Through-bond and through-space J(FF) spin-spin coupling in peridifluoronaphthalenes: accurate DFT evaluation of the four contributions

Distinguishing "Through-Space" from "Through-Bonds" Contribution in Indirect Nuclear Spin-Spin Coupling: General Approaches Applied to Complex JPP and JPSe Scalar Couplings

We present herein two complementary theoretical approaches for analyzing the transmission pathways of indirect nuclear spin-spin couplings in high-resolution nuclear magnetic resonance. This phenomenon is notably conceptually poorly understood in complex experimental situations in which both nonbonded ["through-space" (TS)] and more "classical" bonding ("through-bond") spin-spin coupling pathways are potentially involved. The computational approaches we propose allow the visualization and discussion of individual transmission pathways and estimation of their relative weight from numerical contributions to the spin-spin coupling constant J-value. The first approach is based on the analysis of contributions limited to occupied molecular orbitals [focused on occupied molecular orbitals (FOMO)]. The second approach encompasses the consideration of both occupied and vacant orbitals [global molecular orbital contributions (GMOC)], and, besides the contributions from individual pathways, also considers their cross contributions. Both approaches are applicable to large systems with complex interactions of nuclear magnetic moments. Herein, we have first applied the FOMO and GMOC computational approaches to simple diphosphine models and then extended the analysis to J_PP and J_PSe experimentally measured in a constrained selenated (diphosphino)naphthalene compound. The new computational tools contributed evidence for the importance of the single lone pair not only from phosphorus but also from selenium in TS spin-spin transmission. It evidenced and modeled for the first time the existence of spin-spin transmission pathways mixing classical covalent bonding parts with a lone pair overlap of proximate heteroatoms (P and Se).

PAH/PAH(CF3 )n Donor/Acceptor Charge-Transfer Complexes in Solution and in Solid-State Co-Crystals

A solution, solid-state, and computational study is reported of polycyclic aromatic hydrocarbon PAH/PAH(CF3 )n donor/acceptor (D/A) charge-transfer complexes that involve six PAH(CF3 )n acceptors with known gas-phase electron affinities that range from 2.11(2) to 2.805(15) eV and four PAH donors, including seven CT co-crystal X-ray structures that exhibit hexagonal arrays of mixed π-stacks with 1/1, 1/2, or 2/1 D/A stoichiometries (PAH=anthracene, azulene, coronene, perylene, pyrene, triphenylene; n=5, 6). These are the first D/A CT complexes with PAH(CF3 )n acceptors to be studied in detail. The nine D/A combinations were chosen to allow several structural and electronic comparisons to be made, providing new insights about controlling D/A interactions and the structures of CT co-crystals. The comparisons include, among others, CT complexes of the same PAH(CF3 )n acceptor with four PAH donors and CT complexes of the same donor with four PAH(CF3 )n acceptors. All nine CT complexes exhibit charge-transfer bands in solution with λmax between 467 and 600 nm. A plot of E(λmax ) versus [IE(donor)-EA(acceptor)] for the nine CT complexes studied is linear with a slope of 0.72±0.03 eV eV-1 . This plot is the first of its kind for CT complexes with structurally related donors and acceptors for which precise experimental gas-phase IEs and EAs are known. It demonstrates that conclusions based on the common assumption that the slope of a CT E(λmax ) versus [IE-EA] plot is unity may be incorrect in at least some cases and should be reconsidered.

Weak Pnictogen Bond with Bismuth: Experimental Evidence Based on Bi-P Through-Space Coupling

To study pnictogen bonding involving bismuth, flexible accordion-like molecular complexes of the composition [P(C6 H4 -o-CH2 SCH3 )3 BiX3 ], (X=Cl, Br, I) have been synthesised and characterised. The strength of the weak and mainly electrostatic interaction between the Bi and P centres strongly depends on the character of the halogen substituent on bismuth, which is confirmed by single-crystal X-ray diffraction analyses, DFT and ab initio computations. Significantly, 209 Bi-31 P through-space coupling (J=2560 Hz) is observed in solid-state 31 P NMR spectra, which is so far unprecedented in the literature, delivering direct information on the magnitude of this pnictogen interaction.

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.

Unusual Indirect Nuclear Spin-Spin Exchange Coupling through Solvated Electron

Solvated electrons have been found to exist in various media which also exhibit more intriguing properties such as superconductivity, nonlinear optical response, and so on. However, how they affect the nuclear spin properties has not been proven. In this work, we present the first detailed study on solvated-electron-triggered indirect nuclear spin-spin J-coupling using density functional theory calculations. Taking ¹⁹F as a probe, we verify the presence of unusual J couplings between two distant F atoms in HF-containing anionic clusters. These couplings occur "through solvated electron", rather than through conventional covalent bonds or space. Solvated electron can serve as an additional channel to efficiently realize long-range J-coupling between far separated nuclei because of its dispersivity and Rydberg character. The coupling magnitude strongly depends on the unique distribution of solvated electron and its second-order interaction with solvating HF units. This work provides novel insights into the mediating roles of electrons, possibly opening up potential applications based on weakly bound electrons.

Through-space (19)F-(19)F spin-spin coupling in ortho-fluoro Z-azobenzene

We report through-space (TS) (19)F-(19)F coupling for ortho-fluoro-substituted Z-azobenzenes. The magnitude of the TS-coupling constant ((TS) JFF ) ranged from 2.2-5.9 Hz. Using empirical formulas reported in the literature, these coupling constants correspond to non-bonded F-F distances (dFF) of 3.0-3.5 Å. These non-bonded distances are significantly smaller than those determined by X-ray crystallography or density functional theory, which argues that simple models of (19)F-(19)F TS spin-spin coupling solely based dFF are not applicable. (1)H, (13)C and (19)F data are reported for both the E and Z isomers of ten fluorinated azobenzenes. Density functional theory [B3YLP/6-311++G(d,p)] was used to calculate (19) F chemical shifts, and the calculated values deviated 0.3-10.0 ppm compared with experimental values.

Calculation of Nuclear Spin-Spin Coupling Constants of Molecules with First and Second Row Atoms in Study of Basis Set Dependence

This paper proposes a systematic way to modify standard basis sets for use in NMR spin-spin coupling calculations, which allows the high sensitivity of this property to the basis set to be handled in a manner which remains computationally feasible. The new basis set series is derived by uncontracting a standard basis set, such as correlation-consistent aug-cc-pVTZ, and extending it by systematically adding tight s and d functions. For elements in different rows of the periodic table, different progressions of functions are added. The new basis sets are shown to approach the basis set limit for calculations on a range of molecules containing hydrogen and first and second row atoms.

Investigation of the resonance-assisted hydrogen bond in model β-diketones through localized molecular orbital analysis of the spin-spin coupling constants related to the O-H···O hydrogen bond

The resonance-assisted hydrogen bond (HB) phenomenon has been studied theoretically by a localized molecular orbital (LMO) decomposition of the spin-spin coupling constants between atoms either involved or close to the O-H···O system of some β-diketones and their saturated counterparts. The analysis, carried out at the level of the second-order polarization propagator approximation, shows that the contributions in terms of LMO to the paramagnetic spin orbital and the spin dipolar Ramsey terms proof the importance of the delocalized π-electron structure supporting the idea of the existence of the resonance-assisted HB phenomenon phenomenon. The LMO contributions to the Fermi contact term indicate mainly the presence of the HB that may or not be linked to the π-electrons.

Probing interactions through space using spin–spin coupling

A series of eight 5-(TeAr)-6-(SePh)acenaphthenes (Ar = aryl) were prepared and structurally characterised by X-ray crystallography, solution and solid-state NMR spectroscopy and DFT/B3LYP calculations.

Experimental and theoretical study of the intramolecular C-H···N and C-H···S hydrogen bonding effects in the 1H and 13C NMR spectra of the 2-(alkylsulfanyl)-5-amino-1-vinylpyrroles: a particular state of amine nitrogen

In the (1)H NMR spectra of the 1-vinylpyrroles with amino- and alkylsulfanyl groups in 5 and 2 positions, an extraordinarily large difference between resonance positions of the HA and HB terminal methylene protons of the vinyl group is discovered. Also, the one-bond (1)J(C(β),H(B)) coupling constant is surprisingly greater than the (1)J(C(β),H(A)) coupling constant in pyrroles under investigation, while in all known cases, there was a reverse relationship between these coupling constants. These spectral anomalies are substantiated by quantum chemical calculations. The calculations show that the amine nitrogen lone pair is removed from the conjugation with the π-system of the pyrrole ring so that it is directed toward the HB hydrogen. These factors are favorable to the emergence of the intramolecular C-HB •••N hydrogen bonding in the s-cis(N) conformation. On the other hand, the spatial proximity of the sulfur to the HB hydrogen provides an opportunity of the intramolecular C-HB •••S hydrogen bonding in the s-cis(S) conformation. Presence of the hydrogen bond critical points as well as ring critical point for corresponding chelate ring revealed by a quantum theory of atoms in molecules (QTAIM) approach confirms the existence of the weak intramolecular C-H•••N and C-H•••S hydrogen bonding. Therefore, an unusual high-frequency shift of the HB signal and the increase in the (1)J(C(β),H(B)) coupling constant can be explained by the effects of hydrogen bonding.

Substituent effect in 2-benzoylmethylenequinoline difluoroborates exhibiting through-space couplings. Multinuclear magnetic resonance, X-ray diffraction, and computational study

The series of nine 2-benzoylmethylenequinoline difluoroborates have been synthesized and characterized by multinuclear magnetic resonance, X-ray diffraction (XRD), and computational methods. The through-space spin-spin couplings between (19)F and (1)H/(13)C nuclei have been observed in solution. The NMR chemical shifts have been correlated to the Hammett substituent constants. The crystal structures of six compounds have been solved by XRD. For two derivatives the X-ray wave function refinement was performed to evaluate the character of bonds in the NBF(2)O moiety by topological and integrated bond descriptors.

Accessing long-lived nuclear singlet states between chemically equivalent spins without breaking symmetry

Long-lived nuclear spin states could greatly enhance the applicability of hyperpolarized nuclear magnetic resonance. Using singlet states between inequivalent spin pairs has been shown to extend the signal lifetime by more than an order of magnitude compared to the spin lattice relaxation time (T1), but they have to be prevented from evolving into other states. In the most interesting case the singlet is between chemically equivalent spins, as it can then be inherently an eigenstate. However this presents major challenges in the conversion from bulk magnetization to singlet. In the only case demonstrated so far, a reversible chemical reaction to break symmetry was required. Here we present a pulse sequence technique that interconverts between singlet spin order and bulk magnetization without breaking the symmetry of the spin system. This technique is independent of field strength and is applicable to a broad range of molecules.

Stereospecificity of (1) H, (13) C and (15) N shielding constants in the isomers of methylglyoxal bisdimethylhydrazone: problem with configurational assignment based on (1) H chemical shifts

In the (13) C NMR spectra of methylglyoxal bisdimethylhydrazone, the (13) C-5 signal is shifted to higher frequencies, while the (13) C-6 signal is shifted to lower frequencies on going from the EE to ZE isomer following the trend found previously. Surprisingly, the (1) H-6 chemical shift and (1) J(C-6,H-6) coupling constant are noticeably larger in the ZE isomer than in the EE isomer, although the configuration around the -CH═N- bond does not change. This paradox can be rationalized by the C-H⋯N intramolecular hydrogen bond in the ZE isomer, which is found from the quantum-chemical calculations including Bader's quantum theory of atoms in molecules analysis. This hydrogen bond results in the increase of δ((1) H-6) and (1) J(C-6,H-6) parameters. The effect of the C-H⋯N hydrogen bond on the (1) H shielding and one-bond (13) C-(1) H coupling complicates the configurational assignment of the considered compound because of these spectral parameters. The (1) H, (13) C and (15) N chemical shifts of the 2- and 8-(CH(3) )(2) N groups attached to the -C(CH(3) )═N- and -CH═N- moieties, respectively, reveal pronounced difference. The ab initio calculations show that the 8-(CH(3) )(2) N group conjugate effectively with the π-framework, and the 2-(CH(3) )(2) N group twisted out from the plane of the backbone and loses conjugation. As a result, the degree of charge transfer from the N-2- and N-8- nitrogen lone pairs to the π-framework varies, which affects the (1) H, (13) C and (15) N shieldings.

Unexpected geometrical effects on paramagnetic spin-orbit and spin-dipolar 2J(FF) couplings

The second-rank tensor character of the paramagnetic spin-orbit and spin-dipolar contributions to nuclear spin-spin coupling constants is usually ignored when NMR measurements are carried out in the isotropic phase. However, in this study it is shown that isotropic (2)J(FF) couplings strongly depend on the relative orientation of the C-F bonds containing the coupling nuclei and the eigenvectors of such tensors. Predictions about such effect are obtained using a qualitative approach based on the polarization propagator formalism at the RPA, and results are corroborated performing high-level ab initio spin-spin coupling calculations at the SOPPA(CCSD)/EPR-III//MP2/EPR-III level in a model system. It is highlighted that no calculations at the RPA level were carried out in this work. The quite promising results reported in this paper suggest that similar properties are expected to hold for the second-rank nuclear magnetic shielding tensor.

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).

Pople Style Basis Sets for the Calculation of NMR Spin-Spin Coupling Constants: the 6-31G-J and 6-311G-J Basis Sets

We present a modification of the small and popular Pople basis sets, 6-31G and 6-311G, for density functional theory calculations of Fermi contact dominated NMR indirect nuclear spin-spin coupling constants. These new basis sets, 6-31G-J and 6-311G-J, contain twice the number of contracted s-type functions but the same number of contracted p-type functions as the original Pople basis set. For our test set of 12 one-, two- and three-bond coupling constants, the new basis sets augmented with the standard diffuse and polarization functions, i.e., the 6-31+G*-J and 6-311++G**-J basis sets, lead to a maximum deviation of 5 and 2 Hz, respectively, compared to results obtained with the 6 or more times larger aug-pcJ-4 basis set. In correlated wave function calculations using the second-order polarization propagator approximation, the deviations with respect to the aug-ccJ-pVQZ basis set are 8 and 3 Hz and thus slightly larger.

Study of stereospecificity of 1H, 13C, 15N and 77Se shielding constants in the configurational isomers of the selenophene-2-carbaldehyde azine by NMR spectroscopy and MP2-GIAO calculations

In the (1)H and (13)C NMR spectra of selenophene-2-carbaldehyde azine, the (1)H-5, (13)C-3 and (13)C-5 signals of the selenophene ring are shifted to higher frequencies, whereas those of the (1)H-1, (13)C-1, (13)C-2 and (13)C-4 are shifted to lower frequencies on going from the EE to ZZ isomer or from the E moiety to the Z moiety of EZ isomer. The (15)N chemical shift is significantly larger in the EE isomer relative to the ZZ isomer and in the E moiety relative to the Z moiety of EZ isomer. A very pronounced difference (60-65 mg/g) between the (77)Se resonance positions is revealed in the studied azine isomers, the (77)Se peak being shifted to higher frequencies in the ZZ isomer and in the Z moiety of EZ isomer. The trends in the changes of the measured chemical shifts are reasonably reproduced by the GIAO calculations at the MP2 level of the (1)H, (13)C, (15)N and (77)Se shielding constants in the energy-favorable conformation with the syn orientation of both selenophene rings relative to the C = N groups. The NBO analysis suggests that such an arrangement of the selenophene rings may take place because of a higher energy of some intramolecular interactions.

19F chemical shifts, coupling constants and conformational preferences in monosubstituted perfluoroparacyclophanes

In the process of studying the chemistry of perfluoro[2.2]paracyclophanes (PFPCs), a novel class of compounds, it became necessary to identify some disubstituted products. To achieve this goal, we characterize in this work some monosubstituted PFPCs, identifying their (19)F-(19) F coupling patterns, and establishing a methodology for the assignment of their (19)F chemical shifts. The pattern of coupling constants indicates a skewed geometry in which the upper deck moves towards or away from the substituent, depending on the substituent electron-donor character and size. Quantum chemical calculations, performed at the HF/6-311 + G(d,p)//B3LYP/EPR-III level of theory, confirmed the conformations inferred from coupling constants and reproduced well the values of the couplings. Transmission mechanisms for the FC term of four- and five-bond (19)F-(19) F couplings are discussed in detail. Understanding the conformational preferences of PFPCs and how they are reflected by the coupling constants facilitates the assignment of (19)F chemical shifts in monosubstituted PFPCs and the identification of the disubstituted products.

Benchmarking NMR indirect nuclear spin-spin coupling constants: SOPPA, SOPPA(CC2), and SOPPA(CCSD) versus CCSD

Accurate calculations of NMR indirect nuclear spin-spin coupling constants require especially optimized basis sets and correlated wave function methods such as CCSD or SOPPA(CCSD). Both methods scale as N(6), where N is the number of orbitals, which prevents routine applications to molecules with more than 10-15 nonhydrogen atoms. We have therefore developed a modification of the SOPPA(CCSD) method in which the CCSD singles and doubles amplitudes are replaced by CC2 singles and doubles amplitudes. This new method, called SOPPA(CC2), scales only as N(5), like the original SOPPA-method. The performance of the SOPPA(CC2) method for the calculation of indirect nuclear spin-spin coupling constants is compared to SOPPA and SOPPA(CCSD) employing a set of benchmark molecules. We also investigate the basis set dependence by employing three different basis sets optimized for spin-spin coupling constants, namely the HuzIV-su4, ccJ-pVTZ, and ccJ-pVQZ basis sets. The results of the corresponding CCSD calculations are used as a theoretical reference.

GIAO, DFT, AIM and NBO analysis of the N-H...O intramolecular hydrogen-bond influence on the 1J(N,H) coupling constant in push-pull diaminoenones

In the series of diaminoenones, large high-frequency shifts of the (1)H NMR of the N-H group in the cis-position relative to the carbonyl group suggests strong N-H...O intramolecular hydrogen bonding comprising a six-membered chelate ring. The N-H...O hydrogen bond causes an increase of the (1)J(N,H) coupling constant by 2-4 Hz and high-frequency shift of the (15)N signal by 9-10 ppm despite of the lengthening of the relevant N-H bond. These experimental trends are substantiated by gauge-independent atomic orbital and density functional theory calculations of the shielding and coupling constants in the 3,3-bis(isopropylamino)-1-(aryl)prop-2-en-1-one (12) for conformations with the Z- and E-orientations of the carbonyl group relative to the N-H group. The effects of the N-H...O hydrogen-bond on the NMR parameters are analyzed with the atoms-in-molecules (AIM) and natural bond orbital (NBO) methods. The AIM method indicates a weakening of the N-H...O hydrogen bond as compared with that of 1,1-di(pyrrol-2-yl)-2-formylethene (13) where N-H...O hydrogen bridge establishes a seven-membered chelate ring, and the corresponding (1)J(N,H) coupling constant decreases. The NBO method reveals that the LP(O) --> sigma*(N-H) hyperconjugative interaction is weakened on going from the six-membered chelate ring to the seven-membered one due to a more bent hydrogen bond in the former case. A dominating effect of the N-H bond rehybridization, owing to an electrostatic term in the hydrogen bonding, seems to provide an increase of the (1)J(N,H) value as a consequence of the N-H...O hydrogen bonding in the studied diaminoenones.

Study of conformations and hydrogen bonds in the configurational isomers of pyrrole-2-carbaldehyde oxime by 1H, 13C and 15N NMR spectroscopy combined with MP2 and DFT calculations and NBO analysis

The (1)H, (13)C and (15)N NMR studies have shown that the E and Z isomers of pyrrole-2-carbaldehyde oxime adopt preferable conformation with the syn orientation of the oxime group with respect to the pyrrole ring. The syn conformation of E and Z isomers of pyrrole-2-carbaldehyde oxime is stabilized by the N-H...N and N-H...O intramolecular hydrogen bonds, respectively. The N-H...N hydrogen bond in the E isomer causes the high-frequency shift of the bridge proton signal by about 1 ppm and increase the (1)J(N, H) coupling by approximately 3 Hz. The bridge proton shows further deshielding and higher increase of the (1)J(N, H) coupling constant due to the strengthening of the N-H...O hydrogen bond in the Z isomer. The MP2 calculations indicate that the syn conformation of E and Z isomers is by approximately 3.5 kcal/mol energetically less favorable than the anti conformation. The calculations of (1)H shielding and (1)J(N, H) coupling in the syn and anti conformations allow the contribution to these constants from the N-H...N and N-H...O hydrogen bondings to be estimated. The NBO analysis suggests that the N-H...N hydrogen bond in the E isomer is a pure electrostatic interaction while the charge transfer from the oxygen lone pair to the antibonding orbital of the N-H bond through the N-H...O hydrogen bond occurs in the Z isomer.

Theoretical study of bifurcated hydrogen bonding effects on the 1J(N,H), 1hJ(N,H), 2hJ(N,N) couplings and 1H, 15N shieldings in model pyrroles

According to the density functional theory calculations, the X...H...N (X=N, O) intramolecular bifurcated (three-centered) hydrogen bond with one hydrogen donor and two hydrogen acceptors causes a significant decrease of the (1h)J(N,H) and (2h)J(N,N) coupling constants across the N-H...N hydrogen bond and an increase of the (1)J(N,H) coupling constant across the N-H covalent bond in the 2,5-disubstituted pyrroles. This occurs due to a weakening of the N-H...N hydrogen bridge resulting in a lengthening of the N...H distance and a decrease of the hydrogen bond angle at the bifurcated hydrogen bond formation. The gauge-independent atomic orbital calculations of the shielding constants suggest that a weakening of the N-H...N hydrogen bridge in case of the three-centered hydrogen bond yields a shielding of the bridge proton and deshielding of the acceptor nitrogen atom. The atoms-in-molecules analysis shows that an attenuation of the (1h)J(N,H) and (2h)J(N,N) couplings in the compounds with bifurcated hydrogen bond is connected with a decrease of the electron density rho(H...N) at the hydrogen bond critical point and Laplacian of this electron density nabla(2)rho(H...N). The natural bond orbital analysis suggests that the additional N-H...X interaction partly inhibits the charge transfer from the nitrogen lone pair to the sigma*(N-H) antibonding orbital across hydrogen bond weakening of the (1h)J(N,H) and (2h)J(N,N) trans-hydrogen bond couplings through Fermi-contact mechanism. An increase of the nitrogen s-character percentage of the N-H bond in consequence of the bifurcated hydrogen bonding leads to an increase of the (1)J(N,H) coupling constant across the N-H covalent bond and deshielding of the hydrogen donor nitrogen atom.

C-H...N and C-H...O intramolecular hydrogen bonding effects in the 1H, 13C and 15N NMR spectra of the configurational isomers of 1-vinylpyrrole-2-carbaldehyde oxime substantiated by DFT calculations

According to the (1)H, (13)C and (15)N NMR spectroscopic data and DFT calculations, the E-isomer of 1-vinylpyrrole-2-carbaldehyde adopts preferable conformation with the anti-orientation of the vinyl group relative to the carbaldehyde oxime group and with the syn-arrangement of the carbaldehyde oxime group with reference to the pyrrole ring. This conformation is stabilized by the C-H...N intramolecular hydrogen bond between the alpha-hydrogen of the vinyl group and the oxime group nitrogen, which causes a pronounced high-frequency shift of the alpha-hydrogen signal in (1)H NMR (approximately 0.5 ppm) and an increase in the corresponding one-bond (13)C-(1)H coupling constant (ca 4 Hz). In the Z-isomer, the carbaldehyde oxime group turns to the anti-position with respect to the pyrrole ring. The C-H...O intramolecular hydrogen bond between the H-3 hydrogen of the pyrrole ring and the oxime group oxygen is realized in this case. Due to such hydrogen bonding, the H-3 hydrogen resonance is shifted to a higher frequency by about 1 ppm and the one-bond (13)C-(1)H coupling constant for this proton increases by approximately 5 Hz.