Warren J. Hehre

Self‐consistent molecular orbital methods. XXIII. A polarization‐type basis set for second‐row elements

The 6‐31G* and 6‐31G** basis sets previously introduced for first‐row atoms have been extended through the second‐row of the periodic table. Equilibrium geometries for one‐heavy‐atom hydrides calculated for the two‐basis sets and using Hartree–Fock wave functions are in good agreement both with each other and with the experimental data. HF/6‐31G* structures, obtained for two‐heavy‐atom hydrides and for a variety of hypervalent second‐row molecules, are also in excellent accord with experimental equilibrium geometries. No large deviations between calculated and experimental single bond lengths have been noted, in contrast to previous work on analogous first‐row compounds, where limiting Hartree–Fock distances were in error by up to a tenth of an angstrom. Equilibrium geometries calculated at the HF/6‐31G level are consistently in better agreement with the experimental data than are those previously obtained using the simple split‐valance 3‐21G basis set for both normal‐ and hypervalent compounds. Normal‐mode vibrational frequencies derived from 6‐31G* level calculations are consistently larger than the corresponding experimental values, typically by 10%–15%; they are of much more uniform quality than those obtained from the 3‐21G basis set. Hydrogenation energies calculated for normal‐ and hypervalent compounds are in moderate accord with experimental data, although in some instances large errors appear. Calculated energies relating to the stabilities of single and multiple bonds are in much better accord with the experimental energy differences.

Computation of electron repulsion integrals involving contracted Gaussian basis functions

A computational procedure is outlined for efficient evaluation of four-center coulomb repulsion integrals using contracted Gaussian basis functions. By utilizing information common to a shell of basis functions (such as s, px, py, and pz) and by transforming to alternative axes within the contraction loops, this method achieves high efficiency. The technique has been incorporated into the GAUSSIAN-70 molecular orbital program.

The total synthesis of roquefortine C and a rationale for the thermodynamic stability of isoroquefortine C over roquefortine C.

The first total synthesis of roquefortine C is achieved by implementation of a novel elimination strategy to construct the thermodynamically unstable E-dehydrohistidine moiety. Molecular modeling studies are presented which explain the instability of the roquefortine C structure compared to that of isoroquefortine C.

Geometrical preferences of the crotyl anion, radical and cation

Abstract ab initio MO calculations have been performed on the cis and trans isomers of the crotyl cation, free radical and anion in each of two orientations of the Me rotor about the allylic framework. In agreement with available experimental data, both the crotyl cation and free radical prefer trans skeletal geometries. On the other hand, the cis isomer of the crotyl anion is found to be more stable than the t rans , the same preference as has been noted for alkali metal allyl organometallics in solution, but opposite to that recently reported for the free (gas phase) anion. The Me groups are predicted to eclipse the partial double bond for the trans isomers of all three systems and for the cis cation. These results are rationalized with the aid of perturbation MO theory.

Representations of molecular force fields. I. Ethane: Ab initio and model, harmonic and anharmonic

The quadratic and selected cubic force constants for ethane have been computed, using single determinant molecular orbital wavefunctions at the 4‐31G level, with a view to testing and extending model consistent force fields (CFF) for ’’molecular mechanics’’ calculations. Results agree semiquantitatively with experiment, but experimental force constants of sufficient reliability to provide a definitive comparison are not yet available. In a comparison with the most rational general CFF available, that of Ermer and Lifson, the most significant discrepancies found to occur are those for certain stretch–bend couplings assumed to be zero in the CFF but shown to be appreciable by quantum calculation. It is observed that these couplings, but not the stretch–stretch couplings, are well accounted for by a steric interaction model. The ab initio cubic constants examined display the same pattern of conformity with a steric model. Bend–bend–bend and bend–bend–stretch but not all stretch–stretch–stretch interactions a...

Molecular orbital theory of bond separation

Abstract A bond separation reaction is defined as the process in which a polyatomic molecule is separated into the simplest molecules containing the same component bonds. The energies associated with such reactions are calculated for a series of polyatomic molecules using ab initio molecular orbital theory with a basis of contracted Gaussian Functions. The results are in good agreement with experimental values.

The Effect of Protecting Groups on Chelation Control

Abstract Relative chelating abilities of alcohols, ethers and silyl ethers are rationalized in terms of the π accepting character of the group attached to oxygen. This in turn may be assessed by examination of the bond angle about oxygen.

Diastereofacial selectivity in Diels-Alder cycloadditions involving vinyl sulfoxides

Abstract Diastereofacial selectivities observed for Diels-Alder cycloadditions of achiral dienes with chiral vinyl sulfoxides are rationalized in terms of electrostatics. The “nucleophilic” diene adds to the electron-poor face of the “electrophilic” dienophile.

Models for selectivity in organic reactions

Factors contributing to product selection in organic reactions are analyzed using two different approaches. The first involves consideration only of electrostatic preferences upon initial encounter of reagent and substrate, that is, well in advance of the actual reaction transition state. This model, while computationally very simple and applicable to systems of considerable complexity, is capable only of providing a qualitative account of selectivity preferences. The second approach involves direct evaluation of energy differences for transition states leading to different regio- and stereoproducts, based on well designed and calibrated Hartree-Fock and correlated levels of ab initio molecular orbital theory. This is both generally applicable and, subject to the validity of the underlying transition state model, capable of quantitative accuracy, although it is also computationally very demanding and in practice applicable using only the very simplest molecular orbital methods and then only to very simple systems. Examples are provided, among them stereochemistry in electrophilic and nucleophilic additions and regio- and stereochemistry of Diels-Alder cycloadditions. Generalities regarding the factors which infludence product selection in simple organic reactions are discussed.

Conformational analysis of 3-buten-2-ol: A model asymmetric olefin

The conformational energy of 3-buten-2-ol has been investigated using ab initio molecular orbital theory. Six energy minima have been located, the lowest energy two of which are the same as those assigned in a recent microwave study of the molecule. Rationale for the observed preferences are advanced.