Rapid access to cis-cyclobutane γ-amino acids in enantiomerically pure form

The (+)-(1R,2S) and (−)-(1S,2R) stereoisomers of 2-(aminomethyl)cyclobutane-1-carboxylic acid have been prepared using a short and efficient strategy, which employs the photochemical [2+2] cycloaddition reaction between ethylene and an unsaturated γ-lactam as the key step.     

Electron mobility in liquid and supercritical helium measured using corona discharges: a new semi-empirical model for cavity formation

Electron mobilities in supercritical and liquid helium were investigated as a function of the density. The mobilities were derived from I(V) curves measured in a high-pressure cryogenic cell using a corona discharge in point-plane electrode geometry for charge generation. The presented data spans a wide pressure and temperature range due to the versatility of our experimental set-up. Where data from previous investigations is available for comparison, very good agreement is found. We present a semi-empirical model to calculate electron mobilities both in the liquid and supercritical phase. This model requires the electron-helium scattering length and thermodynamic state equations as the only input and circumvents any need to consider surface tension. Our semi-empirical model reproduces experimental data very well, in particular towards lower densities where transitions from localised to delocalised electron states were observed.     

Understanding (the lack of) homolytic substitution chemistry of sulfones

High level calculations suggest that homolytic substitution (S(H)2) by alkyl radicals at sulfur proceeds through a mechanism that is assisted and dominated by LP → SOMO interactions; in the absence of these interactions, S(H)2 chemistry at sulfur is predicted to be virtually impossible. G3(MP2)-RAD calculations suggest that cyclization of the tert-butylsulfonylbutyl radical 2 (n = 2) proceeds with a rate constant of 1.7 × 10(-24) s(-1) at 80°, some 28 orders of magnitude slower than its sulfide cousin (n = 0).     

Intramolecular homolytic substitution of seleninates – a computational study

Ab initio and density functional theory (DFT) calculations predict that intramolecular homolytic substitution by alkyl radicals at the selenium atom in seleninates proceeds through smooth transition states in which the attacking and leaving radicals adopt a near collinear arrangement. When forming a five-membered ring and the leaving radical is methyl, G3(MP2)-RAD calculations predict that this reaction proceeds with an activation energy (ΔE₁³) of 30.4 kJ mol^?1. ROBHandHLYP/6-311++G(d,p) calculations suggest that the formation of five-membered rings through similar intramolecular homolytic substitution by aryl radicals, with expulsion of phenyl radicals, proceeds with the involvement of a hypervalent intermediate. This intermediate further dissociates to the observed products, with overall energy barriers of about 40 kJ mol^?1. Homolytic addition to the phenyl group was found not to be competitive with substitution, with a calculated barrier of 57.6 kJ mol^?1. This computational study provides insight into homolytic substitution chemistry involving seleninates.     

A two-step valence tautomeric transition in a dinuclear cobalt complex

A dinuclear cobalt complex with cobalt centers bridged by a bis(dioxolene) ligand exhibits a rare two-step valence tautomeric transition.     

The X-Ray Structures of Sulfoxides

Abstract  We have structurally characterized and investigated a range of sulfoxide compounds containing aryl and alkyl substituents. Compounds 1 and 3–6 all crystallize in an orthorhombic space group, where compounds 2 and 7 crystallize in a monoclinic space group. The unit cell parameters of the compounds are as follows: 1 (Fdd2), a = 17.653(5) ?, b = 53.153(14) ?, and c = 10.071(3) ?; 2 (P2 ₁ /c), a = 7.894(13) ?, b = 5.653(10) ?, and c = 27.02(5) ?, β = 97.347(15)°; 3 (P2 ₁ 2 ₁ 2 ₁ ), a = 5.7569(6) ?, b = 12.2139(12) ?, and c = 17.5974(18) ?; 4 (Pca2 ₁ ), a = 8.256(4) ?, b = 5.470(3) ?, and c = 23.995(13) ?; 5 (P2 ₁ 2 ₁ 2 ₁ ), a = 5.848(4) ?, b = 7.568(5) ?, and c = 27.650(17) ?; 6 (Pbca), a = 10.0569(15) ?, b = 9.9403(16) ?, and c = 17.843(3) ?; and 7 (Pc), a = 13.217(4) ?, b = 5.3766(14) ?, and c = 8.370(2) ?, β = 90.673(6)°. The S=O bond distances in these compounds range from 1.489(7) to 1.515(8) ?. In all seven structures, the O(1)–S–C bond angles vary from 105.1(4) to 111.7(30)° and the C(1)–S(1)–C(11) bond angles range from 94.1(4) to 100.56(12)°. The compounds contain unique intra- and intermolecular interactions depending on the groups attached to the sulfoxide moiety. The polarity of the sulfoxide bond in these compounds allows for intramolecular S···O interactions to occur. When the sulfur is bound to alkyl groups, there tends to be a shorter S···O intermolecular distance than when the sulfur is bound to aromatic substituents. Additionally, if the sulfur is flanked by an aryl group, the S–C bond distance is slightly shorter than if flanked by an alkyl group, suggesting a possible weak O···H_aryl intramolecular interaction, weak conjugation of the aromatic ring with the S–O bond or both. Furthermore, if the sulfur is flanked by an alkyl group, a CH₂ proton of S–CH₂–R can be properly oriented to participate in an intermolecular hydrogen bond with the sulfoxide oxygen of another molecule. Index Abstract  We have structurally characterized and investigated a range of sulfoxide compounds containing aryl and alkyl substituents that contain unique intra- and intermolecular interactions depending on the groups attached to the sulfoxide moiety.      

In situ high-pressure X-ray diffraction study of densification of a molecular chalcogenide glass

Structural mechanisms of densification of a molecular chalcogenide glass of composition Ge_2.5As_51.25S_46.25 have been studied in situ at pressures ranging from 1 atm to 11 GPa at ambient temperature as well as ex situ on a sample quenched from 12 GPa and ambient temperature using high-energy X-ray diffraction. The X-ray structure factors display a reduction in height of the first sharp diffraction peak and a growth of the principal diffraction peak with a concomitant shift to higher Q-values with increasing pressure. At low pressures of at least up to 5 GPa the densification of the structure primarily involves an increase in the packing of the As₄S₃ molecules. At higher pressures the As₄S₃ molecules break up and reconnect to form a high-density network with increased extended-range ordering at the highest pressure of 11 GPa indicating a structural transition. This high-density network structure relaxes only slightly on decompression indicating that the pressure-induced structural changes are quenchable.     

Archaeological involvements of physics

Accounts are given of the three main chronological applications of physics in archaeology: thermoluminescence dating, archaeomagnetism, and radiocarbon dating, with mention of direct detection using a Van de Graaff tandem accelerator. Following these, techniques for the location of burried remains are discussed: magnetic prospection using proton free-precession magnetometry, electrical resistivity surveying, electromagnetic techniques, and other possibilities