Recently in our NMR laboratory we have obtained new more accurate values of nuclear magnetic dipole moments for many stable nuclei like 13C, 14N, 15N, 17O, etc. exploring the measurements of resonance frequencies in isolated molecules and the state-of the-art ab initio calculations of nuclear magnetic shielding [1,2]. The new results of magnetic moments are much better than the previous data obtained using similar NMR methods in liquids or solids. As it has been already shown [3] the accurate nuclear magnetic moments permit the direct reading of shielding when helium-3 gas is used as the reference standard. This method is used in the present study of atmospheric gases. Our atmosphere consists of nitrogen, oxygen, water, argon, carbon dioxide and many other gases present at very low concentration. Moreover the atmosphere can also contain other chemicals due to the industrialization process, mostly oxides of carbon, nitrogen and sulfur. Some of the above compounds are paramagnetic (e.g. O2, NO or NO2) and it makes many additional problems. All the atmospheric components are studied by NMR in the gas phase and the measurements are performed for pure gases and gaseous mixtures in the range of pressure up to 300 bar. Then the experimental data are extrapolated to the zero-density limit what gives the shielding and indirect spin-spin coupling in isolated molecules. The present multinuclear NMR measurements performed for gaseous mixtures deliver also some insight into intermolecular interactions. These effects are observed as the change of the basic NMR parameters mostly due to bimolecular collisions. For example, in the case of water we have detected long-lasting dimers with CH3F and CHF3 molecules. It permits the precise measurement of 1JOH spin-spin coupling in an isolated water molecule enriched in oxygen-17. All our experimental results are compared with accurate quantum-chemical calculations available from literature.
References:
[1] A. Antušek, K. Jackowski, M. Jaszuński, W. Makulski, M. Wilczek, Chem. Phys. Letters, 411 (2005) 111.
[2] K. Jackowski, P. Garbacz, Chapter 3 in Gas Phase NMR, Eds. K. Jackowski and M. Jaszuński, pp. 96-126, Royal Society of Chemistry, 2016.
[3] K. Jackowski, M. Jaszuński, M. Wilczek, J. Phys. Chem. A, 114 (2010) 2471.
Acknowledgement:
Support from the National Science Centre (Poland) grant No 2015/19/B/ST4/03757 is gratefully acknowledged.