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

Towards large-scale steady-state enhanced nuclear magnetization with in situ detection

Signal amplification by reversible exchange (SABRE) boosts NMR signals of various nuclei enabling new applications spanning from magnetic resonance imaging to analytical chemistry and fundamental physics. SABRE is especially well positioned for continuous generation of enhanced magnetization on a large scale; however, several challenges need to be addressed for accomplishing this goal. Specifically, SABRE requires (i) a specialized catalyst capable of reversible H₂ activation and (ii) physical transfer of the sample from the point of magnetization generation to the point of detection (e.g., a high-field or a benchtop nuclear magnetic resonance [NMR] spectrometer). Moreover, (iii) continuous parahydrogen bubbling accelerates solvent (e.g., methanol) evaporation, thereby limiting the experimental window to tens of minutes per sample. In this work, we demonstrate a strategy to rapidly generate the best-to-date precatalyst (a compound that is chemically modified in the course of the reaction to yield the catalyst) for SABRE, [Ir(IMes)(COD)Cl] (IMes = 1,3-bis-[2,4,6-trimethylphenyl]-imidazol-2-ylidene; COD = cyclooctadiene) via a highly accessible synthesis. Second, we measure hyperpolarized samples using a home-built zero-field NMR spectrometer and study the field dependence of hyperpolarization directly in the detection apparatus, eliminating the need to physically move the sample during the experiment. Finally, we prolong the measurement time and reduce evaporation by presaturating parahydrogen with the solvent vapor before bubbling into the sample. These advancements extend opportunities for exploring SABRE hyperpolarization by researchers from various fields and pave the way to producing large quantities of hyperpolarized material for long-lasting detection of SABRE-derived nuclear magnetization.

Structural characterisation of natural products by means of quantum chemical calculations of NMR parameters: new insights

In this review, we focus in all aspects of NMR simulation of natural products, from the fundamentals to the new computational toolboxes available, combining advanced quantum chemical calculations with upstream data processing and machine learning.

The analysis of NMR J-couplings of saturated and unsaturated compounds by the localized second order polarization propagator approach method

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).

Difference between ²JC2H3 and ²JC3H2 spin-spin couplings in heterocyclic five- and six-membered rings as a probe for studying σ-ring currents: a quantum chemical analysis

Adequate analyses of canonical molecular orbitals (CMOs) can provide rather detailed information on the importance of different σ-Fermi contact (FC) coupling pathways (FC term transmitted through the σ-skeleton). Knowledge of the spatial distribution of CMOs is obtained by expanding them in terms of natural bond orbitals (NBOs). Their relative importance for transmitting the σ-FC contribution to a given spin-spin coupling constants (SSCCs) is estimated by resorting to the expression of the FC term given by the polarisation propagator formalism. In this way, it is possible to classify the effects affecting such couplings in two different ways: delocalisation interactions taking place in the neighbourhood of the coupling nuclei and 'round the ring' effects. The latter, associated with σ-ring currents, are observed to yield significant differences between the FC terms of (2)J(C2H3) and (2)J(C3H2) SSCCs which, consequently, are taken as probes to gauge the differences in σ-ring currents for the five-membered rings (furan, thiophene, selenophene and pyrrol) and also for the six-membered rings (benzene, pyridine, protonated pyridine and N-oxide pyridine) used in the present study.

Conformational analysis and stereochemical dependences of (31)P-(1)H spin-spin coupling constants of bis(2-phenethyl)vinylphosphine and related phosphine chalcogenides

Theoretical energy-based conformational analysis of bis(2-phenethyl)vinylphosphine and related phosphine oxide, sulfide and selenide synthesized from available secondary phosphine chalcogenides and vinyl sulfoxides is performed at the MP2/6-311G** level to study stereochemical behavior of their (31)P-(1)H spin-spin coupling constants measured experimentally and calculated at different levels of theory. All four title compounds are shown to exist in the equilibrium mixture of two conformers: major planar s-cis and minor orthogonal ones, while (31)P-(1) H spin-spin coupling constants under study are found to demonstrate marked stereochemical dependences with respect to the geometry of the coupling pathways, and to the internal rotation of the vinyl group around the P(X)-C bonds (X = LP, O, S and Se), opening a new guide in the conformational studies of unsaturated phosphines and phosphine chalcogenides.

Divinyl selenide: conformational study and stereochemical behavior of its 77Se-1H spin-spin coupling constants

Theoretical energy-based conformational analysis of divinyl selenide performed at the MP2/6-311G** level is substantiated by the second-order polarization propagator approach (SOPPA) calculations and experimental measurements of its (77)Se-(1)H spin-spin coupling constants, demonstrating marked stereochemical behavior in respect of the internal rotation of both vinyl groups around the Se-C bonds. Based on these data, divinyl selenide is shown to exist in an equilibrium mixture of three nonplanar conformers: one the preferred syn-s-cis-s-trans and two minor anti-s-trans-s-trans and syn-s-trans-s-trans forms.

Experimental and theoretical investigation of NMR 2JHH coupling constant on six-membered ring systems containing oxygen or sulfur atoms

Theoretical and experimental 2JHH coupling constants for six-membered rings containing oxygen or sulfur atoms were studied to investigate whether the 2JHH coupling constant can be used for stereoelectronic studies in heterocyclohexanes, instead of 1JCH, because it is well known that experimental measurements of 2JHH coupling constants at low temperature are much easier to determine than the corresponding 1JCH couplings. For all compounds studied here, the 2JHH coupling constants are affected by sigma*C-H antibonding occupancy together with bond angle effects. For cyclohexane and oxygen-containing compounds, the influence on the geminal coupling for Hax-C2-Heq and for X1-C2-X3 (X=O and C), bond angles are more pronounced than for the sulfur derivatives.

Geminal 2JCCH Spin−Spin Coupling Constants as Probes of the φ Glycosidic Torsion Angle in Oligosaccharides

Two-bond (13)C-(1)H NMR spin-spin coupling constants ((2)J(CCH)) between C2 and H1 of aldopyranosyl rings depend not only on the relative orientation of electronegative substituents on the C1-C2 fragment but also on the C-O torsions involving the same carbons. The latter dependencies were elucidated theoretically using density functional theory and appropriate model pyranosyl rings representing the four relative configurations at C1 and C2, and a 2-deoxy derivative, to probe the relationship between (2)J(C2,H1) magnitude and sign and the C1-O1 (phi, phi) and C2-O2 (alpha) torsion angles. Related calculations were also conducted for the reverse coupling pathway, (2)J(C1,H2). Computed J-couplings were validated by comparison to experimentally measured couplings. (2)J(CCH) displays a primary dependence on the C-O torsion involving the carbon bearing the coupled proton and a secondary dependence on the C-O torsion involving the coupled carbon. These dependencies appear to be caused mainly by the effects of oxygen lone pairs on the C-H and C-C bond lengths along the C-C-H coupling pathway. New parameterized equations are proposed to interpret (2)J(C1,H2) and (2)J(C2,H1) in aldopyranosyl rings. The equation for (2)J(C2,H1) has particular value as a potential NMR structure constraint for the C1-O1 torsion angle (phi) comprising the glycosidic linkages of oligosaccharides.

Non-empirical calculations of NMR indirect carbon-carbon coupling constants. Part 12--aliphatic and alicyclic oximes

One-bond carbon-carbon coupling constants were calculated in a series of nine aliphatic and alicyclic oximes at the SOPPA (second-order polarization propagator approach) level in good agreement with the available experimental data, and several unknown couplings were predicted with high reliability. The experimental difference between J(C,C) of the corresponding carbon-carbon bonds in cis and trans orientations to the nitrogen lone pair is very well reproduced at the SOPPA level, and this provides an additional tool in the configurational assignment at the C=N bond in oximes and related systems.