Hyperconjugation: The Electronic Mechanism That May Underlie the Karplus Curve of Vicinal NMR Indirect Spin Couplings (original) (raw)
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The Journal of Physical Chemistry A, 2012
NMR J-coupling calculations at the secondorder of polarization propagator approach, SOPPA, are among the most reliable. They include a high percentage of the total electron correlation effects in saturated and unsaturated molecular systems. Furthermore, J-couplings are quite sensitive to the whole electronic molecular framework. We present in this article the first study of all three response mechanisms, Fermi contact, FC, spin-dipolar, SD and paramagnetic spin−orbital, PSO, for J-couplings with occupied localized molecular orbitals at the SOPPA level of approach. Even though SOPPA results are not invariant under unitary transformations, the difference between results obtained with canonical and localized molecular orbitals, LMOs, are small enough to permit its application with confidence. The following small-size saturated and unsaturated compounds were analyzed: CH 4 , CH 3 F, C 2 H 6 , NH 3 , C 2 H 4 , CH 2 NH, H 2 CCHF, and FHCCHF. The local character of the FC mechanism that appears in J-couplings of these molecular models is shown through the analysis of contributions from LMOs. The importance of including the electron correlation on the engaged bonding orbitals for one-bond couplings is emphasized. Almost all electron correlation effects are included in such orbitals. Interesting findings were the large contributions by s-type LMOs to the C−H and C−C J-couplings; they are responsible for the variation of 1 J(C−C) when going from ethane to ethene and to 1,2-difluoroethene. The previously proposed hyperconjugative transfer mechanism has been tested. Among other tests we found the difference anti-syn of one-bond 1 J(C−H) in imine as due to both the corresponding σ(C−H) and the lone-pair, LP, contribution. Geminal and vicinal J-couplings were also analyzed. Our findings are in accord with a previous work by Pople and Bothner-by, who considered results taken from calculations or empirical data. For all geminal couplings the pattern of J-couplings, like the change of sign, is originated in the main bondings that participate in the coupling pathways. The finding of asymmetric contributions of LP to vicinal H−H couplings in imine is highlighted. The analysis of J-couplings by contributions from LMOs to the noncontact mechanisms, SD and PSO, show that the π electronic framework makes both terms grow in the specific case of the model compounds studied here. The PSO mechanism is more efficient when a σ bond is vicinal to a π bond. We found in this way an efficient and powerful scheme to get a deeper insight on the electronic molecular framework on which J-couplings are transmitted.
The Journal of Chemical Physics, 2012
Calculations of NMR J-coupling with polarization propagators are not invariant under unitary transformations at second order level of approach, second order polarization propagator approach (SOPPA). They are only invariant at first order or random phase level of approach (RPA). We performed "localized" SOPPA (Loc-SOPPA), calculations of J-couplings applying two different schemes for the localization of molecular orbitals(LMO): Foster-Boys and Pipek-Mezey. We show here that results of such Loc-SOPPA calculations are different though not much: they are less than 6% different in the worst case. Therefore it is possible to apply them with confidence in the analysis of the transmission of different coupling mechanisms within the molecule. We are able now to get reliable information on what LMOs are the most important (and so which are not important) for a given J-coupling in a molecule. This information can then be used for selecting which are the paths that should be described with the highest possible accuracy for that J-coupling calculation. A few unsaturated compounds are analyzed: ethene, trans-difluoroethene or DiF-ethene, and imine. It is shown that different lone pairs (of p z or p x/y type) are responsible for the vicinal F-F J-coupling in DiF-ethene; and also the fact that the main LP contributor is not the same for the fermi contact and the spin-dipolar mechanisms. We also studied phosphorous containing compounds such as phosphine and cis-propylene phosphine. In both cases the analysis of the main LMO contributing to one-bond P-H coupling and through-space P-C coupling were performed. The above mentioned unsaturated molecular systems have quasiinstability problems that arise at RPA level of approach. We show here that they are mostly originated in the antibonding π * LMO, corresponding to the C=C or C=N double bonds. We performed the analysis of the origin of quasiinstabilities for the SD mechanism. The contribution of each kind of excitation terms to SOPPA calculations were considered, meaning the main contributions by single and double excitations. It is shown that one can get more than 97% of the total electron correlation contribution when including terms that mainly contain single excitations (though double-excitation matrix elements should still be calculated).
Magnetic Resonance in Chemistry, 2001
The known lone pair orientation effect of an a-nitrogen atom on 1 J(C,C) couplings was studied using the natural J coupling (NJC) dissection method for the Fermi contact (FC) term. This dissection was performed at the DFT-B3LYP level using three different basis sets. It was verified that non-contact contributions, which are much smaller than the FC term, do not depend on the orientation of the N lone pair. Acetone oxime (1), protonated acetone oxime (2) and ethylamine (3), were taken as model compounds to perform this study. In 1 the difference between 1 J.Z/ and 1 J.E/ is mainly determined by three NJC terms, i.e the nitrogen lone pair, the carbon-carbon bond containing the coupled carbon atoms and the carbon inner core orbitals contributions. In 3 the angular dependence of 1 J(C,C) vs the amine group conformation is dominated by the same three contributions that define the 1 J.Z/ and 1 J.E/ difference in 1.
Journal of Molecular Structure-theochem, 1995
Vicinal J(SnCCSn) and J(SnCCC) spin-spin coupling constants are calculated within the RPA MNDO method for different dihedral angles, θ, determined by the intervening bonds, in model compounds. For both types of couplings, calculated values closely follow Karplus-like dependences. Results are discussed in terms of experimental values, and the effects of substituents attached to the coupled atoms are briefly analyzed for the θ = 0 ° and θ = 180 ° conformations. These last values were also calculated using the RPA AM1 method. Results indicate that the RPA MNDO and RPA AM1 approaches show interesting potential for studying the structural dependences of 3J(SnSn) and 3J(SnC) couplings in tin-containing compounds.
Magnetic Resonance in Chemistry, 2004
New spin-state-selective (S3) NMR pulse sequences exclusively applying cross-polarization schemes to achieve optimum homonuclear and heteronuclear 1HX coherence transfer are reported for the simple and accurate measurement of the magnitude and sign of heteronuclear coupling constants for samples at natural abundance. The proposed spin-edited HCP-TOCSY experiments are based on clean heteronuclear S3 excitation, generated by simultaneous co-addition of two independent in-phase and anti-phase components created during the mixing heteronuclear J-cross-polarization (HCP) step, which is finally transferred to other protons by a conventional homonuclear TOCSY mechanism. Selective 1D and non-selective 2D approaches for the easy determination of long-range proton–carbon and proton–nitrogen coupling constants on any protonated and non-protonated heteronuclei are presented and discussed for several organic molecules. Copyright © 2004 John Wiley & Sons, Ltd.
Magnetic Resonance in Chemistry, 2008
In this work 3 J CH spin-spin coupling constants (SSCCs) for the cis-and trans-conformers for α-X-acetamides (X = F, Cl, Br and CN) (1-4) were studied in detail since they were found to be notably different for both conformers. These differences are rationalized as originating in the changes of the strong negative hyperconjugative interactions that take place within the carbonyl group. Such changes are found to depend not only on conformation, but also on solvent. For the cis-conformers there is a close proximity between the X-substituent and the in-plane oxygen lone pair of pure p character, which affects notably their respective negative hyperconjugative interactions. Both the efficiency for transmitting the Fermi contact (FC) term through the coupling pathway of 3 J CH SSCCs and its potential as a probe to study the stereochemical properties of the XH 2 C group are discussed.
Stereochemical dependence of NMR geminal spin-spin coupling constants
Magnetic Resonance in Chemistry, 2009
In this work it was sought to explore the versatility of geminal spin-spin coupling constants, 2 J XY SSCCs, as probes for stereochemical studies. A set of compounds, where their experimental 2 J XY SSCCs through the X-C-Y molecular fragment are predicted to be sensitive to hyperconjugative interactions involving either bonding or antibonding orbitals containing the C carbon atom ('coupling pathway'), were analyzed. SSCC calculations were performed for some selected examples using the second order polarization propagator approximation (SOPPA) method or within the DFT-B3LYP framework. Hyperconjugative interactions were calculated within the Natural Bond Orbital (NBO) approach. Results are condensed in two qualitative rules: Rule I M -hyperconjugative interactions transferring charge into the coupling pathway yield a positive increase to the Fermi contact (FC), contribution to 2 K XY reduced spin-spin coupling constants (RSSCC), and Rule II M -hyperconjugative interactions transferring charge from the coupling pathway yield a negative increase to the FC contribution to 2 K XY RSSCC.
Viewpoint 8 — polarization propagator analysis of spin-spin coupling constants
Journal of Molecular Structure: THEOCHEM, 1993
The analysis of electronic mechanisms that define indirect NMR spin-spin coupling constants can be efficiently carried out by combining the polarization propagator formalism with the use of localized molecular orbitals. This approach is applied to study several trends of one-bond couplings along the Periodic Table when one of the coupled nuclei bears a lone pair. This analysis is preceded by a revision of the fundamentals of the method in which its approximations, shortcomings and capabilities are commented on. A brief overview of results already published in the literature is also given. The relationship between the classical approach of Pople and Santry and the polarization propagator calculation of the Fermi contact term is discussed.
Molecular Physics, 2001
In this work an implementation of the FPT-DFT approach for calculating the spin-dipolar contribution to NMR spin± spin coupling constants is presented. This method was tested in a set of small molecules, giving results in excellent agreement when comparing them with values taken from the literature, which were obtained with state-of-the-art calculations. To obtain an insight into the relative importance of the spin-dipolar contribution in unsaturated compounds, calculations of J(F, C), J(F, F) and J(F, H) couplings in 1,2-, 1,3-, and 1,4-di¯uorobenzenes were performed. An important spin-dipolar contribution to 3 J(F, F) and 5 J(F, F) was found, suggesting that this term might be important in some cases. When performing DFT calculations the non-singlet instabilities usually found in unsaturated compounds are overcome.