Włodzimierz Makulski

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.

Density-dependent 17O magnetic shielding in the gas phase

Abstract Density-dependent 17 O magnetic shielding has been measured for the first time. For pure CO, CO 2 , OCS and N 2 O gases at 300 K an increase in density linearly diminishes the oxygen shielding constants. It permits to separate quantitatively the shielding contributions due to intermolecular interactions ( σ 1 ) and the shielding parameters of isolated molecules ( σ 0 ). The new experimental results may be used for the better verification of ab initio calculations of oxygen shielding.

NMR shielding constants in PH3, absolute shielding scale, and the nuclear magnetic moment of 31P.

Ab initio values of the absolute shielding constants of phosphorus and hydrogen in PH(3) were determined, and their accuracy is discussed. In particular, we analyzed the relativistic corrections to nuclear magnetic resonance (NMR) shielding constants, comparing the constants computed using the four-component Dirac-Hartree-Fock approach, the four-component density functional theory (DFT), and the Breit-Pauli perturbation theory (BPPT) with nonrelativistic Hartree-Fock or DFT reference functions. For the equilibrium geometry, we obtained σ(P) = 624.309 ppm and σ(H) = 29.761 ppm. Resonance frequencies of both nuclei were measured in gas-phase NMR experiments, and the results were extrapolated to zero density to provide the frequency ratio for an isolated PH(3) molecule. This ratio, together with the computed shielding constants, was used to determine a new value of the nuclear magnetic dipole moment of (31)P: μ(P) = 1.1309246(50) μ(N).

NMR shielding constants in BF3 and magnetic dipole moments of B11 and B10 nuclei

Gas-phase NMR spectra of (11)B, (10)B, and (19)F in BF(3) are reported, and high-level ab initio calculations of the corresponding NMR shielding constants are described. Extrapolation of the measured resonance frequencies to the zero-density limit ensures that the results correspond to the ab initio values for an isolated molecule. Simultaneous measurements of (3)He resonance frequencies and application of the calculated shielding constants allow us to determine improved values of the nuclear magnetic dipole moments of (11)B and (10)B. The magnetic moments of both isotopes are also determined independently by comparing with the (19)F spectral parameters (frequencies and shielding constants). The separately derived nuclear magnetic moments are in good agreement, whereas the literature moments of both (11)B and (10)B are noticeably less accurate.

The 17O nuclear magnetic shielding scale from gas-phase measurements

Abstract 17 O NMR chemical shifts have been measured for 23 liquid chemical compounds and their vapors at natural abundance of oxygen-17 and the temperature of 333 K. In one case the gas-phase measurements were performed over the range of density to show the density-dependent 17 O magnetic shielding in methyl ether and its deuterated analog. With a recently reported evaluation of the absolute oxygen magnetic shielding for 12 C 17 O and some previous 17 O NMR data for gaseous compounds the present results permit one to update the reliable shielding scale for the oxygen-17 nucleus. This scale can directly be used for any verification of ab initio calculations of 17 O magnetic shielding. It is also shown that intermolecular effects are rather large and variable in the oxygen shielding. The appropriate gas-to-liquid shifts are positive (shielding effect) for the carbonyl oxygen nuclei and rather negative (deshielding effect) for the two-coordinated oxygen atoms.

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.

Gas phase photolysis of 1-butene at 147 nm (8.4 eV)

Abstract The photolysis of gaseous 1-pentene was carried out in a static system using the xenon resonance line at 147 nm (8.4 eV) at pressures in the range 0.5 – 400 Torr (0.7 – 533 hPa). Only decomposition processes were studied and no attempt was made to establish the pattern of free radical reactions. The major dissociation products observed were ethylene, allene, propylene, 1,3-butadiene, acetylene and propyne. The minor products included methane, ethane, propane, some C 5 H 8 and C 4 H 6 hydrocarbons, and 1-butene. The radical species were identified using scavengers such as oxygen, H 2 S and HI. The pressure dependence of the yields of the major radicals (C 3 H 5 , CH 3 , C 2 H 5 , C 2 H 3 and C 3 H 7 ) was established. The C 2 H, C 4 H 5 , C 4 H 7 , CH 2 and C 3 H 3 radicals were found to be unimportant. The primary decomposition channels are established. The main processes are the cleavage of a CH bond with a yield φ of 0.45 – 0.48 and the cleavage of a CC bond with a yield φ of 0.47. The allylic CC bond appears to be the only CC bond which undergoes primary rupture. All four primary intermediates, i.e. C 5 H 9 , C 3 H 5 , C 2 H 5 and H, are energized. The radicals either decompose (isomerization prior to decomposition is possible in some cases) or undergo collisional stabilization; some hydrogen atoms add to the double bond prior to thermalization. Some details of the secondary processes are established but the overall mechanism is too complex to be fully interpreted.

Gas phase photolysis of propylene at 8.4 and 10.0 eV

Abstract The photolysis of gaseous propylene was carried out in a static system using the krypton (10.0 eV) and xenon (8.4 eV) resonance lines at pressures in the range 1 – 700 Torr. Only decomposition processes were studied and no attempt was made to establish the pattern of free-radical reactions. The primary decomposition channels were established. With increasing energy the contribution of the processes involving molecular elimination increases at the expense of simple scission of the CC and CH bonds. A comparison of the present data with those obtained by Collin and coworkers at 7.6 eV reveals that in the range 7.6 – 10.0 eV the mechanism for dissociation changes completely. At 7.6 eV atomic hydrogen is formed, while at 10.0 eV this process is virtually absent being replaced by the formation of molecular hydrogen. Both processes occur at an intermediate energy of 8.4 eV. The energy distribution among the products of the primary decomposition exhibits marked deviations from statistical randomization.

13C shielding scale for MAS NMR spectroscopy

We have performed the direct measurements of 13C magnetic shielding for pure liquid TMS, solution of 1% TMS in CDCl3 and solid fullerene. The measurements were carried out in spherical ampoules exploring the relation between the resonance frequencies, shielding constants and magnetic moments of 13C and 3He nuclei. Next the 13C shielding constants of glycine, hexamethylbenzene and adamantane were established on the basis of appropriate chemical shifts measured in the solid state. All the new results are free from susceptibility effects and can be recommended as the reference standards of 13C shielding scale in the magic angle spinning NMR experiments. Copyright

129Xe and 131Xe nuclear magnetic dipole moments from gas phase NMR spectra

3He, 129Xe and 131Xe NMR measurements of resonance frequencies in the magnetic field B0 = 11.7586 T in different gas phase mixtures have been reported. Precise radiofrequency values were extrapolated to the zero gas pressure limit. These results combined with new quantum chemical values of helium and xenon nuclear magnetic shielding constants were used to determine new accurate nuclear magnetic moments of 129Xe and 131Xe in terms of that of the 3He nucleus. They are as follows: μ(129Xe) = −0.7779607(158)μN and μ(131Xe) = +0.6918451(70)μN. By this means, the new ‘helium method’ for estimations of nuclear dipole moments was successfully tested. Gas phase NMR spectra demonstrate the weak intermolecular interactions observed on the 3He and 129Xe and 131Xe shielding in the gaseous mixtures with Xe, CO2 and SF6. Copyright