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139 KiB

Open AccessArticle

The Effect of Substituents on Indirect Nuclear Spin-Spin Coupling Constants: Methan- and Ethanimine, Methanal- and Ethanaloxime

by Patricio F. Provasi, Gustavo A. Aucar and Stephan P. A. Sauer

Int. J. Mol. Sci. 2003, 4(4), 231-248; https://0-doi-org.brum.beds.ac.uk/10.3390/i4040231 - 04 Apr 2003

Cited by 27 | Viewed by 11142

Abstract

Ab initio calculations of the spin-spin coupling constants have been carried out for methan- and ethanimine, methanal- and ethanaloxime at the level of the second order polarization propagator approximation with coupled cluster singles and doubles amplitudes (SOPPA(CCSD)) using the aug-cc-pVTZ-J basis sets. Previously [...] Read more.

Ab initio calculations of the spin-spin coupling constants have been carried out for methan- and ethanimine, methanal- and ethanaloxime at the level of the second order polarization propagator approximation with coupled cluster singles and doubles amplitudes (SOPPA(CCSD)) using the aug-cc-pVTZ-J basis sets. Previously we have shown that this method can reproduce quantitatively the coupling constants for methanimine; our new results for methanal- and ethanaloxime agree also very well with the measured couplings. A study of both purely geometrical and substituent effects on all coupling constants in the title compounds is presented. Analyzing the four contributions to the coupling constants we find that the stereoelectronic effect of the nitrogen lone pair on the one-bond C-H and C-C couplings as well as the corresponding effect for the geminal N-H and N-C couplings is affected strongly by the -OH substituent. For the one-bond C-N couplings we observe that the orbital paramagnetic (OP) contribution is comparable to the Fermi Contact (FC) contribution but opposite in sign and that the spin-dipolar (SD) term amounts to between 40% and 85% of the total coupling constants. Changes in the total one-bond C-N couplings caused by the -OH substituent are also almost entirely due to SD contribution. Full article

(This article belongs to the Special Issue Nuclear Magnetic Resonance Spin-Spin Coupling Constants - Calculations and Measurements - Part II)

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97 KiB

Open AccessArticle

Electric Field Effects on ²J_HH Spin-Spin Coupling Constants

by Martin Grayson

Int. J. Mol. Sci. 2003, 4(4), 218-230; https://0-doi-org.brum.beds.ac.uk/10.3390/i4040218 - 04 Apr 2003

Cited by 9 | Viewed by 8212

Abstract

Some calculations of the electric field derivative of the spin-spin coupling constant on molecules in the series CH₃F, CH₃Cl, CH₃Br, CH₃I, CH₄, CH₃Li, CH₃Na and CH₃K have [...] Read more.

Some calculations of the electric field derivative of the spin-spin coupling constant on molecules in the series CH₃F, CH₃Cl, CH₃Br, CH₃I, CH₄, CH₃Li, CH₃Na and CH₃K have been presented. The data is broken down into the 4 terms of the Ramsey theory: Fermi contact (FC); diamagnetic spin-orbit (DSO); paramagnetic spinorbit (PSO) and spin-dipolar (SD). The FC term is seen to dominate all the calculated Js and their derivatives presented here. The situation where the FC term does not dominate in other molecules is discussed as a contrast. Full article

(This article belongs to the Special Issue Nuclear Magnetic Resonance Spin-Spin Coupling Constants - Calculations and Measurements - Part II)

229 KiB

Open AccessArticle

Electronic Mechanisms of Intra and Intermolecular J Couplings in Systems with C-H···O Interactions

by Claudia G. Giribet, Martin C. Ruiz de Azua, Celia V. Vizioli and Claudio N. Cavasotto

Int. J. Mol. Sci. 2003, 4(4), 203-217; https://0-doi-org.brum.beds.ac.uk/10.3390/i4040203 - 04 Apr 2003

Cited by 14 | Viewed by 11104

Abstract

Correlation effects on the change of ^₁J(CH) couplings in model systems I:NCH...H₂O and II:CH₄...H₂O as a function of the H...O distance are discussed. RPA and SOPPA results follow a similar trend in system II. In system [...] Read more.

Correlation effects on the change of ^₁J(CH) couplings in model systems I:NCH...H₂O and II:CH₄...H₂O as a function of the H...O distance are discussed. RPA and SOPPA results follow a similar trend in system II. In system I RPA values decrease monotonously as the H...O distance decreases, while SOPPA ones exhibit flat maximum near equilibrium. Such different behavior is ascribed to the π-transmitted component. Intermolecular couplings at the equilibrium geometry of I are analyzed by means of the CLOPPA approach. The larger absolute value of ^2hJ(CO) compared to ^1hJ(HO) is found to arise from contributions involving a vacant LMO localized in the C-H...O moiety. Full article

(This article belongs to the Special Issue Nuclear Magnetic Resonance Spin-Spin Coupling Constants - Calculations and Measurements - Part II)

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