Piotr Garbacz

Nuclear Magnetic Shielding for Hydrogen in Selected Isolated Molecules

We present the results of gas-phase NMR measurements designed to yield a new experimental value for the absolute (1)H magnetic shielding for an isolated hydrogen molecule and its deuterium isotopomers. The results are based on the original method of direct shielding measurements (Jackowski et al., 2010) and the density dependence of (1)H, (2)H, and (3)He NMR frequencies for molecular hydrogen and atomic helium-3. The absolute isotropic magnetic shielding measured for molecular hydrogen, σ(0)(H(2)), is 26.293(5) ppm at 300 K, within experimental error of previous measurements based on spin-rotation data and quantum chemistry computations, 26.289(2) ppm (Sundholm and Gauss, 1997), and recent ab initio calculations. We also report isotope effects in shielding for H(2), HD, and D(2) molecules that are consistent with theoretical predictions. In addition, gas-phase (1)H chemical shifts extrapolated to zero density have been measured for numerous small molecules. Our results yield precise absolute shielding data that will be useful in establishing benchmark computational chemistry methods for calculating rovibrational averaged magnetic shielding.

Ground and Excited State Double Hydrogen Transfer in Symmetric and Asymmetric Potentials: Comparison of 2,7,12,17-Tetra-n-propylporphycene with 9-Acetoxy-2,7,12,17-tetra-n-propylporphycene

Analysis of time-resolved anisotropy of transient absorption enabled determination of room temperature ground and excited state rate constants for intramolecular double hydrogen transfer in two similar porphycenes, one of them with symmetric and the other, with asymmetric character of a double minimum potential for hydrogen motion. The perturbation preserves a quasi-symmetric minimum in S(0), but the rate decreases approximately two times. In S(1), the perturbed potential becomes strongly asymmetric, and the downhill hydrogen transfer occurs with a rate higher than that observed for a symmetrical compound.

On variation in Faroese verb placement

In this paper, we present and discuss results from an investigation of verb placement in modern Faroese in which we collected data from speakers from a number of different dialect areas in the Faroe Islands. Altogether we interviewed 54 informants, aged 15–67, 29 women and 25 men. Therefore, our study not only investigates the geographical variation claimed to be present in Faroese with respect to verb placement, but also gender and age variation. O ur results indicate that verb movement in non-V2 contexts is no longer commonly available to the speakers of Faroese. However, our results also suggest that verb movement across adverbs like often and already tends to be more acceptable than movement across negation and other adverbs like never and undoubtedly . Furthermore, movement of finite auxiliaries generally receives a slightly higher score than movement of finite main verbs. Our results do not show any differences in the judgements of speakers below and above the age of 25. Contrary to Jonas (1996), we find that speakers in the North are, if anything, more likely to accept verb movement than speakers in the South.

Spin-rotation and NMR shielding constants in HCl

The spin-rotation and nuclear magnetic shielding constants are analysed for both nuclei in the HCl molecule. Nonrelativistic ab initio calculations at the CCSD(T) level of approximation show that it is essential to include relativistic effects to obtain spin-rotation constants consistent with accurate experimental data. Our best estimates for the spin-rotation constants of (1)H(35)Cl are CCl = -53.914 kHz and C(H) = 42.672 kHz (for the lowest rovibrational level). For the chlorine shielding constant, the ab initio value computed including the relativistic corrections, σ(Cl) = 976.202 ppm, provides a new absolute shielding scale; for hydrogen we find σ(H) = 31.403 ppm (both at 300 K). Combining the theoretical results with our new gas-phase NMR experimental data allows us to improve the accuracy of the magnetic dipole moments of both chlorine isotopes. For the hydrogen shielding constant, including relativistic effects yields better agreement between experimental and computed values.

Weak intermolecular interactions in gas-phase nuclear magnetic resonance

Gas-phase NMR spectra demonstrating the effect of weak intermolecular forces on the NMR shielding constants of the interacting species are reported. We analyse the interaction of the molecular hydrogen isotopomers with He, Ne, and Ar, and the interaction in the He–CO2 dimer. The same effects are studied for all these systems in the ab initio calculations. The comparison of the experimental and computed shielding constants is shown to depend strongly on the treatment of the bulk susceptibility effects, which determine in practice the pressure dependence of the experimental values. Best agreement of the results is obtained when the bulk susceptibility correction in rare gas solvents is evaluated from the analysis of the He-rare gas interactions, and when the shielding of deuterium in D2–rare gas systems is considered.Gas-phase nuclear magnetic resonance (NMR) spectra demonstrating the effect of weak intermolecular forces on the NMR shielding constants of the interacting species are reported. We analyse the interaction of the molecular hydrogen isotopomers with He, Ne, and Ar, and the interaction in the He-CO(2) dimer. The same effects are studied for all these systems in the ab initio calculations. The comparison of the experimental and computed shielding constants is shown to depend strongly on the treatment of the bulk susceptibility effects, which determine in practice the pressure dependence of the experimental values. Best agreement of the results is obtained when the bulk susceptibility correction in rare gas solvents is evaluated from the analysis of the He-rare gas interactions, and when the shielding of deuterium in D(2)-rare gas systems is considered.

The NMR indirect nuclear spin–spin coupling constant of the HD molecule

We present new calculated and experimental values of the NMR indirect nuclear spin–spin coupling constant in HD. In the quantum-chemical ab initio calculations, the full configuration-interaction (FCI) method is used, yielding an equilibrium value of 41.22 Hz in the basis-set limit. Adding a calculated zero-point vibrational correction of 1.89 Hz and a temperature correction of 0.20 Hz at 300 K, we obtain a total calculated spin–spin coupling constant of J FCI(HD) = 43.31(5) Hz at 300 K. This result is within the error bars of the experimental gas-phase NMR value, J exp(HD) = 43.26(6) Hz, obtained by extrapolating values measured in HD–He mixtures to zero density.

1H NMR diffusion studies of water self-diffusion in supercooled aqueous sodium chloride solutions

The physical properties of aqueous sodium chloride solutions have been studied theoretically, but so far no experimental diffusion data have been obtained under supercooled conditions. Here the results of (1)H NMR translational diffusion measurements of water in sodium chloride solutions in the temperature range 230 to 300 K and sodium chloride concentrations up to 4.2 mol/kg are presented. It was found that the diffusion data were well-described by the Vogel-Tamman-Fulcher relationship with concentration-dependent parameters D0, B, and T0. The results indicate that under supercooled conditions the influence of sodium chloride on water diffusion is much smaller than predicted by molecular dynamics simulations.

Experimental characterization of the hydride 1H shielding tensors for HIrX2(PR3)2 and HRhCl2(PR3)2: extremely shielded hydride protons with unusually large magnetic shielding anisotropies.

The hydride proton magnetic shielding tensors for a series of iridium(III) and rhodium(III) complexes are determined. Although it has long been known that hydridic protons for transition-metal hydrides are often extremely shielded, this is the first experimental determination of the shielding tensors for such complexes. Isolating the (1)H NMR signal for a hydride proton requires careful experimental strategies because the spectra are generally dominated by ligand (1)H signals. We show that this can be accomplished for complexes containing as many as 66 ligand protons by substituting the latter with deuterium and by using hyperbolic secant pulses to selectively irradiate the hydride proton signal. We also demonstrate that the quality of the results is improved by performing experiments at the highest practical magnetic field (21.14 T for the work presented here). The hydride protons for iridium hydride complexes HIrX2(PR3)2 (X = Cl, Br, or I; R = isopropyl, cyclohexyl) are highly shielded with isotropic chemical shifts of approximately -50 ppm and are also highly anisotropic, with spans (=δ11 - δ33) ranging from 85.1 to 110.7 ppm. The hydridic protons for related rhodium complexes HRhCl2(PR3)2 also have unusual magnetic shielding properties with chemical shifts and spans of approximately -32 and 85 ppm, respectively. Relativistic density functional theory computations were performed to determine the orientation of the principal components of the hydride proton shielding tensors and to provide insights into the origin of these highly anisotropic shielding tensors. The results of our computations agree well with experiment, and our conclusions concerning the importance of relativistic effects support those recently reported by Kaupp and co-workers.

[Reveiw of:] John D. Sundquist. Morphosyntactic Change in the History of the Mainland Scandinavian Languages

[Reveiw of:] John D. Sundquist. Morphosyntactic Change in the History of the Mainland Scandinavian Languages

Indirect Spin-Spin Coupling Constants in the Hydrogen Isotopologues.

The results of experimental and theoretical studies of indirect spin-spin coupling constants for hydrogen deuteride (HD), hydrogen tritide (HT), and deuterium tritide (DT) are described. The reduced coupling constants obtained from the gas-phase NMR (nuclear magnetic resonance) experiment conducted at 300 K are 2.338(1), 2.334(3), and 2.316(1) × 10(20) T(2) J(-1), while the ab initio values computed at the full configuration interaction level of theory equal 2.349(3), 2.343(3), and 2.322(3) × 10(20) T(2) J(-1) for HD, HT, and DT, respectively. The agreement of the experimental and theoretical results is improved when proper treatment of the influence of nuclear relaxation on the NMR spectrum is applied. However, there is a minor discrepancy between experiment and theory, exceeding the estimated error bars; potential sources of this discrepancy are discussed.