Interpretation of lanthanide-induced shifts in NMR spectra. The case of nonaxial symmetry

JOURNAL

OF MAGVETIC

RESONANCE

52, 169- 18 1 ( 1983)

Interpretation of Lanthanide-InducedShifts in NMR Spectra. The Case of Nonaxial Symmetry T. A. BABUSHKINA, V. F. ZOLIN, AND L. G. KORENEVA Institute of Radioengineering and Electronics, Academy of Sciences USSR, Moscow: USSR Received December 2, 198 1; revised November 9, 1982 The approach to the treatment of low-symmetry systems by the method of lanthanide shift reagents (LSR) is considered. Analysis of the results obtained by optical spectroscopy (mostly by Iuminescense spectroscopy of Eu ‘+ ion) is used for this purpose. From the spectra mentioned, a model of the LSR adduct and parameters describing anisotropy of magnetic susceptibility are obtained. The method is applied to several rare-earth complexes of nonaxial symmetry. Bleaney’s theory connecting pseudocontact NMR shifts to crystal field parameters of the second order is verified and shown to be correct if the crystal field splittings are of the order of kT. The systems Ln(dpm)r and Ln(fod), are discussed. INTRODUCTION

The method of lanthanide shift reagents (LSR) is well known in high-resolution NMR spectroscopy (I-15). The LSR have been introduced for simplification and resolution enhancement of NMR spectra (1-6). They are still used for this purpose, in spite of the introduction of high-frequency spectrometers (7-9). However, the main application of the LSR is in the investigation of structure or conformation of molecules in fluid solutions (3, 7-15). These investigations are based on analysis of the NMR pseudocontact shifts. The expression for dipolar (pseudocontact) shifts is Au/v = D,(3 60s’ 0 - l)re3 + D2 sin2 0 cos 2W3, 0 = xz - 1/3(xX + xy + x2,

02

=

xx

-

xy,

ill

where r, 8, P are spherical polar coordinates of the resonating nucleus with the principal magnetic axes used as reference axes, and the x values are the principal magnetic susceptibilities. A common feature of much work using the LSR method is the assumption that D2 = 0. That means that the system under investigation is assumed to have axial or effectively axial symmetry. Using Eq. [l] one must separate the contact contribution to the observed isotropic shifts. For this aim, the observed shift is considered as the linear combination of contact and dipolar shifts (16): A = K,a, + K,,A,,

PI

where Kc and KIJ are contact and dipolar coefficients that specify dependence of the 169

0022-2364183 $3.00 Copyright 0 1983 by Academic Press. Inc. All rights of reproduction in any form reserved

170

BABUSHKINA,

ZOLIN,

AND KORENEVA

shifts on lanthanide ion, 4, An are contact and dipolar shifts dependent on structure (Eq. [2] is identical to Eq. [3] from (26) but for some minor changes in designations). Values of Kc and KD used here are taken from theoretical works of Golding (I 7) and Bleaney (18); the latter had been verified for the case of axial symmetry (19). Recently, the assumption of axial symmetry was shown to be wrong more frequently than had been expected (20). However, nobody seems to know how to treat the general case of nonaxial LSR systems. It is believed that K, and KD depend on the system symmetry; that is, the common method of separating contact shifts is not adequate for systems with nonaxial symmetry (20). For investigation of low-symmetry complexes, two approaches are usually used. The DI and D2 values are considered as variable parameters (13, 14) or are evaluated from magnetic susceptibility investigations of crystals (IS). In the first case, the structure of the complex and the values of D, and Dz could not be determined unambiguously (13, 24). In the latter, there is a possibility that solid-state structure does not persist in solution. Thus the LSR method is applicable only to systems with axial symmetry. We believe (21-23) that there exists an approach to the treatment of the lowsymmetry systems by the LSR method. One should use for NMR spectra analysis the results obtained by an independent method-by optical spectroscopy. Here one should use Bleaney’s theory (18) which connects pseudocontact NMR shifts to crystal field parameters (CFP) describing the amplitudes of the crystal field harmonics. The application of optical spectroscopy for obtaining the CFP values reduces the number of variable parameters used for structural investigations by means of the LSR. In particular, the use of optical spectroscopy gives an opportunity to analyze the NMR spectra in case of nonaxial symmetry of LSR-substrate adducts. BLEANEY’S

THEORY:

A RELATIONSHIP BETWEEN AND OPTICAL SPECTRA

DIPOLAR

SHIFTS

Bleaney considered (18) the high-temperature limit of the general theory (24) of dipolar shifts. Assuming that only levels of the ground J manifold of the lanthanide ion are populated and that Stark splitting of the levels is small compared with kT, magnetic susceptibility could be expanded as a power series in T-’ (18, 25, 26). In case of fluid solution, the first term (in T-l) is zero. The second term (in T-*) depends on the CFP of the second order B$ and B$ that specify the values of the amplitudes of crystalline field harmonics of the second order; BT = Aq(r*), B$ = A:(r*), A! and A$ are CFP introduced by Stevens. If this term dominates, the dipolar shift is expressed by Au/v = -g*b*J(J + 1)(2J - 1)(2J + 3)60-‘(kT)-2r-3a(2, J) X (2B$(3 cos* 0 - 1) + 2B$ sin* 13cos 2P).

[l’j

Comparing Eqs. [I] and [ 1’1 one reaches the conclusion that D, = CB;,

D2 = CB’,,

where C = -g*P*J(J

+ 1)(2J - 1)(2J + 3)2a(2, J)60-‘(kT)-*.

131 [41

LANTHANIDE-INDUCED

171

SHIFTS

Here 42, J) are the equivalent operator coefficients, /3 is the Bohr magneton, and J is the quantum number for total angular momentum. Europium ion (Eu3’) is diamagnetic in the ground state (J = 0). Thus the excited states should be considered. For this ion C = 467 1 + 2&/&)2a(2,

J)f(T),

where E, and E2 are the energies of the first and second excited states. At T = 300 K, C(Eu”+) = -3 ppm A’/cm-‘. Obviously the use of Eqs. [3] to [5] and the CFP values obtained from Stark splittings gives values of D, and O2 by an independent method. However, these equations should be verified. They should not be precisely correct as the Stark splittings for many cases are at least of the order of kT. These equations had not been verified directly because the CFP of the adducts in solution were not known. Only two predictions of Bleaney’s theory had been checked: (1) the temperature dependence of the shifts, that is, the prediction that dipolar shifts should vary as Te2, and (2) values of the relative shifts due to different rare-earth ions, in the case of fast exchange. Data on the relative shifts were shown to agree with the predicted values only when averaged over all experiments. The causes of the discrepancies were not investigated. They were believed to be due to deviations of the adduct’s symmetry from axial symmetry. However, if Bleaney’s theory is correct, the ratios of shifts induced by different lanthanide ions should not change in case of nonaxial symmetry, because the CFP Bq and B$ have similar variations through the lanthanide series. Lanthanide complexes with ter- and tetradentate ligands are chosen for verification of Bleaney’s theory and for demonstration of possibilities of molecular structure determination. These chelates have nonaxial symmetry. METHODS

OF

INTERPRETATION

OF

OPTICAL

SPECTRA

Second-order CFE are determined from luminescence spectra of Eu3+, chosen because of two advantages. First, CFP of higher orders were shown (3, 26) to contribute less to the pseudocontact shift induced by this ion than by any other paramagnetic rare-earth ion. This seems to be connected to the fact that the first paramagnetic state of Eu3+ is split only by crystal field harmonics of the second order. Second, the procedure for calculation of the CFP of the second order from optical spectra is much simpler for europium than for other lanthanide ions. For evaluation of these parameters one needs to consider sublevels of only one level, ‘I;1 or ‘D,. For example, in the case of C2” point symmetry group B; = -2.5(E(B,)

+ E(B2)),

B: = -2.5(E(B,)

- E(B2));

here E(B,), E(B2) are energy shifts of the Stark components of the 7F1 or ‘D, terms. Wavefunctions of these components transform like B, and B2 representations of the symmetry group. Their positions could be determined by observing ‘DO-‘F1 or 5D17Fo transitions in luminescence spectra. In order to determine the equivalent representations for wavefunctions of the Stark

172

BABUSHKINA,

ZOLIN,

AND

KORENEVA

sublevels, the complete analysis of Eu 3+ luminescence spectra should be undertaken (27, 28). We used the following treatment (“mathematical models”) (22). The symmetry of the crystal field was fixed by angular spherical coordinates 8 and Cpof the “charges” in the first coordination sphere of the ion. Using expressions from (29) specifying the angular dependence of CFP, the relative values of these parameters were calculated. The CFP calculated were used for determination of the Stark splittings of 7FJ and ‘D,,, levels of Eu 3+. Obviously the computation yields relative positions and symmetry representations of sublevels for a definite frame of reference. Dependence of CFP on the distance I between the paramagnetic ion and the charges of the ligands was varied, and the features depending not on r but on number and positions of charges were looked for. These features were used for determination of crystal field symmetry by means of comparison of experimental spectra to those of the appropriate number of models. This approach to symmetry determination is suitable only in the case of solutions, where the long-range forces average to zero. Our investigations show that representations for sublevels of 7FI level could be found unambiguously if the field symmetry is relatively high, that is, if deviations from trigonal or tetragonal symmetry are sufficiently small. If the symmetry is relatively high, degenerate (doublet- or triplet) sublevels of 7FJ levels could be found. In less symmetrical cases of C,, symmetry the sublevels forming “doublets” have representations B, and B2 (if C, (Z) axis was “earlier” C3 or C, axis). Thus existence of “quasi-doublets” enables one to determine the sign and value of D, . The sign of D2 depends on the choice of X and Y axes and usually is determined in NMR data analysis. If the symmetry is low, three sets of D, and D2 were considered.

EXPERIMENTAL

The synthesis of complexes under investigation was described earlier (30-32). Complexes Lnz(salen)3 (salen is the Shiff’s base produced by condensation of two molecules of salicylic aldehyde with one molecule of ethylenediamine) were obtained by mixing ethanol solutions of lanthanide chloride and the ligand, with addition of appropriate quantity of sodium ethylate. Complexes Ln(H&en)(N03)3 were obtained by mixing acetone solutions of lanthanide nitrate and the ligand. Complexes Ln(PLasp) or Ln(Vasp) were obtained by mixing water solutions of aspartic acid (asp), pyridoxal (PL), and lanthanide acetate or water solutions of asp and lanthanide acetate with ethanol solution of o-vanillin (V) or other aromatic aldehyde, raising the pH value to 7.5 or 8 by adding KOH and drying the precipitate in air. PMR spectra were recorded on Bruker spectrometers WP-90 and WM-250. Spectra of Lnzsalen3 and Ln(H2salen)(N03)3 were obtained for solutions in DMSO-d6 with internal standard TMS. Spectra of Ln(PLasp) and Ln(Vasp) complexes were taken on solutions in DzO, with internal standard DSS. Experimental values of paramagnetic shifts thus obtained are given in Table 1. Luminescence spectra were obtained by a glass-prism spectrograph, type ISP-51 (dispersion about 10 A/mm), with photoregistration. Densitograms were obtained on on microphotometer MP-45 1.

LANTHANIDE-INDUCED TABLE

1

EXPERIMENTAL~ALUESOF INDUCEDSHIFTSOF'H LANTHANIDE COMPLEXES Pr3+

Nd’+

EUr+

Tb3+

-48.6 -8.5 7.2 4 10.2 3.2

-17.4 3.6 5.4 2.2 4.6 1.6

28 -14 -8 -3.7 -6.2 -2.5

-391 -147 38 41.3 70 26.4

-54.6 59.8 26.7 28.4 26.5 20.4

-26.3 32.5 23.3 15.2 13.7 9.7

1.9 -40.5 -24.8 -12.7 -8.1 -11.3

-513 -171 50 59 89.4 36.4

Tm3+

Yb3+

-239 -93.5 21 24 40 18

134 21 -19 -16 -24.5 -10

245 50 -26.7 -30 -42 -18.4

99 22.6 -11.7 -12 -17 -7.3

68 -82.2 -50.1 -38.6 -37.8 -30.8

101 -116 -52.5 -58.5 -58.5 -45

22.8 -70.1 -38.6 -32.2 -27.9 -24.5

(in DrO)

-33 -15.7 -7.8

77 67 54

-82 -61 -41.4

-49 -32 -26.5

100 99 73

-87.6 -71.6 -46

-54.9 -39 -33.1

(in 40)

-36.7 -18.9 -10 Ln(n-Vasp)

HCN K HU

Er3+

-274 289.5 71.3 137.9 128.4 106.2

Ln(Vasp) HCN H, HU

HO’+

(m DMSO)

-338.7 282 I.1 126.7 124.9 103.2 Ln(PLasp)

HCN H, HU

(in D,O)

-36.8 -17.8 -10.4

142 108 83

Note. Values are obtained by substraction of the chemical NMR shifts paramagnetic complex. Low-field shifts are positive. H,, averaged values.

INVESTIGATION

FORSEVERAL

(in DMSO)

LnHZsalen(N03X HZC HCN H3 H4 H5 H6

NMR (ppm)

Dy3+ Ln2salen3

HrC HCN H3 H4 HS H6

173

SHIFTS

OF

COMPLEXES

-64.2 -51 -35 of the ligand

from

those

of the

LNZSALEN3

Let us begin the verification of the theory (18) and determination of the structures of complexes in solution with a case of relatively high symmetry-solutions of complexes Ln2salen3 in DMSO. Infrared spectroscopy and the luminescence spectra of polycrystalline samples of Lnzsalen3 doped with 0.5% of europium (added to solution of lanthanide salt in process of the synthesis) show the complexes to be isomotphous. The luminescence spectrum of Euzsalen3 in DMSO (22) is shown by Fig. la.

174

BABUSHKINA,

ZOLIN,

AND KORENEVA

b

a I

17.0

I

I

16.0

I

I

15.0

I

I

14.0

$*io 3 ( CM-‘) FIG. 1. Luminescence spectra of the solutions of europium complexes with salen in DMSO: (a) Eu+alen,, concentration 3 X lo-’ M; (b) Eu(H&en)(NO& concentration IO-’ h4, after heating.

Comparison of this spectrum to those of the models shows that the symmetry of the complex in solution is nearly tetragonal (S,). CFP estimated from the spectrum have values -320 and 107 cn-’ (the 2 axis is the S, axis). Using Eq. [3], we obtain values of D, and 4 (0, = 960, Dz = -320 ppm A’). In PMR spectra of solutions of LnlsalenJ, Ln = Pr - Yb, signals of ethylenediamine protons are identified easily by their intensities. Relative (through lanthanide series) magnitudes of paramagnetic shifts of these signals are shown in Table 2, as well as the theoretical values of the relative dipolar shifts (Z&19). Experimental data appear to be quite near the theoretical ones. These values are expected to represent Ku. Other signals in the NMR spectrum of the europium complex are identified by

LANTHANIDE-INDUCED

175

SHIFTS

TABLE 2 THEORETICALAND EXPERIMENTAL VALUES OF KD, &,

KD

AND C (SEE Eqs. [2] AND [3]) C

KC

Ln’+

Theory a

Exper.’

Theory’

Exper. d

Theory a

Exper. e

Exper. f

Pr Nd EU Th DY Ho Er Tm Yh

-11 -4.2 4.2 -86 -100 -39 33 53 22

-9.5 -3.5 5.5 -16.5 -100 -47 26 41 19

10.4 15.7 -38 -110 -100 -79 -54 -29 -9.4

9 10 -32 -13 -100 -60 -27 -20 -2

6 3 -3 49 54 22.1 -18.8 -30 -13

5.2 2 -3 41 54 25.5 -14 -22 -10

4.5 1.8 -3 42.5 62 30 -17.5 -28.5 -13.5

d From (18). b Relative values of pammagnetic shifts of NMR of CHI protons in Ln&en, complexes, ’ From (I 7). d From optimal linearization of Eq. [2] for azomethyne protons of Lntsalens complexes. ’ From C(Eu) and KD, assuming II! to be constant through the lanthanide series. f From C(Eu) and KD, assuming that @ - (r2). C values are in ppm A3/cm-‘.

taking into account spin-spin splittings and by using the 4-ethoxy derivative of salen. For other ions, relative values of NMR shifts for different protons are considered. Then Eqs. [2] are linearized for each proton of the complex (i.e., optimal linear dependence of A/Ko on KJKD is determined using the values of KD estimated above and the values of K, taken from ( 17 ). The linearization procedure is illustrated by Fig. 2. The linearization gives an opportunity to determine more precise values of Kc. These precise values differ from theoretical predictions for several ions; that appears to demonstrate the significance of interactions not considered in the treatment of the contact shift problem (24). Linearization leads to separation of dipolar and contact contributions to observed shifts. Dipolar shifts obtained for the europium complex were used for structure analysis. For this purpose, shifts obtained for different models of complex structure were compared to experimental values. Shifts were calculated using Eqs. [ 11, with values of D, and D2 obtained above. Sets of shifts for different positions of the ion and variable rotation angle of aromatic rings about the CH2-CH2 bond were computed. Comparison of computed and experimental sets of shifts was performed by means of Hamilton’s R factor (33) minimization. Results obtained are in Table 3a. Consideration of the table leads to the following conclusions. (1) Calculated shifts do correspond to experimental ones; i.e., the value of C(Eu) predicted by the theory is correct. (2) The distance between the ion and the ethylenediamine bond is so large that N atoms could not be coordinated. Ethylenediamine protons are separated from the paramagnetic ion by more than five bonds, i.e., the shifts of their signals are really dipolar in origin.

176

BABUSHKINA,

FIG. 2. Dependence of A/K, protons; 2, H3; 3, H4.

on KJK,

ZOLIN,

for several

AND

protons

KORENEVA

of Ln&en,

(solution

in DMSO);

1, azomethine

If the C coefficient for Eu3+ and relative pseudocontact shifts Kn are known, C coefficients for other lanthanide ions could be evaluated. For this purpose one should assume definite dependence of CFP on the number of the lanthanide ion. For example, one could assume Bi to be constant through the lanthanide series (34, 35) or suppose that B$ - (r’) ((I-~) is t h e mean square value of the lanthanide radii). Values of C thus obtained are given in Table 2. They are seen to differ from the theoretical values by less than 30% in disregard of the assumed dependence of B: on lanthanide number. Thus, we succeeded in verification of Eqs. [3] to [5] in the case of slow exchange, for a chelate that has almost axial symmetry. Deviations from axial symmetry are taken into account by introduction of 4 = CB$. INTERPRETATION

OF

THE

SPECTRA

OF

COMPLEXES

HAVING

LOW

SYMMETRY

Let us now consider examples of low-symmetry complexes of Shiff s bases-products of condensation of asp and aromatic aldehydes: PL, V, and 5-nitro-o-vanillin

LANTHANIDE-INDUCED

177

SHIFfS

TABLE 3 OF COMPARIKIN OF COMPUTED AND EXPERIMENTAL DWOLAR SHIFTS FOR SEVERAL EUROPIUM COMPLEXES: (a) Eu2salen3 in DMSO, (b) Eu(PLasp) IN D20, (c) EuH+len(NO& IN DMSO

RESULTS

(4

H2C

HCN H3 H4 H5 H6 R F’

(4

Exper.

Computed

Exper.

Computed

28 6.8 -3.3 -3.1 -5.3 -2.2

28.37 4.5 -3.1 -2.2 -2.3 -2.4 0.09 604.0

14 -16 -7 -7.8 -7.2 -6.6

12.1 -15.6 -4.2 -8.3 -11.2 -18.4 0.2 03.4

(b) HCN H, HP CH,OH H6 CH RI

Exper. -14 -11 -7.7

Computed -12.7 -11.6 -6.4 -4.4 -1.3 1.8 3.0

-14.2 -12.9 -6.3 -4.6 -1.2 2.0 2.8

Note. R is Hamilton’s R factor, RI is the distance between Eu3+ and ethylenediamine bond of salen or azomethine bond of PLasp (A); I/ is the twisting angle of salen about the CH2-CH2 bond (degrees).

(n-V). Considering luminescence spectra of europium complexes, one should think their symmetry to be trigonal, but deviations from axial symmetry are appreciable. For the PLasp complex, for example, with the 2 axis chosen so that O2 is minimal, D, = -800, D2 = 600 ppm A3. PMR spectra of solutions of Eu, Ho, Er, and Yb complexes of PI-asp, Vasp, and n-Vasp in DzO were obtained, For interpretation of data, the fine structure of the signals was taken into account; in several cases the double-resonance method was applied. Linearization of Eq. [2] for several protons of PLasp complexes is shown by Fig. 3. The linearization is successful, in spite of significant deviations from axial symmetry. Similar results were obtained for other complexes with asp. Making use of D, and D2 values, one can determine the Ln(PLasp) complex structure. As in the case of Ln+alen3 complex, the position of the Ln ion with respect to ligand was varied. It was found convenient to choose the frame of reference so that D, = 80, D2 = - 1500 ppm A’. Several results of comparison of calculated and experimental shifts are shown in Table 3b. Maintenance of proportionality between Bq values obtained from optical spectra

178

BABUSHKINA,

-6

FIG. 3. Dependence of A/& 2, Ha; 3, H,.

ZOLIN,

AND KORENEVA

Er Yb HO I I on KJK,

for several protons Ln PLasp (in D,O): 1, asomethine protons;

and PMR shifts defined by D, parameters, under conditions of considerable variation of values mentioned, is demonstrated by Table 4. In this table, not only complexes containing asp, but complexes of azomethines, derivatives of PL and other amino acids are used. In all cases, the frame of reference was chosen so that D2 has its minimum value. Shifts of the signals of protons located near Z axis are included. Thus there are reasons to believe that Bleaney’s theory correctly describes dipolar shifts in NMR spectra of chelates of different symmetry. The use of optical spectroscopy for evaluation of DI and D2 for the given reference frame permits one to investigate the structures of low-symmetry complexes. Finally let us consider the complexes Ln(H&en)(NO&, Ln = Pr - Yb. These complexes, as well as chelates mentioned before, form isomorphous series. The luminescence spectrum of Eu3+ in solution of the complex is shown on Fig. lb. Judging by the spectrum, the complex has very low symmetry, and 7F, splitting

LANTHANIDE-INDUCED TABLE VALUES (OBTAINED NMR

Ln(PLser),+ Ln(PLala),+ Ln(PLasp) Ln(Vasp) Ln(n-Vasp) Ln(salen),-

4

OF B! PARAMETERS FOR SEVERAL EUROPILJM COMPLEXES FROM LUMINEXEN~E SPWRA) AND VALUES OF DIP~LAR SHIFIX OF “AXIAL” PROTONSIN HOLMIUM COMPLEXES (DEFINED BY D, COEFFICIENTS) B$’ (Eu)

Complex

179

SHIFTS

Exper. -105

-130 260 230 300 -320

(cm-‘) Rel. values 1 1.15 -2.5 -2.3 -3 3.1

0-W (wm) Exper.

Rel. values

-50

1 1

-50 110 120 150 -239

-2.3 -2.4 -3 4

Note. ser and ala are amino acids wine and alanine.

is exceedingly large. In the case of symmetry so low, one should consider three sets of D, and 4 parameters: 620, -3460; -660, 1260; -2130, 800 ppm A3. Dipolar shifts of PMR signals for all three sets of D, and D2 using Eq. [l] were calculated. Ln ion position in the complex was varied. Structure of the complex was determined by minimization of Hamilton’s R factor. PMR spectra of solutions of Ln(Hzsalen)(N03h complexes in DMSO were interpreted in the same manner as those of Ln&ens. For elimination of contact shifts, linearization of Eq. [2] was carried out, using the same values of K, and Kn that were used for other complexes. The procedure is illustrated by Fig. 4. Points corresponding to different lanthanide ions are seen to scatter appreciably, but in spite of that the linearization procedure gives opportunity to eliminate contact shifts, though accuracy of the elimination is less than in cases considered above. The results of comparison of calculated and experimental shifts (for the first set of D, and D2; for other sets even the qualitative agreement could not be achieved) are shown in Table 3b. The R-factor values are seen to be considerable, i.e., reliability of the results is not very high. It should be pointed out, however, that if during the optimization D, and D2 as well as dipolar shifts are slightly varied, the R factor reaches its minimum value at one point, where the ligand is planar, the lanthanide ion is in its plane, and the distance between the ethylenediamine group and lanthanide ion is equal to 3.4 A. Discrepancies between the theory and experiment in the latter case appear to be connected to the failure of Bleaney’s theory in cases where CFP are as large or more than 1000 cm-‘. One cannot, however, exclude influence of the internal degrees of freedom of the l&and, which could change along the lanthanide series (36). CONCLUSION

Thus complete analysis of complex structure in case of nonaxial symmetry of the magnetic interaction between Ln3+ ion and nuclei is shown to be achieved when data derived from optical spectra of solutions are included in the investigation. Application

180

BABUSHKINA,

I

FIG.

4. Dependence of A/KD on KJKD

I

ZOLIN,

AND KORENEVA

I

I

for several protons Ln(H2salen)(N0,)S,

in DMSO, I, H4; 2, H3.

of Bleaney’s theory allows one to calculate parameters of magnetic dipolar interaction by CFP specifying values of the second-order crystal field harmonics. If the CFP are larger than 1000 cm-‘, Bleaney’s expressions cannot hold. However, even in this case the use of Eqs. [3] to [5] leads to determination of the complex structure. The fulfilment of Bleaney’s theory predictions under conditions when the crystal field splittings are of the order of kT seems to be connected to the fact that secondorder harmonics of crystal field comprise the main factor which accounts for the magnetic dipolar interaction. The method described above could also be applied to the case of “classical” shift reagents Ln(dpm)j and Ln(fod),. Our investigations show (23) that for Eu(fodh adduct solutions in CC4 and CHCl, D1 and D2 coefficients are of the order 1100 to 1300 ppm A3 (as a rule these adducts lack the axial symmetry). In the case of Eu(dpmh adduct solutions, D1 coefficient is near 1200 ppm A3. Many adducts have an effective axial symmetry resulting from the rotational averaging and ligand exchange. In par-

LANTHANIDE-INDUCED

SHIFTS

181

titular, 4-picoline and 3,Mutidine adducts have effective axial symmetry; thus, the use of D, and D2 values obtained from investigation of magnetic susceptibilities of crystals (15) where the adducts have nonaxial symmetry (3) is improper. In the cases of low symmetry because of restricted rotation of Eu(dpmh adducts, the D2coefficient could be near - 1000 ppm A3. The values of D, and D2 for Eu(dpm)3 and Eu(fodh adducts mentioned above could be used in dipolar shift analysis for structure investigations by means of LSR. REFERENCES

1. C. C. HINCKLEY, 2. 3. 4. 5. 6. 7. 8. 9. 10. 1I.

12. 13. 14. IS. 16.

17. 18. 19. 20. 21. 22. 23 24 25 26 27 28 29. 30 31. 32. 33. 34. 35. 36.

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