TDDFT study of the structural and spectroscopic properties of Al(III) complexes with 4-nitrocatechol in acidic aqueous solution

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Chemical Physics 340 (2007) 273–282 www.elsevier.com/locate/chemphys

A DFT/TDDFT study of the structural and spectroscopic properties of Al(III) complexes with 4-nitrocatechol in acidic aqueous solution Jean-Paul Cornard *, Christine Lapouge, Jean-Claude Merlin LASIR, CNRS UMR 8516, Universite´ des Sciences et Technologies de Lille, Baˆt C5, 59655 Villeneuve d’Ascq Cedex, France Received 3 July 2007; accepted 7 September 2007 Available online 12 September 2007

Abstract The complexation of 4-nitrocatechol in aqueous solution at pH 5 has been studied by molecular spectroscopy combined with quantum chemical calculations. In these physico-chemical conditions, the formation of the two complexes [4ncatAl(H2O)4]+ and [(4ncat)2Al(H2O)2] has been highlighted. The electronic absorption spectra of the 1:1 and 1:2 complexes of Al(III) with 4-nitrocatechol have been computed using the time-dependent density functional theory and the polarizable continuum model. It turns out that the 6311+G(d,p) basis set provides a good agreement between experimental and theoretical absorption spectra. This good agreement has allowed the determination of the preferential conformation of the 1:2 complex in aqueous solution. A complete assignment of the UV–Vis absorption and Raman spectra of the complexes has been proposed.  2007 Elsevier B.V. All rights reserved. Keywords: 4-Nitrocatechol; Al(III) complexation; DFT; TD-DFT; Electronic spectroscopy; Raman spectroscopy

1. Introduction Natural organic matter (NOM) contains the so-called humic and fulvic acids, oligomers or polymers containing hydroxyaromatic building blocks. The large number of ionisable functional groups in humic substances, mainly carboxylic and phenolic groups, results in an appreciable ability to form stable complexes with metal cations. Because the detailed structure of NOM molecules is uncertain, much of the research explore the metal fixation with model compounds [1–4] to improve the understanding of the underlying mechanisms responsible for the complexation, mobilization or immobilization of pollutant metals by NOM. These studies have demonstrated that a number of dihydroxy-substituted aromatic species play an important role in the metal complexation [5,6]. One can note that interactions between metals and catechol (1,2-dihydroxybenzene) or other ortho-dihydroxy moieties are also being

*

Corresponding author. Tel.: +33 3 20 43 69 26; fax: +33 3 20 43 67 55. E-mail address: [email protected] (J.-P. Cornard).

0301-0104/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2007.09.010

found in an increasing number of biological systems. Studies of metal complexation by catechol have been extensively reported [7–12] and notably with aluminium(III) [13–24]. The presence of a nitro substituent on the aromatic ring enhances the acidity of the catechol function [25] and consequently increases the complexing power of the catechol group with respect to the pyrocatechol (1,2-dihydroxybenzene). In NOM, a competition between catechol and carboxylic groups exists towards metal complexation and the presence of a nitro group induces an evolution in favor of the first one. 4-Nitrocatechol (4-nitro-1,2-benzenediol) has already been used as model compound for nitrohumic acid [26]. The Al(III)–4-nitrocatechol system has already been studied by potentiometry and voltammetry [27–29], and formation constants have been reported for the 1:1, 1:2 and 1:3 (metal:ligand) complexed species. This work aims at investigating the electronic absorption and vibrational spectra of the 4-nitrocatechol–Al(III) system and giving a complete spectral assignment by using quantum chemical approaches. Time-dependent density functional theory (TD-DFT) has become the most widely

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an internal He–Ne laser source (k = 632.8 nm) for excitation and a liquid nitrogen-cooled charge-coupled device (CCD) detector. 2.2. Chemometrics methods

Fig. 1. Atomic numbering of 4ncat used in the text.

used tool for theoretically evaluating excited state energies [30] and has proved to be a very reliable method to simulate the UV–Vis spectra of organic compounds and metal complexes in solution. 4-Nitrocatechol (4ncat), illustrated in Fig. 1, has been studied in aqueous solution by keeping a constant pH 5; a slightly acidic medium close to that observed in the natural environment. The main objectives of this study are (i) to investigate the structural modifications engendered to the ligand upon complexation, and to determine the lowest energy conformation of complexes (ii) to assign the electronic absorption spectra of the complexes and to use the UV–Vis spectra to obtain structural information of the complexes and notably for the complex of 1:2 stoichiometry, and finally (iii) to give an assignment of the wavenumber shifts observed in the Raman spectrum of 4ncat upon aluminium(III) complexation. 2. Experimental and theoretical methods 2.1. Reagents, preparative methods and instrumentation Sample of 4ncat was obtained commercially from Sigma and used without further purification. Al3+ stock solutions were prepared from aluminium chloride AlCl3 Æ 6H2O. The molar ratio method has allowed the determination of the complexes composition in solution from UV–Vis spectra. In this method, the electronic spectra of solutions containing a constant concentration of 4ncat in water (8 · 105 M) and variable concentration of AlCl3 were recorded. The titrations were carried out by incremental additions of aluminium chloride and additions of NaOH to keep a constant pH 5. Sodium chloride has been added to give a constant ionic strength of 0.10 M. A Minipuls II (Gibson) peristaltic pump was used to circulate solution from the thermostated titration cell (25 C) to the flow cell (Hellma) for absorption measurements. Due to the dilution of the ligand solution, all the electronic spectra were corrected before exploitation. UV–Vis spectra were run on a double-beam spectrophotometer (Cary 100-Varian) using flow cells of 1 cm path length. Raman spectra of 4ncat and complex solutions (102 M – ionic strength:1 M) were recorded at different pH with a Jobin Yvon ‘‘LabRam’’ microspectrometer equipped with

The UV–Vis spectra were refined by using a multivariate data analysis program for modelling and fitting equilibrium titration 3D data sets (SPECFIT software – version 3.0.32) [31]. The set of spectra obtained at variable Al(III) concentration were treated by evolving factor analysis (EFA) [32,33] in order to determine the number of different species in the system and the pure absorption spectrum of each species. 2.3. Quantum chemical calculations All calculations were performed at the density functional level of theory with the hybrid functional B3LYP which employs the three parameters Becke exchange functional, B3 [34] with the Lee–Yang–Parr nonlocal correlation functional LYP [35], using the Gaussian (G03) program package [36]. Geometry optimizations were carried out, without any symmetry constraints, using different basis sets: 6-31G(d,p), 6-31+G(p), 6-311G(d,p) and 6311+G(d,p). Vibrational frequency calculations have been performed to ensure that all the optimized structures of complexed ligands correspond to energy minima. Bond orders have been computed with the Border Program (Version 1.0) [37,38] from the Gaussian molecular orbitals, and atomic charges were estimated in the NPA approach [39]. The low-lying excited states were treated within the adiabatic approximation of time dependent density functional theory (DFT-RPA) [40] with the B3LYP hybrid functional. Vertical excitation energies were computed for the first 40 singlet excited states, in order to reproduce the UV–Vis spectra of complexes. As it is well known that the UV– Vis absorption spectra are very sensitive to the solvent effects, these latter were introduced by the SCRF method, via the Polarized Continuum Model (IEF-PCM) [41,42] implemented in the Gaussian program. The calculated absorption energies and intensities were transformed with the GaussSum program into simulated spectra using Gaussian functions assuming a constant bandwidth at the half-height of 4000 cm1. This value constitutes an average width for an absorption band observed in the UV–Vis range. 3. Results and discussion 3.1. UV–Vis absorption spectra of complexed 4ncat Fig. 2 illustrates the evolution of the UV–Vis spectra of the 4ncat–Al(III) system at pH 5 for different [Al(III)]/ [4ncat] molar ratios from 0 to 1.5. The intensity of the double band (345–309 nm) characteristic of 4ncat decreases with the amount of AlCl3 added, whereas a new large band

J.-P. Cornard et al. / Chemical Physics 340 (2007) 273–282

appears in the higher wavelengths range (415 nm). Even if this band seems to be characteristic of the formation of only one complex, the absence of isosbestic point suggests that several complexed species are formed simultaneously during the complexation process. Using the molar ratio method, the curves of absorbance variations versus [Al(III)]/[4ncat] molar ratio plotted at different wavelengths (345 and 415 nm) allow us to observe the formation of two species of stoichiometry (metal:ligand) 1:2 and 1:1. These results are in good agreement with those obtained by Downard et al. [28] using potentiometric methods that reported the various complexed species observed according to the pH. From the UV–Vis spectra set, the determination of the number of different absorbing species was estimated by the EFA method. With this method, three absorption spectra corresponding to pure species have been determined. The characteristic spectra, obtained with the chemometric methods, of 4ncat, [(4ncat)2Al(H2O)2] and [4 ncatAl(H2O)4]+ are represented in Fig. 3. The spectra of these two last species are very close and their absorption maxima differ from less than 10 nm. The concentration variations of the different species versus the quantity of Al(III) added are plotted in Fig. 4. These curves show that the 1:1 and 1:2 complexes are formed simultaneously at the very beginning of the dosage. For molar ratios lower than 0.3 ([Al(III)] added <2.5 · 105 M), the 1:2 complex presents a concentration superior to that of the 1:1 complex, and all the aluminium added to the solution is consumed in the formation of these complexes. From molar ratios higher than 0.3, the 1:1 complex largely predominates, equilibriums are observed between free and complexed metal and a concentration decrease of [(4ncat)2Al(H2O)2] is observed to the advantage to the formation of the 1:1 complex. For a molar ratio of 1, less than 10% of free ligand are present in solution. The ground-state geometry of [4ncatAl(H2O)4]+ was determined by a full optimization of its structural parameters using the DFT/B3LYP theoretical method and the

Fig. 3. UV–Vis absorption spectra of the free and the two complexed forms of 4ncat extracted using the EFA method.

4ncat Concentration (M)

Fig. 2. UV–Vis absorption spectra of 4ncat in aqueous solution (pH 5) in the absence and in the presence of AlCl3. [Al(III)]/[4ncat] molar ratios vary from 0 to 1.5.

275

[4ncatAl(H2O)4]+

5x10-5

Al(III)

[(4ncat)2Al(H2O)2]0 0

.5x10-4 [Al(III)] (M)

10-4

Fig. 4. Distribution curves of the concentrations (mol L1) of free and complexed 4ncat as a function of added metal amount.

6-31G(d,p) basis set. The TD-DFT method was then used to evaluate the excitation energies and oscillator strengths of the electronic excitations of this complex in the UV– Vis region. In Fig. 5, the calculated electronic transitions are characterized by vertical lines with a height proportional to the oscillator strength and the theoretical absorption spectra have been simulated by considering a constant FWHM of 4000 cm1. Even if the general shape of the theoretical spectrum, obtained with the 6-31G(d,p) basis set, reproduces the experimental spectrum (408, 330, 262 and 218 nm), a large spectral shift is observed in the calculated excitation wavelengths (384, 309, 237 and 207 nm). If a triple n basis set (6-311G(d,p)) is used for both geometry optimization and electronic spectrum calculation, only very small changes in the calculated wavelengths are observed (385, 310, 242 and 209 nm) and the theoretical spectrum does not show a distinct improvement with respect to that calculated with the 6-31G(d,p). On the other hand, the

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Epsilon (M-1.cm-1)

104

0

0.2

6-31G(d,p)

0.1 0.05 0.2 0

6-311G(d,p) 0.15

Oscillator Strength Oscillator Strength

0.15

0.1 0.05 0.25 0

6-31+G(d,p)

0.2 0.15 0.1 0.05

0 0.25

6-311+G(d,p)

0.2 0.15 0.1 0.05 0

200

300

400

500

600

Wavelength (nm)

Fig. 5. Experimental absorption spectrum of the 1:1 complex and theoretical spectra calculated at the B3LYP/IEF-PCM level of theory with various basis sets.

addition of diffuse orbitals on heavy atoms, with the 631+G(d,p) basis set, greatly improves the electronic

description of the system insofar as the calculated spectrum (413, 329, 246 and 213 nm) is in good agreement with the experiment. Finally, with the 6-311+G(d,p) basis set, a light improvement in the simulation of the electronic spectrum of [4ncatAl(H2O)4]+ is still observed. As expected, it should be noted that the addition of diffuse functions to the basis sets has been shown to be essential for accurate description of compounds containing lone pairs and makes it possible to obtain a good simulation of the UV–Vis absorption spectrum. However, if the agreement is excellent in wavelengths, in particular for the two transitions of lowest energy, it is less satisfactory with regard to the oscillator strengths. Indeed an inversion of the absorbance of the two bands in the long wavelength range is observed between the experimental and calculated spectra. Test calculations have shown that the inclusion of extra polarisation functions affects neither the excitation energies nor the transition probability. One can note that inclusion of solvent effects in the electronic transitions calculations is absolutely essential, indeed calculations carried out in vacuum give variations in wavelengths between 20 and 40 nm compared to the results obtained with PCM. The wavelength calculated at 410 nm for [4ncatAl(H2O)4]+ could mainly be assigned to the HOMO LUMO transition (Table 1). The HOMO (Fig. 6) has a pronounced p character localized on the benzene ring and also involves non bonding orbitals on oxygen atoms of catecholate, whereas the LUMO has a antibonding p character localized on the benzene ring and especially on the nitro group, with however a bonding character on the C–N bond. The HOMO ! LUMO transition is then characterized by a charge transfer from the catecholate to the nitro group, which little differs from that observed in the free ligand [6]. The HOMO  1 ! LUMO, calculated at 327 nm, also presents a p ! p* character with a charge transfer from the benzene ring to the nitro group. The absorption band observed at 262 nm is less better simulated than the two preceding ones, since it is calculated at 249 nm, and can be also assigned to a p ! p* transition involving the benzene ring. Finally, the calculated wavelength 214 nm is in good agreement with the shoulder observed in the experimental spectrum in the low wavelengths, at 218 nm, and corresponds to different monoexci-

Table 1 Experimental and computed absorption wavelengths of [4ncatAl(H2O)4]+ and of the b structure of [(4ncat)2Al(H2O)2](B3LYP/6-311 + G(d,p)/IEFPCM calculations) [(4ncat)2Al(H2O)2]

[4ncatAl(H2O)4]+ kexptl (nm)

kcalc (nm)

f

MO contribution (%)

kexptl (nm)

kcalc (nm)

f

MO contribution (%)

408 330 262 218

410 327 249 214

0.14 0.19 0.16 0.10

HOMO ! LUMO (82) HOMO  1 ! LUMO (85) HOMO ! LUMO+2 (80) HOMO ! LUMO + 6 (53) HOMO  1 ! LUMO + 2 (21)

420

264

434 419 338 335 253

0.17 0.13 0.38 0.13 0.30

218

215

0.13

HOMO ! LUMO + 1 (79) HOMO  1 ! LUMO (80) HOMO  2 ! LUMO + 1 (72) HOMO  3 ! LUMO (71) HOMO ! LUMO + 4 (38), HOMO ! LUMO + 2 (33) HOMO ! LUMO + 7 (37), HOMO ! LUMO + 9 (19)

336

f is relative to the calculated oscillator strength and the molecular orbitals involved in each transition are given with their contribution.

J.-P. Cornard et al. / Chemical Physics 340 (2007) 273–282

277

Fig. 6. Representation of the main frontier orbitals involved in the lowest energy transitions of [4ncatAl(H2O)4]+ and [(4ncat)2Al(H2O)2].

tations. It can be noticed that the n orbitals corresponding to the nitrogen lone pairs (mainly the HOMO  2 and HOMO  3) do not participate in the optical transitions, they are involved in transitions calculated at 306 and 273 nm with an oscillator strength near zero. With regard to [(4ncat)2Al(H2O)2], two structures can be considered (a and b structures) according to the position of the ligands and water molecules on aluminium ion (Fig. 7). In the structure a, the two ligands are calculated fully coplanar, whereas in the structure b, the two ligand planes are not totally perpendicular. The theoretical

Fig. 7. Conformation of the two possible structures of [(4ncat)2Al(H2O)2].

absorption spectrum of [(4ncat)2Al(H2O)2] has been calculated from these two optimized structures. As previously shown, the use of the 6-311+G(d,p) basis set gives the best simulation of the experimental spectrum. Contrary to what was observed for [4ncatAl(H2O)4]+, the simulation of the absorption bands of [(4ncat)2Al(H2O)2] reveals the implication of a great number of transitions as illustrated in Fig. 8. The calculated spectra from the structures a and b (Fig. 8b and c, respectively) exhibit significant differences in wavelengths, notably for the absorption band observed in the long wavelengths. In both cases, four electronic transitions have been calculated to describe the band with a kmax of 420 nm. The theoretical band obtained by convoluting four Gaussian functions (with a FWHM of 4000 cm1) relative to the four calculated transitions presents a maximum at 461 and 430 nm for the structures a and b, respectively. Undoubtedly, the results obtained starting from the b structure makes it possible to better simulate the experimental absorption band. In a same way, the band located at 336 nm in the experimental spectrum is better calculated for the b structure (338 nm) than for the a structure (345 nm), even if the difference is less marked. The two other bands observed under 300 nm are relatively well reproduced by the calculations whatever the starting structure. The electronic spectrum simulation of [4ncatAl(H2O)4]+ being very good, one can imagine to be able to reproduce, under the same computational conditions, a comparable result in quality for [(4ncat)2Al(H2O)2], and consequently it is reasonable to think that the b structure is more probable than the a one. For

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J.-P. Cornard et al. / Chemical Physics 340 (2007) 273–282

Epsilon (M-1.cm-1)

16000

A

14000

HOMO  2 and HOMO  3. In a same manner, these MOs are quasi-degenerated and obtained by in-phase and out-of-phase linear combinations of the HOMO  1 of [4ncatAl(H2O)4]+. The assignment of the two optical absorption in the low wavelengths is more complex insofar as they result from of a great number of calculated transitions.

a

12000 10000 104 8000 6000 4000 2000 00 0

0.5

B

b 0.25

c

0 0.4

C

3.2. Structural analysis of 4ncat complexes In order to determine the structural changes of the ligand that occur with the chelation of Al(III), the geometries of the two complexes are reported in Table 2. The structural parameters of 4ncat have already been reported at the 6-31G(d,p) level of theory [6] but to make an accurate comparison, these latter were recomputed under the same conditions by increasing the size of the basis set and notably by adding diffuse orbitals. The formation of [4ncatAl(H2O)4]+ induced marked modifications of the ligand structure at the chelation site level.

0.2

0

200

300 400 500 Wavelength (nm)

600

Fig. 8. Experimental absorption spectrum of the 1:2 complex (A) and theoretical spectra calculated at the B3LYP/6-311+G(d,p)/IEF-PCM level of theory for the conformations a (spectrum B) and b (spectrum C).

this reason, the assignment of the UV–Vis spectrum of the 1:2 complex is reported in Table 1 only for the b structure, where only the transitions whose oscillator strength is higher than 0.1 are indicated. The optical transitions observed in the high wavelengths involved the frontier orbitals: HOMO, HOMO  1, LUMO and LUMO + 1. As one can see on Fig. 6, the electronic distribution on each ligand of the HOMO  1 and HOMO orbitals of [(4ncat)2Al(H2O)2] is identical to that calculated for the HOMO of [4ncatAl(H2O)4]+. The HOMO  1 and HOMO of the 1:2 complex correspond, respectively, to in-phase and out-of-phase linear combinations of the HOMO of the 1:1 complex. The HOMO  1 and HOMO orbitals are quasi-degenerated since their respective energy is calculated very near, and present pronounced p character localized on the benzene ring. The same observations can be made for the LUMO and LUMO + 1 orbitals which are obtained from in-phase and out-of-phase linear combinations of the LUMO of the 1:1 complex. These MOs present a p* character localized on the nitro group. The fact that the two highest occupied MOs and that the two lowest unoccupied MOs are quasi-degenerated explains the presence of the four electronic transitions (where these MOs are preponderant) to describe the absorption band observed in the high wavelengths. The absorption band recorded at 336 nm is described by two very close calculated transitions (338 and 335 nm) which involve the

Table 2 ˚ ), main bond angles and dihedral Structural parameters, bond lengths (A angles () of 4ncat, [4ncatAl(H2O)4]+ and [(4ncat)2Al(H2O)2] (b structure) 4ncat C1C2 C2C3 C3C4 C4C5 C5C6 C6C1 C1N7 N7O8 N7O9 C3O10 C4O11 C5H12 C6H13 C2H14 O10H15 O11H16 O10Al O11Al C1C2C3 C2C3C4 C3C4C5 C4C5C6 C5C6C1 C6C1C2 C6C1N7 C1N7O8 C1N7O9 C2C3O10 C3C4O11 C4C5H12 C5C6H13 C1C2H14 C3O10H15 C4O11H16 C3O10Al C4O11Al

1.397 1.389 1.409 1.394 1.394 1.394 1.468 1.233 1.234 1.360 1.370 1.087 1.082 1.082 0.969 0.966

118.7 119.6 120.7 120.1 118.4 122.5 118.9 117.9 117.8 119.6 114.8 119.9 121.5 120.9 109.2 111.3

1:1 complex

1:2 complex

1.398 1.385 1.420 1.392 1.396 1.392 1.471 1.231 1.229 1.373 1.375 1.085 1.082 1.082

1.410 1.389 1.436 1.400 1.395 1.400 1.447 1.242 1.245 1.335 1.335 1.086 1.082 1.082

1.781 1.796 117.7 120.4 120.6 119.3 119.1 122.8 118.9 117.4 117.6 123.9 115.7 119.7 121.6 120.4

1.821 1.864 118.8 119.4 120.5 120 119.1 122.2 118.8 118.6 118.8 124.9 115.1 119.2 121.4 120.6

107.6 107.4

110.6 109.6

J.-P. Cornard et al. / Chemical Physics 340 (2007) 273–282

The two CO bond lengths are almost identical in the complex and longer than in the free ligand; in the same way an important increase of the C3C4 bond length is calculated. An opening of the C2C3O10 angle (more than 4) is also observed. Undoubtedly, the five-membered ring formed with Al(III) in the complex differs from this obtained with the intramolecular hydrogen bond in the free ligand. The two AlO bond lengths slightly differ and show that this five-membered ring is not completely symmetrical. The complexation also affects the nitro group, indeed the CN bond length increases while the two NO bond lengths decrease. These modifications are more sensitive than those generated on the CC bonds of the benzene ring (except for C3C4). Concerning the [(4ncat)2Al(H2O)2] complex, the structural modifications are more important than those calculated for the 1:1 complex. The C3C4 bond is calculated even longer than in the 1:1 complex, in opposition, the two CO bond lengths are considerably reduced and calculated shorter than in the free ligand. In a same way, the CN bond is much more affected than in the 1:1 complex, and presents a strong shortening in [(4ncat)2Al(H2O)2]. At the contrary and in a lesser extent, the NO bond lengths increase. The calculated bond orders, reported in Table 3, are in good agreement with the bond length changes observed between the free ligand and the 1:1 and 1:2 complexes. Table 4 displays the atomic charges calculated for the free and complexed ligands. Even if the total charge of the two complexes differs, the same charge distribution is calculated for [4ncatAl(H2O)4]+ and [(4ncat)2Al(H2O)2]. Indeed the Al atom of each complex presents a charge of ca. +2 whereas the ligand is charged ca. 1.5. The main modifications of atomic charges that occur upon complexation are localized on the O atoms of catechol

Table 3 Bond orders calculated for 4ncat, [4ncatAl(H2O)4]+ and [(4ncat)2Al(H2O)2] (b structure) C1C2 C2C3 C3C4 C4C5 C5C6 C6C1 C1N7 N7O8 N7O9 C3O10 C4O11 C5H12 C6H13 C2H14 O10H15 O11H16 H15—O11 O10Al O11Al

4ncat

1:1 complex

1:2 complex

1.36 1.42 1.27 1.35 1.42 1.4 0.8 1.53 1.53 1.14 1.07 0.95 0.94 0.94 0.93 0.93 0.04

1.36 1.45 1.25 1.42 1.42 1.4 0.8 1.53 1.53 1 1.03 0.96 0.94 0.94

1.29 1.41 1.19 1.4 1.4 1.38 0.83 1.44 1.43 0.94 0.96 0.94 0.92 0.92

0.8 0.79

0.77 0.65

279

Table 4 Calculated NPA charges of 4ncat, [4ncatAl(H2O)4]+ and [(4ncat)2Al(H2O)2] (b structure) C1 C2 C3 C4 C5 C6 N7 O8 O9 O10 O11 H12 H13 H14 H15 H16 Al

4ncat

1:1 complex

1:2 complex

0.07 0.22 0.28 0.28 0.26 0.19 0.48 0.38 0.38 0.66 0.69 0.21 0.24 0.26 0.49 0.49

0.07 0.23 0.24 0.25 0.24 0.19 0.48 0.38 0.37 0.94 0.93 0.23 0.25 0.26

0.04 0.26 0.29 0.32 0.28 0.21 0.47 0.42 0.44 0.86 0.89 0.2 0.23 0.24

1.91

1.98

group, and in a lesser extent on the O atoms of the nitro group for the 1:2 complex. 3.3. Raman spectrum of 4ncat complex Fig. 9 displays the Raman spectra of free 4ncat at pH 2 and of its aluminium complex (for a molar ratio of 1) for various pH from 2 to 5, in aqueous solution. The spectrum of 4ncat at pH 2 presents the same lines as those observed in the spectrum recorded in acidic conditions and already reported with a complete assignment [6]. The two bands observed at 1596 and 1341 cm1 are attributed to the asymmetric and symmetric NO stretching modes, respectively, whereas the 822 cm1 line corresponds to the in-plane NO2 bending mode. The other bands in the spectrum are assigned to vibrational modes of the benzene ring coupled with CO stretchings and OH bendings of the hydroxyl groups. By addition of Al(III), at pH 2, the complex formation is very low and the spectrum b of Fig. 9 mainly corresponds to the signals of the free ligand. However, one can observe some new bands of very low intensity (1320, 810 cm1) which can be attributed to the presence, in small quantities, of complexed ligand. As the pH is increased, the spectrum intensity of the complex increases with the disappearance of the free ligand spectrum. At pH 5 (Fig. 9e) the spectrum of the 4ncat has completely disappeared, and the observed bands are characteristic of the complexed form. This result is in agreement with the evolution of the concentration curves (Fig. 4) which shows that at pH 5, for a molar ratio of 1, it remains less than 10% of free ligand in solution; and the largely predominant species is [4ncatAl(H2O)4]+. Table 5 reports the experimental and the calculated vibrational wavenumbers obtained at the B3LYP/6311+G(d,p) level of theory of the 1:1 complex. The vibrational frequencies of the nitro group are noticeably modi-

J.-P. Cornard et al. / Chemical Physics 340 (2007) 273–282

800

1596

1199

1084

a

1595

b

1200

1577

1577 1595 1577 1595

1595

1501 1501 1501

1427 1427 1427

1230 1230

1000

1230

1084

1124

1084 1124

1084

952 952 952

1124

1286 1320

1501

1341

1230

1124

952

1084

1292 1320

952

791

822

790 810 822 812 823

788 812 823

788

790 810 823

Raman arbitrary units

1295

1341

280

1400

1600

c

d

e

1800

Wavenumbers cm-1 Fig. 9. Raman spectrum of aqueous solution of 4ncat at pH 2 (a) and of Al(III)–4ncat system for a [Al(III)]/[4ncat] molar ratio of 1 at pH 2 (b), pH 2.5 (c), pH 3.5 (d) and pH 5 (e).

Table 5 Experimental and calculated wavenumbers (cm1) of [4ncatAl(H2O)4]+ Exp.

Calc.

Assignment

813 829 954 1084 1127 1233 1286 1320 1427 1577

817 823 936 1057 1112 1234 1248 1339 1427 1591

d(NO2) + 12 d(NO2) + 12 17b 18b + m(CN) 18a 3 + ms(CO) 7a ms(NO2) 19a mas(NO2) + 8b

The Wilson’s notation has been adopted to describe the vibrational modes of benzene ring.

fied with the chelation of Al(III) on the catechol function. In the complex, the asymmetric and symmetric NO2 stretching modes downshift of 20 cm1 in good accordance

with the decrease of the calculated NO bond lengths. The line at 812 cm1 and the shoulder observed at 823 cm1 are assigned to complex modes involving the NO2 in-plane bending, the mode 12 of the benzene ring (according to Wilson’s notation) [43,44] and the O–Al symmetric stretching mode (Fig. 10). The CO stretching modes which were not coupled in the free ligand because of relatively different CO bond lengths, are coupled in the complexes and the CO symmetric mode is observed at 1230 cm1 (calculated at 1234 cm1) mixed with ring mode 3. The CN stretching vibration coupled to the mode 18b does not move with the complexation, indeed the line at 1084 cm1 is observed in the spectrum of free and complexed ligand. The frequencies of the ring modes corresponding to the other bands observed in the spectrum are slightly modified; however these modifications are assigned to mechanical coupling modifications between vibrators. Indeed, for 4ncat, these modes are mixed with in-plane

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281

Fig. 10. Representation of the vibrational normal modes involving the NO2 bending coupled to the 12 mode of the benzene ring observed at 812 (a) and 823 (b) cm1 in the Raman spectrum of [4 ncatAl(H2O)4]+.

bendings of hydroxyl groups; these couplings disappear because of the double deprotonation which occurs with the complex formation. 4. Conclusion We have examined the complexation of Al(III) by the catecholate function of 4ncat in acidic conditions by electronic absorption spectroscopy. Chemometric methods applied to the data set made it possible to characterize the absorption spectra of two complexed species presenting the 1:1 and 1:2 stoichiometry: [4ncatAl(H2O)4]+ and [(4ncat)2Al(H2O)2], respectively. It has been shown that TDDFT method in conjunction with moderate size basis set provides an accurate tool for the determination of the lowest singlet excitation energies of aluminium(III) complexes of 4ncat. Throughout the paper we have compared the performance of the B3LYP functional with different basis sets: 6-31G(d,p), 6311G(d,p), 6-31+G(d,p), 6-311+G(d,p) and have shown that the B3LYP/6-311+G(d,p) method is suitable for the simulation of the 1:1 complex absorption spectrum. This same method was then used in the determination of the conformation of the 1:2 complex. Indeed the UV–Vis spectrum of the two possible conformations of this complex has been calculated and compared to the experimental one; the accuracy of the methodology has allowed unambiguously the determination of the most probable conformation. The frequency shifts observed in the Raman spectrum of 4ncat when the complexation occurs reflect on one hand the structural modifications of the ligand and on the other hand the changes in the couplings between vibrational normal modes. Acknowledgements ‘‘Institut du De´veloppement et des Ressources en Informatique Scientifique’’ (IDRIS – Orsay, France) is thankfully acknowledged for the CPU time allocation. The authors also thank the Lille University Computational Center (C.R.I.). References [1] E. Giannakopoulos, K.C. Christoforidis, A. Tsipis, M. Jerzykiewicz, Y. Deligiannakis, J. Phys. Chem. A 109 (2005) 2223.

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