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 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.
Highlights
In recent years the increasing importance of NMR spectroscopy for the determination of three dimensional structures of even biomolecules [1] leads to an ever increasing demand for very accurate calculations of the spectroscopic parameters defining an NMR spectrum: for instance the chemical shift and the indirect nuclear spin-spin coupling constants of bio-related molecules
In our previous study [16] of the coupling constants in CH2CH2, CH2NH, CH2O and CH2S we found that our second order polarization propagator approximation (SOPPA)(CCSD) coupling constants for methanimine (1) are in very good agreement with the experimental values [65,66], the absolute values and with respect to the differences in couplings syn- or antiperiplanar to the double bond or to the lone pair on nitrogen
The orbital paramagnetic term is important for the geminal coupling constants involving nitrogen and in particular for the one-bond carbon-nitrogen coupling across the double bond, where it is equal in magnitude to the Fermi Contact (FC) term but of opposite sign
Summary
In recent years the increasing importance of NMR spectroscopy for the determination of three dimensional structures of even biomolecules [1] leads to an ever increasing demand for very accurate calculations of the spectroscopic parameters defining an NMR spectrum: for instance the chemical shift and the indirect nuclear spin-spin coupling constants of bio-related molecules Often these molecules are too large for accurate calculations and many authors [2,3,4,5,6,7,8,9,10,11] make use of model-systems which are supposed to include the relevant atoms, e.g. the atoms involved in peptide bonds. These two options can be combined in order to increase the maximal size of molecules in coupling constant calculations without a significant loss of accuracy
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