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Solid state 31P nuclear magnetic resonance of tricyanophosphine
PdyhedronVol. Printed in Great
IO, No. 15, pp. 1831-1833, Britain
1991 0
0277-5387/91 S3.00+.00 1991 Pergamon Press plc
COMMUNICATION SOLID STATE 31P NUCLEAR MAGNETIC TRICYANOPHOSPHINE
RESONANCE
OF
M. A. H. A. AL-JUBOORI, A. FINCH, P. J. GARDNER* and C. J. GROOMBRIDGEt Department of Chemistry, Royal Holloway and Bedford New College, Egham Hill, Egham TW20 OEX, Surrey, U.K. (Received 17 May 199 1; accepted 3 June 199 1)
Abstract-The high resolution 31P NMR spectrum of solid tricyanophosphine is reported. The use of magic angle sample spinning gives the chemical shift much more accurately than the earlier wideline measurement, and spinning sideband analysis gives an improved estimate of the 3‘P chemical shift anisotropy : - 96 + 5 ppm.
The structure-determining ability of nuclear magnetic resonance spectroscopy is well known, and often depends on interpretation of the chemical shift parameter, most commonly through accumulated empirical knowledge. Considerable progress has been made in recent years in improving the theoretical computation of light element chemical shifts, especially 13Cand 14j1‘N, and this is gradually being extended to heavier species, of which 31P is of most widespread interest. Chemical shifts (the term “shielding” tends to be used in the context of theoretical work, and has opposite sign) are actually second rank tensors, i.e. functions of orientation relative to the external influence, and it has been pointed out’ that a full determination (principal components and directions) can lead to a much more complete understanding of the relationship between chemical shifts and structure. It is clearly important to have experimental values for a range of small molecules and it has become increasingly common for high resolution solid state NMR to be used to provide this, particularly by analysis of spinning sideband patterns given by slow magic-angle sample spinning (MAS). Here we report an investigation of P(CN)3, improving upon an earlier wideline 3‘P NMR study. *
* Author to whom correspondence should be addressed. t Present address: BP Research Centre, Chertsey Road, Sunbury-on-Thames TW16 7LN, Middlesex, U.K.
P(CN)3 is pyramidal and crystallizes with tetragonalI42d unit cells in which all molecules are symmetry-equivalent. 3 The bond distances and angles give almost C3” symmetry, but each P--C&N is slightly bent and weak intermolecular N * . . P interactions have been implicated. 3 EXPERIMENTAL Sample preparation
Tricyanophosphine was prepared from PC13 and AgCN by the method of Staats and Morgan4 and was purified by vacuum sublimation. It was a white crystalline material subsequently handled in a dry nitrogen atmosphere. NMR spectroscopy
All spectra were obtained on a Bruker MSL-300 spectrometer using a standard double air bearing magic-angle spinning probe. The P(CN)3 sample was packed into a 7 mm zirconia rotor in a nitrogenfilled glove box. A reasonable air seal was made by the use of a small amount of vacuum grease on the rotor endcap. The sample was spun using dry compressed nitrogen gas boiled from a liquid N2 dewar. The “P frequency was 121.481 MHz and the sample was spun at 4433 Hz. The spectrum was recorded using single 90” pulses with a 64 s recycle. An attempt to acquire a 13C spectrum gave no
1831
1832
Communication
signal. The 3’P chemical shift scale was referenced by replacement with 85% orthophosphoric acid in a sealed glass ampoule ; positive shifts are to high frequency and are precise to + 0.2 ppm. The centreband occurs at the intensity-weighted centre of the spectrum, and this was confirmed by spinning speed adjustment; its position gives the isotropic (i.e. “normal”) chemical shift. An estimate of the 3’P relaxation time, T,, was made using a saturationrecovery pulse sequence, and it was verified that the relative sideband intensities were not affected by incomplete relaxation. Spinning sideband analysis was carried out using the least-squares computer program developed by Sales and co-workers,’ which uses Herzfeld and Berger sideband ratio data. 6 Principal components were determined in the chemical shift sense, and error estimates (95% confidence intervals) were obtained from sum-of-squares derivatives. RESULTS
AND DISCUSSION
The 3’P MAS spectrum of P(CN)3 is shown in Fig. 1. The bands were close to Gaussian shape with 164 Hz full width. The isotropic chemical shift was - 140.0 ppm, close to the reported MeCN solution value, 7 - 137.83 ppm, rather than - 129.72 ppm found in iodopropane. 7 Self-coordination in the crystal3 would thus appear to resemble the MeCN solvent effect. Signal recovery from saturation was
_lA -6
-100
-125
found still to be incomplete at 64 s but the 3’P T, was of the order of 100 s. Chemical shift principal components were found to be 833= -1OO(f4), dZ2= -116(f8), and 6, , = - 204( f 5). Different definitions exist for the anisotropy, but if a1 = l/2(822+833) = - 108 ppm, the anisotropy can be taken as A = 6,,-dl = -96 ppm. Using the definition for asymmetry q = (833-822)/(8iso-811), thisis found to differ significantly (0.24) from axial symmetry. These values are not corrected for bulk magnetic susceptibility since this is likely to be less than experimental error. Jameson et al.* have recently given the shielding on the absolute scale of the phosphoric acid reference, and this can be used to give P(CN)3 shieldings as : Oiso= 468.3 ppm, ~11 = 428, cZ2 = 444, c33 = 532. Recent ab initio molecular calculations of substituted phosphines have suggested’ a significant participation of phosphorus 3d orbitals in the bonding of P(CN),, due to partialprr-dn overlap (3p and 3dcontributions to P-CN bonding were calculated as 0.57 and 0.14). The simple semi-empirical treatment given by Lucken and Williams2 yielded a chemical shift anisotropy of - 115 ppm, but ignored 3d involvement and left considerable flexibility in average excitation energy and electron (r3). An attempt to improve on this would also have to incorporate the intermolecular interaction, since the solvent effect on P(CN)3 shift7 indicates that this may be of the order of 10 ppm.
- 150 Chemical Shift
-17s
-
-50
$/ppm
Fig. 1. “P Magic angle spinning NMR spectrum P(CN)3. Spinning speed 4433 Hz, 400 scans, 64 s recycle.
Communication Acknowledgements-All
NMR spectra were obtained using the University of London Intercollegiate Research Service (ULIRS) solid state NMR facility at RHBNC, with assistance from R. T. Mathavan. Our thanks to Dr K. Sales (Queen Mary and Westfield College, London) for use of the SRCH sideband program. REFERENCES 1. C. J. Jameson, Nucl. Magn. Reson., Spec. Per. Rep. 1985, 14, 11. 2. E. A. C. Lucken and D. F. Williams, Mol. Phys. 1969, 16, 17. [There is an ambiguity in the chemical shift anisotropies given in this paper: different values
1833
appear in the abstract (- 100 f 20 ppm) and Table 1 (- 128+20 ppm).] 3. K. Emerson and D. B&ton, Acta Cryst. 1964, 17, 1134. 4. P. A. Staats and H. W. Morgan, Znorg. Chem. 1960, 6, 84. 5. G. E. Hawkes, K. D. Sales, L. Y. Lian and R. Gobetto, Proc. R. Sot. Lond. A 1989,424,93. 6. J. Herzfeld and A. E. Berger, J. Chem. Phys. 1980,
73, 6021. 7. B. W. Tattersall, Polyhedron 1990, 9, 553. 8. C. J. Jameson, A. De Dios and A. K. Jameson, Chem. Phys. Lett. 1990, 167, 575. 9. E. Magnusson, Phosphorus Sulfur 1986,28, 379.
IO, No. 15, pp. 1831-1833, Britain
1991 0
0277-5387/91 S3.00+.00 1991 Pergamon Press plc
COMMUNICATION SOLID STATE 31P NUCLEAR MAGNETIC TRICYANOPHOSPHINE
RESONANCE
OF
M. A. H. A. AL-JUBOORI, A. FINCH, P. J. GARDNER* and C. J. GROOMBRIDGEt Department of Chemistry, Royal Holloway and Bedford New College, Egham Hill, Egham TW20 OEX, Surrey, U.K. (Received 17 May 199 1; accepted 3 June 199 1)
Abstract-The high resolution 31P NMR spectrum of solid tricyanophosphine is reported. The use of magic angle sample spinning gives the chemical shift much more accurately than the earlier wideline measurement, and spinning sideband analysis gives an improved estimate of the 3‘P chemical shift anisotropy : - 96 + 5 ppm.
The structure-determining ability of nuclear magnetic resonance spectroscopy is well known, and often depends on interpretation of the chemical shift parameter, most commonly through accumulated empirical knowledge. Considerable progress has been made in recent years in improving the theoretical computation of light element chemical shifts, especially 13Cand 14j1‘N, and this is gradually being extended to heavier species, of which 31P is of most widespread interest. Chemical shifts (the term “shielding” tends to be used in the context of theoretical work, and has opposite sign) are actually second rank tensors, i.e. functions of orientation relative to the external influence, and it has been pointed out’ that a full determination (principal components and directions) can lead to a much more complete understanding of the relationship between chemical shifts and structure. It is clearly important to have experimental values for a range of small molecules and it has become increasingly common for high resolution solid state NMR to be used to provide this, particularly by analysis of spinning sideband patterns given by slow magic-angle sample spinning (MAS). Here we report an investigation of P(CN)3, improving upon an earlier wideline 3‘P NMR study. *
* Author to whom correspondence should be addressed. t Present address: BP Research Centre, Chertsey Road, Sunbury-on-Thames TW16 7LN, Middlesex, U.K.
P(CN)3 is pyramidal and crystallizes with tetragonalI42d unit cells in which all molecules are symmetry-equivalent. 3 The bond distances and angles give almost C3” symmetry, but each P--C&N is slightly bent and weak intermolecular N * . . P interactions have been implicated. 3 EXPERIMENTAL Sample preparation
Tricyanophosphine was prepared from PC13 and AgCN by the method of Staats and Morgan4 and was purified by vacuum sublimation. It was a white crystalline material subsequently handled in a dry nitrogen atmosphere. NMR spectroscopy
All spectra were obtained on a Bruker MSL-300 spectrometer using a standard double air bearing magic-angle spinning probe. The P(CN)3 sample was packed into a 7 mm zirconia rotor in a nitrogenfilled glove box. A reasonable air seal was made by the use of a small amount of vacuum grease on the rotor endcap. The sample was spun using dry compressed nitrogen gas boiled from a liquid N2 dewar. The “P frequency was 121.481 MHz and the sample was spun at 4433 Hz. The spectrum was recorded using single 90” pulses with a 64 s recycle. An attempt to acquire a 13C spectrum gave no
1831
1832
Communication
signal. The 3’P chemical shift scale was referenced by replacement with 85% orthophosphoric acid in a sealed glass ampoule ; positive shifts are to high frequency and are precise to + 0.2 ppm. The centreband occurs at the intensity-weighted centre of the spectrum, and this was confirmed by spinning speed adjustment; its position gives the isotropic (i.e. “normal”) chemical shift. An estimate of the 3’P relaxation time, T,, was made using a saturationrecovery pulse sequence, and it was verified that the relative sideband intensities were not affected by incomplete relaxation. Spinning sideband analysis was carried out using the least-squares computer program developed by Sales and co-workers,’ which uses Herzfeld and Berger sideband ratio data. 6 Principal components were determined in the chemical shift sense, and error estimates (95% confidence intervals) were obtained from sum-of-squares derivatives. RESULTS
AND DISCUSSION
The 3’P MAS spectrum of P(CN)3 is shown in Fig. 1. The bands were close to Gaussian shape with 164 Hz full width. The isotropic chemical shift was - 140.0 ppm, close to the reported MeCN solution value, 7 - 137.83 ppm, rather than - 129.72 ppm found in iodopropane. 7 Self-coordination in the crystal3 would thus appear to resemble the MeCN solvent effect. Signal recovery from saturation was
_lA -6
-100
-125
found still to be incomplete at 64 s but the 3’P T, was of the order of 100 s. Chemical shift principal components were found to be 833= -1OO(f4), dZ2= -116(f8), and 6, , = - 204( f 5). Different definitions exist for the anisotropy, but if a1 = l/2(822+833) = - 108 ppm, the anisotropy can be taken as A = 6,,-dl = -96 ppm. Using the definition for asymmetry q = (833-822)/(8iso-811), thisis found to differ significantly (0.24) from axial symmetry. These values are not corrected for bulk magnetic susceptibility since this is likely to be less than experimental error. Jameson et al.* have recently given the shielding on the absolute scale of the phosphoric acid reference, and this can be used to give P(CN)3 shieldings as : Oiso= 468.3 ppm, ~11 = 428, cZ2 = 444, c33 = 532. Recent ab initio molecular calculations of substituted phosphines have suggested’ a significant participation of phosphorus 3d orbitals in the bonding of P(CN),, due to partialprr-dn overlap (3p and 3dcontributions to P-CN bonding were calculated as 0.57 and 0.14). The simple semi-empirical treatment given by Lucken and Williams2 yielded a chemical shift anisotropy of - 115 ppm, but ignored 3d involvement and left considerable flexibility in average excitation energy and electron (r3). An attempt to improve on this would also have to incorporate the intermolecular interaction, since the solvent effect on P(CN)3 shift7 indicates that this may be of the order of 10 ppm.
- 150 Chemical Shift
-17s
-
-50
$/ppm
Fig. 1. “P Magic angle spinning NMR spectrum P(CN)3. Spinning speed 4433 Hz, 400 scans, 64 s recycle.
Communication Acknowledgements-All
NMR spectra were obtained using the University of London Intercollegiate Research Service (ULIRS) solid state NMR facility at RHBNC, with assistance from R. T. Mathavan. Our thanks to Dr K. Sales (Queen Mary and Westfield College, London) for use of the SRCH sideband program. REFERENCES 1. C. J. Jameson, Nucl. Magn. Reson., Spec. Per. Rep. 1985, 14, 11. 2. E. A. C. Lucken and D. F. Williams, Mol. Phys. 1969, 16, 17. [There is an ambiguity in the chemical shift anisotropies given in this paper: different values
1833
appear in the abstract (- 100 f 20 ppm) and Table 1 (- 128+20 ppm).] 3. K. Emerson and D. B&ton, Acta Cryst. 1964, 17, 1134. 4. P. A. Staats and H. W. Morgan, Znorg. Chem. 1960, 6, 84. 5. G. E. Hawkes, K. D. Sales, L. Y. Lian and R. Gobetto, Proc. R. Sot. Lond. A 1989,424,93. 6. J. Herzfeld and A. E. Berger, J. Chem. Phys. 1980,
73, 6021. 7. B. W. Tattersall, Polyhedron 1990, 9, 553. 8. C. J. Jameson, A. De Dios and A. K. Jameson, Chem. Phys. Lett. 1990, 167, 575. 9. E. Magnusson, Phosphorus Sulfur 1986,28, 379.