NaturalJcoupling (NJC) analysis of the electron lone pair effect on NMR couplings: 2. The anomeric effects on¹J(C, H) couplings and its dependence on solvent

Abstract

The electronic origin of the influence of the anomeric effect (negative hyperconjugative interaction, NHI) on the Fermi contact (FC) term of 'J(C,H) couplings has been studied from a theoretical point of view a t the DFT-B3LYP level. The HN=CH, molecule was chosen as the primary model compound, in which both F C 'J(C,H) couplings were decomposed into bond contributions with the natural J coupling dissection approach (NJC). Differences between the I J ( c , H )~~ couplings for C-H bonds in synperiplanar and antiperiplanar orientations with respect to the nitrogen non-bonding electron pair closely follow the experimental trend. They are made up chiefly of three NJC contributions: 'bond', 'direct lone pair' and the 'carbon-core orbitals'. The NHI influence on these terms was studied by applying the natural bond orbital (NBO) deletion procedure to the charge transfer interaction into the antiperiplanar (C-H) antibond (n(N)+(C-H)*) prior to the NJC dissection calculation. The dielectric solvation effect on both the total F C terms and the respective NJC contributions was estimated by carrying out the calculations using the polarization continuum model. Inhibition of the anomeric effect is evident when the solvent polarity is increased. NHI saturates rapidly with increasing solvent dielectric. Specific solute-solvent interaction effects on I J(C,H) couplings were estimated by evaluating molecular complex models of the form CH2=HN. . .S (S = H 2 0 and DMSO).

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

“…The first observation of the lone pair effect on 1 J(C,C) coupling constants was report for a series of oximes including the title compound ethanaloxime [69]. Previous semi-empirical [85,86] and DFT [79] calculations on acetoxime as well as DFT [78] and SOPPA(CCSD) [16] calculations on methanimine showed that only the FC contribution is changed by the lone pair effect. Furthermore an analysis of the contributions of individual localized DFT molecular orbitals to the FC term in the same molecules [78,79] indicated that this effect is primarily due to a direct lone pair contribution from the corresponding localized orbital, which is positive for the coupling synperiplanar to the lone pair and negative for the coupling antiperiplanar to it.…”

“…Previous semi-empirical [85,86] and DFT [79] calculations on acetoxime as well as DFT [78] and SOPPA(CCSD) [16] calculations on methanimine showed that only the FC contribution is changed by the lone pair effect. Furthermore an analysis of the contributions of individual localized DFT molecular orbitals to the FC term in the same molecules [78,79] indicated that this effect is primarily due to a direct lone pair contribution from the corresponding localized orbital, which is positive for the coupling synperiplanar to the lone pair and negative for the coupling antiperiplanar to it. On the other hand, the indirect lone pair contribution via a charge transfer or hyperconjugation interaction from the nitrogen non-bonding lone pair orbital to the antibonding carbon-carbon orbital, the so-called anomeric effect [22][23][24], appeared to be less important [18,78,79].…”

“…Furthermore an analysis of the contributions of individual localized DFT molecular orbitals to the FC term in the same molecules [78,79] indicated that this effect is primarily due to a direct lone pair contribution from the corresponding localized orbital, which is positive for the coupling synperiplanar to the lone pair and negative for the coupling antiperiplanar to it. On the other hand, the indirect lone pair contribution via a charge transfer or hyperconjugation interaction from the nitrogen non-bonding lone pair orbital to the antibonding carbon-carbon orbital, the so-called anomeric effect [22][23][24], appeared to be less important [18,78,79]. The effect of various substituents on experimental 1 J(C,C) coupling constants in imines and oximes has also been studied [18] and an empirical relation between the electronegativity of the substituents and the 1 J(C,C) coupling constants in imines and oximes has even been derived [87,18].…”

Solvent Effects on Nuclear Magnetic Resonance 2J(C,Hf) and 1J(C,Hf) Spin–Spin Coupling Constants in Acetaldehyde

“…In this work the Tomasi et al [26][27] PCM model is chosen since it was used previously [28] to study how the anomeric effect on 1 J(C,H) coupling depends on solvent dielectric and results thus found look promising. The FC term of both 1 J(C c ,H f ) and 2 J(C 1 ,H f ) couplings in acetaldehyde are analyzed first by considering it in the isolated molecule, and then by treating it in the presence of the solvent reaction field for different values of the dielectric constant.…”

“…Besides, they can be used in conjunction with the solvent representation given by the Polarizable Continuum Model, PCM [26][27], to obtain a powerful tool to evaluate NMR couplings in compounds where solvation effects cannot be neglected [28]. and Diamagnetic Spin Orbit, DSO, are calculated within the DFT framework, the dielectric solvent effect analysis is mainly circumscribed to the FC term because in the two couplings considered in this work, it is by far the largest contribution and, therefore even if non-contact terms, SD, PSO and DSO, are solvent-dependent, their contributions to the total solvent effect should be negligible when compared with that of the FC term.…”