Isotope Effects on the 17O, 1H Coupling Constant and the 17O-{1H} Nuclear Overhauser Effect in Water

The NMR spectra of solutions of 30% 17O-enriched H2O and D2O in nitromethane display the resonances of the three isotopomers H2O, HDO, and D2O. All 17O, 1H and 17O, 2H coupling constants and the primary and secondary isotope effects on J(17O, 1H) have been determined. The primary effect is -1.0 +/- 0.2 Hz and the secondary effect is -0.07 +/- 0.04 Hz. Using integrated intensities in the 17O NMR spectra, the equilibrium constant for the reaction H2O + D2O right harpoon over left harpoon 2HDO is found to be 3.68 +/- 0.2 at 343 K. From the relative integrated intensities of proton-coupled and -decoupled spectra the 17O-{1H} NOE is estimated for the first time, resulting in values of 0.908 and 0.945 for H2O and HDO, respectively. This means that dipole-dipole interactions contribute about 2.5% to the overall 17O relaxation rate in H2O dissolved in nitromethane. Copyright 1999 Academic Press.     

A Study of Dipolar Interactions and Dynamic Processes of Water Molecules in Tendon byH andH Homonuclear and Heteronuclear Multiple-Quantum-Filtered NMR Spectroscopy

The effect of proton exchange on the measurement of¹H–¹H,¹H–²H, and²H–²H residual dipolar interactions in water molecules in bovine Achilles tendons was investigated using double-quantum-filtered (DQF) NMR and new pulse sequences based on heteronuclear and homonuclear multiple-quantum filtering (MQF). Derivation of theoretical expressions for these techniques allowed evaluation of the¹H–¹H and¹H–²H residual dipolar interactions and the proton exchange rate at a temperature of 24°C and above, where no dipolar splitting is evident. The values obtained for these parameters at 24°C were 300 and 50 Hz and 3000 s⁻¹, respectively. The results for the residual dipolar interactions were verified by repeating the above measurements at a temperature of 1.5°C, where the spectra of the H₂O molecules were well resolved, so that the¹H–¹H dipolar interaction could be determined directly from the observed splitting. Analysis of the MQF experiments at 1.5°C, where the proton exchange was in the intermediate regime for the¹H–²H dipolar interaction, confirmed the result obtained at 24°C for this interaction. A strong dependence of the intensities of the MQF signals on the proton exchange rate, in the intermediate and the fast exchange regimes, was observed and theoretically interpreted. This leads to the conclusion that the MQF techniques are mostly useful for tissues where the residual dipolar interaction is not significantly smaller than the proton exchange rate. Dependence of the relaxation times and signal intensities of the MQF experiments on the orientation of the tendon with respect to the magnetic field was observed and analyzed. One of the results of the theoretical analysis is that, in the fast exchange regime, the signal decay rates in the MQF experiments as well as in the spin echo or CPMG pulse sequences (T₂) depend on the orientation as the square of the second-rank Legendre polynomial.     

One- and two-dimensional C–H/N–H dipolar correlation experiments under fast magic-angle spinning for determining the peptide dihedral angle φ

A recently proposed ¹³C–¹H recoupling sequence operative under fast magic-angle spinning (MAS) [K. Takegoshi, T. Terao, Solid State Nucl. Magn. Reson. 13 (1999) 203–212.] is applied to observe ¹³C–¹H and ¹⁵N–¹H dipolar powder patterns in the ¹H–¹⁵N–¹³C–¹H system of a peptide bond. Both patterns are correlated by ¹⁵N-to-¹³C cross polarization to observe one- or two-dimensional (1D or 2D) correlation spectra, which can be simulated by using a simple analytical expression to determine the H–N–C–H dihedral angle. The 1D and 2D experiments were applied to N-acetyl[1,2-¹³C,¹⁵N] -valine, and the peptide φ angle was determined with high precision by the 2D experiment to be ±155.0°±1.2°. The positive one is in good agreement with the X-ray value of 154°±5°. The 1D experiment provided the value of φ=±156.0°±0.8°.     

Absolute line intensity measurements in the ν2 bands of HDO and D2O using a tunable diode laser spectrometer

Absolute line intensities for several individual vibration-rotation transitions in the ν₂ bands of HDO and D₂O have been determined at room temperature from laboratory spectra recorded with a tunable diode laser spectrometer. Two tunable semiconductor diode lasers operating in the 1250- to 1350- and 1060- to 1140-cm⁻¹ spectral regions have been used in the recording of the data. Comparisons are made with previously published results where appropriate.     

The Raman spectra and cross-sections of the ν2 band of H2O, D2O, and HDO

We report the experimental Raman spectra of the ν₂ band of H₂O, D₂O, and HDO in the vapor phase at room temperature. A complete interpretation of the Raman intensities is carried out employing the variational rovibrational wavefunctions obtained from a Hamiltonian in Radau coordinates and an ab initio polarizability surface at 514.5 nm. We show the importance of the rotation-vibration coupling to obtain the correct line intensities. Several tables with the assignments of the individual rotational-vibrational transitions and their Raman scattering strengths are reported. From these tables, the ν₂ Raman spectra can be simulated up to 2000 K for H₂O, and up to 300 K for D₂O and HDO.     

Al Chemical Shielding Anisotropy

Accurate values for the²⁷Al chemical shielding anisotropy (CSA) are reported for sapphire (α-Al₂O₃). The values (δ_σ= −17.3 ± 0.6 ppm, η_σ= 0.03 ± 0.06) are obtained from single-crystal²⁷Al NMR and appear to be the first convincing determination of an²⁷Al CSA.     

Detection and Characterization of Boric Acid and Borate Ion Binding to Cytochrome c Using Multiple Quantum Filtered NMR

The application of multiple quantum filtered (MQF) NMR to the identification and characterization of the binding of ligands containing quadrupolar nuclei to proteins is demonstrated. Using relaxation times measured by MQF NMR multiple binding of boric acid and borate ion to ferri and ferrocytochrome c was detected. Borate ion was found to have two different binding sites. One of them was in slow exchange, k_diss = 20 ± 3 s⁻¹ at 5°C and D₂O solution, in agreement with previous findings by ¹H NMR (G. Taler et al., 1998, Inorg. Chim. Acta 273, 388–392). The triple quantum relaxation of the borate in this site was found to be governed by dipolar interaction corresponding to an average B–H distance of 2.06 ± 0.07 Å. Other, fast exchanging sites for borate and boric acid could be detected only by MQF NMR. The binding equilibrium constants at these sites at pH 9.7 were found to be 1800 ± 200 M⁻¹ and 2.6 ± 1.5 M⁻¹ for the borate ion and boric acid, respectively. Thus, detection of binding by MQF NMR proved to be sensitive to fast exchanging ligands as well as to very weak binding that could not be detected using conventional methods.     

Practical aspects of Lee-Goldburg based CRAMPS techniques for high-resolution 1H NMR spectroscopy in solids: implementation and applications

Elucidating the local environment of the hydrogen atoms is an important problem in materials science. Because ¹H spectra in solid-state nuclear magnetic resonance (NMR) suffer from low resolution due to homogeneous broadening, even under magic-angle spinning (MAS), information of chemical interest may only be obtained using certain high-resolution ¹H MAS techniques. ¹H Lee–Goldburg (LG) CRAMPS (Combined Rotation And Multiple-Pulse Spectroscopy) methods are particularly well suited for studying inorganic–organic hybrid materials, rich in ¹H nuclei. However, setting up CRAMPS experiments is time-consuming and not entirely trivial, facts that have discouraged their widespread use by materials scientists. To change this status quo, here we describe and discuss some important aspects of the experimental implementation of CRAMPS techniques based on LG decoupling schemes, such as FSLG (Frequency Switched), and windowed and windowless PMLG (Phase Modulated). In particular, we discuss the influence on the quality of the ¹H NMR spectra of the different parameters at play, for example LG (Lee–Goldburg) pulses, radio-frequency (rf) phase, frequency switching, and pulse imperfections, using glycine and adamantane as model compounds. The efficiency and robustness of the different LG-decoupling schemes is then illustrated on the following materials: organo-phosphorus ligand, N-(phosphonomethyl)iminodiacetic acid [H₄pmida] [I], and inorganic–organic hybrid materials (C₄H₁₂N₂)[Ge₂(pmida)₂OH₂]·4H₂O [II] and (C₂H₅NH₃)[Ti(H_1.5PO₄)(PO₄)]₂·H₂O [III].     

CHHC and H–H magnetization exchange: Analysis by experimental solid-state NMR and 11-spin density-matrix simulations

A protocol is presented for correcting the effect of non-specific cross-polarization in CHHC solid-state MAS NMR experiments, thus allowing the recovery of the ¹H–¹H magnetization exchange functions from the mixing-time dependent buildup of experimental CHHC peak intensity. The presented protocol also incorporates a scaling procedure to take into account the effect of multiplicity of a CH₂ or CH₃ moiety. Experimental CHHC buildup curves are presented for l-tyrosine·HCl samples where either all or only one in 10 molecules are U–¹³C labeled. Good agreement between experiment and 11-spin SPINEVOLUTION simulation (including only isotropic ¹H chemical shifts) is demonstrated for the initial buildup (t_mix < 100 μs) of CHHC peak intensity corresponding to an intramolecular close (2.5 Å) H–H proximity. Differences in the initial CHHC buildup are observed between the one in 10 dilute and 100% samples for cases where there is a close intermolecular H–H proximity in addition to a close intramolecular H–H proximity. For the dilute sample, CHHC cross-peak intensities tended to significantly lower values for long mixing times (500 μs) as compared to the 100% sample. This difference is explained as being due to the dependence of the limiting total magnetization on the ratio N_obs/N_tot between the number of protons that are directly attached to a ¹³C nucleus and hence contribute significantly to the observed ¹³C CHHC NMR signal, and the total number of ¹H spins into the system. ¹H–¹H magnetization exchange curves extracted from CHHC spectra for the 100% l-tyrosine·HCl sample exhibit a clear sensitivity to the root sum squared dipolar coupling, with fast buildup being observed for the shortest intramolecular distances (2.5 Å) and slower, yet observable buildup for the longer intermolecular distances (up to 5 Å).     

Line positions and energy levels of the O substitutions from the HDO/D2O spectra between 5600 and 8800 cm

Absorption spectra of HDO/D₂O mixtures recorded in the 5600-8800 cm⁻¹ region with a total pressure of water from 13 up to 18 hPa and an absorption path length of 600 m have been analyzed in order to obtain new spectroscopic data for HD¹⁸O and D₂¹⁸O. In spite of the low natural ¹⁸O concentration (about 2×10⁻³ with respect to the ¹⁶O one), about 1100 transitions belonging to HD¹⁸O and more than 280 transitions belonging to D₂¹⁸O have been assigned. Most of the D₂¹⁸O transitions belong to the ν₁+ν₂+ν₃ and 2ν₁+ν₃ bands. Sets of energy levels for seven vibrational states of D₂¹⁸O and four states of HD¹⁸O are reported for the first time. The comparison of the experimental data with the calculated values based on Partridge-Schwenke global variational calculations is discussed.     

Line parameters of HDO from high-resolution Fourier transform spectroscopy in the 11 500-23 000 cm spectral region

This work presents new measurements of HDO line parameters in the near-infrared and visible regions (11 500-23 000 cm⁻¹). The measurements consist in high-resolution Fourier transform absorption spectra of H₂O/HDO/D₂O vapor mixtures, obtained using a long absorption path. Spectra with and without nitrogen as the buffer gas were recorded. Due to the simultaneous presence of the three isotopologues H₂O, D₂O, and HDO, the H₂O lines removal and the D₂O lines identification were two necessary preliminary steps to derive the HDO line parameters. The D₂O contribution was small and confined to the well-known 4ν₁ + ν₃ band. An extensive listing of HDO spectroscopic parameters was obtained, for the first time, by fitting some 3256 observed lines to Voigt line profiles. The list contains calibrated line positions, absorption cross-sections and, for many of the lines, N₂-broadening coefficients, as well as N₂-induced frequency shifts. As a result of the low HDO vapor pressures, it was not possible to retrieve the self-broadening parameters. The list is available on the http://www.ulb.ac.be/cpm website.     

Thermal release of trapped deuterium from silica injected by 80-keV D+ ion implantation and RF D2 plasma

When silica is irradiated by 80-keV D⁺ ions or RF plasma of D₂ gas, deuterium is trapped in the silica forming Si-OD bonds. The deuterium, trapped as OD bonds, is desorbed from the silica upon heating to form some release products. The thermal detrapping process corresponds to decrease of OD bonds and was studied by measurement of infrared Fourier transform spectroscopy (FTIR). The release products HDO, D₂O, HD, and D₂ were measured by quadrupole mass spectroscopy (QMS). The detrapping and release processes of trapped deuterium were studied by simultaneous measurement of FTIR and QMS. Since the release spectra of HDO, D₂O, HD, and/or D₂ correspond to the decrease spectra of OD bonds, these release products are formed by thermal decomposition of OD bonds. The formation of water (HDO, D₂O) and hydrogen (HD, D₂) depends upon concentration of pre-existing OH bonds and deuterium injection methods (80-keV D⁺ implantation or RF D₂ plasma irradiation).     

Determination of the Splitting of the 6a0 and 6b0 Bands of C-Hexadeuterobenzene in a Supersonic Jet

By using resonance-enhanced two-photon ionization, rotationally resolved spectra of the 6¹₀ band of ¹²C₆D₆ and (¹³C¹²C₅D₆ molecules have been obtained for the first time at a rotational temperature of 0.7 K in a pulsed supersonic beam. From the former, the values of B″ = 0.1573 ± 0.0008 cm⁻¹, B′ = 0.1508 ± 0.0008 cm⁻¹, and ξ′ = −0.412 ± 0.050 have been derived for rotational and Coriolis constants in the lower and upper levels of ¹²C₆D₆. Also, the spectra corresponding to ¹²C₆H₆ and ¹³C¹²C₅H₆ have been measured and the values B″ = 0.1892 ± 0.0008 cm⁻¹, B′ = 0.1815 ± 0.0008 cm⁻¹, and ξ′ = −0.586 ± 0.050 have been obtained for ¹²C₆H₆, in agreement with previous results. Rotational constants of ¹³C labeled benzene molecules have been geometrically deduced from the constants obtained. Experimental isotopic shifts of the vibronic origins of the 6a¹₀ and 6b¹₀ bands have been determined. There is agreement with previous ¹³C-benzene-h₆ data. The present results are −0.91 ± 0.05 and 3.09 ± 0.05 cm⁻¹ for ¹³C¹²C₅D₆ and −1.64 ± 0.05 and 2.64 ± 0.05 cm⁻¹ for ¹³C¹²C₅H₆. The splittings of vibrational modes 6b and 6a in the ¹B_2u state are 4.00 ± 0.10 cm⁻¹ for ¹³C¹²C₅D₆ and 4.28 ± 0.10 cm⁻¹ for ¹³C¹²C₅H₆.     

Dynamic properties of P4O6S and P4O7: P spin–echo and P MAS–NMR investigations

The rotational dynamics of P₄O₆S and P₄O₇ in the solid state were studied by means of ³¹P NMR spectra of spinning and static powder samples in the temperature range of 153–295 K and 295–388 K, respectively. All spectra were simulated to confirm the type of the motion and to extract the time scales as a function of the temperature. Good agreement between experimental and theoretical data was obtained on the basis of a three-site jump model. For P₄O₆S, the activation energy and the pre-exponential factor derived from the lineshape simulations amount to 51(2) kJ/mol and 6(3)·10¹⁵ s⁻¹. For P₄O₇, the spectral analysis yields an activation energy of 67(1) kJ/mol and a pre-exponential factor of 6(2)·10¹⁴ s⁻¹. The dynamic behavior was checked independently by lineshape analyses under both MAS and static conditions. Activation energies are consistent within the errors for the lineshape analyses. Additionally, we have analyzed spin–lattice relaxation measurements, which show the correct trends for the activation energies.     

Weak water absorption lines around 1.455 and 1.66 μm by CW-CRDS

The absorption spectra of water vapor near 1.455 and 1.66 μm have been recorded with a typical absorption sensitivity of 5 × 10⁻¹⁰ cm⁻¹ by using CW-cavity ring down spectroscopy. A series of 18 distributed feed-back (DFB) lasers was used as sources and allowed for the coverage of the 5911.0-5922.5, 5926-5941.8, 5957.0-6121.6, and 6745-7015.6 cm⁻¹ spectral regions. These regions extend to lower and higher energies our previous study of the water spectrum in the important 1.5 μm transparency window [P. Macko, D. Romanini, S.N. Mikhailenko, O.V. Naumenko, S. Kassi, A. Jenouvrier, Vl.G. Tyuterev, J. Mol. Spectrosc. 227 (2004) 90-108]. The line parameters were determined with the help of an interactive least squares multi-lines fitting program which uses a Voigt function as line profile. More than 1900 water lines with intensities ranging between 10⁻²⁸ and 5 × 10⁻²⁴ cm/molecule at 296 K were measured, about 690 of them being reported for the first time. The rovibrational assignment was performed on the basis of previously determined energy levels and of the results of the variational global calculations [H. Partridge, D.W. Schwenke, J. Chem. Phys. 106 (1997) 4618-4639]. The assignment results were validated by using the Ritz combination principle together with previously reported water transitions. Several new energy levels were determined for the H₂¹⁶O, H₂¹⁷O, and HD¹⁶O isotopologues. The retrieved line lists of the H₂¹⁶O, H₂¹⁷O, H₂¹⁸O, and HD¹⁶O isotopologues are compared with the available calculated and experimental (FTS) databases for water.     

The rotational Raman spectra and cross sections of H2O, D2O, and HDO

We report the experimental rotational Raman spectra of H₂O, and of a mixture of D₂O and HDO in the vapor phase at room temperature, and their interpretation in terms of rotational–vibrational energies, wavefunctions, and transition moments of the molecular polarizability. These transition moments are based on high-level ab initio calculations of the wavelength dependent polarizability surface, and on wavefunctions where the rotational–vibrational coupling is considered in detail. As a byproduct of this analysis several tables have been compiled including scattering strengths and assignments for individual rotational transitions of the three species. From these tables the rotational Raman spectra can be simulated over the range of temperatures up to 2000 K for H₂O, and up to 300 K for D₂O and HDO.     

The Raman spectra and cross-sections of H2O, D2O, and HDO in the OH/OD stretching regions

We report the OH and OD stretching regions of the vapor phase Raman spectra of H₂O, and of a D₂O/HDO mixture, at room temperature. Also, the corresponding spectrum of H₂O at ∼2000 K in a methane/air flame is reported. These spectra are interpreted in terms of transition moments of the molecular polarizability, based on high-level ab initio calculations of the polarizability surface, and on variational wavefunctions considering the rotational-vibrational coupling in full. As a byproduct of this analysis several tables have been compiled including scattering strengths and assignments for individual rotational transitions of the three species. From these tables the Raman spectra in the OH/OD stretching regions can be simulated over the range of temperatures up to 2000 K for H₂O, and up to 300 K for D₂O and HDO.     

Quantitative Evaluation of the Lactate Signal Loss and Its Spatial Dependence in PRESS Localized H NMR Spectroscopy

Localized ¹H NMR spectroscopy using the 90°−t₁−180°−t₁+t₂−180°−t₂−Acq. PRESS sequence can lead to a signal loss for the lactate doublet compared with signals from uncoupled nuclei which is dependent on the choice of t₁ and t₂. The most striking signal loss of up to 78% of the total signal occurs with the symmetrical PRESS sequence (t₁=t₂) at an echo time of 2/J (290 ms). Calculations have shown that this signal loss is related to the pulse angle distributions produced by the two refocusing pulses which leads to the creation of single quantum polarization transfer (PT) as well as to not directly observable states (NDOS) of the lactate AX₃ spin system: zero- and multiple-quantum coherences, and longitudinal spin orders. In addition, the chemical shift dependent voxel displacement (VOD) leads to further signal loss. By calculating the density operator for various of the echo times TE=n/J, n=1, 2, 3, …, we calculated quantitatively the contributions of these effects to the signal loss as well as their spatial distribution. A maximum signal loss of 75% can be expected from theory for the symmetrical PRESS sequence and TE=2/J for Hamming filtered sinc pulses, whereby 47% are due to the creation of NDOS and up to 28% arise from PT. Taking also the VOD effect into account (2 mT/m slice selection gradients, 20-mm slices) leads to 54% signal loss from NDOS and up to 24% from PT, leading to a maximum signal loss of 78%. Using RE-BURP pulses with their more rectangular pulse angle distributions reduces the maximum signal loss to 44%. Experiments at 1.5 T using a lactate solution demonstrated a maximum lactate signal loss for sinc pulses of 82% (52% NDOS, 30% PT) at TE=290 ms using the symmetrical PRESS sequence. The great signal loss and its spatial distribution is of importance for investigations using a symmetrical PRESS sequence at TE=2/J.     

Molecular dynamics and information on possible sites of interaction of intramyocellular metabolites in vivo from resolved dipolar couplings in localized H NMR spectra

Proton NMR resonances of the endogenous metabolites creatine and phosphocreatine ((P)Cr), taurine (Tau), and carnosine (Cs, β-alanyl-l-histidine) were studied with regard to residual dipolar couplings and molecular mobility. We present an analysis of the direct ¹H–¹H interaction that provides information on motional reorientation of subgroups in these molecules in vivo. For this purpose, localized ¹H NMR experiments were performed on m. gastrocnemius of healthy volunteers using a 1.5-T clinical whole-body MR scanner. We evaluated the observable dipolar coupling strength SD₀ (S = order parameter) of the (P)Cr-methyl triplet and the Tau-methylene doublet by means of the apparent line splitting. These were compared to the dipolar coupling strength of the (P)Cr-methylene doublet. In contrast to the aliphatic protons of (P)Cr and Tau, the aromatic H2 (δ = 8 ppm) and H4 (δ = 7 ppm) protons of the imidazole ring of Cs exhibit second-order spectra at 1.5 T. This effect is the consequence of incomplete transition from Zeeman to Paschen-Back regime and allows a determination of SD₀ from H2 and H4 of Cs as an alternative to evaluating the multiplet splitting which can be measured directly in high-resolution ¹H NMR spectra. Experimental data showed striking differences in the mobility of the metabolites when the dipolar coupling constant D₀ (calculated with the internuclear distance known from molecular geometry in the case of complete absence of molecular dynamics and motion) is used for comparison. The aliphatic signals involve very small order parameters S ≈ (1.4 − 3) × 10⁻⁴ indicating rapid reorientation of the corresponding subgroups in these metabolites. In contrast, analysis of the Cs resonances yielded S ≈ (113 − 137) × 10⁻⁴. Thus, the immobilization of the Cs imidazole ring owing to an anisotropic cellular substructure in human m. gastrocnemius is much more effective than for (P)Cr and Tau subgroups. Furthermore, ¹H NMR experiments on aqueous model solutions of histidine and N-acetyl-l-aspartate (NAA) enabled the assignment of an additional signal component at δ = 8 ppm of Cs in vivo to the amide group at the peptide bond. The visibility of this proton could result from hydrogen bonding which would agree with the anticipated stronger motional restriction of Cs. Referring to the observation that all dipolar-coupled multiplets resolved in localized in vivo ¹H NMR spectra of human m. gastrocnemius collapse simultaneously when the fibre structure is tilted towards the magic angle (θ ≈ 55°), a common model for molecular confinement in muscle tissue is proposed on the basis of an interaction of the studied metabolites with myocellular membrane phospholipids.     

Proton conductivity and phase relationship in solid KOH between 248 and 406°C

Existing evidence indicates that between 248°C and the melting point at 406°C, KOH is a rotator phase. We have shown that, as might be expected, this results in enhanced proton conductivity, and a value of 2×10⁻³ ohms⁻¹ cm⁻¹ was found at 350°C, which is the highest reported for proton conducting solid electrolytes in this temperature range. Excess protons are provided by water molecules residing on the normal OH^- sites, and charge compensation is provided by CO²⁻₃ ions in the solid solution of KOH(≈ 1 m/o K₂CO₃, 1.3m/o H₂O). The activation energy for proton hopping between adjacent H₂O and OH⁻ species probably accounts for most of the observed activation energy of 53±3 kJ mol⁻¹. From TGA studies the isobars at 0.05 and 10 Torr were established for KOH-rich compositions in the KOH---H₂O system, and it was shown that the rotator phase of KOH is stable between these vapour pressures.