Andrew Streitwieser

Natural energy decomposition analysis: An energy partitioning procedure for molecular interactions with application to weak hydrogen bonding, strong ionic, and moderate donor–acceptor interactions

We present a procedure for partitioning the Hartree–Fock self‐consistent‐field (SCF) interaction energy into electrostatic, charge transfer, and deformation components. The natural bond orbital (NBO) approach of Weinhold and co‐workers is employed to construct intermediate supermolecule and fragment wave functions that satisfy the Pauli exclusion principle, thereby avoiding the principal deficiency of the popular Kitaura–Morokuma energy decomposition scheme. The function counterpoise method of Boys and Bernardi enters the procedure naturally, providing an estimate of basis set superposition error (BSSE). We find that the energy components exhibit little basis set dependence when BSSE is small. Applications are presented for several representative molecular and ion complexes: the weak hydrogen bond of the water dimer, the strong ionic interaction of the alkali metal hydrides, and the moderate donor–acceptor interactions of BH3NH3 and BH3CO. Electrostatic interaction dominates the long‐range region of the p...

A simple molecular orbital treatment of hyperconjugation

Abstract A modification of the simple molecular orbital theory has been successfully applied to the treatment of ionization potentials of unsaturated molecules. Of the several models examined for the hyperconjugative effect of a methyl group, best results were obtained for a model in which the methyl group is treated as a single “heteroatom” which donates two electrons to the φ system.

Stereochemistry of the primary carbon—X: Stereochemical configurations of some optically active deuterium compounds

Abstract The iso bornyloxymagnesium halide reduction of the appropriate deutero-aldehyde yields benzyl-α- d alcohol and 1-butanol-1- d having the same configuration and about the same level of optical purity. Arguments are presented that these alcohols are close to optical purity. The relative configurations of each alcohol were established by converting each to 1-phenylbutane-1- d using reactions of known or presumed stereochemistry. The rotations and configurations of 18 optically active deuterium compounds are summarized.

SCF-Xα scattered-wave MO study of thorocene and uranocene

Abstract SCF-Xα scattered wave calculations are presented for di-π-cyclooctatetraenethorium (thorocene) and -uranium (uranocene). Relativistic effects are not included; nevertheless, agreement with experimental photoelectron spectra is remarkably good. The calculations show that 5f±2 orbitals of the central metal contribute to ring—metal bonding, as proposed at the time of the first preparations of these compounds, but that 6d orbitals are also at least equally important in such bonding.

Proton magnetic resonance shifts in (bis(cyclooctatetraenyl)uranium(IV)

Abstract The measured proton resonance shifts of U(COT)2 are analyzed on the basis of a crystalline field model and magnetic susceptibility data. It is pointed out that in U(COT)2 the orbital and spin angular momenta are coupled antiparallel for the ground state, so that a positive hyperfine coupling constant due to the Fermi contact term will result in an upfield proton resonance shift. The hyperfine coupling constant arising from the Fermi contact term is found to be AF = +0.90 MHz ± 25%.

An HMO treatment of the reduction of aromatic hydrocarbons with alkali metals; reduction of fluoranthene

Abstract Simple molecular orbital theory is applied to the metal-ammonia reduction of aromatic hydrocarbons. The theory is based on protonation at sites of highest electron density in the corresponding anions. Good agreement is observed between predicted and observed reduction products. The reduction of the non-alternant hydrocarbon fluoranthene takes an anomalous course that is not inconsistent with the theory. Evidence is presented to show that an initial dihydroreduction product, 2,3-dihydrofluoroanthene, undergoes a Michael addition by its conjugate anion to yield 1,2,2′,3,3′,10b-hexahydro-1,2′-bifluoranthyl.

Perspectives on Computational Organic Chemistry

The author reviews how his early love for theoretical organic chemistry led to experimental research and the extended search for quantitative correlations between experiment and quantum calculations. The experimental work led to ion pair acidities of alkali-organic compounds and most recently to equilibria and reactions of lithium and cesium enolates in THF. This chemistry is now being modeled by ab initio calculations. An important consideration is the treatment of solvation in which coordination of the alkali cation with the ether solvent plays a major role.

An electrostatic model of methyllithium tetramer, a non-nonclassical system

Abstract A collection of 4 positive and 4 negative point charges arranged as two interpenetrating tetrahedra subject only to Coulombic forces has a minimum electrostatic energy at a ratio of the two tetrahedra sides of 0.783, a value consistent with the observed LiLi to CC distance ratio of 0.73 in the structure of methyllithium tetramer.

Resonance delocalization in the anion is not the major factor responsible for the higher acidity of carboxylic acids relative to alcohols

Abstract Evidence based on electron populations and charge-flow in formic acid and ethanol is presented, supporting the view that resonance delocalization in carboxylate anions is not the major factor responsible for the higher acidity of carboxylic acids relative to alcohols. Using molecular orbital wave functions obtained from ab initio calculations (RHF/3-21+G//3-21+G) planar projected electron density grids were obtained. These were divided into regions that can be associated with single carbon or oxygen atoms and their attached hydrogens and integrated over these regions. Inkeeping with the traditional view, the hydroxyl groups have the same net charge (−0.46 a.u.) and the total charge in formate ion is delocalized. The carbonyl group is highly polar; the carbonyl oxygen is found to have a total charge of −1.0 a.u. (1.56 pi electrons). The inductive effect caused by the highly polar carbonyl group in formic acid is the major contributor to the higher acidity of formic acid relative to ethanol. Contrary to the traditional view, the total electron-flow during proton removal is similar for the two molecules. The pi electron flow accompanying deprotonation of formic acid qualitatively follows traditional theory, but quantitatively, is much less than this theory predicts (0.15 pi electrons flow away from the hydroxyl oxygen). The total charge-flow accompanying deprotonation of both ethanol and formic acid is dominated by sigma relaxation from the proton to the hydroxyl oxygen. The total number of electrons flowing away from the hydroxyl oxygen in ethanol (0.32) is slightly greater than the total number of electrons flowing away in formic acid (0.29). The acidity difference is, therefore, due to the charge distribution in the neutral molecule and not to an enhanced electron delocalization in the carboxylate ion.

Ab initio modeling of organolithium equilibria.

Experimental ion pair pKs of monomeric contact ion pair lithium salts in THF from our previous studies give good correlations with ab initio calculations at the Hartree-Fock 6-31+g(d) level. PCM methods were found to be inadequate in nonpolar organic solvents, and dielectric solvation was not used in the correlations. Specific coordination of two or three ether solvent molecules with lithium was found to be satisfactory. These correlations include carboxamides, amines, dithianes, sulfones, and sulfoxides, as well as some ketones, beta-diketones, and the lithium salts of dianions.