17O Quadrupole Coupling Constants Research Articles

Solid-State 15N and 17O NMR Studies of S-Nitrosothiols.

We report a solid-state 15N and 17O NMR study of two representative S-nitrosothiols (RSNO): S-nitroso N-acetylpenicillamine (SNAP) and S-nitrosoglutathione (GSNO). The 15N and 17O NMR tensors are experimentally determined for the first time for this important class of nitric-oxide (NO)-related compounds. The observed NMR characteristics for RSNO include large 15N and 17O chemical shift anisotropies and large 17O quadrupole coupling constants. Quantum chemical calculations are also performed for the 15N and 17O NMR tensors in two simple RSNO models: t-BuSNO and MeSNO. On the basis of computational results, we have identified the molecular orbitals that are responsible for the observed large chemical shift anisotropies in RSNO compounds.

A theoretical study on the characteristics of the intermolecular interactions in the active site of human androsterone sulphotransferase: DFT calculations of NQR and NMR parameters and QTAIM analysis

A theoretical study at the level of density functional theory (DFT) was performed to characterize noncovalent intermolecular interactions, especially hydrogen bond interactions, in the active site of enzyme human androsterone sulphotransferase (SULT2A1/ADT). Geometry optimization, interaction energy, 2H, 14N, and 17O electric field gradient (EFG) tensors, 1H, 13C, 17O, and 15N chemical shielding (CS) tensors, Natural Bonding Orbital (NBO) analysis, and quantum theory of atoms in molecules (QTAIM) analysis of this active site were investigated. It was found that androsterone (ADT) is able to form hydrogen bonds with residues Ser80, Ile82, and His99 of the active site. The interaction energy calculations and NBO analysis revealed that the ADT molecule forms the strongest hydrogen bond with Ser80. Results revealed that ADT interacts with the other residues through electrostatic and Van der Waals interactions. Results showed that these hydrogen bonds influence on the calculated 2H, 14N, and 17O quadrupole coupling constants (QCCs), as well as 1H, 13C, 17O, and 15N CS tensors. The magnitude of the QCC and CS changes at each nucleus depends directly on its amount of contribution to the hydrogen bond interaction.

Effective orientation of water in 1,4-dioxane···water: the rotational spectrum of the H217O isotopologue

The measurements of the rotational spectrum of 1,4-dioxane-water complex, performed by pulsed jet Fourier transform microwave spectroscopy, have been extended to the 7–18.5 GHz frequency region and to two additional isotopologues, 1,4-dioxane···H217O and 1,4-dioxane···HOD. The effective orientation of water in the complex has been obtained from the 17O quadrupole coupling constants.

Average orientation of water in CH2F2⋯H2O from the 17O quadrupole effects in the rotational spectrum of CH2F2⋯H217O

The rotational spectrum of the CH2F2⋯H217O adduct has been measured in the 6.5–18.5GHz range using a pulsed jet Fourier-transform microwave spectrometer. The experimental values of the 17O quadrupole coupling constants have been determined to be χaa=−3.853(6), χbb=−3.259(1) and χcc=7.112(9) MHz, respectively. These data correspond to an average value of the angle between the water plane and the FCF plane of about 23°.

Hydrogen Bonds in Cocrystals and Salts of 2-Amino-4,6-dimethylpyrimidine and Carboxylic Acids Studied by Nuclear Quadrupole Resonance

(14)N and (17)O nuclear quadrupole resonance frequencies have been measured in 1:1 cocrystals and salts of 2-amino-4,6-dimethylpyrimidine and several carboxylic acids. A systematic decrease of the (17)O quadrupole coupling constant on increasing strength of the hydrogen bond is observed in cocrystals bound by O-H···N hydrogen bonds. The O-H distances deduced from the line widths of the (17)O NQR lines show that the hydrogen atom is in a hydrogen bond formed by a carboxylic groups for about 0.01 nm displaced from the oxygen atom toward the center of the hydrogen bond. In the O-H···N hydrogen bond formed by the hydroxyl group, which is only slightly longer than the hydrogen bonds formed by the carboxyl group, the hydrogen atom is much less displaced. A linear relation between the (14)N quadrupole coupling constant and the sum of the inverse third powers of the H···A (A = O or N) distances is deduced for the amino group. A linear correlation of the principal values of the (14)N quadrupole coupling tensor in -NH2, as observed in the solid phase and in the gas phase, is analyzed in a simple model assuming a displacement of the electron charge in the N-H σ bond and simultaneous deformation of the nitrogen lone pair electron orbital. At the ring nitrogen position, hydrogen bonding and proton transfer produce a large decrease of the (14)N quadrupole coupling constant. A linear correlation of the principal values of the (14)N quadrupole coupling tensor is observed in cocrystals and salts of 2-amino-4,6-dimethylpyrimidine. This correlation differs from the correlation observed in substituted pyrimidine, where the hydrogen atoms are replaced by other atoms or functional groups. The difference is analyzed in a model, which assumes that the hydrogen bonding and substituents affect the nitrogen lone pair and π electron orbitals. The analysis shows that the two effects are nearly independent.

Solid-state 17O NMR and computational studies of terminal transition metal oxo compounds

We report solid-state NMR characterization of the 17O (I = 5/2) chemical shift and quadrupole coupling tensors in two terminal oxo compounds, 17O≡Ti(IV)(TMP) and 17O≡Cr(IV)(TMP), in which TMP is 5,10,15,20-tetramesitylporphyrin and the oxo ligand is enriched by 17O (ca. 40%). This is the first time that 17O NMR tensors are determined for this important class of compounds. The 17O nuclei in the O≡Ti and O≡Cr triple bonds are found to exhibit very large chemical shift anisotropies but rather small 17O quadrupole coupling constants. Terminal oxo compounds represent one of the rare cases where the atomic nucleus under study simultaneously experiences a highly anisotropic magnetic shielding environment, but a highly symmetrical electric field distribution. The solid-state 17O NMR data are consistent with solid-state 13C, 15N and 31P NMR results on carbido, nitrido and phosphido compounds. The density functional theory (DFT) computations using the zeroth-order regular approximation (ZORA) to account for spin-orbital relativistic effects reproduce the experimental 17O NMR results reasonably well. DFT computations also reveal the origins at the molecular orbital level of the observed 17O NMR properties for terminal oxo compounds.

17 O NQR and 13 C NMR study of hydrogen-bonded organic ferroelectric croconic acid

The 1H17O nuclear quadrupole double resonance spectrum and the 13C CP/MAS NMR spectrum of polycrystalline croconic acid, H2C5O5, have been studied at room temperature. Croconic acid has been recently shown to have the highest switchable spontaneous polarization of all organic ferroelectrics and stays polarized up to the decomposition point at around 450 K. Both the 13C NMR and the 17O nuclear quadrupole resonance (NQR) spectra show that there are five crystallorgaphically non-equivalent carbon and oxygen positions for a given molecule and that therefore the two OH…O bonds are non-equivalent. From the dipolar structure of the 17O quadrupole resonance spectra the OH distance is determined as being 0.099 ± 0.001 nm in both hydrogen bonds. The large 17O quadrupole coupling constant at the COH as well as at the CO…H oxygen position and the short OH distance demonstrate that the OH…O hydrogen bonds are strongly asymmetric. A correlation of the 17OH…O and OH…17O quadrupole coupling constants versus the O…O distance has been observed in several organic acids. The data for croconic acid significantly deviate from this correlation, what may be the result of the strong long range ferroelectric ordering which influences the electron distribution in the hydrogen bonds.

A solid-state 17O NMR, X-ray, and quantum chemical study of N-α-Fmoc-protected amino acids

We report the results of a solid-state 17O NMR and X-ray investigation of 17O-enriched N-α-fluoren-9-ylmethoxycarbonyl- l-alanine (Fmoc- l-ALA) and N-α-fluoren-9-ylmethoxycarbonyl- O- t-butyl- l-serine (Fmoc- l-SER). The present X-ray results for Fmoc- l-SER show that the compound crystallized in the monoclinic space group P2 1 with unit-cell dimensions a = 5.843, b = 11.937, c = 15.042 Å, and β = 96.19°. Analysis of 17O magic-angle spinning (MAS) spectra and stationary NMR spectra recorded at multiple magnetic fields of the present Fmoc-protected amino acids yields the magnitudes of hydroxyl and carbonyl 17O electric-field-gradient (EFG) and chemical shielding (CS) tensors with the relative orientations between the two NMR tensors. The 17O quadrupole coupling constants ( C Q) are found to be 7.05–7.60 MHz and 7.90–8.35 MHz, and the spans of the CS tensors are 218–236 ppm and 450–521 ppm, for hydroxyl and carbonyl oxygen atoms, respectively. We also carry out quantum chemical calculations using density functional theory in order to investigate the effects of hydrogen-bond angles on 17O NMR parameters. It is demonstrated that, in addition to the hydrogen bond distances, hydrogen bond angles are an important factor in determining the magnitudes of these tensor components.

O 17 nuclear quadrupole coupling constants of water bound to a metal ion: A gadolinium(III) case study

Rotational correlation times of metal ion aqua complexes can be determined from 17O NMR relaxation rates if the quadrupole coupling constant of the bound water oxygen-17 nucleus is known. The rotational correlation time is an important parameter for the efficiency of Gd3+ complexes as magnetic resonance imaging contrast agents. Using a combination of density functional theory with classical and Car-Parrinello molecular dynamics simulations we performed a computational study of the 17O quadrupole coupling constants in model aqua ions and the [Gd(DOTA)(H2O)]- complex used in clinical diagnostics. For the inner sphere water molecule in the [Gd(DOTA)(H2O)]- complex the determined quadrupole coupling parameter chi square root of (1 + eta2/3) of 8.7 MHz is very similar to that of the liquid water (9.0 MHz). Very close values were also predicted for the the homoleptic aqua ions of Gd3+ and Ca2+. We conclude that the 17O quadrupole coupling parameters of water molecules coordinated to closed shell and lanthanide metal ions are similar to water molecules in the liquid state.

Influence of N−H···O and C−H···O Hydrogen Bonds on the 17O NMR Tensors in Crystalline Uracil: Computational Study

We report a computational study for the 17O NMR tensors (electric field gradient and chemical shielding tensors) in crystalline uracil. We found that N-H...O and C-H...O hydrogen bonds around the uracil molecule in the crystal lattice have quite different influences on the 17O NMR tensors for the two C=O groups. The computed 17O NMR tensors on O4, which is involved in two strong N-H...O hydrogen bonds, show remarkable sensitivity toward the choice of cluster model, whereas the 17O NMR tensors on O2, which is involved in two weak C-H...O hydrogen bonds, show much smaller improvement when the cluster model includes the C-H...O hydrogen bonds. Our results demonstrate that it is important to have accurate hydrogen atom positions in the molecular models used for 17O NMR tensor calculations. In the absence of low-temperature neutron diffraction data, an effective way to generate reliable hydrogen atom positions in the molecular cluster model is to employ partial geometry optimization for hydrogen atom positions using a cluster model that includes all neighboring hydrogen-bonded molecules. Using an optimized seven-molecule model (a total of 84 atoms), we were able to reproduce the experimental 17O NMR tensors to a reasonably good degree of accuracy. However, we also found that the accuracy for the calculated 17O NMR tensors at O2 is not as good as that found for the corresponding tensors at O4. In particular, at the B3LYP/6-311++G(d,p) level of theory, the individual 17O chemical shielding tensor components differ by less than 10 and 30 ppm from the experimental values for O4 and O2, respectively. For the 17O quadrupole coupling constant, the calculated values differ by 0.30 and 0.87 MHz from the experimental values for O4 and O2, respectively.

A Combined Experimental and Theoretical 17O NMR Study of Crystalline Urea: An Example of Large Hydrogen-bonding Effects

We report the first experimental determination of the carbonyl 17O electric-field-gradient (EFG) tensor and chemical-shift (CS) tensor of a urea-type functional group, R1NHC(O)NHR2. Analysis of magic-angle spinning (MAS) and stationary 17O NMR spectra of crystalline [17O]urea yields not only the principal components of the carbonyl 17O EFG and CS tensors, but also their relative orientations. The carbonyl 17O quadrupole coupling constant (QCC) and the asymmetry parameter (η) in crystalline urea were found to be 7.24 ± 0.01 MHz and 0.92, respectively. The principal components of the 17O CS tensor were determined: δ11 = 300 ± 5, δ22 = 280 ± 5 and δ33 = 20 ± 5 ppm. The direction with the least shielding, δ11, is perpendicular to the CO bond and the principal component corresponding to the largest shielding, δ33, is perpendicular to the NC(O)N plane. The observed 17O CS tensor suggests that, in crystalline urea, the 17O paramagnetic shielding contributions from the σ → π* and π → σ* mixing are greater than t...

Density functional theory and Hartree–Fock-density functional theory calculations of 17O, 33S, and 73Ge quadrupole coupling constants

The performance of several density functional theory and Hartree–Fock density functional theory methods in conjunction with Pople type bases for the calculation of 17O, 33S, and 73Ge quadrupole coupling constants in gaseous state molecules was investigated. Assessment of the several models was made by linear regression analysis of the calculated gradient of the molecular electric field versus the experimental nuclear quadrupole coupling constant (NQCCs). Calculations for oxygen on six molecules with the B3LYP/6-311++G(3df,3p) model yield a residual standard deviation of 0.057 MHz (1.4%); for sulfur on 12 molecules with the B3LYP/6-311G(3df,3p) model, 0.42 MHz (1.8%); and for germanium on nine molecules with the B3P86/6-311G(2d) model, 0.83 MHz (1.0%). In the case of germyl acetylene, our calculations indicate that the experimental NQCC reported some time ago by Thomas and Laurie [J. Chem. Phys. 44 (1966) 2602] was incorrectly assigned with respect to algebraic sign. Predictions are made of the 17O and 33S NQCCs in furan, 4H-pyran-4-one, 4H-pyran-4-thione, and 4H-thiapyran-4-thione; and of the 73Ge NQCC in germyl bromide.

Factors influencing the 17O quadrupole coupling constant in bridging oxygen environments

Ab initio calculations were performed on the series of clusters [(OH) 3MOM(OH) 3] −2 where M=B, Al, and Ga, (OH) 3MOM(OH) 3 where M=C, Si, and Ge, (OH) 2OMOMO(OH) 2 where M=P and As, and (OH)O 2MOMO 2(OH) where M=S and Se. These clusters were constructed to model the effect of changing the coordinating cation and coordinating cation–oxygen distances on the bridging oxygen 17O quadrupole coupling constant. For all clusters studied, we observed a general trend that the magnitude of C q increases linearly with increasing cation–oxygen bond distance and cation group number, suggesting that group number and cation-bridging oxygen distance can serve as a better predictor of the bridging oxygen quadrupole coupling constant than electronegativity differences.

On the density dependence of the rotational dynamics of carbon dioxide and its 17O quadrupole coupling constant

17 O quadrupole relaxation rates 1/T1 of carbon dioxide are measured as a function of the fluid density at T=25 and 46°C. In order to derive both the correlation time τ2 for the reorientational motion of the CO2 molecule and its dependence on the fluid density, we combine these experimental data with ab initio calculations of the nuclear quadrupole coupling constant CQF and its density dependence. These calculations show that the coupling is constant over the range of the experimental phase points and that the asymmetry parameter can be neglected. The resulting correlation times τ2 are compared with those calculated from viscosity data via the Einstein–Debye equation and with those obtained from the extended diffusion (J-diffusion) model. The models give in limited density ranges only a qualitative description of the experimentally found behaviour.

High-resolution oxygen-17 NMR spectroscopy of solids by multiple-quantum magic-angle-spinning

High-resolution solid state 17O NMR spectra of 17O-enriched compounds were observed with multiple-quantum magic-angle-spinning (MQMAS) experiments. The resolution of the 17O MQMAS spectra is approximately 30- to 150-fold higher than that found in conventional 17O MAS spectra, making it possible to detect crystallographically distinct oxygen sites. It is shown that 17O MQMAS studies are feasible for systems with 17O quadrupole coupling constants up to 7 MHz, provided that sufficiently high radio-frequency field strengths (> 120 kHz and spinning speeds (∼ 20 kHz) can be achieved.

Rovibrational dependence of the nuclear quadrupole coupling constants of HF, OH − and NeH +

The 2H, 17O and 21Ne quadrupole coupling constants of rovibrational levels of all quadrupolar isotopomers of HF (X 1 Σ +), OH − (X 1 Σ +) and NeH + (X 1 Σ +) are calculated from molecular wavefunctions which explicitly describe nuclear motion. The strength of the 17O and 21Ne coupling is predicted to be more than one and two orders of magnitude, respectively, larger than that of the deuteron coupling. The vibrational dependence of the nuclear quadrupole coupling constants is found to be quite significant for all isotopomers studied. The change with rotational excitation is unimportant. With the exception of the high- ν levels, the strength of the 17O coupling in OH − increases with vibrational excitation. The quadrupole coupling in 21NeH + is stronger than in any other diatomic first-row hydrides.

The Microwave Spectrum of Carbon Dioxide 17OCO and 18OCO

The J = 1-0 ground state pure rotational transitions of monosubstituted carbon dioxide-17O (17OCO) and -18O (18OCO) isotopomers have been investigated using a waveguide Fourier transform microwave spectrometer especially designed for measurement of very weak signals. Rotational constants could be obtained for 17OCO, B0 = 11 350.539(5) MHz and for 18OCO, B0 = 11 037.887(4) MHz. The 17O quadrupole coupling constant has been determined as −3.92(9) MHz and has been compared with results from quantum chemical calculations.

Nuclear magnetic resonance relaxation, permittivity, viscosity and ultrasonic velocity measurements in binary mixtures of methanol and tetrahydrofuran

Proton NMR relaxation rates of CD3OH and proton and oxygen-17 NMR relaxation rates of CD317OH with 6, 11.7 and 26.8%17O enrichment have been measured in binary mixtures with deuteriated tetrahydrofuran ([2H]THF) containing 10, 20, 30, 50, 60, 75, 85 and 95 mol% methanol at 288, 298 and 308 K. Deuteron NMR relaxation rates of CH3OD have also been measured in tetrahydrofuran (THF) at the same compositions and temperatures. From the measured proton relaxation rates, the 17O-induced intramolecular proton relaxation rates have been extracted. Using these values, the rotational correlation time (τOH2) of the OH bond of methanol has been determined at various compositions. The variation of τOH2 with composition indicates that the hydrogen bonding (self-association) of methanol begins to break up after the addition of ca. 25 mol%[2H]THF and at >40 mol%[2H]THF the structure breaking of methanol becomes much more pronounced. Negative values of the excess isentropic compressibility (KEs), excess permittivity (IµE), excess viscosity (ηE) and excess volume (VE) of CH3OH–THF mixtures over the whole composition range also indicate strong structure breaking, especially in the range 30–60 mol% THF, where all these excess parameters pass through a minimum. Proton NMR measurements also confirm that structure breaking of CH3OH by THF is stronger at >40 mol% THF. The deuteron and 17O quadrupole coupling constants of methanol in various CH3OD–THF and CD317OH–[2H]THF mixtures have been evaluated and a strong variation of the deuteron quadrupole coupling constant with composition and temperature as compared to that of 17O is observed.

The rotational spectrum of the isotopically substituted van der Waals complex ArOCS, obtained using a pulsed beam microwave Fourier transform spectrometer

A sensitive new pulsed beam cavity microwave Fourier transform spectrometer has been constructed. It has been used to measure for the first time the pure rotational spectra of the van der Waals complexes ArO 13CS, Ar 18OCS and Ar 17OCS, and to measure further transitions of ArOC 34S. Rotational and centrifugal distortion constants have been determined for all species, as well as 17O quadrupole coupling constants in Ar 17OCS. A harmonic force field has been obtained for the van der Walls motions, and has been used to reproduce the variations of distortion constants with isotopomer. Both substitution and ground state average structures have been evaluated. It is confirmed that the complex is a loose binding of Ar and OCS, with little change in the structural parameters of OCS from those of the free molecule.

Determination of dynamic parameters in amino acids from 17O NMR line width measurements

AbstractThe viscosity and temperature dependence of the 17O NMR line width of glycine, alanine, proline, leucine, histidine and phenylalanine were investigated at pH 2, 7 and 12.5. The observed viscosity/temperature (η/T) dependence of the reorientation correlation time, τc, is compared with that given by hydrodynamic models. It was found that for amino acids the Gierer–Wirtz model is superior to the Stokes–Einstein–Debye and Hu–Zwanzig models. The 17O quadrupole coupling constant for glycine, alanine and histidine was determined in water solution; the activation energies for molecular tumbling were evaluated, and the hydration of the amino acids in their cationic zwitterionic and anionic ionization states is discussed.