J. Phys. Chem. 1996, 100, 13911-13913
13911
ARTICLES Dynamic Properties of Solid Basketene As Studied by Deuterium and Proton NMR A. Mu1 ller, H. Zimmermann, and U. Haeberlen* Max-Planck-Institut fu¨ r Medizinische Forschung, AG Moleku¨ lkristalle, Jahnstrasse 29, 69120 Heidelberg, Germany
J. J. Wolff Organisch Chemisches Institut, UniVersita¨ t Heidelberg, Neuenheimer Feld 270, 69120 Heidelberg, Germany
R. Poupko and Z. Luz The Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: February 8, 1996; In Final Form: May 30, 1996X
Deuterium and proton NMR line shape and T1 relaxation measurements of solid basketene-d2 are reported. The compound is polymorphic, exhibiting a low-temperature rigid phase that transforms at 221 K to a plastic phase. Kinetic parameters for the molecular reorientation and self-diffusion in the high-temperature phase are derived, yielding the Arrhenius law parameters A ) 4 × 1013 s-1, ∆E ) (13.8 ( 0.5) kJ mol-1 for reorientation and Ad)1.2 × 1013 s-1, ∆Ed ) (39 ( 2) kJ mol-1 for self-diffusion.
Introduction Basketene (bishomocubene or pentacyclo[4.4.0.0.2,503,8.04,7]dec-9-ene, C10H10; 2, RdH) is structurally derived from cubane (1, C8H8) by the substitution of a vinylene bridge (CHdCH) moiety for one of the cube’s edges.
_ R _H _ R _D
cubane, 1
basketene, 2
It was first synthesized in 1966,1,2 and its chemical and photochemical properties in solution, as well as its structure and thermodynamic properties in the gas phase, have been thoroughly investigated.2-5 So far no study of the properties of basketene in the crystalline state has been reported, although it is known to have a high vapor pressure and to be plastic at room temperature (for example, when a powder sample is pressed, it becomes translucent). In the present paper we report deuterium and proton NMR measurements on solid basketene specifically deuterated at the olefinic sites (2, RdD). We show that solid basketene is polymorphic with a low-temperature rigid phase and a high-temperature plastic phase. We report kinetic parameters for molecular reorientation and self-diffusion in the plastic phase and also discuss effects due to residual ordering, which persist even when reorientation and self-diffusion are fast. X
Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00389-9 CCC: $12.00
The analysis of the results follows closely the methodology that we have used in our recent study of solid cubane.6 Experimental Section Sample Preparation. Pentacyclo[6.2.0.0.2,70.3,906,10]decane4,5-dicarboxylic anhydride7 was converted to the cis-dimethyl ester by methanol and concentrated sulfuric acid, mp 81-82 °C (heptane). The ester was equilibrated to the dideuterio transdiester by refluxing it in MeOD/NaOD.8 The deuterated transdiacid was obtained by addition of D2O and further refluxing. The acid was converted to basketene-d2 as described for the unlabeled material.7 Some loss of label was encountered; the product contained about 85% D as judged by integration of 1H solution NMR spectra. These spectra also revealed several weak peaks that must be ascribed to impurities. Their concentration in the original sample is estimated at ∼3%. NMR. Deuterium NMR measurements were made on a home-built FT spectrometer operating for deuterium at 72.13 MHz. The sample temperature was adjusted with a stream of nitrogen gas that was cooled by heat exchange with liquid nitrogen and whose temperature was controlled by an Oxford Instruments ITC4 controller. The temperature of the sample was measured with a 100 Ω Pt resistor positioned close to the sample. The proton spectra were recorded on a Bruker CPX 300 spectrometer at 300.13 MHz and using a BVT 1000 variable temperature controller. Differential Scanning Calorimetry. Measurements were performed on a Mettler DSC 30 system with a scanning rate of 5 K per minute. For our sample of basketene-d2 (2, RdD) they gave the following phase sequence:
solid II
221 K 333 K f solid I f liquid (8.7) (2.4)
where the numbers in parentheses are the transition enthalpies in kJ mol-1. © 1996 American Chemical Society
13912 J. Phys. Chem., Vol. 100, No. 33, 1996
Mu¨ller et al.
Figure 1. Deuterium longitudinal relaxation time, T1, in the solid phases of basketene-d2 as a function of the reciprocal absolute temperature. Inserted Pake doublet spectrum was measured at 190 K and is typical of solid II, while that of the single line was measured at 205 K and is typical of solid I. Latter spectrum corresponds to the supercooled region of the plastic phase.
Results and Discussion Of a powder sample of basketene-d2 we recorded deuterium NMR line shapes and took T1 relaxation data from 180 K to a temperature close to the melting point. The results for T1 and examples of spectra in both phases are shown in Figure 1. In solid II, the deuterium spectrum consists of a Pake doublet corresponding to an uniaxially symmetric quadrupole coupling tensor with a quadrupole coupling constant Qc ) e2qQ/h ) 183 kHz. At the solid II to solid I transition the spectrum changes abruptly to a narrow, essentially Lorentzian line with full width at half-maximum intensity, ∆, of about 1.2 kHz . These results clearly indicate that on the NMR time scale solid II is rigid while in solid I the molecules reorient rapidly with a correlation time, τ, much shorter than Qc-1. The essentially complete averaging out of the quadrupolar coupling also indicates that this solid is orientationally disordered. The kinetic parameters for this reorientation process can be derived as follows from the T1 results shown in Figure 1. In solid II T1 is relatively long (∼200 s), while on transition to solid I it drops discontinuously by 4 decades to almost 10 ms. Thereon, it increases monotonically with increasing temperature, indicating that the T1 results correspond to the fast regime limit, ωoτ , 1, of the BPP equation (ω0 is the Larmor frequency of deuterium). In this limit 9
1 3 2 2 ) π Qc τ T1 2
(1)
from which τ may be derived. A fit to an Arrhenius equation, 1/τ ) A exp(-∆E/RT), yields A ) 4 × 1013 s-1 and ∆E ) (13.8 ( 0.5) kJ mol-1. This value for the activation energy is much lower than the corresponding one for cubane6 (62 kJ mol-1) and reflects a loose packing of the basketene molecules in solid I. From these results alone one cannot tell whether the reorientations in this phase are isotropic or involve some kind of (cubic) jumps. We considered extending the measurements to 13C NMR but felt that this will not provide new information beyond that obtained from deuterium. On the other hand, an
Figure 2. Full line width at half-maximum intensity of the proton (b) and deuterium (O) spectra in solid I of the basketene-d2 sample. Solid line is calculated with the parameters for self-diffusion given in the text, using eq 2.
X-ray structure determination might be useful, since it could confirm (or disprove) that solid I is orientationally disordered. We did not pursue, however, such a study so far. It may be seen that the experimental results for T1 in the supercooled region of solid I gradually fall off the Arrhenius line toward lower T1 values. This corresponds to a slowingdown of the reorientations beyond the values expected from the Arrhenius equation and may reflect pretransition effectswhen the transition to the more rigid solid II phase is approached. We have also measured the proton spectrum of the basketened2 sample over the entire solid-state region. In solid II it consists of a symmetric bell-shaped signal with ∆ ) 44 kHz. Transition to solid I results in a discontinuous change in the line width to ∆ ) 9.9 kHz. On further heating, ∆ decreases continuously, following an S-shaped function (see Figure 2) down to about 560 Hz close to the melting point. Above about 245 K, when the line width reduces to below 5 kHz, the signal becomes asymmetric and sharp features appear. This asymmetry is partly due to the residual olefinic protons that are shifted downfield from those of the cubane moiety, while the sharp features are apparently due to the impurities mentioned above. The proton line width results in Figure 2 correspond to the main signal after subtracting that due to the impurities and the olefinic protons. The molecular reorientations in solid I are always much faster than the intramolecular proton dipolar interactions within the basketene molecules (cf. the Arrhenius equation for the reorientation process). The line width observed in this phase immediately above the transition from solid II must therefore correspond predominantly to intermolecular dipolar interactions. The measured value of 9.9 kHz is very reasonable for this interaction and is almost identical with that found in cubane.6 The narrowing of the proton signal upon heating within solid I can therefore safely be ascribed to an averaging out of this
Solid Basketene
J. Phys. Chem., Vol. 100, No. 33, 1996 13913
intermolecular dipolar interaction by molecular self-diffusion. We can estimate the correlation time, τd, for this process using the relation10
(
)
∆ - ∆∞ 2 2 -1 ) tg [2π(∆ - ∆∞)τd] ∆0 - ∆∞ π
(2)
where ∆∞ is the high-temperature line width (560 Hz) and ∆0 is the line width before the averaging of the intermolecular dipolar interaction (9.9 kHz). The temperature dependence of 1/τd, deduced from eq 2 and the data in Figure 2 for 230 K e T e 270 K follows roughly an Arrhenius behavior with Ad ) 1.2 × 1013 s-1 and ∆Ed ) (39 ( 2) kJ mol-1. The solid line through the experimental points for the protons in Figure 2 is calculated from eq 2 using these parameters. Again, these values are to be compared with those for self-diffusion of cubane, Ad ) 6.1 × 1015 s-1 and ∆Ed ) 83 kJ mol-1. As was the case for molecular reorientation, the activation energies for self-diffusion in basketene and cubane reflect the remarkable fact, which certainly is counterintuitive, that diffusion is less hindered in basketene than it is in its more symmetric parent molecule cubane. We conclude this section with a comment on the residual high-temperature line width of the proton signal in phase I (560 Hz). This corresponds partly to chemical shift differences between the inequivalent protons bound to the tertiary carbons in the molecule and also to inhomogeneities of the sample and of the magnetic field. It was mentioned above that the deuterium line width in solid I, just above the transition from solid II at 221 K, has discontinuously dropped down to about 1.2 kHz. On further heating solid I the line width decreases at a very shallow slope until about 240 K. At about 255 K it drops sharply and then levels off at a value of about 100 Hz. A plot of the deuterium line width as a function of the temperature in solid I is shown in the lower part of Figure 2. On first glance it appears that the deuterium line width follows that of the protons and that, in analogy with the latter, its dispersion as a function of the temperature reflects the averaging out of the deuterium-proton intermolecular dipolar interaction by self-diffusion. This picture seems to be supported by the fact that the initial width of 1.2 kHz is approximately that expected from the proton line width immediately above the solid II to solid I transition, namely, [2γD/ (3γH)] × 9.9 kHz ) 1.01 kHz. However, with the rate parameters for self-diffusion determined in the previous section, we would expect that this intermolecular D-H dipolar interaction would average out at ∼220 K rather than at 255 K, as observed experimentally. The deuterium line width in solid I and its temperature dependence must therefore be due to another broadening mechanism. Such a possible mechanism is the following. The structure of solid I is not yet known, but on the basis of the NMR results presented above, it is most likely a cubic plastic phase. We believe that the extra line width in
the deuterium spectrum of solid I reflects small local deviations from cubic symmetry caused by random distortions of the lattice that extend over several unit cells. Fast reorientation and selfdiffusion within a distorted domain do not completely average out the deuterium quadrupolar coupling, resulting in the observed residual width of 1.2 kHz. Similar effects of residual ordering were observed in the plastic phase of succinonitriled411 and in the high-temperature solid phase of cubane-d2.6 Only when the rate of self-diffusion is sufficiently large to carry basketene molecules within, say, 1 ms over several such distorted domains will this residual broadening become averaged out. For the solid I phase of basketene this apparently occurs at around 255 K. The high-temperature line width of the deuterium spectrum (∼100 Hz) reflects, as did the proton signal, the sample and field inhomogeneities. In conclusion we may say that the physical properties of the basketene solid I phase (low melting enthalpy, high vapor pressure, plasticity) and the fast molecular reorientation and selfdiffusion classify this solid as a plastic phase.12 It is noteworthy that despite the lower symmetry of the basketene molecule compared to that of cubane it undergoes much faster reorientation and self-diffusion when the two compounds are in their respective plastic phases. Acknowledgment. This work was supported by a grant from G.I.F., the German-Israeli Foundation for Scientific Research and Development. One of us (Z.L.) thanks the Humboldt Foundation for a Humboldt Research Award and the MaxPlanck-Institute for Medical Research (Heidelberg) for hospitality. References and Notes (1) Masamune, S.; Cuts, H.; Hogben, M. G., Tetrahedron Lett. 1966, 1017. (2) Furstoss, R.; Lehn, J.-M. Bull. Soc. Chim. Fr. 1966, 2497. (3) Westberg, H. H.; Cain, E. N.; Masamune, S. J. Am. Chem. Soc. 1969, 91, 7512. (4) Allsed, E. L.; Back, B. R.; Mumford, N. A. J. Am. Chem. Soc. 1977, 99, 2694. (5) Khaikin, L. S.; Belyakov, A. V.; Koptev, G. S.; Golubinskii, A. V.; Kiirin, V. N.; Kozmin, A. S.; Vilkov, L. V.; Yarovio, S. S. J. Mol. Struct. 1978, 44, 45. (6) Detken, A.; Zimmermann, H.; Haeberlen, U.; Poupko, R.; Luz, Z. J. Phys. Chem. 1996, 100, 9598. (7) Nelsen, S. F.; Wolff, J. J.; Chang, H.; Powell, D. R. J. Am. Chem. Soc. 1991, 113, 7882. (8) Westberg, H. H.; Cain, E. N.; Masamune, S. J. Am. Chem. Soc. 1969, 91, 7512. (9) Abragam, A. In The Principles of Nuclear Magnetism; Oxford University Press: Oxford, 1961; Chapter VIII. (10) Reference 9, Chapter X. (11) Golemme, A.; Zamir, S.; Poupko, R.; Zimmermann, H.; Luz, Z. Mol. Phys. 1994, 81, 569. (12) Chezeau, J. M.; Strange, J. H. Phys. Rep. 1979, 53, 1.
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