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Ultrahigh precision NMR spectral analysis
JOURNALOFMAGNETTCRESONANCE
16,190-191(1974)
COMMUNICATIONS Ultrahigh Precision NMR Spectral Analysis Using pulsed Fourier tranform methods, very accurate spectral analysis can easily be achieved. This is exemplified by the phosphorus noise-decoupled ‘H NMR spectrum of a 30 mol % solution of o-phenylene phosphorochloridite :
O\
P-Cl
0’
Spectra were measured on a Varian XL-loo-1 2 spectrometer interfaced to a 620L 16K computer equipped with a moving head disc, Ten 90” pulses were accumulated using a spectral width of 100 Hz and 32K data points, giving 0.006 Hz/point in the transformed spectrum. The acquisition time for each pulse was ca. 163 sec. The spectrum was run three times, so due to the inherent symmetry of the AA’BB’ system, six frequency values were obtained for each transition of which the “worst” was disregarded. Probable errors in the determined line positions then ranged from 0.002 to 0.006 Hz. The line positions were obtained through the use of a parabolic centroid function. If v-, vO,and v+ represent the frequencies of the channels below, at, and above the peak maximum, and I-, IO, and 1, the respective intensities, then the “true” center (v) is : 1 I+ + Iv=v~+52~04+-I_’ TABLE
1
CHEMICAL SHIFTS (IN Hz AT 100 MHz) AND SPIN-SPIN COUPLING CONSTA~~FTSFORA 30 MOL%SOLUTION 0~ 0-PHENYLENE PHOSPHOROCHLORIDITEIN CDCI,
Parameter
Value
Probable error
v* = v*,
-6.420 6.420 8.046 1.277 0.454 7.692
0.001 0.001 0.001 0.001 0.001 0.001
Ve = VB’ J,,, = JAW JAB, = JA,B J AA’
JEW Copyright Q 1974 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
190
COMMUNICATIONS
191
After calculating a trial spectrum with the LAOCOON III program (I), 20 out of the possible 24 lines (omitting the outer very weak ones) were assigned. Iteration resulted in the values given in Table 1. Deviations between experimental and calculated line positions ranged from 0.000 to 0.004 Hz, yielding arms error of 0.002 Hz and probable errors for all spectral parameters of 0.001 Hz. Thus the results are about one order of magnitude better than results normally obtained in high-precision NMR work. The efficiency of the interpolation rout.ine for determining the line positions was further tested by accumulating spectra of 800 Hz width using only 8K data points, i.e., 0.195 Hz/point. Iterative fitting with LAOCOON III gave 0.011 Hz rms error for the measured line positions. REFERENCE 1. S.~ASTELLANOAND A. A. BOTHNER-BY, J.Chem.Phys.
41, 3863 (1964).
LUDGERERNST DAVID N. LINCOLN Gesellschaft fiir Molekularbiologische D-3300 Braunschweig-StBckheim Federal Republic of Germany Received July 9, 1974
Forschung mbH
16,190-191(1974)
COMMUNICATIONS Ultrahigh Precision NMR Spectral Analysis Using pulsed Fourier tranform methods, very accurate spectral analysis can easily be achieved. This is exemplified by the phosphorus noise-decoupled ‘H NMR spectrum of a 30 mol % solution of o-phenylene phosphorochloridite :
O\
P-Cl
0’
Spectra were measured on a Varian XL-loo-1 2 spectrometer interfaced to a 620L 16K computer equipped with a moving head disc, Ten 90” pulses were accumulated using a spectral width of 100 Hz and 32K data points, giving 0.006 Hz/point in the transformed spectrum. The acquisition time for each pulse was ca. 163 sec. The spectrum was run three times, so due to the inherent symmetry of the AA’BB’ system, six frequency values were obtained for each transition of which the “worst” was disregarded. Probable errors in the determined line positions then ranged from 0.002 to 0.006 Hz. The line positions were obtained through the use of a parabolic centroid function. If v-, vO,and v+ represent the frequencies of the channels below, at, and above the peak maximum, and I-, IO, and 1, the respective intensities, then the “true” center (v) is : 1 I+ + Iv=v~+52~04+-I_’ TABLE
1
CHEMICAL SHIFTS (IN Hz AT 100 MHz) AND SPIN-SPIN COUPLING CONSTA~~FTSFORA 30 MOL%SOLUTION 0~ 0-PHENYLENE PHOSPHOROCHLORIDITEIN CDCI,
Parameter
Value
Probable error
v* = v*,
-6.420 6.420 8.046 1.277 0.454 7.692
0.001 0.001 0.001 0.001 0.001 0.001
Ve = VB’ J,,, = JAW JAB, = JA,B J AA’
JEW Copyright Q 1974 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
190
COMMUNICATIONS
191
After calculating a trial spectrum with the LAOCOON III program (I), 20 out of the possible 24 lines (omitting the outer very weak ones) were assigned. Iteration resulted in the values given in Table 1. Deviations between experimental and calculated line positions ranged from 0.000 to 0.004 Hz, yielding arms error of 0.002 Hz and probable errors for all spectral parameters of 0.001 Hz. Thus the results are about one order of magnitude better than results normally obtained in high-precision NMR work. The efficiency of the interpolation rout.ine for determining the line positions was further tested by accumulating spectra of 800 Hz width using only 8K data points, i.e., 0.195 Hz/point. Iterative fitting with LAOCOON III gave 0.011 Hz rms error for the measured line positions. REFERENCE 1. S.~ASTELLANOAND A. A. BOTHNER-BY, J.Chem.Phys.
41, 3863 (1964).
LUDGERERNST DAVID N. LINCOLN Gesellschaft fiir Molekularbiologische D-3300 Braunschweig-StBckheim Federal Republic of Germany Received July 9, 1974
Forschung mbH