3578
J. Phys. Chem. 1991,95, 3578-3581
increased from the fluoro to the chloro and finally to the bromo compound because of the increased steric interaction between the 2-position halogen and the carbonyl oxygen atom. These similar energies for the transition states and increased energies for the trans minimum in the potential result in the decreasing trend in the trans-cis barriers. Since the radius of the bromine atom is only slightly larger than that of chlorine, the calculated s-trans-
s-cis barrier decreases only slightly from CH,CCICFO to CH2CBrCFO. In contrast, this barrier decreases significantly from CH,CHC(O)F or CH,CFCFO to CH,CCICFO. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this study.
Combined Sample Rotation and MultiplePulse NMR Spectroscopic Studies on Protons Bonded to "N Nuclel in Solid Amino Acids Akira Naito, Andrew Root, and Charles A. McDowell* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T I Y6 (Received: July 16, 1990)
Solid-state high-resolution proton nuclear magnetic m n a n c e spectra (NMR) of glycine, alanine, N-acetylglycine, and L-histidine hydrochloride monohydrate were recorded by using the CRAMPS technique. The protons bonded to I4N nuclei showed characteristic line shapes which are explained by the "N quadrupole effects on the I4N-H dipolar interactions. The BR-24 pulse sequence was generally more effective than the MREV-8 sequence in yielding well-resolved IH CRAMPS spectra. In the case of the peptide proton bonded to an "N nucleus in N-acetylglycine, however, better resolved spectra were obtained by decoupling with the MREV-8, than with BR-24 pulse sequence, because a faster rotor-spinning frequency was required in this case to reduce the N-H dipolar interactions. Theoretical computer simulations of the line shapes show good agreement with those observed experimentally.
Introduction The introduction of cross-polarization and high-power dipolar decoupling in nuclear magnetic resonance spectroscopy made it possible in many cases to suppress heteronuclear dipolar broadening and to obtain signals with high sensitivity in solid-state samples.'v2 When combined with magic-angle spinning of sample~,~,' this led to cross-polarization magic-angle-spinnings (CPMAS) solid-state nuclear magnetic resonance spectroscopy! This was particularly the case for protons attached to dilute spins such as I3C, 15N, 29Si,etc. For other situations with abundant spins such as 'H, 19F, 31P,etc., it was shown that by combining a multiple-pulse experiment with magic-angle spinning, it was possible to reduce both dipolar broadening caused by homonuclear interactions and also magnetic shielding anisotropies. This technique of combined sample rotation and multiple-pulse NMR spectroscopy was called CRAMPS by G e r ~ t e i n 'and ~ ~ his collaborators, who pioneered the method. This innovation has proved to be most effective in yielding high-resolution proton spectra for many different types of compounds, including rigid solids where very high sample spinning rates may otherwise be necessary to average the homonuclear proton dipolar broadening. The CRAMPS technique has been successfully applied to obtain S.R.; Hahn, E. L. Phys. Reo. 1962, 128, 2042. (2) Pines, A.; Gibby, M. G.; Waugh, J. S.J . Chem. Phys. 1973,59, 569. (3) Andrew, E. R.; Bradbury, A. E.;Eades, R. G. Nature 1959,162, 1659. (4) Lowe, 1. J. Phys. Rev. Lrtt. 1959, 2, 285. ( 5 ) Schaefer, J.; Stejskal, E. 0. J . Am. Chem. Soc. 1976, 98, 1031. (6) Mehring, M. High Resolution NMR in Solids, 2nd 4.Springer; (1) Hartmann,
Verlag: West Berlin, 1982. (7) Gerstein, B. C. Philos. Trans. R . Soc. London 1981, 521, A299. (8) Gentein, B. C.; Pemklton, R. G.; Wilson, R. C.; Ryan, L. M. J. Chem. Phys. 1977, 66, 361. Ryan, L. M.; Taylor, R.E.; Paff, A. T.; Gerstein, B. C. J . Chem. Phys. 1980, 72, 508. (9) Gerstein, B. C.; Dybowski, C. Transient Techniques in Solid State NMR; Academic Press: New York, 1985.
0022-3654/91/2095-3578$02.50/0
high-resolution proton spectra of complex solids such as organic polymer^,^*^ humic acid,9 bituminous,8 and Turkish coals.9 Extensive studies have been made of the high-resolution 'HN M R spectra of the different stages of hydration of silica gel, silicaalumina, a-alumina, and H Y zeolite.'*I2 Recent attempts to combine the CRAMPS method with MAS imaging, though yielding interesting results, have not completely fulfilled expect a t i o n ~ . ' ~Extensive 'H CRAMPS studies of hydrogen-bonding phenomena in carboxylic acids and phosphoric acid derivatives have been carried out by Harris et aI.l4 and by Scheler et al.ls The results were used to establish clearly the relationship between proton isotropic chemical shifts and hydrogen-bond distances. These studies have recently been extended to study the 0.10bond distances of hydrogen bonds in solid N-benzoyl-L-phenylalanine and, in particular, to provide information concerning the thermal phase alterations of amino acids.I6 The use of I9F CRAMPS N M R in studying fluorinated organic compounds hsa been assessed by Harris et al." in their detailed studies on perfluoronaphthalene in which they detected splittings arising from different (10) Bronnimann, C. E.;Hawkins, B. L.; Zhang, M.; Maciel, G. E.Anal. Chem. 1988, 60, 1743. ( 1 1) Bronnimann, C. E.; Zigler, R. C.; Maciel, G. E. J . Am. Chem. Soc.
1988, 110, 2023. (12) Dec. S. F.: Bronnimann. C. E.:Wind. R. A.: Maciel. G. E.J . Mam. Reson: 1989, 82, 454. (1 3) Veeman, W. S.;Cory, D. G. In Advances in Magnetic Resonance; Academic Press: New York, 1989; Vol. 13, p 43. (14) Harris, R. K.;Jackson, P.; Merwin, L. H.; Say, B. J.; Hagele, G. J. Chem. Soc., Faraday Trans. 1 1988,84, 2649. (15) Scheler, G.; Hubenreiser, U.;Rosenberger, H. J. Magn. Reson. 1981, 44, 134. (16) Chu, P. J.; Potzebowski, J.; Gao, Y.; Scott, A. 1. J . Am. Chem. Soc. 1990, 112, 881. (17) Harris, R. K.;Jackson, P.; Nesbitt, G. J. J. Magn. Reson. 1989,85, 294. (18) Smith, K. A.; Burum, D. P. J . Magn. Reson. 1989, 84, 85-94.
0 1991 American Chemical Society
CRAMPS Studies on Solid Amino Acids
n
I-alanine H
H,N+-
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3579
I
N-Acetylglycine
c -cooI
n
CH3
18.0 14.0 10.0 6.0
2.0 -2.0 20 I8 I6 I4 I2 IO 8 6 4 2 0 -2
qlycine
PPM
H
I
H ~ N + - c-coo-
I
H
I \
180 140 100 6 0
MREV-8
I \
20 - 2 0
PPM
Figure 1. IH CRAMPS NMR spectra of L-alanine (a) and glycine (b).
20 18 16 14 12
I
)
!
IO 8 6 4 2 0 -2 PPM
crystallographic sites. CRAMPS also has shown to be an important method for the study of inorganic compounds containing I9F nuclei.' Smith and Buruml* have reported an extensive study of the I9F CRAMPS NMR spectra of mixtures of calcium fluoride and fluoroapatite. It is that I3C nuclei bonded to 14Nnuclei show characteristic line shapes. These observations have been theoretically explained22-26by showing that the l3C-I4N dipolar interaction cannot be completely averaged out by magic-angle sample spinning, since, in this case, the quantization axis of the 14N nuclei is not aligned along the direction of the static magnetic field because of the I4N quadrupolar interactions. This type of characteristic line shape has been observed in other than the I3C-l4N system. It has been n~ted~'-~O in the NMR spectra of compounds containing various quadrupolar nuclei bonded to spin S = nuclei, e.g., '3C-35CI, 35C1-29Si, 75A~-'3C,D-13C, and 63CU-31P. Although it is also known that the protons bonded to nitrogen nuclei show broad line shapes which are brought about by the I4N quadrupole effect, no detailed analysis of the broad line has, however, been presented. The characteristic line shape as observed for carbon bonded to 14N has not, until now, been observed in 'H CRAMPS experiments. In this paper, we show that N M R line shapes of the protons bonded to I4N nuclei clearly show characteristic line shapes in solid-state amino acids. These 'H broadened CRAMPS spectra can be simulated successfully by theoretical calculations that take into account quadrupolar effects and the scaling factor appropriate for the particular multiple-pulse sequences used in the experiments. (19) Opella, S.J.; Frey, M. H.; Cross, T. A. J . Am. Chem. Soc. 1979,101, 5856. (20) Groombridge, C. L.; Harris, R. K.; Packer, K. J.; Say, B. J.; Tanner, S. F. J. Chem. SOC.,Chem. Commun. 1980,4, 174. (21) Frey, M. H.; Opella, S.J. J . Chem. SOC.,Chem. Commun. 1980, 11, 474. (22) Naito, A.; Ganapathy, S.;McDowell, C. A. J . Chem. Phys. 1981, 24, 5191
(23) Zumbulyadis, N.; Henrichs, P. M.; Young, R. H. J . Chem. Phys.
1981. 15, 1603.
(24) Hexem. J. G.;Frey. M. H.; Opella, S.J. J. Am. Chem. SOC.1981, 103, 224. (25) Hexem, J. G.;Frey, M. H.; Opella, S. J. J . Chem. Phys. 1982, 77, 3847. (26) Naito. A.; Ganapathy, S.;McDowell. C. A. J . Magn. Reson. 1982, 48, 367. (27) Menger, E. M.; Veeman, W. S. J . Magn. Reson. 1982, 46, 257. (28) Bohm, J.; Fenzke, D.; Pfeifer, H. J . Magn. Reson. 1983, 55, 197. (29) Sastry, D. L.; Naito, A.; McDowell, C. A. Chem. Phys. Lett. 1988, 146, 422. (30) Scott, D.; Ganapathy, S.;Bryant, R. G.J . Magn. Reson. 1987, 72, 376.
Figure 2. 'H CRAMPS NMR spectrum of N-acetylglycine. The spectrum shown in (a) was recorded by using the BR-24 pulse sequence while that exhibited in (b) was obtained by employing the MREV-8 pulse sequence.
Experimental Section The amino acids alanine, glycine, 1-histidine hydrochloride monohydrate, and N-acetylglycine were recrystallized from aqueous solutions. The crystals were finely ground and packed in the rotor sample tube by using specially constructed hemispherical concave-ended compression cylindrical plugs so that spherical samples resulted. The improved resolution resulting from the use of spherical samples has been clearly d e m o n ~ t r a t e d . ~ l J ~ * ~ ~ The 'H CRAMPS N M R spectra were recorded on a Bruker MSL-200 FT N M R spectrometer with a MAS probe supplied by Doty Scientific Inc. A 2.5-ps 90' pulse was used for both the MREV-831and BR-2432pulse sequences. The multiple-pulse cycle times, tC)s, were 42 and 126 ps for MREV-8 and BR-24, respectively, corresponding to a pulse spacing 7 of 3.5 ps. The spinning frequency was adjusted to be 3.7 and 1.2 kHz for the MREV-8 and the BR-24 pulse sequence, respectively, by using an electronic air spinning speed contr~ller.~'Bronnimann et a1.I0 state that they obtained the best spectral resolution when using the BR24 pulse sequence with MAS speeds of -1.5-2.0 kHz. Adamantane was used as an internal chemical shift reference standard, namely, at 1.74 ppm lower field from TMS.
Results and Discussion Proton NMR Spectra of Amino Acids. Figure 1 displays the 'HCRAMPS N M R spectra of polycrystalline samples of the amino acids glycine and alanine measured as described above. The signals were assigned to be the methyl protons, a-proton, and the amino proton going from high to low magnetic field in alanine (Figure la). The two a-protons in glycine show slightly different chemical shift values of 2.9 and 3.7 ppm, while the amino proton at 7.5 ppm exhibits a fairly broad line shape compared with the a-protons (Figure 1 b). In Figure 2 we show the IH CRAMPS NMR spectra of N-acetylglycine. Figure 2a was obtained by using the BR-24 pulse sequence with a spinning frequency of 1.2 kHz, and Figure 2b was recorded by using the M R E V - 8 pulse sequence with a spinning frequency of 3.7 kHz. These proton signals were (31) Rhim, W.-K.; Elleman, D. D.; Vaughan, R. W. J . Chem. Phys. 1973,
59, 3740.
(32) Burum, D.; Rhim, W.-K. J . Chem. Phys. 1979, 71, 964. (33) Jackson, P.; Harris, R. K. M a g . Reson. Chem. 1988, 26, 1003. (34) Maciel, G.E.; Bronnimann, C. E.; Hawkins, B. L. In Adounces in Magnetic Resonance; Academic Press: New York. 1990; Vol. 14, p 125. (35) Lee, J. N.; Alderman, D. W.; Jin, J. Y.; Zalm, K. N.; Mayne. C. L.; Pugrine, R. J.; Grant, D. M. Rev. Sci. Instrum. 1984, 55, 516.
3580 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 L-tiistidine. HCI- H 2 0 -CHZ-
'
$0 IO 16 I4
15
I6 8 6 4
0 -2
PPM
Figure 3. 'H CRAMPS N M R spectrum of L-histidine hydrochloride monohydrate obtained by using the MREV-8 pulse sequence.
assigned to arise from the methyl, CY-, peptide, and carboxyl protons on going from high to low field. It will be noticed that the BR-24 pulse sequence gives better resolution for the methyl, CY-, and carboxyl proton resonances than is the case with the MREV-8 sequence. The MREV-8 pulse sequence, however, gives higher resolution for the proton bonded to the peptide nitrogen in Nacetylglycine than does the BR-24 sequence. This latter observation is interesting and may be significant, because others have also found that, in general, the best resolved 'H CRAMPS spectra are usually obtained by using the BR-24 pulse sequence. In the experiments employing the BR-24 pulse sequence, a 1.2-kHz spinning frequency was used, but the I4N-IH dipolar interaction, -yN-yHh/(4$rNH3),for the peptide proton is calculated to be 2.92 kHz after taking into account a scaling factor S,of 0.422 for the BR-24 pulse sequence. Therefore, the 1.2-kHz spinning frequency used for the BR-24 pulse sequence cannot completely average out the I4N-IH dipolar interaction, thus causing a residual line broadening. On the other hand, the 3.7-kHz spinning frequency used for the MREV-8 pulse sequence is sufficient to average the I4N-'H dipolar interaction of 3.57 kHz scaled by the factor S,= 0.517, leading to a clear characteristic line shape for the peptide proton signal. The line shape of the peptide proton obtained with MREV-8 pulse sequence shows the pattern which is characteristic of a S = nucleus bonded to an I4N nucleus. It is to be noted that in their study of the IH CRAMPS spectra of N-benzoyl-L-phenylaniline, Chu et a1.,I6who only used the BR-24 pulse sequence, obtained broad signals for the protons that they assumed were located in N-H...O==C type H bonds. In that study, the aromatic and the peptide proton signals overlapped each other, and hence Chu et a1.I6 could not observe the expected characteristic line shape. It was shown early by Schnabel et al.,36 who used the WAHUHA3' pulse sequence with MAS, that theory indicates there is to be expected an interference effect between multiple-pulse sequences and sample rotation. They found experimental evidence for that phenomenon. Gerstein and Dybowski9 made an elegant and interesting analysis of the efficiencies of various multiple-pulse schemes for use in CRAMPS studies. They showed clearly that the BR-24 sequence has advantages over MREV-8 pulse sequence in that, like the latter, it compensates for both pulse timing and phase errors, while eliminating the zeroth-order and all odd-order terms in the Magnus expansion of the effective dipolar Hamiltonian. In a spinning sample, however, a fast spinning rate may cause line broadening by the destructive interference interaction between the MAS and the multiple-pulse averaging processes if the MAS rotation period becomes comparable to the pulse cycle time of the multiple-pulse s e q ~ e n c e . ' ~Thus, ~ ~ J the ~ upper limit of the spinning frequency required to give better resolution with (36) Schnabel, B.; Hubenreiser, U.; Scheler, G.; Muller, R. Magn. Reson. Relat. Phenom., Proc. Congr. Ampere, 19th 1976, 414. ( 3 7 ) Waugh, J. S.; Huber, L. M.; Haeberlcn, U. Phys. Reo. Lett. 1968, 20. 180.
Naito et al. the BR-24 sequence is lower than that for the MREV-8 because the pulse cycle time for the BR-24 sequence is 3 times longer than that of the MREV-8 sequence. The lH CRAMPS NMR spectrum of L-histidine hydrochloride monohydrate is shown is Figure 3. The ' H NMR signals were assigned to arise from the methylene protons, a-C-H, water of crystallization, two imidazole ring C-H, and the two ring imino protons (1 1.5 and 15.6 ppm) as we go from high to low field, by comparing them with the reported values.14 In this case the imino protons bonded to the N2 and N3 nitrogen nuclei did not show the characteristic doublet pattern although they showed broad line shapes. The imino proton bonded to N2 nitrogen nucleus is reported to have strong hydrogen bonding:* and hence thus would be expected to exhibit a larger chemical shift value than the hydrogen bonded to the N3 nitrogen nucleus. Therefore, the lowest field peak (15.6 ppm) and the second lowest field peak (1 1.5 ppm) were assigned to be the protons bonded to N 2 and N3 nitrogen nuclei, respectively. N M R Line Shapes of Protons Bonded to I4N. The most striking feature of the CRAMPS NMR of protons which we observe in the amino acids here studied is the characteristic line shape of protons bonded to a peptide I4N nucleus. This characteristic line shape can be explained by taking into account the I4N quadrupole effects on the I4N-lH dipolar interaction just as in the case of carbon bonded to a I4N nucleus. To explain the line shape, first we consider the appropriate I4N Zeeman quadrupolar Hamiltonian for this case as follows21-26
where Z'is the direction of the static magnetic field and Z is the direction of the unique axis, ZEFG.The asymmetry parameter 7 = (qxx- qyy)/q,,, in this expression eqii is the principal value of the electric field gradient tensor at the I4N nucleus, and as usual Iq,l 1 1q I 1 1q.J. The quantity eZQqii (i = x , y, z ) is one of the principafialues of the quadrupole coupling tensor, and e2Qqzzis the quadrupole coupling constant. The Hamiltonian 7fZQcan easily be solved by using the adiabatic approximation. The Hamiltonian representing the I4N-IH dipolar interaction in the Zeeman coordinate system when a I4N-lH is rotating about an axis inclined at an angle of Bo from the static magnetic field can be expressed as where
Here (X,Y,Z) are the elements of the N-H vector with respect to the I4N quadrupole coupling tensor principal axis system. S, is the scaling factor for the multiple-pulse scheme used. S, is the scaling factor caused by a group rotation where 0 is the angle between the rotation axis and the N-H direction. It is necessary to use the appropriate value of S,when the N-H bonded is rotating rapidly; otherwise S, = 1. As outlined in our earlier paper^,^^^^^ the dipolar interactions for each orientation were calculated and averaged over one spinning cycle. This is necessary because the time-dependent I4N-IH dipolar interaction is less than the MAS (38) Fuess, H.; Hohlhein, D.; Mason, S. A. Acfa Crystallogr. 1977,833, 654.
CRAMPS Studies on Solid Amino Acids
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3581
value (qyr) of the electric field gradient tensor of the peptide nitrogen IS parallel to the N-H direction, it later became clear that X, is parallel to the N-H direction, and the sign of the quadrupole coupling constant was negative as seen by inspecting the pattern of 13CCPMAS spectra of the peptide carbon nuclei.za The simulated spectrum clearly shows the asymmetric doublet pattern, and hence it is clear that feature arises from the quadrupole effect of the I4N nucleus. Spectral simulation was also performed for the amino protons in glycine (Figure 4b). In the case of the amino protons, it was assumed that the N-H bond A c' A A rotated rapidly about the C3axis of amino group, making an angle of 109O. In the calculation, we used the I4N quadrupole coupling constant of 1.18 MHz and the asymmetry parameter 7 = 0.54 determined from a single-crystal study.40 When fairly broad isotropic line widths of 120 Hz were included in the calculations, a line shape was obtained that was similar to those observed. These 4c€l 100 0 -200 100 0 -200 200 0 -200 2aQ 0 -2oon: broadening features can be explained by considering that the three Figure 4. Expanded IH CRAMPS NMR spectra for protons bonded to amino protons have slightly different chemical shift values since peptide (a'), amino (b'), and two imino (N2,c'; N,, d') nitrogen nuclei N-H bond distances for three amino protons are expected to be in N-acetylglycine, glycine, and L-histidine hydrochloride monohydrate, different. The spectra for the protons bonded to imidazole nitrogen respectively. The corresponding computer-simulated spectra are shown nuclei were also simulated by using the I4N quadrupole coupling in (a)-(d). The top figures indicate the directions of the principal axes constants determined in our earlier I4N NMR single-crystal (X,, Y,,and 2,)of the 14N electric field gradient tensors. The paramstudies4' (Figure 4c,d). The simulated spectra show that a broad eters used in calculations are as follows. a (ref 39): 2qQ/h = -3.21 line is to be expected, and a clear asymmetric doublet pattern MHZ, 7) = 0.32, Y N Y H ~ / ( ~ T ~ ~=N 6.91 $ ) kHz, sm 0.517, and a cannot be anticipated in this case. It is also expected that the half-width of 60 Hz. b (ref 40): e2qQ/h = 1.18 MHz, 7) = 0.54, proton bonded to the N2 nitrogen should show a broader line kHz, s, 0.517, s, 3 0.447, and a half-width Y N ~ H ~ / ( ~ + N H=' 7.341 ) Of 120 HZ. C (ref 41): &Q/h = 1.465,T = 0.2676, Y N Y H ~ / ( ~ ? T ~ ~ N Hcompared ~) with that for a proton bonded to the N3 nitrogen nucleus. = 6.716 kHz, S, = 0.517, and a half-width of 100 Hz. d (ref 41): In conclusion, it has been observed that the protons bonded to &Q/h = -1.287 MHZ, 0.9456, Y N Y H ~ / ( ~ ? T ~ = ~ N7.618 $ ) kHz, sm nitrogen nuclei in amino acids show broad characteristic doublet = 0.517, and a half-width of 100 Hz. line shapes even at high magnetic fields. Although it has generally been accepted that the BR-24 pulse sequence is the most efficient frequency. The values of the transition energies for the protons to yield narrow CRAMPS signals, the MREV-8 pulse sequence aE'(t)were calculated for the three nuclear spin states of the I4N gave better resolution in the case of a proton bonded to a I4N nucleus. The average AEb(t) was calculated for steps of 5 O in nucleus in N-acetylglycine, Because the I4N-IH dipolar inter8, the angle between the rNH vector and the magic-angle-spinning action is quite large, a relatively high spinning frequency is required axis. These calculated resonances were finally convoluted by to average out the interaction. Therefore, MREV-8 has the applying a Gaussian line shape function to give the actual line advantage of yielding better resolution for the proton bonded to shape.2226.29 a I4N nucleus, because the upper limit of the spinning frequency In Figure 4 we show the expanded 'H CRAMPS NMR spectra for that sequence is higher than that of the BR-24 pulse sequence. observed for protons bonded to a peptide, amino, and two imino Similar effects may also be observed in the 'H CRAMPS spectra nitrogen nuclei in N-acetylglycine, glycine, and histidine hydroof compounds containing protons bonded to other quadrupolar chloride monohydrate, respectively. An asymmetric doublet nuclei. powder pattern is clearly shown for the peptide proton signal (Figure 4a). This type of well-defined asymmetric pattern in the Acknowledgment. We thank the Natural Sciences and EnIH CRAMPS spectra has not been previously clearly observed gineering Research Council of Canada for financial support. A.N. in these types of studies. The spectrum shown in Figure 4a was acknowledges the receipt of a travel grant from the Murata simulated by using the I4N quadrupole coupling constant of -3.2 Foundation, Japan. MHz, and 7 = 0.32 for a peptide proton; these parameters had Registry No. N2, 7727-37-9; H-Gly-OH, 56-40-6; H-Ala-OH, 56been determined earlier in an I4N NMR study on N-acetylvaline 41-7; Ac-Gly-OH, 543-24-8; H-His-OHeHCI, 645-35-2. single crystals.39 Although it was reported in that single-crystal study that the direction ( Y,) of the Y component of the principal dl
J
-
U
L
(39) Stark, R. E.; Haberkorn, R. A.; Griffin, R. G. J . Chem. Phys. 1978, 68, 1996-1997.
(40) Haberkorn, R. A.; Stark, R. E.; van Willigen, H.; Griffin, R. G. J . Am. Chem. Soc. 1981, 103, 2534. (41) McDowell, C. A.; Naito, A.; Sastry, D. L.; Takegoshi, K. J. Magn. Reson. 1986, 69, 283-292.
