Research article Received: 5 July 2014
Revised: 4 October 2014
Accepted: 22 October 2014
Published online in Wiley Online Library: 16 January 2015
(wileyonlinelibrary.com) DOI 10.1002/mrc.4191
129Xe
and 131Xe nuclear magnetic dipole moments from gas phase NMR spectra Włodzimierz Makulski* 3
He, 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 131 Xe shielding in the gaseous mixtures with Xe, CO2 and SF6. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: 3He; 129Xe and 131Xe NMR spectra; gas phase measurements; 129Xe and 131Xe nuclear magnetic moments
Introduction
Magn. Reson. Chem. 2015, 53, 273–279
* Correspondence to: Włodzimierz Makulski, Laboratory of NMR Spectroscopy, Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland. E-mail: [email protected] Laboratory of NMR Spectroscopy, Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093, Warsaw, Poland
Copyright © 2015 John Wiley & Sons, Ltd.
273
In the last few years, we have observed spectacular interest in xenon NMR spectroscopy in the context of characterizing different intermolecular interactions. Xenon itself is present in the atmosphere at a concentration of 0.0087 ppm and is readily extractable. Nine stable isotopes of natural xenon are present from 124Xe up to 136 Xe. In the magnetic resonance analysis, two isotopes of natural abundance xenon can be utilized: 129Xe (26.44% with 75 neutrons and I = 1/2) and 131Xe (21.18% with 77 neutrons and I = +3/2). The 129Xe isotope is usually preferred because of its high relative sensitivity 2.12 × 102 vs 131Xe 2.76 × 103 as compared with 1 H = 1.[1,2] The xenon spectral range attains ~8000 ppm as a result of the large number of electrons (54) in its internal structure. The total solvent effects on 129Xe shielding reach 250 ppm.[3] The favorable 129Xe nucleus usually has long spin-lattice relaxation times in the gas phase of up to several minutes (T1 up to ~4.1 h).[4] The nuclear relaxation in the noble gases with ½ spin number is purely intermolecular; thus, with increasing pressure, we are able to observe the decrease in relaxation times. The relaxation of 131Xe is much faster because of the quadrupolar mechanism present (Q ~ 0.116(4) barn). Xenon is an excellent ‘spin spy’ because of its inertness as it does not affect the structure of the surrounding partner under study.[1,2] Monoatomic xenon is used to study many systems, from organic isotropic liquids and host molecules, polymers, clathrates, liquid crystals and porous materials to large biomolecule species. Another attribute of xenon atom nuclei are their high sensitivity to NMR analysis as a result of their high electric polarizability (α). These advantages were supported by hyperpolarized methods (HP) that enhanced and improved 129Xe (such as 3He) NMR spectra and MRI images.[3] Contrary to xenon, the shielding range of 3He atom in different solvents is very small exceeding only 1.024 ppm.[5] Helium, possibly the most chemically inert substance, is small (0.024 nm) compared with xenon (0.048 nm) and can be a useful probe of the local magnetic fields of different molecular species. Its limited natural abundance 1.3 × 104% means that enriched material is needed.
Fortunately, this is the by-product of tritium decay, commercially available from the nuclear weapons program. From the first molecular beam magnetic resonance experiments, it is known that resonance techniques give rise to estimations of nuclear magnetic moments.[6] The latter studies of magnetic moments were carried out by classical NMR experiments in stable constant magnetic fields via measurements of the spin precession frequencies (firstly proposed in ref. [7]). Usually, they are observed in bulk matter systems where intermolecular interactions and internal motions within a molecule are always present. To obtain the magnetic moments of bare nuclei, one must allow both these aspects. Recently, nuclear magnetic moments of some nuclei were revisited mostly by the NMR measurements employing gas phase experiments and the comparison with the 1H magnetic moment.[8–12] The main advantage of this approach lies in the characteristic gaseous features. Each physicochemical gas property can be expressed as virial expansion in terms of density (concentration). This can be performed for the nuclear magnetic shielding values of any nucleus in a given molecule. So, it is relatively easy to extrapolate shieldings that are usually measured as chemical shifts to the zero pressure (density) limit.[13] These values are completely free from intermolecular interactions, and these can be easily verified in theoretical calculations. Bulk susceptibility effects can also be omitted. The origin of this work had two helpful circumstances. The first of these was establishing an analytical procedure that takes advantage of helium-3 NMR measurements and its nuclear magnetic moment.[14] The second was providing new shielding calculations of the magnetically active nuclei under study.[15,16]
W. Makulski 381.3587
Experimental
He frequency: 381.3586618 MHz 129/131
Methods and results
He/Xe/Xe
Xe resonance frequencies [MHz]
He/Xe/SF He/Xe/CO
381.3585 138.4735
Xe frequency: 138.4703657 MHz 138.4725
138.4715 He/Xe/Xe
138.4705
He/Xe/SF He/Xe/CO
41.0490
Xe frequency: 41.0474996 MHz
41.0480 He/Xe/Xe He/Xe/SF He/Xe/CO
41.0470 0
0.4
0.8
1.2
Gaseous xenon was frequently studied by the NMR method at pressures up to 200 atm. The virial expansion in terms of density can be developed for any macroscopic quantity measured in a real fluid. The nuclear magnetic shielding of pure gas was described as power series in the density[21]: σ ðρ; TÞ ¼ σ 0 ðTÞ þ σ 1 ðTÞρ þ σ 2 ðTÞρ2 þ σ 3 ðTÞρ3 þ …
(1)
274
where σ 0 is the shielding of the ‘isolated’ xenon atom at a given temperature and σ 1, σ 2, σ 3 are virial coefficients from many body collisions. Below ~40 atm, the dependence is strictly linear with σ 1 = 0.548, 0.539, 0.553 ppm/amagat at 298 K (where 1 amagat = 2.6867805 × 1025
1.6
Density [mol/L] 3
129
131
Figure 1. Density dependences of He, Xe and Xe NMR frequencies in 3 3 3 gaseous mixtures: He/Xe, He/Xe/CO2 and He/Xe/SF6.
molecules in 1 m3).[22,23] At higher pressures, σ 2 = 0.17 × 103 ppm/ amagat2 and the σ 3 = 0.16 × 105 ppm/amagat3 appear because of the many body collisions.[24] When mixtures of gaseous substances are investigated, slightly modified virial equations can be treated. These systems can be considered as the binary mixtures of gaseous solute substance A (He or Xe in this work) containing the nucleus X whose shielding σ(X) is measured, and gas B (Xe, CO2 or SF6) is the solvent. Eqn (1) can then be formulated as follows: σ ðXÞ ¼ σ 0 ðXÞ þ σ AA ðXÞρA þ σ AB ðXÞρB þ …
Gas phase measurements
wileyonlinelibrary.com/journal/mrc
381.3586
Xe,
In this work, the He and Xe frequencies/chemical shifts were examined in gaseous mixtures of helium-3 (Aldrich, min. 99.95%) and xenon (Air Products Ltd., R.G.), CO2 (99.8%, Aldrich) and SF6 (99.75%, Aldrich). The glass ampules containing small amounts of (1 : 1) mixture of 3He/Xe
Revised: 4 October 2014
Accepted: 22 October 2014
Published online in Wiley Online Library: 16 January 2015
(wileyonlinelibrary.com) DOI 10.1002/mrc.4191
129Xe
and 131Xe nuclear magnetic dipole moments from gas phase NMR spectra Włodzimierz Makulski* 3
He, 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 131 Xe shielding in the gaseous mixtures with Xe, CO2 and SF6. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: 3He; 129Xe and 131Xe NMR spectra; gas phase measurements; 129Xe and 131Xe nuclear magnetic moments
Introduction
Magn. Reson. Chem. 2015, 53, 273–279
* Correspondence to: Włodzimierz Makulski, Laboratory of NMR Spectroscopy, Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland. E-mail: [email protected] Laboratory of NMR Spectroscopy, Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093, Warsaw, Poland
Copyright © 2015 John Wiley & Sons, Ltd.
273
In the last few years, we have observed spectacular interest in xenon NMR spectroscopy in the context of characterizing different intermolecular interactions. Xenon itself is present in the atmosphere at a concentration of 0.0087 ppm and is readily extractable. Nine stable isotopes of natural xenon are present from 124Xe up to 136 Xe. In the magnetic resonance analysis, two isotopes of natural abundance xenon can be utilized: 129Xe (26.44% with 75 neutrons and I = 1/2) and 131Xe (21.18% with 77 neutrons and I = +3/2). The 129Xe isotope is usually preferred because of its high relative sensitivity 2.12 × 102 vs 131Xe 2.76 × 103 as compared with 1 H = 1.[1,2] The xenon spectral range attains ~8000 ppm as a result of the large number of electrons (54) in its internal structure. The total solvent effects on 129Xe shielding reach 250 ppm.[3] The favorable 129Xe nucleus usually has long spin-lattice relaxation times in the gas phase of up to several minutes (T1 up to ~4.1 h).[4] The nuclear relaxation in the noble gases with ½ spin number is purely intermolecular; thus, with increasing pressure, we are able to observe the decrease in relaxation times. The relaxation of 131Xe is much faster because of the quadrupolar mechanism present (Q ~ 0.116(4) barn). Xenon is an excellent ‘spin spy’ because of its inertness as it does not affect the structure of the surrounding partner under study.[1,2] Monoatomic xenon is used to study many systems, from organic isotropic liquids and host molecules, polymers, clathrates, liquid crystals and porous materials to large biomolecule species. Another attribute of xenon atom nuclei are their high sensitivity to NMR analysis as a result of their high electric polarizability (α). These advantages were supported by hyperpolarized methods (HP) that enhanced and improved 129Xe (such as 3He) NMR spectra and MRI images.[3] Contrary to xenon, the shielding range of 3He atom in different solvents is very small exceeding only 1.024 ppm.[5] Helium, possibly the most chemically inert substance, is small (0.024 nm) compared with xenon (0.048 nm) and can be a useful probe of the local magnetic fields of different molecular species. Its limited natural abundance 1.3 × 104% means that enriched material is needed.
Fortunately, this is the by-product of tritium decay, commercially available from the nuclear weapons program. From the first molecular beam magnetic resonance experiments, it is known that resonance techniques give rise to estimations of nuclear magnetic moments.[6] The latter studies of magnetic moments were carried out by classical NMR experiments in stable constant magnetic fields via measurements of the spin precession frequencies (firstly proposed in ref. [7]). Usually, they are observed in bulk matter systems where intermolecular interactions and internal motions within a molecule are always present. To obtain the magnetic moments of bare nuclei, one must allow both these aspects. Recently, nuclear magnetic moments of some nuclei were revisited mostly by the NMR measurements employing gas phase experiments and the comparison with the 1H magnetic moment.[8–12] The main advantage of this approach lies in the characteristic gaseous features. Each physicochemical gas property can be expressed as virial expansion in terms of density (concentration). This can be performed for the nuclear magnetic shielding values of any nucleus in a given molecule. So, it is relatively easy to extrapolate shieldings that are usually measured as chemical shifts to the zero pressure (density) limit.[13] These values are completely free from intermolecular interactions, and these can be easily verified in theoretical calculations. Bulk susceptibility effects can also be omitted. The origin of this work had two helpful circumstances. The first of these was establishing an analytical procedure that takes advantage of helium-3 NMR measurements and its nuclear magnetic moment.[14] The second was providing new shielding calculations of the magnetically active nuclei under study.[15,16]
W. Makulski 381.3587
Experimental
He frequency: 381.3586618 MHz 129/131
Methods and results
He/Xe/Xe
Xe resonance frequencies [MHz]
He/Xe/SF He/Xe/CO
381.3585 138.4735
Xe frequency: 138.4703657 MHz 138.4725
138.4715 He/Xe/Xe
138.4705
He/Xe/SF He/Xe/CO
41.0490
Xe frequency: 41.0474996 MHz
41.0480 He/Xe/Xe He/Xe/SF He/Xe/CO
41.0470 0
0.4
0.8
1.2
Gaseous xenon was frequently studied by the NMR method at pressures up to 200 atm. The virial expansion in terms of density can be developed for any macroscopic quantity measured in a real fluid. The nuclear magnetic shielding of pure gas was described as power series in the density[21]: σ ðρ; TÞ ¼ σ 0 ðTÞ þ σ 1 ðTÞρ þ σ 2 ðTÞρ2 þ σ 3 ðTÞρ3 þ …
(1)
274
where σ 0 is the shielding of the ‘isolated’ xenon atom at a given temperature and σ 1, σ 2, σ 3 are virial coefficients from many body collisions. Below ~40 atm, the dependence is strictly linear with σ 1 = 0.548, 0.539, 0.553 ppm/amagat at 298 K (where 1 amagat = 2.6867805 × 1025
1.6
Density [mol/L] 3
129
131
Figure 1. Density dependences of He, Xe and Xe NMR frequencies in 3 3 3 gaseous mixtures: He/Xe, He/Xe/CO2 and He/Xe/SF6.
molecules in 1 m3).[22,23] At higher pressures, σ 2 = 0.17 × 103 ppm/ amagat2 and the σ 3 = 0.16 × 105 ppm/amagat3 appear because of the many body collisions.[24] When mixtures of gaseous substances are investigated, slightly modified virial equations can be treated. These systems can be considered as the binary mixtures of gaseous solute substance A (He or Xe in this work) containing the nucleus X whose shielding σ(X) is measured, and gas B (Xe, CO2 or SF6) is the solvent. Eqn (1) can then be formulated as follows: σ ðXÞ ¼ σ 0 ðXÞ þ σ AA ðXÞρA þ σ AB ðXÞρB þ …
Gas phase measurements
wileyonlinelibrary.com/journal/mrc
381.3586
Xe,
In this work, the He and Xe frequencies/chemical shifts were examined in gaseous mixtures of helium-3 (Aldrich, min. 99.95%) and xenon (Air Products Ltd., R.G.), CO2 (99.8%, Aldrich) and SF6 (99.75%, Aldrich). The glass ampules containing small amounts of (1 : 1) mixture of 3He/Xe
