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l -Histidine-europium(III) complex: a spectroscopical study
Spectrochimica Acta Part A 55 (1999) 1185 – 1191
L-Histidine-europium(III)
complex: a spectroscopical study
Sergio R. de Andrade Leite a,*, Marco A. Couto dos Santos a, Ce´lia R. Carubelli b, Ana M. Galindo Massabni a a
Uni6ersidade Estadual Paulista, Instituto de Quı´mica, Caixa postal 355, 14801 -970 Araraquara-SP, Brazil b Uni6ersidade Estadual de Ponta Grossa, Ponta Grossa-PR, Brazil Received 25 February 1998; accepted 28 August 1998
Abstract Chemical characterization as well as spectroscopical study of the L-histidine-europium(III) complex were developed both experimental and theoretically. Molecular mechanics (MM) simulation was performed in order to have indication of the compound structure and the Eu3 + chemical environment. The Simple Overlap Model (SOM) was applied to predict spectroscopic quantities as 5D0 7F0/5D0 7F2 intensity ratio, 5D0 7F1 transition splitting and the intensity Vl parameters (l=2 and 4). Satisfactory results are obtained with 0.1 and 2/3 as the effective charges of the nitrogen (gN) and oxygen (gO) respectively, and their polarizabilities (a) depend on the distance. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Histidine; Europium; Spectroscopy; Luminescence; Molecular mechanics; Simple overlap model
1. Introduction About a third of all proteins in their native state contain bound metal ions, or require metal ions for a variety of metabolic pathways [1]. These metal ions are bound directly to protein amino acids residues or to prosthetic groups. This is the case of haemoproteins, in which iron ion is complexed to a porfirin. The amino acids molecules, which are the block units for the peptidic chain, have amine and carboxylate groups able to bind metal ions, and some amino acids * Corresponding author. Fax: +55-16-222-7932. E-mail address: [email protected] (S.R. de Andrade Leite)
have even other potentially complexing groups. Among the amino acids, L-histidine is a unique case in relation to the considerations above: it has a imidazolic group that may bond to a metal ion and this amino acid itself is a component of enzimes active sites [2]; otherwise, histidine is present in haemoproteins, like haemoglobin and mioglobin, where its imidazolic group binds the iron ion present at the porfirin center [2]. Then, this amino acid interacting with metals is an excellent model for the study of many metalloproteins that contain the imidazolic group as a site of metal complexing. We have studied the L-histidine complexation to the trivalent lanthanide ion Eu3 + . It is a good system to study on account of its optical spectro-
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scopic properties. Thus, Eu3 + shows a luminescent emission that can be very intense due to ligand(L)-to-metal energy transfer mechanism (antenna effect), according to the ligand constitution, having long lifetime and line-like emission bands in the visible region [3,4]. In addition to the luminescence study, we have also characterized this complex by vibrational infrared spectroscopy (IR) and 13C nuclear magnetic resonance spectroscopy (13C-NMR). Beyond the experimental procedures, molecular mechanics (MM) simulation [5,6] was used to obtain the relative spatial positions of the Eu3 + and the ligands, because it was not possible to have them experimentally and to test molecular mechanics calculation to this type of compound. The Simple Overlap Model (SOM) [7 – 9] was used to discuss the 5D0 7F1 transition splitting (DE0 – 1) [10], the 5 D0 7F0/5D0 7F2 (I0 – 0/I0 – 2) intensity ratio and intensity parameters Vl (l =2 and 4) of this system. The motivation for applying the SOM is that recently some modifications in this model has allowed the input of very reasonable values of effective charge (gN and gO) and polarizability (a) of the ligands [4,11,12].
2. Experimental The complex of Eu3 + and L-histidine was obtained from the reaction of this compound with europium perchlorate in water-ethanol solution. Europium perchlorate was prepared by reaction of stoichiometric quantities of 1 M perchloric acid solution with europium(III) oxide (Eu2O3) under heating. The lanthanide perchlorate and histidine solutions were mixed up in the molar ratio one to three. The mixture was concentrated by solvent evaporation through a gentle heating under vacuum and successive addition of ethanol until the volume of remaining water was minimum. A further lowering of temperature was sufficient to induce precipitation of the complex. It was dried in a desiccator under vacuum. Stoichiometry of the compound was determined by C, H, N elemental analysis and the metal content was determined by complexometric titration with EDTA.
IR spectra were obtained using a Nicolet 730 FT-IR spectrophotometer and samples as KBr pellets. A Bruker AC 200 spectrometer was used for recording the 13C-NMR spectra (field of 4.7 T and frequency of 50 MHz). The sample was dissolved in deuterium oxyde, D2O. Luminescence spectra (excitation and emission) were obtained from a Spex Fluorolog F212I, with powder samples supported between silica windows at room temperature. Continuous and pulsed Xenon lamps were used for obtention of the spectra and decay curves, respectively. Powder X-rays diffratograms was obtained in a Veb Freiberger Prazisionsmechanik HGZ 4/B diffractometer, utilizing copper Ka lines. Histidine was obtained from Merck and europium (III) oxide (99.99%) from Aldrich. The other chemicals were of analytical grade, from Merck or Carlo Erba.
3. Theoretical The spectroscopic quantities analysed in this work are the 5D0 7F0/5D0 7F2 intensity ratio (I0 – 0/I0 – 2), 5D0 7F1 transition splitting (DE0 – 1) and the Vl (l=2.4) intensity parameters. Their experimental values are obtained as follows:
I0 − 0 S0 − 0 s0 = I0 − 2 S0 − 0 s2
2
(1)
S0 – J being the area related to the 0 J transition in the spectrum recorded in nm. DE0 – 1 is the difference between the upper and the lower lines of the 01 transition. Vl (l=2.4) parameters are given by: V2 =
A0 − 2 2.33× 108s 22xDE
(2)
A0 − 4 2.40× 108s 24xDE
(3)
and V4 =
xDE is the Lorentz local field correction. In the case of the Eu3 + ion, l coincides to J. So, A0 – l is obtained through:
S.R. de Andrade Leite et al. / Spectrochimica Acta Part A 55 (1999) 1185–1191
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Table 1 Elemental analysis results Compound
% Metal
[Eu(L-his)3](ClO4)3
Found 16.25
A0 − J S0 − J sJ = A0 − 1 S0 − 1 s1
%C Calculated 16.59
%H
Found 24.08
(4)
sJ is the barycenter position of the 0 –J transition in cm − 1 and A0 – 1 is the Einstein coefficient of the 0–1 magnetic dipole transition, taken as reference. It is given by A0 – 1 =0,31 ×10 − 11h 3s 31. MM calculation was performed by using the HyperMM+ program [13] through a modified MM2 force field (named MM+) [5,6,14], and the Polack– Ribiere minimum energy search procedure [15]. The compound potential energy surface was explored by repeated molecular dynamics sessions (50 times), each one followed by the energy minimization procedure. The minimum energy structure was considered as a basis for other calculations. With the ligand coordinates obtained through MM calculation, the SOM [7 – 9] was applied in order to predict the spectroscopic quantities presented above. This model attempts to describe the chemical environment effects on the luminescent site by an effective potential produced by the effective charge located around the middle of the metal– ligand distance (metal means 4f as well as 3d ions). This effective charge is proportional to the overlap between 4f and ligand wave functions. Recent modifications in the SOM has allowed the input of very reasonable values of effective charge and polarizability [4,12]. Particularly, the r 8 radial integral value necessary for the calculation of the intensity parameters, was extrap2.5454 olated via r k =0.884e0.02425k . This function reproduces the Freeman – Desclaux [16] values within an average relative deviation less than 6% in the case of the Eu3 + ion.For a detailed discussion on the SOM and the recent modifications, the reader is encouraged to investigate [4,7,8,12].
Calculated 23.61
%N
Found 2.98
Calculated 2.97
Found 14.03
Calculated 13.76
4. Results and discussion The results of elemental analysis (Table 1) agree with the formula [Eu (L-his)3] (ClO4)3. Under heating, the compound do not melt but decomposes at 225.4°C. For comparison, L-histidine melts at 277–287°C [17]. Table 2 X-ray data of L-histidine and L-histidine-Eu3+ L-Histidine
[Eu(L-his)3](ClO4)3
d(A)
I/I0
d(A)
I/I0
9.36 5.81 5.13 4.71 – – – – 4.23 – – 4.02 3.97 3.71 – – – – – 3.00 2.96 2.88 2.80 – 2.67 2.43 2.39 2.35 2.26 2.16
9.89 5.91 4.14 100 – – – – 19.0 – – 4.03 5.59 13.4 – – – – – 5.48 3.44 8.06 3.33 – 4.73 3.17 3.55 3.01 4.09 9.35
– – – – 4.68 4.58 4.50 4.48 – 4.17 4.09 – 3.99 – 3.67 3.44 3.24 3.21 3.19 – 2.93 2.90 2.79 2.76 – – – 2.34 – –
– – – – 30.5 33.3 41.7 44.4 – 43.8 39.6 – 43.0 – 33.3 100 31.2 33.3 31.9 – 45.8 33.3 43.0 34.7 – – – 31.9 – –
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Table 3 Infrared data of L-histidine and L-histidine-Eu3+ complex. Wavenumbers (cm−1)
Assignment
L-Histidine
[Eu(L-his)3](ClO4)3
3138–2600 (s) 2022 (w) 1634 (s) 1594 (m) 1575 (m) 1507 (m)
– 1998 1634 1579
nN–H dasNH+ 3 dasNH+ 3 nasCOO−, nringCC
1523
nCN(ring), dNH3, nring
1468 1421 1345 1318 1254 1120 1096 1068 973 930 914 843 803 787 692 661 629
(s) (s) (s) (s) (s) (w) (w) (w) (m) (m) (sh) (m) (m) (sh) (w) (w) (s)
1483 1413 1334 1286 1231
542 (m)
542
nsCOO− 6CH2 dC–H(ring), dCH2
*
nasCCN
*
rCH2
– 780
dring, dsCCN dC–H(ring)
708 669 622
gC–H(ring), 6COO− dC–H(ring) dCOO−, ring vibration rCOO−
results indicate the participation of these groups in the coordination. The dasNH3+ 1634 cm − 1 band does not change, showing that the positive amine group is not a binding site. Changes in the large band between 3200 and 2400 cm − 1, a classic dN–H region, do not mean amine implication in the bonding, but may be related with the NH–CO hydrogen bond breaking following carboxylate coordination to europium. The bands at 1110 and 626 cm − 1 are assigned to the perchlorate anion. The well defined and sharp band at 626 cm − 1 shows that the perchlorate local symmetry changes very little in relation to the Td one presented by the free ion [18]. Otherwise, this band would be split as a result of symmetry lowering. Thus, perchlorate probably is not directly coordinated to lanthanide ion and remains in the outer sphere. The 13C-NMR spectrum (Table 4) also confirms coordination via carboxylate group in aqueous solution, as C1 signal at d 175.2 in the free ligand disappears in the Eu3 + compound, probably due to the T2 relaxation time increasing following lanthanide coordination [19,20]. The C5 signal at d 137.5 in the histidine shifts to higher field, indicating that coordination by the imidazolic ring occurs through N7 and not N8, because the former nitrogen atom is neighbor C5 and the
* Overlaped by perchlorate bands.
The crystal interplanar distances (Table 2), determined by X-ray powder diffraction analysis, indicate that the L-histidine-Eu3 + compound crystalizes under a different pattern as the free L-histidine. IR spectrum of the compound shows changes in the positions and profiles of some bands, as compared to those of the free L-histidine (Table 3), showing the participation of the corresponding groups in the coordination bonding to the europium. The carboxylate stretching band at 1421 cm − 1 (nsCOO − ) shifts to lower frequency, and the nasCOO − at 1594 cm − 1 and nCC (ring) at 1575 cm − 1 merge to a single band at 1579 cm − 1. Bands belonging to imidazolic ring, nN–H at 3090 cm − 1, nC =N at 1507 cm − 1 and n ring at 1468 cm − 1, shift to higher wavenumbers. These
Table 4 13 C NMR data of L-histidine and L-histidine-Eu3+ complex d (ppm)
L-Histidine
[Eu(L-his)3](ClO4)3
Assignment
175.2 137.5 133.4 117.9 55.9 29.4
– 135.6 129.4 118.6 54.0 27.3
C1 C6 C5 C4 C2 C3
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Fig. 1. Luminescence spectra of [Eu (L-his)3] (ClO4)3. From top to bottom: (a) emission spectrum; (b) excitation spectrum.
latter is located beside C4, whose signal changes very little after coordination. The luminescence spectra (Fig. 1) show the following features: the Eu3 + compound, excited at 393.0 nm, exhibits bands at 579.4 nm (transition 5D0 7F0), 591.6 nm and 595.0 nm (5D0 7 F1), 611.8 and 615.4 nm (5D0 7F2), 649.6 nm 5 ( Do 7F3), 628.6, 636.2 and 699.0 nm (5D0 7 F4). A low structure symmetry is evidenced by the presence of the Laporte prohibited 5D0 7F0
band and by the very intense 5D0 7F2 transition in comparison to the 5D0 7F1 band. The excitation spectrum of the same compound, measured at 611.8 nm, shows bands at 361.0 nm (7F0 5 D4), 374.4 and 379.0 nm (7F0 5GJ ), 393.0 nm 7 ( F0 5L6), 414.6 nm (7F0 5D3), 464.2 nm (7F0 5D2), 525.6 and 534.8 nm (7F0 5D1). The five first bands are located over a large band that covers the region between 300 and 440 nm, probably due to oxygen to europium(III) charge trans-
S.R. de Andrade Leite et al. / Spectrochimica Acta Part A 55 (1999) 1185–1191
1190
Table 6 Experimental results and theoretical predictions of the I0–0/I0– 2, DE0–1 in cm−1 and Vl (l =2,4) (in units of 10–20 cm2)
Fig. 2. Molecular structure of the complex ion [Eu (L-his)3]3 + obtained by molecular mechanics MM + method.
fer. The excited state average lifetime measured through the emission decay is 0.26 ms, at room temperature. For comparison, aqueous Eu3 + lifetime is about 0.20 ms [21], showing a faster de-excitation in water solution, as expected. The Fig. 2 shows the minimum energy structure of the system obtained by molecular mechanics calculation. Table 5 exhibits the ligand coordinates in the framework of the Eu3 + ion and polarizabilities. As a rule, the magnitude of the polarizability is smaller, the greater in the metal-ligand distance without a defined relation Table 5 Ligand coordinates in the framework of the Eu3+ ion and their respective polarizability Ligand
˚) R(A
u (°)
f(°)
˚ 3) a(A
N1 O1 O2 N2 O3 O4 N3 O5 O6
2.543 2.557 2.527 2.537 2.527 2.529 2.541 2.556 2.545
13.659 81.342 89.798 137.519 72.085 78.259 67.375 119.445 118.016
191.073 162.418 231.621 88.816 116.12 34.496 −55.706 267.924 11.929
2.6 0.1 4.5 2.7 4.5 3.9 2.6 0.1 0.2
Parameters
Experimental
SOM
I0–0/I0–2 DE0–1 V2 V4
0.011 230 4.1 4.0
0.008 226 4.1 2.9
[22,23]. Table 6 brings the comparison between the experimental results and theoretical predictions. The latter has been developed with the polarizabilities lying on Table 5 and gN=0.1 and gO = 2/3. In the same sequence, we point out that the Mu¨lliken charges obtained by quantum mechanics AM1 method using the molecule structure obtained by our MM simulation are 0.1 and 0.6.
5. Conclusions Eu3 + complex with L-histidine has been obtained. It is a white crystalline powder, in which the metal interaction with the ligand occurs via the oxygen atoms of the carboxylate group and one of the imidazolic ring nitrogen atom. Such interactions occur in the solid state, as shown by IR spectroscopy measurements, and also in solution, as shown by the 13C-NMR spectrum in deuterium oxide. The emission spectrum, investigated with solid samples, shows that the luminescent ion occupies a low symmetry site, since the 0 0 transition is present. The perchlorate ion stays outer the coordination sphere. Good agreement between experimental results and theoretical predictions of the DE0 – 1, the I0 – 0/I0 – 2 intensity ratio and intensity parameters Vl (l=2 and 4) was achieved, using reasonable values of polarizabilities and nitrogen and oxygen charges. For both good SOM predictions and reproduction of Mu¨lliken charges, we have a strong indication that molecular mechanics simulation may provides very satisfactory structure for quite large rare earth complexes.
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Acknowledgements We thank CAPES and FAPESP (Brazilian agencies) for financial support.
References [1] W.D. Horrocks Jr., D.R. Sudnick, Acc. Chem. Res. 14 (1981) 384. [2] J.A. Cowan, Inorganic Biochemistry: an Introduction, VCH, New York, 1993. [3] M.E. Mesquita, G.F. Sa´, M.A.B. Lopes, O.L. Malta, J. Chem. Res. 5 (1996) 120. [4] O.L. Malta, H.F. Brito, J.F.S. Menezes, F.R. Gonc¸alves e Silva, S. Alves Jr., F.S. Farias Jr., A.V.M. Andrade, J. Luminesc. 75 (1997) 255. [5] N.L. Allinger, J. Am. Chem. Soc. 99 (1977) 8127. [6] U. Burhert, N.L. Allinger, Molecular Mechanics, ACS, Washington, 1982, p. 177. [7] O.L. Malta, Chem. Phys. Lett. 88 (1982) 353. [8] O.L. Malta, S.J.L. Ribeiro, M. Faucher, P. Porcher, J. Phys. Chem. Sol. 52 (1991) 587. [9] O.L. Malta, M.A. Couto dos Santos, L.C. Thompson, N.K. Ito, J. Luminesc. 69 (1996) 77. [10] O.L. Malta, E. Antic-Fidancev, M. Lemaitre-Blaise, A. Milicic-Tang, M. Taibi, J. Alloys Comp. 228 (1995) 41.
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[11] E.B. Stucchi, S.L. Scarpari, M.A. Couto dos Santos, S.R.A. Leite, J. Alloys Comp. 89 (1998) 275– 277. [12] O.L. Malta, A.E. Mauro, M.P.D. Mattioli, V. Sargentelli, H.F. Brito, J. Braz. Chem. Soc. 3 (1998) 243. [13] HyperMM +, in HyperChem H.S. Software, Hypercube, Waterloo, Ontario, Canada. License number 545– 10001953. [14] N.L. Allinger, Y.H. Yuh, Quantum Chemistry Program Exchange, Bloomington, in program c 395. [15] HiperChem., Computational Chemistry, Hypercube, Waterloo, Canada, 1995. [16] A.J. Freeman, J.P. Desclaux, J. Mag. Magn. Mat. 12 (1979) 11. [17] The Merck Index, 10th ed., Merck, New Jersey, USA, 1983. [18] B.J. Hathaway, A.E. Underhill, J. Chem. Soc. Part III (1961) 3091. [19] P.F. Knowles, D. Marsh, H.W.E. Rattle, Magnetic Resonance of Biomolecules, Wiley, London, 1976. [20] E.D. Becker, High resolution NMR-Theory and Chemical Applications, Academic Press, New York, 1980. [21] S.R.A. Leite, J. Dexpert-Ghys, B.P. Roques, J. Braz. Chem. Soc. 1 (1990) 41. [22] E.B. Stucchi, A. Castro, M.A. Couto dos Santos, P. Melnikov, J. Alloys Comp. 86 (1998) 275 – 277. [23] A.M.G. Massabni, G.J. Montandon, M.A. Couto dos Santos, Material Res. 1 (1998) 1.
L-Histidine-europium(III)
complex: a spectroscopical study
Sergio R. de Andrade Leite a,*, Marco A. Couto dos Santos a, Ce´lia R. Carubelli b, Ana M. Galindo Massabni a a
Uni6ersidade Estadual Paulista, Instituto de Quı´mica, Caixa postal 355, 14801 -970 Araraquara-SP, Brazil b Uni6ersidade Estadual de Ponta Grossa, Ponta Grossa-PR, Brazil Received 25 February 1998; accepted 28 August 1998
Abstract Chemical characterization as well as spectroscopical study of the L-histidine-europium(III) complex were developed both experimental and theoretically. Molecular mechanics (MM) simulation was performed in order to have indication of the compound structure and the Eu3 + chemical environment. The Simple Overlap Model (SOM) was applied to predict spectroscopic quantities as 5D0 7F0/5D0 7F2 intensity ratio, 5D0 7F1 transition splitting and the intensity Vl parameters (l=2 and 4). Satisfactory results are obtained with 0.1 and 2/3 as the effective charges of the nitrogen (gN) and oxygen (gO) respectively, and their polarizabilities (a) depend on the distance. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Histidine; Europium; Spectroscopy; Luminescence; Molecular mechanics; Simple overlap model
1. Introduction About a third of all proteins in their native state contain bound metal ions, or require metal ions for a variety of metabolic pathways [1]. These metal ions are bound directly to protein amino acids residues or to prosthetic groups. This is the case of haemoproteins, in which iron ion is complexed to a porfirin. The amino acids molecules, which are the block units for the peptidic chain, have amine and carboxylate groups able to bind metal ions, and some amino acids * Corresponding author. Fax: +55-16-222-7932. E-mail address: [email protected] (S.R. de Andrade Leite)
have even other potentially complexing groups. Among the amino acids, L-histidine is a unique case in relation to the considerations above: it has a imidazolic group that may bond to a metal ion and this amino acid itself is a component of enzimes active sites [2]; otherwise, histidine is present in haemoproteins, like haemoglobin and mioglobin, where its imidazolic group binds the iron ion present at the porfirin center [2]. Then, this amino acid interacting with metals is an excellent model for the study of many metalloproteins that contain the imidazolic group as a site of metal complexing. We have studied the L-histidine complexation to the trivalent lanthanide ion Eu3 + . It is a good system to study on account of its optical spectro-
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scopic properties. Thus, Eu3 + shows a luminescent emission that can be very intense due to ligand(L)-to-metal energy transfer mechanism (antenna effect), according to the ligand constitution, having long lifetime and line-like emission bands in the visible region [3,4]. In addition to the luminescence study, we have also characterized this complex by vibrational infrared spectroscopy (IR) and 13C nuclear magnetic resonance spectroscopy (13C-NMR). Beyond the experimental procedures, molecular mechanics (MM) simulation [5,6] was used to obtain the relative spatial positions of the Eu3 + and the ligands, because it was not possible to have them experimentally and to test molecular mechanics calculation to this type of compound. The Simple Overlap Model (SOM) [7 – 9] was used to discuss the 5D0 7F1 transition splitting (DE0 – 1) [10], the 5 D0 7F0/5D0 7F2 (I0 – 0/I0 – 2) intensity ratio and intensity parameters Vl (l =2 and 4) of this system. The motivation for applying the SOM is that recently some modifications in this model has allowed the input of very reasonable values of effective charge (gN and gO) and polarizability (a) of the ligands [4,11,12].
2. Experimental The complex of Eu3 + and L-histidine was obtained from the reaction of this compound with europium perchlorate in water-ethanol solution. Europium perchlorate was prepared by reaction of stoichiometric quantities of 1 M perchloric acid solution with europium(III) oxide (Eu2O3) under heating. The lanthanide perchlorate and histidine solutions were mixed up in the molar ratio one to three. The mixture was concentrated by solvent evaporation through a gentle heating under vacuum and successive addition of ethanol until the volume of remaining water was minimum. A further lowering of temperature was sufficient to induce precipitation of the complex. It was dried in a desiccator under vacuum. Stoichiometry of the compound was determined by C, H, N elemental analysis and the metal content was determined by complexometric titration with EDTA.
IR spectra were obtained using a Nicolet 730 FT-IR spectrophotometer and samples as KBr pellets. A Bruker AC 200 spectrometer was used for recording the 13C-NMR spectra (field of 4.7 T and frequency of 50 MHz). The sample was dissolved in deuterium oxyde, D2O. Luminescence spectra (excitation and emission) were obtained from a Spex Fluorolog F212I, with powder samples supported between silica windows at room temperature. Continuous and pulsed Xenon lamps were used for obtention of the spectra and decay curves, respectively. Powder X-rays diffratograms was obtained in a Veb Freiberger Prazisionsmechanik HGZ 4/B diffractometer, utilizing copper Ka lines. Histidine was obtained from Merck and europium (III) oxide (99.99%) from Aldrich. The other chemicals were of analytical grade, from Merck or Carlo Erba.
3. Theoretical The spectroscopic quantities analysed in this work are the 5D0 7F0/5D0 7F2 intensity ratio (I0 – 0/I0 – 2), 5D0 7F1 transition splitting (DE0 – 1) and the Vl (l=2.4) intensity parameters. Their experimental values are obtained as follows:
I0 − 0 S0 − 0 s0 = I0 − 2 S0 − 0 s2
2
(1)
S0 – J being the area related to the 0 J transition in the spectrum recorded in nm. DE0 – 1 is the difference between the upper and the lower lines of the 01 transition. Vl (l=2.4) parameters are given by: V2 =
A0 − 2 2.33× 108s 22xDE
(2)
A0 − 4 2.40× 108s 24xDE
(3)
and V4 =
xDE is the Lorentz local field correction. In the case of the Eu3 + ion, l coincides to J. So, A0 – l is obtained through:
S.R. de Andrade Leite et al. / Spectrochimica Acta Part A 55 (1999) 1185–1191
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Table 1 Elemental analysis results Compound
% Metal
[Eu(L-his)3](ClO4)3
Found 16.25
A0 − J S0 − J sJ = A0 − 1 S0 − 1 s1
%C Calculated 16.59
%H
Found 24.08
(4)
sJ is the barycenter position of the 0 –J transition in cm − 1 and A0 – 1 is the Einstein coefficient of the 0–1 magnetic dipole transition, taken as reference. It is given by A0 – 1 =0,31 ×10 − 11h 3s 31. MM calculation was performed by using the HyperMM+ program [13] through a modified MM2 force field (named MM+) [5,6,14], and the Polack– Ribiere minimum energy search procedure [15]. The compound potential energy surface was explored by repeated molecular dynamics sessions (50 times), each one followed by the energy minimization procedure. The minimum energy structure was considered as a basis for other calculations. With the ligand coordinates obtained through MM calculation, the SOM [7 – 9] was applied in order to predict the spectroscopic quantities presented above. This model attempts to describe the chemical environment effects on the luminescent site by an effective potential produced by the effective charge located around the middle of the metal– ligand distance (metal means 4f as well as 3d ions). This effective charge is proportional to the overlap between 4f and ligand wave functions. Recent modifications in the SOM has allowed the input of very reasonable values of effective charge and polarizability [4,12]. Particularly, the r 8 radial integral value necessary for the calculation of the intensity parameters, was extrap2.5454 olated via r k =0.884e0.02425k . This function reproduces the Freeman – Desclaux [16] values within an average relative deviation less than 6% in the case of the Eu3 + ion.For a detailed discussion on the SOM and the recent modifications, the reader is encouraged to investigate [4,7,8,12].
Calculated 23.61
%N
Found 2.98
Calculated 2.97
Found 14.03
Calculated 13.76
4. Results and discussion The results of elemental analysis (Table 1) agree with the formula [Eu (L-his)3] (ClO4)3. Under heating, the compound do not melt but decomposes at 225.4°C. For comparison, L-histidine melts at 277–287°C [17]. Table 2 X-ray data of L-histidine and L-histidine-Eu3+ L-Histidine
[Eu(L-his)3](ClO4)3
d(A)
I/I0
d(A)
I/I0
9.36 5.81 5.13 4.71 – – – – 4.23 – – 4.02 3.97 3.71 – – – – – 3.00 2.96 2.88 2.80 – 2.67 2.43 2.39 2.35 2.26 2.16
9.89 5.91 4.14 100 – – – – 19.0 – – 4.03 5.59 13.4 – – – – – 5.48 3.44 8.06 3.33 – 4.73 3.17 3.55 3.01 4.09 9.35
– – – – 4.68 4.58 4.50 4.48 – 4.17 4.09 – 3.99 – 3.67 3.44 3.24 3.21 3.19 – 2.93 2.90 2.79 2.76 – – – 2.34 – –
– – – – 30.5 33.3 41.7 44.4 – 43.8 39.6 – 43.0 – 33.3 100 31.2 33.3 31.9 – 45.8 33.3 43.0 34.7 – – – 31.9 – –
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Table 3 Infrared data of L-histidine and L-histidine-Eu3+ complex. Wavenumbers (cm−1)
Assignment
L-Histidine
[Eu(L-his)3](ClO4)3
3138–2600 (s) 2022 (w) 1634 (s) 1594 (m) 1575 (m) 1507 (m)
– 1998 1634 1579
nN–H dasNH+ 3 dasNH+ 3 nasCOO−, nringCC
1523
nCN(ring), dNH3, nring
1468 1421 1345 1318 1254 1120 1096 1068 973 930 914 843 803 787 692 661 629
(s) (s) (s) (s) (s) (w) (w) (w) (m) (m) (sh) (m) (m) (sh) (w) (w) (s)
1483 1413 1334 1286 1231
542 (m)
542
nsCOO− 6CH2 dC–H(ring), dCH2
*
nasCCN
*
rCH2
– 780
dring, dsCCN dC–H(ring)
708 669 622
gC–H(ring), 6COO− dC–H(ring) dCOO−, ring vibration rCOO−
results indicate the participation of these groups in the coordination. The dasNH3+ 1634 cm − 1 band does not change, showing that the positive amine group is not a binding site. Changes in the large band between 3200 and 2400 cm − 1, a classic dN–H region, do not mean amine implication in the bonding, but may be related with the NH–CO hydrogen bond breaking following carboxylate coordination to europium. The bands at 1110 and 626 cm − 1 are assigned to the perchlorate anion. The well defined and sharp band at 626 cm − 1 shows that the perchlorate local symmetry changes very little in relation to the Td one presented by the free ion [18]. Otherwise, this band would be split as a result of symmetry lowering. Thus, perchlorate probably is not directly coordinated to lanthanide ion and remains in the outer sphere. The 13C-NMR spectrum (Table 4) also confirms coordination via carboxylate group in aqueous solution, as C1 signal at d 175.2 in the free ligand disappears in the Eu3 + compound, probably due to the T2 relaxation time increasing following lanthanide coordination [19,20]. The C5 signal at d 137.5 in the histidine shifts to higher field, indicating that coordination by the imidazolic ring occurs through N7 and not N8, because the former nitrogen atom is neighbor C5 and the
* Overlaped by perchlorate bands.
The crystal interplanar distances (Table 2), determined by X-ray powder diffraction analysis, indicate that the L-histidine-Eu3 + compound crystalizes under a different pattern as the free L-histidine. IR spectrum of the compound shows changes in the positions and profiles of some bands, as compared to those of the free L-histidine (Table 3), showing the participation of the corresponding groups in the coordination bonding to the europium. The carboxylate stretching band at 1421 cm − 1 (nsCOO − ) shifts to lower frequency, and the nasCOO − at 1594 cm − 1 and nCC (ring) at 1575 cm − 1 merge to a single band at 1579 cm − 1. Bands belonging to imidazolic ring, nN–H at 3090 cm − 1, nC =N at 1507 cm − 1 and n ring at 1468 cm − 1, shift to higher wavenumbers. These
Table 4 13 C NMR data of L-histidine and L-histidine-Eu3+ complex d (ppm)
L-Histidine
[Eu(L-his)3](ClO4)3
Assignment
175.2 137.5 133.4 117.9 55.9 29.4
– 135.6 129.4 118.6 54.0 27.3
C1 C6 C5 C4 C2 C3
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Fig. 1. Luminescence spectra of [Eu (L-his)3] (ClO4)3. From top to bottom: (a) emission spectrum; (b) excitation spectrum.
latter is located beside C4, whose signal changes very little after coordination. The luminescence spectra (Fig. 1) show the following features: the Eu3 + compound, excited at 393.0 nm, exhibits bands at 579.4 nm (transition 5D0 7F0), 591.6 nm and 595.0 nm (5D0 7 F1), 611.8 and 615.4 nm (5D0 7F2), 649.6 nm 5 ( Do 7F3), 628.6, 636.2 and 699.0 nm (5D0 7 F4). A low structure symmetry is evidenced by the presence of the Laporte prohibited 5D0 7F0
band and by the very intense 5D0 7F2 transition in comparison to the 5D0 7F1 band. The excitation spectrum of the same compound, measured at 611.8 nm, shows bands at 361.0 nm (7F0 5 D4), 374.4 and 379.0 nm (7F0 5GJ ), 393.0 nm 7 ( F0 5L6), 414.6 nm (7F0 5D3), 464.2 nm (7F0 5D2), 525.6 and 534.8 nm (7F0 5D1). The five first bands are located over a large band that covers the region between 300 and 440 nm, probably due to oxygen to europium(III) charge trans-
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Table 6 Experimental results and theoretical predictions of the I0–0/I0– 2, DE0–1 in cm−1 and Vl (l =2,4) (in units of 10–20 cm2)
Fig. 2. Molecular structure of the complex ion [Eu (L-his)3]3 + obtained by molecular mechanics MM + method.
fer. The excited state average lifetime measured through the emission decay is 0.26 ms, at room temperature. For comparison, aqueous Eu3 + lifetime is about 0.20 ms [21], showing a faster de-excitation in water solution, as expected. The Fig. 2 shows the minimum energy structure of the system obtained by molecular mechanics calculation. Table 5 exhibits the ligand coordinates in the framework of the Eu3 + ion and polarizabilities. As a rule, the magnitude of the polarizability is smaller, the greater in the metal-ligand distance without a defined relation Table 5 Ligand coordinates in the framework of the Eu3+ ion and their respective polarizability Ligand
˚) R(A
u (°)
f(°)
˚ 3) a(A
N1 O1 O2 N2 O3 O4 N3 O5 O6
2.543 2.557 2.527 2.537 2.527 2.529 2.541 2.556 2.545
13.659 81.342 89.798 137.519 72.085 78.259 67.375 119.445 118.016
191.073 162.418 231.621 88.816 116.12 34.496 −55.706 267.924 11.929
2.6 0.1 4.5 2.7 4.5 3.9 2.6 0.1 0.2
Parameters
Experimental
SOM
I0–0/I0–2 DE0–1 V2 V4
0.011 230 4.1 4.0
0.008 226 4.1 2.9
[22,23]. Table 6 brings the comparison between the experimental results and theoretical predictions. The latter has been developed with the polarizabilities lying on Table 5 and gN=0.1 and gO = 2/3. In the same sequence, we point out that the Mu¨lliken charges obtained by quantum mechanics AM1 method using the molecule structure obtained by our MM simulation are 0.1 and 0.6.
5. Conclusions Eu3 + complex with L-histidine has been obtained. It is a white crystalline powder, in which the metal interaction with the ligand occurs via the oxygen atoms of the carboxylate group and one of the imidazolic ring nitrogen atom. Such interactions occur in the solid state, as shown by IR spectroscopy measurements, and also in solution, as shown by the 13C-NMR spectrum in deuterium oxide. The emission spectrum, investigated with solid samples, shows that the luminescent ion occupies a low symmetry site, since the 0 0 transition is present. The perchlorate ion stays outer the coordination sphere. Good agreement between experimental results and theoretical predictions of the DE0 – 1, the I0 – 0/I0 – 2 intensity ratio and intensity parameters Vl (l=2 and 4) was achieved, using reasonable values of polarizabilities and nitrogen and oxygen charges. For both good SOM predictions and reproduction of Mu¨lliken charges, we have a strong indication that molecular mechanics simulation may provides very satisfactory structure for quite large rare earth complexes.
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Acknowledgements We thank CAPES and FAPESP (Brazilian agencies) for financial support.
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