Model calculations of hydrogen isotope effects for non-linear transition states

1970 ◽  
pp. 785 ◽  
Author(s):  
R. A. More O'Ferrall
Keyword(s):  
Hydrogen Isotope ◽  
Isotope Effects ◽  
Transition States ◽  
Model Calculations ◽  
Non Linear

Model Calculations of Isotope Effects. IV. Transition State Structures and the Question of Tunnelling in Proton Transfer Reactions

1979 ◽  
Vol 32 (9) ◽  
pp. 1883 ◽  
Author(s):  
DJ McLennan
Keyword(s):  
Proton Transfer ◽  
Transition State ◽  
Normal Values ◽  
Isotope Effects ◽  
Transition States ◽  
Transfer Reactions ◽  
Model Calculations ◽  
Proton Transfer Reactions ◽  
Donor Acceptor ◽  
State Models

Model calculations of primary hydrogen isotope effects in proton transfer reactions are reported. The geometries and force fields of transition state models have been systematically varied with respect to both reactant-like and product-like character and to tight against loose character. The models include both hypothetical cut-off molecules and 2-nitropropane. Values of kH/kD greater than 17 are calculated for loose, symmetrical transition states in which the sum of the bond orders pertaining to the transferring proton is set at 0.6, and higher than normal values of (ED-EH) and ADIAH are also associated with such transition states. It is suggested that transition state looseness is a consequence of repulsive donor-acceptor steric interactions, and that several sets of experimental results which have hitherto been rationalized by the invocation of proton tunnelling may equally well be explained by postulating loose transition states.

scholarly journals Unravelling the process of petroleum hydrocarbon biodegradation in different filter materials of constructed wetlands by stable isotope fractionation and labelling studies

2021 ◽  
Author(s):  
Andrea Watzinger ◽  
Melanie Hager ◽  
Thomas Reichenauer ◽  
Gerhard Soja ◽  
Paul Kinner
Keyword(s):  
Stable Isotope ◽  
Constructed Wetlands ◽  
Hydrogen Isotope ◽  
Petroleum Hydrocarbon ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Filter Material ◽  
Hydrocarbon Biodegradation ◽  
Stable Isotope Fractionation ◽  
Filter Materials

AbstractMaintaining and supporting complete biodegradation during remediation of petroleum hydrocarbon contaminated groundwater in constructed wetlands is vital for the final destruction and removal of contaminants. We aimed to compare and gain insight into biodegradation and explore possible limitations in different filter materials (sand, sand amended with biochar, expanded clay). These filters were collected from constructed wetlands after two years of operation and batch experiments were conducted using two stable isotope techniques; (i) carbon isotope labelling of hexadecane and (ii) hydrogen isotope fractionation of decane. Both hydrocarbon compounds hexadecane and decane were biodegraded. The mineralization rate of hexadecane was higher in the sandy filter material (3.6 µg CO2 g−1 day−1) than in the expanded clay (1.0 µg CO2 g−1 day−1). The microbial community of the constructed wetland microcosms was dominated by Gram negative bacteria and fungi and was specific for the different filter materials while hexadecane was primarily anabolized by bacteria. Adsorption / desorption of petroleum hydrocarbons in expanded clay was observed, which might not hinder but delay biodegradation. Very few cases of hydrogen isotope fractionation were recorded in expanded clay and sand & biochar filters during decane biodegradation. In sand filters, decane was biodegraded more slowly and hydrogen isotope fractionation was visible. Still, the range of observed apparent kinetic hydrogen isotope effects (AKIEH = 1.072–1.500) and apparent decane biodegradation rates (k = − 0.017 to − 0.067 day−1) of the sand filter were low. To conclude, low biodegradation rates, small hydrogen isotope fractionation, zero order mineralization kinetics and lack of microbial biomass growth indicated that mass transfer controlled biodegradation.

scholarly journals Mass spectrometric measurement of hydrogen isotope fractionation for the reactions of chloromethane with OH and Cl

2018 ◽  
Author(s):  
Frank Keppler ◽  
Enno Bahlmann ◽  
Markus Greule ◽  
Heinz Friedrich Schöler ◽  
Julian Wittmer ◽  
...  
Keyword(s):  
Hydrogen Isotope ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Isotope Analysis ◽  
Isotope Ratio Mass Spectrometry ◽  
Detailed Knowledge ◽  
Mass Spectrometric Measurement ◽  
Isotopic Analyses ◽  
Chlorine Radicals ◽  
Stable Hydrogen Isotope

Abstract. Chloromethane (CH3Cl) is an important provider of chlorine to the stratosphere but yet lacks detailed knowledge of its budget. Stable isotope analysis is potentially a powerful tool to constrain CH3Cl flux estimates. The largest degree of isotope fractionation is expected to occur for deuterium in CH3Cl in the hydrogen abstraction reactions with its main sink reactant tropospheric OH and its minor sink reactant Cl atoms. We determined the isotope fractionation by stable hydrogen isotope analysis of the fraction of CH3Cl remaining after reaction with hydroxyl and chlorine radicals in a 3.5 m3 Teflon smog-chamber at 293 ± 1 K. We measured the increasing stable hydrogen isotope values of the unreacted CH3Cl using compound specific thermal conversion isotope ratio mass spectrometry. The isotope fractionations of CH3Cl for the reactions with hydroxyl and chlorine radicals were found to be −242 ± 7 mUr (or ‰) and −280 ± 11 mUr, respectively. For comparison, we performed similar experiments using methane (CH4) as the target compound with OH and obtained a fractionation constant of −205 ± 6 mUr which is in good agreement with values previously reported. The observed large kinetic isotope effects are helpful when employing isotopic analyses of CH3Cl in the atmosphere to improve our knowledge of its atmospheric budget.

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