Unrestricted perfect pairing: the simplest wave-function-based model chemistry beyond mean field

The perfect pairing (PP) approximation from generalized valence bond theory is formulated in an unrestricted fashion for both closed- and open-shell systems using a coupled cluster ansatz. In the model chemistry proposed here, active electron pairs are correlated, but the unpaired or radical electrons remain uncorrelated, leading to a linear number of decoupled cluster amplitudes which can be solved for analytically. The alpha and beta spatial orbitals are variationally optimized independently. This minimal treatment of electron-electron correlation noticeably improves upon symmetry-breaking problems and other pathologies in Hartree-Fock (HF) theory and may be computed using the resolution of the identity approximation at only a factor of several times more effort than HF itself. PP also generally predicts improved molecular structures over HF. This compact, correlated wave function potentially provides a useful starting point for dynamical correlation corrections.     

Porphyrins

Extended Hückel calculations are reported for tetravalent porphin complexes of Si(OH)₂, Ge(OH)₂, GeCl₂, and SnCl₂ and divalent complexes of Ge, Sn, and Pb. Divalent Ge porphin is expected to be planar and have the extra two electrons in the ring. Divalent Sn and Pb porphins are expected to be non-planar and have the extra two electrons on the metal. The possibility of a charge transfer transition a ₁(p _z )e _g ^* () is noted, and its identication in available spectra of Sn and PbTPP is made. The electronic structure of the tetravalent species is similar to other metalloporphyrins except for the possibility of low lying ligand to porphin charge transfer states in the hydroxy complexes.

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Zusammenfassung Für die vierwertigen Porphin-Komplexe des Si(OH)₂, Ge(OH)₂, GeCl₂ und SnCl₂ und die zweiwertigen Komplexe des Ge, Sn und Pb werden Berechnungen nach der erweiterten Hückel-methode durchgeführt. Von dem zweiwertigen Ge-Porphin wird erwartet, da\ es planar ist und da\ sich die beiden zusÄtzlichen Elektronen im Ring befinden, wÄhrend von den zweiwertigen Sn- und Pb-Porphinen zu erwarten ist, da\ sie nicht planar sind und die beiden zusÄtzlichen Elektronen sich am Metall befinden. Auf die Möglichkeit eines Charge-Transfer-übergangs a ₁(p _z )e _g ^* () wird hingewiesen, und dieser übergang wird in gemessenen Spektren von Sn und PbTPP identifiziert. Die Elektronenstruktur der vierwertigen Verbindung ist denjenigen anderer Metallporphyrine Ähnlich, au\er der Möglichkeit niedrig liegender Ligand-Porphin-Charge-Transfer-ZustÄnde in den Hydroxy-Komplexen.

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Résumé Calculs de type Hückel étendu pour des complexes tétravalents de la porphine avec Si(OH)₂, Ge(OH)₂, GeCl₂ et SnCl₂ et des complexes divalents avec Ge, Sn et Pb. La porphine de Ge divalente est prévue plane avec les deux électrons supplémentaires dans le cycle. Les porphines divalentes de Sn et Pb sont prévues non planes avec les deux électrons supplémentaires sur le métal. On remarque la possibilité d'une transition de transfert de charge a ₁(p _z)e _g ^* () et on l'identifie dans les spectres disponsibles pour Sn et PbTPP. La structure électronique des espèces tétravalentes est semblable à celle des autres métalloporphyrines à l'existence possible près dans les complexes hydroxy d'états de transfert de charge de basse énergie entre le ligand et la porphine.

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Paper XX: Ake, R. L, Gouterman, M.: Theoret. chim. Acta (Berl.) 17, 408–416 (1970).     

Redox control of the P450cam catalytic cycle: effects of Y96F active site mutation and binding of a non-natural substrate

Spectroelectrochemistry measurements are used to demonstrate that active site mutation and binding of an non-natural substrate to P450cam (CYP101) reduces the shift in the redox potential caused by substrate-binding, and thereby results in slower catalytic turnover rate relative to wild-type enzyme with the natural camphor substrate.     

Curvy-steps approach to constraint-free extended-Lagrangian ab initio molecular dynamics, using atom-centered basis functions: convergence toward Born-Oppenheimer trajectories

A dynamical extension of the "curvy-steps" approach to linear-scaling self-consistent field calculations is presented, which yields an extended-Lagrangian formulation of ab initio molecular dynamics. An exponential parametrization of the one-electron density matrix, expressed in terms of atom-centered Gaussian basis functions, facilitates propagation along the manifold of density matrices in a geometrically correct fashion that automatically enforces idempotency constraints. The extended Lagrangian itself is constraint free, thus neither density matrix purification nor expensive, iterative solution for Lagrange multipliers is required. Propagation is highly efficient, and time steps compare favorably to those used in Car-Parrinello molecular dynamics simulations. The behavior of the method, especially with regard to the maintenance of adiabatic decoupling of nuclei and electrons, is examined for a sequence of diatomic molecules, and comparison is made to trajectories propagated on the converged Born-Oppenheimer surface. Certain claims to the contrary notwithstanding, our results demonstrate that vibrational frequencies may depend on the value of the fictitious mass parameter, even in an atom-centered basis. Light-atom stretching frequencies can be significantly redshifted, even when the nuclear and electronic energy scales are well separated. With a sufficiently small fictitious mass and a short time step, accurate frequencies can be obtained; we characterize appropriate values of these parameters for a wide range of vibrational frequencies.     

Structure activity relationship by NMR and by computer: a comparative study

There has recently been considerable interest in using NMR spectroscopy to identify ligand binding sites of macromolecules. In particular, a modular approach has been put forward by Fesik et al. (Shuker, S. B.; Hajduk, P. J.; Meadows, R. P.; Fesik, S. W. Science 1996, 274, 1531-1534) in which small ligands that bind to a particular target are identified in a first round of screening and subsequently linked together to form ligands of higher affinity. Similar strategies have also been proposed for in silico drug design, where the binding sites of small chemical groups are identified, and complete ligands are subsequently assembled from different groups that have favorable interactions with the macromolecular target. In this paper, we compare experimental and computational results on a selected target (FKBP12). The binding sites of three small ligands ((2S)1-acetylprolinemethylester, 1-formylpiperidine, 1-piperidinecarboxamide) in FKBP12 were identified independently by NMR and by computational methods. The subsequent comparison of the experimental and computational data showed that the computational method identified and ranked favorably ligand positions that satisfy the experimental NOE constraints.     

γ‐HIO3 – a Metastable,Centrosymmetric Polymorph of Iodic Acid

From solutions of chromium(III) perchlorate and periodic acid, single crystals of γ‐HIO₃ were obtained and characterized by single‐crystal X‐ray diffraction, Raman spectroscopy and thermal analysis. The compound crystallizes in the orthorhombic crystal system, space group Pbca (a = 563.92, b = 611.10, c = 1507.16 pm). The structure is built up by dimers (HIO₃)₂, which are formed by hydrogen bonds. The crystals are metastable and transform into the stable modification, α‐HIO₃, within a couple of weeks.     

On the stability and decomposition of η6-(2-lithiochlorobenzene)tricarbonylchromium(0)

The decomposition of η⁶-(2-lithiochlorobenzene)tricarbonylchromium(0) (I) was found to follow first order kinetics with k_dec  5.1 x 10^-3 min^-1 at 0°C, the half life of I being 136 min at 0°C. While this dependence strongly suggests intermediacy of η⁶-(benzyne)tricarbonylchromium, trapping experiments were successful in only low yield.     

Ionization energies of cesium and cesium oxide clusters

The vertical ionization potentials of 7 cesium and 86 oxidized cesium clusters were determined using the technique of photoionization mass spectrometry. The spectra were obtained using a tunablecw dye laser for clusters in a mass range 1 to 2024 amu. The vertical ionization potentials (IP) are presented as a function of size and composition. The ionization energies of cesium clusters, Cs_n, decrease with cluster size. Unusually low IP were observed for the enneamer, Cs₉, and for the cesium monoxide Cs₁₁ O. With increasing oxidation of the cesium metal clusters the IP decreases (suboxides) reaches a minimum at Cs(Cs₂O)_n and then increases (superoxides).     

Synthesis of oxa-bridged analogues of farnesyltransferase inhibitor RPR 115135

Two synthetic routes to new oxygen-bridged analogues of farnesyltransferase inhibitors are described that follow either a [3 + 2]/[4 + 2] or a [4 + 2]/[3 + 2] sequence of reactions. The first approach has been achieved by reacting the in situ generated phenylisobenzofuran (PIBF) 4 with pyrroline 5a and has led stereoselectively to racemic 18, which was transformed in a few steps into the target molecule 2. The second pathway relies on a key intermediate 6, obtained either by condensation of PIBF with methyl acrylate, followed by a deprotonation/selenation and an oxidation/elimination sequence, or by cycloaddition between PIBF and alpha-phenylselenoacrylate 11, followed by the same oxidation/elimination sequence. The reaction of 6 with amino dipole 7 gives diastereoselective access to pyrrolidine 25, a precursor of the second target 3, an epimer of 2.     

Spectroscopy of hydrothermal reactions, 19: pH and salt dependence of decarboxylation of α‐alanine at 280–330°C in an FT‐IR spectroscopy flow reactor

The spontaneous decarboxylation of 0.5 m aqueous α‐alanine solutions as a function of pH (1–9 at 320°C, where neutrality is approximately 6) was determined with a flow reactor at 280–330°C and 275 bar by FT‐IR spectroscopy. The kinetics for the cationic and anionic forms have not been previously reported. The rate constants for the cationic form [CH₃(NH₃⁺)CHCO₂H], the anionic form [CH₃(NH₂)CHCO₂^?], and the zwitterion form [CH₃(NH₃⁺)CHCO₂^?] were obtained and followed the first‐order rate law. The rate of decarboxylation of the zwitterion is three times greater than that of the cationic and anionic forms in the temperature and pH ranges of study. The corresponding Arrhenius parameters were determined and compared with previously reported data. The addition of KCl (1 and 2 m) at the natural pH of α‐alanine resulted in a reduction of the decarboxylation rate, suggesting that the transition state is less polar than the zwitterion and/or that the activity of the zwitterion has been reduced. The α‐alanine solution is therefore somewhat more robust in solutions of high ionic strength, such as seawater, than it is in pure water. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 271–277, 2002