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31P NMR reference standards for aqueous samples
JOURNAL
OF MAGNETIC
RESONANCE
49, 172- 174 ( 1982)
31PNMR ReferenceStandards for AqueousSamples MICHAEL School
of Chemistry,
Macquarie
BATLEY University, Received
AND North May
JOHN
W.
Ryde,
New
REDMOND South
Wales
21 I3, Australia
17, 1982
It is well known that chemical shifts measured relative to a standard in a solvent of different diamagnetic susceptibility depend on sample geometry (1). The susceptibility correction appears to have been neglected, however, when reporting phosphate chemical shifts. Consequently, values differing by 1 ppm have been reported for the same compounds measured on instruments with iron core and superconducting magnets (2,3). As shown in Fig. 1, the signal from 85% phosphoric acid shifts upfield relative to the secondary standard, aqueous tetrahydroxyphosphonium perchlorate (4) when measured using a superconducting magnet. The correction to the applied field is calculated assuming that the molecules occupy spherical cavities in the solvent. If the sample shape is also spherical the local field inside the cavity is the same as the external field (5). A spherical reference sample, therefore, gives chemical shifts that are independent of the difference in magnetic susceptibilities between the sample and reference material. Usually, however, concentric cylindrical tubes are used. The local fields inside spherical cavities in the sample and reference solutions then differ by (2*/3)& - xsample)if the field is perpendicular and -4/3(xref - x~~,,,~,Jif the field is parallel to the axis of the tubes, where x is the magnetic susceptibility per unit volume. The shape factors are more frequently quoted in connection with dielectric polarization (5). To make these corrections we found it necessary to measure the diamagnetic susceptibility of 85% phosphoric acid using the method of Frei and Bernstein (1). A commercial spherical microcell was filled with the phosphoric acid and placed inside a 5-mm-diameter cylindrical tube (6). Calibration of the shape factor was performed by comparing frequencies observed with the spherical part of the reference surrounded by air and water at 25°C. Using the reported value for the susceptibility of water (I), it was found that a shape factor 3.1% higher than the theoretical value of 47r/3 was required. This compares favorably with the increase of 1.8% employed by Frei and Bernstein. The volume susceptibility of air was taken as 0.028 X 10e6, calculated by assuming a composition of 21% oxygen and 79% nitrogen and using reported molar susceptibilities (7). The measured susceptibility difference between water and phosphoric acid was -0.197 X 10e6. This implies that the diamagnetic susceptibility of 85% phosphoric acid is 9% higher than that calculated on the basis of the reported molar susceptibilities of phosphoric acid and water (7). The measurement was independent of whether HZ0 or D20 was used. 172 0022-2364/82/100172-03$02.00/O Copyright 0 1982 by Academic Press. Inc. All rights of reproduction in any form reserved.
173
COMMUNICATIONS
I 1.0
I 0.5
I 0 Chemical
I -0.5 Shift
I - 1.0 (ppm
I -1.5
1
FIG. I. “P NMR spectrum of 85% phosphoric acid externally referenced to aqueous tetrahydroxyphosphonium perchlorate. Measurement carried out with a Varian XL-200 spectrometer at 80.98 MHz. The observed chemical-shift difference is 0.822 ppm.
It follows that chemical-shift values measured relative to 85% phosphoric acid in a cylindrical tube should be corrected by +0.37 ppm if measured using an iron magnet and by -0.73 ppm for a superconducting magnet. The corrections are accurate to kO.01 ppm. The resulting values represent chemical shifts relative to 85% phosphoric acid in an ideal sphere. When these corrections are applied to the chemical shift of the secondary 3’P standard, 1.0 A4 methylene diphosphonate at pH 9.5 in D20 (8), the values obtained with the two types of instrument are in excellent agreement. Using the convention that high frequencies are positive (9) our correction to the value measured using an iron magnet (8) is 16.7 ppm. We obtained a corrected value of 16.72 + 0.02 ppm at 25°C using a solenoid magnet. This agreement indicates that the diamagnetic susceptibility of the reference sample is the same as that of pure water. Chemical shifts in aqueous solution may therefore be measured relative to this sample without further correction. Unfortunately, the same is not true for the other secondary standard, 0.2 M phosphoric acid in 14% perchloric acid. The susceptibility of this sample appears to be 3% higher than that for pure water. Therefore, aqueous samples measured relative to this material also require correction, as is the case for 85% phosphoric acid. The corrections of 0.07 and -0.14 ppm are, however, much smaller. The chemical shift of this signal relative to a spherical sample of 85% phosphoric acid is 0.229 ppm. There is an alternative approach to the avoidance of geometry-dependent chem-
174
COMMUNICATIONS
ical shifts. The secondary standards could be referred to external 85% phosphoric acid, using concentric tubes in a conventional spectrometer. The advantage of this approach is that the presently greater volume of published results would require no adjustment. All chemical shifts obtained on instruments with superconducting magnets could then be measured relative to the secondary standard and would require no further correction. We do not recommend this practice. There would be no difference in the ease of reporting new results and the two types of spectrometer would still require the use of different values for the tetrahydroxyphosphonium perchlorate secondary standard. There is, on the other hand, a significant disadvantage in having a primary standard that cannot be directly measured (in the case of a superconducting magnet). Furthermore, results obtained in different solvents would not be directly comparable, and if corrections were attempted they would have to be in a form appropriate for conventional geometry, even for results obtained with a superconducting magnet; this could cause considerable confusion. This comment applies a fortiori to proton spectra where aqueous samples are often referred to external standards. In summary, we recommend that 31P chemical shifts in aqueous samples be measured relative to 85% phosphoric acid in an ideal spherical container inside the solution. This may be achieved practically by using concentric cylindrical tubes and one of the following reference samples: (1) phosphoric acid (85%) at 0.37 ppm (iron magnet) or -0.73 ppm (solenoid); (2) methylene diphosphonate ( 1.O M, pH 9.5) at 16.72 ppm (either geometry); (3) aqueous tetrahydroxyphosphonium perchlorate at 0.30 ppm (iron magnet) or 0.09 ppm (solenoid). For other solvents, measurements using both geometries can rapidly determine the appropriate correction. The corrected chemical shift of a compound in the solvent is given by (%a, + Y3aB), where (TAis the apparent chemical shift measured relative to 85% phosphoric acid in a cylindrical tube on an iron magnet spectrometer, and (TVis that measured using a solenoid. Once a single chemical shift has been so determined, it may be used as a secondary standard for chemical shifts in that solvent. ACKNOWLEDGMENTS This of New
work South
was supported Wales,
by the Australian
is thanked
for
Research
Grants
Committee.
Dr. M. Gallagher,
University
discussion. REFERENCES
1. K. FREI AND H. J. BERNSTEIN, J. Chem. Phys. 37, 189 1 (1962). 2. P. D. RICK, L. W.-M. FUNG, C. Ho, AND M. J. OSBORN, J. Eiol.
Chem.
252,
4904
(1977).
3. M. R. ROSNER, H. G. KHORANA, AND A. C. SATTERTHWAIT, J. Viol. Chem. 254, 5918 (1979). 4. T. GLONEK AND J. R. VAN WASER, J. Magn. Reson. 13, 390 (1974). 3rd ed., p. 378, Wiley, New York, 1966; (b) 5. (a) C. KITTEL, “Introduction to Solid State Physics,” 6.
J. K. BECCONSALL, G. D. DAVE& AND W. R. ANDERSON, J. Am. Chem. R. J. MYERS, “Molecular Magnetism and Magnetic Resonance Spectroscopy,” Hall, Englewood Cliffs, N.J., 1973.
7. G. FoEx, “Tables of Constants and Numerical Data (UICPA), Diamagnetisme et Paramagnetisme,” Masson, Paris, 1957. 8. C. T. BURT, T. GLONEK, AND M. BARANY, J. Biol. Chem. 251, 9. Pure Appl. Chem. 45, 217 (1976).
No. 2584
Sot.
92,430 p. 188,
7, Constantes (1976).
(1970). Prentice-
Selectionees
OF MAGNETIC
RESONANCE
49, 172- 174 ( 1982)
31PNMR ReferenceStandards for AqueousSamples MICHAEL School
of Chemistry,
Macquarie
BATLEY University, Received
AND North May
JOHN
W.
Ryde,
New
REDMOND South
Wales
21 I3, Australia
17, 1982
It is well known that chemical shifts measured relative to a standard in a solvent of different diamagnetic susceptibility depend on sample geometry (1). The susceptibility correction appears to have been neglected, however, when reporting phosphate chemical shifts. Consequently, values differing by 1 ppm have been reported for the same compounds measured on instruments with iron core and superconducting magnets (2,3). As shown in Fig. 1, the signal from 85% phosphoric acid shifts upfield relative to the secondary standard, aqueous tetrahydroxyphosphonium perchlorate (4) when measured using a superconducting magnet. The correction to the applied field is calculated assuming that the molecules occupy spherical cavities in the solvent. If the sample shape is also spherical the local field inside the cavity is the same as the external field (5). A spherical reference sample, therefore, gives chemical shifts that are independent of the difference in magnetic susceptibilities between the sample and reference material. Usually, however, concentric cylindrical tubes are used. The local fields inside spherical cavities in the sample and reference solutions then differ by (2*/3)& - xsample)if the field is perpendicular and -4/3(xref - x~~,,,~,Jif the field is parallel to the axis of the tubes, where x is the magnetic susceptibility per unit volume. The shape factors are more frequently quoted in connection with dielectric polarization (5). To make these corrections we found it necessary to measure the diamagnetic susceptibility of 85% phosphoric acid using the method of Frei and Bernstein (1). A commercial spherical microcell was filled with the phosphoric acid and placed inside a 5-mm-diameter cylindrical tube (6). Calibration of the shape factor was performed by comparing frequencies observed with the spherical part of the reference surrounded by air and water at 25°C. Using the reported value for the susceptibility of water (I), it was found that a shape factor 3.1% higher than the theoretical value of 47r/3 was required. This compares favorably with the increase of 1.8% employed by Frei and Bernstein. The volume susceptibility of air was taken as 0.028 X 10e6, calculated by assuming a composition of 21% oxygen and 79% nitrogen and using reported molar susceptibilities (7). The measured susceptibility difference between water and phosphoric acid was -0.197 X 10e6. This implies that the diamagnetic susceptibility of 85% phosphoric acid is 9% higher than that calculated on the basis of the reported molar susceptibilities of phosphoric acid and water (7). The measurement was independent of whether HZ0 or D20 was used. 172 0022-2364/82/100172-03$02.00/O Copyright 0 1982 by Academic Press. Inc. All rights of reproduction in any form reserved.
173
COMMUNICATIONS
I 1.0
I 0.5
I 0 Chemical
I -0.5 Shift
I - 1.0 (ppm
I -1.5
1
FIG. I. “P NMR spectrum of 85% phosphoric acid externally referenced to aqueous tetrahydroxyphosphonium perchlorate. Measurement carried out with a Varian XL-200 spectrometer at 80.98 MHz. The observed chemical-shift difference is 0.822 ppm.
It follows that chemical-shift values measured relative to 85% phosphoric acid in a cylindrical tube should be corrected by +0.37 ppm if measured using an iron magnet and by -0.73 ppm for a superconducting magnet. The corrections are accurate to kO.01 ppm. The resulting values represent chemical shifts relative to 85% phosphoric acid in an ideal sphere. When these corrections are applied to the chemical shift of the secondary 3’P standard, 1.0 A4 methylene diphosphonate at pH 9.5 in D20 (8), the values obtained with the two types of instrument are in excellent agreement. Using the convention that high frequencies are positive (9) our correction to the value measured using an iron magnet (8) is 16.7 ppm. We obtained a corrected value of 16.72 + 0.02 ppm at 25°C using a solenoid magnet. This agreement indicates that the diamagnetic susceptibility of the reference sample is the same as that of pure water. Chemical shifts in aqueous solution may therefore be measured relative to this sample without further correction. Unfortunately, the same is not true for the other secondary standard, 0.2 M phosphoric acid in 14% perchloric acid. The susceptibility of this sample appears to be 3% higher than that for pure water. Therefore, aqueous samples measured relative to this material also require correction, as is the case for 85% phosphoric acid. The corrections of 0.07 and -0.14 ppm are, however, much smaller. The chemical shift of this signal relative to a spherical sample of 85% phosphoric acid is 0.229 ppm. There is an alternative approach to the avoidance of geometry-dependent chem-
174
COMMUNICATIONS
ical shifts. The secondary standards could be referred to external 85% phosphoric acid, using concentric tubes in a conventional spectrometer. The advantage of this approach is that the presently greater volume of published results would require no adjustment. All chemical shifts obtained on instruments with superconducting magnets could then be measured relative to the secondary standard and would require no further correction. We do not recommend this practice. There would be no difference in the ease of reporting new results and the two types of spectrometer would still require the use of different values for the tetrahydroxyphosphonium perchlorate secondary standard. There is, on the other hand, a significant disadvantage in having a primary standard that cannot be directly measured (in the case of a superconducting magnet). Furthermore, results obtained in different solvents would not be directly comparable, and if corrections were attempted they would have to be in a form appropriate for conventional geometry, even for results obtained with a superconducting magnet; this could cause considerable confusion. This comment applies a fortiori to proton spectra where aqueous samples are often referred to external standards. In summary, we recommend that 31P chemical shifts in aqueous samples be measured relative to 85% phosphoric acid in an ideal spherical container inside the solution. This may be achieved practically by using concentric cylindrical tubes and one of the following reference samples: (1) phosphoric acid (85%) at 0.37 ppm (iron magnet) or -0.73 ppm (solenoid); (2) methylene diphosphonate ( 1.O M, pH 9.5) at 16.72 ppm (either geometry); (3) aqueous tetrahydroxyphosphonium perchlorate at 0.30 ppm (iron magnet) or 0.09 ppm (solenoid). For other solvents, measurements using both geometries can rapidly determine the appropriate correction. The corrected chemical shift of a compound in the solvent is given by (%a, + Y3aB), where (TAis the apparent chemical shift measured relative to 85% phosphoric acid in a cylindrical tube on an iron magnet spectrometer, and (TVis that measured using a solenoid. Once a single chemical shift has been so determined, it may be used as a secondary standard for chemical shifts in that solvent. ACKNOWLEDGMENTS This of New
work South
was supported Wales,
by the Australian
is thanked
for
Research
Grants
Committee.
Dr. M. Gallagher,
University
discussion. REFERENCES
1. K. FREI AND H. J. BERNSTEIN, J. Chem. Phys. 37, 189 1 (1962). 2. P. D. RICK, L. W.-M. FUNG, C. Ho, AND M. J. OSBORN, J. Eiol.
Chem.
252,
4904
(1977).
3. M. R. ROSNER, H. G. KHORANA, AND A. C. SATTERTHWAIT, J. Viol. Chem. 254, 5918 (1979). 4. T. GLONEK AND J. R. VAN WASER, J. Magn. Reson. 13, 390 (1974). 3rd ed., p. 378, Wiley, New York, 1966; (b) 5. (a) C. KITTEL, “Introduction to Solid State Physics,” 6.
J. K. BECCONSALL, G. D. DAVE& AND W. R. ANDERSON, J. Am. Chem. R. J. MYERS, “Molecular Magnetism and Magnetic Resonance Spectroscopy,” Hall, Englewood Cliffs, N.J., 1973.
7. G. FoEx, “Tables of Constants and Numerical Data (UICPA), Diamagnetisme et Paramagnetisme,” Masson, Paris, 1957. 8. C. T. BURT, T. GLONEK, AND M. BARANY, J. Biol. Chem. 251, 9. Pure Appl. Chem. 45, 217 (1976).
No. 2584
Sot.
92,430 p. 188,
7, Constantes (1976).
(1970). Prentice-
Selectionees