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31P relaxation in some organophosphorus compounds
JOURNAL
OF MAGNETIC
RESONANCE
25, 375-378
(1977)
31PRelaxation in some Organophosphorus Compounds In pulse FT NMR spectroscopy the measurement of spin-lattice relaxation times has nowadays become a routine experiment. Most relaxation studies are concerned with the nuclei lH and 13C. Nevertheless, relaxation data for other nuclei may serve equally well as valuable probes for chemical information. Moreover, knowledge of relaxation behavior is needed for efficient pulse NMR experiments. We investigated the 31P relaxation in some organophosphorus compounds in order to study the role of the different relaxation mechanisms in relation to the molecular properties of these compounds and to gain insight into the numerical values of the T1’s of 31P. As far as we know the only report on 31P relaxation data in organophosphorus compounds has been given by Dale and Hobbs (I). For five compounds, triethylphosphine (Et,P), tetraethylphosphonium iodide (Et,P+I-), triphenylphosphine (Ph,P), tetraphenylphosphonium bromide (Ph,P+Br-), and triphenylphosphine oxide (Ph,PO), we measured the Tl's and the NOE factors at two or more temperatures. An example is shown in Fig. 1; the results are listed in Table 1. Three contributions (3) may play a role in the relaxation of the 31P nucleus in (S.-St1
FIG. 1. T1 measurementat 40.5 MHz on a Varian XL-loo-15 spectrometerof the 31Pnucleusin tetraethylphosphonium iodide at 58°C by the method of modified inversionrecovery(2). The T1values were determined from a semilogplot and a least-squaresanalysisof the slope. The correlation coefficients(ten t values; t < 2 x T1)were better than 0.999.
our compounds: the dipole-dipole (DD) contribution, the spin-rotation (SR) contribution, and the contribution of the chemical shift anisotropy (CSA). The dipoledipole contribution can be separated from the other terms by using the nuclear Overhauser effect, l/TTD = (r//1.235) (l/T,), Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
375 ISSN 0022-2364
376
COMMUNICATIONS TABLE RELAXATION
Compound
TIMES AND
Concentration” (moles/liter)
NOE FACTORS
1
IN SOME ORGANOPHOSPHORUS
COMPOUNDS
Temperature (k2”c’)
(se4
‘I”
TDD1 (set)
47 64 -85
l-1
Et3P
0.7 0.7 0.7
-19 29 58
15.3 10.4 3.4
0.4 0.2 o.05
Et,P+I-
0.7 0.7
29 58
9.0 11.0
1.0 0.9
11 15
Ph,P
0.7 0.7
29 50
30.8 25.8
0.4 0.3
95 106
Ph4PfBr-
1.0 1.0
29 63
11.8 9.4
0.5 0.3
29 39
Ph,PO
1.0 1.0
29 63
19.8 25.4
0.4 0.3
61 105
” In CDCI,. b The NOE factors q were obtained as an average from three separate experiments and have an estimated error of 5 % in the (q + 1) values.
where 1.235 is the maximum NOE factor for the 31P-1H interaction. The TyD values obtained in this way are also listed in Table 1. At normal probe temperature (29°C) the dominant relaxation mechanism for 31P in triethylphosphine is obviously the spin-rotation mechanism. The small NOE factors q indicate that dipolar relaxation makes only a minor contribution to the total. Furthermore the Tl value is drastically decreased at higher temperatures, whereas the effect of higher temperatures on TFD (see Table 1) and TTSA1s an increase in the Tl value. Only at temperatures below -20°C may the DD contribution compete with or even dominate the SR contribution. This result is in agreement with the study by Dale and Hobbs (I). For a comparable compound such as trimethylphosphite, (CH30)3P, they found the same temperature effect and concluded that the SR mechanism is of major importance. For the phosphonium salt Et4PfI- the reverse situation holds. From the NOE factors it can readily be deduced that at both temperatures the DD relaxation is the most important contribution. The sixfold increase in the DD relaxation rate with respect to Et3P may partly be due to the larger number of interacting protons in the Et,P+ moiety. However, a more important factor seems to us to be the formation of E&P+-Iion pairs in solution. As a result the tumbling rate of the phosphonium ions will be restricted and consequently the effective correlation time z, becomes longer. This lengthening of the z, value in the region of motional narrowing may well explain the improved efficiency of the DD mechanism. A similar effect has recently been reported in the case of transient molecular association from hydrogen bonding (4). The sensitivity of the correlation times to the molecular environment indicates the usefulness of relaxation data in studies of intermolecular association and complexation processes. The apparent decrease in the SR relaxation rate for Et4P+I- with respect to E&P
COMMUNICATIONS
317
cannot easily be explained. As the spin-rotation interaction constant C is directly related to the paramagnetic shielding term (.5), the 60-ppm downfield shift of Et,P+Iwith respect to Et,P should imply an increase in the relaxation rate. However, changes in the molecular symmetry and in the spin-rotation correlation time TSRmay have the opposite effect. The results for triphenylphosphine and its derivative tetraphenylphosphonium bromide show a behavior for the 31P relaxation similar to that for the previous compounds. Again the SR contribution dominates the relaxation in Ph3P at normal temperatures, although the SR mechanism is considerably less efficient in Ph3P than in Et,P. The TyD of 31P in PPh3 is relatively large, probably because of the larger average distance of the neighboring protons to the phosphorus atom with respect to E&P. Nevertheless, the DD relaxation increases strongly on going from Ph,P to Ph,P+Br-, Also in this case we assume that the formation of ion pairs slows down the tumbling rate of the Ph,P+ species, which consequently leads to more effective DD mechanism. In contrast with the results for Et4P+I-, the DD mechanism in Ph,P+Brcompetes at probe temperature with another mechanism, probably the SR mechanism, as can be seen from the measurement at 63°C. Finally, the data for triphenylphosphine oxide are interesting because from the NOE measurements and the temperature dependence of T,, it appears that neither the DD mechanism nor the SR mechanism plays the dominant role in the relaxation of 31P. The only mechanism that is consistent with our observations is the CSAmechanism. The presence of the CSA mechanism has also been noted in some phosphoryl compounds (I), containing the P=O bond. Presumably, the P=O bond disturbs the isotropic electron distribution around the 31Pnucleus, resulting in substantial values for the anisotropy tensors. A large anisotropy of the chemical shift has been observed for the 31P nucleus in phosphate groups in biochemical compounds (6). It is clear, even from the few compounds studied, that the prediction of relaxation behavior is more complex for 31P than for 13C. For 31P we have three contributing mechanisms, dipolar, spin-rotation, and CSA, ofwhich the relative weights are seen only from experiment. CSA seems to be important only in phosphorus compounds containing the P=O bond. It is remarkable that spin-rotation is important for a molecule of the size of triphenylphosphine. The data in Table 1 indicate a large range of relaxation times (factor 10) even for closely related compounds at the same temperature. The efficiency of the 31P pulse NMR experiment may be improved in some cases by lowering the temperature (e.g., Et,P+I-) and in other cases by elevating the temperature (e.g., Et,P). For a more complete understanding of the relaxation behavior of 31P, more extensive studies will be needed. ACKNOWLEDGMENT This investigation was supported by The Netherlands Foundation for Chemical Research (S.O.N.) with financial aid from The Netherlands Organisation for the Advancement of Pure Research (Z.W.O.). REFERENCES 1.
S. W. DALE
2.
R. FREEMAN
AND M.
E. HOBBS, J. Phyx
AND H. D. W. HILL,
Chem. 75,3537 J. Chem. Phys. 54,3367
(1971). (1971).
378
COMMUNICATIONS
3. A. ABRAGAM, “Principles of Nuclear Magnetism,” Chap. VIII, Oxford University Press, London, 1961. 4. I. D. CAMPBELL, R. FREEMAN, AND D. L. TURNER, J. Maglz. Resonance 20,172 (1975). 5. W. H. FLYGARE, J. Chem. Phys. 41,7903 (1964). 6. (a) J. KOHLER AND M. P. KLEIN, Biochemistry 15,967 (1976). (b) W. NIEDERBERGER AND J. SEELIG, J. Amer. Chem. Sot. 98,3704 (1976). (c) A. C. MCLAUGHLIN, P. R. CULLIS, J. A. BERDEN, AND R. E. RICHARDS, J. Magn. Resonance 20, 146 (1975). N. J. KOOLE A. J. DE KONING M. J. A. DE BIE Laboratory for Organic Chemistry State University at Utrecht Croesestraat 79 Utrecht The Netherlands Received
September
12, 1976
OF MAGNETIC
RESONANCE
25, 375-378
(1977)
31PRelaxation in some Organophosphorus Compounds In pulse FT NMR spectroscopy the measurement of spin-lattice relaxation times has nowadays become a routine experiment. Most relaxation studies are concerned with the nuclei lH and 13C. Nevertheless, relaxation data for other nuclei may serve equally well as valuable probes for chemical information. Moreover, knowledge of relaxation behavior is needed for efficient pulse NMR experiments. We investigated the 31P relaxation in some organophosphorus compounds in order to study the role of the different relaxation mechanisms in relation to the molecular properties of these compounds and to gain insight into the numerical values of the T1’s of 31P. As far as we know the only report on 31P relaxation data in organophosphorus compounds has been given by Dale and Hobbs (I). For five compounds, triethylphosphine (Et,P), tetraethylphosphonium iodide (Et,P+I-), triphenylphosphine (Ph,P), tetraphenylphosphonium bromide (Ph,P+Br-), and triphenylphosphine oxide (Ph,PO), we measured the Tl's and the NOE factors at two or more temperatures. An example is shown in Fig. 1; the results are listed in Table 1. Three contributions (3) may play a role in the relaxation of the 31P nucleus in (S.-St1
FIG. 1. T1 measurementat 40.5 MHz on a Varian XL-loo-15 spectrometerof the 31Pnucleusin tetraethylphosphonium iodide at 58°C by the method of modified inversionrecovery(2). The T1values were determined from a semilogplot and a least-squaresanalysisof the slope. The correlation coefficients(ten t values; t < 2 x T1)were better than 0.999.
our compounds: the dipole-dipole (DD) contribution, the spin-rotation (SR) contribution, and the contribution of the chemical shift anisotropy (CSA). The dipoledipole contribution can be separated from the other terms by using the nuclear Overhauser effect, l/TTD = (r//1.235) (l/T,), Copyright 0 1977 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
375 ISSN 0022-2364
376
COMMUNICATIONS TABLE RELAXATION
Compound
TIMES AND
Concentration” (moles/liter)
NOE FACTORS
1
IN SOME ORGANOPHOSPHORUS
COMPOUNDS
Temperature (k2”c’)
(se4
‘I”
TDD1 (set)
47 64 -85
l-1
Et3P
0.7 0.7 0.7
-19 29 58
15.3 10.4 3.4
0.4 0.2 o.05
Et,P+I-
0.7 0.7
29 58
9.0 11.0
1.0 0.9
11 15
Ph,P
0.7 0.7
29 50
30.8 25.8
0.4 0.3
95 106
Ph4PfBr-
1.0 1.0
29 63
11.8 9.4
0.5 0.3
29 39
Ph,PO
1.0 1.0
29 63
19.8 25.4
0.4 0.3
61 105
” In CDCI,. b The NOE factors q were obtained as an average from three separate experiments and have an estimated error of 5 % in the (q + 1) values.
where 1.235 is the maximum NOE factor for the 31P-1H interaction. The TyD values obtained in this way are also listed in Table 1. At normal probe temperature (29°C) the dominant relaxation mechanism for 31P in triethylphosphine is obviously the spin-rotation mechanism. The small NOE factors q indicate that dipolar relaxation makes only a minor contribution to the total. Furthermore the Tl value is drastically decreased at higher temperatures, whereas the effect of higher temperatures on TFD (see Table 1) and TTSA1s an increase in the Tl value. Only at temperatures below -20°C may the DD contribution compete with or even dominate the SR contribution. This result is in agreement with the study by Dale and Hobbs (I). For a comparable compound such as trimethylphosphite, (CH30)3P, they found the same temperature effect and concluded that the SR mechanism is of major importance. For the phosphonium salt Et4PfI- the reverse situation holds. From the NOE factors it can readily be deduced that at both temperatures the DD relaxation is the most important contribution. The sixfold increase in the DD relaxation rate with respect to Et3P may partly be due to the larger number of interacting protons in the Et,P+ moiety. However, a more important factor seems to us to be the formation of E&P+-Iion pairs in solution. As a result the tumbling rate of the phosphonium ions will be restricted and consequently the effective correlation time z, becomes longer. This lengthening of the z, value in the region of motional narrowing may well explain the improved efficiency of the DD mechanism. A similar effect has recently been reported in the case of transient molecular association from hydrogen bonding (4). The sensitivity of the correlation times to the molecular environment indicates the usefulness of relaxation data in studies of intermolecular association and complexation processes. The apparent decrease in the SR relaxation rate for Et4P+I- with respect to E&P
COMMUNICATIONS
317
cannot easily be explained. As the spin-rotation interaction constant C is directly related to the paramagnetic shielding term (.5), the 60-ppm downfield shift of Et,P+Iwith respect to Et,P should imply an increase in the relaxation rate. However, changes in the molecular symmetry and in the spin-rotation correlation time TSRmay have the opposite effect. The results for triphenylphosphine and its derivative tetraphenylphosphonium bromide show a behavior for the 31P relaxation similar to that for the previous compounds. Again the SR contribution dominates the relaxation in Ph3P at normal temperatures, although the SR mechanism is considerably less efficient in Ph3P than in Et,P. The TyD of 31P in PPh3 is relatively large, probably because of the larger average distance of the neighboring protons to the phosphorus atom with respect to E&P. Nevertheless, the DD relaxation increases strongly on going from Ph,P to Ph,P+Br-, Also in this case we assume that the formation of ion pairs slows down the tumbling rate of the Ph,P+ species, which consequently leads to more effective DD mechanism. In contrast with the results for Et4P+I-, the DD mechanism in Ph,P+Brcompetes at probe temperature with another mechanism, probably the SR mechanism, as can be seen from the measurement at 63°C. Finally, the data for triphenylphosphine oxide are interesting because from the NOE measurements and the temperature dependence of T,, it appears that neither the DD mechanism nor the SR mechanism plays the dominant role in the relaxation of 31P. The only mechanism that is consistent with our observations is the CSAmechanism. The presence of the CSA mechanism has also been noted in some phosphoryl compounds (I), containing the P=O bond. Presumably, the P=O bond disturbs the isotropic electron distribution around the 31Pnucleus, resulting in substantial values for the anisotropy tensors. A large anisotropy of the chemical shift has been observed for the 31P nucleus in phosphate groups in biochemical compounds (6). It is clear, even from the few compounds studied, that the prediction of relaxation behavior is more complex for 31P than for 13C. For 31P we have three contributing mechanisms, dipolar, spin-rotation, and CSA, ofwhich the relative weights are seen only from experiment. CSA seems to be important only in phosphorus compounds containing the P=O bond. It is remarkable that spin-rotation is important for a molecule of the size of triphenylphosphine. The data in Table 1 indicate a large range of relaxation times (factor 10) even for closely related compounds at the same temperature. The efficiency of the 31P pulse NMR experiment may be improved in some cases by lowering the temperature (e.g., Et,P+I-) and in other cases by elevating the temperature (e.g., Et,P). For a more complete understanding of the relaxation behavior of 31P, more extensive studies will be needed. ACKNOWLEDGMENT This investigation was supported by The Netherlands Foundation for Chemical Research (S.O.N.) with financial aid from The Netherlands Organisation for the Advancement of Pure Research (Z.W.O.). REFERENCES 1.
S. W. DALE
2.
R. FREEMAN
AND M.
E. HOBBS, J. Phyx
AND H. D. W. HILL,
Chem. 75,3537 J. Chem. Phys. 54,3367
(1971). (1971).
378
COMMUNICATIONS
3. A. ABRAGAM, “Principles of Nuclear Magnetism,” Chap. VIII, Oxford University Press, London, 1961. 4. I. D. CAMPBELL, R. FREEMAN, AND D. L. TURNER, J. Maglz. Resonance 20,172 (1975). 5. W. H. FLYGARE, J. Chem. Phys. 41,7903 (1964). 6. (a) J. KOHLER AND M. P. KLEIN, Biochemistry 15,967 (1976). (b) W. NIEDERBERGER AND J. SEELIG, J. Amer. Chem. Sot. 98,3704 (1976). (c) A. C. MCLAUGHLIN, P. R. CULLIS, J. A. BERDEN, AND R. E. RICHARDS, J. Magn. Resonance 20, 146 (1975). N. J. KOOLE A. J. DE KONING M. J. A. DE BIE Laboratory for Organic Chemistry State University at Utrecht Croesestraat 79 Utrecht The Netherlands Received
September
12, 1976