A combined quantum mechanical (QM) and molecular mechanical (MM) potential has been developed for the study of reactions in condensed phases. For the quantum mechanical calculations semiempirical methods of the MNDO and AM1 type are used, while the molecular mechanics part is treated with the CHARMM force field. Specific prescriptions are given for the interactions between the QM and MM portions of the system; cases in which the QM and MM methodology is applied to parts of the same molecule or to different molecules are considered. The details of the method and a range of test calculations, including comparisons with ab initio and experimental results, are given. It is found that in many cases satisfactory results are obtained. However, there are limitations to this type of approach, some of which arise from the AM1 or MNDO methods themselves and others from the present QM/MM implementation. This suggests that it is important to test the applicability of the method to each particular case prior to its use. Possible areas of improvement in the methodology are discussed.

A combined quantum mechanical and molecular mechanical potential for molecular dynamics simulations

The hydrogen bond and crystal engineering

The field of cluster research can trace its origins back to the mid-nineteenth century when early studies of colloids, aerosols, and nucleation phenomena were reported. The field underwent a resurgence of interest several decades ago when well-defined clusters were observed in supersonic expansions that could be investigated using mass spectrometers. The advent of the laser provided a new dimension, enabling detailed spectroscopic observations through the probing of systems of varying size and degree of solvation. Modern interest derives from recognition that interrogating clusters provides a way of studying the energetics and dynamics of intermediate states of matter as cluster systems evolve from the gas toward the condensed state. Herein, we endeavor to highlight some of the significant advances which have been made during the past several decades that have led to a nearly explosive growth of interest in the field of cluster science. Finally, we conclude that the field will continue to expand through i...

Clusters: Structure, Energetics, and Dynamics of Intermediate States of Matter

1.1. Historical Background Hydrogen bonds1,2 are one of the principal intermolecular forces. A special class are ionic hydrogen bonds (IHBs) that form between ions and molecules with bonds strengths of 5-35 kcal/mol, up to a third of the strength of covalent bonds. These strong interactions are critical, for example, in ionic clusters and nucleation, in electrolytes, ion solvation, and acid-base chemistry, in the structures of ionic crystals, surfaces, silicates, and clays, in surface adsorption, and in self-assembly in supramolecular chemistry and molecular crystals.3 IHBs are also important in bioenergetics including protein folding, enzyme active centers, formation of membranes and proton transport, and biomolecular recognition. With such wide-ranging roles, the fundamental properties of IHB interactions need to be understood. The energetics of IHB interactions cannot be isolated and quantified in the condensed phase. However, these interactions can be isolated and studied quantitatively in gas phase. These studies lead to a fundamental understanding of relations between IHB bond strengths and molecular structure, the solvation of ions, especially in the critical inner shells, and acid-based phenomena and bioenergetics. This review will present the basic insights that have been obtained in the past four decades. It will present a comprehensive review of the thermochemistry and its structural implications obtained from ab initio calculations. Relevant recent results from spectroscopy will be also illustrated. The advent of variable temperature high-pressure mass spectrometry (HPMS) introduced by Field and co-workers in the 1960s4 and pulsed high-pressure mass spectrometry (PHPMS) introduced by Kebarle in the late 1960s5 have been particularly significant. These workers realized that at pressures of several torrs and gas densities of 1016-1017 cm-3 in ion sources, ions can undergo 103-106 collisions with neutral molecules during typical residence times of 0.1-10 ms and establish thermal ion populations and * E-mail: m.mautner@solis1.com. 213 Chem. Rev. 2005, 105, 213−284

http://www.astro-ecology.com/PDFTheIonicHydrogenBondChemicalReviews2005Paper.pdf

P. Kebarle

Papers

Hydrogen-bonding energies to negative ions from gas-phase measurements of ionic equilibriums