (dppe = 1,2-bis(diphenylphosphino)ethane) gave self-assembled tetranuclear complexes [M2{S(C6H4)nS}]2(OTf)4 (n = 1, 2). The same reaction with 1,4-benezenedimethanethiol yielded octanuclear supramolecular coordination complexes (SCC) [M2{SCH2C6H4CH2S}]4(OTf)8. These complexes were characterized by NMR, mass, and UV-vis spectroscopies, cyclic voltammetry, as well as density functional theory studies and represent the first examples of SCCs constructed by thiolate groups and square-planar metal ions. The rectangular shape of tetranuclear complexes and square shape of octanuclear complex are confirmed by single-crystal structures and computational studies. The palladium complexes showed excellent catalytic activity in Suzuki C-C cross-coupling reactions with high turnover numbers (2 × 10(7)), even with low catalyst loading.

Instability range of microsolvated multiply charged negative ions: prediction from detachment energy of stable hydrated clusters

We have presented a first-principle theory-based derivation of an exact expression for the solvent number-dependent electron-detachment energy of a solvated species in the thermodynamic limit. We also propose a generalized equation bridging the electron detachment energies for small and infinitely large clusters, thus providing a new route to calculate the ionization potential of a negatively charged ion from the electron-detachment energies of its stable hydrated clusters. Most importantly, it has the ability to predict the instability range of microhydrated anions. The calculated results for the ionization potential for a number of ions are found to be in good agreement with the available experimental results, and the predicted instability range for the doubly charged anions SO₄²⁻ and C₂O₄²⁻ is also consistent with experimental and ab initio results.

Solvent polarity induced structural changes in 2,6-diamino-9,10-anthraquinone dye

Photophysical properties of 2,6-diamino-9,10-anthraquinone (2,6-DAAQ) dye have been investigated in different solvents and solvent mixtures. The fluorescence quantum yields, fluorescence lifetimes, radiative rate constants, nonradiative rate constants and absorption and fluorescence spectral characteristics show unusual deviations in the lower polarity aprotic solvents in comparison to those in other aprotic solvents of medium to higher polarities. The results indicate that the dye exists in different structural forms in the lower and in the medium to higher polarity solvents. Drawing an analogy with the results reported for other amino-substituted dyes, it is inferred that 2,6-DAAQ dye adopts a planar intramolecular charge transfer (ICT) structure in medium to higher polarity solvents, where the amino lone pairs are in good resonance with the anthraquinone pi-cloud. In the lower polarity solvents, however, the dye is inferred to exist in a nonplanar structure where the amino lone pairs are not in good resonance with the anthraquinone pi-cloud. Due to these structural differences, the dye displays significantly different photophysical behavior in the lower polarity solvents than in the other solvents of medium to higher polarities. Supportive evidence for the above structural changes has been obtained from ab initio quantum chemical calculations on the structures of the dye under different conditions. Unusual deviations in the photophysical properties of 2,6-DAAQ dye in protic solvents in comparison to those in aprotic solvents of similar polarities are attributed to the intermolecular hydrogen bonding effect involving the OH groups of the protic solvents and the quinonoid oxygens of the dye.

Theoretical studies on photoelectron and IR spectral properties of Br2∙−(H2O)n clusters

We report vertical detachment energy (VDE) and IR spectra of Br2.-.(H2O)n clusters (n=1-8) based on first principles electronic structure calculations. Cluster structures and IR spectra are calculated at Becke's half-and-half hybrid exchange-correlation functional (BHHLYP) with a triple split valence basis function, 6-311++G(d,p). VDE for the hydrated clusters is calculated based on second order Moller-Plesset perturbation (MP2) theory with the same set of basis function. On full geometry optimization, it is observed that conformers having interwater hydrogen bonding among solvent water molecules are more stable than the structures having double or single hydrogen bonded structures between the anionic solute, Br2.-, and solvent water molecules. Moreover, a conformer having cyclic interwater hydrogen bonded network is predicted to be more stable for each size hydrated cluster. It is also noticed that up to four solvent H2O units can reside around the solute in a cyclic interwater hydrogen bonded network. The excess electron in these hydrated clusters is localized over the solute atoms. Weighted average VDE is calculated for each size (n) cluster based on statistical population of the conformers at 150 K. A linear relationship is obtained for VDE versus (n+3)(-1/3) and bulk VDE of Br2.- aqueous solution is calculated as 10.01 eV at MP2 level of theory. BHHLYP density functional is seen to make a systematic overestimation in VDE values by approximately 0.5 eV compared to MP2 data in all the hydrated clusters. It is observed that hydration increases VDE of bromine dimer anion system by approximately 6.4 eV. Calculated IR spectra show that the formation of Br2.--water clusters induces large shifts from the normal O-H stretching bands of isolated water keeping bending modes rather insensitive. Hydrated clusters, Br2.-.(H2O)n, show characteristic sharp features of O-H stretching bands of water in the small size clusters.

Microhydration of Cs+ ion: A density functional theory study on Cs+–(H2O)n clusters (n=1–10)

Structure, energy enthalpy, and IR frequency of hydrated cesium ion clusters, Cs+-(H2O)n (n=1-10), are reported based on all electron calculations. Calculations have been carried out with a hybrid density functional, namely, Becke's three-parameter nonlocal hybrid exchange-correlation functional B3LYP applying cc-PVDZ correlated basis function for H and O atoms and a split valence 3-21G basis function for Cs atom. Geometry optimizations for all the cesium ion-water clusters have been carried out with several possible initial guess structures following Newton-Raphson procedure leading to many conformers close in energy. The calculated values of binding enthalpy obtained from present density functional based all electron calculations are in good agreement with the available measured data. Binding enthalpy profile of the hydrated clusters shows a saturation behavior indicating geometrical shell closing in hydrated structure. Significant shifts of O-H stretching bands with respect to free water molecule in IR spectra of hydrated clusters are observed in all the hydrated clusters.

Geometrical and electronic properties of neutral and charged cesium clusters Cs(n) (n=2-10): a theoretical study

We have investigated the structure and electronic properties of cesium clusters following all electron ab initio theoretical methods based on configuration interaction, second-order Moller-Plesset (MP2) perturbation theory, and density-functional theory. Becke's three-parameter nonlocal hybrid exchange-correlation functional (B3LYP) is found to perform best on the present systems with a split valence 3-21G basis function. We have calculated the optimized geometries of neutral and singly charged cesium clusters having up to ten atoms, their binding energy per atom, ionization potentials (IPs), and adiabatic electron affinity (EA). Geometry optimizations for all the clusters are carried out without imposing any symmetry restriction. The neutral clusters having up to six atoms prefer planar structure and three-dimensional structure is preferred only when the number of atoms in a cluster is more than six. There is a good agreement between the present theoretical and reported experimental IP values for the neutral clusters with cluster size n<or=10. Calculated EA is also in good agreement with the available experimental data for the small Cs clusters. Similar calculations have also been carried out for small Rb clusters and the performance of B3LYP is equally good. Thermodynamic properties such as enthalpy and entropy for small Cs and Rb clusters have also been calculated and are well compared with the experimental data.

Structure, energy, and IR spectra of I2∙−.nH2O clusters (n=1–8): A theoretical study

The authors report theoretical results on structure, bonding, energy, and infrared spectra of iodine dimer radical anion hydrated clusters, I(2) (-).nH(2)O (n=1-8), based on a systematic study following density functional theory. Several initial guess structures are considered for each size cluster to locate minimum energy conformers with a Gaussian 6-311++G(d,p) split valence basis function (triple split valence 6-311 basis set is applied for iodine). It is observed that three different types of hydrogen bonded structures, namely, symmetrical double hydrogen bonding, single hydrogen bonding, and interwater hydrogen bonding structures, are possible in these hydrated clusters. But conformers having interwater hydrogen bonding arrangements are more stable compared to those of double or single hydrogen bonded structures. It is also noticed that up to four solvent H(2)O units can reside around the solute in interwater hydrogen bonding network. At the maximum six H(2)O units are independently linked to the dimer anion having four double hydrogen bonding and two single hydrogen bonding, suggesting the hydration number of I(2) (-) to be 6. However, conformers having H(2)O units independently linked to the iodine dimer anion are not the most stable structures. In all these hydrated clusters, the odd electron is found to be localized over two I atoms and the two atoms are bound by a three-electron hemi bond. The solvation, interaction, and vertical detachment energies are calculated for all I(2) (-).nH(2)O clusters. Energy of interaction and vertical detachment energy profiles show stepwise saturation, indicating geometrical shell closing in the hydrated clusters, but solvation energy profile fails to show such behavior. A linear correlation is observed between the calculated energy of interaction and vertical detachment energy. It is observed that formation of I(2) (-)-water cluster induces significant shifts from the normal O-H stretching modes of isolated H(2)O. However, bending mode of H(2)O remains insensitive to the successive addition of solvent H(2)O units. Weighted average energy profiles and IR spectra are reported for all the hydrated clusters based on the statistical population of individual conformers at room temperature.

Microhydration shell structure in Cl2∙−∙nH2O clusters: A theoretical study

We present the results of a detailed study on structure and electronic properties of hydrated cluster Cl2*-.nH2O (n = 1-7) based on a nonlocal density functional, namely, Becke's [J. Chem. Phys. 98, 1372 (1993)] half and half hybrid exchange-correlation functional with a split valence 6-311++G(d,p) basis function. Geometry optimizations for all the clusters are carried out with various possible initial guess structures without any symmetry restriction. Several minimum energy structures (conformers) are predicted with a small difference in total energy. There is a competition between the binding of solvent H2O units with Cl2*- dimer radical anion directly through ion-molecule interaction and forming interwater hydrogen-bonding network in Cl2*-.nH2O (n > or = 2) hydrated cluster. Structure having interwater H-bonded network is more stable over the structure where H2O units are connected to the solute dimer radical anion Cl2*- rather independently either by single or double H bonding in a particular size (n) of hydrated cluster Cl2*-.nH2O. At the maximum four solvent H2O units reside in interwater H-bonding network present in these hydrated clusters. It is observed that up to six H2O units are independently linked to the anion having four double H bondings and two single H bondings suggesting the primary hydration number of Cl2*- to be 6. In all these clusters, the odd electron is found to be mostly localized over the two Cl atoms and these two atoms are bound by a three-electron hemibond. Calculated interaction (between solute and different water clusters) and vertical detachment energy profiles show saturation at n = 6 in the hydrated cluster Cl2*-.nH2O (n = 1-7). However, calculated solvation energy increases with the increase in number of solvent H2O molecules in the cluster. Interaction energy varies linearly with vertical detachment energy for the hydrated clusters Cl2*-.nH2O (n < or = 6). Calculation of the vibration frequencies show that the formation of Cl2*(-)-water clusters induces significant shifts from the normal stretching modes of isolated water. A clear difference in the pattern of IR spectra is observed in the O-H stretching region of water from hexa- to heptahydrated cluster.

A comparative ab initio study of Br2•− and Br2 water clusters

The work presents ab initio results on structure and electronic properties of Br2*-.nH2O(n=1-10) and Br2.nH2O(n=1-8) hydrated clusters to study the effects of an excess electron on the microhydration of the halide dimer. A nonlocal density functional, namely, Becke's half-and-half hybrid exchange-correlation functional is found to perform well on the present systems with a split valence 6-31++G(d,p) basis function. Geometry optimizations for all the clusters are carried out with several initial guess structures and without imposing any symmetry restriction. Br2*-.nH2O clusters prefer to have symmetrical double hydrogen-bonding structures. Results on Br2.nH2O(n>or=2) cluster show that the O atom of one H2O is oriented towards one Br atom and the H atom of another H2O is directed to other Br atom making Br2 to exist as Br+-Br- entity in the cluster. The binding and solvation energies are calculated for the Br2*-.nH2O and Br2.nH2O clusters. Calculations of the vibrational frequencies show that the formation of Br2*- and Br2 water clusters induces significant shifts from the normal modes of isolated water. Excited-state calculations are carried out on Br2*-.nH2O clusters following configuration interaction with single electron excitation procedure and UV-VIS absorption profiles are simulated. There is an excellent agreement between the present theoretical UV-VIS spectra of Br2*-.10H2O cluster and the reported transient optical spectra for Br2*- in aqueous solution.

One Electron Oxidation Induced Dimerization of 5-Hydroxytryptophol: Role of 5-Hydroxy Substitution

Reaction of one-electron oxidant (Br(2)(*-)) with tryptophol (TP) and 5-hydroxytryptophol (HTP) have been studied in aqueous solution in the pH range from 3 to 10, employing nanosecond pulse radiolysis technique and the transients detected by kinetic spectrophotometry. One-electron oxidation of TP has produced an indolyl radical that absorbs in the 300-600 nm region with radical pK(a) = 4.9 +/- 0.2, while the reaction with HTP has produced an indoloxyl radical with lambda(max) at 420 nm and radical pK(a) < 3. Hydroxyl radicals ((*)OH) react with these two compounds producing (*)OH radical adducts that undergo water elimination to give one-electron-oxidized indolyl and indoloxyl radical species, respectively. The indoloxyl radicals react with the parent compound to form dimer radicals with an average association constant of (6.7 +/- 0.4) x 10(4) M(-1). No such dimerization is observed with indolyl radical, indicating that the presence of the 5-hydroxy group markedly alters its ability to form a dimer. A possible explanation behind such a difference in reactivity has been supported with ab initio quantum chemical calculations.

Photophysical properties of 2-amino-9,10-anthraquinone: evidence for structural changes in the molecule with solvent polarity

The photophysical properties of 2-amino-9,10-anthraquinone (2AAQ) have been investigated in different solvents and solvent mixtures and correlated with the Lippert-Mataga solvent polarity parameter, Deltaf. In the low solvent polarity region with Deltaf < ca. 0.1, the dye shows unusually high fluorescence quantum yields (Phif) and lifetimes (tauf) in comparison to those in other solvents of medium to high polarities. Similarly, the radiative rate constants (kf) are relatively lower and the non-radiative rate constants (knr) are relatively higher in the low polarity solvents in comparison to those in the medium to high polarity solvents. The current results have been rationalized assuming that the dye adopts different structural forms below and above the Deltaf value of approximately 0.1. It is inferred that in the low solvent polarity region the dye exists in a non-planar structure, with its 2-NH2 plane away from that of the 9,10-anthraquinone moiety in the ground state. In solvents of medium to high polarities, the dye exists in a polar intramolecular charge transfer (ICT) structure, where the amino lone pair of the 2-NH2 group is in strong resonance with the anthraquinone pi-cloud in the ground state. In all the solvents, however the dye is inferred to exist in the ICT structure in its excited (S1) state. Supportive evidence for the above hypothesis has been obtained from the solvent polarity effect on the Stokes' shifts for the dye. Quantum chemical studies on the structures of 2AAQ dye in the gas phase also give qualitative support for the inferences drawn from the photophysical properties of the dye in different solvents.

Multiphoton dissociation/ionization of CHCl3 and CFCl3 at 355 nm: an experimental and theoretical study

Nonresonant laser-induced multiphoton dissociation/ionization studies have been conducted for trichloromethane (CHCl3) and trichlorofluoromethane (CFCl3) at 355 nm, using time-of-flight mass spectrometry (TOFMS). The molecular ion signal was found to be missing for both these compounds, and very similar fragmentation patterns were observed. Ab initio molecular electronic structure calculations were performed to help understand the fragmentation pattern of these molecules in the laser field. The energetics of different dissociation channels in the ground states of [CHCl3]+*, [CHCl2]+, [CFCl3]+* and [CFCl2]+, as well as neutral CHCl3, CHCl2*, CFCl3 and CFCl2* systems, were calculated. On comparing theoretical results with experimentally observed ion signals and their relative abundances in TOFMS, it is inferred that these molecules undergo sequential Cl atom elimination followed by photoionization of the fragments. The absence of [CFCl]+ has been interpreted on the basis of resonant A state-mediated two-photon absorption by CFCl, and the subsequent prompt photodissociation processes occurring for this state.