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Nikitin K, O'Gara R. Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy. Chemistry 2019; 25:4551-4589. [PMID: 30421834 DOI: 10.1002/chem.201804123] [Citation(s) in RCA: 34] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2018] [Indexed: 12/31/2022]
Abstract
Detailed mechanistic information is crucial to our understanding of reaction pathways and selectivity. Dynamic exchange NMR techniques, in particular 2D exchange spectroscopy (EXSY) and its modifications, provide indispensable intricate information on the mechanisms of organic and inorganic reactions and other phenomena, for example, the dynamics of interfacial processes. In this Review, key results from exchange NMR studies of small molecules over the last few decades are systemised and discussed. After a brief introduction to the theory, the key types of dynamic processes are identified and fundamental examples given of intra- and intermolecular reactions, which, in turn, could involve, or not, bond-making and bond-breaking events. Following that logic, internal molecular rotation, intramolecular stereomutation and molecular recognition will first be considered because they do not typically involve bond breaking. Then, rearrangements, substitution-type reactions, cyclisations, additions and other processes affecting chemical bonds will be discussed. Finally, interfacial molecular dynamics and unexpected combinations of different types of fluxional processes will also be highlighted. How exchange NMR spectroscopy helps to identify conformational changes, coordination and molecular recognition processes as well as quantify reaction energy barriers and extract detailed mechanistic information by using reaction rate theory in conjunction with computational techniques will be shown.
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Affiliation(s)
- Kirill Nikitin
- School of Chemistry, University College Dublin, Belfield, Dublin, Ireland
| | - Ryan O'Gara
- School of Chemistry, University College Dublin, Belfield, Dublin, Ireland
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Ok S, Hoyt DW, Andersen A, Sheets J, Welch SA, Cole DR, Mueller KT, Washton NM. Surface Interactions and Confinement of Methane: A High Pressure Magic Angle Spinning NMR and Computational Chemistry Study. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2017; 33:1359-1367. [PMID: 28099024 DOI: 10.1021/acs.langmuir.6b03590] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Characterization and modeling of the molecular-level behavior of simple hydrocarbon gases, such as methane, in the presence of both nonporous and nanoporous mineral matrices allows for predictive understanding of important processes in engineered and natural systems. In this study, changes in local electromagnetic environments of the carbon atoms in methane under conditions of high pressure (up to 130 bar) and moderate temperature (up to 346 K) were observed with 13C magic-angle spinning (MAS) NMR spectroscopy while the methane gas was mixed with two model solid substrates: a fumed nonporous, 12 nm particle size silica and a mesoporous silica with 200 nm particle size and 4 nm average pore diameter. Examination of the interactions between methane and the silica systems over temperatures and pressures that include the supercritical regime was allowed by a novel high pressure MAS sample containment system, which provided high resolution spectra collected under in situ conditions. For pure methane, no significant thermal effects were found for the observed 13C chemical shifts at all pressures studied here (28.2, 32.6, 56.4, 65.1, 112.7, and 130.3 bar). However, the 13C chemical shifts of resonances arising from confined methane changed slightly with changes in temperature in mixtures with mesoporous silica. The chemical shift values of 13C nuclides in methane change measurably as a function of pressure both in the pure state and in mixtures with both silica matrices, with a more pronounced shift when meso-porous silica is present. Molecular-level simulations utilizing GCMC, MD, and DFT confirm qualitatively that the experimentally measured changes are attributed to interactions of methane with the hydroxylated silica surfaces as well as densification of methane within nanopores and on pore surfaces.
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Affiliation(s)
- Salim Ok
- School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States
- Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States
| | - David W Hoyt
- School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States
- Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States
| | - Amity Andersen
- School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States
- Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States
| | - Julie Sheets
- School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States
- Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States
| | - Susan A Welch
- School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States
- Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States
| | - David R Cole
- School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States
- Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States
| | - Karl T Mueller
- School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States
- Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States
| | - Nancy M Washton
- School of Earth Sciences and ‡Department of Chemistry, The Ohio State University , Columbus, Ohio 43210, United States
- Environmental Molecular Sciences Laboratory and ∥Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99354, United States
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