Cartesian coupled coherent states simulations: Ne(n)Br2 dissociation as a test case

In this article, we describe coupled coherent states (CCS) simulations of vibrational predissociation of weakly bounded complexes. The CCS method is implemented in the Cartesian frame in a manner that is similar to classical molecular dynamics. The calculated lifetimes of the vibrationally excited Ne-Br(2)(ν) complexes agree with experiment and previous calculations. Although the CCS method is, in principle, a fully quantum approach, in practice it typically becomes a semiclassical technique at long times. This is especially true following dissociation events. Consequently, it is very difficult to converge the quantum calculations of the final Br(2) vibrational distributions after predissociation and of the autocorrelation functions. However, the main advantage of the method is that it can be applied with relative ease to determine the lifetimes of larger complexes and, in order to demonstrate this, preliminary results for tetra- and penta-atomic clusters are reported.     

Structure and stability of Ne+He(n): experiment and diffusion quantum Monte Carlo theory with "on the fly" electronic structure

New data are reported for the mass-spectrometry fragmentation patterns of helium clusters, either pure or containing a Ne or an Ar atom. The patterns for He(n)+ and Ar+He(n) show clear evidence of structure, while those of Ne+He(n) do not. To better understand the surprising result for the Ne+He(n) fragments, diffusion quantum Monte Carlo (DMC) calculations of the energies and structural properties of these ions were performed using a diatomics-in-molecule (DIM) parametrization of the potential energy. Using DIM for electronic energy evaluation allows us to sample 10(9) configurations even for a cluster as large as Ne+He14. The results of the DMC calculation are very surprising. For n > 7, the DMC random walkers rarely venture within 100 cm(-1) of the minimum potential energy. Analysis of the resulting particle density distributions shows that the zero-point energy does more than spread the wave function around the potential-energy minima, resulting in very diffuse wave functions. For some of the clusters the quantum effects nearly exclude the region of the potential minimum from the overall wave function. An important result of this effect is that the incremental bonding energy of the nth helium atom varies quite smoothly with n, for n > 5. This eliminates the expected shell structure and explains the lack of magic-number-type features in the data.     

Nonadiabatic molecular dynamics of photoexcited Li2(+) Ne(n) clusters

We investigate the relaxation of photoexcited Li(2)(+) chromophores solvated in Ne(n) clusters (n = 2-22) by means of molecular dynamics with surface hopping. The simplicity of the electronic structure of these ideal systems is exploited to design an accurate and computationally efficient model. These systems present two series of conical intersections between the states correlated with the Li+Li(2s) and Li+Li(2p) dissociation limits of the Li(2)(+) molecule. Frank-Condon transition from the ground state to one of the three lowest excited states, hereafter indexed by ascending energy from 1 to 3, quickly drives the system toward the first series of conical intersections, which have a tremendous influence on the issue of the dynamics. The states 1 and 2, which originate in the Frank-Condon area from the degenerated nondissociative 1(2)Π(u) states of the bare Li(2)(+) molecule, relax mainly to Li+Li(2s) with a complete atomization of the clusters in the whole range of size n investigated here. The third state, which originates in the Frank-Condon area from the dissociative 1(2)Σ(u)(+) state of the bare Li(2)(+) molecule, exhibits a richer relaxation dynamics. Contrary to intuition, excitation into state 3 leads to less molecular dissociation, though the amount of energy deposited in the cluster by the excitation process is larger than for excitation into state 1 and 2. This extra amount of energy allows the system to reach the second series of conical intersections so that approximately 20% of the clusters are stabilized in the 2(2)Σ(g)(+) state potential well for cluster sizes n larger than 6.     

Fragmentation dynamics of ionized neon clusters (Ne(n), n=3-14) embedded in helium nanodroplets

We report a theoretical study of the nonadiabatic fragmentation dynamics of ionized neon clusters embedded in helium nanodroplets for cluster sizes up to n=14 atoms. The dynamics of the neon atoms is modeled using the molecular dynamics with quantum transitions method of Tully [J. Chem. Phys. 93, 1061 (1990)] with the nuclei treated classically and transitions between electronic states quantum mechanically. The potential-energy surfaces are derived from a diatomics-in-molecules model to which induced dipole-induced dipole interactions are added. The effect of the spin-orbit interaction is also discussed. The helium environment is modeled by a friction force acting on charged atoms whose speed exceeds the critical Landau velocity. The dependence of the fragment size distribution on the friction strength and on the initial nanodroplet size is investigated. By comparing with the available experimental data obtained for Ne3+ and Ne4+, a reasonable value for the friction coefficient, the only parameter of the model, is deduced. This value is then used to predict the effect of the helium environment on the dissociation dynamics of larger neon clusters, n=5-14. The results show stabilization of larger fragments than in the gas phase, but fragmentation is not completely caged. In addition, two types of dynamics are characterized for Ne4+: fast and explosive, therefore leaving no time for friction to cool down the process when dynamics starts on one of the highest electronic states, and slower, therefore leading to some stabilization by helium when it starts on one of the lowest electronic states.     

Predicting atomic dopant solvation in helium clusters: the MgHe(n) case

We present a quantum Monte Carlo study of the solvation and spectroscopic properties of the Mg-doped helium clusters MgHe(n) with n=2-50. Three high-level [MP4, CCSD(T), and CCSDT] MgHe interaction potentials have been used to study the sensitivity of the dopant location on the shape of the pair interaction. Despite the similar MgHe well depth, the pair distribution functions obtained in the diffusion Monte Carlo simulations markedly differ for the three pair potentials, therefore indicating different solubility properties for Mg in He(n). Moreover, we found interesting size effects for the behavior of the Mg impurity. As a sensitive probe of the solvation properties, the Mg excitation spectra have been simulated for various cluster sizes and compared with the available experimental results. The interaction between the excited 1P Mg atom and the He moiety has been approximated using the diatomics-in-molecules method and the two excited 1pi and 1sigma MgHe potentials. The shape of the simulated MgHe50 spectra shows a substantial dependency on the location of the Mg impurity, and hence on the MgHe pair interaction employed. To unravel the dependency of the solvation behavior on the shape of the computed potentials, exact density-functional theory has been adapted to the case of doped He(n) and various energy distributions have been computed. The results indicate the shape of the repulsive part of the MgHe potential as an important cause of the different behaviors.     

Bonding in SCln (n = 1-6): a quantum chemical study

Following a previous study of bonding and isomerism in the SF(n) and singly chloro-substituted SF(n-1)Cl (n = 1-6) series, we describe bonding in the ground and low-lying excited states of the completely substituted series, SCl(n) (n = 1-6). All structures were characterized at least at the RCCSD(T)/aug-cc-pV(Q+d)Z level of theory. Both differences and similarities were observed between SCl(n) and our previous results on SF(n-1)Cl and SF(n). Several minimum structures that exist in SF(n) and SF(n-1)Cl are absent in SCl(n). For example, the optimized structure of SCl(2)((3)A(2)) is a transition state in C(s) symmetry, whereas the analogous states are minima in SF(n) and SF(n-1)Cl. Second, we found a continuation of a trend discovered in the SF(n-1)Cl series, where Cl substitution has a destabilizing effect that weakens bonds with respect to SF(n). This effect is much stronger in the SCl(n) series than it is in the SF(n-1)Cl series, which is why SCl(2) is the most stable observed species in the family and why SCl(4), SCl(5), and SCl(6) are unstable (SCl(n-2) + Cl(2) additions are endothermic for n = 4-6).     

Dissociative ionization of neon clusters Ne(n), n=3 to 14: a realistic multisurface dynamical study

The molecular dynamics with quantum transitions (MDQT) method is applied to study the fragmentation dynamics of neon clusters following vertical ionization of neutral clusters with 3 to 14 atoms. The motion of the neon atoms is treated classically, while transitions between the adiabatic electronic states of the ionic clusters are treated quantum mechanically. The potential energy surfaces are described by the diatomics-in-molecules model in a minimal basis set consisting of the effective 2p orbitals on each neon atom for the missing electron. The fragmentation mechanism is found to be rather explosive, with a large number of events where several atoms simultaneously dissociate. This is in contrast with evaporative atom by atom fragmentation. The dynamics are highly nonadiabatic, especially at shorter times and for the larger clusters. Initial excitation of the neutral clusters does not affect the fragmentation pattern. The influence of spin-orbit coupling is also examined and found to be small, except for the smaller size systems for which the proportion of the Ne+ fragment is increased up to 43%. From the methodological point of view, most of the usual momentum adjustment methods at hopping events are shown to induce nonconservation of the total nuclear angular momentum because of the nonzero electronic to rotation coupling in these systems. A new method for separating out this coupling and enforcing the conservation of the total nuclear momentum is proposed. It is applied here to the MDQT method of Tully but it is very general and can be applied to other surface hopping methods.     

Energy transfer kinetics of the np5(n + 1)p excited states of Ne and Kr

Energy transfer rate constants for Ne(2p(5)3p) and Kr(4p(5)5p) atoms colliding with ground state rare gas atoms (Rg) have been measured. In part, this study is motivated by the possibility of using excited rare gas atoms as the active species in optically pumped laser systems. Rg(np(5)(n + 1)s) metastable states may be produced using low-power electrical discharges. The potential then exits for optical pumping and laser action on the np(5)(n + 1)p ? np(5)(n + 1)s transitions. Knowledge of the rate constants for collisional energy transfer and deactivation of the np(5)(n + 1)p states is required to evaluate the laser potential for various Rg + buffer gas combinations. In the present study we have characterized energy transfer processes for Ne (2p(5)3p) + He for the six lowest energy states of the multiplet. Rate constants for state-to-state transfer have been determined. Deactivation of the lowest energy level of Kr (4p(5)5p) by He, Ne, and Kr has also been characterized. Initial results suggest that Kr (4p(5)5p) + Ne mixtures may be the best suited for optically pumped laser applications.     

Large-amplitude quantum mechanics in polyatomic hydrides. II. A particle-on-a-sphere model for XH(n) (n=4,5)

This paper describes the application of a relatively simple, but computationally tractable, "particle-on-a-sphere" (POS) model for quantum-mechanical calculation of large-amplitude, H atom dynamics in polyatomic hydrides (XH(n)), based on radially relaxed, two-dimensional angular motion of H atoms on the surface of a sphere. This work focuses on systems with many degrees of freedom, i.e., XH(4) (eight dimensional) and XH(5) (ten dimensional), with corresponding molecular analogs of CH(4) and CH(5) (+) and is applicable to rovibrationally excited states with J> or =0. A pairwise-additive potential fit for CH(5) (+), which yields remarkable agreement with geometries, energies, and barrier heights on the full-dimensional surface of Brown et al. [J. Chem. Phys. 121, 4105 (2004)] is presented. Comparisons with experimental data and diffusion quantum Monte Carlo (DMC) methods test convergence for the POS model and provide insight into multidimensional quantum rovibrational dynamics. In particular, POS energy-level patterns for a series of scaled CH(5) (+) potentials indicate an absence of strong tunneling behavior, consistent with the highly delocalized wave functions, large zero-point energies, and small interconversion barriers noted in previous DMC studies of Brown et al.     

A quantum chemistry study on boron nitrides (BNN)2n and (BNN)2n2+ (n = 3–8)

(BNN)_2n (n = 3–8) with eclipsed D_nh conformations (Series I), except (BNN)₈ with O_h conformation, (BNN)_2n²⁺ (n = 3–8) with staggered D_nd conformations (Series II), and the analogous BCO‐based compounds have been studied in the present paper using density functional theory. All the species are located as genuine minima and show high stability with respect to the dissociation into BNN (BCO) or N₂ (CO). It is found that the strong aromaticity is one of the important factors in the stability of Series II and the angular strain dominantly affects the stability of Series I with relatively weak aromaticity. The same is also observed for the corresponding BCO analogies. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

The importance of methylation in the binding of (ferrocenylmethyl)tempammonium guests by cucurbit[n]uril (n=7, 8) hosts

The extent of methylation of the central nitrogen in (ferrocenylmethyl)tempammonium guests determines the main binding site for complexation by the cucurbit[8]uril host.     

The collision-induced electric dipole moment of H(n=2) in H-He,Ne and Ar collisions

The collision-induced electric dipole moment of H(n=2) in H(1s)-Ne and H(1s)-Ar collisions was measured in an energy range of 1 to 25 keV. For these systems we observe a positive electric dipole moment which corresponds to an electron lagging behind the proton. This behaviour is in contrast to recent measurements for the H-He systems, where a negative dipole moment corresponding to an electron moving in front of the proton was observed. A simple explanation for this difference is given.     

Stereographic projection path integral simulations of (HCl)n clusters (n=2-5): evidence of quantum induced melting in small hydrogen bonded networks

We investigate the quantum thermodynamic properties of small (HCl)(n) clusters using stereographic projection path integral simulations. The HCl stretches are rigid, the orientations are mapped with stereographic projection coordinates, and we make use of the reweighted random series techniques to obtain cubic convergence with respect to the number of path coefficients. Path integral simulations are converged at and above 10 K for the pentamer and above 15 K for the dimer and the trimer. None of the systems display a melting feature in the classical limit. We find an evidence of quantum induced melting between 15 and 45 K.     

Experimental and computational study of the Zn(n)S(n) and Zn(n)S(n)+ clusters

Zinc sulfide clusters produced by direct laser ablation and analyzed in a time-of-flight mass-spectrometer, showed evidence that clusters composed of 3, 6, and 13 monomer units were ultrastable. The geometry and energies of neutral and positively charged Zn(n)S(n) clusters, up to n = 16, were obtained computationally at the B3LYP/6-311+G level of theory with the assistance of an algorithm to generate all possible structures having predefined constraints. Small neutral and positive clusters were found to have planar geometries, neutral three-dimensional clusters have the geometry of closed-cage polyhedra, and cationic three-dimensional clusters have structures with a pair of two-coordinated atoms. Physical properties of the clusters as a function of size are reported. The relative stability of the positive stoichiometric clusters provides a thermodynamic rationale for the experimental results.     

Bonding and isomerism in SF(n-1)Cl (n = 1-6): a quantum chemical study

Using high-level MRCI and CCSD(T) quantum chemical calculations, we report structures, energetics, and other properties of the sulfur fluoromonochloride family (SF(n-1)Cl, n = 1-6). Our group previously studied the sulfur fluoride family (SF(n), n = 1-6) and found that several of the excited states of SF and SF(2) as well as the ground states of SF(3)-SF(6) exhibited a new type of bonding, called recoupled pair bonding. Comparing the SF(n-1)Cl and SF(n) species allows us to study isomerism, apicophilicities, and substituent effects due to the Cl substitution. The primary findings of this work are twofold. First, replacing F with Cl weakens the adjacent S-F bonds by destabilizing the molecule with respect to the pure SF(n) analog. Second, an isomer with a singly occupied S-Cl antibonding orbital is more stable than the analogous isomer with a singly occupied S-F antibonding orbital, thus explaining apicophilicities. This work has also allowed us to further refine and expand our understanding of the nature of the recoupled pair bond model. Finally, we discovered the presence of bond-stretch isomers in the first excited ((3)A') state of SFCl.     

Thermochemistry of halomethanes CF(n)Br(4-n) (n = 0-3) based on iPEPICO experiments and quantum chemical computations

Internal energy selected bromofluoromethane cations were prepared and their internal energy dependent fragmentation pathways were recorded by imaging photoelectron photoion coincidence spectroscopy (iPEPICO). The first dissociation reaction is bromine atom loss, which is followed by fluorine atom loss in CF(3)Br and CF(2)Br(2) at higher energies. Accurate 0 K appearance energies have been obtained for these processes, which are complemented by ab initio isodesmic reaction energy calculations. A thermochemical network is set up to obtain updated heats of formation of the samples and their dissociative photoionization products. Several computational methods have been benchmarked against the well-known interhalogen heats of formation. As a corollary, we stumbled upon an assignment issue for the ClF heat of formation leading to a 5.7 kJ mol(-1) error, resolved some time ago, but still lacking closure because of outdated compilations. Our CF(3)(+) appearance energy from CF(3)Br confirms the measurements of Asher and Ruscic (J. Chem. Phys. 1997, 106, 210) and Garcia et al. (J. Phys. Chem. A 2001, 105, 8296) as opposed to the most recent result of Clay et al. (J. Phys. Chem. A 2005, 109, 1541). The ionization energy of CF(3) is determined to be 9.02-9.08 eV on the basis of a previous CF(3)-Br neutral bond energy and the CF(3) heat of formation, respectively. We also show that the breakdown diagram of CFBr(3)(+), a weakly bound parent ion, can be used to obtain the accurate adiabatic ionization energy of the neutral of 10.625 ± 0.010 eV. The updated 298 K enthalpies of formation Δ(f)H(o)(g) for CF(3)Br, CF(2)Br(2), CFBr(3), and CBr(4) are reported to be -647.0 ± 3.5, -361.0 ± 7.4, -111.6 ± 7.7, and 113.7 ± 4 kJ mol(-1), respectively.     

Structures and energetics of Be(n)Si(n) and Be(2n)Si(n) (n = 1-4) clusters

The structures and energies of Be(n)Si(n) and Be(2n)Si(n) (n = 1-4) clusters have been examined in ab initio theoretical electronic structure calculations. Cluster geometries have been established in B3LYP/6-31G(2df) calculations and accurate relative energies determined by the G3XMP2 method. The two atoms readily bond to each other and to other atoms of their own kind. The result is a great variety of low-energy clusters in a variety of structural types.     

Chemical effects of (n, γ) reactions in periodate systems

Szilard-Chalmers effect in crystalline periodates NaIO₄, Na₃H₂IO₆ and Na₄I₂O₉·3H₂O containing¹²⁷I and¹²⁷I+¹²⁹I was investigated. The initial yields and thermal annealing behaviour of the¹²⁸I and¹³⁰I induced activities were found to be different for each type of periodate; both isotopes kept their own behaviour in systems having at the same time¹²⁸I+¹³⁰I and an isotope effect was evident. The de-excitation of the hot atoms through Auger processes and subsequent relaxation steps in a particular chemical and structural environment seems to explain the results better than nuclear recoil.     

Nb(9)PdAs(7): a unique arrangement in the M(n2+3n+2)X(n2+n)Y family of hexagonal structures

The ternary transition-metal arsenide Nb(9)PdAs(7) has been prepared through reaction of the elements, and its structure has been determined by single-crystal X-ray diffraction methods. It adopts a new structure type (Pearson symbol hP51, hexagonal, space group P6, Z = 3), with unit cell parameters a = 16.6955(6) and c = 3.5582(1) A. The structure contains assemblies of As-centered trigonal prisms that extend as triangular columns through sharing of the triangular faces. Not only does Nb(9)PdAs(7) extend a family of hexagonal structures with general formula M(n2+3n+2)X(n2+n)Y to n = 4, the highest member known thus far, but it also displays the unique feature in which there are two distinct types of triangular columns, one having corner atoms (Pd) different from the other atoms (Nb). Structural relationships between members of the M(n2+3n+2)X(n2+n)Y family are presented. The chemical bonding in Nb(9)PdAs(7) was analyzed through an extended Hückel band structure calculation.     

Solvation effects on angular distributions in H- (NH3)n and NH2(-) (NH3)n photodetachment: role of solute electronic structure

We report 355 and 532 nm photoelectron imaging results for H(-)(NH(3))(n) and NH(2)(-)(NH(3))(n), n = 0-5. The photoelectron spectra are consistent with the electrostatic picture of a charged solute (H(-) or NH(2)(-)) solvated by n ammonia molecules. For a given number of solvent molecules, the NH(2)(-) core anion is stabilized more strongly than H(-), yet the photoelectron angular distributions for solvated H(-) deviate more strongly from the unsolvated limit than those for solvated NH(2)(-). Hence, we conclude that solvation effects on photoelectron angular distributions are dependent on the electronic structure of the anion, i.e., the type of the initial orbital of the photodetached electron, rather than merely the strength of solvation interactions. We also find evidence of photofragmentation and autodetachment of NH(2)(-)(NH(3))(2-5), as well as autodetachment of H(-)(NH(3))(5), upon 532 nm excitation of these species.