Unbiased expectation values from diffusion quantum Monte Carlo simulations with a fixed number of walkers

We append forward walking to a diffusion Monte Carlo algorithm which maintains a fixed number of walkers. This removes the importance sampling bias of expectation values of operators which do not commute with the Hamiltonian. We demonstrate the effectiveness of this approach by employing three importance sampling functions for the hydrogen atom ground state, two very crude. We estimate moments of the electron-nuclear distance, static polarizabilities, and high-order hyperpolarizabilites up to the fourth power in the electric field, where no use is made of the finite field approximation. The results agree with the analytical values, with a statistical error which increases substantially with decreasing overlap of the guiding function with the exact wave function.     

A unique cyanide-bridged three-dimensional (3-D) copper(II)-copper(I) mixed-valence polymer containing 1-D water tapes with cyclic pentamer units

A unique cyano-bridged copper(II)-copper(I) mixed-valence polymer, {[Cu(3)(CN)(4)(H(2)O)(3)](H(2)O)(2)}(n) (1), has been prepared and structurally determined by X-ray diffraction. The cyanide anions with (mu(2),eta(2)) and (mu(3),eta(2)) bridging modes connect the copper centers into a complicated zeolite-like 3-D network. Of further interest, coordinated and lattice water molecules in this structure form 1-D hydrogen-bonded water tapes consisting of linked cyclic pentamer clusters.     

First organophosphorus radical-mediated cyclisations to afford medium-sized rings: eight-membered lactones and seven- and eight-membered lactams

The phosphorus based radical precursors N-ethylpiperidine hypophosphite (EPHP) and diethylphosphine oxide (DEPO) are efficient reagents for carrying out the formation of seven- and eight-membered rings. Esters and amides were successfully converted into the corresponding eight-membered lactones and seven- and eight-membered lactams in good to excellent yields.     

Nonlocality as a feature of the physical world

Aprediction puzzle leads to a form ofnecessary realism which forces the rejection of a central tenet of the Copenhagen interpretation. This leads to reconsidering conceptual difficulties related to Bell's locality premise. It is shown that aparadox of elementary probability theory puts new light on Bell's assumption that causality and statistical independence are mutually incompatible. Nemo dat quod non habet : No one can offer more than he has ability to give; in that spirit, this paper is dedicated to the memory of J.S. Bell.     

The microwave spectrum of thioformaldehyde,CD2S,and CH2S: Average structure,dipole moments,and 33S quadrupole coupling

Microwave transitions up to J = 53 in the ground vibrational state of deuterothioformaldehyde, CD₂S, were studied between 8 and 40 GHz. A detailed centrifugal distortion analysis yields accurate constants for comparison with force field values. The isotopic species ¹³CH₂S, CH₂³⁴S, CH₂³³S, ¹³CD₂S, CD₂³⁴S, and CD₂³³S were studied in natural abundance. Accurate average zero-point structures were determined for both CD₂S and CH₂S:

$\text{CH}{}_2\text{S}\text{CS}\text{=1.6138(4)}\text{CH}\text{= 1.0962(6)}\text{A}\text{?}\text{∠}\text{HCH}\text{=116° 16(6)′,}\text{CD}{}_2\text{S}\text{CS}\text{=1.6136(4)}\text{CD}\text{= 1.0931(4)}\text{A}\text{?}\text{∠}\text{DCD}\text{=116° 25(5)′}$

Changes in the zero-point geometry for deuterium substitution were established. Quadrupole fine structure arising from the ³³S nucleus has been measured in CH₂³³S and CD₂³³S. Analysis gives the following coupling constants (for both molecules) as χ_aa = ?11.7 and χ_bb - χ_cc = 88.1 MHz. The dipole moment of CD₂S was measured to be 1.6588(8)D and an accurate comparison with CH₂S was made; the ratio of dipole moments $\text{CD}{}_2\text{S}\text{CH}{}_2\text{S}$ was found to be 1.0062(4). The spectroscopic and bonding properties of CH₂S will be compared with formaldehyde and other molecules.     

Surface forces,supramolecular polymers,and inversion symmetry

The effects of supramolecular equilibrium polymers on surface forces are studied by both a phenomenological Landau type analysis and a molecular model based on a Bethe-Guggenheim approximation. We point out that surface forces brought about by equilibrium polymers may be completely different from what can be found with "ordinary" polymers. The new feature is the role of inversion (a)symmetry or "directionality" of the associating unit molecules ("monomers"). Symmetric B-B monomers (where B denotes a self-complementary binding group) give rise to nondirectional chains and lead to attractive forces between similar surfaces. Asymmetric A-D monomers (where A and D denote complementary acceptor and donor groups, respectively) produce directional chains and can cause strong repulsion. The range of the attractive force has a maximum at intermediate concentration, while the range of the repulsive force increases over the whole concentration range.     

Diversity-oriented synthesis of biaryl-containing medium rings using a one bead/one stock solution platform

Diversity-oriented synthesis of structurally complex and diverse small molecules can be used as the first step in a process to explore cellular and organismal pathways. The success of this process is likely going to be dependent on advances in the synthesis of small molecules having natural product-like structures in an efficient and stereoselective manner. The development, scope, and mechanism of the oxidation of organocuprates was investigated and exploited in the atropdiastereoselective synthesis of biaryl-containing medium rings (9-, 10-, and 11-membered rings). The methodology was performed on high-capacity, large polystyrene beads by metalating aryl bromides with i-PrBu(2)MgLi, followed by transmetalating with CuCN x 2LiBr and then oxidizing with 1,3-dinitrobenzene, and was used in a diversity-oriented synthesis of biaryl-containing medium rings (library total theoretical maximum 1412 members). The high capacity beads were arrayed into 384-well plates and, using a process optimized during the development of a one bead/one stock solution technology platform, converted into arrays of stock solutions, with each stock solution containing largely one compound. These stock solutions were used in numerous phenotypic and protein-binding assays. The process described outlines a pathway that we feel will contribute to a comprehensive and systematic chemical approach to exploring biology (chemical genetics).     

Reduced local energy as a criterion for the accuracy of approximate H2 wave-functions

The goodness of the local fit of an approximate wave-function, \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \psi $\end{document}, to the exact function, ψ₀, is \documentclass{article}\pagestyle{empty}\begin{document}$ |\tilde \psi - \psi _0 | $\end{document}. From this quantity the global accuracy of \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \psi $\end{document} is defined and a “working supposition” is presented, which quantitatively relates the global accuracy to the accuracy of expectation values. Two criteria based on the accuracy of the reduced local energy and the density respectively, are presented as alternatives to \documentclass{article}\pagestyle{empty}\begin{document}$ |\tilde \psi - \psi _0 | $\end{document}. The relative global accuracies of eight wave-functions for H₂ are determined using the two criteria. The ‘working supposition’ is applied and predictions are made concerning the relative accuracies of the expectation values of the following operators: z², r², x² + y², 3z² ?; r², ξ, r, r, and E_L (the reduced local energy). The success rate is high (>90%) except for those operators which are sensitive to interelectron coordinates or derivatives of the wave-function.     

Et3SiH + KOtBu provide multiple reactive intermediates that compete in the reactions and rearrangements of benzylnitriles and indolenines

The combination of potassium tert-butoxide and triethylsilane is unusual because it generates multiple different types of reactive intermediates simultaneously that provide access to (i) silyl radical reactions, (ii) hydrogen atom transfer reactions to closed shell molecules and to radicals, (iii) electron transfer reductions and (iv) hydride ion chemistry, giving scope for unprecedented outcomes. Until now, reactions with this reagent pair have generally been explained by reference to one of the intermediates, but we now highlight the interplay and competition between them.

The combination of potassium tert-butoxide and triethylsilane provides simultaneous access to multiple reactive intermediates, radicals, H-atom donors, hydride donors and electron donors, giving scope for unprecedented reaction outcomes.     

A Testable Theory for the Emergence of the Classical World

The transition from the quantum to the classical world is not yet understood. Here, we take a new approach. Central to this is the understanding that measurement and actualization cannot occur except on some specific basis. However, we have no established theory for the emergence of a specific basis. Our framework entails the following: (i) Sets of N entangled quantum variables can mutually actualize one another. (ii) Such actualization must occur in only one of the 2^N possible bases. (iii) Mutual actualization progressively breaks symmetry among the 2^N bases. (iv) An emerging “amplitude” for any basis can be amplified by further measurements in that basis, and it can decay between measurements. (v) The emergence of any basis is driven by mutual measurements among the N variables and decoherence with the environment. Quantum Zeno interactions among the N variables mediates the mutual measurements. (vi) As the number of variables, N, increases, the number of Quantum Zeno mediated measurements among the N variables increases. We note that decoherence alone does not yield a specific basis. (vii) Quantum ordered, quantum critical, and quantum chaotic peptides that decohere at nanosecond versus femtosecond time scales can be used as test objects. (viii) By varying the number of amino acids, N, and the use of quantum ordered, critical, or chaotic peptides, the ratio of decoherence to Quantum Zeno effects can be tuned. This enables new means to probe the emergence of one among a set of initially entangled bases via weak measurements after preparing the system in a mixed basis condition. (ix) Use of the three stable isotopes of carbon, oxygen, and nitrogen and the five stable isotopes of sulfur allows any ten atoms in the test protein to be discriminably labeled and the basis of emergence for those labeled atoms can be detected by weak measurements. We present an initial mathematical framework for this theory, and we propose experiments.