Pionic x-ray transitions in 3He

Pionic ³He atoms have been produced in a gaseous target. Energies and strong absorption broadening of the 2 → 1 and the 3 → 1 transition lines have been measured. The 1s level is shifted by 44 ± 5 eV by the (attractive) strong interaction, and the natural 1s level width is 42 ± 14 eV.     

Chemically activated methylcyclobutane from the 337.1 nm photolysis of ketene in the presence of cyclobutane. Dependence of S/D on extent of photolysis

The 3371 A photolysis of ketene/cyclobutane/oxygen mixtures gave stabilization/decomposition ratios for chemically activated methylcyclobutane that varied with the extent of photolysis. Experiments designed to circumvent this complication gave a 10 ± 2 kcal/mole collisional deactivation step size for cyclobutane.     

Electron attachment step in electron capture dissociation (ECD) and electron transfer dissociation (ETD)

We have made use of classical dynamics trajectory simultions and ab initio electronic structure calculations to estimate the cross sections with which electrons are attached (in electron capture dissociation (ECD)) or transferred (in electron transfer dissociation (ETD)) to a model system that contained both an S-S bond that is cleaved and a -NH(3)(+) positively charged site. We used a Landau-Zener-Stueckelberg curve-crossing approximation to estimate the ETD rates for electron transfer from a CH(3)(-) anion to the -NH(3)(+) Rydberg orbital or the S-S sigma* orbital. We draw conclusions about ECD from our ETD results and from known experimental electron-attachment cross sections for cations and sigma-bonds. We predict the cross section for ETD at the positive site of our model compound to be an order of magnitude larger than that for transfer to the Coulomb-stabilized S-S bond site. We also predict that, in ECD, the cross section for electron capture at the positive site will be up to 3 orders of magnitude larger than that for capture at the S-S bond site. These results seem to suggest that attachment to such positive sites should dominate in producing S-S bond cleavage in our compound. However, we also note that cleavage induced by capture at the positive site will be diminished by an amount that is related to the distance from the positive site to the S-S bond. This dimunition can render cleavage through Coulomb-assisted S-S sigma* attachment competitive for our model compound. Implications for ECD and ETD of peptides and proteins in which SS or N-C(alpha) bonds are cleaved are also discussed, and we explain that such events are most likely susceptible to Coulomb-assisted attachment, because the S-S sigma* and C=O pi* orbitals are the lowest-lying antibonding orbitals in most peptides and proteins.     

Measurement of the parity mixing of the 501 keV transition in180Hf by use of a backscattering polarimeter

The circular polarization of the 501 keV gamma radiation in¹⁸⁰Hf was remeasured by use of a Compton backscattering polarimeter with the single pulse counting technique. The resultP _c=?(1.9±0.6) · 10^?3 confirms the strong parity mixing which was previously reported by other groups.     

Chemically activated 1,1-dimethylcyclopropane from photolysis of isobutene–neopentane–diazomethane–oxygen mixtures

A study of the structural isomerization rate of chemically activated 1,1-dimethylcyclopropane from singlet methylene addition to the double bond of isobutene is reported. Singlet methylenes were produced from the 4358- and 3660-Å photolysis of diazomethane in the presence of added oxygen. Theoretical rates calculated via RRKM theory are in excellent agreement with experiment for calculations utilizing activated complex structures and critical energies consistent with known thermal Arrhenius parameters, and excitation energies consistent with previous determinations of ΔH(CH₂) + E*(CH₂) = 116.1 and 112.6 kcal/mole for diazomethane photolyses at 3660 and 4358 Å, respectively.     

Neutral current effects in muonic atoms: E1-E2 interferences

We study the parity-violating neutral current effects in muonic atoms which might result from an interference between E1 and E2 amplitudes in 3D → 1S transitions. We conclude that polarization asymmetries may be measurable in heavy atoms (Z ~ 70) exhibiting large nuclear deformations.     

The role of excited Rydberg States in electron transfer dissociation

Ab initio electronic structure methods are used to estimate the cross sections for electron transfer from donor anions having electron binding energies ranging from 0.001 to 0.6 eV to each of three sites in a model disulfide-linked molecular cation. The three sites are (1) the S-S sigma(*) orbital to which electron attachment is rendered exothermic by Coulomb stabilization from the nearby positive site, (2) the ground Rydberg orbital of the -NH(3)(+) site, and (3) excited Rydberg orbitals of the same -NH(3)(+) site. It is found that attachment to the ground Rydberg orbital has a somewhat higher cross section than attachment to either the sigma orbital or the excited Rydberg orbital. However, it is through attachment either to the sigma(*) orbital or to certain excited Rydberg orbitals that cleavage of the S-S bond is most likely to occur. Attachment to the sigma(*) orbital causes prompt cleavage because the sigma energy surface is repulsive (except at very long range). Attachment to the ground or excited Rydberg state causes the S-S bond to rupture only once a through-bond electron transfer from the Rydberg orbital to the S-S sigma(*) orbital takes place. For the ground Rydberg state, this transfer requires surmounting an approximately 0.4 eV barrier that renders the S-S bond cleavage rate slow. However, for the excited Rydberg state, the intramolecular electron transfer has a much smaller barrier and is prompt.     

Solid-state NMR spectra and long intradimer bonds in the pi-[TCNE]22- dianion

The principal (13)C chemical-shift values for the pi-[TCNE](2)(2-) dimer anion within an array of counterions have been measured to understand better the electronic structure of these atypical chemical species in several related TCNE-based structures. The structure of pi-[TCNE](2)(2-) is unusual as it contains two very long C-C bond lengths (ca. 2.9 Angstroms) between the two monomeric units and has been found to exist as a singlet state, suggestive of a (1)A(1g) (b(2u)(2)b(1g)(0)) electronic configuration. A systematic study of several oxidation states of [TCNE](n) (n = 0, 1-, 2-) was conducted to determine how the NMR chemical-shift tensor values change as a function of electronic structure and to understand the interactions that lead to spin-pairing of the monomer units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. Such theoretical methods provide information on the tensor magnitudes and orientations of their principal tensor components with respect to the molecular frame. Both theoretical and experimental ethylenic chemical-shielding tensors reveal high sensitivity in the component, delta(perpendicular), lying in the monomer molecular plane and perpendicular to the pi-electron plane. This largest shift dependence on charge density is observed to be about -111 ppm/e(-) for delta(perpendicular). The component in the molecular plane but parallel to the central C=C bond, delta(parallel), exhibits a sensitivity of approximately -43 ppm/e(-). However, the out-of-plane component delta'(perpendicular) shows a minimal dependence of -2.6 ppm/e(-) on the oxidation state (n) of [TCNE](n). These relative values support the claim that it is changes within the ethylenic pi-electrons and not the sigma-electrons that best account for the dramatic variations in bonding and shift tensors in this series of compounds. Concerning the intraion bonding, relatively weak Wiberg bond orders between the two monomeric components of the dimer correlate with the long bonds linking the two [TCNE(*)](-) monomers. The chemical-shift tensors for the cyano group, compared to the ethylene shifts, exhibit a reduced sensitivity on the TCNE oxidation state. The experimental principal chemical-shift components agree (within typical errors) with the calculated quantum mechanical shieldings used to correlate the bonding. The embedded ion model (EIM) was used to investigate the typically large electrostatic lattice potential in these ionic materials. Chemical-shielding principal values calculated with the EIM model differ from experiment by +/-3.82 ppm on average, whereas in the absence of an electrostatic field model, the experimental and theoretical results agree by +/-4.42 ppm, which is only a modest increase in error considering the overall ionic magnitudes associated with the tensor variations. Apparently, the effects of the sizable long-range electrostatic fields cancel when the shifts are computed because of lattice symmetry.     

Design and Synthesis of Imidazole and Triazole Pyrazoles as Mycobacterium Tuberculosis CYP121A1 Inhibitors

The emergence of untreatable drug-resistant strains of Mycobacterium tuberculosis is a major public health problem worldwide, and the identification of new efficient treatments is urgently needed. Mycobacterium tuberculosis cytochrome P450 CYP121A1 is a promising drug target for the treatment of tuberculosis owing to its essential role in mycobacterial growth. Using a rational approach, which includes molecular modelling studies, three series of azole pyrazole derivatives were designed through two synthetic pathways. The synthesized compounds were biologically evaluated for their inhibitory activity towards M. tuberculosis and their protein binding affinity (K_D). Series 3 biarylpyrazole imidazole derivatives were the most effective with the isobutyl ( 10 f ) and tert-butyl ( 10 g ) compounds displaying optimal activity (MIC 1.562 μg/mL, K_D 0.22 μM ( 10 f ) and 4.81 μM ( 10 g )). The spectroscopic data showed that all the synthesised compounds produced a type II red shift of the heme Soret band indicating either direct binding to heme iron or (where less extensive Soret shifts are observed) putative indirect binding via an interstitial water molecule. Evaluation of biological and physicochemical properties identified the following as requirements for activity: LogP >4, H-bond acceptors/H-bond donors 4/0, number of rotatable bonds 5–6, molecular volume >340 ų, topological polar surface area <40 Ų.