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Anal. Chem. 1982, 5 4 , 610-615
Europium Luminescence Lifetimes and Spectra for Evaluation of 11 Europium Complexes as Aqueous Shift Reagents for Nuclear Magnetic Resonance Spectrometry Charles C. Bryden‘” and Charles N. Rellley Kenan Laboratories of Chemistty, University of North Carollna, Chapel Hill, North Carolina 275 74
Water coordlnatlon numbers and crystal fleld spllttings have been deduced from Eu( I I I ) lumlnescence measurements and have been shown to govern contact shifts, dlpolar shifts, and the symmetry of europium complexes. Small contact shifts, large dipolar shifts, and axial symmetry are all deslrable propertles of shift reagents in most appllcatlons. On the bask of these crlterla, the Eu( I I I ) complex of the eight-coordinate macrocycle, 1,4,7,10-tetraazacyciododecane-N ,N’,N’’,N‘’’tetraacetate, Is the best aqueous shlft reagent.
pared as described previously (9),and NOTA was prepared as in ref 10. The europium complexes were prepared in H20or D20 from anhydrous EuC13 and adjusted to pH 6.5-7.0 with NaOH or NaOD (pH 2 for Eu aquo ion). The concentration of the complexes was 0.1 M except for Eu aquo, 0.2 M, EuEDTA, 0.03 M, EuNTA, 0.007 M, and Eu(NTA)~,0.04 M. Solutions of Eu(II1) complexes were placed in a standard Raman cell of about 1mL capacity and their luminescence spectra obtained at a resolution of 1cm-l using the laser Raman system described in ref 11. The argon laser line at 21 468 cm-l was used to excite the 7F0 5D2Eu(II1)transition at 21 500 cm-’. The Ar+ plasma lines at 17 198.80 cm-l and 16348.90 cm-I were employed for wavenumber calibration (12). The europium ion luminescence lifetimes were determined by using the apparatus described in ref 2. Solutions of Eu(II1) complexes in H20or D20were placed in standard 1-cm fluorimeter cuvettes, and the strong Eu(I1I) absorption band at 25 340 cm-I was excited by a nitrogen-pumped dye laser which was operated at 10 pps. Luminescence was collected at 16900 and 16250 cm-l (220 cm-’ band-pass), and 400-800 luminescence decay transients were averaged for each measurement (lifetimes measured at 16900 and 16250 cm-’ were identical). When two different time bases were used in the transient digitizer, lifetimes measured on the same solution were identical within kl%. The computer-calculated luminescence decay lifetimes were checked in several cases by manual calculations from decay plots. -+
Recently there has been considerable interest in using the luminescence properties of Eu(II1) and Tb(II1) to study the nature of meM coordination in proteins (1-4, ion-containing polymers (5),and NMR shift reagents (6, 7). The Eu(II1) or Tb(1II) luminescence lifetime in aqueous solution can be used to determine the number of coordinated water molecules displaced by the binding site or ligand, while high-resolution luminescence spectra of complexed Eu(II1) can be used to determine the number of different coordination sites, the symmetry of the coordination site, the magnitude of the crystal field splitting, and whether there is an equilibrium between different numbers of coordinated water molecules a t a single metal site. However, few researchers have fully exploited the information available from Eu(II1) luminescence spectra; in addition, no study of europium ion coordination in aqueous solution has included both luminescence spectra and luminescence lifetimes. In this work we show that much more information on Eu(II1) coordination can be obtained by combining lifetime and high-resolution spectral measurements, and that valuable information can be obtained concerning the solution properties of aqueous NMR shift reagents. To obtain europium complexes with a range of symmetries and coordination numbers, we prepared complexes with ethylenediaminetetraacetate (EDTA4-), cyclohexanediaminetetraacetate (CyDTA4-), N-hydroxyethylenediaminetetraacetate (HEDTA3-), nitrilotriacetate (NTA3-), pyridine-2,6-dicarboxylate (DPA2-), diethylenetriaminepentaacetate (DTPA&), 1,4,7-triazacyclononane-N,”fl”-triacetate (NOTA3-), 1,4~7,1”tetraazacyclododecane-N,N’,N’’,N”’tetraacetate (DOTA“), and 1,4,8,11-tetraazacyclotetradecane-N,N”N”,N”’-tetraacetate(TETA4-), the structures of which are shown in Figure 1. In a recent NMR study of LnEDTA, LnDOTA, and LnNOTA complexes (8), we concluded that LnDOTA complexes are superior aqueous shift reagents; in this luminescence study on a wider range of complexes, that conclusion is confirmed. EXPERIMENTAL SECTION The ligands EDTA, CyDTA, HEDTA, NTA, DPA, and DTPA were obtained commercially, while DOTA and TETA were prePresent address: Research Center, Hercules Inc., Wilmington, DE 19899. 0003-2700/82/0354-0610$01.25/0
RESULTS The spectra in Figures 2-5 have been normalized to the largest peak in each spectrum, except for Eu(DPA)~.Lowresolution spectra of Eu(aquo), EuEDTA, and EuDTPA have been reported previously (13), and are identical with the spectra in Figures 2 and 3 except for the 5Do 7Fopeaks where high resolution is important. In Figures 2 and 3, five groups of peaks appear, corresponding to the 5Do 7Fo transition a t 17 230-17 270 cm-‘, the 5Do 7F,transition at 16800-17 000 cm-I, the 5Do ‘F2transition at 16 000-16 500 cm-l, the 5Do 7F3transition at 15300-15 500 cm-l, and the 5Do 7F4transition at 14 200-14 700 cm-l; in Figures 4 and 5, the 6Do ‘Fo and sDo 7F1regions appear in expanded form. Water coordination numbers (q in Table I) were calculated by placing the measured Ak values on the Ak vs. y plot in ref 1. As described in more detail in the discussion, Eu(II1) luminescence lifetimes can be correlated with the number of water molecules coordinated to the Eu(1II) complex. Luminescence lifetimes of some of the complexes have been measured previously, with the reported q values differing by 0.5-1.3 water molecules, depending on the complex. For the Eu aquo complex, Ah values of 8.3 ms-l (2),8.5 ms-l (14),8.55 ms-’ (this work), 9.27 ms-l ( I ) , and 9.56 ms-I (15) have been reported, corresponding to a range of 8.7-10.0 coordinated water molecules. Recently, in a careful X-ray diffraction study of concentrated LnC13 solutions, a y of 8.3 was reported for the Eu aquo complex (16);although this value may be slightly low because of the concentrated solution employed, it supports the lower Q values obtained from luminescence lifetimes measured in more dilute solutions. X-ray crystal structures
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
Table I. Peak Positions and Luminescence Decay Constants for Eu(II1) Complexes 5D, .+ ?FOpeaks
5D0-+ 'F, peaks
PWHMpa cm-'
k(H,?),
h(D,?)> ms"
ms-'
4
cm-
Eu aquo
8.90
0.35
8.55
9.0
17 270
EuNTA
5.24
0.46
4.78
5.0
EuNOTA,
3.98
0.81
3.1'7
3.3
Eu( NTA),
1.60
0.38
1* 2!1
1.3
EuDOTA.
1.66
0.47
1.19
1.2
17 256 17 236 17 251 17 240 17 256 17 236 17 244
Eu( DPA),
0.65
0.34
0.31
0.3
17231
9
EuHEDTA
3.34
0.57
2.77
2.9
17 250 -17235
9
EuCVDTA
3.13
0.52
2.61
2.7
12=
EuEDTA
2.90
0.48
2.441
2.5
EuDTPA
1.59
0.44
1.151
1.2
17 245 -17235 17 250 17235 17 240
EuTETA
1.17
0.63
0.54
0.6
17252 -17 260
10
complex
ms-
a Peak width at half-maximum. peaks.
b
Ah,
Loo-
rcoo-
- 0 o c 4 N-N:\COO-
DOTA
EDTA
fi
-ooc\ /N
-ooc2
QN,ncoo-oocJN
'."
co 0-
/cooN\
TETA
CyDTA
HEDTA
POS;
-
- 11
8 1 BC
9 10
9 9 10
cooDTPA
Flgure 1. Structures of the Ilgands.
of lanthanide bromates and ethylsulfates show nine coordinated water molecules in d l cases (see ref 17 and references therein), in agreement with our solution measurement. In addition, as will be seen, the Eu(aquo) luminescence spectrum provides evidence that the Eu(aquo) complex in solution has an integral y, and that q is 9. For the EuEDTA complex, Ak values of 2.4 ms-l (14), 2.42 ms-l (this work), 3.2 ms-l ( I ) , and 3.36 ms-l (15) have been reported, corresponding to a range of 2.5-3.5 water molecules, while for the TbEDTA complex, q values of 2.4 ( 4 ) and 2.8 (1) have been reported. X-ray crystal structures of LaEDTA (18),TbEDTA (19),DyEDTA (ZO),and YbEDTA (20) show a water Coordination number of three through DyEDfI'A,
sePpb cm-'
POS;
cm-
16920 16880 16900 16840 16930 16790 16900 16800 16990 16920 16 800 16930 16 820 16930 16870 16790 16910 16820 16950 16870 16960 16810 17 020 16840 16 790
Separation between the highest and lowest frequency 5D0-+ 'F, peaks.
NT A
-0ocz
611
__-
-___
40 60 140 100 190 110 140 90 80 150 230
C
Overlapping
decreasing to two for YbEDTA. These results on crystals support the lower q values obtained in solution for EuEDTA and TbEDTA complexes. Luminescence lifetime measurements on solutions of EuNTA complexes have resulted in q values of 5.0 (this work), and 6.0 ( I ) , while q values of 4.4 (4) and 5.0 (1) have been reported for solutions of TbNTA complexes. X-ray crystal structures of PrNTA and DyNTA show total coordination numbers of nine and eight, respectively (21,22). As NTA ii3 quadridentate, these results strongly suggest a water coordination number of five for PrNTA in solution and four for DyNTA in solution, which again supports the lower q values obtained from EuNTA and TbNTA from luminescence lifetime measurements.
DISCUSSION The ground manifold of Eu(II1) is split by spin-orbit cow pling into seven states 7 F (J ~ = 0, ...,6) spaced about 400-1100 cm-' apart, each of which is split by the crystal field into a maximum of 2J -k 1 states spaced about 50-200 cm-l apart. When the europium ion is excited from the 'Fo ground state to higher states (e.g., a t 21 500 cm-l), it undergoes radiationless decay to the 6Dostate, from which virtually all luminescence to the 'FJ manifold arises in solutions at ambient temperature. Eu(II1) spectra are relatively simple, as the transitions are between states of low J values. Consequently, useful spectral interpretations are possible without a detailed and difficult analysis of the crystal field splitting. Of particular interest is the 6D, 'F1 transition, which gives rise to three peaks for complexes of low symmetry, and two peaks for complexes with C3 or higher symmetry (7). The Eu(I11) luminescence from the 5Dostate is in competition with radiationless decay processes, including a highly efficient process involving a vibronic coupling of the 5Dostate to the OH oscillators of coordinated water molecules (15). This process is much slower in deuterated water, and the difference, Ak, between the luminescence decay rates in H,O and in D,O has been shown to be proportional to the number of water molecules coordinated to the Eu(II1) ion (1). +
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
Eu AQUO
/
17030
,
I
16003
,
l
15ooo
,
I
14coO
,
,
13ooo
CM -1
Flgure 3.
Luminescence spectra of low-symmetry Eu(1I I) complexes.
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Flgure 2. Luminescence spectra of high-symmetry Eu(II1) complexes.
In the following sections, we will relate europium luminescence spectra and lifetimes to important features of Eu(111) coordination. Water Coordination Number. The spectra of all 11 Eu complexes have one or two sharp peaks at about 17 250 cm-l which can be assigned to the 5Do 7Fotransition in Eu(II1). As the 5Doand 7F0states cannot be split by a crystal field, the presence of two peaks in the vicinity of 17250 cm-l is evidence that the Eu complex exists in two forms. For EuEDTA and EuCyDTA, it has been shown that these two forms differ by one coordinated water molecule, with the lower frequency peak corresponding to the form with the fewer water molecules (23,24). The same authors showed that the molar absorptivities of the two peaks are the same, so that the peak areas represent a measure of the relative amounts of the two forms of complexes present. Thus the 6Do 7Fotransition can be used as a cross-check on the accuracy of the water coordination number determined from the luminescence lifetime. Because the rate of equilibration is faster than the luminescence decay, only a single lifetime is obtained for the decay.
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For the series of closely related complexes EuEDTA, EuCyDTA, and EuHEDTA, a comparison of the 5Do 7F0peaks to the q values obtained from Eu(II1) luminescence lifetimes shows close agreement as to the average number of coordinated water molecules. The areas of the two peaks in the EuEDTA spectrum (Figure 5) are nearly equal, and q is 2.5 (Table I), while in the EuCyDTA spectrum the peak corresponding to the form with the greater number of water molecules dominates (about 70% of the totalarea), suggesting a larger average water coordination number, and indeed q was determined to be 2.7 from the lifetime measurements. The EuHEDTA spectrum shows that the higher frequency peak has at least 90% of the total area, again in agreement with the measured q of 2.9. As HEDTA has a weakly coordinating 4 H 2 0 H group in place of a $00-group and would be bound for a smaller fraction of the total time, we expect a larger water coordination number for the EuHEDTA complex than for the EuEDTA and EuCyDTA complexes. Four other complexes, EuNOTA, EuTETA, EuNTA, and Eu(NTA)~,have a second 5D0 'FOpeak in the luminescence spectrum, but in the latter two complexes, the extra peak is due to the presence of a small amount of E u ( N T A ) ~and EuNTA, respectively (see the 6Do 7Fopeak frequencies in Table I). In the case of the EuNOTA and EuTETA spectra, the presence of two 6Do 7Fopeaks shows that two forms of these complexes exist in solution. The spectrum of the EuNOTA complex shows a shoulder at about 17 240 cm-l, representing about 30% of the total area. When this result is compared with the q of 3.3, we conclude
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ANALYTICAL. CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
P
L
/ \ x
17@0
I (
25
I
174w
-
l
-
-
i
17200
l
!
17000
,
I
-
!
17200
,
I
,
17000
I
16800
8
1
1
16600
GM-'
EJ~DPA
Figure 5. Luminescence spectra of low-symmetry Eu(1I I) complexes: 5Do 7F0 and 5D0 7F, transitions.
( L
of the TETA ligand. The cavity in the 14-membered ring of the TETA ligand can easily accommodate the Eu3+ ion, allowing nine coordinated ligand to wrap itself farther around the ion than is possible for the DOTA, DTPA, and NTA ligands. For the EuDOTA complex, a q of 1 f 0.3 was calculated from the 170NMR contact shifts (8),in agreement with the value of 1.2 obtained in this work. Although the EuDOTA spectrum shows only one peak in the 5Do ?FOregion, thie large crystal field splitting allows the observation of a small peak (at 16 920 em-') in the 5Do 7F1region; this suggests that a small amount of another form of the complex is present. The small peak we expect in the 5Do 7F0region must be obscured by the large peak due to the major form of the EuDOTA complex. The spectrum of EuDOTA is unique among those studied here in that the ratio of the hypersensitive 5Do 7Fztransition intensity to the 5D0 7F1transition intensity is not significantly enhanced relative to that of the Eu aquo complex (Figure 2). Another interesting characteristic of the EuDOTA complex which it shares with the EuTETA complex (both of which are tetraaza macrocycles) is the high intensity of the 5D0---c 7F4transition relative to the 5Do 7F, transition (Figurea 2 and 3). An enhancement of the 5Do-* 7F4luminescence intensity has also been observed for Eu(terpyridyl),, which has nine coordinated nitrogens (25). In contrast to all of the other complexes, the 5Do 7F, transition is very weak for the Eu(aquo) and Eu(DPA), complexes. A complex with 0, symmetry would show no 5Do 'Fo peak and two 5Do 7F, peaks (251, so the spectra of the Eu(aquo1 and Eu(DPAl3 complexes in solution are consistent
1 In\
I
,
613
16800
1
1
1
16600
CM-'
Figure 4. Luminescence spectra of hlgh-symmetry Eu(I1I) complexes: 'bo 7F0and %, 7F, transitions.
that the lower frequency peak corresponds to the form with the greater number of water molecules. A q of 3 f 1 was calculated from the I7O NMR contact shifts (8), in agreement with the present results. The water coordination number of the six-coordinate EuNOTA complex is larger than for the six-coordinate EuEDTA and EuCyDTA complexes, which is to be expected in view of the small ring size in the NOTA ligand. The cavity in the nine-membered ring of the NOTA ligand cannot accommodate the Eu3+ion, and consequently the ligand is unable to wrap itself around the ion as well as the EDTA and CyDTA ligands can. The luminescence spectrum of the EuTETA complex also shows a shoulder in the Q0 'Fo region, and two shoulders are evident in the 5Do 7F1region (arrows in Figure 5). The small size of the 5D0 7F0shoulder suggests that the measured q of 0.6 is low andl should be about 0.9. The difference of 0.3 is withii the accuracy of the luminescence lifetime experiment (I). The crystal field splitting in this complex is the largest of all those studied (Table I), and allows the observation of two of the three rimall peaks we expect to see in the :'Do 7F1region. The water coordination number of this eight-coordinate complex is the smallest of the eight-coordinate complexes !studied (EuDOTA, EuDTPA, and EU(NTA)~ are the others), which is expected in view of the large ring size
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
with a slight deviation from the D3 symmetry found for the nine-coordinate crystal structures of both types of complexes (17, 26, 27). Only one 5Do 7Fopeak is evident in each spectrum, which suggests integral water coordination numbers for each complex (zero for Eu(DPA)~and nine for Eu(aquo), by comparison to the q values in Table I). NMR Shift Reagent Evaluation. Several aspects of these luminescence measurements can be used to evaluate the relative utility of lanthanide complexes as NMR shift reagents. The number of peaks in the 5Do 'F1 region shows whether the complex is axially symmetric, while the magnitude of the splitting of the 5D0 7F1peaks in the axially symmetric case is proportional to the magnitude of the dipolar NMR shift. Also, for aqueous shift reagents, the number of coordinated water molecules can be used as a measure of the probable contact contribution to the paramagnetic shift. As is well-known, the symmetry of the crystal field determines the number of peaks which will appear in each 'FJ multiplet of Eu(III), with a maximum number of W C 1peaks per multiplet (28). Recently it has been proposed that the number of peaks in the 5D0 7F1multiplet be used as a simple method of determining the symmetry of lanthanide shift reagent complexes in solution (6). For a shift reagent which has axial symmetry (at least a C3 axis of symmetry), the 7F1 multiplet will show only two peaks, while three peaks (the maximum) will be observed for shift reagents of lower symmetry. As the expression for the NMR dipolar shift is greatly simplified for shift reagents which are axially symmetric (29), an experimental method for the determination of the symmetry of shift reagents in solution would be quite useful. To evaluate the utility of this method of determining the symmetry of shift reagents in solution, we obtained the Iuminescence spectra for Eu complexes of six ligands expected to be axially symmetric on the basis of crystal structure or ligand symmetry (Figure 4) and of five complexes expected to have lower symmetry (Figure 5). All of the six complexes in Figure 4 have the expected two peaks in the 5D0 7F1 region of the spectrum. However, in most cases, the peak widths are so large relative to the crystal field splitting that a third peak could be obscured. Of the five complexes in Figure 5, the spectra of two (EuHEDTA and EuTETA) clearly show three peaks for the 7F1multiplet, while the EuDTPA spectrum has a flat-topped peak at 16 810 cm-' which is probably due to two nearly fused peaks. Unfortunately, the crystal field splitting for EuCyDTA and EuEDTA is not large enough to allow the resolution of the three peaks we expect. It is clear that inspection of the room-temperature luminescence spectra of Eu(II1) complexes does not always allow one to determine unambiguously the symmetry of the complex in solution. As luminescence bands iq Eu complexes have been reported to sharpen in glasses and powders at low temperature (25, 30-32) and as the 5Do 7F1region of the EuEDTA complex has been resolved into three peaks at 77 K (33),it is likely that a substantial increase in resolution could be achieved by measuring the luminescence of Eu complexes as glasses at liquid nitrogen temperatures. Such low-temperature spectra (not obtainable with our present instrumentation) would allow the unambiguous assessment of shift reagent symmetries. It is well-known that for axially symmetric Eu(1II) complexes, the separation of the two peaks in the 7F1multiplet is proportional to the crystal field coefficient Azo (32, 34). Recently it was shown that for axially symmetric lanthanide complexes, the dipolar NMR shift is, to a close approximation, proportional to the same crystal field coefficient A$ (35,36). Thus one can evaluate the relative abilities of axially symmetric dipolar shift reagents to induce dipolar shifts in sub-+
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strates simply by measuring the splitting of the two 6Do 7F1peaks. This measure of the magnitude of the dipolar shift of shift reagents is especially useful because it is independent of shift reagent-substrate stability constants, which can vary substantially from one shift reagent to another. From Table I, we see that the EuDOTA complex has the largest crystal field splitting (190cm-') of the six axially symmetric complexes studied, and thus is predicted to induce the largest dipolar shifts in substrate molecules, while the Eu aquo complex has the smallest crystal field splitting (40 cm-l) and thus can be expected to induce dipolar shifts which are only one-fifth as large as those induced by the EuDOTA complex. In fact, for the case of the 'H (or 2H) shifts of water, the dipolar shift induced by the Eu(aquo) complex was recently found to be l/lsth of the dipolar shift induced by the EuDOTA complex (8);that the shift ratio is even smaller than expected is the result of an averaging process due to rapid ligand exchange in the Eu(aquo) complex. In most cases it is desirable to minimize the contact contribution to the paramagnetic shift induced in substrate molecules by shift reagents. The largest contact shifts will occur when the substrate is directly coordinated to the metal ion in the shift reagent. In the absence of direct coordination, structural information may still be obtained as a result of a geometrically selective interaction (e.g., hydrogen bonding, ion pairing, or hydrophilic or hydrophobic bonding effects), and this structural information will be more purely dipolar because of minimal contact contribution to the paramagnetic shift. Structural information without direct coordination has been obtained by using shift reagents in nonaqueous solution (37, 38), and in aqueous solution (8, 39, 40). As the direct coordination of the substrate is achieved by displacement of one or more solvent molecules coordinated to the lanthanide ion, smaller contact shifts will be induced by shift reagents with small numbers of coordinated solvent molecules. This prediction is in agreement with the observation that the 170 NMR contact shifts of water in solutions of LnDOTA complexes were one-third of the 1 7 0 contact shifts of water and in solutions of LnEDTA and LnNOTA complexes (8). With only the criterion of fewest coordinated water molecules, the Eu(DPA)~and EuTETA complexes would qualify as good shift reagents. However, when the three criteria of axial symmetry, large crystal field splitting, and low water coordination number are all applied, it is evident that the EuDOTA complex is the best shift reagent among the 11 complexes studied.
ACKNOWLEDGMENT The authors thank Jean F. Desreux for providing the DOTA and TETA macrocycles, Dennis S. Everhart and Ronald F. Evilia for providing the NOTA macrocycle, Martha M. Sarasua for generously loaning equipment and supplies for the luminescence lifetime measurements, and Henry H. Dearman for helpful discussions. LITERATURE CITED Horrocks, William Dew., Jr.; Sudnick, Daniel J. Am. Chem. SOC. 1979, 101, 334-340. Sarasua, Martha M.; Scott, Mary E.; Helpern, Joseph A,; Ten Kortenaar, Paul B. W.; Boggs, Norman T., 111; Pedersen, Lee G.; Koehler, Karl A.; Hiskey, Richard G. J. Am. Chem. SOC. 1980. 102, 3404-341 2. Scott, Mary E.; Sarasua, Martha M.; Marsh, Henry C.; Harris, David L.; Hiskey, Richard G.; Koehler, Karl A. J. Am. Chem. SOC.1980, 102, 3413-3419. Brewer, John M.; Carrelra, L. A.; Irwin, R. M.; Elliott, J. I. J. Inorg. Blochem. 1981, 1 4 , 33-44. Okamoto, Y.; Ueba, Y.; Dzanibekov, N. F.; Banks, E. Macromolecules 1981, 14, 17-22. Zolln, V. F.; Koreneva, L. G. Zh. Strukt. Khim. 1980, 21, 66-71. Rlchardson. Frederlck S.: Brittain.. Harm . G. J. Am. Chem. SOC. 1081, 103, 18-24. Bryden, Charles C.; Desreux, Jean F.; Rellley, Charles N. Anal. Chem. 1981, 53, 1418-1425.
Anal. Chem. 1982, 5 4 , 615-621
(9) Desreux, J. F.; Merclny, E.; Loncln, M. F. Inorg. Chem. 1981, 20, 987-991. ( I O ) Everhart, Dnnnls S.; Evllla, Ronald F., University of New Orleans, unpublished work. (11) Pemberton, Jeanne E.; Buck, Rlchard P. S. fhys. Chem. 1981, 85, 248-262. (12) Cralg, Norman C.; Levln, Ira W. Appl. Spectrosc. 1979, 33, 475-476. (13) Gallagher, P. K. J. Chem. Phys. 1984, 4 1 , 3061-3069. (14) Sarasua, Martha M. Ph.D. Thesls, Unlverslty of North Carolina, 1980. (15) Kropp, John L.; Wlndsor, Maurice W. J. Chem. fhys. 1985, 4 2 , 1599- 1608. (16) Habenschuss, Anton; Speddlng, Frank H. J. Chem. Phys. 1980, 7 3 , 442-450. (17) Albertson, JBrgen; Eldhg. Inga Acta Crysfallogr.. Sect. 8 1977. 833, 1480-1469. (18) Hoard, J. L.; Lee, Byungkook; Llnd, M. D. J . Am. Chem. Sot. 1985, 87, 1612-1613. (19) Lee, Byungkook Ph.D. Thesis, Cornell Unlversity, 1967. (20) Nasslmbenl, Lulgl R.; Wrlght. M. Robert W.; van Nlekeirk, Jill C.; McCallum, IPamela PI. Acta Crysfallogr., Sect. 8 1979, 835, 1341- 1345. (21) . . Martln. Leslie L.: Jacobson. Robert A. Inom. Chem. 1972,. 1 1., 2785-2789. (22) Martln, Leslie L.; Jacobson, Robert A. Inorg. Chem. ‘1972, 11, 2789-2795. (23) Kostromlna, N. A.; Tananaeva, N. N. Russ. J. Inorg. Cham. 1971,
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(24) Geler, G.; Jargensen. C. K. Chem. Phys. Lett. 1971, 9 , 263-265. (25) Durham, D. A; Frost, (3. H.; Hart, F. A. J. Inorg. Nucl. Chom. 1969, 39, 633-838. (26) Sayre, Edward V.; Freed, Simon J. Chem. Phys. 1956, 2 4 , 1213-12 19.
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(27) Albertsson, Jorgen Acta Chem. Scand. 1972, 26, 1023-1044. (28) Slnha, Shyama P. “Europium”; Sprlnger-Verlag: New York, 1967; p 116. (29) Rellley, Charles N.; Good, Bennle W.; Ailendoerfer, Robert D. Anal. Chem. 1978, 48, 1446-1458. (30) Sayre, Edward V.; Miller, Donald G.; Freed, Simon J . Chem. Phys. 1957, 26, 109-113. (31) Melo, S6rglo M.; Serra, Osvaldo A. “The Rare Earths In Modern Sclence and Technology”; McCarthy, Gregory J., Rhyne, James J., Silber, Herbert B., Eds.; Plenum Press: New York, 1980; Vol. 2. (32) Kuya, M. K.; Serra, Osvaldo A. J. Coord. Chem. 1980, 10, 13-18. (33) Galduk, M. [.; Zolin, V. F.; Galgerova, L. S. “The Luminescence Spectra of Europium” [In Russian]; Nauka: Moscow, 1974; p 166. (34) Judd, B. R. Mol. Phys. 1959, 2 , 407-414. (35) Bleaney, B. J. Magn. Reson. 1972, 8 . 91-100. (36) Golding, R. M.; Pyykko, P. Mol. Phys. 1973, 26, 1389-1396. (37) Evans, Dennis F.; Tucker, John N.; de Vlllardi, George C. J. Chem. SOC..Chem. Commun. 1975, 205-208. (38) Wentzel, T. J.; Sievers, R. E. Anal. Chem. 1961, 53, 393-399. (39) Elgavish, Gabriel A.;Reuben, Jacques J. Am. Chem. Soc. 1977, 99, 1762- 1785. (40) Perrln, Charles L. Org. Magn. Reson. 1981, 16, 11-13.
RECEIVED for review August 5,1981. Accepted December 18, 1981. We gratefully acknowledge research support from the National Science Foundation. The laser laboratory facility a t the University of North Carolina at Chapel Hill was established through Grant CHEM 77-14547 of the National Science Foundation.
Water- SioIubIe Paramagnellic ReIaxat ion Reagents f or Carbon- 13 Nuclear Magnetic Resonance Spectrometry Thomas J. We!nzel,I Martin E. Ashley, and Robert E. Sievers” Depatiment of Ctremlstry and Cooperative Institute for Research In Environmental Sciences, University of Colorado, Boulder, Colorado 80309
Studies have boen performed In whlch metal complexes of the llgands ethylenedlamlnetetraacetlc acld (H,EDYA), dlethylenetrlamlneipentaacetlc acld (H,DTPA), and trlethylenetetraamlnehexaacetlc acld (HelTHA) were utlllzed ais paramagnetic relaxatlon reagents for ‘‘C NMR spectrometry In aqueous solutions. The Fe( I I I ) and Cr( I I I ) compllexes of DTPA are especlally siilted for facllltatlng spin relsrxatlon. These complexes can be used to decrease the relaxatlon tlmes of carbon nuclel, resultlng In enhanced slgnal to nolse ratlos. They allso quench the nuclear Overhauser effect (NOE), allowing one to obtain quantltatlve 13C NMR spectra. Through the use of these reagents, pulse angles can be Increased and scan tlmes can be decreased, resultlng in the acqulsltlon of hlgher signal lntensltles in shorter limes. Examples of modell cornpounds wlth whlch these reagents are effectlve Include1 arginlne, ascorblc acld, nlcotlnamlcle, and 5,5-dImethylhydimtoIn.
Paramagnetic metal complexes can be added to solutions of organic compounds in order to decrease carbon-13 spinlattice relaxation times as well as suppress the nuclear Overhauser effect (PJOE)observed for protonated carbonri (1-4). The addition of these reagents to a sample causes changes to ‘Present addrens: D e p a r t m e n t of Chemistry, Bates College, Lewiston, ME 04240.
the relaxation times that render the collection of 13CNMR data faster and more efficiently. The signal intensities for carbons not attached to hydrogen are enhanced, pulse angles can be increased so that more signal is obtained per scan, and the delay time between pulses can be decreased, significantly reducing the total data acquisition time. Also, if the NOE is completely suppressed, quantitative integration of 13CNMR spectra may be performed. Paramagnetic relaxation reagents suitable for organic solvents have been studied quite extensively (5-13); however, only a few reports have appeared dealing with water-soluble relaxation reagents (64-19). Gansow et al. have described the use of gadolinium(II1) cryptand (14,15). Bose et al. studied the ethylenediamine complex of chromium, Cr(er),Cl3-3H2O, with a number of amino acids (16). Lettvin and Sherry studied the triethylenetetraaminehexaaceticacid (TTHA) complex of Gd(II1) as an aid in l8C NMR (17). Dechter and Levy have reported the utilization of Gd(TTHA),- and the Gd(II1) complex of diethylenetriaminepentaacetic acid, Gd(DTPA)2-, as relaxation reagents for 15NNMR in aqueous solutions (18), and Bearden et al. (19)reported an aqueous relaxation reagent formed in solution from Gd(N03)3.6H20and inositol. In this paper we compare the Gd(II1) complexes of EDTA, DTPA, and TTHA for their applicability to analytical 13C NMR spectrometry. Dechter and Levy have reported that the ‘ITHA complex interacts the least with nitrogen bases (18). In our studies which include compounds containing oxygen functionalities as well as nitrogen (ascorbic acid, arginine, nicotinamide, and 5,5-dimethylhydantoin), some interesting 0 1982 American Chemlcal Soclety
