Homonuclear J-couplings and rotationally induced sideband enhancements in NMR spectra of rotating solids

We demonstrate direct detection of ¹³C-¹³C J-couplings in magic angle sample spinning NMR spectra despite the presence of the much larger homogeneous broadening caused by the homonuclear dipolar couplings. Carbon-carbon J-couplings were observed in doubly ¹³C-enriched samples of sodium acetate, glycine and glucose. The resolved J-coupling permits carbon-carbon connectivities to be established with standard two-dimensional techniques. Interesting spectral features are observed when the rotational sidebands of the coupled spins overlap: when a sideband from a dipolar-coupled pair approaches the centerband of its partner, a significant enhancement in sideband intensity is observed as well as small shifts in the resonance frequency.     

One-bond CC spin—spin coupling constants in derivatives of benzene. Non-linearity of 1J(CC) vs substituent electronegativity

A set of one-bond CC coupling constants has been determined for mono- and disubstituted benzenes. Large ¹J(CC) values have been found within the benzene rings bearing highly electronegative substituents such as halogens, methoxy and nitro groups and small values for those with electropositive substituents. The total range of ¹J(CC) couplings observed in our work is larger than 50 Hz. These large variations of CC spin—spin couplings are interpreted in terms of Fermi-contact contributions and the redistribution of s-electrons within a CC bond under influence of substituents. Contrary to some previous findings the data obtained in the present work indicate that the relationship between ¹J(CC) and the substituent electronegativity is not linear.     

Measurement of carbon-proton dipolar couplings in liquid crystals using DAPT

Dipolar couplings provide valuable information on order and dynamics in liquid crystals. For measuring heteronuclear dipolar couplings in oriented systems, a new separated local field experiment is presented here. The method is based on the dipolar assisted polarization transfer (DAPT) pulse sequence proposed recently (Chem. Phys. Lett. 2007, 439, 407) for transfer of polarization between two spins I and S. DAPT utilizes the evolution of magnetization of the I and S spins under two blocks of phase shifted BLEW-12 pulses on the I spin separated by a 90 degree pulse on the S spin. Compared to the rotating frame techniques based on Hartmann-Hahn match, this approach is easy to implement and is independent of any matching conditions. DAPT can be utilized either as a proton encoded local field (PELF) technique or as a separated local field (SLF) technique, which means that the heteronuclear dipolar coupling can be obtained by following either the evolution of the abundant spin like proton (PELF) or that of the rare spin such as carbon (SLF). We have demonstrated the use of DAPT both as a PELF and as a SLF technique on an oriented liquid crystalline sample at room temperature and also have compared its performance with PISEMA. We have also incorporated modifications to the original DAPT pulse sequence for (i) improving its sensitivity and (ii) removing carrier offset dependence.     

Theoretical studies of d-d and d-π-d magnetic interactions in (EDT-TTFVO)2FeBr4 crystals

The effective exchange integrals (J(HB)) of the Heisenberg spin model have been evaluated by using the ab initio MO and based on Hartree-Fock (HF) and density functional theory (DFT) for organic magnetic metals, the (EDT-TTFVO)₂FeBr₄ crystal based on the X-ray crystallographic structures at 113 K. In order to study the magnetic properties, we proposed some of the pairs, where the direct (d-d) and indirect (d-π-d) magnetic couplings between Fe(III) d-spins (S = 5/2) without/with π-dimer spins (S = 1/2) were calculated, respectively. The effective exchange integrals were evaluated by using UB3LYP method, and principal J values were 0.5, −0.1 and 0.4 K. From these results, it is found that there were three dimensional spin arrangements of Fe(III) d-spins. The Quantum Monte Carlo (QMC) simulations had been carried out with our calculated J values to evaluate the magnetic susceptibility for this molecular crystal, reproducing the experimental tendency.     

Long range 13C1H coupling constants at bridgehead carbons in some naphthalene and quinoline derivatives, “cross” ring and “through” ring interactions

The ³J_CH couplings at bridgehead carbons in naphthalene and quinoline derivatives have been classified as “through” ring and “cross” ring interactions. Further divisions of the couplings have been introduced in an attempt to rationalize the interactions such that they may be used for structural assignments. The influence of central π-bond order and of the heterocyclic nitrogen atom on the couplings has been discussed. From a study of the ³J couplings at C-9 and C-10 in 45 quinoline derivatives the coupling constant ranges observed were “through” ring (via 9,10-bond): J₉₄ 4.9–6.1 Hz, J₉₅ 5.5–7.3 Hz, J_10.8 4.1–5.3 Hz; “cross” ring: J₉₂ 11.0–13.5 Hz, J₉₇ 6.7–9.8 Hz, J_10.3 4.4–7.3 Hz, J_10.6 5.2–9.1 Hz.     

Magnetic susceptibilities of UO2ThO2 solid solutions

The magnetic susceptibility of UO₂ThO₂ solid solutions has been measured from room temperature to 2.0 K. The magnetic moment and the Weiss constant have been determined in the temperature range in which the Curie-Weiss law holds. For the solid solutions showing antiferromagnetic transition, the Néel temperature has been also determined. These values decrease monotonically with increasing ThO₂ concentration. The results were analyzed using the molecular field theory which includes the interaction between next-nearest neighbor spins. The interactions between nearest neighbor spins, J₁, and those between next-nearest neighbor spins, J₂, both decrease with increasing ThO₂ concentration. The change of J₁ with composition is larger than that of J₂. The effect of magnetic dilution with ThO₂ is considered to be stronger on the interaction between nearest neighbor uranium ions.     

Structural studies on aryl‐substituted enaminoketones and their thio analogues. Part II. Experimental and DFT calculated carbon–carbon coupling constants,nJ(CC)'s (n = 1–3)

Spin–spin carbon–carbon coupling constants across one, two and three bonds, J(CC), have been measured for a series of aryl‐substituted Z‐s‐Z‐s‐E enaminoketones and their thio analogues. As a result, a large set, altogether 178, of J(CC)s has been obtained. It consists of 82 couplings across one bond, 31 couplings across two bonds and 65 couplings across three bonds. Independently, the DFT calculations at the B3PW91/6‐311++G(d,p)//B3PW91/6‐311++G(d,p) level yielded a set of theoretical J(CC) values. A comparison of these two sets of data gave an excellent linear correlation with parameters a and b close to ideal; a = 0.9978 which is not far from unity and b = 0.22 Hz which is close to zero. The ¹J(CC) couplings determined for the crucial fragment of the molecules, i.e. ? C?C? C?O (or ? C?C? C?S), are: ¹J(C?C) ≈ 68 Hz (67 Hz) and ¹J(C? C) = 60.5 Hz (60.0 Hz). The corresponding couplings found for the Z‐s‐Z‐s‐E isomer of the parent enaminoketone, 4‐methylamino‐but‐3‐en‐2‐one are 64.1 and 59.3 Hz, respectively. The most sensitive towards substitution of the oxygen atom by sulfur are two‐bond couplings between the α‐vinylic and aromatic C_ipso carbon atoms, which attain 12 Hz in the enaminoketone derivatives and decrease to 5 Hz in their thio analogues. Copyright © 2009 John Wiley & Sons, Ltd.     

Molecular orbital concept on spin-flip transport in molecular junctions

The spin-dependent electron transport correlated with spin-flip dynamics in a molecular junction was investigated in the wave-packet and Green??s function approaches. The molecular junction adopted in this work is described by a simple one-dimensional tight-binding chain including a localized spin. The spin exchange coupling J between the localized and conduction electron spins was taken into account through the s-d Hamiltonian. The wave-packet simulations showed that the transmission probabilities in both the spin-flip and no-flip processes show large peaks at the eigenvalues of the spin singlet (?3J/4) and triplet (J/4) states, and that, different transmission properties appear at the mid-gap of the two eigenvalues: the spin-flip process shows a moderate decrease, whereas the no-flip process an abrupt drop. Dividing the s-d Hamiltonian into two submatrices and referring to the molecular orbital concept for the coherent electron transport, we found that the moderate decrease in the spin-flip process at the mid-gap is the result of a coherent-and-cooperative contribution from the singlet and triplet states of the conduction and localized electron spins, and that, the abrupt drop in the no-flip process at the mid-gap is mainly caused by the coherent cancellation from the singlet and triplet states. The molecular orbital concept available for the electron transport including spin-flip scattering processes is described in Green??s function method, in analogy to the one derived for the spinless electron transport.     

Detection and use of small J couplings in solid state NMR experiments

Over the years, solid-state nuclear magnetic resonance (NMR) spectroscopy has become an important tool for materials science, with its local point of view that is highly complementary to the structural information provided by diffraction techniques, electron microscopy, and molecular modeling, for example. As compared to other interactions that determine the spectral expression of the local structure of the observed nuclei in solid-state NMR experiments, the J coupling, characteristic of the chemical bonds, has received far less attention because of its being generally so small that it is masked in the line-widths. Nevertheless, the scalar or isotropic part of J couplings, which is not averaged by magic angle spinning (MAS), can be evidenced in many systems, and exploited to unequivocally characterize the extended coordination sphere. In a first step we describe the different experiments that permit the observation and the measurement of J couplings, even when dealing with quadrupolar nuclei. We then present new and recently-published results that illustrate the state of the art of NMR methodologies based on or intended for measuring J couplings in solids and the novel perspectives that they open towards better understanding of ordered and disordered materials at the subnanometric scale, a length scale that is otherwise difficult to access.     

A 1H NMR and molecular modelling investigation of diastereotopic methylene hydrogen atoms

The ¹H NMR spectra of methyl 3‐bromo‐2‐methylpropionate (1a) and the corresponding chloro compound (2a) show no long‐range coupling between the methyl and methylene protons. In contrast, in the analogous dihalocompounds, methyl 2,3‐dibromo‐2‐methylpropionate (1b) and methyl 2,3‐dichloro‐2‐methylpropionate (2b), one of the methylene protons exhibits a large ⁴J_HH coupling (0.8 Hz) to the methyl group, but the other proton shows no observable splitting. This can be explained quantitatively by calculations of the conformational preferences in these compounds combined with the known orientation dependence of the ⁴J_HHcouplings. One conformer predominates in the dihalo compounds 1b and 2b, and this is responsible for the ⁴J_HH coupling. In 1a and 2a all three conformers are populated and the ⁴J_HH couplings average to zero. The technique is a potentially general method of unambiguously assigning diastereotopic methylene protons. Copyright © 2002 John Wiley & Sons, Ltd.     

Magnitude of Finite‐Nucleus‐Size Effects in Relativistic Density Functional Computations of Indirect NMR Nuclear Spin–Spin Coupling Constants

A spherical Gaussian nuclear charge distribution model has been implemented for spin‐free (scalar) and two‐component (spin–orbit) relativistic density functional calculations of indirect NMR nuclear spin–spin coupling (J‐coupling) constants. The finite nuclear volume effects on the hyperfine integrals are quite pronounced and as a consequence they noticeably alter coupling constants involving heavy NMR nuclei such as W, Pt, Hg, Tl, and Pb. Typically, the isotropic J‐couplings are reduced in magnitude by about 10 to 15 % for couplings between one of the heaviest NMR nuclei and a light atomic ligand, and even more so for couplings between two heavy atoms. For a subset of the systems studied, viz. the Hg atom, Hg₂²⁺, and Tl? X where X=Br, I, the basis set convergence of the hyperfine integrals and the coupling constants was monitored. For the Hg atom, numerical and basis set calculations of the electron density and the 1s and 6s orbital hyperfine integrals are directly compared. The coupling anisotropies of TlBr and TlI increase by about 2 % due to finite‐nucleus effects.     

Magnetic susceptibilities of UO2ZrO2 solid solutions

The magnetic susceptibility of cubic solid solutions of UO₂ and ZrO₂ with a fluorite structure has been measured from room temperature to 2.3 K. The magnetic moment and the Weiss constant have been determined in the temperature range where the Curie-Weiss law holds. These values decrease monotonically with increasing ZrO₂ concentration. The solid solutions of UO₂ diluted with 0 ～ 20 mole% ZrO₂ show an antiferromagnetic transition and have a linear dependence of Néel temperature on concentration, with a critical concentration of 78 mole% UO₂. The molecular field theory which includes next-nearest-neighbor interaction was applied to the results. The interaction between nearest-neighbor spins, J₁, decreases with increasing ZrO₂ concentration, whereas that between next-nearest-neighbor spins, J₂, increases. The behavior of J₁ against composition is thought to be caused from the direct effect of magnetic dilution with ZrO₂. Regarding the behavior of J₂, the effect of increasing magnetic interaction due to the smaller distance of uranium ions is considered to be stronger than the magnetic dilution effect.     

Exchange couplings in one-dimensionally arrayed 4f-3d heterometallic [Ln2Cu2]n compounds: A chemical trend of the coupling parameter

Heterometallic polymeric coordination compounds [{Ln(hfac)₂(CH₃OH)}₂{Cu(dmg)(Hdmg)}₂]_n (abbreviated as [Ln₂Cu₂]_n) involving light lanthanoid ions, Pr, Nd, Sm, and Eu, were synthesized, where H₂dmg and Hhfac stand for dimethylglyoxime and 1,1,1,5,5,5-hexafluoropentane-2,4-dione, respectively. The X-ray crystallographic analysis shows that their structures are isomorphous to those of the known heavy lanthanoid analogs. The exchange couplings were evaluated by high-frequency electron paramagnetic resonance and pulsed-field magnetization studies, giving ferromagnetic exchange parameters: J_Pr-Cu/k_B = 0.039(1) K, J_Nd-Cu/k_B = 0.38(1) K, J_Sm-Cu/k_B = 0.95(2) K, and no interaction between Eu and Cu ions. A significant trend is found in the order of the periodic table. The ferro- and antiferromagnetic 4f-3d couplings were characterized for the light and heavy lanthanoid derivatives, respectively. The magnitude of J_Ln-Cu decreases with a decrease of the 4f-spin portion, i.e., the number of unpaired electrons, whether the coupling is ferro- or antiferromagnetic.     

STRONG INADEQUATE,an experiment for detection of small J(C,C) couplings in symmetrical molecules

A new version of the two-dimensional INADEQUATE experiment was designed for detection of small couplings between equivalent carbon atoms separated in the molecule by several bonds, where other techniques fail due to rich line splitting and mutual peak cancellation in many molecules. As the proposed method is suitable for detection of couplings in strongly coupled systems in general, we propose the name STRONG INADEQUATE in the paper. Similar to other methods for detection of couplings between equivalent carbons, the STRONG INADEQUATE experiment utilizes one-bond carbon–proton coupling for creation of the effective chemical shift differences. The STRONG INADEQUATE experiment works superbly for ⁿJ_CC, where n ≥ 3. Then the F1 pattern is reduced to a simple antiphase doublet with ⁿJ_CC separation, and this pattern is also preserved when a symmetrical HC···C′H′ system is coupled to other protons. Even in the measurement of ²J_CC couplings, the STRONG INADEQUATE experiment generates a much simpler pattern than the original pulse sequences for measurement of couplings between equivalent carbons.     

Complete analysis of the 1H NMR spectra of acetylated glycals—a conformational study

The complete analysis of the ¹H NMR spectra of the six acetylated glycals as well as that of tetra-O-acetyl-2-hydroxy-D -glucal has been carried out. The conformation of these compounds, as determined from the proton couplings (±0.05Hz) obtained, has been interpreted in terms of a rapid interconversion equilibrium between the two possible dihydropyran half-chair conformations. A computer treatment of all observed couplings has been carried out to obtain optimized values for the populations and couplings characteristic of the two alternative half-chair conformations. The rotamer populations of the acetoxy–methyl group are discussed on the basis of the measured ⁵J₁₆ and ⁵J₃₆ couplings.     

Nonlinear level crossing spectroscopy of highly excited levels in yttrium using stepwise laser excitation

A technique based on the detection of nonlinear level-crossing resonances in finite magnetic fields has been extended to the study of high-lying electronic levels excited by stepwise excitation with two cw dye lasers. The method has been applied to the determination of |A/g _J | values for four highly excited even-parity levels and two odd-parity levels in yttrium I. Using recently determined values of the magnetic hyperfine interaction constantA, Landég _J -factors have been deduced for these levels with an accuracy of about 1%. The results show significant differences from previous determinations.     

3J(PCNH) in phosphorus substituted thioformamides as a configurational probe

Coupling between P and (N)? H has been observed in the ¹H{¹⁴N}NMR spectra of a series of phosphorus substituted thioformamides, R¹₂/P(X)C(S)NHR². For R² = H one of the two couplings constants ³J(PCNH) is much larger than the other. The larger constant is assumed to be ³J(PCNH) (trans) and the magnitude of ³J(PCNH) for several compounds with R² = Me or Ph is used to assign the configuration about the C(S)? N bond.     

Theoretical studies on magnetic interactions between Cu(II) ions in salen nucleobases

The magnetic and other physical properties between Cu²⁺ ions coordinated by salen–base pairs (Cu²⁺–DNA) are examined by using DFT calculations. In order to consider effects of entanglement and dis-entanglement of the double helix chain, three types of structural disorders i.e. distance, rotation angle and discrepancy in XY-plane, are changed in the model dimer structure. All calculated results show that J_ab values are weak anti-ferromagnetic couplings. It is also found that the J_ab values strongly depend on the salen structure.     

A General Method for Extracting Individual Coupling Constants from Crowded 1H NMR Spectra

Couplings between protons, whether scalar or dipolar, provide a wealth of structural information. Unfortunately, the high number of ¹H‐¹H couplings gives rise to complex multiplets and severe overlap in crowded spectra, greatly complicating their measurement. Many different methods exist for disentangling couplings, but none approaches optimum resolution. Here, we present a general new 2D J‐resolved method, PSYCHEDELIC, in which all homonuclear couplings are suppressed in F₂, and only the couplings to chosen spins appear, as simple doublets, in F₁. This approaches the theoretical limit for resolving ¹H‐¹H couplings, with close to natural linewidths and with only chemical shifts in F₂. With the same high sensitivity and spectral purity as the parent PSYCHE pure shift experiment, PSYCHEDELIC offers a robust method for chemists seeking to exploit couplings for structural, conformational, or stereochemical analyses.