Two-dimensional NMR spectra of oriented molecules

JOURNAL

OF

MAGNETIC

RESONANCE

37,349-351

(1980)

COMMUNICATIONS Two-Dimensional NMR Spectra of Oriented Molecules In an earlier communication (I), it was demonstrated theoretically that the two-dimensional NMR spectroscopy of “oriented” molecules may provide additional information on the spin systems and the appearance of lines with negative intensities was predicted in the 2D spin-echo spectra of strongly coupled oriented systems such as AB, or AB2. The 2D experiments considered consisted of a nonselective 180’ pulse in the middle of the evolution period (ri) and nonselective detection during the detection period (tz) (2). On Fourier transformation with respect to the two time variables tl and t2, one obtains the spectrum with two frequency variables o1 and WZ, respectively. In the present note, we report the first experimental result on the 2D spectra of oriented molecules. The proton NMR spectrum of 1-thia-3-selenole-2-thione (without the 13C-lH and the “Se-‘H satellites) forms an AB system. The spectrum of a 4 mol% solution of the compound in iV-( pmethoxybenxylidene)-p-nbutylaniline was recorded at 21°C on a Bruker WH-270 FT NMR spectrometer coupled with a BNC-12 Computer. It is shown in Fig. 1. The software of the 2D experiment was developed at the Bangalore NMR Facility. The lines with negative intensities are clearly observed in Fig. 1. The value of lJ~n+2DmI (where JAB and D An define the indirect and the direct spin-spin couplings between nuclei A and B) determined unambiguously from the four lines with equal intensities is 93 Hz.

FIG. 1. Two-dimensional proton NMR spectrum (270 MHz) of an AB system oriented in the nematic phase of N-(pmethoxybenxylidene)~p-n-butylaniline, obtained using a nonselective 180” pulse in the middle of the evolution period (tl) and nonselective detection during the detection period (rz). 0022-2364/80/020349-03502.00/O 349 Cowrinht

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COMMUNICATIONS

The conditions of “deceptive simplicity” (3) of the 2D spin-echo spectra of strongly coupled systems of oriented molecules were examined and the results are reported in the present communication. In deceptively simple spectra, one, several, or all subspectra are degenerate (4). Whereas deceptive simplicity in isotropic media involves collapse of the subspectra into singlets when chemical shifts (S’s) tend to zero, the degenerate subspectra of oriented molecules may be relatively complex (5). Unlike the situation in isotropic cases, the subspectral degeneracy in oriented systems may also arise because of special relations between the parameters. In this note, the discussion is, however, restricted to the case S -D 0. The spin systems examined are ABX and ABzX, for which the AB and the AB2 parts of the spectra are superpositions of two AB and two AB2 subspectra (6), respectively. The experiments considered involve selective excitation of transitions of the strongly coupled spins by a 180” pulse. For this purpose, the expressions reported in Ref. (I) for the intensities and the frequencies of the allowed transitions in an oriented AB system were used and those for the AB&pe spectra were obtained with the help of the procedure outlines in Ref. (2). The expressions leading to deceptively simple ABXand ABzX-type spectra were derived with a view to finding out whether the same conditions as those for the one-dimensional spectra exist (5) or whether, because of the larger number of transitions in the 2D spectra, the conditions are different. For S = 0, the 1D and the 2D spectra are, however, similar (1). The expressions for the 2D spectra of type AB (1) show that, analogous to the corresponding 1 D cases, the degeneracy of the AB subspectra arises only from the small intensity of lines rather than from the overlap. Since in the 2D spin-echo experiments, the number of allowed transitions differs from that for the 1D and the expressions for the intensities are not identical to those for the 1D cases, the conditions leading to deceptive simplicity may be different in the two cases. It can be shown that the negative intensity lines are observed only if

where i is the minimum observable intensity. However, appearance of the four lines of equal intensities is

the condition

for the

PI Condition [l] is identical to that derived for the 1D degenerate AB subspectra (5). It shows that if i - 0.1, IJAB -DAM] - 500 Hz, 8~~ should be larger than 220 Hz before its effect is observed on the spectrum. On the other hand, condition [23 shows that the effect of SAB on the 2D spectra of AB type will be noticeable for S - 160 Hz for similar values of i and ]JAB -DAB]. The results, therefore, show that the 2D spectra may be more sensitive than the corresponding 1D spectra for the determination of SAB, especially when the value is small. In an AB2 system the subspectral degeneracy arises from the overlap of the lines (5) rather than from the nonobservability of certain transitions. In such cases, on the other hand, the deceptive simplicity conditions are identical in 1D and 2D spectra.

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The experiments provide the first 2D spin-echo spectrum of an oriented system. The value of the parameter l./~u + 2DAB( is unambiguously determined for the AB system. It is also shown that for the oriented AB type, the deceptive simplicity conditions in 1D and 2D spectra are not identical, whereas for cases like AB2, they are the same.

REFERENCES 1. A. KUMAR AND C. L. KHETRAPAL, J. Magn. Reson. 30,137 (1978). 2. A. KUMAR, J. Magn. Reson. 30,227 (1978); R. FREEMAN AND G. A. MORRIS, Bull. Magn. Reson. 1,5 (1979). 3. R. J. ABRAHAM AND H. J. BERNSTEIN, Can. J. Chem. 39,216 (1961). 4. P. DIEHL, Heiu. Chim. Acta 47,l (1964). 5. P. DIEHL, C. L. KHETRAPAL. AND U. LIENHARD, Org. Magn. Reson. 1,93 (1969). 6. P. DIEHL, R. K. HARRIS, AND R. G. JONES, “Nuclear Magnetic Resonance Spectroscopy,” Chap. 3, p. 1, Pergamon, Oxford, 1967.

C. L.

KHETRAPAL*?

ANIL

KIJMAR*‘$

A. C. KImwARt P. C. MATHIAS* K. V. RAMANATHAN* * Bangalore NMR Facility Indian Institute of Science Bangalore 560 012, India tRaman Research Institute Bangalore 560 006, India SPh ysics Department Indian Institute of Science Bangalore 560 012, India Received July 9, 1979