Molecular motion studied by NMR powder spectra. II. Experimental results for solid P4 and solid Fe(CO)5

CHEMICALPHYSICS 6 (1974) 226-234. Q NORTH-HOLLAND

PUBLlSHlNG COMPANY

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MOLECULARMOTIONSTUDlEDBYNMRPOWDERSPECTRA. H.W.SPIESS, R. CROSESCU’ and U. HAEBERLEN Max_Plmck-Institute. Department of hfolecuhr Physics, 6900 Heidelberg, FRG Received 9 August 1974

Experimentalexamples of the effect of molecular motion on NMR powder spectra governedby axially symmetric shieldingtensors u are presented. In solid white phosphorus, P4, the “P resonancewas studied at 92 MHzand temperaturesdown to 4.2 K. At low temperaturesthe usual powder pattern for a rigid solid is observed,from which we obtain the shieldinganisotropy Au = aa - a1 = -405 f 10 ppm. At room temperatureon the other hand the 31P spectrum of solid P4 consistsof a sharp symmetric line only, the frequency shifts due to the anisotlopy ofabeing averagedout by rapid molecule motion.J: the p-phaseof this solid at tempe_r;turubetween 80 K and 120 K intermediite values for the jump frequency r were found, such that Aw and r differ by not more than about one order of magnitude. The spectra observedshow the characteristicfeatures calculated in the precedingpaper. Values for the jump frequencies wele obtained independently from the spin-lattice relaxation time T, studied &o at 92 MHz. At this frequency the relaxation at low temperaturesis strongly dominated by relaxation through anisotropic shielding, 50 that the relationshipbetween I -’ and TI is very simple and unambiguous.The agreementof 7-I ob tained by analysis of the spectra or TI, respe’ctively,providesa quantiladve check of the linesha+ calculation. As a first example, where the analysis of such NhlR spectra is applied to obtain information about molecuku dynamics in a solid, ‘“C spectra in solid Fe(CO)s were studied at 61 MHz..In contrast to Ni(CO)a, where we observe the usual powder specuum due to an axially symmetric shidding tensor, strong deviations from such a pattern are observed in Fe(CO)s. The values found for the shielding anisotropy in these metal carbonylr are quite close to the one in free ‘%O: Ao = 401,395 and 425 f 15 ppm, for 13C0, Ni(‘3C0)4 and Fe(13C0)5, respectively. From the analysis of the spectra in solid Fe(CO)s it follows that the intmmolecuku exchange between axial and equatorial carbonyls in the trigcnal bipyramid Fe(CO)s postulated for the free molecule takes place even in the solid with exchange frequenciesup to 25 kHz. A comparison is given with values for t-’ in the liquid, where they are found to be higher by about six ordersof magnitude,

I. lntrduction In this paper we want to present two experimental examples for the influence of motion of the spins on NMR powder spectra governed by axiaUysymmetric shielding tensors and discussedtheoretically in the preceding paper [ 11, to which we will refer to as I. In fact, the calculations described in I were stimulated by the observation of peculiar lineshapesof the 13C resonance in solid ironpentacarbonyl, Fe(CO)5, in contrast to solid nickeltetracarbonyl, Ni(C0)4, for which we obtained the usual powder pattern charac-

l

Resent address:Innilute of Atomic Physics, Bucharest, RUlUonia.

teristic for an axially symmetric shielding tensor. Fe(CO)5 is a trigonal bipyramid 121,for which in the liquid (or gaseous)phase an intramolecular exchange of axial and equatorial carbonyl groups has been postulated [3]. This exchange is fast on the NMR time scale so that the t3C spectrum in the liquid consists of a single line only [4-S]. Measurementsof the transverse relaxation time T2 at 61 MHz in the liquid also did not show any measurable effect due to the exchange [6] indicating that the exchange rate might be exceedingly high. As we will show, the lineshapes of the 13Cresonance suggestthat the exchange occurs even in solid Fe(CO)5, but the range of jump frequencies r-l is rather limited. In order to support our interpretation

H. W. Spies et aL. Molecular

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of the NhlR lineshapesin this solid, we looked for a more favourable system where the whole range from fast to slowjump frequencies is accessibleto experiments. Such a model system we found to be solid white phosphorus consistingof P4 tetrahedra. In addition, in this case we could obtain values for the jump frequencies independently from the analysis of the spectra (see I) by studying the spin-lattice relaxation time ?“I. This allowed us to test our lineshape calculation quantitatively. Therefore we will discuss the 31P resonance in solid P4 first and then come back to the t3C spectra of solid Fe(C0)5.

2. Experimental White phosphorus was purified by two subsequent vacuum distillations in the dark starting from commercial material. The preparation of the 13Cenriched metal carbonyls, Ni(C0)4 and Fe(CO)5, has been described before [6]. Fourier transform NMRspectra were taken at 61 MHz for 13Cand 92 MHz for 31P using a superconducting magnet. Two different probes were used depending on the temperature region at which spectra were taken. The electronic circuit in both caseswas the same as described in ref. [7], so that tuning and matching could be achieved with capacitors outside the magnet for temperatures down to 4.2 K. For temperatures above 100 K the probe was connected by a copper rod to a dewar with liquid nitrogen and for temperatures down to 4.2 K the probe was put on a heat exchanger of a homebuilt flow cryostat cooled by liquid helium. If one wants to reach temperatures below about 20 K by such a flow cryostat, it is advantageousto put the heat exchanger as close as possible to the liquid helium reservoir and use small diameters for the liquid helium pipe [8]. Therefore the liquid helium vesselwas put directly underneath the superconducting magnet in a ditch and the Liquid helium pipe had an inner diameter of 0.8 mm. The flow of helium was regulated with an electromagnetic valve. The cryostat which has an outer diameter of 30 mm works in the range from room temperature down to 4.2 K. For the powder spectra of 13C typically about 10 free induction decays (FID) were accumulated, whereas most of the 31P spectra were obtained from single

227

FIDs. The rate of digitization was I MHzin most CX.t?S.

3. Results and discussion 3.1. Solid white phosphors

P,

Solid state spectra for solid P4 at 92 MHzare shown in fig. 1 for two temperatures. At 25 K (tip. la) we observe the usual powder pattern far an tialiy symmetric shieldingtensor u. whereas at room temperature (fig. 1b) the spectrum even in the solid consists only of a single symmetric line. A considerable part of its width is determined by the inhomogeneity of our superconducting magnet. It is clear from fig. la that the dipolar linewidth is far below the frequency shifts due to the anisotropy of a. which leads to a total spread of the powder spectrum of about 37 kt-fz at 92 MHz. From a computer fit we obtain: o, = -272 f 10 ppm, a, =f 133 + lOppm,rtndUi,= -2 f 5 ppm relative to the isotropic shift of solid F,t measured directly at higher temperatures (see fig. 1b). The dipolar Linewidth obtained in this tit is 6/21r = 2.3 kHz (for a definition of 6 see ref. [9] ch. Iv). Ibis value can be completely accounted for by intramofec-

alar dipole-dipole interaction from which one calculates S/27r = 2.5 kHz. In the P4 tetrahedra the 31P nuclei occupy posi-

Fig. 1. 3LP spectra of solid whitephosphorusat 92 MHz.

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H. W. Spiezr et al, Male&u

marion from NMR powder rpeccnz U

tions on threefold axes, therefore the shieldingtensors should be axially symmetric as observed.The value of the shieldinganisotropy Au = uI - u1 = -405 f 10 ppm falls in the same range as found for other phosphorus compounds [lo], where typically IAol is of the order of severalhundred of ppm. In view of the values observed for the shieldinganisotropiesfor 15N [ 1I], one would not be surprisedif !Aol was even larger for %P since phosphorus is the second row. analog of nitrogen. At room temperature, on the other hand, although P4 is still a solid the frequency shifts due to the anisotropy of a arc almost completely averagedout due to rapid reorientation of the P4 moleculesas shown in fig. 1b. Similareffects have been observed for the 31P resonance in solid P4S3. At room temperature the singlecrystal spectra of P,& show that the molecules rotate fast about their threefold axes [lo], leaving only a residualshieldinganisotropy which is still Au = 480 ppm, however. P4S3 undergoesa phase transition at 313.9 K and above this temperature the frequency splittingsdue to the anisotropy of a are averagedout completely [ 121. Low temperature spectra, however, have not been studied in this system. In solid P4, on the other h.and,the slow reorientational limit can be studied as shown in fig. 1a and therefore we have the possibility to study spectra for intermediate jump frequencies as well. Another advantageof studying solid P4 is that by measuring the spin-lattice relaxation time Tr we can get valuesfor the jump frequencies independently from the spectra (see below). Furthermore, as we will see below, several phasesexist for solid white phosphorus and the jump frequencies are different for different phasesat the same temperature. This leads to different valuesof Tt or its temperature dependence. ‘Thusby studying Tt we can detect phase transitions and can make sure that we are dealingwith singlephasesonly. Note also that the measurementof Tt and the recording of the spectra is done under equal conditions in pulsed NhiR. Therefore we shall now first discussthe relaxation time measurements and then come back to the discussionof the spectra. 3.1.1. Relamtion times in solid Pd The spin-lattice relaxation time If, in solid white phosphorus has been studied before [ 13,141 at frequencies up to 30 MHz As mentioned above, one of the reasons for studying T1 againat 92 MHzwas that

11

IO

9

0

7

6

5

.!

3

Fig. 2. Experimental s1P relaxation times 7’1 in solid white phosphorus at 92 MHz. The points labclled 6, to (P correspond to spectra in tig. 4.

from the values of Tt and its temperature dependence we can determine which phase of solid white phosphorus we have at a given time. In fig. 2 the experimental relaxation times are plotted versus the reciprocal temperature. Wesee three different branches, each belongingto a different phase. ‘IIreplastic crystalline cc.and the crystalline &phasehave been described before (see, e.g., refs. [ 131 and [ 141). It does not seem to have been noticed before, however, that the o-phase can be supercooled down to about 108 K, where it slowly undergoesa phase transition taking up to severalhours time, to what we call the y-phase. A spectrum obtained during the period where the phase transition had not yet been completed is shown in fig. 3. The sharp peak in the center corresponds to the a-phase, whereasthe broad powder pattern, which has the same width as the low temperature spectrum in fig. la, belongsto the r-phase. By heating

H. W.

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et al, Molecuhv motion from NMR

the sample to 158 K a fast phase transition from the by-to the &phase occurs, the latter being stable down to 4.2 K, and by heating up to 196 K going back to the a-phase. Note that at the phase transitions Tr changes drastically so that they can easily be detected by measuringT,. The possible relaxation mechanismsfor solid white phosphorus have been discussedbefore [ 13,141, i.e., dipole-dipole interaction, relaxation due to anisotropic shielding,and spin-rotation interaction. We shall mainly be concerned with low temperature spectra, where the last mechanism,which is important for the @phaseabove about 200 K (see fig. 2) and for the liquid and gaseousphase, can be neglected. As noted above, the frequency splitting due to the anisotropy of the shielding by far exceeds the dipolar coupling at 92 MHz. Boden and Folland [14] have shown that the dipolar relaxation is strongly dominated by the intmmolecular contribution. Therefore we may tentatively use the same value for the correlation time 7c for relaxation through ciipolarcoupling and through the anisotropy ofa. From the values given above for Au and the dipolar constant 6 it follows that at 92 MHzthe dipolar contribution to the relaxation rate is only about 3% of that due to the anisotropy of u. Therefore the dipolar relaxation rate can be neglected and at low temperatures at 92 MHz the relaxation rate is simply given by [9] : 7’i’ = &(AcT)~~~r/(1 + c@),

(1)

where wD is the Larmor frequency and T is the correlation time. The anisotropy of the shielding, Au, is determined from the low temperature spectrum (see above). For the &phase we are still in the extreme narrowing limit, where ~$7~ Q 1, but for the pphase, on the contrary, f.@ S 1 as seen from the temperature dependence of Tt , so that at least below 150 K the expression for the relaxation rate further simplifiesto q’

= &(Ao)~T-'

,

(2)

Therefore from our T1 data and our experimental value for Au we can get directly the correlation time and thus the jump frequencies 7-l from eqs. (1) and (2) without further assumptions. Note the simplicity of the connection of the correlation time with ob servable quantities expressed in eqs. (I) and (2) in comparison with the corresponding relationships in proton NMR [9].

powder tpectra.Ii

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Fig. 3. NP spectrumobtained during tie slow conversionof the supercaoled a-phase to the -,--phaseof ralid white phosphoms.

We note in passingthat the agreement -withprevious Tt measurements 113,141 is good. !n the &phase the correlation times extracted by Boden and Folland [ 141by analysisof the dipolar relaxation rates, however, are consistently shorter by a factor of about 2.2 compared with the correlation times obtained directly from relaxation due to the anisotropy of b. This explains why Boden acd Folland obtained a value for lAoI= 285 ppm only from their relaxation data at 10 and 30 MHz. 3.I .2. 31P spectra of solid P4 We shah be mainly concerned with the 31P spectra of the bphase of solid white phosphorus. The reason is that in the &phase rbere exists a region of intermediate jump frequencies, such that Awr z= 1. In the o-phase, on the other hand, the correlation times are very short so that even w&* 4 1, as noted above. As a consequence, the spectrum always consists of a singlesymmetric line whose width increasesas the temperature is lowered (see fig. lb). In the Tphase the correlation time is long, such that Awr B 1, and the spectra are very Similarto the spectra of the p phase at low temperatures discussedbelow (see also fig. 3). Representative spectra of the &phase are shown in fig. 4. Above 126 K the spectrum always consists of a singleline. The spectra 0 to Q correspond to the marked points in the relaxation time diagram, fig. 2. The values for the jump frequencies r-t obtained from the relaxation data are indicated at the right. We see that the spectrum is very sensitive against r-I intheregion~a7-1(moUrcase~=UOkHz).

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H. W.Spies et al.. MolecularmotionfromNM? &xx&r spectru II

Another observation from the spectra in fig. 4 is that the spectrum does not directly become the usual powder pattern when the temperature is lowered further and the jump frequency decreases. Instead we observe an extra jump near w, as found in the lineshape calculations (see I). In fig. 5 calculated spectra are shown for a tetrahedron which jumps between its equilibrium positions. ‘The values of the jump fre-

quencies were chosen so that the calculated spectra closely resemble the observed ones. The characteristic features of the experimental spectra are clearly reproduced by the calculation and the agreement between

Fig, 4. 3*P spectra of the r?-ph;rse of solid white phosphorus at various temperatures. The spectra &belled 0 to 0 correspond to the relaxation times marked in fig 2. Values for the jump frequency 7-I from Tr are givenat the right.

At fmt sight, spectr?m @(or @) looks like a superposition of a usual powder pattern and a single averaged line in the center (see for comparison fig. 3). Similar spectra have been observed for the 13C resonance in benzene [ 151 and for the lgF resonance in C,F6 [ 161 adsorbed at charcoal and have been inter-

the jump frequencies necessary to explain the spectra within our calculation (I) with the values for 7-l obtained from the T1 data is highly satisfactory. From the comparison of lineshape calculations for solids and liquids given in I it is clear that the reorientation of the P4 tetrahedra in the &phase of solid white phosphorus must be described by a solid model, where the molecule jumps between fured positions. In particular, the spectra Q and Q in fig. 4 cannotbe

explained by reorientational modelsapplicableto liquids which do not give the double Peak spectrum 0. Since an experimental verification for the rotational jump model has also been established recently [ 171, it is clear that from lineshapes observed in NhIR or ESR one can obtain information not only about the time scale, but also about the type of reorientation.

preted as evidence for the simultaneouspresence of moleculesin different states of motional narrowing. It is, therefore, important to realize that such spectra can also be observed in single phases. From our Tl

measurement,wherewe did not find deviationsfrom a singleexponential, we know that the spectra in fig. 4 are single phase spectra and that the relaxation accm with a tingle time constant for the whole spectrum. This is also confumed by the observation that rhe lineshape is independent of the repetition rate of the fipulses. This is not so for the two-phase spectrum shown in fig. 3, where the relative intensity of the middle peak increases considerably if the repetition rate is high due to the fact that TI is much

shorter in the &phase than in the -y-phase(see fig. 2). By changingthe repetition rate, Kaplan et al. also were ableto showthat their 13C spectra of benzene adsorbed at charcoal were due to molecules in different states of motional narrowing [ 1S].

Fig. 5. Calculated spectra for P4 jumping between its

librium poGtions.

equt-

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h? W.Spiess et ai, Molcakr motion from NMR powder rpecrrn II

3.2. Solid ironpntacxarbonyl Fe(CO)5 Whereas in the o-phase of white phosphorus we were able to observe spectra for the whole range from fast to slow exchange, for the 13Cresonance in solid Fe(C0)5 at 61 MHz the jump frequencies are always in the region of slow exchange. This can be seen in fig. 6 where t3C spectra of solid Fe(CO)5 between 213 and 4.2 K are shown. This covers most of the solid range (the melting point is 252 K). Note that a peak in the center of the spectrum, characteristic for intermediate and fast exchange rates, I, is absent in all spectra. At high temperatures, the extra hump near wr is SO strong that the spectrum at first sight looks like a “two line” spectrum. From such a spectrum alone one might be tempted to attribute the two peaks to two chemically different sites. Note, however, that the frequency splitting between the two peaks (more than 10 kHz) is far too big to allow such an interpretation for CO in metal carbonyls (see below). Further-

more, the spectrum approaches the usual powder pattern governed by an axially symmetric tensor at low temperatures. At temperatures below 25 K the spinlattice relaxation time T, becomes exceedingly long and also shows a significant angular dependence, such that the part of the spectrum close to oL relaxes more slowly than the rest of the spectrum. Fe(CO)s is a trigonal bipyramid and the t3C shieldingtensors for the axial and the equatorial carbony1 groups are not necessarily the same. In fact, only the shielding tensors for the axial CO’sare required to be axially symmetric by symmetry. Nevertheless, our data clearly do not allow to extract more than the principal elements of one axially symmetric shieldingtensor, as given in table 1. For comparison, in table 1 also the principal shieldingcomponents for free CO [ 181 and Ni(CO)a are given(see also fig. 7 which shows the 13&spectra of solid Ni(CO)4 and Fe(C0)5 together with the isotropic shifts in the liquids). It is clear from table 1 that the differences in the shieldingcomponents between free CO and CO bound in metal carbonyls are rather small. This also means that the d$firences for the shieldingtensors of the axial and equatorial CO groups in Fe(CO)s ace probably small compared with the shielding anisotropy Au itself. The shieldingcomponents and their relevance with regard to the bonding in metal carbonyls have been discussedelsewhere [ 191. It is also worth mentioning that the values for Au obtained directly in the solid are in good agreement with the vaIues extracted previously from Tt measurements in the liquid state [6]. The most remarkable difference between the powder spectra for Ni(CO), and Fe(CO)s. however. is the lineshape(see figs.6 and 7). Whereas the powder spectrum for Ni(CO)d presents itself almost as a textTable 1 Principalelements of “C shielding tensors in Ni(C0)4 and Fe(CO)s relative to CS2. The enor limits are t 15 ppm

I

, 60

,

, LO

I

&pm,

V[kHz’ I

,

2

0

Ni(CO)a

Fe(CO)s co a)

Fig, 6. 13C spectra of solid Fe(CO)5 at 61 MHz. Experimek tal values of the spin-lattice relaxation time TI are givvenat

the right.

a) SW Rf. 1x81.

262 263 278

Ql

AU

@pm)

(Ppm)

-133 -162 -123

395 425 401

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,

-

v

CkHzJ

-

Fig. 1. 13C spectra of solid Ni(CO)d and Fe(CO)S at a ten pcratue cf about 150 K. book example for an axially symmetric shielding tensor, for Fe(CO& the deviations from the usual powder pattern are very strong. In both cases it is clear that the dipolar linewidth is far below the splitting due to the anisotropy of the shielding (6/2a = 0.5 kHz in agreement with the calculated second moment). In case of Ni(CO)a jumps of the molecule in the solid can occur only at a rate far below the NMRline splitting of Aw = 150 kHz. Spin exchange between the t3C spins in solid Fe(C0)5, on the other hand, must be considerably faster. In case of Fe(C0)5 one immediately thinks of the possibility of exchange of axial and equatorial CO groups (see above). Therefore, the spectra were analyzed under the assumption that each 13C spin can take on all five positions in the molecule with equal probability. Again the spin-lattice relaxation times T, were measured as a check of the lineshape calculation. The values are given in fig. 6 together with the corresponding spectra. Note that the relaxation times range from about i - 2a h, and it would certainly be dangerous to try to obtain reliable values for correlation times from these data alone. Unless these values are affected by paramagnetic impurities, the major relaxation mechanism will again be relaxation due to the anisotropy of the shielding and eq. (2) applies. The

from MHR powder

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reason for this has been discussed above for P4. We then note first that the temperature dependence of T, is very small leading to an apparent activation energy of about 1 kcal/mol only. ‘Ibis means that the correlation time also changes very little with temperature. ‘Ihii explains immediately why the changes in the spectra shown in fig. 6 are small as the temperature is lowered. In fig. 8 fmally, observed and calculated spectra for Fe(CO& are p!otted for comparison. As the dipolar linewidth is so small, even minor changes in the spectrum can be observed and are also reproduced quite well in the calculation, e.g., the disappearance of the extra hump in the center of the spectrum and the decrease of the intensity of the second peak near wr as the temperature is lowered. Note also the excellent agreement of the values for the jump frequency obtained from Tl data and from the spectra. In the calculation we had to assume that the 13C spin jumps between all five positions of the molecules as described in 1. This is nothing else than an intramolecular exchange of axial and equatorial CO groups in the solid. An alternative motion would be rotation about the C3 axis of the molecule. Such a rotation would leave the spectra due to the axial CO groups unchanged. As long as the rotation is slow, the powder spectrum due to the equatorial carbonyls alone

60

10

20

0

Fig. 8. ‘“C spectra of solid Fe(CO)s. (a) Observed, (b) Calculated assuming exchange of carbonyl groups between all fnrepasition9 inthemolecule. The values for the exchange frequency I-’ in (a) were obtained from the 7’1data (W fig. 6).

H. lb! Spiess et aL, Molecular molbn from NMR powda

would look quite similar to the calculated spectra in fig. 8b. As discussed in I, the lineshapes in the slow exchange region arise mainly from the distortion of the usual powder pattern due to the broadening of the individual lines caused by the exchange, and therefore the lineshape is relatively insensitive against the details of the motion. The complete powder spectra calculated for such a rotation about the Cl axis, therefore, still looks quite similar to the calculated ones in fig. Bb. The intensity of the second peak near We, however, is then reduced by a factor of 3/5, which makes the agreement with the experiment much less satisfactory. Therefore, our 13C spectra in solid FE!(CO)~are strong evidence that the exchange of axial and equatorial carbonyl groups takes place even in the solid with a rate of the order up to about 25 kHz (see fig. 8). As discussed in I, the lineshapes for glow and even intermediate exchange frequencies are not sensitive enough against the details of the motion to make more specific statements as to how the exchange actually occurs. This might be possible if the fast exchange limit could be observed in the solid. Consider, e.g., spectra to be expected for Fe(CO),: (a) rotating rapidly about the C, axis, or (b) for the fast exchange of all carbonyls. In case (a) the powder spectrum would be a superposition of two powder patterns due to the axially symmetric shielding tensors with intensity 2 to 3 arising from the axial and the equatorial carbonyls, respectively. The first one being described by Au as obtained in the rigid solid and the second having Au’ =- k Au. The case (b) was discussed explicitly in I, where we showed that in fast exchange a powder pattern described by a single axially symmetric tensor results whose width is determined by Au’ = $ Au. In the case of Fe(CO)S, however, the exchange is always relatively slow in the solid as noted above, so that the fast exchange limit cannot be reached in high magnetic fields, where the frequency shifts due to the anisotropy of u can be determined easily. Therefore, one has probably to study single crystals to get more information. The fact that the exchange occurs even in the solid and with such a low activation energy (see above) suggests that the exchange should occur very much faster in the liquid state. From measurements of the trans. verse relaxation times T2 at 61 MHz in the liquid, pre-

specltn

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viously we obtained a limit [6] : &(SUim)2 Oif 4 IO-’ Hz,

(3)

where 6Oix, is the difference of the isotropic shielding for axial and equatorial carbonyl groups. This quantity, which could not be measured directly until now, has recently been extracted [2Oj from temperature dependent t3C NMR and IR studies on the series of the PF, substituted iron carbonyls Fe(CO)5_x(PF&. Using the value obtained [20] hot, = 17.7 +5 ppm, one obtains 7 < 9 X 10-l 1 s in the liquid at the melting point 1201. We can also give a lower Limitfor t from the measurements of the spin-lattice relaxation time TI in the Liquid [6]. From T, we were able to obtain the correlation time ~c, with a value rc = 1.0 X IO-‘L sat the same temperature. The mean lifetime of a given configuration in the liquid cannot be shorter than rc because otherwise the exchange of axial and equatoriai carbonyl groups would determine the correlation time and not the hindered reorientation of the molecule in the liquid as usual. From this consideration the exchange frequency in liquid Fe(CO)s at 250 K must be in the range I .I X lOlo Hz G r-l d IOtt Hz. This means that the exchange frequency in the liquid at the melting point is about six orders of magnitude higher than in the solid. We feel that this makes the obsenration, that exchange still occurs in the solid, quite plausible.

4. summary We have demonstrated experimentaliy that NMR lineshapes in powder samples governed by the shieIding anisotropy are affected in a sensitive manner by molecular motion, especially if the mean lifetime of a given configuration is such that Aw = I. We were able to do this by simply applying high magnetic fields of about SO-60 kG. ‘This increases AU, such that the changes of the lineshape due to the motion can be observed more easily. The application of high magnetic fields is probab!y the simplest way to increase the resolution in solids. The sirnpticity of the technique without a need of homo- or heteronuclear decoupling makes it easy to cover a large temperature range needed to study the effects of the motion quantitatively.

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Furthermore, we could prove the validity of the model used in our lineshape calculation because we were able to obtain independent values for the jump frequencies 7-t from measurementsof the spinlattice relaxation time Tt , which at these fields is dominated by relaxation tfuough the anisotropy of the shielding.The agreement between obsenred and calculated spectra shows that from such NMRspectra one can obtain not only jump frequencies but can also distinguish,at least to some extent, between different models for the motion of the spins.

Acknowledgement

We would like to thank Professor K.H. Hausser for his interest and continuous encouragement throughout this work. Special thanks are due to J. huli for constructing the helium flow cryostat and for his help in designingit. We would also like to thank H. Zimmerman for preparation of the sample of solid white phosphorus and Dr. H. Mahnke for the synthesis of the 13Cenriched carbonyls. One of us, R. Grosescu, acknowledgesfinancialsupport from the “Gesellschaft fir Kemforschungm.b.H., Karlsruhe”.

References 111 H.W. Spiess. Chem. Phys. 6 (1974) 217. 121 1. Donohue and A. Caron, Acta Cryst. 17 (1964) 663.

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[ 31 For a reviewon pentacoordinate systems see, e.g., EL. Muctlerties and R-A. Schunn, Quvterly Rev. 20 (1%6) 244. (41 F.A. Cotton, A. Danti. JS. Wau@ and R.W.Fessenden. I. Chem. Phys. 29 (1958) 1427. 15j R. Bnmley, B.N. F&is and RS. Nyholm, Trans. Farrday Sot. 58 (1962) 1893. [6] H.W. Spiess and H. Mahnke, Ber. Bunsenges. Phys. Chem. 76 (1972) 990. [7] J. Kemp& H.W.Spicss. U. Haeberlen and H. Zimmerman, Chem. Phys. 4 (1974) 269. [8l J. Haupt, 2. Angew.Phys. 23 (1967) 377. [9] A. Abragam, The principles of nuclear magnetism (Oxford University Press, London, 1961). [lo] M.C. Cibby, A. pines, WX. Rhim and JS. Waugh, J. Chem. Phys. 56 (1972) 991. [I I] D. Schweitzer and H.W. Spicss, J. Magn. Res. (1974) in Press. [ 12) E.R. Andrew, WS. Hinshaw and A. Jasinski, Chem. Phys. Letters 24 (1974) 399. [ 13 1 H.A. Resin& J. Chem. Phys. 37 (1962) 2575. [ 141 N. Bodm and R. Folland, Mol. Phys. 21(1971) 1123. [IS] S. Kaplan, H.A. Resing and JS. Waugh, J. Chem. Phys. 59 (1973) 5681. [ 16 J R.W. Vaughan, private communication. [ 171 K. Hensen, W.O. Riede, H. Sillcscu and A. von Wittgenstein, I. Chem. Phys., to be published. [IS] 1. Ozier.K.M. Crapa and N.F. Ramsey, J. Chem. Phys. 49 (1968) 2314. 1191 H. Mahnke, R.K. Sheline and H.W. Spiess, J. Chem. Phys. 61(1974) 55. [ZO]H. Mahnke.RJ. Clark. R. Rosanske and R.K. Sheline. J. Chem. Phyr. 60 (1974) 2997.