Saul Wolfe

Abinitio computation of force constants. The second and third period hydrides

The force method has been employed to calculate the force constants of HF, H2O, NH3, CH4, HCl, H2S, PH3, and SiH4. The computations were performed using wavefunctions generated by both the STO‐3G and 4‐31G basis sets that are standard in the Gaussian 70 program system. The 4‐31G basis set provides reliable harmonic force constants, but those computed with the STO‐3G basis set are poor. Cubic force constants computed at the 4‐31G level are fair to very good. Cubic stretching force constants are reproduced especially well, regardless of the basis set. Quartic stretching force constants are good, but quartic bending constants are in error. A component analysis has been developed and relations have been found that may be useful for qualitative and quantitative discussions.

On the magnitudes and origins of the “anomeric effects”, “exo-anomeric effects”, “reverse anomeric effects”, and CX and CY bond -lengths in XCH2YH molecules

Ab-initio molecular-orbital calculations have been performed at the STO-3G level on the series of molecules XCH2YH in which X  H2N, CH3, OH, F, and Cl, and Y  O and S, and on the series of fluoromethanes CHnF4-n. The CX and CY bond lengths in the gauche and antiperiplanar conformations of XCH2YH have been optimized. In all cases, the experimental trends in the bond lengths from conformation to conformation and from molecule to molecule, and in the torsional behavior as a function of X and Y have been reproduced. Thus, the “anomeric effects” are found to be Cl > F > OH, as a function of X, and O > S, as a function of Y. Molecules in which X  CH3 and NH2 exhibit “reverse anomeric effects”; in methanediol, the “anomeric effect” and the “exo-anomeric effect” are equal. The results have been analyzed by a quantitative, perturbational molecular-orbital (PMO) treatment that calculates orbital interactions between XCH2 and YH in the cases of the “anomeric” and “exo-anomeric effects”, and between X and CH2YH in the case of the “reverse anomeric effect”, by using fragments and fragment orbitals generated from the ab-initio wavefunction. Attention has been focused, especially, upon the stabilizing interactions between the lone pair of Y and antibonding orbitals of XCH2, and between the highest-lying orbital of X and antibonding orbitals of CH2YH. The justification for this choice is that these orbitals make a dominant contribution to the highest occupied molecular orbital (HOMO) of XCH2YH, and the stereochemical behavior of the HOMO parallels that of the total energy. In all cases, the trends in these stabilizing orbital interactions parallel the trends in the “anomeric” and “reverse anomeric effects”, suggesting that, within the framework of the PMO model, such interactions may be regarded as the “origin” of these effects. Although both σ* and π* antibonding orbitals have to be taken into account to achieve quantitative agreement between the calculated orbital interactions and the total energy-differences, only those interactions associated wit σ* vary significantly as X and Y are varied, as suggested originally by Lucken and by Altona. In the series of fluoromethanes, a linear relationships exist between the stabilizing orbital interactions and the experimental CF bond energies, and also between stabilizing orbital interactions and the number of double bond-no bond resonance structure. This result provides quantitative support for a description of these various effects in terms of the classical concept of double bond-no bond resonance. Finally, the in the CX and CY bond lengths in XCH2YH and XCH2X molecules have found to be the result of the combination of coulombic effects and the principle of maximization of overlap between fragments.

Ab initio computation of force constants. IV. The theoretical anharmonic force fields and vibrational frequencies of methylamine, methanol, and methanethiol

The anharmonic force fields of methylamine, methanol, and methanethiol have been computed ab initio. The harmonic force constants agree fairly well with experiment. Better agreement is found between the experimental frequencies and the theoretical anharmonic frequencies. The stretching modes are predicted especially well. The calculated ν13 and ν14 for CH3NH2 agree with the solid phase but not the gas phase experimental results. Methylamine, methanol, and methanethiol exhibit Bohlmann bands, i.e., a lowering of the stretching frequency of a C–H bond antiperiplanar to a lone pair. The computations reveal that these bands are linked to a smaller force constant for the anti CH bond and to Fermi resonance between the symmetric C–H stretch and CH bending overtones.

The gauche effect : On the nature of the interaction between electronegative substituents in trans-1,2-disubstituted cyclohexanes

Abstract The positions of the aa/ee conformational equilibria of a series of trans -1,2-disubstituted cyclohexanes containing etectronegative substituents have been determined experimentally by NMR, and calculated theoretically by classical means. It is found that the classical analysis of the results, which takes into account explicitly steric and electrostatic interactions between gauche substituents, is not adequate, because residual attraction remains between the substituents.

Cloning and nucleotide sequence determination of the isopenicillin N synthetase gene from Streptomyces clavuligerus.

The isopenicillin N synthetase (IPNS) gene from Streptomyces clavuligerus was isolated from an Escherichia coli plasmid library of S. clavuligerus genomic DNA fragments using a 44-mer mixed oligodeoxynucleotide probe. The nucleotide sequence of a 3-kb region of the cloned fragment from the plasmid, pBL1, was determined and analysis of the sequence showed an open reading frame that could encode a protein of 329 amino acids with an Mr of 36,917. When the S. clavuligerus DNA from pBL1 was introduced into an IPNS-deficient mutant of S. clavuligerus on the Streptomyces vector pIJ941, the recombinant plasmid was able to complement the mutation and restore IPNS activity. The protein coding region of the S. clavuligerus IPNS gene shows about 63% and 62% similarity to the Cephalosporium acremonium and Penicillium chrysogenum IPNS nucleotide sequences, respectively, and the predicted amino acid sequence of the encoded protein showed about 56% similarity to both fungal sequences.

A theoretical reexamination of carbanions adjacent to sulfoxide, sulfone and sulfonium centres

Abstract The structures and proton affinities of CH 2 SR 2 (R = H,CH 3 ), − H 2 SOR (R = H,CH 3 ) and − CH 2 SO 2 R (R = H,CH 3 ) have been found at the 3-21G* (d-orbitals on sulfur) and 3-21G (no d-orbitals on sulfur) levels, using gradient optimization procedures. d-Orbital effects are important in these optimized structures. The factors leading to the structures now found, and the role of the d-orbitals, have been evaluated by quantitative PMO analyses of the wavefunctions.

Cell-free synthesis of δ-(L-α-aminoadipyl)-L-cysteine, the first intermediate of penicillin and cephalosporin biosynthesis

Abstract δ-(L-α-aminoadipyl)-L-cysteine synthetase (LL-AC synthetase) activity has been found in extracts of Cephalosporium acremonium C-10. The enzyme extract carries out a linear synthesis of LL-AC from its constituent amino acids for at least 6 hours. The reaction is dependent on active enzyme, time, L-α-aminoadipate, L-cysteine, ATP and Mn2+ or Mg2+. The activity is stabilized by glycerol.

Ab initio computation of force constants. V. The theoretical anharmonic force fields and vibrational frequencies of methyl fluoride and methyl chloride

The anharmonic force fields of methyl fluoride and methyl chloride have been calculated ab initio. The theoretical and experimental harmonic force fields agree fairly well, but certain small discrepancies are found for CH3F. Agreement between calculated and observed anharmonic frequencies is good. It is suggested that application of the theoretical anharmonicity corrections to the observed frequencies can lead to better estimates of the experimental harmonic frequencies and force constants.

A theoretical study of the Edward–Lemieux effect (the anomeric effect). The stereochemical requirements of adjacent electron pairs and polar bonds

Many data, collected from the literature, support the postulation of two general rules which describe the static and dynamic results of having in a molecule or generating in an intermediate adjacent electron pairs and/or polar bonds. In disagreement with current chemical intuition, structures which contain the maximum number of gauche-interactions between lone electron pairs or polar bonds represent energy minima and, frequently, the lowest minima. The stereochemical implications of the phenomenon are discussed, along with possible physical explanations. It is suggested that partitioning of the total energy of the system into attractive-dominant and repulsive-dominant interactions provides the most helpful framework for the construction of a physical picture of the phenomenon. The total energy and its components can be obtained by ab initio molecular quantum mechanical calculations.The only apparent exception to the phenomenon that has been found corresponds to a polar bond adjacent to two lone pairs (as in the case of two hetero-atoms attached to the same carbon atom). This exception, here termed the Edward–Lemieux effect, has been examined theoretically by an ab initio(Hartree–Fock) calculation using fluoromethanol as a model compound. The calculation has reproduced the Edward–Lemieux effect; the stable conformation has the C–F bond trans to one ‘electron pair’ and gauche to another, and the conformation in which the C–F bond bisects the ‘electron pairs’ is the energy maximum. The partitioning of the total energy into its components of attraction and repulsion and comparison of the results with other systems for which barriers to internal rotation have been obtained by ab initio methods reveals a similarity between fluoromethanol, hydrazine, hydroxylamine, and hydrogen peroxide. An interpretation of this result is provided in which an early suggestion by Lemieux and Chu is supported and the concept of ‘rabbit-ears’ is not.

Near‐molecular Hartree‐Fock wavefunctions for CH3O−, CH3OH, and CH3OH2+

A total of nine molecular Hartree‐Fock wavefunctions have been computed for CH3O− (I), CH3–OH (IIA–IId), and CH3–OH2+ (IIIa–IIId). The lowest energies obtained, in hartree atomic units, were: − 114.34285 (CH3O−), − 115.01162 (CH3OH), and − 115.32787 (CH3OH2+). The two proton affinity values for the successive protonations (CH3O−→ CH3OH→ CH3OH2+) were calculated to be − 419.4 and − 198.9 kcal/mole, respectively. For CH3–OH, the barriers to rotation and to in‐plane inversion were computed to be 1.44 and 32.5 kcal/mole, respectively; CH3–OH2+ showed no barrier to either rotation or inversion. Some excitation energies, dipole moments, and charge distributions have also been computed.