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Heteronuclear chemical-shift correlation utilizing homonuclear decoupling and distortionless enhancement by polarization transfer
Volume 106, number 4
HET~RO~~LEA~
C~M~~AL~~FT
AND DISTORTIONLESS V. RUTAR
27 Ai.lril 1984
CHEMICAL PHYSICS LETTERS
COLLATION
ENHANCEMENT
~I~ZtN~
BY POLARIZATION
HO~ONUCLEAR
DECOUPL~G
TRANSFER
*
Depnrtment of Chemiktry. University of Missour& Coiumbia. Missouri 65211, USA Received 18 January 1984; in fmal form 15 February 1984
NMR spectroscopy, selective i?ips of distant
’ H--13C c~e~~~~t~o~elation by polarization
transfer are combined
into a new two-dimensionaI
technique.
protons and distortionless
Correlation maps are simpfifkd
enhancement by suppressing
most homonuclear couplings, thus improving the signal-to-noise ratio. Applications include accurate measurements signments of t H chemical shifts as well as couplings with additional nuclear spins (lgF, 31Pl etc.) in the molecule.
The DEPT pulse sequence fer of polarization
from
[I ] promotes
abundant
(usually
--ill
the translI3) to di-
lute spins (t3C, 15N, etc.). Compared to the more traditional INEPT approach [2,3], some distinctive advantages have been noticed: mu~tiplets are not distorted and spectral editing is reliable even in the presence of non-uniform one-bond coupling constants (tJCw)_ Recently [4,5], the DEPT pulse sequence has been successfully adopted to generate heieronuclear chemical-shift correlation. Since any two-dimensional (Z-D) NMR technique requires a certain time commitment for data acquisition, the experiment must be carefully programmed to avoid long measuring times [6]. “Bilinear pulses”, as ~troduced by Garbow et al. [7], ~tention~y simplify data sets by suppressing homonuclear couplings. This letter descriies a new pulse sequence (A) (see scheme 1) which combines distortionless enhancement by polarization transfer with additional pulses(B) f7 lH: 1%:
and as-
“H:
_*.n/2(~~)-?--n(yl)--T-a/2((71)
Q:
-7Q3
...
)-
(B)
of the evolution time tt . If the delay T is set equal to r. = 1/21JCH, pulses (3) apparently do not affect those protons (Ha) which evolve for 180“ due to one-bond coupiing with directly attached 1363 spins. Other protons in the molecule are ‘distant” (Hd), and their homonucfear and heteronuciear couplings are small compared to ~~~~ ; tfterefore, they practically do not evolve ciuring the intervals 27 and they are flipped over by pulses (B)_ Precession resulting from Coup~gs~HaHd is refocused during the second half of the evolution time. The subsequent mixing period accomplishes polarization transfer from the 1H to 13C spin system and the variable pulse 6 adjusts enhancements for CH, at the middle
-~--B((F)~)-T--dec.
~/2((3I)-tl/2--irl2(~7)-~--n(y1)-~-n/2(~2~tl/2-7-
-acq .
-7dx~+
--n(Y3)_
evolution
t
illixing
69
I
Scheme I * Permarrent address: J. Stefan Institute, 6111 I Ljubljana, Yugoslavia.
258
P.O. Box 53,
0 009-2614/84/S 03.00 0 Elsevier Science Publishers born-Ho~and Physics Publishing Division)
B.V.
Volume
106. number
4
Table 1 Relative
phases a) of radio-frequency puiscs in sequence (A)
CHEMICAL
Step
rpl
1
-Y
7
--x
3 4
a) As denoted
92
Y x
in the doubly
rotating
77 April
LETTERS
1981
cJ3
X --J’
PHYSICS
x -X
--x
x
.,
--x
frame.
groups (m = 1,2,3) [ 1 ] _ A four-step phase cycle (table 1) together with alternate addition and subtraction of carbon signals ensures quadrature detection along both dimension [ 121 and suppression of coherencetransfer echoes [ 131. Results of 2-D data processing can be displayed as contour plots (fig. 1) which unambiguously correlate 1H and 13C chemical shifts. When single or chemically equivalent protons are attached to the same carbon nucleus, pulse sequence (A) eliminates all homonuclear J couplings from the F, dimension, thus giving rise to sharp single peaks. The signal-to-noise ratio and
Fig. 2. Picked-out decoupled spectra of protons at positions in I-bromoheptanc. if the variable pulse e is incrsosed fromt9t =a/4toe2= 3~~14. the spectrum of the CH3 group does not change (a and b), while the signal of the CHz goup is reversed (c and d).
accuracy of 1 H chemical-shift measurements are better than in the :raditional versions [ 14-161. Cross sections aiong the F, dimension have been picked OUI for each non-equivalent 13C signal and zero-filled up to 8000 data points to increase digital resolution (fig. 2). Chemical shifts (a,,) of protons at positions 1.2 and 7 in 1-bromoheprane are obvious even from one-dimensional 1 H spectra, while rhe remaining protons (positions 3-6) are so strongly coupled that rhey cannot be resolved even by 2-D homonuclear J spectroscopy [ 17). Here, pulse sequence (-4) represents an esclusive approach and, utilizing assignment of l;C resonances [ 181 , all 1H chemical shifts have been determined (rable 2). If two non-equivalent protons are srtached directly Table 2 Proton chemical
Fig. 1. Assigned chemical-shift correlation map for l-bromoheptane as obtained by pulse sequence (A) and 13= n/4- Since homonuclear couplings have been suppressed. signals are shown as *‘sharp” points (6~. 6c) where 6H and 6~ denote corresponding ‘H (horizontal scale) and 13C chemical shifts (vertical).
1
and 7
shifrs a) in l-bromohrptane
Position
Chemical
1 2 3 4 5 6 7
3.362 l-S37 1.120 1.316 1.7-83 1.325 0.890
shift
a) ln ppm downfield from internal tetramethylsilane. m3red nandvd deviation kO.005 ppm.
Esti-
3.59
Volume
106, number 4
CHEMICAL
PHYSICS LETTERS
to the same carbon, both are apparently not affected by pulses (B) and precession resulting from their geminal coupling is not refocused during the second half of the evolution time. Cross sections along the F, dimen-
sion show four resonances which are detemlined by the 1H chemical shifts and geminal couplings. It is evident that the signal-to-noise ratio drops, but on the other hand pulse sequence (A) facilitates selective measurements of geminal couplings even without resolving the complete pattern of complex molecules. By varying the pulse 8, a complete spectral editing can be achieved [I ,4] at the expense of a slightly longer measuring time. However, there is also an interesting “time-saving” possibility [S] : if two experiments are performed with 8, = a/4 and 62 = 3n/4, only signals of CH, groups are opposite (fig. 2). By subtracting (or adding) the corresponding cross sections, signals of CH2 groups can be separately enhanced (or suppressed) without any increase of the measuring time. The use of pulse sequences (A) is not limited to molecules which contain only protons and carbons. If another nucleus (such as t9F, 3tP, etc.) is present, it
b
Z
--3 I 7.3
”
”
I 7.2
’
n #‘I
“‘I’ 7.
I
7.9
I-
a “‘I”’ 6.9
PPY
F& 3. Assigned ‘H-l” C chemical+rifr correlation map of fluorobenzene as obtained by pulse sequence (A) and e = 42. The presence of fluorine splits resonance
peaks and they are shifred in the same direction along both axes, thus confiinning that coupling with 19F has the same Sign for the 1 H and 13C spin in each pair. Benzene (b) has been added as a secondary
reference.
260
27 April 1984
can be coupled to both tH and 13C spins and the correlation map provides interesting information (fig. 3). In fluorobenzene, all directly attached protons (H,) experience ak0 heteronuclear couplings.&,F with lgF, but this interaction is not refocused during the evolution time because pulses (B) apparently do not affect H,. Those protons which arc coupled to fluorine spins in the “up” state give rise to a peak at 6B, + $IB,F along the Ft dimension, while the directly bonded carbon nuclei are coupled to lgF in the same state and the resonance must be detected at 6, + iJCF along the F7 dimension. Fig. 3 indeed shows pairs of peaks at @Ha + $JH,F, 6C + ;+F) and @Ha - $B,F, 6c - &F). To avoid limitations imposed by a finite size of computer memory, the original data matrix was divided into blocks which were processed separately to improve digital resolution along the F, dimension. In this way, all couplings between 1 H and tgF have been determined (3JB2F = 8.9 + 0.2 Hz, 4JB3 F = 5.7 i 0.2 Hz, 5JB,F = 0.4 +-0.2 Hz) and assigned even without resolving the complicated tH spectrum. Furthermore, the presence of fluorine shifts peaks in the same direction along both dimensions (fig. 3); therefore, “+~JH,~F and “Jc,,~ (n = 2,3,4) must have the same sign. The results are in perfect agreement with the previous study [ 191. The efficiency of pulse sequence (A) depends on adjustment of the delay T, which cannot be equal to ho = 1/2lJ,, in the general case, because values of ‘JCB range from 120 up to 250 Hz. Effects of non-uniform one-bond coupling constants are well documented for the DEPT pulse sequence [ 1] , while the selective flip of distant protons (pulses(B)) is still surrounded with some controversy, and mis-adjustments for more than 10% were claimed intolerable for the delay 7 [lo] _ This discouraging conclusion does not hold, since missetting of 7 merely increases the probability that the attached protons are reversed by pulses(B) [l l] . The precession resulting from the chemical-shift dispersion is refocused, while precession resulting from heteronuclear (rJc.t) and homonuclear couplings (JH,H~) is continued during the second half of the evolution time. The majority of the attached protons, however, evolve as supposed in the description and they give rise to “real” peaks determined by 1 H chemical shifts and geminal couplings. The intensity may be reduced, but “desired” measurements are not seriously hampered,
Volume 106. number 4
CHEMICAL
PHYSICS
I
27 April
LETTERS
19S1
The original data matris contained typically 128 blocks. while extensive zero-filling along the F, dimension improved digital resolution. Pulse sequence (A) does not refocus precession resulting from 13C chemical-shift dispersion_ A phase roll occurs, but it can be compensated for individual cross sections (figs. 2 and 4) which can be presented in the phase-sensitive mode. Magnitude spectra must be calculated, however. before the contour maps are plotted (figs. 1 and 3). Samples were prepared as approximately 50% solutions in CDCIJ. A few drops of tetramethylsilnne were added as a primary internal reference for chrmicalshift measurements.
n-l----.--:r Fig. 4. Cross section along the F1 dimension as obtained by pulse sequence (A) and 0 = n/7_ in transdichlorocthene. The delay T was intentionally mis-set (7 = 0.5~0) to illustrate the appearance of negative spurious peaks. which are centered at k’JCH/3- and further split by homonuclear coupling 3JHH. They can be easily distinguished from the positive “real” sigtul at the t H chemical shift 6 H. In most spin systems. however, several homonuclear couplinps $vc rise to even more compticared patterns of spurious peaks and their intensity is usually reduced below the noise level. hlis-setting of the delay t does not seriously hamper ‘H-“C chemicai-shift-correla-
tion
spectroscopy.
as illustrated in fig. 4 which exaggerates the effects. The delay T was Dztentionahy m&et by as much as 50% and a very simple molecule was selected to show that negative spurious peaks are split by ‘JCH = 198.8 however, Hz and 3JHH = 12.7 Hz. In most molecules, several homonuclear couplings are present and they give rise to complicated patterns for spurious peaks,
thus reducing their intensity below the noise level. It is also obvious that some information about IJCH is usually available for the sample and mis-setting of the delay 7 should not exceed ~30% in practice. Strong coupling in the 1 H spin system and other apparent obstacles also produce spurious peaks, but the signals are distributed among a large number of relatively weak resonances. Pulse sequence (A) will provide an indispensable link between lH and ‘SC resonances, thus facilitating assignment of peaks, accurate measurements of chemical shifts and specialized studies of hetcronuclear couplings with additional spins. Experimental. ti experiments were performed on a Nicolet NT-300 spectrometer using a 20 mm probe. The rr/2 pulse width was 45 F for 13C and 60 ps for IH spins. Pulse sequences were programmed and data sets were processed using the standard NMC software.
The NT-300 spectrometer was purchased paniall~ through a grant from the National Science Foundation (PCM-8115599).
References [l]
D.T. Perzs, D.11. DoddreU and M.R. BrndnlJ, J. Chcm. Phys. 77 (1981) 2735. [ 21 S..i\. \lorris and R. Freeman, J. dm. Chum. Sot. 101 [3]
(1979) 76C. D.P. Burum and R.R.
Ernst,
J. \la_en. Rcson.
39 (1980)
163. 141 11-H. Levitt,
O.\V. Sorensen and R.R. Ernst. Chrm. Phys. Letters 91 (1983) 5-l-10. [5] 11-R. BendaB and D.T. Peep, J. Slagn. Rcson. 53 (1983) 1-W. [6l A. Bx\ and R. Frrsman, J. Am. Chem. Sot. 104 t1962.l
1099. 171 J.R. Carbo\v.
D.P. \Vttitekamp and A. Pines. Chcm. Phys. Letters 93 (1983) 50-l. [ 81 A. Ba\. J. IrlaSn. Reson. 53 (19S3) 330. [9] V. Rutar, J. Am. Chem. Sot. 105 (19S3) 1095. [ 101 A. Bax, J. Magn. Reson. 53 (1983) 5 17. [ 11] V. Rurar. J. Magn. Rcson. 56 (1961) 67. [ 171 A. Bax and C..A. Morris, J. \lapn. Reson. 12 (19Sl) 501. [ 131 A.A. Maudslry, A. N’okaun and R.R. Ernst, Chcm. Phys. Letters 55 (1976) 9. [ 141 A.A. Maudsley and R.R. Ernst, Chcm. Phys. Letters 50 (1977) 366. [ 151 C. Bodenhausrn and R. Freeman, J. Xm. Chem. Sot. 100 (1978) 330. [ 161 L.D. Hall, G.A. Morris and S. Sukumar, J. Xm. Chem. Sot. 101(1980) 175. ] 171 W.P. Aue, J. Karhan and R.R. Ernst, J. Chcm. Phys. 64 (1976) 327-6. [ 181 Y.K. Levine, K.J.M. Birdsall, A.G. Let, J.C. Wxcaife, P. Partington and G.C.K. Roberts, J. Chem. Phys. 60 (1974) 2890. [ 191 W.S. Brey. L.W. Jaques and H.J. Jacobsen, Org. htngo. Resort. 13 (1979) 243.
261
HET~RO~~LEA~
C~M~~AL~~FT
AND DISTORTIONLESS V. RUTAR
27 Ai.lril 1984
CHEMICAL PHYSICS LETTERS
COLLATION
ENHANCEMENT
~I~ZtN~
BY POLARIZATION
HO~ONUCLEAR
DECOUPL~G
TRANSFER
*
Depnrtment of Chemiktry. University of Missour& Coiumbia. Missouri 65211, USA Received 18 January 1984; in fmal form 15 February 1984
NMR spectroscopy, selective i?ips of distant
’ H--13C c~e~~~~t~o~elation by polarization
transfer are combined
into a new two-dimensionaI
technique.
protons and distortionless
Correlation maps are simpfifkd
enhancement by suppressing
most homonuclear couplings, thus improving the signal-to-noise ratio. Applications include accurate measurements signments of t H chemical shifts as well as couplings with additional nuclear spins (lgF, 31Pl etc.) in the molecule.
The DEPT pulse sequence fer of polarization
from
[I ] promotes
abundant
(usually
--ill
the translI3) to di-
lute spins (t3C, 15N, etc.). Compared to the more traditional INEPT approach [2,3], some distinctive advantages have been noticed: mu~tiplets are not distorted and spectral editing is reliable even in the presence of non-uniform one-bond coupling constants (tJCw)_ Recently [4,5], the DEPT pulse sequence has been successfully adopted to generate heieronuclear chemical-shift correlation. Since any two-dimensional (Z-D) NMR technique requires a certain time commitment for data acquisition, the experiment must be carefully programmed to avoid long measuring times [6]. “Bilinear pulses”, as ~troduced by Garbow et al. [7], ~tention~y simplify data sets by suppressing homonuclear couplings. This letter descriies a new pulse sequence (A) (see scheme 1) which combines distortionless enhancement by polarization transfer with additional pulses(B) f7 lH: 1%:
and as-
“H:
_*.n/2(~~)-?--n(yl)--T-a/2((71)
Q:
-7Q3
...
)-
(B)
of the evolution time tt . If the delay T is set equal to r. = 1/21JCH, pulses (3) apparently do not affect those protons (Ha) which evolve for 180“ due to one-bond coupiing with directly attached 1363 spins. Other protons in the molecule are ‘distant” (Hd), and their homonucfear and heteronuciear couplings are small compared to ~~~~ ; tfterefore, they practically do not evolve ciuring the intervals 27 and they are flipped over by pulses (B)_ Precession resulting from Coup~gs~HaHd is refocused during the second half of the evolution time. The subsequent mixing period accomplishes polarization transfer from the 1H to 13C spin system and the variable pulse 6 adjusts enhancements for CH, at the middle
-~--B((F)~)-T--dec.
~/2((3I)-tl/2--irl2(~7)-~--n(y1)-~-n/2(~2~tl/2-7-
-acq .
-7dx~+
--n(Y3)_
evolution
t
illixing
69
I
Scheme I * Permarrent address: J. Stefan Institute, 6111 I Ljubljana, Yugoslavia.
258
P.O. Box 53,
0 009-2614/84/S 03.00 0 Elsevier Science Publishers born-Ho~and Physics Publishing Division)
B.V.
Volume
106. number
4
Table 1 Relative
phases a) of radio-frequency puiscs in sequence (A)
CHEMICAL
Step
rpl
1
-Y
7
--x
3 4
a) As denoted
92
Y x
in the doubly
rotating
77 April
LETTERS
1981
cJ3
X --J’
PHYSICS
x -X
--x
x
.,
--x
frame.
groups (m = 1,2,3) [ 1 ] _ A four-step phase cycle (table 1) together with alternate addition and subtraction of carbon signals ensures quadrature detection along both dimension [ 121 and suppression of coherencetransfer echoes [ 131. Results of 2-D data processing can be displayed as contour plots (fig. 1) which unambiguously correlate 1H and 13C chemical shifts. When single or chemically equivalent protons are attached to the same carbon nucleus, pulse sequence (A) eliminates all homonuclear J couplings from the F, dimension, thus giving rise to sharp single peaks. The signal-to-noise ratio and
Fig. 2. Picked-out decoupled spectra of protons at positions in I-bromoheptanc. if the variable pulse e is incrsosed fromt9t =a/4toe2= 3~~14. the spectrum of the CH3 group does not change (a and b), while the signal of the CHz goup is reversed (c and d).
accuracy of 1 H chemical-shift measurements are better than in the :raditional versions [ 14-161. Cross sections aiong the F, dimension have been picked OUI for each non-equivalent 13C signal and zero-filled up to 8000 data points to increase digital resolution (fig. 2). Chemical shifts (a,,) of protons at positions 1.2 and 7 in 1-bromoheprane are obvious even from one-dimensional 1 H spectra, while rhe remaining protons (positions 3-6) are so strongly coupled that rhey cannot be resolved even by 2-D homonuclear J spectroscopy [ 17). Here, pulse sequence (-4) represents an esclusive approach and, utilizing assignment of l;C resonances [ 181 , all 1H chemical shifts have been determined (rable 2). If two non-equivalent protons are srtached directly Table 2 Proton chemical
Fig. 1. Assigned chemical-shift correlation map for l-bromoheptane as obtained by pulse sequence (A) and 13= n/4- Since homonuclear couplings have been suppressed. signals are shown as *‘sharp” points (6~. 6c) where 6H and 6~ denote corresponding ‘H (horizontal scale) and 13C chemical shifts (vertical).
1
and 7
shifrs a) in l-bromohrptane
Position
Chemical
1 2 3 4 5 6 7
3.362 l-S37 1.120 1.316 1.7-83 1.325 0.890
shift
a) ln ppm downfield from internal tetramethylsilane. m3red nandvd deviation kO.005 ppm.
Esti-
3.59
Volume
106, number 4
CHEMICAL
PHYSICS LETTERS
to the same carbon, both are apparently not affected by pulses (B) and precession resulting from their geminal coupling is not refocused during the second half of the evolution time. Cross sections along the F, dimen-
sion show four resonances which are detemlined by the 1H chemical shifts and geminal couplings. It is evident that the signal-to-noise ratio drops, but on the other hand pulse sequence (A) facilitates selective measurements of geminal couplings even without resolving the complete pattern of complex molecules. By varying the pulse 8, a complete spectral editing can be achieved [I ,4] at the expense of a slightly longer measuring time. However, there is also an interesting “time-saving” possibility [S] : if two experiments are performed with 8, = a/4 and 62 = 3n/4, only signals of CH, groups are opposite (fig. 2). By subtracting (or adding) the corresponding cross sections, signals of CH2 groups can be separately enhanced (or suppressed) without any increase of the measuring time. The use of pulse sequences (A) is not limited to molecules which contain only protons and carbons. If another nucleus (such as t9F, 3tP, etc.) is present, it
b
Z
--3 I 7.3
”
”
I 7.2
’
n #‘I
“‘I’ 7.
I
7.9
I-
a “‘I”’ 6.9
PPY
F& 3. Assigned ‘H-l” C chemical+rifr correlation map of fluorobenzene as obtained by pulse sequence (A) and e = 42. The presence of fluorine splits resonance
peaks and they are shifred in the same direction along both axes, thus confiinning that coupling with 19F has the same Sign for the 1 H and 13C spin in each pair. Benzene (b) has been added as a secondary
reference.
260
27 April 1984
can be coupled to both tH and 13C spins and the correlation map provides interesting information (fig. 3). In fluorobenzene, all directly attached protons (H,) experience ak0 heteronuclear couplings.&,F with lgF, but this interaction is not refocused during the evolution time because pulses (B) apparently do not affect H,. Those protons which arc coupled to fluorine spins in the “up” state give rise to a peak at 6B, + $IB,F along the Ft dimension, while the directly bonded carbon nuclei are coupled to lgF in the same state and the resonance must be detected at 6, + iJCF along the F7 dimension. Fig. 3 indeed shows pairs of peaks at @Ha + $JH,F, 6C + ;+F) and @Ha - $B,F, 6c - &F). To avoid limitations imposed by a finite size of computer memory, the original data matrix was divided into blocks which were processed separately to improve digital resolution along the F, dimension. In this way, all couplings between 1 H and tgF have been determined (3JB2F = 8.9 + 0.2 Hz, 4JB3 F = 5.7 i 0.2 Hz, 5JB,F = 0.4 +-0.2 Hz) and assigned even without resolving the complicated tH spectrum. Furthermore, the presence of fluorine shifts peaks in the same direction along both dimensions (fig. 3); therefore, “+~JH,~F and “Jc,,~ (n = 2,3,4) must have the same sign. The results are in perfect agreement with the previous study [ 191. The efficiency of pulse sequence (A) depends on adjustment of the delay T, which cannot be equal to ho = 1/2lJ,, in the general case, because values of ‘JCB range from 120 up to 250 Hz. Effects of non-uniform one-bond coupling constants are well documented for the DEPT pulse sequence [ 1] , while the selective flip of distant protons (pulses(B)) is still surrounded with some controversy, and mis-adjustments for more than 10% were claimed intolerable for the delay 7 [lo] _ This discouraging conclusion does not hold, since missetting of 7 merely increases the probability that the attached protons are reversed by pulses(B) [l l] . The precession resulting from the chemical-shift dispersion is refocused, while precession resulting from heteronuclear (rJc.t) and homonuclear couplings (JH,H~) is continued during the second half of the evolution time. The majority of the attached protons, however, evolve as supposed in the description and they give rise to “real” peaks determined by 1 H chemical shifts and geminal couplings. The intensity may be reduced, but “desired” measurements are not seriously hampered,
Volume 106. number 4
CHEMICAL
PHYSICS
I
27 April
LETTERS
19S1
The original data matris contained typically 128 blocks. while extensive zero-filling along the F, dimension improved digital resolution. Pulse sequence (A) does not refocus precession resulting from 13C chemical-shift dispersion_ A phase roll occurs, but it can be compensated for individual cross sections (figs. 2 and 4) which can be presented in the phase-sensitive mode. Magnitude spectra must be calculated, however. before the contour maps are plotted (figs. 1 and 3). Samples were prepared as approximately 50% solutions in CDCIJ. A few drops of tetramethylsilnne were added as a primary internal reference for chrmicalshift measurements.
n-l----.--:r Fig. 4. Cross section along the F1 dimension as obtained by pulse sequence (A) and 0 = n/7_ in transdichlorocthene. The delay T was intentionally mis-set (7 = 0.5~0) to illustrate the appearance of negative spurious peaks. which are centered at k’JCH/3- and further split by homonuclear coupling 3JHH. They can be easily distinguished from the positive “real” sigtul at the t H chemical shift 6 H. In most spin systems. however, several homonuclear couplinps $vc rise to even more compticared patterns of spurious peaks and their intensity is usually reduced below the noise level. hlis-setting of the delay t does not seriously hamper ‘H-“C chemicai-shift-correla-
tion
spectroscopy.
as illustrated in fig. 4 which exaggerates the effects. The delay T was Dztentionahy m&et by as much as 50% and a very simple molecule was selected to show that negative spurious peaks are split by ‘JCH = 198.8 however, Hz and 3JHH = 12.7 Hz. In most molecules, several homonuclear couplings are present and they give rise to complicated patterns for spurious peaks,
thus reducing their intensity below the noise level. It is also obvious that some information about IJCH is usually available for the sample and mis-setting of the delay 7 should not exceed ~30% in practice. Strong coupling in the 1 H spin system and other apparent obstacles also produce spurious peaks, but the signals are distributed among a large number of relatively weak resonances. Pulse sequence (A) will provide an indispensable link between lH and ‘SC resonances, thus facilitating assignment of peaks, accurate measurements of chemical shifts and specialized studies of hetcronuclear couplings with additional spins. Experimental. ti experiments were performed on a Nicolet NT-300 spectrometer using a 20 mm probe. The rr/2 pulse width was 45 F for 13C and 60 ps for IH spins. Pulse sequences were programmed and data sets were processed using the standard NMC software.
The NT-300 spectrometer was purchased paniall~ through a grant from the National Science Foundation (PCM-8115599).
References [l]
D.T. Perzs, D.11. DoddreU and M.R. BrndnlJ, J. Chcm. Phys. 77 (1981) 2735. [ 21 S..i\. \lorris and R. Freeman, J. dm. Chum. Sot. 101 [3]
(1979) 76C. D.P. Burum and R.R.
Ernst,
J. \la_en. Rcson.
39 (1980)
163. 141 11-H. Levitt,
O.\V. Sorensen and R.R. Ernst. Chrm. Phys. Letters 91 (1983) 5-l-10. [5] 11-R. BendaB and D.T. Peep, J. Slagn. Rcson. 53 (1983) 1-W. [6l A. Bx\ and R. Frrsman, J. Am. Chem. Sot. 104 t1962.l
1099. 171 J.R. Carbo\v.
D.P. \Vttitekamp and A. Pines. Chcm. Phys. Letters 93 (1983) 50-l. [ 81 A. Ba\. J. IrlaSn. Reson. 53 (19S3) 330. [9] V. Rutar, J. Am. Chem. Sot. 105 (19S3) 1095. [ 101 A. Bax, J. Magn. Reson. 53 (1983) 5 17. [ 11] V. Rurar. J. Magn. Rcson. 56 (1961) 67. [ 171 A. Bax and C..A. Morris, J. \lapn. Reson. 12 (19Sl) 501. [ 131 A.A. Maudslry, A. N’okaun and R.R. Ernst, Chcm. Phys. Letters 55 (1976) 9. [ 141 A.A. Maudsley and R.R. Ernst, Chcm. Phys. Letters 50 (1977) 366. [ 151 C. Bodenhausrn and R. Freeman, J. Xm. Chem. Sot. 100 (1978) 330. [ 161 L.D. Hall, G.A. Morris and S. Sukumar, J. Xm. Chem. Sot. 101(1980) 175. ] 171 W.P. Aue, J. Karhan and R.R. Ernst, J. Chcm. Phys. 64 (1976) 327-6. [ 181 Y.K. Levine, K.J.M. Birdsall, A.G. Let, J.C. Wxcaife, P. Partington and G.C.K. Roberts, J. Chem. Phys. 60 (1974) 2890. [ 191 W.S. Brey. L.W. Jaques and H.J. Jacobsen, Org. htngo. Resort. 13 (1979) 243.
261