Joey W. Storer

Class IV charge models: A new semiempirical approach in quantum chemistry

SummaryWe propose a new criterion for defining partial charges on atoms in molecules, namely that physical observables calculated from those partial charges should be as accurate as possible. We also propose a method to obtain such charges based on a mapping from approximate electronic wave functions. The method is illustrated by parameterizing two new charge models called AM1-CM1A and PM3-CM1P, based on experimental dipole moments and, respectively, on AM1 and PM3 semiempirical electronic wave functions. These charge models yield rms errors of 0.30 and 0.26 D, respectively, in the dipole moments of a set of 195 neutral molecules consisting of 103 molecules containing H, C, N and O, covering variations of multiple common organic functional groups, 68 fluorides, chlorides, bromides and iodides, 15 compounds containing H, C, Si or S, and 9 compounds containing C-S-O or C-N-O linkages. In addition, partial charges computed with this method agree extremely well with high-level ab initio calculations for both neutral compounds and ions. The CM1 charge models provide a more accurate point charge representation of the dipole moment than provided by most previously available partial charges, and they are far less expensive to compute.

Factors controlling relative stability of anomers and hydroxymethyl conformers of glucopyranose

The relative energies of 11 different conformers of D‐glucose, including different exo‐anomeric orientations and at least one of each hydroxymethyl conformer (G−, G+, and T) for each of the two anomeric forms (α and β), were calculated at much more complete levels of quantum mechanical (QM) electronic structure theory than previously available, and relative free energies in solution were calculated by adding rotational, vibrational, and aqueous solvation effects. The gas‐phase results are based on very large basis sets (up to 624 contracted basis functions) and the coupled cluster method for electron correlation. Solvation Model 5.4/ AM1 was used to calculate the effects of aqueous solvation. Factors contributing to the relative energies of these conformers have been analyzed. Relative energies varied considerably (up to 4.5 kcal/mol), depending on the theoretical level, and different levels of theory disagreed as to which anomer was lower in energy. The highest‐level gas‐phase calculations predicted the α‐anomer to be lower in free energy by 0.4 kcal/mol (Boltzmann average). Gas‐phase energies from several different classical force fields were compared to QM results. The QM structures optimized at the MP2/cc‐pVDZ level of theory compared well with experiment for three different crystal structures. In water, the β‐anomers were better solvated than the α‐anomers by 0.6 kcal/mol (Boltzmann average). Contributions of individual hydrophilic groups to the solvation free energies were analyzed.u2003© 1998 John Wiley & Sons, Inc.u2003J Comput Chem 19: 1111–1129, 1998

Full valence complete active space SCF, multireference CI, and density functional calculations of 1A1—3B1 singlet—triplet gaps for the valence-isoelectronic series BH-2, CH2, NH+2, AlH-2, SiH2, PH+2, GaH-2, GeH2, and AsH+2

Abstract MRCISD(Q)/CASSCF calculations have been performed for the title dihydrides. The agreement with available experimental data is quite good for the 1 A 1 - 3 B 1 singlet—triplet (S—T) energy gaps, as is agreement with other calculations of similar quality. DFT calculations employing basis sets of similar quality with non-local exchange and correlation corrections are two to three orders of magnitude faster than MRCI. DFT agrees well with MRCI for the first and third rows, although the use of different correlation functionals is required. Results in the second row are only fair with either correlation functional.

Concerted and stepwise mechanisms in cycloaddition reactions: potential surfaces and isotope effects

CASSCF/6-31G* calculations have been performed on concerted and stepwise Diels–Alder reactions of butadiene with ethene, the dimerization of butadiene and the dimerization of cyclobutadiene. The relative energies of concerted and stepwise mechanisms are compared, and the factors influencing these “energies of concert” are discussed. The comparison of calculated isotope effects to experimental data provides support for theoretical results.