Low pressure gas electron diffraction: An experimental setup and case studies.

Vishnevskiy Y, Blomeyer S, Reuter C. Low pressure gas electron diffraction: An experimental setup and case studies. The Review of scientific instruments. 2020;91(7): 074104.Principles of low pressure gas electron diffraction are introduced. An experimental setup has been constructed for measuring the electron diffraction patterns of gaseous samples at pressures below 10-3 mbar. Test measurements have been performed for benzoic acid at T = 287 K corresponding to a vapor pressure of the substance P = 2 * 10-4 mbar, for iodoform CHI3 at T = 288 K (P = 4 * 10-4 mbar), and for carbon tetraiodide CI4 at T = 290 K (P = 1 * 10-4 mbar). Due to the low experimental temperature, thermal decomposition of CI4 has been prevented, which was unavoidable in previous classical measurements at higher temperatures. From the obtained data, the molecular structures have been successfully refined. The most important semi-empirical equilibrium molecular parameters are re(Car-Car)av = 1.387(5) A in benzoic acid, re(C-I) = 2.123(3) A in iodoform, and re(C-I) = 2.133(7) A in carbon tetraiodide. The determined parameters showed consistency with the theoretically predicted values. A critical comparison with the results of the earlier investigations has also been done

Molecular Structure of Pyrazinamide: A Critical Assessment of Modern Gas Electron Diffraction Data from Three Laboratories

Otlyotov AA, Girichev, Georgiy V, Rykov AN, Glodde T, Vishnevskiy Y. Molecular Structure of Pyrazinamide: A Critical Assessment of Modern Gas Electron Diffraction Data from Three Laboratories. JOURNAL OF PHYSICAL CHEMISTRY A. 2020;124(25):5204-5211.Accuracy and precision of molecular parameters determined by modern gas electron diffraction have been investigated. Diffraction patterns of gaseous pyrazinamide have been measured independently in three laboratories, in Bielefeld (Germany), Ivanovo (Russia), and Moscow (Russia). All data sets have been analyzed in equal manner using a highly controlled background elimination procedure and flexible restraints in molecular structure refinement. In detailed examination and comparison of the obtained results we have determined the average experimental precision of 0.004 angstrom for bond lengths and 0.2 degrees for angles. The corresponding average deviations of the refined parameters from the ae-CCSD(T)/cc-pwCVTZ theoretical values were 0.003 angstrom and 0.2 degrees. The average precision for refined amplitudes of interatomic vibrations was determined to be 0.005 angstrom. It is recommended to take into account these values in calculations of total errors for refined parameters of other molecules with comparable complexity

Regiochemical Control in Triptycene Formation-An Exercise in Subtle Balancing Multiple Factors

Lamm J-H, Vishnevskiy Y, Ziemann E, Neumann B, Stammler H-G, Mitzel NW. Regiochemical Control in Triptycene Formation-An Exercise in Subtle Balancing Multiple Factors. ChemistryOpen. 2018;7(1):111-114.Reactions between 1,8-dichloroanthracenes with substituents in position 10 and ortho-chloroaryne afford mixtures of 1,8,13- (syn) and 1,8,16-trichlorotriptycenes (anti). The syn/anti ratio is dependent on these substituents. Electropositive substituents like SiMe3 and GeMe3 lead to preferred formation of the syn-isomer, whereas CMe3 groups exclusively afford the anti-isomer. Different quantum chemical calculations including location of transition states give conflicting results, but indicate the importance of dispersion forces for an at least qualitative prediction of results. The syn-trichlorotriptycenes with SiMe3 and GeMe3 substituents were characterized by using NMR spectroscopy, mass spectrometry, and X-ray diffraction experiments. Triptycene represents one of a few rigid organic frameworks of D3h symmetry without any (Lewis-basic) heteroatoms. It was first synthesized by Bartlett et al. in 1942 using a multi-step procedure starting from anthracene and p-benzoquinone.[1] In 1956, Wittig and Ludwig reported a more efficient access to triptycene in one step from anthracene by reacting it with in situ-formed benzyne.[2] The symmetry and rigidity of triptycene have inspired a plethora of applications in fundamental and applied chemical research.[3-5] Substituted triptycenes are widely used, for example, as building blocks for fluorescent or non-fluorescent organic macromolecules, polymers, and liquid crystals,[3, 6] as rigid spacers in several Pd complexes used for cross coupling reactions,[7] as devices in molecular machines,[8] in crystal engineering processes,[9, 10] and as a basis for the design of highly porous organic materials with numerous applications.[11] Although the chemistry of triptycenes and their functionalization is generally in an advanced state, the 1,8,13-trisubstitution motif remains a challenge for synthesis. However, exactly this pattern is interesting to introduce three functionalities oriented in the same direction. We try to make use of such 1,8,13-trisubstituted triptycenes (also called syn-triptycenes) as rigid organic frameworks for constructing directed polydentate Lewis acids,[12, 13] but many other applications might be envisioned. syn-Triptycenes can be obtained through Diels–Alder reactions of 1,8-disubstituted anthracenes with ortho-functionalized arynes, a protocol introduced by Rogers and Averill in 1986.[14] The drawback of this method is that the corresponding anti-trisubstituted 1,8,16-isomer is always formed as the main product when, for example, Cl-functionalized anthracenes and arynes are used.[12, 14] In 2010, we reported attempts to increase the syn/anti ratio by making use of the steric interference of the (bulky) anthracene substituent at C-10 with the chlorine atom of the chloroaryne (Scheme 1). We expected this strategy to provide an increased formation of the syn-isomer. However, the steric influence of the C-10 substituent turned out to be minimal, whereas the electronic properties are dominant;[12] of all substituents tested, the biggest R=C(CH3)3 led to the formation of 100 % anti-isomer, despite the formation of an extremely deformed product by mutual repulsion of the Cl and R substituents, as indicated in Scheme 1 b

Influence of Antipodally Coupled Iodine and Carbon Atoms on the Cage Structure of 9,12-I2-closo-1,2-C2B10H10 : An Electron Diffraction and Computational Study

Because of the comparable electron scattering abilities of carbon and boron, the electron diffraction structure of the C2v-symmetric molecule closo-1,2-C2B10H12 (1), one of the building blocks of boron cluster chemistry, is not as accurate as it could be. On that basis, we have prepared the known diiodo derivative of 1, 9,12-I2-closo-1,2-C2B10H10 (2), which has the same point-group symmetry as 1 but in which the presence of iodine atoms, with their much stronger ability to scatter electrons, ensures much better structural characterization of the C2B10 icosahedral core. Furthermore, the influence on the C2B10 geometry in 2 of the antipodally positioned iodine substituents with respect to both carbon atoms has been examined using the concerted application of gas electron diffraction and quantum chemical calculations at the MP2 and density functional theory (DFT) levels. The experimental and computed molecular geometries are in good overall agreement. Molecular dynamics simulations used to obtain vibrational parameters, which are needed for analyzing the electron diffraction data, have been performed for the first time for this class of compound. According to DFT calculations at the ZORA-SO/BP86 level, the 11B chemical shifts of the boron atoms to which the iodine substituents are bonded are dominated by spin-orbit coupling. Magnetically induced currents within 2 have been calculated and compared to those for [B12H12]2-, the latter adopting a regular icosahedral structure with Ih point-group symmetry. Similar total current strengths are found but with a certain anisotropy, suggesting that spherical aromaticity is present; electron delocalization in the plane of the hetero atoms in 2 is slightly hindered compared to that for [B12H12]2-, presumably because of the departure from ideal icosahedral symmetry

Synchronization and Timing in CMS HCAL

The synchronization and timing of the hadron calorimeter (HCAL) for the Compact Muon Solenoid has been extensively studied with test beams at CERN during the period 2003-4, including runs with 40 MHz structured beam. The relative phases of the signals from different calorimeter segments are timed to 1 ns accuracy using a laser and equalized using programmable delay settings in the front-end electronics. The beam was used to verify the timing and to map out the entire range of pulse shapes over the 25 ns interval between beam crossings. These data were used to make detailed measurements of energy-dependent time slewing effects and to tune the electronics for optimal performance

Energy Response and Longitudinal Shower Profiles Measured in CMS HCAL and Comparison With Geant4

The response of the CMS combined electromagnetic and hadron calorimeter to beams of pions with momenta in the range 5-300 GeV/c has been measured in the H2 test beam at CERN. The raw response with the electromagnetic compartment calibrated to electrons and the hadron compartment calibrated to 300 GeV pions may be represented by sigma = (1.2) sqrt{E} oplus (0.095) E. The fraction of energy visible in the calorimeter ranges from 0.72 at 5 GeV to 0.95 at 300 GeV, indicating a substantial nonlinearity. The intrinsic electron to hadron ratios are fit as a function of energy and found to be in the range 1.3-2.7 for the electromagnetic compartment and 1.4-1.8 for the hadronic compartment. The fits are used to correct the non-linearity of the e pi response to 5% over the entire measured range resulting in a substantially improved resolution at low energy. Longitudinal shower profile have been measured in detail and compared to Geant4 models, LHEP-3.7 and QGSP-2.8. At energies below 30 GeV, the data, LHEP and QGSP are in agreement. Above 30 GeV, LHEP gives a more accurate simulation of the longitudinal shower profile

Design, Performance, and Calibration of CMS Hadron Endcap Calorimeters

Detailed measurements have been made with the CMS hadron calorimeter endcaps (HE) in response to beams of muons, electrons, and pions. Readout of HE with custom electronics and hybrid photodiodes (HPDs) shows no change of performance compared to readout with commercial electronics and photomultipliers. When combined with lead-tungstenate crystals, an energy resolution of 8\% is achieved with 300 GeV/c pions. A laser calibration system is used to set the timing and monitor operation of the complete electronics chain. Data taken with radioactive sources in comparison with test beam pions provides an absolute initial calibration of HE to approximately 4\% to 5\%

Design, Performance, and Calibration of the CMS Hadron-Outer Calorimeter

The CMS hadron calorimeter is a sampling calorimeter with brass absorber and plastic scintillator tiles with wavelength shifting fibres for carrying the light to the readout device. The barrel hadron calorimeter is complemented with an outer calorimeter to ensure high energy shower containment in the calorimeter. Fabrication, testing and calibration of the outer hadron calorimeter are carried out keeping in mind its importance in the energy measurement of jets in view of linearity and resolution. It will provide a net improvement in missing \et measurements at LHC energies. The outer hadron calorimeter will also be used for the muon trigger in coincidence with other muon chambers in CMS

Differential cross section measurements for the production of a W boson in association with jets in proton–proton collisions at √s = 7 TeV

Measurements are reported of differential cross sections for the production of a W boson, which decays into a muon and a neutrino, in association with jets, as a function of several variables, including the transverse momenta (pT) and pseudorapidities of the four leading jets, the scalar sum of jet transverse momenta (HT), and the difference in azimuthal angle between the directions of each jet and the muon. The data sample of pp collisions at a centre-of-mass energy of 7 TeV was collected with the CMS detector at the LHC and corresponds to an integrated luminosity of 5.0 fb[superscript −1]. The measured cross sections are compared to predictions from Monte Carlo generators, MadGraph + pythia and sherpa, and to next-to-leading-order calculations from BlackHat + sherpa. The differential cross sections are found to be in agreement with the predictions, apart from the pT distributions of the leading jets at high pT values, the distributions of the HT at high-HT and low jet multiplicity, and the distribution of the difference in azimuthal angle between the leading jet and the muon at low values.United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio