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that for the triplet state of the 0-trans isomer (see Figure 9). The shortest lifetime limit of triplet-state molecule that we can detect by our apparatus of sensitized phosphorescence excitation spectroscopy is 40 ps. From the absence of the 0-cis band in the sensitized phosphorescence excitation spectrum, the triplet-state lifetime of the 0-cis isomer should be much shorter than 40 ps. The enol type biradical is also in its triplet state. If it lives more than 40 ps, it should also appear in the sensitized phosphorescence excitation spectrum. The absence of the 0-cis isomer in the sensitized phosphorescence excitation spectrum means that the lifetime of the radical also should be less than 40 ps. We measured the (1 + 1’) REMPI spectrum by taking the delay time of about 100 ns to the ionization laser and found the strong appearance of the band of the 0-cis isomer. It is concluded that the lifetime of the biradical is longer than 100 ns but shorter than 40 ps. This agrees with the reported value of 1500 ns in the solution.28 In the above, we assumed that the enolization occurs in the triplet state of the 0-cis isomer. This can be confirmed by the comparison between the sensitized phosphorescence excitation spectrum and the one-color REMPI spectrum due to the direct Tl(n,.lr*) Sotransition shown in Figure 7. While the 0 band of the T1 So transition of the 0-cis isomer is missing in the former spectrum (Figure 7a), it appears strongly in the latter (Figure 7b). This provides the evidence that the enolization reaction occurs in the triplet state. The selective chemical reaction of the 0-cis isomer of omethylbenzaldehyde found in this work has a profound implication in the photochemistry of rotational isomers. A similar selective reaction is also expected for other carbonyl compounds whose studies are now in progress in our laboratory.
--
0-trans
0-cis
enol
Figure 9. Relaxation scheme of the rotational isomers of o-methylbenzaldehyde. v1 and v2 are the frequencies of the pumping and probing laser lights, respectively. The waving arrow indicates phosphorescence.
enol type biradical, namely, triplet state enol resulting from the intramolecular hydrogen abstraction of the 0-cis isomer. Therefore, the appearance of the 0-cis isomer in (1 1’) REMPI is interpreted as due to the ionization of the biradical produced from the 0-cis isomer in the triplet state by assuming a much smaller energy is needed for the ionization of the biradical than
+
Acknowledgment. We are grateful to T. Niwa for her experimental assistance. Registry No. o-Methylbenzaldehyde, 529-20-4; m-methylbenzaldehyde, 620-23-5; p-methylbenzaldehyde, 104-87-0; o-fluorobenzaldehyde, 446-52-6; m-fluorobenzaldehyde, 456-48-4; p-fluorobenzaldehyde, 459-57-4.
Nuclear Spin Relaxation and Dipolar Interactions in Maiononitrile/Dichioromethane Solutions L. Foucat> M. T. Chenon,*f and L. Werbelowt*§ LASIR, CNRS, 94320 Thiais, France and Laboratoire des Methodes Spectroscopiques, Centre de St. Jerome, Boite 541, 13397 Marseille, Cedex 13, France (Received: December 21, 1989)
A low-temperature (-20 “C)NMR relaxation study of malononitrile dissolved in dichloromethane or dichlorodideuteriomethane was performed. Using a multispin formalism, both intermolecular and intramolecular dipolar relaxation rate constants were determined. Cross-correlation between various intramolecular dipolar interactions yielded a very exacting microdynamical description of malononitrile in the solution state. The plausibility of the relative ratio of intermolecular dipolar relaxation rates in the protonated and deuterated solvents was demonstrated. It is suggested that multispin NMR relaxation may provide a very sensitive probe of liquid-state structures. Further extensions of this work are proposed.
Introduction The study of nuclear spin relaxation in coupled multispin systems provides a very powerful method for determination of solution-state local structure and site-specific dynamics.lV2 In many applications, the relaxation characteristics of a multispin system are modeled as being affected by time-dependent random-filed and intramolecular dipolar couplings. The temporal ‘Current address: Department of Chemistry, Institute of Mining and
Technology, Socorro, New Mexico 87801.
*
LASIR, CNRS. (Centre de St. Jerome.
0022-3654/90/2094-579 1$02.50/0
correlation (interference) between various dipolar interactions leads to creation of various degrees of multispin order, each possessing an informationally rich unique Inclusion of random-field interactions acknowledges the fact that “other” relax(1) Werbelow, L. G.; Grant, D. M. Ado. Mugn. Reson. 1977, 9, 189. (2) Canet, D. Prog. NMR Spectrosc. 1989, 21, 237. (3) Hartzell, C. J.; Lynch, T. J.; Stein, P.C.; Werbelow, L.G.;Earl, W. L. J. Am. Chem. SOC.1989, 111, 51 14, and references cited therein. (4) Oschkinat, H.; Limat, D.; Emsley, L.; Bodenhausen, G. J. Mugn. Reson. 1989,81, 13. Wimperis, S.; Bodenhausen, G. Mol. Phys. 1989, 66, 897. Dalvit, C.; Bodenhausen, G. Adu. Mugn.Reson., in press.
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ation mechanisms may be operative yet enables one to focus attention on the dipolar couplings. Generally, it is assumed that the random-field and dipolar interactions are not correlated, which implicitly presumes chemical shift anisotropy and quadrupolar couplings are absent. Further comments related to the appropriateness and limitations of the random-field approximation can be found in the l i t e r a t ~ r e . ~ - ~ Although not immediately obvious, intermolecular dipolar interactions often can be modeled as random-field-like interactions.68 Thus, studies that monitor coupled multispin relaxation can provide access to both the intramolecular and intermolecular dipolar relaxation rates. Interpretation of these rates yields microdynamical details of reorientational and translational motions. Recently, the study of multispin relaxation was used to determine the intermolecular dipolar contribution to nuclear multispin relaxation rates of I3CH2Cl2dissolved in acetone and its fully deuterated i s o t ~ p o m e r . ~However, in this previous study, certain anomalous features were noted. In particular, the experimental ratio of intermolecular dipolar relaxation rates in protonated and deuterated solvent was incompatible with simplified assumptions often invoked to separate intra- and intermolecular dipolar contributions. Tentatively, the discrepancy between theory and experiment was ascribed to dynamical differences in the two isotopomers. [It is important to note that failure to acknowledge similar considerations renders suspect many previous studies in this field.] The present work examines the multispin relaxation features of selectively enriched malononitrile, 13CH2(CN)2,dissolved in protonated and deuterated methylene chloride. The dynamics of this solvent should be much less immune to isotopic substitution. By choosing a solvent with another magnetically active nucleus (chlorine), it is demonstrated that very detailed information regarding solute-solvent interaction is sampled. Experimental Section
The malononitrile used in this study was 90% enriched in carbon-I 3 at the methylene position. The dichloromethane was obtained from Prolabo (Paris) and the deuterated dichloromethane (99.3%) was provided by CEA (Saclay). The two solutions, one with the deuterated solvent, solution D, and the other with the protonated solvent, solution H, were prepared in an identical fashion. Dissolved oxygen was removed by successive freeze-pump-thaw cycles. The (malononitrile) molalities of solutions D and H were 1.02 and 1.04 m, respectively. Solution H retained a relatively small amount (20%) of deuterated dichloromethane to serve as a lock signal. To suppress the diffusion of resonant nuclei out of the probe coil, 9-mm-0.d. microcells were used, Spectra were obtained on a Bruker WH 90 spectrometer. All measurements were done on solutions held at 253 K. The important acquisition parameters for the I3C (IH) spectra were the following: spectral width 1000 Hz (1000 Hz); acquisition time 8.19 s (4.10 s); 90° pulse-calibrated for each experiment-ca. 9 p s (35 11s): number of accumulations 8 (12). The time waited between two acquisitions was on the order of 90 s (Le., at least 10 times greater than either the proton or carbon relaxation “time” which were on the order of 5 and 7 s, respectively). The response characteristics of the carbon magnetization of the I3CH2spin grouping in malononitrile were observed subsequent to various perturbation schemes: inversion of the carbon triplet (carbon inversion recovery: CIR) inversion of the proton doublet (proton hard pulse; HP) selective inversion of one line in the proton doublet (proton soft pulse; SP). In addition, a standard proton inversion recovery experiment was performed on the methylene protons for the malononitrile in the deuterated solvent. ( 5 ) Kumar, A.; Nageswara Rao, B. D. J . Mugn. Reson. 1971.8, 1. Nageswara Rao, B. D. Adu. Magn. Reson. 1972, 4 , 271. (6) Vold, R. L.; Vold, R. R. Prog. N M R Spectrosc. 1978, 12, 79. (7) Khazanovich, T. N.; Zitserman, V. Mol. Phys. 1971, 21, 65. (8) Krishna, N. R.: Gordon, S.L. J . Chem. Phys. 1973, 58, 5687. (9) C‘henon. M T ; Bernassau. J. M.: Coupry. C.Mol. Phys. 198554,277.
Foucat et al. TABLE I: Values of Various Relaxation Parameters Determined from the Five Perturbation-Response Schemes: SVHh CIR[Hl, SPIDl. CIRIDL and PIRlDP parameter solution H solution D
D,,, 10’0 s-1 DYY’10’0 s-1 D,,,10’0 s-1 JCHCH, S-’ JHHfHH,, IO-2 S-’ JCHHH,, IO-2 S-’ JCHCHP, IOw2 S-’ Jcc, 10-2 s-I JHH, Io-’ s-’ JHH,, IO-2 S-’ ~ C I Rdeg , asp, deg ~ P I R ,deg
2.38 f 0.03 2.38 f 0.03 12.6 f 0.5 2.02 f 0.02 1.74 f 0.02 1.32 f 0.01 0.44 f 0.03 0.3 f 0.1 2.57 f 0.06 2.0 f 0.3 157 f 1 145 f 1
2.25 f 0.02 2.25 f 0.05 10.8 f 0.2 2.23 f 0.01 I .92 f 0.01 1.42 f 0.01 0.39 f 0.03
