Nuclear Magnetic Shielding for Hydrogen in Selected Isolated Molecules

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Nuclear Magnetic Shielding for Hydrogen in Selected Isolated Molecules Piotr Garbacz, Karol Jackowski, Wlodzimierz Makulski, and Roderick Ernest Wasylishen J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 01 Nov 2012 Downloaded from http://pubs.acs.org on November 8, 2012

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The Journal of Physical Chemistry

Nuclear Magnetic Shielding for Hydrogen in Selected Isolated Molecules Piotr Garbacz,† Karol Jackowski,*,†Włodzimierz Makulski,† Roderick E. Wasylishen,*,‡ †

‡

University of Warsaw, Faculty of Chemistry, Pasteura 1, 02-093 Warszawa, Poland

University of Alberta, Gunning/Lemieux Chemistry Centre, Edmonton, AB T6G 2G2 Canada

Corresponding Authors *E-mail: K.J., kjack@ chem.uw.edu.pl; R.W., [email protected]

ACS Paragon Plus Environment

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ABSTRACT We present the results of gas-phase NMR measurements designed to yield a new experimental value for the absolute 1H 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 1H, 2H and 3He NMR frequencies for molecular hydrogen and atomic helium-3. The absolute isotropic magnetic shielding measured for molecular hydrogen, σ0(H2), 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 H2, HD and D2 molecules that are consistent with theoretical predictions. In addition, gas-phase 1H 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.

KEYWORDS Gas-phase NMR, shielding measurements, primary isotope effects on magnetic shielding, 1H chemical shifts, experimental value for the absolute magnetic shielding for dihydrogen, a new absolute magnetic shielding scale for hydrogen.


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INTRODUCTION

The magnetic shielding experienced by a nucleus in an atom or molecule is a property of fundamental interest to chemists and chemical physicists.1−5 The absolute shielding experienced by a nucleus describes the magnetic shielding relative to the bare nucleus of that isotope. For almost 60 years there was no simple direct experimental method of determining the absolute shielding experienced by a nucleus. Instead, experimentalists generally measured a parameter known as the chemical shift. The chemical shift for a nucleus S is defined as:6 ߜௌ ൌ

ఔೄ ିఔೃ ఔೃ

ൌ

ఙೃ ିఙೄ ଵିఙೃ

ൎ ߪோ െ ߪௌ

(1)

In eq 1 νS and νR are the resonance frequencies of the nucleus of interest and of some reference sample, respectively. In 1H,

13

C and

29

Si NMR studies the accepted reference sample is a 1%

solution of TMS (tetramethylsilane, Si(CH3)4) in chloroform-d6. From eq 1, it is clear that if one knows the absolute magnetic shielding for a nucleus of a particular isotope in one sample (σR), the absolute shielding of any nucleus of that isotope in any sample (σS) can be determined when the chemical shift is measured. While a 1% solution of TMS in deuterated chloroform serves as an excellent internal standard in solution NMR studies, efforts to determine absolute shielding for 1H nuclei in solution are fraught with difficulties.

Most important is the problem of

accounting for intermolecular interactions, and bulk susceptibility effects if an external reference is used. These problems are particularly acute for hydrogen isotopes since the chemical shift range is typically less than 15 ppm, while intermolecular interactions and bulk susceptibility effects can be a few ppm.7−9 Efforts to determine the absolute magnetic shielding for the 1H nuclei of the hydrogen molecule date back to early molecular beam experiments elegantly described by Ramsey in his


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classic textbook, Molecular Beams,10 where he indicates, σ0(H2) = 26.2(4) ppm for the first rotational state of molecular hydrogen.

Notably, Ramsey demonstrated how experimental

nuclear spin-rotation tensors are related to the paramagnetic shielding experienced by a nucleus, as was later discussed by other authors.11−14 Many years after Ramsey’s first studies, Reid obtained σ0(H2) = 26.23(18) ppm for hydrogen gas at 295 K.15 In 1983, Raynes and Panteli further refined this value reporting σ0(H2) = 26.363(4) ppm for hydrogen gas at 34 ºC.16 More recently, Sundholm and Gauss used earlier experimental spin-rotation data and their computational results to yield an isotropic shielding of 26.2886(15) ppm17 for molecular hydrogen at 296 K. The purpose of this contribution is two-fold. First, we wanted to obtain an independent experimental measurement of the hydrogen and deuterium absolute shielding for isolated H2, HD and D2 molecules using the method of direct shielding measurements as previously described.18 This method requires the measurement of the NMR resonance frequencies of 1H in the hydrogen isotopomer of interest and of 3He in atomic helium as a function of pressure. The fundamental parameters that our measurements are based on as well as all errors will be discussed. Our second goal was to determine accurate absolute values of isotropic 1H magnetic shielding for a set of small molecules using the shielding of molecular hydrogen as a reference.

All

measurements for the latter part of the study were performed on gaseous samples sealed in glass tubes for a wide range of densities. Observed chemical shifts were extrapolated to zero-density to yield shielding values at 300 K that are unaffected by intermolecular interactions and that are accurate to within ±0.01 ppm. Such data will be particularly valuable to the computational quantum chemistry community because it allows theoreticians to assess the accuracy of their computations on polyatomic molecules significantly more complex than hydrogen (e.g.,


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tetramethylsilane, benzene, pyridine, etc.) Finally, we wish to emphasize that rovibrational averaging effects are typically less than 1.0 ppm,19 so in comparing results obtained from computational methods with those obtained experimentally, it is important that the latter be accurate.


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EXPERIMENTAL SECTION NMR Measurements of Shielding in H2, HD, D2 and 3He An efficient high-pressure system was constructed in our laboratory (University of Warsaw) which permitted the NMR investigation of hydrogen and its deuterated isotopomers over high densities with pressure up to 300 bar (1 bar = 0.986923 atm). NMR spectra of gases were obtained in a stationary zirconia tube (Daedalus Innovations, USA). The tube contained a set of three coaxial capillaries: the inner, central and outer chambers contained pure liquid TMS, liquid nitromethane-d3 and the sample of interest, respectively (see Fig. 1A).

More detailed

information about our experiments at high pressure are given in a previous paper.20 The 1H, 2H and 3He NMR frequencies were measured using a Varian INOVA 500 spectrometer at 300 K operating at 500.6057, 76.8464 and 381.3586 MHz, respectively. Deuterium NMR spectra were acquired with a standard two channel Varian switchable 5 mm probe while 3He and 1H NMR spectra were acquired in a probe modified to observe 3He NMR spectra of atomic helium.21 Gases: H2 (Air Product, 99.9999%), HD (Isotec, 98% D), D2 (Isotec, 99.96%), 3He (Isotec, 99.96%), 4He (Air Product, 99.9%), Ne (Air Product, 99.999%) and Ar (Air Product, 99.9999%) from lecture bottles were used without further purification for the preparation of samples.

Measurement of Proton Shielding in Gaseous Solutions of Small Molecules For the measurement of chemical shifts relative to molecular hydrogen, gas samples were prepared by condensing pure gases or vapors from an accurately calibrated portion of a vacuum line into glass ampules using standard vacuum gas-rack procedures (vide infra). Ammonia–15N (98%

15

NH3), benzene–13C1 (99%

13

CC5H6), methanethiol (CH3SH ≥ 99.5%), pyridine (C5H5N


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99.8%, anhydrous), chloroform-d (CDCl3, 99.8% D), TMS (Si(CH3)4) and others were used to fill the ampules. Most of the gases and liquids were purchased from Aldrich and used without further purification. For gas–phase measurements xenon (99.9%, Messer Duisburg, Germany) was used as a gaseous solvent to prevent condensation. Glass tubes with outside diameters of 4 mm (approx. 5.5 cm long) were used for the preparation of the gas ampules (Fig. 1B). The volumes of the ampules and of the vacuum line were measured using mercury. The ampules were filled with pure gas (or gaseous mixtures), cooled with liquid nitrogen and sealed with a propane-butane torch. The sealed 4 mm o.d. gas samples were fit into standard 5 mm o.d. thinwalled NMR tubes (Wilmad 528-PP) with liquid toluene-d8 in the annular space. NMR chemical shifts were measured relative to 1% TMS in CDCl3 as an external reference standard always using the same lock solvent for the given measurement. The absolute frequency of the reference standard was determined with the lock system tuned to the CD3 signal of external toluene-d8. The constant 2H NMR frequency of the lock system (76.8464 MHz for our spectrometer) allowed us to preserve the same external magnetic field for all measurements. Standard onedimensional 1H NMR spectra were acquired at 300 K. The FID acquisition time was generally set to 2 s, the 45° pulse width was 6 µs and the spectral width varied from 400 to 800 Hz.


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RESULTS AND DISCUSSION Influence of Intermolecular Interactions on Nuclear Magnetic Resonance Frequency In the gas phase, the magnetic shielding of a nucleus in a molecule is modified by both intermolecular interactions and intramolecular motion, i.e., the shielding σ(ρ,T) depends on the density and temperature:22−26

σ(ρ,T) = σ0(T) + σ1(T)ρ + σ2(T)ρ2 + ...

(2)

where σ0(T) is the shielding in a molecule free of intermolecular interactions and therefore equivalent to an isolated molecule while σ1(T) is a measure of the effects on nuclear magnetic shielding of binary collisions. The higher-order terms, starting from σ2(T), are usually negligible for low-density samples. Thus, the observed dependence of the shielding on density is linear and the two shielding parameters, σ0(T) and σ1(T), are easily available for a given temperature (e.g., 300 K):

σ(ρ) = σ0 + σ1ρ

(3)

Eq 3 shows how σ0 and σ1 can be obtained from the density dependence of magnetic shielding for pure gases. However some compounds investigated here have insufficient vapor pressure at room temperature to obtain density-dependent chemical shifts. This difficulty can be overcome if the NMR measurements are performed for gaseous solutions (i.e., gaseous matrices). For a binary mixture of gas A, containing the nucleus whose shielding σA is observed, and gas B as the solvent, eq 3 can be modified: σA(ρA,ρB) = σ0A + σ1AAρA + σ1ABρB

(4)


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where ρA and ρB are the densities of A and B, respectively and σ0A is the shielding at the zerodensity limit. If the density of A is kept very low to eliminate the solute-solute molecular interactions eq 4 can be simplified to: σA(ρB) = σ0A + σ1ABρB

(5)

Thus the shielding parameter σ0A of the isolated molecule A can be obtained. The procedure entails undertaking NMR measurements in gaseous matrices. For compounds whose vapor pressure was sufficient to allow the determination of the pressure dependence of σ, we obtained identical values for σ0 using either eqs 3 or 5. Modern NMR spectrometers allow the measurement of absolute resonance frequencies with high accuracy. Assuming that the external magnetic field remains constant, the resonance frequency of a reference standard, νR, is also constant during the course of a set of NMR measurements. From eq 1, it is clear that the absolute resonance frequency νS is proportional to the shielding value σS with the opposite sign. Eqs 3-5 and can be rewritten using absolute frequencies instead of shielding parameters but for the present work it is useful just to rewrite eq 5:

νA = ν0A + ν1ABρB

(6)

In the case of pure gases, A = B andν0A is the resonance frequency of a nucleus in an isolated molecule A. This method was used to measure the 1H and 2H resonance frequencies of the hydrogen isotopomers (A = H2, HD and D2) in noble gases (B = He, Ne and Ar). Fig. 2 shows the density-dependent 1H and 2H resonance frequencies of the hydrogen isotopomers in neon solutions as an example.

As shown all the 1H and 2H resonance frequencies are linearly

dependent on density allowing the accurate extrapolation of measurements to zero-density.


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From the ν0A values for the investigated molecules we can determinate the 1H and 2H shielding values according to eq 7 as described below.

1

H and 2H Shielding in Hydrogen Molecules

The measurements of magnetic shielding in hydrogen isotopomers (H2, HD and D2) were carried out using a method described earlier,18 based on eq 7: σX = 1 −

µ νX I ⋅ He ⋅ X (1 − σ He ) ν He µX I He

(7)

where ν, µ and I are the resonance frequencies, the nuclear magnetic moments and the spin quantum numbers of appropriate nuclei (X = 1H or 2H). The constants used in eq 7 are: •

the best-known proton and deuteron magnetic moments, µH = 2.792 847 356(23)µN and µD = 0.8574382308(72)µN,27 where µN is the nuclear magneton;

•

the magnetic moment for the helion, µHe is −2.127 625 306(25)µN;27

•

the recent best value of magnetic shielding in an isolated helium-3 atom, σHe = 59.96743(10) ppm;28

•

our previous study of the resonance frequency for an isolated helium-3 atom, ν0(3He) = 381 357 225.1(1) Hz for B0 ≈ 11.75 T;21

•

the extrapolated 1H and 2H resonance frequencies obtained in the present work: ν0(H2) = 500 609 004.0(5) Hz, ν0(HD) = 500 608 987.1(5) Hz, ν0(HD) = 76 846 533.6(5) Hz and ν0(D2) = 76 846 530.3(5) Hz (see Fig. 2). Following the steps outlined in the previous section, we determined the shielding of H2,

HD and D2 molecules free from intermolecular interactions, which is equivalent to the shielding


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of isolated molecules. As summarized in Table 1, application of this method yields σ0(H2) = 26.293(5) ppm at 300 K. Values for σ0(HD), σ0(HD) and σ0(D2) are also given in Table 1. Eq 7 can be equivalently written in terms of the magnetogyric ratios, γX and γHe, or by the nuclear g factors, ge and gHe. Using accepted values of these constants27 and the frequency ratios given above, identical shielding parameters as presented in Table 1 were obtained for the various isotopomers of the hydrogen molecule. Table 1 presents our results together with earlier experimental and theoretical data given by Sundholm and Gauss17 and recent calculations performed by Jaszuński et al.29 The discrepancy between our experimental and recent calculated results for any given molecule is less than 0.015 ppm, suggesting that the absolute shielding scale for protons is known to a similar accuracy. With the possible exception of σ0(HD), our direct measurements of shielding are of slightly lower precision than the experimental results deduced from spin-rotation constants by Sundholm and Gauss.17 Our experimental value for the primary isotope effect (σ0(HD) − σ0(HD)) is 12(6) ppb while the earlier result is 11(6) ppb.17 The good agreement is important since quantumchemical calculations are unable to estimate the primary isotope effect. In conclusion, we are satisfied that the direct measurement of nuclear magnetic shielding for hydrogen reported here yields reliable results; therefore, for other applications described here we used σ0(H2) = 26.293(5) ppm as the shielding of an isolated hydrogen molecule at 300 K (Fig. 3).

Proton Shielding of Gaseous Solutions of Small Molecules Table 2 lists 1H NMR chemical shifts and proton shielding values for 71 molecules,30−52 including 48 reported in this study; the shielding for H2 reported here is used as the shielding reference standard.

A typical 1H spectrum, of CH3SH, is shown in Fig 4; the pressure


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dependence of the shielding for this molecule is illustrated in Fig. 5. With the exception of H2, these results were first obtained as chemical shifts measured relative to liquid TMS and converted to the shifts relative to 1% TMS in CDCl3 on the basis of experimental relation: δS(1% TMS in CDCl3) = δS(pure TMS) + 0.665 ppm. Consequently the precision of the chemical shift measurements is slightly better than those for proton shielding which also include the error in the shielding measurements for a hydrogen molecule. All the values presented in Table 2 were obtained from the linear extrapolation of appropriate experimental data according to eqs 3 or 5. Therefore the shielding values are those expected for molecules free from intermolecular interactions and can be directly compared with any calculated or experimental shielding parameters for isolated molecules at 300K. See Fig. 3 for an illustration of the relationship between the chemical shifts of some key compounds. Proton shielding values for several small molecules were obtained in the gas phase 50 years ago; these are summarized in Rummens’ review.8 Values for several compounds can also be found in early papers by Schneider and coworkers.53,54 However the early data are generally of lower precision. For example, in Rummen’s review,8 three values are given for the chemical shift of H2 relative to ethane (C2H6): 3.35, 3.55 and 3.58 ppm; this compares to the value for

σ0(13C2H6) – σ0(H2) of 3.593(2) ppm listed in Table 2. The secondary isotope effect, σ0(13C2H6) – σ0(12C2H6), can result in slightly greater proton shielding for

13

C2H6 compared to that for

12

C2H6, but this effect is expected to be on the order of 2 ppb55,56 and hence does not account for

the discrepancies with the previous three results. Later studies of proton shielding in gaseous compounds were less common.

For example, Jameson’s 1991 review on gas-phase NMR

spectroscopy57 does not include proton shielding data for isolated molecules. Nevertheless, some gas-phase 1H shielding data have been reported but the results are usually not extrapolated to


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zero-density. For example, in a recent 1H NMR study of gaseous compounds, Fujiwara et al.58 reported the 1H chemical shift of H2 as δ = 7.40(1) ppm at 0.18 MPa and 298 K; the correction for pressure and temperature conditions is less than the experimental error. This value is slightly different from our measurements which give δ = 7.187(2) ppm. However, it is important to note that neither value is corrected for susceptibility effects. Kantola et al.59 determined the proton shielding tensors in methyl halides and they also reported the σ0(1H) values for CH3F, CH3Cl, CH3Br and CH3I, 26.623(26), 27.924(25), 28.319(26) and 28.779(34) ppm, respectively. Within experimental error, these results are in agreement with the corresponding values listed in Table 2. Also listed in Table 2 is the shielding of water in the gas phase at 300 K, σ0(H2O) = 30.102(8) ppm, which, as outlined below, is consistent with the accepted value for liquid water. Using data presented by Hindman,60 at 300 K the proton shielding for water in the gas phase is 4.347(5) ppm greater than that in the liquid. That is, at 300 K, σ(H2O(l)) = 25.755 ppm. Using results from Petley and Donaldson,61 it is clear that the hydrogen shielding for liquid water will decrease by approximately 0.019 ppm on decreasing the temperature from 300.0 to 298.15 K. Thus, using our data in Table 2 and the corrections outlined above, we calculate σ(H2O(l), 25 oC) = 25.736(10) ppm, comparable to earlier reported values62,63 and to the currently accepted CODATA value of 25.694(14) ppm.27 Any attempt to account for the discrepancy of 0.042 ppm between the latter value and our value would be speculation on our part. We would like to point out that we have noticed that the σ0(14NH3) value for ammonia is often quoted as 32.10 ± 0.02 ppm14 at 300 K; however, this value is clearly out of line with our value of 30.727(8) ppm. Our value is within experimental error of the early value that one can


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derive from early results presented by Schneider et al.53,54 therefore we suggest it be adopted as the accepted value.


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Further Comparisons With Earlier Work For many years, the hydrogen shielding of gaseous methane was taken as the absolute shielding reference standard. The accepted value generally quoted in the literature, σ = 30.611 ± 0.024 ppm, was derived by Raynes in 197864 from the mean value for σ(H2O(g)) obtained by Hindman at various temperatures and pressures.60 It is instructive to compare some fundamental values of shielding parameters reported in earlier studies60,64,65 with those obtained in the current study; these are listed in Table 3. For all parameters, the early reported parameters are very close or within experimental error of the current values. Given the quality of NMR instrumentation available at that time, this agreement is remarkable. Proton chemical shift values relative to methane for several of the compounds investigated in the present study were first reported in 1958 by Schneider and coworkers.53 A comparison of the early and current data is presented in Table 4. Again, the early data, acquired at 40 MHz, is in remarkable agreement with the much more accurate data reported here. It is important to note that the data from Schneider et al. was obtained at relatively high pressures (e.g., 10 atm.) and generally at 295 K; the interested reader should consult the original reference for details.53 Although the chemical shift of hydrogen was not reported in this paper, it was later reported in the classic 1959 text by the same authors.66 The reported value relative to methane was 4.20 ppm, i.e., σ(CH4, gas) − σ (H2, gas) = 4.20 ppm, approximately 0.13 ppm less than the value reported here, 4.340 (6) ppm. Note that at 40 MHz, 0.13 ppm corresponds to 5.2 Hz and given that the 1H NMR spectrum of gaseous hydrogen is broad, this agreement is regarded as very good.9 For a comprehensive summary of early 1H gas-phase chemical shifts, the reader should consult Table 18 (pp. 37−39) of ref. 8.


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Finally we want to emphasize that the chemical shifts reported in Table 2 are not corrected for the bulk magnetic susceptibility of our reference, 1% TMS in chloroform-d. Using the bulk magnetic susceptibility of chloroform reported by Hoffman, we estimate that this correction due to the chloroform-d solvent is approximately 3.10 ppm. That is, the chemical shifts given in Table 2 can be corrected for the bulk susceptibility of chloroform by subtracting 3.10 ppm from each value. For example, for H2(g) the "corrected" 1H chemical shift is 7.187 − 3.10 or 4.09 ppm. This value is much closer to the value one obtains by dissolving H2 in a chloroform-d, 1% TMS solution, 4.622 ppm. The difference, 4.09 − 4.62 or −0.53 ppm is an intrinsic solvent effect arising from H2-chloroform intermolecular interactions. hydrogen have been reported and discussed by Evans.9

Such solvent shifts for molecular This of course emphasizes the

advantages of working with the magnetic shielding data given in Table 2 which are rid of bulk magnetic susceptibility effects and intermolecular interactions (i.e., solvent effects).


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CONCLUSIONS Our experimental values for σ0(H2), σ0(HD), σ0(HD) and σ0(D2) at 300 K are in excellent agreement with previous values derived using sound but less direct methods.17 Our value of σ0 for dihydrogen, 26.293(5) ppm, is within experimental error of the highly accurate rovibrational averaged value at 300 K recently calculated by Jaszuński et al.29 Measurements of the density-dependent 1H chemical shifts in 71 molecules, including dihydrogen, have allowed us to establish 115 benchmark values of 1H isotropic magnetic shielding values. These range over more than 20 ppm, from a maximum value of 43.92 ppm for hydrogen iodide to a minimum of 19.258 ppm for CF3COOH (see Fig. 3). Because the errors in these 115 values are generally less than 0.01 ppm, they should prove invaluable to computational scientists interested in testing procedures to deal with rovibrational averaging of magnetic shielding parameters (e.g., H2O, NH3),19 relativistic effects in magnetic shielding parameters (e.g., values for HCl, HBr and HI)67−69 as well as testing the general reliability of their computational methods.


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ACKNOWLEDGMENTS We acknowledge financial support from the Polish Ministry of Science and Higher Education, research grant N N204 244134 (2008-2011). This project was partly co-operated within the Foundation for Polish Science MPD Program co-financed by the European Union (EU) European Regional Development Fund. REW acknowledges the financial support of the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Government of Canada for a Canada Research Chair in Physical Chemistry. We thank Dr. Guy Bernard for his interest in this work and for many helpful suggestions.


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Jackowski, K.; Jaszuński, M. Concepts Magn. Reson. A 2007, 30, 246−260.

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Harris, R. K.; Becker, E. D.; Cabral de Menezes, S. M.; Goodfellow, R.; Granger, P.; Pure Appl. Chem. 2001, 73, 1795−1818. Reprinted in Magn. Reson. Chem. 2002, 40, 489−505.

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Hoffman, R. E. Magn. Reson. Chem. 2006, 44, 606−616.

8

Rummens, F. H. A. NMR Basic Principles and Progress; Diehl, P.; Fluck, E; Kosfeld, R., Eds.; Springer-Verlag: New York, 1975;Vol. 10, pp. 37−39.

9

Evans, D. F. J. Magn. Reson. 1977, 26, 369−371.

10

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11

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Table 1. Experimental and Calculated Values for the Shielding Parameters of Isolated H2, HD and D2 at 300 K. σ0(H2) Gauss et al.a (Exp) This work (Exp)

σ0(HD)

σ0(HD)

σ0(D2)

26.2886(15) 26.3329(12) 26.3436(48) 26.3884(20) 26.293(5)

26.327(3)

26.339(3)

26.388(3)

Gauss et al.a (Calc)

26.2983

26.3434

26.3434

26.3958

Jaszuński et al.b (Calc)

26.2980

26.3416

26.3416

26.3930

a

Ref. 17; experimental values deduced from spin-rotation constants.

b

Ref. 29.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2.

1

Page 24 of 43

H NMR Chemical Shifts and Absolute Shielding Values (ppm) for Molecules Free

From Intermolecular Interactions at 300 K. No

Molecule

1

Hydrogen

Nucleus

δ(ppm)a

σ0(ppm)b

H2

7.187(2)

26.293(5)

HD

7.128(3)

26.327(3)

Ref. This work

2

Hydrogen chloride

HCl

2.356(2)

31.124(7)

This work

3

Hydrogen bromide

HBr

−1.557(2)

35.037(7)

This work

4

Hydrogen iodide

HI

−10.440(5)

43.920(10)

This work

5

Methane

13

CH4

2.847(1)

30.633(6)

30

15

NH3

2.752(7)

30.728(12)

14

NH3

2.753(3)

30.727(8)

H2 O

3.378(3)

30.102(8)

HOD

3.339(3)

30.141(8)

6

7

Ammonia

31

32

Water

8

Hydrogen sulfide

H2 S

2.908(1)

30.572(6)

This work

9

Phosphine

PH3

4.269(5)

29.211(10)

33

10

Silane

SiH4

5.815(1)

27.665(6)

34

11

Germane

GeH4

5.666(1)

27.814(6)

34

12

Acetylene

4.163(1)

29.317(6)

35

13

Ethylene

8.017(3)

25.463(7)

36

14

Ethane

3.593(2)

29.887(7)

36

(CH3)2CH2

3.648(2)

29.832(7)

15

Propane (CH3)2CH2

4.095(2)

29.385(7)

(CH3)2(CH2)2

3.653(2)

29.827(7)

(CH3)2(CH2)2

4.071(3)

29.409(8)

(CH3)3CH

3.636(2)

29.844(7)

(CH3)3CH

4.453(3)

29.027(8)

16

17

13

C2H2

C2H4 13

C2H6

This work This work

n-Butane

i-Butane

This work


24

Page 25 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18

Allene

19

Propyne

H2CCCH2

7.201(1)

26.279(4)

CH3CCH

4.354(1)

29.126(4)

CH3CCH

4.126(1)

29.354(4)

This work This work

20

Cyclopropane

(CH2)3

2.910(1)

30.570(6)

37

21

Cyclopentane

(CH2)5

4.298(1)

29.182(6)

This work

22

Cyclohexane

(CH2)6

4.240(1)

29.240(6)

This work

23

Cycloheptane

(CH2)7

4.347(2)

29.133(7)

This work

CH3CHCH2

4.379(1)

29.101(6)

CH3CHCH2

8.510(1)

24.970(6)

CH3CHCHHcis

7.684(1)

25.796(6)

CH3CHCHHtrans

7.569(1)

25.911(6)

CH3I

4.687(3)

28.793(8)

39

CH3Br

5.152(3)

28.328(8)

40

CH3Cl

5.548(4)

27.932(9)

41

24

38

Propylene

13

25

Iodomethane

26

Bromomethane

13

27

Chloromethane

13

28

Fluoromethane

CH3F

6.845(3)

26.635(8)

42

29

Difluoromethane

CH2F2

8.149(3)

25.996(8)

43

30

Trifluoromethane

CHF3

8.897(5)

24.583(10)

44

31

Nitromethane

CH3NO2

6.729(1)

26.751(6)

This work

32

Dichloromethane

CH2Cl2

7.810(2)

25.670(7)

This work

33

Trichlormethane

CHCl3

9.821(2)

23.659(7)

This work

34

1,2-Dichloroethane

C2H4Cl2

6.241(2)

27.239(7)

This work

CH313C15N

4.327(1)

29.224(6)

45

CH313CN

4.331(3)

29.149(8)

46

CH3OH

6.130(5)

27.350(10)

CH3OD

6.121(5)

27.359(10)

CH3OH

2.809(25)

30.671(30)

35

36

Acetonitrile

Methanol

13

47


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

37

38

39

40

41

42

43

Ethanol

Page 26 of 43

CH3CH2OH

4.088(1)

29.392(6)

CH3CH2OH

6.350(1)

27.130(6)

CH3CH2OH

3.154(1)

30.326(6)

CH3CHO

4.623(3)

28.857(8)

CH3CHO

12.412(2)

21.068(7)

HCOOCH3

10.541(1)

22.939(6)

HCOOCH3

6.347(1)

27.133(6)

CH3COOCH3

4.556(1)

28.924(6)

CH3COOCH3

6.286(1)

27.194(6)

CH3COOCH2CH3

3.903(2)

29.577(7)

CH3COOCH2CH3

4.552(2)

28.928(7)

CH3COOCH2CH3

6.792(2)

26.688(7)

CH3COCH2CH3

4.631(1)

28.849(6)

CH3COCH2CH3

4.985(2)

27.830(6)

CH3COCH2CH3

3.747(1)

29.733(6)

CF3CH2OH

6.484(1)

26.996(6)

CF3CH2OH

3.941(1)

29.539(6)

Acetaldehyde

This work

Methylformate

This work

Methyl acetate

Ethyl acetate

2-Butanone

This work

This work

This work

This work

This work

2,2,2,-Trifluoroethanol

44

Trifluoroacetic acid

CF3COOH

14.222(1)

19.258(6)

This work

45

Acetone

(CH3)2CO

4.630(1)

28.850(6)

This work

CH3CHOCH2

3.899(2)

29.234(7)

CH3CHOCH2(cis)

5.429(5)

28.051(10)

CH3CHOCH2(trans)

4.836(5)

28.644(10)

CH3CHOCH2

5.196(5)

28.284(10)

CH3COCF3

4.896(3)

28.584(8)

CH3OCH3

5.940(2)

27.540(7)

CH3OCD3

5.935(2)

27.545(7)

46

This work

Propylene oxide

47

1,1,1-Trifluoroacetone

48

Dimethyl ether

This work 48


26

Page 27 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


49

50

51

52

53 54

55

(CH3CH2)2O

3.864(1)

29.616(6)

(CH3CH2)2O

6.115(1)

27.365(6)

(CH3CH2CH2)2O

3.674(5)

29.806(10)

(CH3CH2CH2)2O

4.299(5)

29.181(10)

(CH3CH2CH2)2O

6.048(5)

27.432(10)

CH315NH2

5.175(4)

28.305(9)

CH315NH2

3.113(4)

30.367(9)

(CH3)2NH

5.115(2)

28.365(7)

(CH3)2NH

2.892(33)

30.588(38)

CH3SH

4.649(2)

28.831(7)

CH3SH

3.592(2)

29.888(7)

C6H6

9.945(5)

23.535(10)

ortho-H

9.660(5)

23.820(10)

meta-H

9.912(5)

23.568(10)

para-H

9.697(5)

23.783(10)

Diethyl ether

Dipropyl ether

Methyl amine

This work

49

Dimethyl amine

This work

This work

Methanethiol Benzene

Fluorobenzene

This work

This work

This work

56

1,3,5-Trifluorobenzene

C6H3F3

9.207(2)

24.273(7)

This work

57

Pentafluorobenzene

C6HF5

9.366(2)

24.114(7)

This work

ortho-H

10.331(2)

23.149(7)

meta-H

10.059(2)

23.421(7)

para-H

10.104(2)

23.376(7)

H-2,H-6

11.277(7)

22.203(12)

H-3,H-5

9.743(7)

23.737(12)

H-4

10.149(6)

23.331(11)

H-3

8.974(1)

24.506(6)

H-2

10.023(1)

23.457(6)

(CH2)4O2

6.285(1)

27.195(6)

58

59

60 61

α,α,α-Trifluorotoluene

Pyridine

Furan 1,4-Dioxane

This work

This work

This work This work


27


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62

Page 28 of 43

CH2(OCH3)2

7.185(2)

26.295(7)

CH2(OCH3)2

6.000(2)

27.480(7)

Dimethoxymethane

This work

63

Neopentane

C(CH3)4

3.654(2)

29.826(7)

This work

64

Tetramethylsilane (TMS)

Si(CH3)4

2.697(2)

30.783(7)

This work

65

Tetramethylgermane

Ge(CH3)4

2.802(2)

30.678(7)

This work

66

Tetramethyltin

Sn(CH3)4

2.739(2)

30.741(7)

50

67

Tetramethyllead

Pb(CH3)4

3.398(2)

30.082(7)

This work

Si(CH3CH2)4

3.730(2)

29.750(7)

68

Tetraethylsilane Si(CH3CH2)4

3.316(2)

30.164(7)

CF3CHClBr

8.304 (1)

25.176(6)

(CF3)CHOCH2F

6.950(3)

26.530(8)

(CF3)CHOCH2F

7.976(3)

25.504(8)

CF3CHFOCHF2

8.488(1)

24.992(6)

CF3CHFOCHF2

8.902(1)

24.578(6)

69

Halotane

70

Sevoflurane

71

This work 51 52

Desflurane

This work

a

Relative to external 1% TMS in liquid CDCl3. Note that the chemical shifts are not corrected for the magnetic susceptibility of the latter.

b

Assuming the absolute shielding of protons in the H2 molecule is 26.293(5) ppm. This yields a value of σ0 = 32.815(5) ppm for liquid TMS at 300 K and σ0 = 33.480(5) ppm for 1 % TMS in CDCl3 at 300 K, but the latter two values are not corrected for magnetic susceptibility. However, σ0 values in the table are not affected by magnetic susceptibility.


28

Page 29 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Comparison of Early and Current 1H Shielding Values. 1

H Shielding Parameter

Early Work/ppm

This Work/ppm

Ref.

σ0(CH4(g)) – σ0(H2O(g))

0.559(19)

0.531(8)

60

σ0(CH4(g))

30.611(24)

30.633(6)

60,64

σ0(CH4(g)) – σ0(H2(g))

4.35(15)

4.340(6)

65

σ0(H2(g))

26.26(15)

26.293(5)

64,65


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 43

Table 4. Comparison of Early and Current 1H Chemical Shifts.

a

Early Work/ppmb

This Work/ppmc

CH4

0.00

0.000

C2H6

0.75

0.746

C2H4

5.18

5.170

C2H2

1.35

1.316

NH3

−0.08

−0.095

H2O

0.60

0.531

SiH4

3.00

2.968

PH3

1.48

1.422

H2S

0.08

0.061

HCl

−0.45

−0.491

HBr HI

−4.35 −13.25

−4.404 −13.287

All compounds were measured in the gas phase; values are relative to methane.

b

c

Compounda

From Ref. 53.

These are the values of σ0(CH4) - σ0(compound).


30

Page 31 of 43

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Figure Captions.

Fig. 1. (A) Zirconia tube used in the measurement of the proton shielding for the hydrogen molecule and (B) the NMR tube containing the gaseous solution of molecules in a glass ampule. Fig. 2. Density-dependent 2H and 1H resonance frequencies of hydrogen isotopomers in gaseous neon solutions. Fig. 3. Proton absolute shielding, σ(1H), and chemical shift, δ(1H), scales. The chemical shifts were determined relative to a 1% solution of TMS in chloroform-d (labelled TMS* above). Note that the latter is uncorrected for bulk magnetic susceptibility. Fig. 4. 1H NMR spectra of (A) liquid and (B) gaseous (≈ 5 bar) methanethiol (CH3SH) acquired at 300 K. Fig. 5. Dependence of the 1H chemical shifts of methanethiol(g) on density.


31


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Page 32 of 43

Fig. 1. (A) Zirconia tube used in the measurement of the proton shielding for the hydrogen molecule and (B) the NMR tube containing the gaseous solution of molecules in a glass ampule.


32

Page 33 of 43

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Fig. 2. Density-dependent 2H and 1H resonance frequencies of hydrogen isotopomers in gaseous neon solutions.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 43

Fig. 3. Proton absolute shielding, σ(1H), and chemical shift, δ(1H), scales. The chemical shifts were determined relative to a 1% solution of TMS in chloroform-d (labelled TMS* above). Note that the latter is uncorrected for bulk magnetic susceptibility.


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Fig. 4. 1H NMR spectra of (A) liquid and (B) gaseous (≈ 5 bar) methanethiol (CH3SH) acquired at 300 K.


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Fig. 5. Dependence of the 1H chemical shifts of methanethiol(g) on density.


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Graphical Abstract.


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Fig. 1. (A) Zirconia tube used in the measurement of the proton shielding for the hydrogen molecule and (B) the NMR tube containing the gaseous solution of molecules in a glass ampule. 188x95mm (120 x 120 DPI)


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Fig. 2. Density-dependent 2H and 1H resonance frequencies of hydrogen isotopomers in gaseous neon solutions. 406x284mm (96 x 96 DPI)



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Fig. 3. Proton absolute shielding, σ(1H), and chemical shift, δ(1H), scales. The chemical shifts were determined relative to a 1% solution of TMS in chloroform d (labelled TMS* above). Note that the latter is uncorrected for bulk magnetic susceptibility. 307x125mm (120 x 120 DPI)


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Fig. 4.1H NMR spectra of (A) liquid and (B) gaseous (≈ 5 bar) methanethiol (CH3SH) acquired at 300 K. 1965x1304mm (96 x 96 DPI)



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Fig. 5. Dependence of the 1H chemical shifts of methanethiol(g) on density. 63x65mm (300 x 300 DPI)


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graphical abrstract 227x112mm (120 x 120 DPI)


Nuclear Magnetic Shielding for Hydrogen in Selected Isolated Molecules

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