This article describes the parameterization and performance of MMFF94 for molecular geometries and deformations. It defines the form used for the valence-coordinate terms that represent variations in bond lengths and angles, and it describes the derivation of quadratic force constants from HF/6-31G* data and the derivation of reference bond lengths and angles from fits to MP2/6-31G*-optimized geometries. Comparisons offered show that MMFF94 accurately reproduces the computational data used in its parameterization and demonstrate that its derivation from such data simultaneously confers the ability to reproduce experiment. In particular, MMFF94 reproduces experimentally determined bond lengths and angles for 30 organic molecules with root mean square (rms) deviations of 0.014 A and 1.2°, respectively. MM3 reproduces bond angles to the same accuracy, but reproduces experimental bond lengths more accurately, in part because it was fit directly to thermally averaged experimental bond lengths; MMFF94, in contrast, was fit to (usually shorter) energy-minimum values, as is proper for an anharmonic force field intended for use in molecular-dynamics simulations. The comparisons also show that UFF and a recent version of CHARMm (QUANTA 3.3 parameterization) are less accurate for molecular geometries than either MMFF94 or MM3. For vibrational frequencies, MMFF94 and MM3 give comparable overall rms deviations versus experiment of 61 cm−1 and 57 cm−1, respectively, for 15 small, mostly organic molecules. In a number of instances, MM3's derivation employed observed frequencies that differ substantially—by nearly 400 cm−1 in one case—from other published frequencies which had themselves been confirmed theoretically by good-quality ab initio calculations. Overall, the comparisons to experimental geometries and vibrational frequencies demonstrate that MMFF94 achieves MM3-like accuracy for organic systems for which MM3 has been parameterized. Because MMFF94 is derived mainly from computational data, however, it has been possible to parameterize MMFF94 with equal rigor for a wide variety of additional systems for which little or no useful experimental data exist. Equally good performance can be expected for such systems. © John Wiley & Sons, Inc.

Merck Molecular Force Field. III. Molecular Geometries and Vibrational Frequencies for MMFF94

An empirical potential based on permanent atomic multipoles and atomic induced dipoles is reported for alkanes, alcohols, amines, sulfides, aldehydes, carboxylic acids, amides, aromatics, and other small organic molecules. Permanent atomic multipole moments through quadrupole moments have been derived from gas phase ab initio molecular orbital calculations. The van der Waals parameters are obtained by fitting to gas phase homodimer QM energies and structures, as well as experimental densities and heats of vaporization of neat liquids. As a validation, the hydrogen bonding energies and structures of gas phase heterodimers with water are evaluated using the resulting potential. For 32 homo- and heterodimers, the association energy agrees with ab initio results to within 0.4 kcal/mol. The RMS deviation of the hydrogen bond distance from QM optimized geometry is less than 0.06 A. In addition, liquid self-diffusion and static dielectric constants computed from a molecular dynamics simulation are consistent wit...

Polarizable Atomic Multipole-based Molecular Mechanics for Organic Molecules

Wahrend sich die klassische Organische Chemie bisher weitgehend mit der Synthese individueller Molekule und der Untersuchung ihrer Eigenschaften beschaftigte, gewinnen nunmehr zunehmend Forschungsaktivitaten an Bedeutung, die sich mit dem Verstandnis und dem Ausnutzen von Wechselwirkungen zwischen Molekulen befassen. Zwei fur diese Entwicklung reprasentative Gebiete sind die supramolekulare Chemie und die molekulare Erkennung. Die Wechselwirkungen zwischen Molekulen werden durch intermolekulare Krafte bestimmt, deren energetische und geometrische Eigenschaften viel weniger gut verstanden sind als die von klassischen chemischen Bindungen zwischen Atomen. Zu den starksten dieser Wechselwirkungen gehoren die H-Brucken, deren gerichtete Eigenschaften auf lokalem Niveau (d. h. fur nur eine H-Brucke) besser als die vieler anderer Typen nichtbindender Wechselwirkungen verstanden werden. Trotzdem ist noch weithin ungeklart, wie man vorgehen muste, um die Konsequenzen der Bildung vieler H-Brucken zwischen Molekulen und den daraus folgenden Aufbau von molekularen Aggregaten (im mikroskopischen Bereich) oder von Kristallen (im makroskopischen Bereich) zu charakterisieren, zu verstehen und vorherzusagen. Einer der vielversprechendsten systematischen Ansatze, um dieses Problem zu losen, wurde ursprunglich von der inzwischen verstorbenen M. C. Etter entwickelt. In den letzten Jahren wurden ihre Ideen von anderen ubernommen und angewendet. Diese Arbeiten bewiesen einerseits die Bedeutung und die potentielle Nutzlichkeit der ursprunglichen Ideen und Ansatze, zeigten andererseits aber auch die Notwendigkeit auf, Etters Formalismus zu erweitern. Gerade letzteres war der Ausloser fur die hier vorgestellten Arbeiten.

Muster aus H‐Brücken: ihre Funktionalität und ihre graphentheoretische Analyse in Kristallen

This article focusses on the crystallographic research aimed at a better understanding of hydrogen bonds which has been published since January 1990. In the interest of continuity, earlier work is quoted when it relates to that published during this period.

Hydrogen-Bonding: An Update

The hydrolysis of sterically hindered and unhindered alkyl nitriles, and also of benzyl and phenyl nitriles RCN (R = Me, CH2Cl, Et, n-Pr, i-Pr, n-Bu, t-Bu, p-MeOC6H4CH2, Ph), to carboxamides is cat...

Zinc(II)/ketoxime system as a simple and efficient catalyst for hydrolysis of organonitriles.

Neutron diffraction at 23 K and ab initio molecular-orbital studies of the molecular structure of acetamide

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J. S. Binkley

Papers

Neutron diffraction at 23 K and ab initio molecular-orbital studies of the molecular structure of acetamide

Ab initiomolecular orbital versus low-temperature neutron diffraction molecular structures for some organic molecules