17O Quadrupole Coupling Tensors Research Articles

Unusual Electron Charge Density in Carboxylic Acid. 17O Quadrupole Coupling in cis-Cyclobutane-1,2-dicarboxylic Acid

The (17)O NQR frequencies have been measured in cis-cyclobutane-1,2-dicarboxylic acid and the quadrupole coupling tensors have been determined at various temperatures. Two O···H oxygen positions and two O-H oxygen positions are observed, showing the presence of two different types of O-H···O hydrogen bonds in the unit cell. The quadrupole coupling constants at the O-H oxygen positions are approximately 30% lower than the lowest quadrupole coupling constants experimentally observed at the C-O-H positions in other carboxylic acids with either ordered or disordered hydrogen bonds. The O-H distances have been calculated from the (17)O-(1)H dipole-dipole interaction at the O-H oxygen positions. The obtained values are longer than the O-H distances usually found in O-H···O hydrogen bonds with comparable O···O distance, in agreement with the proposed proton exchange O-H···O ↔ O···H-O, which partially averages the dipole-dipole interaction. The energy difference of the two proton configurations, O-H···O and O···H-O, is calculated from the O-H distances determined by NQR. The temperature dependence of the (17)O quadrupole coupling tensors at the (17)O···H-O oxygen positions is analyzed in the model of proton exchange and the energy differences of the two proton configurations obtained by this analysis agree with the values obtained from the O-H distances. The quadrupole coupling tensors are analyzed in a model based on the Townes and Dailey model. The model shows that the population of an oxygen lone pair orbital is at this oxygen position reduced from 2 to approximately 1.3. The electron electric charge is most probably transferred to the oxygen σ and π electron orbitals. This may be associated with the structure of the cyclobutane ring, where the X-ray data show the presence of two unusually short C-C bonds.

A theoretical study of repeating sequence in HRP II: A combination of molecular dynamics simulations and 17O quadrupole coupling tensors

Histidine rich protein II derived peptide (HRP II 169–182) was investigated by molecular dynamics, MD, simulation and 17O electric field gradient, EFG, tensor calculations. MD simulation was performed in water at 300 K with α-helix initial structure. It was found that peptide loses its initial α-helix structure rapidly and is converted to random coil and bent secondary structures. To understand how peptide structure affects EFG tensors, initial structure and final conformations resulting from MD simulations were used to calculate 17O EFG tensors of backbone carbonyl oxygens. Calculations were performed using B3LYP method and 6–31 + G⁎ basis set. Calculated 17O EFG tensors were used to evaluate quadrupole coupling constants, QCC, and asymmetry parameters, η Q . Difference between the calculated QCC and η Q values revealed how hydrogen-bonding interactions affect EFG tensors at the sites of each oxygen nucleus.

14N and 17O electric field gradient tensors in benzamide clusters: Theoretical evidence for cooperative and electronic delocalization effects in N–H⋯O hydrogen bonding

Nuclear quadrupole coupling tensors of 14N and 17O nuclei in benzamide clusters (up to n = 6) were calculated via density functional theory. Results revealed that N–H⋯O hydrogen bonds around the benzamide molecule in crystalline lattice have significant influence on 14N and 17O nuclear quadrupole coupling tensors. n-dependent trend in 14N quadrupole coupling tensors significantly correlates with cooperative effects of R(N–H⋯O) hydrogen bond distance. Natural bonding orbital analysis, NBO, was used to rationalize the quadrupole coupling results in terms of substantial n O → σ N – H ∗ charge transfer in (benzamide) n=1−6 clusters. It was found that intermolecular n O → σ N – H ∗ interactions play a key role in cooperative change of 14N and 17O quadrupole coupling tensors. There is an acceptable linear correlation between 14N or 17O quadrupole coupling tensors with strength of Fock matrix elements (Fij). This suggests that both 14N and 17O quadrupole coupling measurements can provide a useful probe for electron delocalization effects in both gaseous and condensed media.

17O NMR and crystalline hydrates

17O NMR is complementary to 2H NMR for studies of hydrate structure and dynamics in solids, insofar as the quadrupole coupling parameters can be accurately determined. At a Larmor frequency of 34.2 MHz, the effects of a large 17O quadrupole coupling ( e 2qQ h ≈ 7 MHz ) on both the orientational dependence of the central transition resonance frequency and its observation by cross-polarization are examined in powders and a single crystal of oxalic acid dihydrate. Comparison with the exact energies shows that the variation of the central transition frequency with tensor orientation is accurately described as a second-order perturbation, even when e 2qQ h and the Larmor frequency are comparable. The combined chemical and quadrupole-induced shifts of the central transition frequency are separated by their distinct transformation properties, and we report the H 2 17O quadrupole coupling tensor in α-oxalic acid dihydrate, which is compared with the hydrogen bonding structure. Excitation of the central transition by cross-polarization is examined in terms of the 17O nutation frequency. Distortions in powder patterns and the loss of lines in single-crystal spectra owing to the orientational dependence of the first-order quadrupole coupling are verified, and simple methods for mitigating the problem in single-crystal and powder experiments are presented.

Oxygen‐17 quadrupole coupling in liquid acetone

Abstract17O NMR spectroscopy of liquid acetone, oriented in a strong applied electric field, was used to measure one of the principal components of the 17O quadrupole coupling tensor (qct). The component along the carbonyl bond was found to be (±)3.0±0.2 MHz. By comparison with other data in the literature concerning acetone and related molecules, it can be concluded that this value refers to the smallest of the principal components. The proposed NMR method is a powerful technique for obtaining information about qcts in normal liquids.

17O quadrupole coupling and nuclear magnetic shielding tensors in benzophenone

The 17O quadrupole-coupling tensor in a single crystal of benzophenone was completely determined using high-field NMR, with B 0 of 8.4 T. The sample crystals were 50% 17O enriched; the carbon in the carbonyl position was 90% 13C. All possible transitions of the 17O nucleus were observed. The 17O quadrupole-coupling tensor (QCT) was determined from the ± 5 2 ↔ ± 3 2 and the ± 3 2 ↔ ± 1 2 quadrupole splittings. The quadrupolecoupling constant and asymmetry parameter are ezgQih = 10.808 MHz and η = 0.369. The principal axis system of the QCT conforms to the local C 2 v symmetry of the carbonyl group. The fine structure of the 17O transitions, caused by 17O 13C coupling, allowed determination of the 17O 13C coupling tensor. The resulting CO distance r = 1.213Å agrees well with the X-ray data. By taking into account the second-order quadrupole shifts the 17O chemical-shift tensor could be determined from the ± 1 2 ↔ − 1 2 trasitions. The principal axis that corresponds to the largest paramagnetic shift is parallel to the CO bond. The largest diamagnetic shift is perpendicular to the-CO plane. These findings are in good agreement with a Hartree-Fock calculation on the related compound formaldehyde.

The first oxygen-17 N.M.R. spectrum of a polar liquid aligned by an electric field

From the 2H and 17O N.M.R. spectra of liquid nitromethane, partially oriented by an external electric field, the component of the 17O quadrupole coupling tensor along the molecular dipole moment direction could be obtained: 9·1 ± 0·4 MHz. For 14N the analogous component was found to be 1·75 ± 0·05 MHz.

17O and 2H quadrupole double resonance in some acid carboxylates

An 17O and 2H quadrupole double resonance study has been made of a number of strong O—H—O hydrogen bonds in acid carboxylates and related compounds and an analysis of the 17O⋯1H dipolar structure in one (potassium hydrogen maleate). The compounds selected show a range of hydrogen-bond structures varying from symmetric and centred to asymmetric and non-centred; with certain assumptions, it is possible to follow the consequent changes in sign and orientation of the 17O quadrupole coupling tensor. The arguments are based on the proposal that the 17O quadrupole coupling constant is negative in symmetric hydrogen bonds, its orientation being related to that of the 17O tensor in the C—17OH group in carboxylic acids by a rotation of 25 ° with respect to the O—H direction. The behaviour of the 2H quadrupole coupling constant and asymmetry parameter in strong O—H—O hydrogen bonds is discussed in terms of the position of the hydrogen atom.

17O and 2H quadrupole double resonance in some carboxylic acid dimers

17 O quadrupole coupling constants and asymmetry parameters for the C17O and C—17OH nuclei in benzoic acid, eleven substituted derivatives and β-oxalic acid have been determined at 291 K by double resonance techniques together with the 2H values for the hydrogen-bonding deuteron. Many of these compounds form planar, or nearly planar, cyclic hydrogen-bonded units in the solid state, and the maximum principal components of the 17O quadrupole coupling tensor, e2qzzQ/h, lie probably in this plane, with the minimum component, e2qxxQ/h, perpendicular to the plane. The changes in these two components, Δqxx and Δqzz, due to substituents display an almost linear relationship to each other with a ratio Δqzz/Δqxx of ca.–0.5 for C—17OH. This relationship can be understood if the changes in the 17O quadrupole coupling constants are predominantly due to variations in the pπ oxygen populations, with smaller but proportional changes in pσ. Such changes can be brought about by a variety of effects, including molecular vibration, and their special effects on the C—17OH and C17O quadrupole coupling tensors may be related to the fixed geometry which these tensors tend to assume in the bonding system.