Gilles H. Peslherbe

Semiempirical MNDO, AM1, and PM3 direct dynamics trajectory studies of formaldehyde unimolecular dissociation

Direct dynamics calculations are performed, using the semiempirical neglect of diatomic differential overlap (NDDO) molecular orbital theory, to explore the level of electronic structure theory required to accurately describe the product energy partitioning when formaldehyde dissociates into hydrogen and carbon monoxide. Trajectories are initiated at the saddlepoint and are propagated for the short time needed to form products, by obtaining the energy and gradient directly from the NDDO theory. The resulting product energy partitioning is compared to available experimental data and the findings of two previous trajectory studies, including one ab initio trajectory study at the HF/6‐31G** level of theory [Chem. Phys. Lett. 228, 436 (1994)]. The MNDO, AM1, and PM3 semiempirical Hamiltonians are studied, as well as Hamiltonians based on specific reaction parameters (SRP). For the latter, the original PM3 and AM1 parameters are adjusted to reproduce some ab initio potential energy surface properties, such as ...

Statistical anharmonic unimolecular rate constants for the dissociation of fluxional molecules: Application to aluminum clusters

Anharmonic densities of state are determined for the Aln (n=5,6,12,13) clusters using a model analytic potential energy function. Relative anharmonic densities of state are calculated by the multiple histogram/Nose dynamics method. Absolute densities for Al5 and Al6 are determined by Monte Carlo evaluation of the phase integral, while for Al12 and Al13 they are determined by adiabatic switching. The anharmonic densities of state are orders of magnitude larger than harmonic values based on the deepest potential energy minimum. At an energy equal to the cluster dissociation threshold, the anharmonic density is 56 and 4600 times larger than the harmonic density for Al6 and Al13, respectively. The anharmonic densities of state are used to determine anharmonic phase space theory rate constants for Al6→Al5+Al and Al13→Al12+Al dissociation. These rate constants are within a factor of 2 of the anharmonic microcanonical rate constants determined by using classical trajectories to calculate the initial decay rates ...

Structure of NaI ion pairs in water clusters

Abstract The structural properties of the thermodynamically stable NaI salt ion pairs in water clusters have been investigated by means of Monte Carlo simulations with model potentials. Attention was also paid to the structure of single ion–water Na + (H 2 O) n and I − (H 2 O) n clusters, which are found to be non-spherically symmetric at room temperature. In agreement with earlier studies, I − (H 2 O) n clusters exhibit surface structures, with the “hydrophobic” iodide ion sitting at the surface of a water network, while room-temperature Na + (H 2 O) n clusters exhibit a solvation shell structure, where solvent molecules beyond the first solvation shell tend to accumulate on one side of the cluster instead of forming a spherical droplet. Both “contact” ion pairs (CIP) and solvent-separated ion pairs (SSIP) are found to have surface structures for the smaller clusters while both interior and surface structures may exist at room temperature for cluster size of 32. A remarkable feature of the ion pair cluster structural properties is that they are very much akin to those for individual ion–water clusters, especially for SSIPs, and some insight into the ion pair cluster structures can thus be gained from single ion–water cluster structures. We propose that the (small) extent of solvent–solvent hydrogen bonding and the magnitude of the (large) solvent dipole moments in the clusters can be used to illustrate the extent of the perturbation introduced by the ions or the ion pairs in the solvent environments. In contrast to the ion pair free energetics investigated in previous work, ion–water and salt-water cluster structural properties are rather insensitive to the choice of model potentials, whether one employs non-polarizable optimized potentials for liquid simulations (OPLS) such as TIP4P/OPLS or polarizable optimized potentials for cluster simulations. The structure of NaI(H 2 O) n CIP and SSIP clusters have implications for the NaI(H 2 O) n cluster photodissociation dynamics. The large solvent dipole moments obtained for NaI(H 2 O) n clusters are indicative of increasingly larger local solvent dipoles in the clusters, which may then grow large enough to dipole-bind an electron upon cluster photoexcitation. Photoexcitation of the larger clusters might then proceed via a different route than it does for the small clusters and isolated NaI possibly involving a charge-transfer-to-solvent excited state akin to that of I − (H 2 O) n clusters. For the NaI photodissociation pathway, the surface structure of the small NaI(H 2 O) n clusters may imply a slow change in the reaction dynamics with cluster size.

Unimolecular dynamics of Cl−...CH3Cl intermolecular complexes formed by Cl−+CH3Cl association

A previous trajectory study of the dissociation of Cl−...CH3Cl complexes formed by Cl−+CH3Cl association is further analyzed to determine (1) the relationship between classical and quantum Rice–Ramsperger–Kassel–Marcus (RRKM) rate constants for Cl−...CH3Cl→Cl−+CH3Cl dissociation; (2) the importance of anharmonicity in calculating the RRKM dissociation rate constant; (3) the role of angular momentum in interpreting the trajectory distribution N(t)/N(0) of Cl−...CH3Cl complexes versus time; and (4) the pressure‐dependent collision‐averaged rate constant k(ω,E) for Cl−...CH3Cl dissociation. It is found that only the low‐frequency intermolecular modes of Cl−...CH3Cl are initially excited by Cl−+CH3Cl association. Classical and quantum RRKM rate constants for dissociation of this intermolecular complex are in excellent agreement. Anharmonicity lowers the rate constant by a factor of 4–8 from its harmonic value. The dissociation rate for the long‐time tail of the trajectory N(t)/N(0) distribution is much smalle...

Analysis and extension of a model for constraining zero‐point energy flow in classical trajectory simulations

In previous work a method, based on an instantaneous normal mode analysis and time reversal of the momentum of any local normal mode whose energy falls below its quantum harmonic zero‐point value, has been proposed for controlling zero‐point energy flow during classical trajectory simulations. From the previous work it is not clear that this ZPE constraint method, which was developed for nonrotating systems, conserves linear and angular momenta. The projection of infinitesimal rotations and translations from the Cartesian force constant matrix, during the instantaneous normal mode analysis, results in specific orthogonality relations for the vibrational eigenvectors. Since the vibrating molecular system is defined to obey the instantaneous Eckart conditions, it is shown that these orthogonality relations have a form which preserves the linear and angular momenta as constants of motion when the ZPE constraint is applied. Based on this property, an extended ZPE constraint scheme is proposed for molecular sy...

Design, synthesis, and solution behaviour of small polyamines as switchable water additives

The practice of adding salt to water to induce salting out of contaminants or to break emulsions and suspensions is generally avoided industrially because of the expense of the necessary treatment of the salty water afterwards. However, the use of switchable water, an aqueous solvent with switchable ionic strength, allows for reversible generation and elimination of salts in aqueous solution, through the introduction and removal of CO2. In the effort to improve the efficiency of these switchable salts, a physical study modeling their reactivity and solution behaviour has been performed, resulting in a set of design principles for future switchable water additives. The resulting polyamines synthesized using this template show the highest efficiency recorded for a switchable water additive.

A comparison of classical trajectory and statistical unimolecular rate theory calculations of Al3 decomposition

Classical trajectories are used to simulate the dissociation of microcanonical ensembles of Al3 clusters with 0.25–2.00 kcal/mol energy in excess of the dissociation threshold. Unimolecular lifetime distributions for the ensembles are in accord with the random lifetime prescription of Rice–Ramsperger–Kassel–Marcus (RRKM) theory and Al3 is identified as an intrinsic RRKM molecule. Unimolecular rate constants determined from the trajectories are compared with the predictions of variational RRKM theory with harmonic vibrator and flexible transition state models, phase space theory (PST), and the orbiting transition state model of phase space theory (OTS/PST). The flexible RRKM model, PST, and OTS/PST give Al2‐ ‐ ‐Al transition state sums of state which agree to within 1%. The harmonic vibrator RRKM model gives a sum of states which varies from only 40% to 10% larger as the excess energy is increased from 0.25 to 2.0 kcal/mol. Adiabatic switching and direct integration of the phase integral are used to determ...

On the transition from surface to interior solvation in iodide–water clusters

A quantitative investigation of surface vs. interior solvation in iodide–water clusters was performed by evaluating the potentials of mean force and structural properties of I−(H2O)n clusters (n=32, 64) from Monte Carlo simulations with both non-polarizable and polarizable model potentials. Simulation results clearly indicate that the iodide ion tends to reside at the surface of a water cluster of size 32, whereas entropy and polarization effects make the interior solvation state more likely for a cluster size of 64. This is consistent with previous analyses of cluster experimental and model data, which suggest a transition from surface to bulk behavior around a cluster size of 60.

Quasiclassical trajectory calculations for the OH(X 2Π) and OD(X 2Π)+HBr reactions: Energy partitioning and rate constants

The quasiclassical trajectory (QCT) method was used to study the dynamics of the OH(X 2Π) and OD(X 2Π)+HBr chemical reactions on an empirical potential energy surface (PES). The main emphasis in the calculation was the vibrational energy distributions of H2O (and HDO) and the magnitude and temperature dependence of the rate constant. However, this PES also serves as a generic model for the dynamics of direct H atom abstraction by OH radicals. Since this PES has no formal potential energy barrier, variational transition‐state theory was used to obtain rate constants for comparison with the QCT calculations and experimental results. The parameters of the potential energy surface were adjusted to obtain better agreement with the experimentally measured fraction of H2O vibrational energy, 〈fV(H2O)〉=0.6, without significantly changing the entrance channel. No isotope effect for the partition of energy to H2O vs HOD was found. Analysis of the trajectories indicates that the reactant OH(OD) bond is a spectator, ...

Conformational Analysis of 18-Azacrown-6 and Its Bonding with Late First Transition Series Divalent Metals: Insight from DFT Combined with NPA and QTAIM Analyses

Density functional theory calculations, together with quantum theory of atoms in molecules (QTAIM) analyses, have been performed to investigate 18-azacrown-6 complexes of the high-spin late first transition series divalent metal ions in the gas phase and, in some cases, in aqueous solution simulated by a polarizable continuum model. Six intramolecular H-H bonding interactions in the meso-complexes are found to arise from folding of the ligand upon its electrostatic interaction with the metal ions, which are largely absent in the lowest-energy C(2h) conformer of the free ligand. The ligand-to-metal charge transfer obtained from QTAIM analysis, among other things, is found to be an important factor that controls the stability of these complexes. The inter-relationship between the ligand preorganization energy, the zero-point corrected formation energy of the metal complexes, and the H-H bonding pair distances, as well as the dependence of the electron density and the total energy density at the H-H bond critical points on the H-H bonding pair distances, provides a physical basis for understanding and explaining the stabilizing nature of these closed-shell interactions, which are often viewed as steric clashes that lead to complex destabilization.