Search results for " Isomers"
showing 10 items of 39 documents
Experimental and theoretical NMR and IR studies of the side‐chain orientation effects on the backbone conformation of dehydrophenylalanine residue
2011
Conformation of N‐acetyl‐(E)‐dehydrophenylalanine N′, N′‐dimethylamide (Ac‐(E)‐ΔPhe‐NMe2) in solution, a member of (E)‐α, β‐dehydroamino acids, was studied by NMR and infrared spectroscopy and the results were compared with those obtained for (Z) isomer. To support the spectroscopic interpretation, the ϕ, ψ potential energy surfaces were calculated at the MP2/6‐31 + G(d,p) level of theory in chloroform solution modeled by the self‐consistent reaction field‐polarizable continuum model method. All minima were fully optimized by the MP2 method and their relative stabilities were analyzed in terms of π‐conjugation, internal H‐bonds and dipole interactions between carbonyl groups. The obtained N…
Towards large‐scale steady‐state enhanced nuclear magnetization with in situ detection
2021
Magnetic resonance in chemistry 59(12), 1208 - 1215 (2021). doi:10.1002/mrc.5161
Tetrazola hemiamināls kā hirāla palīggrupa
2017
Tetrazola hemiamināls kā hirāla palīggrupa. Sējējs M., zinātniskais vadītājs Prof., Dr. chem. Sūna E., konsultēja MSc. chem., Kinēns A. Maģistra darbs uzrakstīts latviešu valodā, tā apjoms 58 lapaspuses. Darbs satur 56 attēlus, 5 tabulas. Maģistra darbs veltīts terazolu hemiaminālu hirālās palīggrupas iegūšanai un pielietojumam diastereoselektīvajā sintēzē. Izstrādātās hirālās palīggrupas efektivitāte demonstrēta, ar augstu diastereoselektivitāti veicot karbonskābju α-pozīcijas fluorēšanas un nitrozo-aldola reakcijas. Darba rezultātā izstrādāts tetrazola hemiamināla hirālās palīggrupas stereoindukcijas modelis. Demonstrēta hirālās palīggrupas šķelšana maigos reakcijas apstākļos. Tetrazola h…
Cavitation of electron bubbles in liquid parahydrogen
2011
Within a finite-temperature density functional approach, we have investigated the structure of electron bubbles in liquid parahydrogen below the saturated vapour pressure, determining the critical pressure at which electron bubbles explode as a function of temperature. The electron-parahydrogen interaction has been modelled by a Hartree-type local potential fitted to the experimental value of the conduction band-edge for a delocalized electron in pH(2). We have found that the pressure for bubble explosion is, in absolute value, about a factor of two smaller than that of the homogeneous cavitation pressure in the liquid. Comparison with the results obtained within the capillary model shows t…
Nucleation and cavitation in parahydrogen
2012
We have used a density functional approach to investigate thermal homogeneous nucleation and cavitation in parahydrogen. The effect of electrons as seeds of heterogeneous cavitation in liquid parahydrogen is also discussed within the capillary model. (C) 2011 Elsevier B.V. All rights reserved.
Structure, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. IV. Dihydrophospholophosphole isomers
2005
The geometry of the heteropentalenes formed by two phosphole units has been determined at the DFT level. The magnetic susceptibility and the nuclear magnetic shielding at the nuclei of these systems have also been calculated using gauge-including atomic orbitals and a large Gaussian basis set to achieve near Hartree-Fock estimates. A comparative study of the various isomers, of their flattened analogs, and of the parent phosphole molecule, shows that the [3,4-c] isomer is the most aromatic system in the set considered, assuming diatropicity and degree of planarity as indicators, even if it is the less stable in terms of total molecular energy. Plots of magnetic field-induced current densiti…
Thermal effects on small para-hydrogen clusters
2010
A brief review of different quantum Monte Carlo simulations of small (p-H2)N clusters is presented. The clusters are viewed as a set of N structureless p-H2 molecules, interacting via an isotropic pairwise potential. Properties as superfluidity, magic numbers, radial structure, excitation spectra, and abundance production of (p-H2)N clusters are discussed and, whenever possible, a comparison with 4HeN droplets is presented. All together, the simulations indicate that temperature has a paradoxical effect of the properties of (p-H2)N clusters, as they are solid-like at high T and liquid-like at low T, due to quantum delocalization at the lowest temperature. © 2010 Wiley Periodicals, Inc. Int …
Current density maps, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. III. Thieno-thiophene isomers
2005
Near Hartree–Fock values of the magnetic susceptibility and nuclear magnetic shielding of bis-heteropentalenes consisting of two thiophene units ([2,3-b], [3,2-b], [3,4-b], and [3,4-c] isomers) have been estimated via computational schemes relying on continuous transformation of the origin of the current density within the coupled Hartree–Fock approximation and extended gaugeless Gaussian basis sets. The results are compared with those obtained via London gauge-including orbitals. Maps of streamlines and the modulus of the ring current density induced by a magnetic field normal to the molecular plane are reported for the three isomers of higher symmetry, showing that the intense diamagnetic…
Conformation resolved induced infrared activity: trans- and cis-formic acid isolated in solid molecular hydrogen
2011
We report combined experimental and theoretical studies of infrared absorptions induced in solid molecular hydrogen by different conformers of formic acid (HCOOH, FA). FTIR spectra recorded in the H(2) fundamental region (4120-4160 cm(-1)) reveal a number of relatively strong trans-FA induced Q-branch absorptions that are assigned by studying both FA-doped parahydrogen (pH(2)) and normal hydrogen (nH(2)) samples. The induced H(2) absorptions are also studied for HCOOD doped nH(2) crystals for both the trans and cis conformers that show resolvable differences. Samples containing90% of the higher energy cis-HCOOD conformer are produced by in situ IR pumping of the OD stretching overtone of tr…
Singlet‐Contrast Magnetic Resonance Imaging: Unlocking Hyperpolarization with Metabolism
2020
Abstract Hyperpolarization‐enhanced magnetic resonance imaging can be used to study biomolecular processes in the body, but typically requires nuclei such as 13C, 15N, or 129Xe due to their long spin‐polarization lifetimes and the absence of a proton‐background signal from water and fat in the images. Here we present a novel type of 1H imaging, in which hyperpolarized spin order is locked in a nonmagnetic long‐lived correlated (singlet) state, and is only liberated for imaging by a specific biochemical reaction. In this work we produce hyperpolarized fumarate via chemical reaction of a precursor molecule with para‐enriched hydrogen gas, and the proton singlet order in fumarate is released a…