Carbon nuclear magnetic resonance studies of conformational equilibria
<p>One bond coupling constants were measured as a function of temperature for bonds between carbon and carbon (<sup>l</sup>J<sub>CC</sub>) and, less commonly, hydrogen (<sup>l</sup>J<sub>CH</sub>) or fluorine (<sup>l&...
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| Format: | Thesis |
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1981
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| _version_ | 1797104056515166208 |
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| author | Barna, J |
| author_facet | Barna, J |
| author_sort | Barna, J |
| collection | OXFORD |
| description | <p>One bond coupling constants were measured as a function of temperature for bonds between carbon and carbon (<sup>l</sup>J<sub>CC</sub>) and, less commonly, hydrogen (<sup>l</sup>J<sub>CH</sub>) or fluorine (<sup>l</sup>J<sub>CF</sub>). Precisions of <sup>&pm;</sup>0.01 Hz were achieved using double isotopic substitution (for <sup>l</sup>J<sub>CC</sub>) and least squares curve fitting. Temperatures were measured to <sup>&pm;</sup>0.5 K using carbon chemical shift thermometers.</p> <p>Molecules having no conformational equilibria showed substituent, temperature and solvent effects on the couplings, throwing doubt on some conformational uses of higher order couplings.</p> <p>Comparison of <sup>l</sup>J<sub>CC</sub> values and their temperature dependences (ΔJ/ΔT) for some amines and appropriate models allowed determination of conformational free energies (ΔG°) . ΔG° was estimated for some ethylamines (<em>trans</em> and <em>gauche</em> conformers) and for piperidine (axial and equatorial N-H) . The method depends on the differences in <sup>l</sup>J<sub>CC</sub> for a bond in an ethyl group (ethylamines) or ring (piperidines) according to whether it is <em>cis</em>, <em>gauche</em>, or <em>trans</em> to a nitrogen lone pair. Conformational information was generally obtained from comparison of absolute <sup>l</sup>J<sub>CC</sub> , values, not from ΔJ/ΔT.</p> <p>A hindered internal rotation of the ethyl in <em>r</em>-2,<em>c</em>-4,<em>c</em>-6-trimethyl-<em>N</em>-ethylpiperidine was discovered; <sup>l</sup>J<sub>CC</sub> was observed for separate conformers at low temperature and ΔG° was estimated at room temperature. The result was compared with ΔG°s obtained using other carbon n.m.r. methods, i.e., measurement of line broadening, integrated resonances at low temperature and averaged chemical shifts. Axial /equatorial equilibr were studied by known n.m.r. methods for <em>N</em>-ethyl- and analogous <em>N</em>-methyl-piperidines .</p> <p><sup>l</sup>J<sub>CC</sub> is dependent on conformation in [(<sup>l3</sup>C<sub>2</sub>)ethyl] benzene and its mesityl analogue; mathematical treatment should provide an estimate of the barrier to rotation in ethylbenzene. <sup>l</sup>J<sub>CC</sub> is dependent on conformation in ketones (e.g. diethylketone) and on the configuration (<em>syn/anti</em>) of their derivatives.</p> <p>Hyperconjugation with unsaturated centres explains the conformational dependences of <sup>l</sup>J<sub>CC</sub> , analogous to those known for <sup>l</sup>J<sub>CH</sub>.</p> |
| first_indexed | 2024-03-07T06:28:29Z |
| format | Thesis |
| id | oxford-uuid:f524b599-7eb0-4d12-a171-87903989cc2f |
| institution | University of Oxford |
| last_indexed | 2024-03-07T06:28:29Z |
| publishDate | 1981 |
| record_format | dspace |
| spelling | oxford-uuid:f524b599-7eb0-4d12-a171-87903989cc2f2022-03-27T12:25:07ZCarbon nuclear magnetic resonance studies of conformational equilibriaThesishttp://purl.org/coar/resource_type/c_db06uuid:f524b599-7eb0-4d12-a171-87903989cc2fPolonsky Theses Digitisation Project1981Barna, J<p>One bond coupling constants were measured as a function of temperature for bonds between carbon and carbon (<sup>l</sup>J<sub>CC</sub>) and, less commonly, hydrogen (<sup>l</sup>J<sub>CH</sub>) or fluorine (<sup>l</sup>J<sub>CF</sub>). Precisions of <sup>&pm;</sup>0.01 Hz were achieved using double isotopic substitution (for <sup>l</sup>J<sub>CC</sub>) and least squares curve fitting. Temperatures were measured to <sup>&pm;</sup>0.5 K using carbon chemical shift thermometers.</p> <p>Molecules having no conformational equilibria showed substituent, temperature and solvent effects on the couplings, throwing doubt on some conformational uses of higher order couplings.</p> <p>Comparison of <sup>l</sup>J<sub>CC</sub> values and their temperature dependences (ΔJ/ΔT) for some amines and appropriate models allowed determination of conformational free energies (ΔG°) . ΔG° was estimated for some ethylamines (<em>trans</em> and <em>gauche</em> conformers) and for piperidine (axial and equatorial N-H) . The method depends on the differences in <sup>l</sup>J<sub>CC</sub> for a bond in an ethyl group (ethylamines) or ring (piperidines) according to whether it is <em>cis</em>, <em>gauche</em>, or <em>trans</em> to a nitrogen lone pair. Conformational information was generally obtained from comparison of absolute <sup>l</sup>J<sub>CC</sub> , values, not from ΔJ/ΔT.</p> <p>A hindered internal rotation of the ethyl in <em>r</em>-2,<em>c</em>-4,<em>c</em>-6-trimethyl-<em>N</em>-ethylpiperidine was discovered; <sup>l</sup>J<sub>CC</sub> was observed for separate conformers at low temperature and ΔG° was estimated at room temperature. The result was compared with ΔG°s obtained using other carbon n.m.r. methods, i.e., measurement of line broadening, integrated resonances at low temperature and averaged chemical shifts. Axial /equatorial equilibr were studied by known n.m.r. methods for <em>N</em>-ethyl- and analogous <em>N</em>-methyl-piperidines .</p> <p><sup>l</sup>J<sub>CC</sub> is dependent on conformation in [(<sup>l3</sup>C<sub>2</sub>)ethyl] benzene and its mesityl analogue; mathematical treatment should provide an estimate of the barrier to rotation in ethylbenzene. <sup>l</sup>J<sub>CC</sub> is dependent on conformation in ketones (e.g. diethylketone) and on the configuration (<em>syn/anti</em>) of their derivatives.</p> <p>Hyperconjugation with unsaturated centres explains the conformational dependences of <sup>l</sup>J<sub>CC</sub> , analogous to those known for <sup>l</sup>J<sub>CH</sub>.</p> |
| spellingShingle | Barna, J Carbon nuclear magnetic resonance studies of conformational equilibria |
| title | Carbon nuclear magnetic resonance studies of conformational equilibria |
| title_full | Carbon nuclear magnetic resonance studies of conformational equilibria |
| title_fullStr | Carbon nuclear magnetic resonance studies of conformational equilibria |
| title_full_unstemmed | Carbon nuclear magnetic resonance studies of conformational equilibria |
| title_short | Carbon nuclear magnetic resonance studies of conformational equilibria |
| title_sort | carbon nuclear magnetic resonance studies of conformational equilibria |
| work_keys_str_mv | AT barnaj carbonnuclearmagneticresonancestudiesofconformationalequilibria |