Experimental and Theoretical Study of Hyperconjugative Interaction Effects on NMR 1JCH Scalar Couplings

Computational 1 H and 13 C NMR in structural and stereochemical studies

Present review outlines the advances and perspectives of computational ¹ H and ¹³ C NMR applied to the stereochemical studies of inorganic, organic, and bioorganic compounds, involving in particular natural products, carbohydrates, and carbonium ions. The first part of the review briefly outlines theoretical background of the modern computational methods applied to the calculation of chemical shifts and spin-spin coupling constants at the DFT and the non-empirical levels. The second part of the review deals with the achievements of the computational ¹ H and ¹³ C NMR in the stereochemical investigation of a variety of inorganic, organic, and bioorganic compounds, providing in an abridged form the material partly discussed by the author in a series of parent reviews. Major attention is focused herewith on the publications of the recent years, which were not reviewed elsewhere.

1 J CH Coupling in Benzaldehyde Derivatives: Ortho Substitution Effect

The natural J-coupling (NJC) method is applied to analyze the Fermi contact contribution of the NMR spin-spin coupling constant decomposing this contribution in terms of natural localized molecular orbitals. We investigated the influence of the basis set on the NJC analysis for the formyl group coupling constant (¹ J _CHf) of benzaldehyde derivatives. NJC and other NBO analyses, like steric and natural Coulombic energy, were chosen to explain the influence of electron-donating and electron-withdrawing groups on ¹ J _CHf for some substituted benzaldehydes (Me, OH, OMe, F, Cl, Br, I, and NO₂). For the ortho derivatives, electronegative substituents near the C-Hf bond increase the ¹ J _CHf coupling. This effect could be related to an increase in formyl carbon s character and changes in the carbon and hydrogen natural charges. This indicates that the substituents in ortho have a proximity effect on ¹ J _CHf coupling mainly of electrostatic origin instead of the expected hyperconjugative interactions.

Theoretical calculations of carbon-hydrogen spin-spin coupling constants

Structural applications of theoretical calculations of carbon-hydrogen spin-spin coupling constants are reviewed covering papers published mainly during the last 10-15 years with a special emphasis on the most notable studies of hybridization, substitution and stereoelectronic effects together with the investigation of hydrogen bonding and intermolecular interactions. The wide scope of different applications of calculated carbon-hydrogen couplings in the structural elucidation of particular classes of organic and bioorganic molecules is reviewed, concentrating mainly on saturated, unsaturated, aromatic and heteroaromatic compounds and their functional derivatives, as well as on natural compounds and carbohydrates. The review is dedicated to Professor Emeritus Michael Barfield in view of his invaluable pioneering contribution to this field.

NMR spin-spin coupling constants: bond angle dependence of the sign and magnitude of the vicinal (3)JHF coupling

The dependence of the magnitude and sign of (3)JHFF on the bond angle in fluoro-cycloalkene compounds is evaluated by electronic structure calculations using different levels of theory, viz. DFT, SOPPA(CCSD) and SOPPA(CC2). Localized molecular orbital contributions to (3)JHFF are analyzed to assess which orbitals are responsible for (3)JHFF and which are the most important coupling transmission mechanisms for each compound. Fluoro-ethylene is used as a model system to evaluate the dependence of the (3)JHFF coupling constant on the angle between the σCα-F and σCα'-HF vectors. Through-space and hyperconjugative transmission pathways and ring strain are identified as responsible for the opposite trend between (3)JHFF and bond angle, and for the negative signs obtained for the two molecules, respectively. One of the fluorine lone pairs, σCα'-HF, σCα-F, σCα'-Cβ' bonding orbitals and the σ*Cα-F antibonding orbital are involved in the J-coupling pathways, according to analyses of pairwise-steric and hyperconjugative energies.

Conformational analysis of small molecules: NMR and quantum mechanics calculations

This review deals with conformational analysis in small organic molecules, and describes the stereoelectronic interactions responsible for conformational stability. Conformational analysis is usually performed using NMR spectroscopy through measurement of coupling constants at room or low temperature in different solvents to determine the populations of conformers in solution. Quantum mechanical calculations are used to address the interactions responsible for conformer stability. The conformational analysis of a large number of small molecules is described, using coupling constant measurements in different solvents and at low temperature, as well as recent applications of through-space and through-hydrogen bond coupling constants JFH as tools for the conformational analysis of fluorinated molecules. Besides NMR parameters, stereoelectronic interactions such as conjugative, hyperconjugative, steric and intramolecular hydrogen bond interactions involved in conformational preferences are discussed.

Lone-Pair Orientation Effect of an α-Oxygen Atom on (1)JCC NMR Spin-Spin Coupling Constants in o-Substituted Phenols. Experimental and DFT Study

The well-known N lone-pair orientation effect on (1)JCC spin-spin coupling constants (SSCCs) in oximes and their derivatives was used to study how negative hyperconjugative interactions of type LP1(O) → σ*CC depend on ortho interactions involving the OH group. This study demanded the following analyses:  (i) a qualitative estimation of how (1)JCC SSCCs are affected by hyperconjugative interactions, (ii) a study of similar stereochemical effects to those in oximes, but in (1)JC1C2 and (1)JC1C6in a series of 2-substituted phenols, and (iii) a quantitative estimation, with the natural bond order approach, of some key electron delocalization interactions. A few unexpected results are quoted. LP1(O) → σ*CC interactions are affected by proximity interactions as follows:  (a) they are enhanced by hydrogen bonds transferring charge into the (O-H)* antibonding orbital; (b) they are enhanced by proximity interactions of type LP1(O)···H-C; (c) they are inhibited by interactions of type LP(O1)···H-O. Consequences of these observations are discussed.

The electronic origin of unusually large (n)J(FN) coupling constants in some fluoroximes

SOPPA(CCSD) calculations show that the FC term is the most important contribution to the through-space transmission of JFN coupling constants for the fluoroximes studied in this work. Because of the well-known behavior of FC term, a new rationalization for the experimental (TS)JFN SSCC is presented. It is mainly based on the overlap matrix (Sij) between fluorine and nitrogen lone pairs obtained from NBO analyses. An expression is proposed to take into account the influence of the electronic density (Dij) between coupled nuclei as well as the s% character at the site of the coupling nuclei of bonds and non-bonding electron pairs involved in Dij. In using this approach, a linear correlation between (TS)JFN versus Dij is obtained. The most important aspect of this rationalization is related to the facility for understanding the behavior of some unusual experimental coupling constants. It is shown that, at least in this case, the electronic origin of the so-called through-space coupling is transmitted through to the overlap of orbitals on the coupled atoms, suggesting that, at least for these compounds, instead of through-space coupling, it should better be dubbed as 'through overlapping orbital coupling'.

F···HO intramolecular hydrogen bond as the main transmission mechanism for (1h)J(F,H(O)) coupling constant in 2'-fluoroflavonol

Flavonoids are useful compounds in medicinal chemistry and exhibit conformational isomerism, which is ruled by intramolecular interactions. One of the main intramolecular forces governing the stability of conformations is the hydrogen bond. Hydrogen bond involving fluorine covalently bonded to carbon has been found to be rare, but it appears in 2'-fluoroflavonol, although the F···HO hydrogen bond cannot be considered the main effect governing the conformational stability of this compound. Because (19)F is magnetically active and suitable for NMR studies, the (1h)J(F,H(O)) coupling constant can be used as a probe for such an interaction in 2'-fluoroflavonol. In fact, the (1h)J(F,H(O)) coupling was computationally analyzed in this work, and the F···HO hydrogen bond was found to be its main transmission mechanism, which modulates this coupling in 2'-fluoroflavonol, rather than overlap of proximate electronic clouds, such as in 2-fluorophenol.

Real-time mechanistic monitoring of an acetal hydrolysis using ultrafast 2D NMR

Ultrafast (UF) 2D NMR makes it possible to obtain a 2D NMR spectrum in less than a second. Here, UF-HSQC experiments are used for the real-time mechanistic study of an acetal hydrolysis at ¹³C natural abundance, and it is possible to characterize the presence of the hemiacetal, an intermediate with a well-known short lifetime. The assignments are confirmed and rationalized by quantum calculations of ¹H and ¹³C NMR chemical shifts and natural bonding orbital analysis.

Thermal and solvent effects on NMR indirect spin-spin coupling constants of a prototypical Chagas disease drug

NMR J-couplings across hydrogen bonds reflect the static and dynamic character of hydrogen bonding. They are affected by thermal and solvent effects and can therefore be used to probe such effects. We have applied density functional theory (DFT) to compute the NMR (n)J(N,H) scalar couplings of a prototypical Chagas disease drug (metronidazole). The calculations were done for the molecule in vacuo, in microsolvated cluster models with one or few water molecules, in snapshots obtained from molecular dynamics simulations with explicit water solvent, and in a polarizable dielectric continuum. Hyperconjugative and electrostatic effects on spin-spin coupling constants were assessed through DFT calculations using natural bond orbital (NBO) analysis and atoms in molecules (AIM) theory. In the calculations with explicit solvent molecules, special attention was given to the nature of the hydrogen bonds formed with the solvent molecules. The results highlight the importance of properly incorporating thermal and solvent effects into NMR calculations in the condensed phase.

Spatial structure and NMR spectra of strained [2.2.2]cyclophanes

The chemical shifts and coupling constants in the NMR spectra of the title compounds are influenced by their strain and the proximity of the aromatic rings. These parameters are studied by density functional theory (DFT) calculations for 1-4 and measured for 3 and 4. We find that, in spite of the strain, the calculations reproduce the experimental values satisfactorily. This finding is of special importance for novel hypothetical molecules like hexahydrosuperphane 5 for their future identification and in searches of hydrocarbons exhibiting unusual NMR parameters. Our results demonstrate the influence of strain on the parameters studied. Most proton and carbon chemical shifts for the molecules under study having nonplanar aromatic rings differ considerably from the corresponding values for the relatively unstrained trimethylbenzenes and ethylbenzene. In addition, the calculated values of the coupling constants through three bonds in most cases do not follow Karplus relation.

Are the silyl group hydrogens in peri-substituted-9-silyltriptycenes engaged in blue-shifting hydrogen bonds?

In three 9-silyltriptycenes bearing chlorine, bromine and the methyl group in one of the peri positions, the silyl group suffers extreme hindrance of its reorientational motion. Owing to this, separate signals of the individual silyl group protons could be observed in NMR spectra at relatively high temperatures. For each compound, the measured values of J couplings of these protons to the 29Si nucleus are strongly diversified, especially when a halogen atom occurs in the peri position. These differences can be qualitatively interpreted in terms of electron density transfers from the peri substituent, causing the individual Si-H bonds to differentiate. The stereoelectronic picture revealed by natural bond orbital analysis fits the pattern normally interpreted as that of the blue-shifting hydrogen bond. However, an essential similarity of the stereoelectronic effects for both the halogen and methyl substituents raises a serious question about the applicability of that concept to the present case.

Stereochemical dependence of NMR geminal spin-spin coupling constants

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.

On the unusual 2J(C2-H(f)) coupling dependence on syn/anti CHO conformation in 5-X-furan-2-carboxaldehydes

A remarkable difference for (2)J(C(2)-H(f)) coupling constant in syn and anti conformers of 5-X-furan-2-carboxaldehydes (X = CH(3), Ph, NO(2), Br) and a rationalization of this difference are reported. On the basis of the current knowledge of the Fermi-contact term transmission, a rather unusual dual-coupling pathway in the syn conformer is presented. The additional coupling pathway resembles somewhat that of the J(H-H) in homoallylic couplings, which are transmitted by hyperconjugative interactions involving the pi(C=C) electronic system. The homoallylic coupling pathway can be labeled as sigma*(C-H) <-- pi(C=C) --> sigma*(C-H). In the present case, this additional coupling pathway, using an analogous notation, can be labeled as sigma*(C(2)-C(C)) <-- LP(1)(O(1))...LP(2)(O(C)) --> sigma*(C(C)-H(f)) (sigma*(C(2)-C(C))) where O(1) and O(C) stand for the ring and carbonyl O atoms, respectively. This additional coupling pathway is not activated in the anti conformers since both oxygen lone pairs do not overlap.

Manifestations of stereoelectronic interactions in 1JC-H one-bond coupling constants

Two-electron/two-orbital hyperconjugative interactions depend on the relative orientation of bonds and lone pairs in a molecule and are also inversely proportional to the energy difference between the interacting orbitals. Spectroscopic manifestations of stereoelectronic interactions are particularly useful experimental signatures of these effects which can be utilized for testing molecular models. Empirical observations together with theoretical interpretations in cyclohexane and six-membered heterocycles confirm the relevance of sigma C-H ax --> sigma* C-H ax , n X --> sigma* C-H ax (X = O or N), sigma C-S --> sigma* C-H eq , beta-n O --> sigma* C-H eq , sigma C(2)-H ax --> pi* CY (Y = O, S, or CH 2), and sigma C(2)-H ax --> sigma* S-O ax two-electron/two-orbital stereoelectronic interactions that weaken the acceptor (or donor) C-H bonds and attenuate the Fermi contribution to the one-bond (13)C- (1)H coupling constants.

Stereochemical study of iminodihydrofurans based on experimental measurements and SOPPA calculations of (13)C-(13)C spin-spin coupling constants

Configurational assignment and conformational analysis of a series of iminodihydrofurans obtained from cyanoacetylenic alcohols were performed on the basis of experimental measurements and high-level ab initio calculations of their (13)C-(13)C spin-spin coupling constants. The title compounds were shown to form and exist in solution as the individual Z isomers, adopting the orthogonal orientation of the amino, alkylamino and dialkylamino groups and the s-trans orientation of the CONH(2) group at the C(4) position of the 2,5-dihydro-2-iminofuran moiety.

Experimental and DFT studies on the transmission mechanisms of analogous NMR JCH and JCC couplings in 1-X- and 1-X-3-methylbicyclo[1.1.1]-pentanes

The main aim of this work is to compare the transmission mechanisms for the Fermi contact term of spin-spin couplings, SSCCs, in series 1-X-bicyclo[1.1.1]-pentane, (1), and 1-X-3-methylbicyclo[1.1.1]pentane, (2), and from that comparison to gain insight into some subtle aspects of the FC transmission. To this end, 18 members of the latter series were isotopically enriched in (13)C at the methyl position and the following couplings were measured; 1JC3CMe, 3JC1CMe and 4JCXCMe. These three types of SSCCs in (2) are compared, respectively, with 1JC3H3, 3JC1H3 and 4JCXH in (1); these latter values were taken from previous works. Since electron delocalization plays an important role in the transmission of the FC interaction, the natural bond orbital (NBO) method is employed to quantify electron delocalization interactions within selected members of series (1) and (2). It is found that 1JC3H3 SSCCs in (1) is more efficiently transmitted than 1JC3CMe SSCCs in (2). On the other hand, 3JC1H3 and 4JCXH SSCCs in (1) are notably less efficiently transmitted than 3JC1CMe and 4JCXCMe SSCCs in (2), although substituent effects on these two SSCCs show the opposite trends. These different efficiencies are rationalized in terms of different sigma-hyperconjugative interactions in both series of compounds.

Calculation and analysis of NMR spin-spin coupling constants

The analysis of NMR spin-spin coupling leads to a unique insight into the electronic structure of closed-shell molecules, provided one is able to decode the different features of the spin-spin coupling mechanism. For this purpose, the physics of spin-spin coupling is described and the way how spin-spin coupling constants (SSCCs) can be quantum mechanically determined. Based on this insight, a set of requirements is derived that guide the development of a quantum mechanical analysis of spin-spin coupling. It is demonstrated that the J-OC-PSP (=J-OC-OC-PSP: Decomposition of J into orbital contributions using orbital currents and partial spin polarization) analysis method fulfills all requirements. J-OC-PSP makes it possible to partition the isotropic indirect SSCC J or its reduced analogue K as well as the four Ramsey terms (Fermi contact (FC), spin dipole (SD), diamagnetic spin orbit (DSO), paramagnetic spin orbit (PSO)) leading to J (or K) into Cartesian components (for the anisotropic Ramsey terms SD, DSO, PSO), orbital contributions or electron interaction terms. For the purpose of decoding the spin-spin coupling mechanism, FC, SD, DSO, and PSO coupling is discussed in detail and related to electronic and bonding features of the molecules in question. The myth of empirical and semiempirical relationships between SSCCs and bonding features is unveiled. It is found that most relationships are only of limited, partly dubious value, often arising from a fortuitous cancellation of terms that cannot be expected in general. These relationships are replaced by quantum chemical relations and descriptions that directly reflect the complex electronic processes leading to spin-spin coupling.