Quantitative Assessment of Force Fields on Both Low-Energy Conformational Basins and Transition-State Regions of the (ϕ−ψ) Space

2010 ◽  
Vol 7 (2) ◽  
pp. 402-419 ◽  
Author(s):  
Zhiwei Liu ◽  
Bernd Ensing ◽  
Preston B. Moore
Keyword(s):  
Transition State ◽  
Quantitative Assessment ◽  
Force Fields ◽  
Low Energy

scholarly journals Application of Q2MM to predictions in stereoselective synthesis

2018 ◽  
Vol 54 (60) ◽  
pp. 8294-8311 ◽  
Author(s):  
Anthony R. Rosales ◽  
Taylor R. Quinn ◽  
Jessica Wahlers ◽  
Anna Tomberg ◽  
Xin Zhang ◽  
...  
Keyword(s):  
Transition State ◽  
Molecular Mechanics ◽  
Stereoselective Synthesis ◽  
Force Fields ◽  
Accurate Prediction ◽  
State Force

Transition state force fields derived by Quantum Guided Molecular Mechanics (Q2MM) allows the rapid and accurate prediction of stereoselectivity.

Model Calculations of Isotope Effects. III. Isotope Effects in Hydrogen Transfer Reactions, and Transition State Force Fields

1979 ◽  
Vol 32 (9) ◽  
pp. 1869
Author(s):  
DJ McLennan
Keyword(s):  
Transition State ◽  
Hydrogen Transfer ◽  
Force Fields ◽  
Isotope Effects ◽  
Semiempirical Method ◽  
Model Calculations ◽  
Surface Hydrogen ◽  
Kinetic Isotope ◽  
State Force ◽  
Better Than

Model calculations of kinetic isotope effects for the reactions: C2H6+·CH3 → ·C2H5+CH4 CH4 + ·CF3 → ·CH3+CHF3 are reported. Transition state geometries were those calculated by Dewar and coworkers using the MNDO semiempirical method. Transition state force fields were formulated from empirical expressions for stretching, bending, linear bending and interaction valence force constants by using bond orders as disposable parameters. Although it proved impossible to assign a best force field to either reaction, the calculated isotope effects generally were in satisfactory agreement with experiment, and were better than those calculated from the MNDO potential energy surface. Hydrogen tunnelling is apparently implicated in the CH4+ CF3 reaction.

scholarly journals Comparison of low energy and medium energy collimators in the quantitative assessment of 123I-MIBG cardiac sympathetic innervation imaging

2021 ◽  
Vol 22 (Supplement_3) ◽  
Author(s):  
I Casans Tormo ◽  
R Ruiz Granell
Keyword(s):  
Image Quality ◽  
Quantitative Assessment ◽  
Sympathetic Innervation ◽  
High Energy ◽  
Mean Difference ◽  
Low Energy ◽  
Medium Energy ◽  
Cardiac Sympathetic Innervation ◽  
Cardiac Uptake ◽  
Planar Images

Abstract Funding Acknowledgements Type of funding sources: Public hospital(s). Main funding source(s): University Clinic Hospital of Valencia Aim The assessment of cardiac sympathetic innervation by 123I-metayodobenzylguanidine (123I-MIBG) has proved useful in patients with heart failure and neurodegenerative disorders. The standard quantification of global cardiac uptake is to obtain the heart to mediastinum (HM) ratio in planar images, while SPECT images provide better evaluation of regional extent of denervated areas. Although low energy (LE) collimator has been widely used in these patients, septal penetration of high-energy photons of 123I could affect image quality and quantification accuracy. We have compared in the same patients the effect of collimator type on image quality and quantitative assessment of HM ratio. Methods In a group of 14 patients (11 men, 57-77 y/o, 66.4 ± 5.6) submitted for cardiac sympathetic study, we obtained successive planar anterior chest images 4h after the IV administration of 10 mCi of 123I-MIBG with a low-energy-high-resolution (LEHR) and a medium-energy (ME) collimators. Images were obtained with the same gammacamera Brightview Philips for 10 min on 256x256 matrix, with 159 Kev photopeak and unchanged patient position. For quantification of HM ratio, we use in each patient the same manual heart ROI and rectangular upper mediastinum ROI in the two acquired images to obtain the corresponding HM ratio. Results The image quality was better in all the patients with ME collimator acquisition than with LEHR collimator acquisition, and the HM ratio showed higher values with ME: 1.65-2.61 (mean 2.15 ± 0.28) than with LEHR: 1.27-1.85 (mean: 1.51 ± 0.18) with a mean difference of 0.64 ± 0.15 (0.38-0.88) between ME and LEHR and a mean ratio LEHR/ME of 0.70 ± 0.04 (0.64-0.79). In 9 patients with HM ratio ≤ 1.60 obtained from LEHR collimator acquisition, the mean difference with HM ratio obtained with ME collimator was 0.61 ± 0.12 and mean ratio LEHR/ME was 0.69 ± 0.03 and in 5 patients with HM ratio(LEHR) >1.60, mean difference with HM ratio(ME) was 0.69 ± 0.19 and mean ratio LEHR/ME was 0.71 ± 0.02. Conclusion Use of a ME collimator provides better image quality than LEHR collimator in planar images and higher values of HM ratio, providing a more accurate quantification of cardiac uptake in the patients submitted for evaluation of cardiac sympathetic innervation by 123I-MIBG, and could also improve the evaluation of regional impairment and extent of denervated areas by SPECT.

Transition-state modeling with empirical force fields

1993 ◽  
Vol 93 (7) ◽  
pp. 2439-2461 ◽  
Author(s):  
John E. Eksterowicz ◽  
K. N. Houk
Keyword(s):  
Transition State ◽  
Force Fields

scholarly journals Molecular Electrochemical Reductive Splitting of Dinitrogen with a Molybdenum Complex

2021 ◽  
Author(s):  
Lydia Merakeb ◽  
Soukaina Bennaamane ◽  
Eric Clot ◽  
Nicolas Mézailles ◽  
Marc Robert
Keyword(s):  
Nitrogen Fixation ◽  
Transition State ◽  
Atmospheric Pressure ◽  
Room Temperature ◽  
Energy Transition ◽  
Low Energy ◽  
Short List ◽  
Molybdenum Complex ◽  
Nmr Studies ◽  
Mild Conditions

Nitrogen reduction in mild conditions (i.e. room temperature and atmospheric pressure) and using a non-fossil source of hydrogen remains a high chemistry challenge. Molecular metal complexes, notably Mo based, have recently shown to be active for such nitrogen fixation. In this work, we report about the electrochemical N2 splitting with MoIII triphosphino com-plex ((PPP)MoI3), at room temperature and a moderately negative potential. A MoIV nitride species was generated, which was confirmed by electrochemistry and NMR studies. The reaction goes through the bi-electronic reduction of the starting Mo species, coordination of an N2 molecule, and further splitting to a MoIV nitride complex. Preliminary DFT investigation supports the intermediacy of a bridging MoIN2MoI dinitrogen dimer evolving to the Mo nitride via a low energy transition state. This example joins a short list of molecular electrochemical complexes for N2 reductive cleavage. It opens a door to molecular electrochemical PCET conversion studies of N2 to NH3.

A Unified Interpretation of Mass and Charge Distribution and Prompt Neutron Evaporation in Low-Energy Nuclear Fission

1969 ◽  
Vol 24 (12) ◽  
pp. 2000-2003 ◽  
Author(s):  
H. O . Denschlag ◽  
S. M. Qaim
Keyword(s):  
Experimental Data ◽  
Simple Model ◽  
Transition State ◽  
Charge Distribution ◽  
Nuclear Fission ◽  
Low Energy ◽  
Prompt Neutron ◽  
Neutron Evaporation ◽  
State Configuration

AbstractA simple model for a transition state configuration in low-energy fission is given. In its light the latest experimental data on mass and charge distribution and on prompt neutron evaporation are discussed.

Peculiarities of Quantitative Assessment of Country Risk in Case of International Oil and Gas Projects Implementation

2018 ◽  
Vol 7 (1) ◽  
pp. 4-9
Author(s):  
Д. Власов ◽  
D. Vlasov
Keyword(s):  
Alternative Energy ◽  
Quantitative Assessment ◽  
Oil And Gas ◽  
Country Risk ◽  
Low Energy ◽  
Energy Sources ◽  
Energy Prices ◽  
Alternative Energy Sources ◽  
International Projects ◽  
Oil And Gas Companies

Mitigation of risks becomes one of the key problematic areas of national oil and gas companies in case of low energy prices, prolongation of sanctions regarding Russia and development of alternative energy sources. The study is devoted to development of approach to quantitative assessment of country risk as a tool of oil and gas international projects selection.

Structure and Conformations of GABA-Transaminase Inhibitors. IV. Transition State Analogs

1988 ◽  
Vol 41 (4) ◽  
pp. 493 ◽  
Author(s):  
PR Andrews ◽  
JM Gulbis ◽  
MN Iskander ◽  
MF Mackay ◽  
C Dipaola ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Hydrogen Bonding ◽  
Transition State ◽  
Crystal Structures ◽  
Minimum Energy ◽  
Low Energy ◽  
Gaba Transaminase ◽  
Transition State Analogs ◽  
Potential Inhibitors ◽  
Low Energy Conformations

Crystal structures of three potential inhibitors [salicylamide derivative C16H15NO4 (5), pyridoxazine C11H16N2O5S (6) and benzoxazone C12H13NO4 (7).H2O] of GABA- transaminase (E.C.2.6.1.19, GABA-T) based on the calculated transition state of GABA-T were determined. The conformational analyses of these structures (non-bonded energies, MNDO,AM1) indicate that they can all fit the transition state in relatively low energy conformations. The crystal structures appear to be close to the calculated minimum energy conformations, except for the salicylamide derivative (5), which features internal hydrogen-bonding. The AM1 parametrization has been used successfully to predict two possible hydrogen-bonded conformations of (5), one of which is found in the crystal structure.

On the mechanism of Wittig reactions with cyclic anhydrides. II.

1993 ◽  
Vol 71 (7) ◽  
pp. 1010-1021 ◽  
Author(s):  
Margaret M. Kayser ◽  
Krista L. Hatt ◽  
Heshui Yu ◽  
Donald L. Hooper
Keyword(s):  
Transition State ◽  
Low Energy ◽  
Spectroscopic Methods ◽  
Selective Transformation ◽  
Π Stacking ◽  
Wittig Olefination ◽  
Reversible Formation ◽  
Cyclic Anhydrides ◽  
Wittig Reactions

A study by NMR spectroscopic methods and trapping experiments of the mechanism of Wittig reactions between stabilized phosphoranes and unsymmetrically substituted cyclic anhydrides suggests that two reactions are involved: (1) a low-energy, reversible formation of acyclic adducts; and (2) a higher energy "Wittig olefination" reaction leading to enol-lactones. The latter, more selective, transformation requires a more highly organized transition state in which π-stacking and stabilizing complexations are important factors.

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