- Email: [email protected]
Multinuclear NMR spectroscopy in the gas phase
Journal of Molecular Structure 786 (2006) 215–219 www.elsevier.com/locate/molstruc
Multinuclear NMR spectroscopy in the gas phase K. Jackowski * Laboratory of NMR Spectroscopy, Department of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warszawa, Poland Received 19 September 2005; accepted 3 October 2005 Available online 9 February 2006
Abstract Nuclear magnetic resonance (NMR) of some nuclei (e.g. 1H, 13C, 19F, 29Si or 31P, IZ1/2) gives strong signals which allow analytical studies of gaseous compounds. The other magnetic nuclei have low natural abundance or/and contain an electric quadrupole moment and their NMR signals are rather weak. In our laboratory we have developed new experimental techniques, which permit us to monitor several micrograms of chemical compounds in gaseous matrices. Applying this approach we have observed magnetic shielding of various nuclei, including 17O and 33S at the natural abundance, in the gas phase as a function of density. Density-dependent spin–spin couplings were also found for many chemical compounds. It has been shown that NMR gas-phase studies can easily be extended on molecules, which exhibit strong intermolecular interactions and are liquids at room temperature. All the latter NMR experimental results obtained for gaseous matrices are reviewed in this paper. q 2005 Elsevier B.V. All rights reserved. Keywords: NMR spectra; Nuclear magnetic shielding; Spin–spin coupling; Molecular interactions; Gaseous matrices
1. Introduction Modern NMR spectrometers with supercoducting magnets permit the observation of 1H and 19F spectra for micrograms of chemical compounds. It is especially useful for investigations of gases when low density limits the number of molecules. Fig. 1 shows the 1H NMR spectrum of atmospheric air closed in a 4 mm o.d. glass tube. The distinct signal comes from 3 mg of water present in normal air where nitrogen and oxygen are used as the gaseous solvents. The observation of water molecules as monomers would be hardly possible without the presence of gaseous solvents because the H2O molecules can easily form hydrogen bonds. Moreover, the H2O signal in Fig. 1 is relatively wide (Dn1/2Z81 Hz) because at atmospheric pressure the spin-rotation relaxation of protons in water molecules is still very efficient. The same signal can be much more narrow (at least one order of magnitude) if another chemically inert gas is used as a solvent in order to increase significantly the total pressure in a sample; it is deduced from our preliminary measurements for water and it means that the significant improvement of a gaseous H2O signal can still be expected in gaseous matrices. Let us summarize that the molecules of gas solvents (O2, N2) hold the solute molecules * Tel.: C48 22 822 0211x315; fax: C48 22 822 59 96. E-mail address: [email protected]
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.10.018
(H2O) apart and simultaneously slow their rotational movement due to molecular collisions. The similar problem is shown by another sample spectrum. We have investigated methyl fluoride-d3 (CD3F) in the gas phase [1] and found that the commercial product (99% CD3F from Aldrich) contains other isotopomers of methyl fluoride-d3 as impurities. Fig. 2 presents the 19F NMR spectrum of the latter gas mixture at 20.4 atm. NMR signals of the 19F NMR spectrum in Fig. 2 are much narrow than the previous one in Fig. 1 for gaseous water because the density of gaseous solution is much higher here. It is interesting that in this case the 12CD3F component can be considered as a gas solvent for all the other isotopomers. It efficiently diminishes the rate of spin–rotation relaxation of fluorine nuclei and the 19F NMR signals are really narrow. The 19F NMR spectral parameters (e.g. chemical shifts, spin–spin coupling constants and isotope effects) of CD3F isotopomers shown in Fig. 2 were precisely measured and analyzed in our previous paper [1]. As shown the NMR spectra of gaseous matrices can give important information for qualitative and quantitative analysis. This review shows that such an analysis is precise and the NMR spectral parameters of an isolated solute molecule can accurately be determined.
2. Experimental Gas samples are prepared condensing pure gases or their mixtures from the calibrated part of vacuum line to NMR
216
K. Jackowski / Journal of Molecular Structure 786 (2006) 215–219
spectrometer) allows one to preserve the same Bo for all measurements. Later the chemical shifts are converted into the absolute shielding constants. 3. Results and discussion 3.1. Intermolecular effects in NMR spectral parameters
Fig. 1. 500 MHz 1H NMR spectrum of water in atmospheric air at normal pressure (758 Torr), temperature (22 8C) and humidity (w50%). The origin of the wide signal at 1 ppm remains unknown. The zero-point is assigned to liquid TMS.
tubes. Small quantities of solute gases were obtained by gradual dissolution in gaseous solvents. Usually, 4 mm o.d. glass tubes (approx. 5 cm long) are used for gas-phase measurements. The volumes of sample tubes and the vacuum line are measured using mercury. The sample tubes are filled with gaseous mixtures and sealed. The sealed 4 mm o.d. gas samples are fitted into the standard 5 mm o.d. thin-walled NMR tube (Wilmad 528-PP) with liquid toluene-d8 in the annular space. NMR chemical shifts are measured relative to the external reference standards; the secondary liquid standards are appropriate to observed nuclei, e.g. TMS for 1H and 13C NMR spectra, CH3NO2 for 15N NMR, liquid water for 17O NMR measurements etc. For this purpose the absolute frequency of the reference standard is determined in the conditions of lock system tuned to the CD3 signal of external toluene-d8. The constant frequency of the lock system (76.8464 MHz at our Varian UNITYplus-500 multinuclear
The nuclear magnetic shielding of a nucleus in a molecule is affected by both intermolecular interactions and intramolecular motion. In the gas phase, these effects are observed as a dependence of the shielding constant (s (T,r)) on density and temperature [2]: sðT; rÞ Z s0 ðTÞ C s1 ðTÞr C s2 ðTÞr2 C .
(1)
where so(T) is the shielding for an isolated molecule and the higher terms (s1(T), s2(T).) are dependent on the density r and describe the intermolecular interactions in gases. For most gaseous compounds at constant temperature the shielding s (T) varies linearly with density [3] if the pressure of gas does not exceed 40 atm. In such a case the s2(T) and higher-order coefficients in Eq. (1) can be safely ignored and the remaining parameters, i.e. so(T) and s1(T), can be precisely determined. Gas mixtures can also be used for the determination of the latter shielding parameters. For a binary mixture of gaseous compound A, containing the nucleus X whose shielding s (X) is of interest, and gas B as the solvent, Eq. (2) can be rewritten as follows: sðXÞ Z s0 ðXÞ C sAA ðXÞrA C sAB ðXÞrB C .
(2)
where rA and rB are the densities of A and B, respectively, and so(X) is the shielding at the zero-density limit. The coefficients sAA(X) and sAB(X) contain the bulk susceptibility corrections ((sA)b and (sB)b) and the terms taking account of intermolecular interactions during the binary collisions of A–A and A–B molecules are (s(A–A)(X) and s(A–B)(X)), respectively. The shielding parameters in Eq. (2) are obviously temperature dependent and for this reason the measurements for various densities must be performed at constant temperature. In the gas phase, nuclear spin–spin coupling is also modified by interactions in pairs of molecules and multiple interactions, the appropriate equation for spin–spin coupling in the binary mixtures of gases (A and B) is similar to Eq. (2) at constant temperature: JðXYÞ Z J0 ðXYÞ C JAA ðXYÞrA C JAB ðXYÞrB C .
(3)
where Jo(XY) is the spin–spin coupling between X and Y nuclei at the zero-density limit and JAA(XY), JAB(XY) are solely due to intermolecular effects in the binary collisions of A–A and A–B molecules, respectively. Usually, the density of A is kept sufficiently low for Eqs. (2) and (3) to simplify to Fig. 2. 470.4 MHz 19F NMR spectrum of commercial 99% CD3F gas at 20.4 atm. The quartet comes from CH3F (Dn1/2Z1.83 Hz) and arises from the spin–spin coupling of a fluorine nucleus with three protons. The doublet of quintets (Dn1/2Z1.82 Hz) is from CHD2F, the strongest singlet (Dn1/2Z 1.53 Hz) represents 12CD3F and the doublet of septets (Dn1/2Z1.81 Hz) shows 13 CD3F at the natural abundance of carbon C-13 (1.11%). The 19F NMR chemical shifts are measured relative to liquid CFCl3.
sðXÞ Z s0 ðXÞ C sAB ðXÞrB
(4)
JðXYÞ Z J0 ðXYÞ C JAB ðXYÞrB
(5)
and the terms sAA(X) and JAA(XY) can be ignored if micrograms of gas A are diluted in gaseous solutions.
K. Jackowski / Journal of Molecular Structure 786 (2006) 215–219
3.2. Nuclear magnetic shielding measured in gaseous solutions According to Eq. (4) the measurements of nuclear magnetic shielding extrapolated to the zero-density limit allow one to determine the shielding constants of isolated molecules. The sAB(X) parameter gives information on intermolecular interactions between the solute and solvent molecules. Using gaseous solvents one can easily perform the exact measurements of shielding and estimate the magnitude of molecular interactions in the gas phase. We have performed such studies using isotopically enriched acetylene-13C2 (H13Cb13CH) and two gaseous solvents: xenon (Xe) and carbon dioxide (CO2)
217
[4]. It is worth noting that using at least two gaseous solvents we have verified if the extrapolation gives the same value so(X), otherwise Eq. (4) would be not sufficient for the analysis of shielding parameters. This method is also good for many molecules, which exhibit strong intermolecular interactions and form the liquid phase at room temperature and atmospheric pressure. We have used this approach investigating the shielding parameters of acetonitrile (CH3CN) [5,6] and also in this case the isotopomers containing the 13C and 15N nuclei have been very useful. We could use micrograms of the solutes and successfully obtain all the NMR spectral parameters. The shielding results of CH3CN are shown in Table 1 and the so(X) data can be directly used for the verification of ab initio
Table 1 NMR shielding parameters recently determined from the measurements of chemical shifts in gaseous mixtures Solute gas (A) 13
13
H Cb CH
15 CH13 3 C N
13
CH13 3 CN
13
CH3I
15
NH2CH3
C6H6 CD17 3 OCD3 CH17 3 OCH3 C17O2 N17 2 O 17 OCS C17O S17O2 33 SO2 S17O3 CH3F CHD2F 33 SF6
a b
Solvent gas (B)
Nucleus (X)
s0(X) (ppm)a
s(A–B)(X) (ppm mL molK1)
Ref.
Xe CO2 Xe CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 Xe SF6 Xe SF6 Xe SF6 Xe CO2 Xe CD3OCD3 CH3OCH3 CO2 N2O OCS CO SO2 SO2 SO3 CD3F CD3F SF6 Xe CO2 NH3
1
29.278(1) 29.276(1) 116.58(1) 116.58(1) 29.113(1) 29.113(1) 74.021(3) 74.019(12) K8.873(5) K8.843(22) 29.107(3) 29.111(3) 73.984(12) 73.961(12) 187.753(3) 187.747(3) 28.753(3) 28.753(1) 220.594(7) 220.592(2) 28.269(4) 28.262(3) 30.330(2) 30.324(2) 158.259(2) 158.260(3) 249.55(1) 249.52(2) 57.113(4) 57.105(9) 339.5(1) 337.1(1) 222.5(1) 181.0(1) 87.3(1) K62.7(1) K231.0(1) K152.5(20) 55.3(1) 470.98(1) 472.11(1) 379.90(4) 379.90(4) 379.90(4) 379.90(4)
K29(3) K27(3) K267(5) K180(5) K13(3) K16(2) K181(7) K627(26) 51(9) 927(39) K5(5) K19(3) K204(11) K590(13) K199(5) K209(6) K4(4) K15(1) K426(10) K643(3) 0b 0b 0b 0b K102(3) K245(3) K280(12) K785(32) K99(2) K150(3) K1440(50) K1550(50) K345(12) K256(25) K585(12) K70(12) 945(50) K11,650(700) K828(150) K539.9(22) K551.9(22) K132(7) K225(20) K117(22) K233(9)
[4]
H H 13 C 13 C 1 H 1 H 13 C (in CN) 13 C (in CN) 15 N 15 N 1 H 1 H 13 C (in CN) 13 C (in CN) 13 C (in CH3) 13 C (in CH3) 1 H 1 H 13 C 13 C 1 H (in CH3) 1 H (in CH3) 1 H (in NH2) 1 H (in NH2) 13 C 13 C 15 N 15 N 13 C 13 C 17 O 17 O 17 O 17 O 17 O 17 O 17 O 33 S 17 O 19 F 19 F 33 S 33 S 33 S 33 S 1
Absolute shielding constants. Independent of density within the limit of experimental error.
[5]
[6]
[19]
[20]
[21] [7] [7] [7,8] [7,8] [7,8] [7,8] [18] [18] [18] [1] [1] [9,10]
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K. Jackowski / Journal of Molecular Structure 786 (2006) 215–219
Table 2 Spin–spin coupling parameters recently determined from the NMR spectra of gaseous mixtures Solute gas (A) 13
13
H Cb CH 15 CH13 3 C N
CH13 3 CN
13
CD3F
13
CH3I
15
NH2CH3
C6H6
Solvent gas (B)
n
Xe CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 CD3F CD3F CD3F SF6 CO2 SF6 Xe SF6 Xe SF6 Xe SF6 Xe CO2 Xe
1
J (XY)
J(CC) J(CC) 1 J(NC) 1 J(NC) 2 J(CH) 2 J(CH) 3 J(NH) 3 J(NH) 1 J(CC) 1 J(CC) 1 J(CH) 1 J(CH) 1 J(FC) 1 J(CD) 2 J(FD) 1 J(CH) 1 J(CH) 1 J(NH) 1 J(NH) 1 J(CH) 1 J(CH) 1 J(NC) 1 J(NC) 3 J(HH) 3 J(HH) 1 J(CH) 1 J(CH) 1
J0(XY) (Hz)
JAB(XY) (Hz mL molK1)
Ref.
174.77(2) 174.78(2) K16.20(1) K16.23(2) K10.18(2) K10.19(2) K1.34(2) K1.43(9) 60.12(5) 60.12(3) 134.03(1) 134.05(1) K160.72(5) 22.45(5) 7.20(5) 149.38(1) 149.39(1) K65.4(1) K65.4(1) 132.5(1) 132.5(1) K5.4(5) K5.4(5) 7.0(1) 7.0(1) 157.77(2) 157.78(2)
K30(20) K301(20) K62(14) K191(44) K13(37) 6(46) K325(77) K134(171) K86(87) K151(56) 95(24) 148(23) K1008(72) 43(46) 4.8(44) 111(5) 151(5) 0a 0a 0a 0a 0a 0a 0a 0a 0a 0a
[4] [5]
[6]
[1]
[19] [20]
[21]
a a
Independent of density within the limit of experimental error.
calculations of CH3CN shielding. For acetonitrile and other chemical compounds the experimental so(X) results have been already converted to the absolute shielding constants using the accepted standards: liquid TMS for 1H and 13C NMR spectra [4], gaseous ammonia for 15N NMR [11], liquid CFCl3 for 19F NMR [12], gaseous CO for 17O NMR [13] and gaseous OCS for 33S NMR [10,14]. Table 1 displays also the s(A–B)(X) parameters which describe intermolecular interactions in the gas phase. They are usually negative (deshielding effects) with one pronounced exception: the nitrogen nucleus in CH3CN is more shielded due to the interactions. It is consistent with our recent study of acetonitrile in liquid solutions and the sign of 15 N gas-to-solution shifts [15]. In the liquid state the selfassociation of CH3CN molecules is really strong, its boiling point is 81.6 8C. Nevertheless, we could still investigate it precisely in the gas phase using micrograms of suitable isotopomers of acetonitrile. The 17O and 33S NMR investigations have required some improvements in the detection technique of these weak signals. All the procedures are carefully described in our original papers [9,16]. 3.3. Spin–spin coupling in gaseous mixtures It is well-known that spin–spin coupling constants are less dependent on intermolecular interactions than shielding constants for the same nuclei. This problem has recently been reviewed [17] and many new examples of densitydependent spin–spin coupling constants have been reported. It
looks like that there is only a problem of precision of NMR measurements because every coupling constant is more or less dependent on intermolecular interactions. Some modest examples of it are shown in Table 2 and they prove that spin–spin coupling constants can also be detected and precisely analyzed in the gas phase. As shown the Jo(XY) parameters for isolated molecules are available from the samples of gaseous mixtures. Some gaseous solvents are really helpful for the more precise measurements of Jo(XY), cf. for example CO2 which gives the distinct density dependence of 1J(CC) in acetylene. It is interesting that in the same molecule some spin–spin couplings are strongly dependent on intermolecular interactions (cf. 1J(CF) in CD3F) and the others are almost independent of density, e.g. 1J(CD) and 2J(FD) in CD3F. 4. Conclusions New measurements of shielding and coupling constants for isolated molecules are available from the NMR measurements performed for diluted gaseous solutions (gaseous matrices). Such mixtures contain usually micrograms of investigated chemical compounds and a gas solvent. The measurements in gaseous matrices have been successfully performed for the chemical compound which exhibits strong molecular interactions in the liquid state, e.g. for acetonitrile. Similar investigations can be carried out for many other compounds being liquids at standard temperature and pressure. It allows the determination of NMR parameters free from intermolecular
K. Jackowski / Journal of Molecular Structure 786 (2006) 215–219
effects for many chemical compounds and such experimental data are excellent for many applications in chemical analysis. In particular, they allow one to estimate intermolecular effects in the condensed phases and they can be used for the reliable verification of ab initio calculations of NMR spectral parameters. Acknowledgements This work was supported by the Polish State Committee for Scientific Research as the research grant number 4 T09A 035 23 available in years 2002–2005. References [1] K. Jackowski, M. Kubiszewski, W. Makulski, J. Mol. Struct. 614 (2002) 267. [2] W.T. Raynes, A.D. Buckingham, H.J. Bernstein, J. Chem. Phys. 36 (1962) 3481. [3] C.J. Jameson, Bull. Magn. Reson. 3 (1980) 3. [4] K. Jackowski, M. Wilczek, M. Pecul, J. Sadlej, J. Phys. Chem. A 104 (2000) 5955; K. Jackowski, M. Wilczek, M. Pecul, J. Sadlej, J. Phys. Chem. A 104 (2000) 9806 (Erratum).
219
[5] M. Wilczek, W. Koz´min´ski, K. Jackowski, Chem. Phys. Lett. 358 (2002) 263. [6] K. Jackowski, M. Wilczek, J. Mol. Struct. 651–653 (2003) 259. [7] W. Makulski, K. Jackowski, J. Mol. Struct. 651–653 (2003) 265. [8] W. Makulski, K. Jackowski, Chem. Phys. Lett. 341 (2001) 369. [9] K. Jackowski, M. Wilczek, W. Makulski, W. Koz´min´ski, J. Phys. Chem. A 106 (2002) 2829. [10] K. Jackowski, W. Makulski, W. Koz´min´ski, Magn. Res. Chem. 40 (2002) 563. [11] C.J. Jameson, A.K. Jameson, D. Oppusunggu, S. Wille, P.M. Burell, J. Mason, J. Chem. Phys. 74 (1981) 81. [12] C.J. Jameson, A.K. Jameson, P.M. Burell, J. Chem. Phys. 73 (1980) 6013. [13] R.E. Wasylishen, D.L. Bryce, J. Chem. Phys. 117 (2002) 10061. [14] R.E. Wasylishen, C. Connor, J.A. Friedrich, Can. J. Chem. 62 (1984) 981. [15] K. Jackowski, P. Bernatowicz, E. Wielogo´rska, Z. Phys. Chem. 216 (2002) 1401. [16] W. Koz´min´ski, K. Jackowski, Magn. Reson. Chem. 38 (2000) 459. [17] K. Jackowski, Int. J. Mol. Sci. 4 (2003) 135. [18] W. Makulski, K. Jackowski, J. Mol. Struct. 704 (2004) 219. [19] M. Wilczek, M. Kubiszewski, K. Jackowski, J. Mol. Struct. 704 (2004) 311. [20] E. Wielogo´rska, W. Makulski, W. Koz´min´ski, K. Jackowski, J. Mol. Struct. 704 (2004) 305. [21] K. Jackowski, E. Macia˛ga, M. Wilczek, J. Mol. Struct. 744–747 (2005) 101.
Multinuclear NMR spectroscopy in the gas phase K. Jackowski * Laboratory of NMR Spectroscopy, Department of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warszawa, Poland Received 19 September 2005; accepted 3 October 2005 Available online 9 February 2006
Abstract Nuclear magnetic resonance (NMR) of some nuclei (e.g. 1H, 13C, 19F, 29Si or 31P, IZ1/2) gives strong signals which allow analytical studies of gaseous compounds. The other magnetic nuclei have low natural abundance or/and contain an electric quadrupole moment and their NMR signals are rather weak. In our laboratory we have developed new experimental techniques, which permit us to monitor several micrograms of chemical compounds in gaseous matrices. Applying this approach we have observed magnetic shielding of various nuclei, including 17O and 33S at the natural abundance, in the gas phase as a function of density. Density-dependent spin–spin couplings were also found for many chemical compounds. It has been shown that NMR gas-phase studies can easily be extended on molecules, which exhibit strong intermolecular interactions and are liquids at room temperature. All the latter NMR experimental results obtained for gaseous matrices are reviewed in this paper. q 2005 Elsevier B.V. All rights reserved. Keywords: NMR spectra; Nuclear magnetic shielding; Spin–spin coupling; Molecular interactions; Gaseous matrices
1. Introduction Modern NMR spectrometers with supercoducting magnets permit the observation of 1H and 19F spectra for micrograms of chemical compounds. It is especially useful for investigations of gases when low density limits the number of molecules. Fig. 1 shows the 1H NMR spectrum of atmospheric air closed in a 4 mm o.d. glass tube. The distinct signal comes from 3 mg of water present in normal air where nitrogen and oxygen are used as the gaseous solvents. The observation of water molecules as monomers would be hardly possible without the presence of gaseous solvents because the H2O molecules can easily form hydrogen bonds. Moreover, the H2O signal in Fig. 1 is relatively wide (Dn1/2Z81 Hz) because at atmospheric pressure the spin-rotation relaxation of protons in water molecules is still very efficient. The same signal can be much more narrow (at least one order of magnitude) if another chemically inert gas is used as a solvent in order to increase significantly the total pressure in a sample; it is deduced from our preliminary measurements for water and it means that the significant improvement of a gaseous H2O signal can still be expected in gaseous matrices. Let us summarize that the molecules of gas solvents (O2, N2) hold the solute molecules * Tel.: C48 22 822 0211x315; fax: C48 22 822 59 96. E-mail address: [email protected]
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.10.018
(H2O) apart and simultaneously slow their rotational movement due to molecular collisions. The similar problem is shown by another sample spectrum. We have investigated methyl fluoride-d3 (CD3F) in the gas phase [1] and found that the commercial product (99% CD3F from Aldrich) contains other isotopomers of methyl fluoride-d3 as impurities. Fig. 2 presents the 19F NMR spectrum of the latter gas mixture at 20.4 atm. NMR signals of the 19F NMR spectrum in Fig. 2 are much narrow than the previous one in Fig. 1 for gaseous water because the density of gaseous solution is much higher here. It is interesting that in this case the 12CD3F component can be considered as a gas solvent for all the other isotopomers. It efficiently diminishes the rate of spin–rotation relaxation of fluorine nuclei and the 19F NMR signals are really narrow. The 19F NMR spectral parameters (e.g. chemical shifts, spin–spin coupling constants and isotope effects) of CD3F isotopomers shown in Fig. 2 were precisely measured and analyzed in our previous paper [1]. As shown the NMR spectra of gaseous matrices can give important information for qualitative and quantitative analysis. This review shows that such an analysis is precise and the NMR spectral parameters of an isolated solute molecule can accurately be determined.
2. Experimental Gas samples are prepared condensing pure gases or their mixtures from the calibrated part of vacuum line to NMR
216
K. Jackowski / Journal of Molecular Structure 786 (2006) 215–219
spectrometer) allows one to preserve the same Bo for all measurements. Later the chemical shifts are converted into the absolute shielding constants. 3. Results and discussion 3.1. Intermolecular effects in NMR spectral parameters
Fig. 1. 500 MHz 1H NMR spectrum of water in atmospheric air at normal pressure (758 Torr), temperature (22 8C) and humidity (w50%). The origin of the wide signal at 1 ppm remains unknown. The zero-point is assigned to liquid TMS.
tubes. Small quantities of solute gases were obtained by gradual dissolution in gaseous solvents. Usually, 4 mm o.d. glass tubes (approx. 5 cm long) are used for gas-phase measurements. The volumes of sample tubes and the vacuum line are measured using mercury. The sample tubes are filled with gaseous mixtures and sealed. The sealed 4 mm o.d. gas samples are fitted into the standard 5 mm o.d. thin-walled NMR tube (Wilmad 528-PP) with liquid toluene-d8 in the annular space. NMR chemical shifts are measured relative to the external reference standards; the secondary liquid standards are appropriate to observed nuclei, e.g. TMS for 1H and 13C NMR spectra, CH3NO2 for 15N NMR, liquid water for 17O NMR measurements etc. For this purpose the absolute frequency of the reference standard is determined in the conditions of lock system tuned to the CD3 signal of external toluene-d8. The constant frequency of the lock system (76.8464 MHz at our Varian UNITYplus-500 multinuclear
The nuclear magnetic shielding of a nucleus in a molecule is affected by both intermolecular interactions and intramolecular motion. In the gas phase, these effects are observed as a dependence of the shielding constant (s (T,r)) on density and temperature [2]: sðT; rÞ Z s0 ðTÞ C s1 ðTÞr C s2 ðTÞr2 C .
(1)
where so(T) is the shielding for an isolated molecule and the higher terms (s1(T), s2(T).) are dependent on the density r and describe the intermolecular interactions in gases. For most gaseous compounds at constant temperature the shielding s (T) varies linearly with density [3] if the pressure of gas does not exceed 40 atm. In such a case the s2(T) and higher-order coefficients in Eq. (1) can be safely ignored and the remaining parameters, i.e. so(T) and s1(T), can be precisely determined. Gas mixtures can also be used for the determination of the latter shielding parameters. For a binary mixture of gaseous compound A, containing the nucleus X whose shielding s (X) is of interest, and gas B as the solvent, Eq. (2) can be rewritten as follows: sðXÞ Z s0 ðXÞ C sAA ðXÞrA C sAB ðXÞrB C .
(2)
where rA and rB are the densities of A and B, respectively, and so(X) is the shielding at the zero-density limit. The coefficients sAA(X) and sAB(X) contain the bulk susceptibility corrections ((sA)b and (sB)b) and the terms taking account of intermolecular interactions during the binary collisions of A–A and A–B molecules are (s(A–A)(X) and s(A–B)(X)), respectively. The shielding parameters in Eq. (2) are obviously temperature dependent and for this reason the measurements for various densities must be performed at constant temperature. In the gas phase, nuclear spin–spin coupling is also modified by interactions in pairs of molecules and multiple interactions, the appropriate equation for spin–spin coupling in the binary mixtures of gases (A and B) is similar to Eq. (2) at constant temperature: JðXYÞ Z J0 ðXYÞ C JAA ðXYÞrA C JAB ðXYÞrB C .
(3)
where Jo(XY) is the spin–spin coupling between X and Y nuclei at the zero-density limit and JAA(XY), JAB(XY) are solely due to intermolecular effects in the binary collisions of A–A and A–B molecules, respectively. Usually, the density of A is kept sufficiently low for Eqs. (2) and (3) to simplify to Fig. 2. 470.4 MHz 19F NMR spectrum of commercial 99% CD3F gas at 20.4 atm. The quartet comes from CH3F (Dn1/2Z1.83 Hz) and arises from the spin–spin coupling of a fluorine nucleus with three protons. The doublet of quintets (Dn1/2Z1.82 Hz) is from CHD2F, the strongest singlet (Dn1/2Z 1.53 Hz) represents 12CD3F and the doublet of septets (Dn1/2Z1.81 Hz) shows 13 CD3F at the natural abundance of carbon C-13 (1.11%). The 19F NMR chemical shifts are measured relative to liquid CFCl3.
sðXÞ Z s0 ðXÞ C sAB ðXÞrB
(4)
JðXYÞ Z J0 ðXYÞ C JAB ðXYÞrB
(5)
and the terms sAA(X) and JAA(XY) can be ignored if micrograms of gas A are diluted in gaseous solutions.
K. Jackowski / Journal of Molecular Structure 786 (2006) 215–219
3.2. Nuclear magnetic shielding measured in gaseous solutions According to Eq. (4) the measurements of nuclear magnetic shielding extrapolated to the zero-density limit allow one to determine the shielding constants of isolated molecules. The sAB(X) parameter gives information on intermolecular interactions between the solute and solvent molecules. Using gaseous solvents one can easily perform the exact measurements of shielding and estimate the magnitude of molecular interactions in the gas phase. We have performed such studies using isotopically enriched acetylene-13C2 (H13Cb13CH) and two gaseous solvents: xenon (Xe) and carbon dioxide (CO2)
217
[4]. It is worth noting that using at least two gaseous solvents we have verified if the extrapolation gives the same value so(X), otherwise Eq. (4) would be not sufficient for the analysis of shielding parameters. This method is also good for many molecules, which exhibit strong intermolecular interactions and form the liquid phase at room temperature and atmospheric pressure. We have used this approach investigating the shielding parameters of acetonitrile (CH3CN) [5,6] and also in this case the isotopomers containing the 13C and 15N nuclei have been very useful. We could use micrograms of the solutes and successfully obtain all the NMR spectral parameters. The shielding results of CH3CN are shown in Table 1 and the so(X) data can be directly used for the verification of ab initio
Table 1 NMR shielding parameters recently determined from the measurements of chemical shifts in gaseous mixtures Solute gas (A) 13
13
H Cb CH
15 CH13 3 C N
13
CH13 3 CN
13
CH3I
15
NH2CH3
C6H6 CD17 3 OCD3 CH17 3 OCH3 C17O2 N17 2 O 17 OCS C17O S17O2 33 SO2 S17O3 CH3F CHD2F 33 SF6
a b
Solvent gas (B)
Nucleus (X)
s0(X) (ppm)a
s(A–B)(X) (ppm mL molK1)
Ref.
Xe CO2 Xe CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 Xe SF6 Xe SF6 Xe SF6 Xe CO2 Xe CD3OCD3 CH3OCH3 CO2 N2O OCS CO SO2 SO2 SO3 CD3F CD3F SF6 Xe CO2 NH3
1
29.278(1) 29.276(1) 116.58(1) 116.58(1) 29.113(1) 29.113(1) 74.021(3) 74.019(12) K8.873(5) K8.843(22) 29.107(3) 29.111(3) 73.984(12) 73.961(12) 187.753(3) 187.747(3) 28.753(3) 28.753(1) 220.594(7) 220.592(2) 28.269(4) 28.262(3) 30.330(2) 30.324(2) 158.259(2) 158.260(3) 249.55(1) 249.52(2) 57.113(4) 57.105(9) 339.5(1) 337.1(1) 222.5(1) 181.0(1) 87.3(1) K62.7(1) K231.0(1) K152.5(20) 55.3(1) 470.98(1) 472.11(1) 379.90(4) 379.90(4) 379.90(4) 379.90(4)
K29(3) K27(3) K267(5) K180(5) K13(3) K16(2) K181(7) K627(26) 51(9) 927(39) K5(5) K19(3) K204(11) K590(13) K199(5) K209(6) K4(4) K15(1) K426(10) K643(3) 0b 0b 0b 0b K102(3) K245(3) K280(12) K785(32) K99(2) K150(3) K1440(50) K1550(50) K345(12) K256(25) K585(12) K70(12) 945(50) K11,650(700) K828(150) K539.9(22) K551.9(22) K132(7) K225(20) K117(22) K233(9)
[4]
H H 13 C 13 C 1 H 1 H 13 C (in CN) 13 C (in CN) 15 N 15 N 1 H 1 H 13 C (in CN) 13 C (in CN) 13 C (in CH3) 13 C (in CH3) 1 H 1 H 13 C 13 C 1 H (in CH3) 1 H (in CH3) 1 H (in NH2) 1 H (in NH2) 13 C 13 C 15 N 15 N 13 C 13 C 17 O 17 O 17 O 17 O 17 O 17 O 17 O 33 S 17 O 19 F 19 F 33 S 33 S 33 S 33 S 1
Absolute shielding constants. Independent of density within the limit of experimental error.
[5]
[6]
[19]
[20]
[21] [7] [7] [7,8] [7,8] [7,8] [7,8] [18] [18] [18] [1] [1] [9,10]
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K. Jackowski / Journal of Molecular Structure 786 (2006) 215–219
Table 2 Spin–spin coupling parameters recently determined from the NMR spectra of gaseous mixtures Solute gas (A) 13
13
H Cb CH 15 CH13 3 C N
CH13 3 CN
13
CD3F
13
CH3I
15
NH2CH3
C6H6
Solvent gas (B)
n
Xe CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 SF6 CO2 CD3F CD3F CD3F SF6 CO2 SF6 Xe SF6 Xe SF6 Xe SF6 Xe CO2 Xe
1
J (XY)
J(CC) J(CC) 1 J(NC) 1 J(NC) 2 J(CH) 2 J(CH) 3 J(NH) 3 J(NH) 1 J(CC) 1 J(CC) 1 J(CH) 1 J(CH) 1 J(FC) 1 J(CD) 2 J(FD) 1 J(CH) 1 J(CH) 1 J(NH) 1 J(NH) 1 J(CH) 1 J(CH) 1 J(NC) 1 J(NC) 3 J(HH) 3 J(HH) 1 J(CH) 1 J(CH) 1
J0(XY) (Hz)
JAB(XY) (Hz mL molK1)
Ref.
174.77(2) 174.78(2) K16.20(1) K16.23(2) K10.18(2) K10.19(2) K1.34(2) K1.43(9) 60.12(5) 60.12(3) 134.03(1) 134.05(1) K160.72(5) 22.45(5) 7.20(5) 149.38(1) 149.39(1) K65.4(1) K65.4(1) 132.5(1) 132.5(1) K5.4(5) K5.4(5) 7.0(1) 7.0(1) 157.77(2) 157.78(2)
K30(20) K301(20) K62(14) K191(44) K13(37) 6(46) K325(77) K134(171) K86(87) K151(56) 95(24) 148(23) K1008(72) 43(46) 4.8(44) 111(5) 151(5) 0a 0a 0a 0a 0a 0a 0a 0a 0a 0a
[4] [5]
[6]
[1]
[19] [20]
[21]
a a
Independent of density within the limit of experimental error.
calculations of CH3CN shielding. For acetonitrile and other chemical compounds the experimental so(X) results have been already converted to the absolute shielding constants using the accepted standards: liquid TMS for 1H and 13C NMR spectra [4], gaseous ammonia for 15N NMR [11], liquid CFCl3 for 19F NMR [12], gaseous CO for 17O NMR [13] and gaseous OCS for 33S NMR [10,14]. Table 1 displays also the s(A–B)(X) parameters which describe intermolecular interactions in the gas phase. They are usually negative (deshielding effects) with one pronounced exception: the nitrogen nucleus in CH3CN is more shielded due to the interactions. It is consistent with our recent study of acetonitrile in liquid solutions and the sign of 15 N gas-to-solution shifts [15]. In the liquid state the selfassociation of CH3CN molecules is really strong, its boiling point is 81.6 8C. Nevertheless, we could still investigate it precisely in the gas phase using micrograms of suitable isotopomers of acetonitrile. The 17O and 33S NMR investigations have required some improvements in the detection technique of these weak signals. All the procedures are carefully described in our original papers [9,16]. 3.3. Spin–spin coupling in gaseous mixtures It is well-known that spin–spin coupling constants are less dependent on intermolecular interactions than shielding constants for the same nuclei. This problem has recently been reviewed [17] and many new examples of densitydependent spin–spin coupling constants have been reported. It
looks like that there is only a problem of precision of NMR measurements because every coupling constant is more or less dependent on intermolecular interactions. Some modest examples of it are shown in Table 2 and they prove that spin–spin coupling constants can also be detected and precisely analyzed in the gas phase. As shown the Jo(XY) parameters for isolated molecules are available from the samples of gaseous mixtures. Some gaseous solvents are really helpful for the more precise measurements of Jo(XY), cf. for example CO2 which gives the distinct density dependence of 1J(CC) in acetylene. It is interesting that in the same molecule some spin–spin couplings are strongly dependent on intermolecular interactions (cf. 1J(CF) in CD3F) and the others are almost independent of density, e.g. 1J(CD) and 2J(FD) in CD3F. 4. Conclusions New measurements of shielding and coupling constants for isolated molecules are available from the NMR measurements performed for diluted gaseous solutions (gaseous matrices). Such mixtures contain usually micrograms of investigated chemical compounds and a gas solvent. The measurements in gaseous matrices have been successfully performed for the chemical compound which exhibits strong molecular interactions in the liquid state, e.g. for acetonitrile. Similar investigations can be carried out for many other compounds being liquids at standard temperature and pressure. It allows the determination of NMR parameters free from intermolecular
K. Jackowski / Journal of Molecular Structure 786 (2006) 215–219
effects for many chemical compounds and such experimental data are excellent for many applications in chemical analysis. In particular, they allow one to estimate intermolecular effects in the condensed phases and they can be used for the reliable verification of ab initio calculations of NMR spectral parameters. Acknowledgements This work was supported by the Polish State Committee for Scientific Research as the research grant number 4 T09A 035 23 available in years 2002–2005. References [1] K. Jackowski, M. Kubiszewski, W. Makulski, J. Mol. Struct. 614 (2002) 267. [2] W.T. Raynes, A.D. Buckingham, H.J. Bernstein, J. Chem. Phys. 36 (1962) 3481. [3] C.J. Jameson, Bull. Magn. Reson. 3 (1980) 3. [4] K. Jackowski, M. Wilczek, M. Pecul, J. Sadlej, J. Phys. Chem. A 104 (2000) 5955; K. Jackowski, M. Wilczek, M. Pecul, J. Sadlej, J. Phys. Chem. A 104 (2000) 9806 (Erratum).
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[5] M. Wilczek, W. Koz´min´ski, K. Jackowski, Chem. Phys. Lett. 358 (2002) 263. [6] K. Jackowski, M. Wilczek, J. Mol. Struct. 651–653 (2003) 259. [7] W. Makulski, K. Jackowski, J. Mol. Struct. 651–653 (2003) 265. [8] W. Makulski, K. Jackowski, Chem. Phys. Lett. 341 (2001) 369. [9] K. Jackowski, M. Wilczek, W. Makulski, W. Koz´min´ski, J. Phys. Chem. A 106 (2002) 2829. [10] K. Jackowski, W. Makulski, W. Koz´min´ski, Magn. Res. Chem. 40 (2002) 563. [11] C.J. Jameson, A.K. Jameson, D. Oppusunggu, S. Wille, P.M. Burell, J. Mason, J. Chem. Phys. 74 (1981) 81. [12] C.J. Jameson, A.K. Jameson, P.M. Burell, J. Chem. Phys. 73 (1980) 6013. [13] R.E. Wasylishen, D.L. Bryce, J. Chem. Phys. 117 (2002) 10061. [14] R.E. Wasylishen, C. Connor, J.A. Friedrich, Can. J. Chem. 62 (1984) 981. [15] K. Jackowski, P. Bernatowicz, E. Wielogo´rska, Z. Phys. Chem. 216 (2002) 1401. [16] W. Koz´min´ski, K. Jackowski, Magn. Reson. Chem. 38 (2000) 459. [17] K. Jackowski, Int. J. Mol. Sci. 4 (2003) 135. [18] W. Makulski, K. Jackowski, J. Mol. Struct. 704 (2004) 219. [19] M. Wilczek, M. Kubiszewski, K. Jackowski, J. Mol. Struct. 704 (2004) 311. [20] E. Wielogo´rska, W. Makulski, W. Koz´min´ski, K. Jackowski, J. Mol. Struct. 704 (2004) 305. [21] K. Jackowski, E. Macia˛ga, M. Wilczek, J. Mol. Struct. 744–747 (2005) 101.