Neutral Trimer Research Articles

Fragmentation channels of non-fullerene cationic carbon clusters.

The unimolecular fragmentation channels of highly excited small cationic carbon clusters have been measured with a time-of-flight mass spectrometer after photofragmentation. The dominant channel is loss of the neutral trimer, for all CN+N = 10-27 clusters except for N = 11, 12 which decay by monomer emission, and C25+ which shows competing loss of C2 and C3. The results permit to quantify the role of the rotational entropy in the competition between monomer and trimer decays with the help of energies calculated with density functional theory.

Vibrational spectroscopic signatures of hydrogen bond induced NH stretch-bend Fermi-resonance in amines: The methylamine clusters and other N-H⋯N hydrogen-bonded complexes.

The appearance of multiple bands in the N-H stretching region of the infrared spectra of the neutral methylamine dimer and trimer is a sign of NH bend-stretch anharmonic coupling. Ab initio anharmonic calculations were carried out in a step-wise manner to reveal the origin of various bands observed in the spectrum of the methylamine dimer. A seven-dimensional potential energy surface involving symmetric and asymmetric stretching and bending vibrations of both the hydrogen bond donor and the acceptor along intermolecular-translational modes was constructed using the discrete variable representation approach. The resulting spectrum of the dimer shows five bands that can be attributed to the symmetric stretching (νsym D), asymmetric stretchin (νasym D), and bending overtone (2νbend D) of the donor moiety. These appear along with the combination band arising out of bending vibrations of the donor and acceptor (νbend D + νbend A) and with the combination of the intermolecular translational mode over the donor bending overtone (νtrans + 2νbend D). The spectrum of the trimer essentially consists of all the features seen in the dimer with marginal changes in band positions. The analysis of the experimental spectra based on the two-state deperturbation model and ab initio anharmonic calculations yield a matrix element of about 40 cm-1 for the N-H bend-stretch Fermi resonance coupling. In general, the IR spectra of the hydrogen-bonded amino group depict three sets of bands that arise due to bend-stretch Fermi resonance coupling.

Timescale of π‐Stacking Formation in a Benzene Trimer Cation Formed by Ionization of the Parent Neutral Trimer: A Direct Ab Initio Molecular Dynamics Study

AbstractThe π‐stacking interaction plays a crucial role in structural arrangements and molecular assemblies in several molecular systems. In the present study, π‐stacking formation in the benzene trimer cation, (Bz)3+, following ionization of the parent neutral benzene trimer, (Bz)3, was investigated using direct ab initio molecular dynamics to elucidate the timescale of π‐stacking formation and reaction mechanism in the ionized state. Cyclic (Bz)3 was chosen as the benzene cluster. After ionizing (Bz)3, the structure drastically changed to a double π‐stacking form. The (Bz)3+ product vibrated between two structural forms, (Bz‐Bzc)+—Bz ↔ Bz—(Bzc‐Bz)+, where Bzc is a benzene molecule located in the central position of (Bz)3+ and (Bzc‐Bz)+ represents a dimer core composed of the benzene dimer cation. The timescale of π‐stacking formation was calculated as 300–380 fs. The absorption spectrum of (Bz)3+ was blue‐shifted as a function of time. We discuss the reaction mechanism based on the theoretical results.

Structures, Thermochemical Properties, and Bonding of Mixed Alkaline-Earth-Metal Silicon Trimers Si3M(+/0/-) with M = Be, Mg, Ca.

The ground state geometries, electronic structures, and thermochemical properties of binary alkaline-earth-metal silicon clusters Si3M with M = Be, Mg, Ca in neutral, cationic, and anionic states were investigated using quantum chemical computations. Lowest-lying isomers of the clusters were determined on the basis of the composite G4 energies. Along with total atomization energies, thermochemical parameters were determined for the first time by means of the G4 and coupled-cluster theory with complete basis set CCSD(T)/CBS approaches. The most favored equilibrium formation sequences for Si3M clusters emerge as follows: all Si3M(+/0/-) clusters are formed by attaching the M atom into the corresponding cation, neutral and anion silicon trimer Si3(+/0/-), except for the Si3Mg(+) and Si3Ca(+) where the metal cations are bound to the neutral Si3. The resulting mixed tetramers exhibit geometrical and electronic features similar to those of the pure silicon tetramer Si4(+/0/-). Electron localization function (ELF) and ring current analyses point out that the σ-aromatic character of silicon tetramer remains unchanged upon substituting one Si atom by one alkaline-earth-metal atom.

Double π–π stacking dynamics of benzene trimer cation: direct ab initio molecular dynamics (AIMD) study

Mechanism of π–π double stacking process of benzene trimer cation has been investigated by means of direct ab initio molecular dynamics method. It was found that H-shaped form of neutral benzene trimer is drastically changed to the π–π double stacking form composed of benzene trimer cation following the ionization. Absorption spectrum of charge resonance band of (Bz) 3 + was gradually blueshifted as a function of time. Time scale of the double stacking process (the formation of trimer cation from the H-shaped form) was calculated to be 0.7–1.5 ps, which is comparable to that of dimer cation formation. After the ionization of benzene trimer, (Bz)(Bz)c(Bz′), the hole is distributed around three benzene molecules with a branching ratio of (Bz)+0.3(Bz)c+0.4(Bz′)+0.3, and the structure of (Bz)c+ was rapidly deformed due to the Jahn–Teller effect. Next, the molecular rotation of central benzene (Bz)c+ occurred gradually. Immediately, both benzene molecules in the wing positions were also rotated synchronicity. Finally, the π–π double stacking form composed of benzene trimer cation was formed. The formation mechanism of benzene trimer cation was discussed on the basis of theoretical results.

Backbone effects on the charge transport in poly-imidazole membranes: a theoretical study

In this paper we present a theoretical investigation of proton conduction in a 2-tethered poly-vinyl-imidazole system, an N-heterocyclic-based membrane used as electrolyte in proton exchange membrane fuel cells (PEMFCs). In particular a detailed analysis, combining Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations, shows the underpinning role of the backbone in determining the proton transport mechanisms. DFT calculations, carried out on a neutral trimer, suggest that proton conduction is based on a Grotthuss-like mechanism, which requires a series of proton transfers along the chain followed by a rotation of all the imidazoles. This latter represents the limiting step and it is clearly observed during the MD simulations on a larger model (oligomer of 15 imidazoles) at a typical operative temperature. From the computational point of view, this is the first time that such a mechanism is clearly evidenced in N-heterocycle-based membranes and the results give a clear explanation for the difference in proton mobility conductivities observed between P2VI and the parent poly(4-vinyl-imidazole) (P4VI). These results underline the relevant role of imidazole connectivity to the polymer backbone in determining proton conduction and could give valuable insights for the design of new and better performing protogenic groups with minimum reorientation barriers.

Structures of Thermal, Mass-Selected Water Clusters Probed with Hydrophobic Ion Tags and Infrared Photodissociation Spectroscopy

The structures of tetramethylammonium (TMA(+)) and phenyltrimethylammonium (PTMA(+)) with 1-8 water molecules attached were investigated using infrared photodissociation spectroscopy (IRPD), blackbody infrared radiative dissociation, and theory to elucidate the effects of ion-water interactions and water-water hydrogen bonding in clusters containing these hydrophobic ions. Several pieces of evidence suggest that these ions have only a small effect on water structure. The binding energy of a water molecule to TMA(+) is 44 kJ/mol, a value that is significantly lower than that reported for other cations and close to the condensation energy of bulk water. The OH asymmetric stretch of the water molecule in TMA(+).(H(2)O)(1) at 3718 cm(-1) is less red-shifted from that of an isolated water molecule (3756 cm(-1)) than for those of other cations, and water-water hydrogen bonding is extensive for all clusters with two or more water molecules. These results indicate that charge transfer from water to these hydrophobic ions is much less than that for other cations. There are two predominant structures for these ions with three water molecules: one in which water molecules form a chain and one with a cyclic structure analogous to that of neutral water trimer. With four water molecules attached, water forms a cyclic structure similar to that of neutral water tetramer. The spectra of the larger clusters follow trends reported for neutral water molecule clusters, which indicates that these hydrophobic ions have only a minimal effect on the structure of water in clusters with four or more water molecules. These results suggest that these hydrophobic ions can be used as "tags" that could make possible acquisition of IRPD spectra of even larger clusters, such as clathrates, for which precise mass selection of neutral clusters is not possible by other methods.

Fragmentation Dynamics of Size-Selected Pyrrole Clusters Prepared by Electron Impact Ionization: Forming a Solvated Dimer Ion Core

Structure and dynamics of size-selected charged pyrrole clusters have been studied by means of molecular beam scattering experiments and ab initio calculations. Small neutral Pyn clusters were produced in Py/He mixture expansions, and the scattering experiment with a secondary beam of He-atoms was exploited to select the neutral clusters of different sizes. The complete size-selected fragmentation patterns for the neutral dimer to the tetramer after an electron impact ionization at 70 eV were obtained from the measurements of the angular and velocity distributions at different fragment masses. All the investigated cluster sizes decay mainly to the monomer ions Py+1 (from 60 to 80% of the corresponding neutral size) and to the dimer ion Py+2 (20-30%). The trimer ions Py+3 are generated to less than 10% from the neutral trimer and tetramer. To explain the observed results, we have calculated the structures and energetics of pyrrole clusters up to the trimer for the neutral and the ionic state using DFT and PMP2 methods. The ab initio calculations show that ionized pyrrole clusters are formed with a dimeric core that is solvated by neutral pyrrole molecules. In addition, the ground and ionic state of Py-Ar complexes were calculated at CCSD(T) level with extended basis in relevance to the mixed clusters produced in supersonic expansions of Py seeded in Ar. The calculated dissociation energies of the Py-Ar and (Py-Ar)+ complexes indicate that Ar atoms are able to rapidly evaporate after ionization. The combined analysis of the fragmentation probabilities, and calculations allowed us to estimate the distribution of energy deposited in the clusters after the electron impact, which peaks above 1 eV and has a tail up to 5 eV.

Competitive bond breaking in floppy molecular trimers: HeNeH and HeNeH− calculations

Calculations for the J=0 bound states associated with the two title molecular systems are carried out using distributed Gaussian function approach and using the sum of two-body potential approximation to describe the overall interactions. The results yield one bound state for the neutral trimer and three bound states for the anionic triatom. The relative values of dissociation energies place the first dissociation threshold at the H emission for the neutral complex and at the He emission for the corresponding anion. The general spatial properties of the various bound states are analyzed on both systems.

Partial Oxidation of Propylene to Propylene Oxide over a Neutral Gold Trimer in the Gas Phase: A Density Functional Theory Study

We report a B3LYP study of a novel mechanism for propylene epoxidation using H(2) and O(2) on a neutral Au(3) cluster, including full thermodynamics and pre-exponential factors. A side-on O(2) adsorption on Au(3) is followed by dissociative addition of H(2) across one of the Au-O bonds (DeltaE(act) = 2.2 kcal/mol), forming a hydroperoxy intermediate (OOH) and a lone H atom situated on the Au(3) cluster. The more electrophilic O atom (proximal to the Au) of the Au-OOH group attacks the C=C of an approaching propylene to form propylene oxide (PO) with an activation barrier of 19.6 kcal/mol. We predict the PO desorption energy from the Au(3) cluster with residual OH and H to be 11.5 kcal/mol. The catalytic cycle can be closed in two different ways. In the first subpathway, OH and H, hosted by the same terminal Au atom, combine to form water (DeltaE(act) = 26.5 kcal/mol). We attribute rather a high activation barrier of this step to the breaking of the partial bond between the H atom and the central Au atom in the transition state. Upon water desorption (DeltaE(des) = 9.9 kcal/mol), the Au(3) is regenerated (closure). In the second subpathway, H(2) is added across the Au-OH bond to form water and another Au-H bond (DeltaE(act) = 22.6 kcal/mol). Water spontaneously desorbs to form an obtuse angle Au(3) dihydride, with one H atom on the terminal Au atom and the other bridging the same terminal Au atom and the central Au atom. A slightly activated rearrangement to a symmetric triangular Au(3) intermediate with two equivalent Au-H bonds, addition of O(2) into the Au-H bond, and rotation reforms the hydroperoxy intermediate in the main cycle. On the basis of the DeltaG(act), which contains contribution from both pre-exponetial factor and activation energy, we identify the propylene epoxidation step as the actual rate-determining step (RDS) in both the pathways. The activation barrier of the RDS (epoxidation step: DeltaE(act) = 19.6 kcal/mol) is in the same range as that in the published computationally investigated olefin epoxidation mechanisms involving Ti sites (without Au involved) indicating that isolated Au clusters and possibly Au clusters on non-Ti supports can be active for gas-phase partial oxidation, even though cooperative mechanisms involving Au clusters/Ti-based-supports may be favored.

A Density Functional Study of Structures and Vibrations of Ta3O and Ta3O−

Density functional calculations of neutral and anionic tantalum trimer monoxide are presented. The calculations were performed employing scalar quasi–relativistic effective core potentials. Different isomers of Ta3O and Ta3O- were studied in order to determinethe ground state structures. For both systems a planar C2vstructure with an edge-boundoxygen atom was found as ground state. Equilibrium structure parameters, harmonic frequencies, adiabatic electron affinity and Kohn-Sham orbital diagrams are reported. The calculated values are in good agreement with the available experimental data obtained from negative ion photoelectron spectroscopy. The correlation diagram between the neutral and anionic Ta3O shows that, in agreement with the experimental prediction, the extra electron in the anionic system occupies a nonbonding orbital.

Full configuration interaction calculation of singlet excited states of Be3.

The full configuration interaction (FCI) study of the singlets vertical spectrum of the neutral beryllium trimer has been performed using atomic natural orbitals [3s2p1d] basis set. The FCI triangular equilibrium structure of the ground state has been used to calculate the FCI vertical excitation energies up to 4.8 eV. The FCI vertical ionization potential for the same geometry and basis set amounts to 7.6292 eV. The FCI dipole and quadrupole transition moments from the ground state are reported as well. The FCI electric quadrupole moment of the X (3)A(1) (') ground state has been also calculated with the same basis set (Theta(zz)=-2.6461 a.u., Theta(xx)=Theta(yy)=-1/2Theta(zz)). Twelve of the 19 calculated excited singlets are doubly excited states. Most of the states have large multiconfigurational character. These results provide benchmark values for electronic correlation multireference methods. (4ex6MO)CAS-SDCI values for the same energies and properties are also reported.

Structures and vibrations of Nb3O and Nb3O-: a density functional study.

Density functional calculations of neutral and anionic niobium trimer monoxides are presented. The calculations were performed employing scalar quasirelativistic effective core potentials. In order to test the accuracy of the used effective core potentials in the framework of density functional theory the pulsed field ionization-zero electron kinetic energy photoelectron spectrum of Nb(3)O was simulated and compared to experiment. Different isomers of Nb(3)O and Nb(3)O(-) were studied in order to determine the ground state structures. For both neutral and anionic systems a planar C(2v) structure with an edge-bound oxygen atom was found as a ground state. Equilibrium structure parameters, harmonic frequencies, and adiabatic electron affinity are reported. The calculated electron affinity and frequencies are in good agreement with the available experimental data obtained recently from vibrationally resolved negative ion photoelectron spectroscopy.

Fragmentation dynamics of ionized neon trimer inside helium nanodroplets: a theoretical study.

We report a theoretical study of the fragmentation dynamics of Ne(3) (+) inside helium nanodroplets, following vertical ionization of the neutral neon trimer. The motion of the neon atoms is treated classically, while transitions between the electronic states of the ionic cluster are treated quantum mechanically. A diatomics-in-molecules description of the potential energy surfaces is used, in a minimal basis set consisting of three effective p orbitals on each neon atom for the missing electron. The helium environment is modeled by a friction force acting on the neon atoms when their speed exceeds the Landau velocity. A reasonable range of values for the corresponding friction coefficient is obtained by comparison with existing experimental measurements.

Formation of hydrogen peroxide from H 2 and O 2 over a neutral gold trimer: a DFT study

Our density-functional theory study of the formation of hydrogen peroxide over a neutral Au 3 cluster details a reaction path with activation barriers less than 10 kcal/mol. The reactions proceed on the edges and one side of the triangular Au 3 cluster which makes this mechanism viable for a cluster in contact with a support surface. The Au 3 cluster remains in a triangular geometry throughout the reaction but the electron population on the Au trimer during the catalytic cycle proper, as calculated with the Natural Bond Orbital method, varies from a charge of +0.304 (cationic) (Au 3O 2H 2) to −0.138 (anionic) (Au 3H 2). Au 3 in the reaction initiation intermediate, Au 3O 2, is also cationic in character with a charge of +0.390. It is interesting to note that the interaction of Au 3 with a model oxidic support, TS-1, was essentially neutral in character, the Au 3 charge population being −0.044. Formation of hydrogen peroxide does not involve breaking the OO bond, but does break the HH bond in a step that is rate limiting under standard conditions. The highest energy barrier in the cycle is 8.6 kcal/mol for desorption of H 2O 2 from Au 3H 2. Adsorption of H 2O 2 on this site is unactivated. This route to formation of hydrogen peroxide combined with existing mechanisms for epoxidation by H 2O 2 over TS-1 gives a fully plausible, energetically favorable, closed cycle for epoxidation of propylene by H 2 and O 2 over Au/TS-1 catalysts. Thus, isolated molecular gold clusters can act as viable sites for this reaction.

Full configuration interaction calculation of Be3.

The full configuration interaction (FCI) study of the ground state of the neutral beryllium trimer has been performed using an atomic natural orbitals [3s2p1d] basis set. Both triangular and linear structures have been considered for the Be(3) cluster. The optimal geometry for the equilateral triangle has been calculated. The potential energy cut sections along the normal a(1)(') mode and one of the components of the e(') mode have then been studied. The FCI symmetric atomization potential of the linear cluster is also reported. It shows a secondary van der Waals minimum at a long bond distance. All singular points in the potential energy curves are characterized. Other properties, like dissociation energies D(e) and vibrational frequencies, have been estimated from a fourth-order fitting of a large range of points around the minima. The calculated FCI wave number values for the nu(1) and nu(2) normal modes are (467.33+/-0.43) cm(-1) and (390.77+/-0.56) cm(-1).

Binding Energies of the Neutral and Ionic Clusters of Naphthalene in Their Ground Electronic States

The binding energies of the neutral and ionic clusters of naphthalene have been determined from the two-color two-photon measurements of the ionization potentials and appearance potentials. The measured binding energies (D0) of the neutral dimer is significantly smaller than the binding energies (De) computed by correlated (MP2) quantum chemistry calculations. Interestingly, the experimental binding energy of the naphthalene dimer ion is very close to half the transition energy of its charge resonance absorption band, indicating that the dimer ion is stabilized mostly by charge resonance interactions. The slow increase in the dimer ion current with increasing photon energy, near the threshold of the excimer-mediated photoionization, suggests that the geometry of the dimer ion is significantly different from the sandwich-pair geometry of the excimer. Finally, the measured evaporation energies of the neutral tetramer, trimer ion, and tetramer ion support the trimer core structure of the neutral tetramer and the dimer core structure of the ionic clusters.

Ag 3 Born–Oppenheimer potential hypersurfaces

The full Born–Oppenheimer potential energy hypersurfaces of the Ag3 system have been explored by the local spin-density scheme using an analytic potential. Our calculated physical properties, such as dissociation energies and barriers to isomerization, compare well with the available high-level configuration interaction calculations and experiments at the equilibrium geometry. Despite its simplicity, the analytic potential provides an excellent description of the neutral trimer system for silver and the other transition metals.

Negative ion photoelectron spectra simulation of V3O from a density functional study

A density functional study of neutral and anionic vanadium trimer monoxides is presented. The calculations were of all-electron type employing a newly developed basis set for the vanadium atom. Different isomers of V3O and V3O− were studied in order to determine the ground state structures. For both systems a planar C2v structure with an edge-bonded oxygen atom was found as the ground state. Equilibrium structure parameters of ground states as well as low-lying excited states, harmonic frequencies, the adiabatic electron affinity, and Kohn–Sham orbital diagrams are reported. The experimental negative ion photoelectron spectra of V3O was simulated by calculating multidimensional Franck–Condon factors, using the geometries and harmonic frequencies of the calculated ground states of V3O− and V3O. The good agreement between the experimental and the theoretical spectra allows the determination of the ground state structure of V3O and V3O−. This represents the first work in which a simulation of a vibrationally resolved negative ion photoelectron spectra of a transition metal oxide is presented.

Structures and vibrations of V3O and V3O−: A density functional study

AbstractDensity functional calculations of neutral and anionic vanadium trimer monoxide are presented. The calculations were of all‐electron type, employing a newly developed basis set for the vanadium atom. Different isomers of V3O and V3O− were studied to determine the ground‐state structures. For both systems, a planar C2v structure with an edge‐bound oxygen atom was found as ground state. Equilibrium‐structure parameters, harmonic frequencies, adiabatic electron affinity and Kohn–Sham orbital diagrams are reported. The calculated values are in good agreement with the available experimental data obtained from negative ion photoelectron spectroscopy. The correlation diagram between the neutral and anionic V3O shows that, in agreement with the experimental prediction, the extra electron in the anionic system occupies a nonbonding orbital. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003