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Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These artieks are *ztended to serve the readers of THIB JOURNAL by calling attention to new developmats in the t h e w , design, or availability of chemical laboratory instrumentation, a by presenting useful inaights and explanations of topics that are of practical importance to those who use, or teach the use of, modern instrumentation and instrumenla1 techniques. The editor invites correspondence from prospective contributors.
LVI. Nuclear Quadrupole Resonance Spectroscopy. Part Three-Pulse Methods; chemical Applications J. A. 5. SMITH. School of Molecular Sciences, Universify o f Worwick Coventry C V 4
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PULSED METHODS
We now turn to the second group of instruments which has been used recently in the study of nuclear quadrupole resonance, in which the rf power is pulsed and signals are detected only in the quiescent periods. That such techniques e m he applied to quadrupole phenomena has been known since 1934 (29) but recent years have seen a. remarkable increese in the range and variety of the experiments that call be conducted. Pulse methods give us the relaxation times of tho system of nuclear spins more accurately and reliably than any other technique. Suppose t.hab by analogy with nuclear magnetic resonance we can describepart of the behavior af a quadropole system by means of a magnetization M , aligned initially along the direction of the maximum component of the electric field gradient; in nuclear magnetic resonance, this would correspond to the magnetic polarization of the nuclei produced by spplication of a magnetic field Ha (assumed to be parallel to z ; Fig. 13a). We now apply an
Figure 13. ments.
intense pulse of radiofrequency power to the sample coil, generating thenew field HI (Fig. 13b) which rotates about Oz a t a rate equal to the applied radio frequency. The nuclear magnetization now precesses about H, a t a rate ?,HI as well as about Oa a t n rate ?,Ha,where 7. is the nuclear gyromagnetic ratio (defined as the ratio of angular frequency to field); in a frame of reference rotating about 0 z a t the applied radio frequency, the nuclear magnetization appears to rotabe ahout the direction of HI. We leave the puke on for sosufficient time to rotate M by 90'-a called 90" pulse (Fig. 13~)-and then switch it off. Left to itself, the magnetisation dl might well continue rotating about 0 5 , but in condensed states, the system is not left to itself and various inturnctions are present which can change M. Far example, there will exist inhomogeneities in HO (or e.g., in the effective electric field gradient in a nuclear quadrupole resonance experiment) which can change the rate a t ~hihich M precesses about 0 z . Different groups of nuclei
Magnetization vectors (upper line) and pulse responses (lower line) in puked experi-
within the system therefore precess a t slightly different rates: in the rotating frame, their magnetization vectors "fan" out (Fig. 13d) and the 90" magnetiaation induced by the first pulse is gradually destroyed. Experimentally, therefore, we see an induced rf voltage in the sample coil as M precesses about Oz, gradually subsidine as the "fannine-out" oroceeds. the inverse line-width parameter, since there should exist an inverse relationship between the line width Au as observed in any spectrometer giving true line-shapes and T2*. Consider now a slightly different kind of experiment in which we apply two pulses; the original SOP-pulse, after which we wait a time r (to which there are strict limits), and then apply s. second pulse in phase with the first hut of twice t,he duration (a. 180'-pulse). The fan diagram is then rotated by 180' about H I (Fig. 13e) so that after a time 7 the fan callspses and the original magnetization (Fig. 13c) builds up again. At least, i t would do so had there been no loss of magnetization transverse to Oz during the time 27; if such loss is small, the reconstructed magnetization a t Z r produces an "echo" signal (Fig. 13f), the maximum amplitude of which is a measure of d l . Figure 14 shows an oscilloscope display of such an "echo" for "Cl quddrpole resonance a t 36.991 M H s in CHL% a t 77"K, together with the free induction decay (I) after the first 90"-pulse. The pulses themselves are not visible on the trace but are operative during the discontinuities. However, in practice there are mechanisms which cause M to decay during the time 27 in which M is perpendicular to Os,and which do no1 produce a recoverable signal in the way we have described; for example, in C H d X , there will exist a t the 3iCI nuclei 8. local magnetio field originating from the magnetic moment of the protons in the same or neighboring molecules. Fluctustions in the longitudinal component of this field a t the 3SCI nuclei therefore causes fluctuations in their quadrupole resonance frequency and irrecoverable changes of phase in Figure 13d. The amplitude of the echo envelope is then not as strong as we would have expected, and furthermore is a function of r, so that a. plot of the echo amplitude as a function of r will he a decay curve defining a second relaxation time, T2, the so-called spin-phase memory time. I n general, in nuclear quadrupole resonance, T2*< T2;for example, in the case of CH2CIs already referred to, oscilloscope traces like those of Figure 14 lead to values of T2*= 90 rsec and T. = 870 M e e a t 77'K. The common feature of both these relaxation times is that they govern relaxstion processes transverse to Oa. Suppose now that the first pulse rotated the rnagnetiae, tion M b y 180' and the second by 90°, (Continued on page A148)
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Chemical Instrumentation with a period r in between. The signal induced in the coil immediately following the second pulse is now a measure of the magnetiestion along Oe immediately preceding this pulse. Hence as 7 is lengthened, we now see yet another decay curve describing the loss of megnetiaation parallel to 08. The t i e constant governing this decay is T,, the so-called spin-lattice relsxation time. In general in solids, and in all nuclear quadrupole resonance experiments, TIdoes not equal T e e n e r a l l y TI > T*, for example in CHQCLat 77% TI is close to 100 q e o . Three quanthes, TI, TI and Tpa, are therefore needed to describe the relaxation properties of our spin system. Although we have illustrated their mode of actioG by reference to a nuclear m w e t i e resonance experiment, the definitions apply with some important modifications (50) to describe relaxation in nuclear quadrupole resonance, although when axial symmetry is lacking no semi-classical model would seem to be valid. In order to illustrclte the usefulness of pulse methods, we discuss three rather arbitrarily-selected examples from a wide range of important applications. TI is of fundamental importance in all nuclear quadrupole resonance experiments since it governs the rate at which the energy acquired by the spin system from the radio-frequency field can be dissipated into thermal energy. Intense radiofrequency fields applied (say) to W 1 at the quadmpole resonance frequency tend to equalize the populations of the &'/> and +a/. states, whereas spin-lattice relaxation acts in such a way as to restore the surplus population in the lower state. Without this surplus population, there would he no net absorption of energy by the spin system and no signal would be observed in a cw (cantinuous-wave) experiment. We may relate T,to a transition probability W according to the equa, tion
Two transition probabilities are important
in quadrupole relaxation, viz., WI, correspondingto processes for which Am = & 1 (viz., + I / , -+ or vice versa) and W. for which Am = &2 (vie., - I / > -+ '/* or vice versa). In pdichlorobenzene, for example, at room temperature TI = [ ~ ( W I WI)]-I = 22msec, with W, = 4.8 and WS = 17.9 see-' (&3.0 sec-1) (for I = a / 9 , a smdl magnetic field has to be applied in order to derive W, and WS separately). These values of WI and Wn and, in particular, their ratio, provide a critical test of any model of relaxation in solids. In one such theory, first proposed by Bayer, the torsional motions of the molecule in the crystal are continuously interrupted by the action of other vibrational and torsional modes, leading to random changes in the electric field gradient at the nuclear site. We may describe these random changes by means of a frequency distribution, which will have components at the frequencies carresponding to Am = *1 and Am = *2. These components are equivalent to oscillating electric field gradients a t these frequencies and can effect electric quadrupole transitions from the nuclear spin states. The spin populations m e therefore changed by a mechanism which is thermal in origin, and which will act so as to restore the s u r ~ l u sno~ulstionin the lowr spin l n r w . I t i. ( h r lhhl I!', and 11'; c ~ ~ r l r s p mto d pnwisr.; uccurring in dillermt paru of the f r r q w n c y spectrum, and their relative values over a range of temperature provide the critical test of any relaxation model. In p-dichlorobenzene a t room temperature, this ratio is near 7; at 77%, it is near to 1. From the Raman spectra, frequencies of 27 and 54 em-' have been observed, and if these are now assigned to torsional modes of oscillation about two axes perpendicular to the C-CI bond (the direction of V,), the Bayer theory fits the temperature variation of W1/W1 and predicts the reasonable lifetime for any torsional state of 6.7 X 10-IP sec. Relaxation in p-dichlorobeneene therefore seems to be governed by the random thermal interup tions of the molecular torsional oscillations. Such investigations are therefore not only important in giving us a mechanism for relaxation, but also give us useful information about molecular motions in
+
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Figure 14. Free induction decoy (I)and rtimulated echo (E) of "CI quodrvpole resonance in p l y crystalline CH8C18at 77%. One division of the x a l e = 1 0 0 #*.
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nystds. It is easy to see the importance of deuteron quadrupole relaxation me* surements in the study of proton and deuteron ferroelectrics. Since the response of the spin system to pulsed radiofrequency power contains at least as much information as the spectrum we observe from a super-regenerative oscillator, in principal we should be able to search far unknown resonances provided thrtt we heve a broadband pulsed system whose frequency can be varied over a wide range. The signal could be detected for example by sccumulating the free induction decays or stimulated echoes in s. oomputer of average transients or a signal integrator. The critical question is then: would such a system have a better signalto-noise ratio with respect to signal d e tection than a cw or super-regenerative oscillator? The question is not easy to answer theoretically, particularly as the ability to vary coherence in a super-re generative oscilllttor enables one to vary the linewidth in the power spectrum, as Figure 8 shows. However, it has been claimed ( 8 1 ) that pulse methods do have better signal-to-noise ratios when the ratio (v/Au), where u is the signal frequency and A v its width, is less than 10J to 10% The "QBi resonance in BirOs has not yet been detected by ew techniques but a. quadrupole echo signal has been observed (58). In CO~(CO)~, pulse methods of detecting the W a resonances near 9,12, and 19 MHz ( I = 7/52, with nf 0) lead to signalto-noise ratios of 200: 1 a t 7 7 T with s. sample size of 5 cc (33). It seems therefore that under certain circumstances, not yet fully defined, pulse methods can lead to superior signal-tl-to-noiseratios in signal detection, an instrumental advance that has yet to be fully exploited. To conclude this section, we turn to another recent variation of pulse techniques in quadrupole resonance, which also seems to have considemble potential applications in chemistry. This is the method of double resonance, first exploited by Herzog and Hahn (54). We discuss as an example double nuclear quadrupole resonance of W1 and =H in solid CDnCb at 77°K. In this compound, the W l signal near 36 MHz is strong whereas no Tl pure quadrupole resonance signal has yet been observed, the major experimental difficulty being the low radio frequencies needed for pure deuteron quadrupole resonance (less than 300 kHz). We begin by msking the plausible assumption that TSfor W l contains an important contribution from the fluctuating dipole fields produced by the ZHnuclei (which means that there should be a significant change in Tz for W 1 reson8*nce on going from CHLL to CDzCla). Consequently, if we simultaneously irradiate a t the 'H eero-field quadrupole resonance frequency whilst ohsewing the strong "CI signal under pulse conditions, there will he a marked attenuation of a stimulated echo. Such a change is observed (36)at a, double resonance frequency of 127.2 kHa at 77"K, corresponding to 8. 'H quadrupole coupling constant of 169.6 kHz, assuming n = 0 (a disadvantage of the method applied to C 1 polycrystalline samples). The strong W signal therefore serves as an indicator for (Continued on page A169)
Chemical instrumentation
I
the quadrupole resonance of a nucleus whose zero-field signal has hitherto been too weak to observe. For the various experimental conditions under which this kind of experiment has been performed or is likely to he successful, the reader is referred to recent literature articles ( 9 4 4 7 ) ; one remarkable feature of the technique is its extraordinary sensitivity when the right conditions do apply. For example in a single crystal of CaF? using "F magnetic resonance as an indicator, double resonance techniques can measure the relaxation timm of the W a nuclei (I = 7/2), present with an abundance of only 0.13% (57).
7. INTRODUCTION TO CHEMICAL APPLICATIONS We willnow turn to some recent applications of nuclear quadrupole resonance spectroscopy to chemistry and chemical physics. In view of the existence of some recent reviews (98-40) and one book (41) on the subject, we intend here to make a. selection of topics rather than a survey, discussing five aspects of the subject where something of significance has been recently established, or is Likely to be established in the near future. It must be emphasized that the subject is still a growing one, where many important problems remain unsolved. Since the technique we Itre discussing has been hitherto applied only to solids, it is appropriate to discuss first of all the effect of the lattice on the nuclear quadrupole coupling constant. To do this, we may need to use values of the coupling constant obtained by microwave spectroscopy, magnetic resonance in liquid crystal media, .and MBssbsuer speetroscapy, although none of these three techniques will he discawed in great detail. Values of quadrGpole coupling constants to which no specific references are given have been taken from the several uieful compilations which now exist (42-46).
8.
SOLID STATE EFFECTS
Effect of Molecular Poinf-Symmetry in the Cryrtol It is immediately obvious after one has studied a few nuclear ouadruoole reso-
of the gaseous molecule, being generally greater than one would predict. As run example, hexachlorocyclopentadiene was referred to in the previous article as giving six lines in the a-solid phsje a t 77", wherees the symmetry of the "ideal" molecule predicts only three of equal intensity. The molecule is expected to
have two mirror plans, one containing the ring atoms, and the other perpen(Continuedm page A160) .
~~
~.~ . . ..... ~~ ~.
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Chemical Instrumentation dicular t o it pmsing through the three atoms of the CC12 group; the former relates Cl. to Cls and t,he latter CII to C14 and Cl? to CL. Admittedly, the lines do fall into two groups, one of four nem 37 MHz, which we may therefore associate with the ring CCI, and the other of two near 39 MHz which we associate with CCh. Clearly what matters is the point symmetry of the molecule in the cr?~slal and this is one of the first parameters that may be obtained from a complete nuclear quadrupole resonance spectrum. We emphasize the verb 'may', because a, number of other factors may intervene; for example, lines may overlap badly, or a fund* mental may be confused with a sideband response in a super-regenerative oscillator, or piezoelectric responses may appear if the oscillator is frequency-modulated, If however the point symmetry of the molecule is known from sn X-ray structure analysi?, or more simply from the space group, unit cell dimensions, and density, then the number of lines may be predicted and this number must be consistent with the nuclear quadrupole resonance spectrum; if it is not, one or the other is at fault. U n l w this consistency is obvious, one cannot be absolutely sure that the complete spectrum has been recorded, although a reasonable certainty can be es tahlishd. For example, to return to the case of hexachlorocyelopentadiene, careful measurements of the a-phase reveal the presence of six lines and since there are six chlorine nuclei in the molecule, it seems reasonable that the asymmetric unit on which the unit cell is based must contain m e molecule and one molecule only. This, then, is a prediction which can be made from the nuclear quadrupole resonance spectrum and provided the criterion is applied with care, it can be most useful in the determination of molecular point symmetries in elystak. As an example, we consider the case of 8'C1 resonance in (BCI..NMe&, which shows two signals a t 21.008 and 21.156 MHz at 273% (48). The crystal space group is C2/m; with two molecules in the unit cell, the molecular point symmetry must be 2/m. The molecule is believed to contain a planar BN ring with the structure shown below. The information is sufficient to settle the orientation of the two-fold axis in the molecule and hence the orientation of the molecule with respect to the b-miis of the monoclinic cell. There are three possible positions for the two-fold axis: passing
through the two B atoms, or through the two N atoms, or perpendicular to the BNBN plane. The former and the latter predict only one W 1 frequency. For example, the two-fold axis'through B . . B (Continued on page A162)
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Chemical Instrumentation .elates ~ 1to, ~ 1and % ~1~to CL, and adding relates CI, ;he perpelldicular mirror CB to c l c so a,( ,.hlorines are I , cis w o ~ , ~be d truefor a 5quivslent; the ~ .f..d dn.xis nernendieulsr the- ~ BNBN . r - ~ r ~ ~ to - ~ plane. On the other hand, if the two-fold ixis passes through the two N atoms, it relstes C1, to Cb and CL to CI.; adding the perpendicular mirror plane then adds nothing further, since the four CL atoms
the melting-point. One might therefore conclude that in,,,ing thermal vibrations are persuading the molecule in the crystal to move towards yet a third conformation, possibly that d s o found in the melt, in which the molecule shows only three frequencies, one stronger than the other two, and has a probable point of 2/m. Fffects of Temperofure
To return to Figure 15, we see that the temperature coefficients of the a'Cl
Transition temperature
Melting
TW) Figure 15. Temperature variation of the P'CI quadrupole resonance frequencies ( v q ) in the K-form 1. and T-form ( e l of (PNClnlc
must lie in it,, so t,hat two lines of equal intensit,y are predicted, as observed. If a single crystal can he studied, the method becomes of even greater utility; for the example, in 1,2,4,5-tetraehlorohenaene, directions of V , (along two crystallographically independent C-C1 bonds) were found by a study of the Zeeman effect an a. single crystal. This information gave the direction cosines of the molecular plane which, since the molecular center of symmetry lies a t the origin of the unit cell, wss sufficient to proceed directly with a reNernent of the X-ray diffraction data (48). The relationship between the moleculsr point-symmetry and the signal multiplicity means t,hat nuclear quadrupole resanrtnce spectra m e very sensitive to phase changes and lambda-transitions. The molecule (PNCl&, for example, exists in a t least two_eonformations in the solid state; one has 4 point-symmetry (K-form) and shows therefore two W l nuclear quadrupole- resonence signals and the other has 1 point symmetry and shows four (T-form). On heating the K-form, the two l i n s separate into four near 63-C which thus locates the transition temperature (49) Figure 15 shows a plot of the temperature variation of the "C1 frequencies in the two forms. The first feature of interest is the negative temperature coefficient of the frequency, s topic t,o which *e return in the next section. The second is that the two inner line8 of the T-form a t first diverge as the temperature ' r i s e and then converge, apparently extrapolating to coincidence a t
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spectra range from 2 kHz (OK)-' a t low temperatures to 4 kHz (OK)-' near the melting-point; these figures are typical of the behavior of many nuclear quadrupole resonance signals in molecular crystds as the temperature is varied. The negative temperature coefficient is explained by the increasing amplitude of the thermal motions of the molecule which also influence significantly the qudrupole spin-lattice relaxation time, TI (section 6); these can be lattice modes, corresponding, for example, to hindered rotation or translation of the molecule in the crystal, or they may be largely internal modw, which can also affect the quadrupole coupling constant determined in the vapour phase, as for W l in the ion-pair Kt"Cl-(60) or .Re in 187ReOnF (61). In the lattice modes the molecular motion changes the direction of V,.; a t the high frequencies characteristic of such modes, the nuclei cannot follow, so a partial averaging of the angular factor (3 cos28 - 1) in eqn. ( 2 3 ) (section 3) will occur. Bayer (58)treated this as a problem in simple harmonic motion; for a, rigid molecole undergoing hindered rotation about one moment-ofinertia axiv coincident (say) with the principal z-axis of the electric field gradient, he deduced the equation:
where
is the angular frequency of the (Continued on page A164)
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Chemical instrumentation torsional mode and I, is a constant characteristic of each mode, and equaling the moment of inertia for a hindered rotation. Similar expressions hold for hindered rotations %hoot the other two axes. If we can now fit the observed temperature variation to equations of this form, we can in principle derive the important quantities no, I and w . u, is the nuclear quadrupole resonance frequency in a 'rigid' lattice, or a t least one in which the effects of the assigned frequencies have been removed. The magnitude of I tells us something about the kind of motion. w is its characteristic or Einstein frequency, and should he comparable with values obtained from other experimental methods, e.g., the study of specific heats, R s m m spectroscopy, or neutron inelastic scattering. Unfortunately the Bayer theory fails to allow for the effects of lattice expansion, which also changes the motional amplitudes of the molecule. One way round this difficulty is to assume that w is a function of temperature; as zn example, we refer to the work of Utton (65)on ' C I resonance in KClOo where the hindered rotation modes about axes parallel to the plane of the three oxygen atoms (Raman frequencies of 127 and 145 em-') are claimed to have a squared dependence on temperature. Between 77°K and room tempera ture, the change would amount to some 2074, of the frequency, and if it were a
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general phenomenon should be readily detectable in the temperature variation of the Raman spectrum or neutron inelastic scattering. An alternative procedure is to study the nuclear quadrupole resonance frequency as a function of pressure. Writing vQ = f(wi,V,T) where thew; are t.he lattice frequencies and V the volume, then differentiation gives (54)
'QBrresonance in TiBr4below 200°K (66), ( a u ~ / b V is ) found to be positive and (avplbp) to he negative; the second term in (4.5) then overcomes the effects of the first and third, leading t o ~ apositive temoerature coefficient s t constant oressure. that are sometimes observed to an increase in V Q with volume. Effeds of the Lotfice
In eqn. (46), the lest term ( a v p / a T ) v represents the contribution of the Bayer model and the others make dlowance for the finite dependence of the lattice frequencies on temperature and of u p on the molecular volume. Using eqn. (46) and experimental values of u p at various pressures, one can derive estimates of several interesting quantities (66). For example, v, can be found, and its volume dependence; in Cu?O, u, .r 1/V, which would he expected for an ionic lattice, where the charges on 0 govern the electric field gradient at copper. I n some cases, e.g.,
Two effects of the environment of a molecule in a crystal are immediately obvious. The first we have already referred to, namely that the molecular point symmetry is no longer that of the gaseous molecule but that of the molecule in the crystal. Chlorine nuclei which in the gas reside in chemically equivalent atonx, and would therefore have identical quadrupole coupling constants in the microwave spectrum, may have in the solid different frequencies. The second is illustrrtt,ed by the data in Table 2: the frequency itself also changes on going from the gas to the solid. Now it is clear tthat an important contribution to this change comes from the effects of lattice modes which we have discussed in the previous section. For the rigid-body torsions of a molecule like CHaCl, however, such changes would always be to low frequency on going from gas to solid, i.e., A in Table 2 is negative, whereas positive values of A, albhough fairly rare, are also observed. Other kinds of lattice effect clearly exist. Their (Continued on page A168)
Chemical Instrumentation precise nature would be a mattar of considerable significance to the study of molecules in condensed stat,es, but surprisingly little h a . been done about the problem. I n molecular halogen com~ o u n d s ,i t has been supposed (67) that one import,ant effect of the cryst,al is to increase the ionic character of t,he bonds to t,he halogen, so that for example in ICl, the 1P'I coupling constant increases whereas that for W 1 decreases (Table 2). Bersohn in s n interesting paper (68) pointed out that the electric fields within the crystal can influence the quadrupole coupling constant, producing increases as well as decreases of the JSCl frequencies according to the field direct,ion with respect to the bond axis. Thus in 1,3,.5trichlorohenrene his calculations show that the internal field should increase t,he coupling constant. This would not necessarily be obvious in the t,emperattture dependence a t constant pressure which could be dominated by 8. large Bayer term, but i t is clear from the arguments presented a t the end of the previous section that (avQ/ap) might well be positive; this has yet to be tested. Hydrogen-bonding is an extreme exemple of strong intermolecular interactions, and the dat,a,in Table 2 for W I resonance in HCI, "N resonance in NHa, and %H resonance in D 2 0 are examples of this. All these molecules show large negative values of A. Another noteworthy exam-
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Table 2. Quadrupole Coupling Constants (inMHz) and some asymmetry parameters in the gaseous and solid state
Molecule Nucleus
("!)
sw
A(so1idgas) (%I
* Calculated assuming 7 = 0. ple for W 1 is that of the biehloride ion, HCl%-; in the ~ e l tCsCl.j(H3OtHCI2-), where the H C k ion is known from the X-ray structure analysis to have a mirror plane perpendicular to the C1. . . . CI direction, the W 1 frequency a t 294' is 11.89 MHz, whereas in NMe4+ H C k , where this mirror plane is lacking, the a6CI frequency a t the same temperature is 19..51 MHz (69). CsHCb belongs to the 'high-frequency' class (20.47 MHz a t 294') whereas NEt4+ H C G helonm to the 'low frequency' class (11.89 MHz at 294'). The nuclear quadrupole resonance data thus distinguish between two very different kinds of hydrogen-bonding in
this simple system. Since in HCI a t 77"K, the W 1 frequency is 26.469 MHz, and the hydrogen-bond is almost certainly asymmetric, the 'high-frequency' class of HCI1- ion is also presumably asymmetric. I n which case the 'low frequency' class may correspond to the symmetric ion, so that quite a. small change in the size of the positive ion suffices to swiech the H C k ion from one configuration t o another. The present data do not clearly establish whether the symmetric ion contains a. true single potential well or s. double well with tunneling. (Continued on page A170)
I REFERENCES (29) BLOOM. M., and NonseRa, R. E.. Phys. Rea., 93,638 (1954); H ~ a uE. . L.. and HERZO@, U., Phya. R N . , 93,639 (1954). (30) BLOOM,M., HAAN.E. L., and 1I~nzoO.B., Phys. Re".. 97,1699 (1955). (31) S A ~ N I.,A,. PAVLOV, B. N.. and STERN.D. YA..Zauod. Lnb.. 30,676 (1964). (32) SAFIN,I. A,, Zh. SLIUIL.Khim., 4,267 (1963). E. 6., S P I E ~H. ~ ,W.. GARRETT. (33) MOOBERRY, B. B.. and SEELINE, R. K., J. Cham. Phus.. 59, 1970(1969).
I
(34) HERZO(I.B., and HAHN. E. L., P h w ~ .Re%. 103,148(1956). (35) RAOLE,J. L., and Sxena, K. L.. J . Chem. Phya.. 50,3553 (1969). (36) HARTMANN. 6. R., and HAHN,E. L.. Phm. Rcu., 128.2042 (1962). (37) H ~ a r * ,E. L., Proe. XIV Coll. A n p i r c , North Holland. Amsterdam (1967).
1
(38) J ~ r ~ n e G. u , A,. and SAITO,T.. Pvooi. Solid Stole Chem.. 1,380 (1964). (39) Kuso. M., and N*K*M"R*. D., Ad". Inoro. Chem.RadGxhem., 8,257 (1966). (40) S ~ M I NG. . K.. and F ~ o r a ,E. I.. in "The Massbauer Effect and i e A~pliehtionato Chemistry." V. I. Gol'dsnskii, ed., Consultants Bureau, New York. 1864. (41) SeeRef. (19). (42) Ref. (401.PP.93-112. (43) SE(IEL.S. L., and B ~ n s e a .R. G.. "Catalog of ~ u o l e a rQuadrupole Interactions and Resonance Frequencies in Solids." U. 6. ~ationa~ l u r e a uof Standards, Washington. 1962. 1865.
(45) ~ ~ n u u x o vI.. P.,Yonoaaov, M. G., and SAFIN,I. A,, ''Table. of Nuolear Quadr u ~ o l eResonanoe Frquenciea," Leningrad, 1968. , A. 6.. and Toso, D. A,. Chem. (46) S r r ~ n J. Commun.. l,3 (1965). 2 e B . f . K G l . , 118,361 (1063). (47) HEBS.H.,
C., Polr~hx.M.. CRAVEN, B. M.. and (48) DEAN, JBPFREY, G. A,. A d a Cvwst.. 11, 710 (1958).
(49) DIXON,M., JENXINB. H. D. B., SMITH. J. A. 6.. and T o m D. A,, T ~ m aFamd. . Soc.. 63.2852 (1967). W~cx~w R.., and D Y M A N u ~ , A.. J . Chem. Phys.. 46,3749 (1967).
(50)
VAN
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J. F., JAVAN. A,. and E~am.A,, J. C h m . Phys., 31, 633 (1959); L o r s ~ e m n J. , F.. J . Chen. Phvs.. 31,643 (1959). BRECHT,
(52) BAYER.H.,Z.Phy*k, 130,227 (1951). (53) Urrorr. D. B., J . Chom. Phys., 47, 371. (1967). H. S., and W I L ~ ~ MG. S ,A,. (54) GOTOWSKY. Phva. Rm.. 105,464 (1957). (55) BENEDEX.G. B.. "Magnetic Resonance at High Pramre." Interscience, NBIVYork and London, 1963, p. 29.
(56) BARNEB,R, G.. ~ n dENOLRDT,R. D.. J . Chem.Phus., 29,248 (1958). (57) See an interesting'di&asion in Ref. (4). p. 166. n , J . Appl. Phys.. Suppl. to vol. (58) B ~ a s o ~R., 33. 286 (1962). (59) Lnoht*~. C. J., W * o n l m ~ o r . T. C.. SALTHOOBE, J. A.. LYNOB,R. J. SMITE. J. A. S.,Chm. Commun.. 6, 405 (1970).
To be cacluded in the April 1971 issue .... . ~..
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