Abstracto

Analysis of Nuclear Magnetic Shielding in Medium-Sized Molecules

Karol Jackowski

In nuclear magnetic resonance (NMR) spectroscopy the chemical shift describes the displacement of a signal due to variations in nuclear magnetic shielding and it exactly means the difference between magnetic shielding observed in two molecules: the reference and unknown molecules, respectively. At present the absolute shielding values of numerous molecules are well-known from advanced ab initio calculations. NMR measurements in the gas phase allow the determination of chemical shifts and shielding free from intermolecular interactions. It is equivalent to the appropriate parameters of isolated molecules which are available from the quantum chemical calculations. In our laboratory we have developed many new techniques of NMR experiments in the gas phase. We found that applying an inert gas as a solvent it was possible to obtain gas-phase NMR spectra also for chemical compounds which are liquid at room temperature, e.g., for water. Next, it was shown that the simultaneous observation of resonance frequencies for 1H and another X nucleus leads to the accurate determination of the nuclear magnetic dipole moment of X what is important for nuclear physics and obviously for molecular spectroscopy. Next, we have presented a new method of the standardization of NMR spectra based on the direct measurements of magnetic shielding. All the above ideas are illustrated by our new experimental 1H, 13C, 15N and 17O NMR studies of atmospheric gases. The NMR spectra are adequately applied for dinitrogen (N2), nitrous oxide (N2O), carbon dioxide (CO2) and water (H2O) in many various gaseous mixtures, including also paramagnetic dioxygen (O2).