Andrew Willetts

Ab initio calculation of anharmonic constants for a transition state, with application to semiclassical transition state tunneling probabilities

“Good” (i.e. conserved) action variables exist in the vicinity of a saddle point (i.e. transition state) of a potential energy surface in complete analogy to those related to a minimum on the surface. Transition state theory tunneling (or transmission) probabilities can be expressed semiclassically in terms of these “good” action variables, including the effects of non-separable coupling of all degrees of freedom with each other. This paper shows how ab initio quantum chemistry methods recently developed for calculating anharmonic constants about a potential minimum (i.e. for ordinary vibrational energy levels) can be readily adapted to obtain those related to a transition state, thus providing a rigorous and practical way to apply this non-separable transition state theory. Application is made to the transition state for he reaction D2CO→D2 + CO.

Structural studies and synthetic applications of Baeyer-Villiger monooxygenases.

Baeyer-Villiger monooxygenases (BVMOs) are enzymes able to perform highly regio-plus steroselective nucleophilic and electrophilic biooxygenations on various substrates. The resultant chiral products (lactones and sulfoxides) can be valuable for the chemoenzymatic synthesis of a wide range of useful compounds. Recent studies have provided a number of alternative active-site models that attempt to explain the exquisite and unusual selectivity of BVMOs. This article reviews some of the established applications, and considers the merits of the various predictive models.

Anharmonic vibrational properties of CH2F2 : A comparison of theory and experiment

Ab initio theoretical chemistry is used to provide a complete understanding of the infrared spectroscopy of CH2F2. Second‐order Mo/ller–Plesset perturbation theory (MP2) with a 631G extended basis set is used to provide a quartic expansion of the potential energy surface and a cubic expansion of the dipole surface. Standard perturbation theory is then used to determine effective vibrational and rotational Hamiltonians for fundamentals, selected overtones, and combination bands. Effects of Fermi resonance, Darling–Dennison resonance, and Coriolis resonance are included by matrix diagonalization. Empirical (x,K) relations are used to demonstrate that the anharmonic constants for C–H are in good agreement with those determined from CH2Cl2. The local mode nature of the CH overtones is demonstrated. Important resonances are found to be (ν3,2ν4), (ν8,ν4+ν9), and (ν1,2ν2,2ν8,ν4+ν8+ν9, 2ν4+2ν9,ν3+2ν9). Rotational constants, quartic and sextic centrifugal distortion constants, vibration rotation interaction consta...

The prediction of spectroscopic properties from quartic correlated force fields: HCCF, HFCO, SiH+3

Knowledge of a force field expanded through quartic displacements, together with a dipole field expanded through cubic displacements, yields all the harmonic and anharmonic molecular properties of interest to infrared spectroscopists. Such force fields may also explain much of the mechanism behind intramolecular vibrational energy redistribution. The ab initio quantum chemist can now calculate these fields, either at the self‐consistent field level or with the inclusion of electron correlation effects. For accurate predictions, it is important to include electron correlations effects for at least the quadratic part of the force fields. Here we report studies using the second‐order Mo/ller–Plesset method for the full quartic fields. We examine the effects of using large basis sets. The quadratic force constants are calculated analytically; cubic and quartic constants are calculated using central differences of second derivatives in reduced normal coordinates. Three molecules are studied. HCCF, for which a ...

Vibrational contributions to static polarizabilities and hyperpolarizabilities

Calculations of the vibrational contribution to diagonal components of static polarizabilities and hyperpolarizabilities of HF and H2O are compared with the results of perturbation theory. The calculations are based on finite difference derivatives of the vibrational energy. Effects of mechanical and electrical anharmonicity are shown to be important in the calculation of these properties.

A study of the ground electronic state of hydrogen peroxide

Abstract A quartic force field for H 2 O 2 is presented in terms of curvilinear displacement coordinates, the result of an ab initio calculation with DZP, TZ2P, and TZ2P+ f basis sets. The MP2 method was used for the quadratic part with the SCF method used for the cubic and quartic parts. The MP2 calculations suggest that the equilibrium torsional angle is near 115°, rather than the currently accepted experimental value of 112°. Use of second-order perturbation theory leads to predictions for fundamentals which are within 1% of observed values. The quartic field was then changed so that the torsional part was represented in terms of cos nφ , to give a better description of the large amplitude motion. The vibrational energy levels of this new force field were then calculated by a six-dimensional variational vibrational analysis using an exact representation of the kinetic energy operator. Good agreement with experimental data was again achieved including low-lying torsional vibrations. Variationally calculated vibrational levels up to 6000 cm −1 are presented. The new potential is therefore shown, by the variational method, to be a good representation of the potential for H 2 O 2 in the region of the equilibrium geometry.

Biological Baeyer–Villiger oxidation of some monocyclic and bicyclic ketones using monooxygenases from Acinetobacter calcoaceticus NCIMB 9871 and Pseudomonas putida NCIMB 10007

A. calcoaceticus NCIMB 9871 and Ps. putida NCIMB 10007 [grown on (+)-camphor] have been utilized as biocatalysts in Baeyer–Villiger oxidations. The former microorganism oxidized the racemic ketone 6 non-selectively but transformed the dihalogeno ketone (±)-8 into optically active lactone 10 and recovered ketone. Ps. putida NCIMB 10007 oxidized the two enantiomers of the ketone 6 at different rates while both enantiomers of ketone (±)-1 were converted into lactones, one enantiomer giving 3-oxabicyclooctenone preferentially, while the other enantiomer gave 2-oxabicyclooctenone. Ps. putida NCIMB 10007 contains two quite different types of monooxygenase enzyme, one using NADH as cofactor (labelled MO1) the other employing NADPH as cofactor (labelled MO2). Monooxygenase MO1 proved to be a selective efficient biocatalyst for the oxidation of bicyclic ketones such as 1 and 6 while monooxygenase MO2 is a useful catalyst for the oxidation of cyclopentanones 15–17. Cofactor recycling was effected using dehydrogenase enzymes in preparative-scale experiments.

Characterization and partial purification of an enantioselective arylacetonitrilase from Pseudomonas fluorescens DSM 7155

Pseudomonas fluorescens DSM 7155 after growth on phenylacetonitrile as sole nitrogen source contained an inducible nitrilase which consists of two different functional subunits (40 and 38 kDa). The nitrilase catalysed the exclusive hydrolysis of arylacetonitrile substrates into the equivalent carboxylic acids plus ammonia as major products. The corresponding amides were formed at low levels (<5%) during nitrile hydrolysis but were not substrates for the purified enzyme. The native enzyme, which had a pH optimum of 9 and a temperature optimum of 55°C, was activated (140–160%) by the thiol protectant 2-mercaptoethanol (50–100 mM). The purified nitrilase catalysed the hydrolysis of the two enantiomers of racemic 2-(methoxy)-mandelonitrile to the corresponding acid at significantly different rates: at 50% overall conversion the predominant product was the (R)-acid (enantiomeric excess=92%) whereas at 85% overall conversion the ee% of the (R)-acid had decreased to 27%.

Ab initio prediction of fundamental, overtone and combination band infrared intensities

Abstract Programs have been implemented for calculating ab initio anharmonic force fields which are quartic in displacement coordinates and, likewise, dipole-moment expansions which are cubic in displacement coordinates. Knowledge of these, and the use of second-order perturbation theory, leads to predictions for fundamental, overtone and combination band infrared intensities as well as the usual spectroscopic constants. Here we use the ab initio MP2 method, with large basis sets, applied to H 2 O and H 2 CO. The results, including those for the overtone and combination band intensities, are in good agreement with the available experimental data.

Nitrile hydratase enzymes in organic synthesis: Enantioselective synthesis of the lactone moiety of the mevinic acids

Abstract ( R )-4-Hydroxy-5-cyanopentene (−)- 9 , a known precursor of the protected lactone moiety of the mevinic acids 1 , has been prepared in 9 steps from ( S )-3-(benzyloxy)-4-cyanobutanoic acid 5 (88% e.e.), which was obtained by the asymmetric 2 step hydrolysis of 3-benzyloxyglutaronitrile 4 involving the successive activity of a nitrile hydratase and an amidase enzyme.