The geometry of the trianionic (23-) ligand form was fur-ther optimized by density functional theory with Becke’s three-parameter hybrid method and the correlation functional of Lee, Yang and Parr (B3LYP) with 6–31G(d) basis set [18Becke, A.D. Density functional thermochemistry. III. The role of the exact exchange. J. Chem. Phys., 1993, 98(7), 5648-5652.], without constraints. The atomic charges were calculat-ed using the Natural Population Analysis (NPA) scheme [19Kostova, I.; Trendafilova, N.; Momekovc, G. Theoretical, spectral characterization and antineoplastic activity of new lanthanide complexes. J. Trace Elem. Med. Biol., 2008, 22, 100-111. [and refer-ences therein.].]. All calculations were performed with Gaussian09 program package [20Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.P.; Izmaylov, A.F.; Bloino, J.; Zheng, G.; Sonnenberg, J.L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J.A.; Peralta, J.E.; Ogliaro, F.; Bearpark, M.; Heyd, J.J.; Brothers, E.; Kudin, K.N.; Staroverov, V.N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J.C.; Iyengar, S.S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J.M.; Klene, M.; Knox, J.E.; Cross, J.B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.E.; Yazyev, O.; Austin, A.J.; Cammi, R.; Pomelli, C.; Ochterski, J.W.; Martin, K.; Morokuma, V.G.; Zakrzewski, G.A.; Voth, S.P.; Dannenberg, J.J.; Dapprich, S.; Dan-iels, A.D.; Farkas, O.; Foresman, J.B.; Ortiz, J.V.; Cioslowski, J.; Fox, D.J. Gaussian09. Wallingford, CT: Gaussian Inc.; 2009. p. A.2; Gaussian 09, Revision A.02.].

It should be noted that the formation of compound 2 led to the generation of two stereogenic nitrogens (N2 and N4). Single-bonded nitrogen is pyramidal in shape, with the nonbonding electron pair pointing to the unoccupied corner of a tetrahedral region. Since each nitrogen in these compounds is bonded to three different groups, its configuration is chiral, and, due to the presence of the stereogenic centers in compounds 2 and 4, a mixture of diastereomers will result.

In the absence of the X-ray crystal structure, the ground state geometries of Gd-2 or [Gd (2)(H₂O)] were obtained by molecular mechanics methods, and all the potential starting arrangements of the donor atoms around the Gd (III) metal ion for ligand 2 were considered. For molecular mechanics calculations, the MM+ force field for both complexes was used [21 http://www.sparkle.pro.br/tutorial/drawing-complexes (Accessed February 02, 2018).].

The geometries of the complexes were calculated by us-ing the Sparkle/PM6 model implemented in the MOPAC 2012 package [22(a) Stewart, J.P. 2018. MOPAC2012, Stewart Computational Chemistry, Version 15.301W web: HTTP://OpenMOPAC.net (Accessed Feb-ruary 02, 2018)(b) Maia, J.D.C.; Carvalho, G.A.U.; Mangueira, C.P., Jr; Santana, S.R.; Cabral, L.A.F.; Rocha, G.B. GPU linear alge-bra libraries and GPGPU programming for accelerating MOPAC semiempirical quantum chemistry calculations. J. Chem. Theory Comput., 2012, 8(9), 3072-3081.]. The MOPAC keywords used in all Sparkle/PM6 calculations were: GNORM = 0.01, SCFCRT = 1.D-10 (to increase the SCF convergence criterion), PRE-CISE (the criteria to end all electronic and geometric optimi-zations are to be increased by a factor, normally 100, using this keyword), EF and XYZ (the geometric optimizations were performed in Cartesian coordinates). To establish whether the structures obtained were consistent with a mini-mum of energy, the force constants of the vibrational modes were analyzed on the output*.arc files and using the key-words: PM6, SPARKLE, XYZ, AUX (output auxiliary in-formation for use by other programs, for example: GABEDIT 2.4.8 [23Allouche, A.R. Computational Chemistry Softwares. J. Comput. Chem., 2011, 32, 174-182.]), and FORCE (force-calculation is to be run). The vibrational frequencies, measured by infrared, are proportional to the square root of the force constants. So, when one of the vibrational force constants is negative, the corresponding vibrational frequency is imaginary. A valid optimized geometry must have all vibrational force constants positive (in which case one can safely say that the geometry is sitting at a true minimum at the potential energy hypersur-face), CHARGE = 0, Singlet (ground state of multiplicity) and LET (which means that the supplied geometry is to be used, even if the gradients are large).

The deprotonated form of the ligand was conformational-ly analyzed in the gas phase by using the molecular mechan-ics method MM+. The relative energy (stability) orders ob-tained showed some differences but five conformers always showed comparatively lower energy and were thus selected for the calculations at the higher level of theory (Fig. 4). To simplify the visualization and analysis, structures are pre-sented without the corresponding hydrogen atoms.

It is worth noting that the computational analyses were not performed for the neutral form of the ligand in either solvent or gas phase. The reason for this decision was based on the fact that the objective of these analyses is to establish the O atoms that would present a greater tendency to participate in the coordination of Gd. It is for this reason that, in the calculations made, the water was assumed to be solvent because of its dielectric constant and because, given the low concentrations of the complex used, it is the medium that would present greater similarity to the real reaction conditions.

Among the many factors that influence the formation or not of a chemical, two of the most important are the steric and electronic effects. To theoretically estimate the incidence of the latter, as mentioned above, the value of the charges was analyzed by the NPA method, which has been developed to calculate the atomic charges and orbital populations of molecular wave functions in general atomic orbital basis sets. This method is an alternative to conventional Mulliken population analysis and seems to exhibit improved numerical stability and to better describe the electron distribution in compounds of high ionic character [19Kostova, I.; Trendafilova, N.; Momekovc, G. Theoretical, spectral characterization and antineoplastic activity of new lanthanide complexes. J. Trace Elem. Med. Biol., 2008, 22, 100-111. [and refer-ences therein.].], such as the conformers of ligand 2 in its trianionic form.

Fig. (4) Geometries of compound 2 3- , obtained by search confor-mational at MM+ level of the theory.

Table 1 shows the values of NPA (qNPA) for the atoms defined as it is required to be observed in Fig. (4).

Table 1
Atomic charges (q, a.u.) at selected atoms in the neutral and deprotonated forms of the ligand 2.

A rapid analysis led to the conclusion that atom O43 had a greater tendency to interact with the metal center. However, this is only if the electronic factor is taken into account and we do not know the steric hindrance. This is why we decided to study all possible structures of the complex in which oxygen atoms participate [Gd(2)(H₂O)] or not [Gd(2)].

There is no unified criterion on how macrocycle 1 coor-dinates a given metal center (Mⁿ⁺). Some authors indicate that the OH group is involved in the coordination sphere where Mⁿ⁺ = Ni²⁺, which was checked by the for obtained structure diffraction X-ray. However, Janda et al. represented the same family of complexes with a structure where the OH group was not coordinated, and this information was based on infrared spectroscopy data [28Berg, T.; Vandersteen, A. M.; Janda, K.D. High-throughput synthesis and direct screening for the discovery of novel hydrolytic metal , 1998, 8(10), 1221-1224. ].

In this same sense, these authors observed that when the metal center is Ni²⁺ and the proton of the alcohol group pre-sent in macrocycle 1 is replaced by a substituent R (R = Me, Bn, β-Naphthyl), the rate of ester hydrolysis increases as the size of R present in the ligand increases. The authors indicat-ed that this would be due to the steric hindrance exerted by the substituent on the probability that the oxygen is coordi-nated to the metallic center.

Therefore, the interactions between the cation and the carbonyl oxygen atoms of the carboxylic acid groups of lig-and 2, will be more similar to the interactions proposed by Yan et al. [29Liu, J-L.; Yan, B. Lanthanide-centered organicin organic hybrids through a functionalized. Aza-Crown Ether Bridge: Coordination bonding Assembly, microstructure and multicolor luminescence. Dalton Trans., 2011, 40(9), 1961-1968.]. They synthesized derivatives ligand 1 where-in the N atoms have organic/inorganic hybrid substituents and whose complex derivatives with M = Eu³⁺, Tb³⁺, Nd³⁺, are represented by a possible structure where the authors proposed a coordination mode of one Ln³⁺ to two crown ether moieties. In this case, each N-substituted chelating group (–C=O) is unable to complex the same Ln³⁺ (which is located in the ring of the crown ether molecule) because it is not in an appropriate geometry.

The coordination will thus take place in the inter-molecular mode but not in the intramolecular one [29Liu, J-L.; Yan, B. Lanthanide-centered organicin organic hybrids through a functionalized. Aza-Crown Ether Bridge: Coordination bonding Assembly, microstructure and multicolor luminescence. Dalton Trans., 2011, 40(9), 1961-1968.]. Ac-cording to the proposed structure, the OH group would not be coordinated to the metal center.

Subsequently, Janda et al. [11Berg, T.; Simeonov, A.; Janda, K.D. A combined parallel synthesis and screening of macrocyclic lanthanide complexes for the cleavage of phospho di- and triesters and double-stranded DNA. Comb. Chem., 1999, 1(1), 96-100.] carried out a similar study, but using phospho di- and triesters and double-stranded DNA as substrates, and using different Ln³⁺ instead of the aforementioned cations. The authors reached conclu-sions similar to those of the previous work and provided similar explanations related to the way in which the metallic center is coordinated, depending on the identity of substituent R. Finally, the authors indicated that they would do addi-tional work to study how the metal centers were linked to the different ligands [11Berg, T.; Simeonov, A.; Janda, K.D. A combined parallel synthesis and screening of macrocyclic lanthanide complexes for the cleavage of phospho di- and triesters and double-stranded DNA. Comb. Chem., 1999, 1(1), 96-100., 28Berg, T.; Vandersteen, A. M.; Janda, K.D. High-throughput synthesis and direct screening for the discovery of novel hydrolytic metal , 1998, 8(10), 1221-1224. ]. However, after a detailed biblio-graphic search, we found no further studies on the way in which the Ln⁺³ cations interacts with the ligand. A feature worth noting is the absence of the acetate groups in the structure of ligand 1 compared to that described for compound 2. This latter characteristic would generate that the structure of the complexes with ligand 1 has fewer restrictions since the groups with negative net electric charge (X^-1) that confirm the sphere of coordination of the metallic center are not covalently united.

Based on the experimental evidence obtained in the pre-sent study by ESI-HRMS, we can conclude that this last mode of coordination does not explain in finished form since, in addition to the obvious structural differences, the Gd/compound 2 ratio in the Gd-2 complex would be 1:1 (Fig. 5).

Fig. (5) Region of ion molecular of complex [Gd(2)] (ESI-HRMS, negative mode), with characteristic isotopic distribution.

As mentioned above, the complex obtained could not be recrystallized in such a way as to be able to obtain its structure by x-ray. This led us to obtain the ground state geometries by molecular mechanic methods. All the potential starting arrangements of the donor atoms around the Gd (III) metal ion for ligand 2^3-, q = 9 and a variable number of coordinated water molecules nH₂O where n varies from 0 to 3 were considered. Possible donor atoms were defined according to the atomic charges calculated using the NPA scheme [19Kostova, I.; Trendafilova, N.; Momekovc, G. Theoretical, spectral characterization and antineoplastic activity of new lanthanide complexes. J. Trace Elem. Med. Biol., 2008, 22, 100-111. [and refer-ences therein.].] and the COSMO model (conductor-like screening solvation model) as implemented in Gaussian09 [28Berg, T.; Vandersteen, A. M.; Janda, K.D. High-throughput synthesis and direct screening for the discovery of novel hydrolytic metal , 1998, 8(10), 1221-1224. ], considering water as a solvent.

The resulting structures were then refined using quantum chemical semiempirical methods, giving rise to a set of low-energy structures.

For molecular mechanics calculations, an extension of the MM+ force field for Gd (III) complexes was used [21 http://www.sparkle.pro.br/tutorial/drawing-complexes (Accessed February 02, 2018).]. Semiempirical molecular orbital calculations use the Spar-kle/PM6 method [22(a) Stewart, J.P. 2018. MOPAC2012, Stewart Computational Chemistry, Version 15.301W web: HTTP://OpenMOPAC.net (Accessed Feb-ruary 02, 2018)(b) Maia, J.D.C.; Carvalho, G.A.U.; Mangueira, C.P., Jr; Santana, S.R.; Cabral, L.A.F.; Rocha, G.B. GPU linear alge-bra libraries and GPGPU programming for accelerating MOPAC semiempirical quantum chemistry calculations. J. Chem. Theory Comput., 2012, 8(9), 3072-3081.], a computational chemistry model faster than ab initio/ECP calculations [30Dolg, M.; Stoll, H.; Savin, A.; Preuss, H. Energy-adjusted pseudo- potentials for the rare-earth elements. Theor. Chim. Acta, 1989, 75(3), 173-194.], with a comparable accuracy for ligands with directly coordinating nitrogen and/or oxygen atoms [31Daniel, C.R.A.; Rodrigues, N.M.; da Costa, N.B. Are quantum chemistry semiempirical methods effective to predict solid state structure and adsorption in metal organic frameworks? J. Phys. Chem. C, 2015, 19(41), 23398-23406.]. It is noteworthy that regardless of whether the structure of the complex responds to the form [Gd(2)] or [Gd(2)(H₂O)], the ligand has three independent components of chirality, N-C-C-N torsion angle of the chelate ring, the helicity of the side arms. Depending on the sign of the N-C-C-N torsion angle, the conformation of each ethylene group in the macrocyclic ring can be either left-handed, designated as λ (negative N-C-C-N torsion angle), or right-handed, designated as δ (positive N-C-C-N torsion angle), the orientation of the pendant arms can be either clockwise, (positive N-C-C-O torsion angle), or counter-clockwise, Λ (negative N-C-C-O torsion angle). The third independent component of chirality is generated by the ori-entation of the side arms (SAs) (-CH2COO-) respect to the surface which contains the Gd nucleus, and whose vertexes were defined by the nuclei O38-39 and N2-4. The description of this property will be represented by the following nomenclature: if two arms joined to two contiguous nuclei of N are on the same side of the surface described, they will be called syn(SAsNn,Nn+1), whereas if they are oriented on oppo-site sides, they will be called anti(SAsNn,Nn+1). Fig. (6) shows the two minimal energy geometries of complexes [Gd(2)] and [Gd(2)(H₂O)], calculated with PM6/Sparkle and imple-mented in the program MOPAC2012.

Fig. (6) Minimal energy geometries of complexes [Gd(2)] and [Gd(2) (H2O)], calculated with PM6/Sparkle.

Also, the fact that the ninth position (q = 1) in the latter complex is occupied by the oxygen of a water molecule (O73) would make the complex more stable due to a de-crease in the effect of electronic repulsion and steric hin-drance (which would be comparatively higher in the case of the first complex), wherein the ninth coordinating position (q=0) of the Gd (III) is occupied by the oxygen nucleus O43 corresponding to the hydroxyl group located at C40.

“Points on a Sphere” repulsion calculations incisively identify the geometry most favorable polytopal form for an ML9 coordination complex, which will be discussed later [32Guggenberger, L.J.; Muetterties, E.L. Reaction path Analysis. 2. The nine-atom family. J. Am. Chem. Soc., 1976, 98(3), 7221-7225.].

The IR spectrum of each optimized structure was calcu-lated by PM6/Sparkle and AM1/Sparkle models implemented in MOPAC, and Hartree-Fock method. The best results were obtained with the latter method using the with 6-31G basis set and, we applied a quasi-relativistic ECP as de-scribed by Dolg et al. for the metal atom [30Dolg, M.; Stoll, H.; Savin, A.; Preuss, H. Energy-adjusted pseudo- potentials for the rare-earth elements. Theor. Chim. Acta, 1989, 75(3), 173-194.].

The signals on which special attention was given were those that would allow proposing which are the oxygen at-oms corresponding to the Ar–O–CH2 vibrations and ν(C-O) stretching of the alcohol groups [11Berg, T.; Simeonov, A.; Janda, K.D. A combined parallel synthesis and screening of macrocyclic lanthanide complexes for the cleavage of phospho di- and triesters and double-stranded DNA. Comb. Chem., 1999, 1(1), 96-100., 28Berg, T.; Vandersteen, A. M.; Janda, K.D. High-throughput synthesis and direct screening for the discovery of novel hydrolytic metal , 1998, 8(10), 1221-1224. , 29, 33(a) Eisenstein, O.; Maron, L.J. DFT Studies of some structures and reactions of lanthanides complexes. Organomet. Chem., 2002, 647(1-2), 190-197.(b) Cao, X.; Dolg, M. Density functional studies on lanthanide (III) texaphyrins (Ln-Tex²⁺), Ln = La, Gd, Lu): Structure, stability and electronic excitation spectrum. Mol. Phys., 2009, 101(15), 2427-2435.(c) Cosentino, U.; Villa, A.; Pitea, D.; Moro, G.; Barone, V.; Maiocchi, V. Conformational characterization of lanthanide(III)-DOTA complexes by ab Initio investigation in vacuo and in aqueous solution. J. Am. Chem. Soc., 2002, 124(17), 4901-4909.(d) Perrin, L.; Maron, L.; Eisenstein, O. Modelling Me5C5 for reactivity studies in (Eta(5)-C5Me5)(2)Ln-R: full DFT and QM/MM approaches. New J. Chem., 2004, 28(10), 1255-1259.(e) Pérez-Mayoral, E.; Soriano, E.; Cerdán, S.; Ballesteros, P. Experimental and theoretical study of lanthanide complexes based on linear and macrocyclic polyaminopolycarboxylic acids containing pyrazolylethyl arms. Molecules, 2006, 11(5), 345-356.]. In this sense, the area of the ν(C = O) vibration modes was also ana-lyzed to understand the most probable geometry for the for-mation of the -COO-Gd bonds (Fig. 7a).

The experimental spectra did not allow verifying the existence or not of the interaction of the tertiary amino groups with Gd, because the area in the spectrum in which the signals corresponding to the stretching vibrations of the bonds C-N (1210-1150 cm), N not coordinated) [34Coates, J. Interpretation of infrared spectra, A practical approach. Encyclopedia of Analytical Chemistry, 2006, 1.] were observed was similar to that in which the signals of the stretches of groups such as Ar-O-C- and -C-OH were observed. In addition, it should not be forgotten that the comparative analysis of the infrared spectra performed was based on the spectrum of the ligand in its salt form Fig. (7b). Considering that the amino groups may be some of those having the greatest interaction with the trifluoroacetate counter ion, precautions should be taken when comparing the observed variations of the functional groups before the formation reaction of the coordination compound, and the spectrum obtained therefrom the latest. However, the analysis of the modes of vibration calculated by computational tools indicates that the tertiary amino groups are coordinating a Gd (III) (Fig. 7b).

Fig. (7a) Comparison of simulated (upper) versus experimental (bottom) FT-IR for [Gd(2)(H2O)].

Fig. (7b) FTIR spectra of compound ([2Arnaud, F-X.; Hissene, A.; Métivier, D.; Dutasta, F.; Berets, O.; N’Guema, B.; A’Teriitéhau, C.; Baccialone, J.; Potet, J. Gadolinium enhancement in brain magnetic resonance imaging in progressive multifocal leukoencephalopathy after natalizumab monotherapy: Is it really atypical? J. Neuroradiol., 2012, 39(4), 267-270.-F3CCOOH]•H2O) (upper) and complex and complex [Gd(2)(H2O)n] (bottom).

After checking the stoichiometry of complex formed by ligand 2 and Gd (III) by mass spectrometry and being able to propose the most probable structure from computational cal-culations and the comparison of the experimental infrared spectra with those obtained for the different geometries calculated from the theoretical tool mentioned above, we decided to perform a comparative analysis of different geometric parameters of the proposed structure with those reported for different complexes of different lanthanides whose CN = 9. The analysis of x-ray data allowed concluding that the observed geometry of many Gd (III) complexes with the CN mentioned above gives rise to a nine coordinated distorted tricapped trigonal prism (D_h3 geometry) [32Guggenberger, L.J.; Muetterties, E.L. Reaction path Analysis. 2. The nine-atom family. J. Am. Chem. Soc., 1976, 98(3), 7221-7225.]. In the geometry of other complexes, such as Gadobutrol, the coordination polyhedron can be approximated by a distorted monocapped square antiprism (C_4v) symmetry [35Platzek, J.; Blaszkiewicz, P.; Gries, H.; Luger, P.; Michl, G. Muller-Fahrnow, A.; Radu1chel, B.; Su1lzle, D. Synthesis and structure of a new macrocyclic polyhydroxylated gadolinium chelate used as a contrast agent for magnetic resonance imaging. Inorg. Chem., 1997, 36(26), 6086-6093.].

Fig. (7c) shows an enlargement of the regions of the experimental infrared spectra of compound [2Arnaud, F-X.; Hissene, A.; Métivier, D.; Dutasta, F.; Berets, O.; N’Guema, B.; A’Teriitéhau, C.; Baccialone, J.; Potet, J. Gadolinium enhancement in brain magnetic resonance imaging in progressive multifocal leukoencephalopathy after natalizumab monotherapy: Is it really atypical? J. Neuroradiol., 2012, 39(4), 267-270.-F₃CCOOH] and complex [Gd(2)(H₂O)] where the most important differences can be verified and which would allow to make some of the affirmations previously made.

Fig. (7c) Regions FTIR spectra of compound ([2Arnaud, F-X.; Hissene, A.; Métivier, D.; Dutasta, F.; Berets, O.; N’Guema, B.; A’Teriitéhau, C.; Baccialone, J.; Potet, J. Gadolinium enhancement in brain magnetic resonance imaging in progressive multifocal leukoencephalopathy after natalizumab monotherapy: Is it really atypical? J. Neuroradiol., 2012, 39(4), 267-270.-F3CCOOH]•H2O) (upper) and complex and complex [Gd(2)(H2O)n] (bottom).

As indicated earlier, the theoretical basis to determine the structure of the [Gd(2)(H₂O)] complex was based on “Points on a Sphere” repulsion calculations [32Guggenberger, L.J.; Muetterties, E.L. Reaction path Analysis. 2. The nine-atom family. J. Am. Chem. Soc., 1976, 98(3), 7221-7225.] The polyhedron corresponding for the most probable structures of the com-plex obtained is shown in Fig. (8a). Fig. (8b) shows that the capping positions are occupied by the N3 nitrogen atom and the O13 and O17 oxygen atoms of the polyaminopolycar-boxylate ligand, while that the vertices of the two triangular faces of the coordination polyhedron are constituted by O73 (Ow), O4, 039 and O38, O7 and N2, respectively. Fig. (8c) intends to show the D_h3 geometry.

Fig. (8) a) the most probable structures of the complex, b) graph-ical representation of positions occupied by the heteroatoms of ligand, c) Coordination polyhedron showing Dh3 geometry.

Table 2 shows selected bond lengths, bond angles and torsion angles for [Gd(2)(H₂O)] complexes, which allowed a comparative analysis with the same parameters reported for other lanthanide complexes with a CN = 9.

The observation of the geometric data of the structure proposed for the [Gd(2)(H₂O)] complex allows stating that Gd (III) is not found in the center of the polyhedron but closer to the faces formed by the atoms O7-O38-O39-N4 and N1-N3-O7-O(W)73, and capped by the atoms O13, O17 and N3, respectively.

The average length of Gd-O carboxylate and the bond angles are similar to those previously reported for other Gd (III) complexes [27a, 36Grenthe, J. Structural studies on the rare Earth carboxylates. 14. A structural study of the orthorhombic trishydroxyacetates of Lanthanum(III) and Gadolinium(III). Acta Chem. Scand., 1972, 26, 1479-1489.] In this same sense, the calculated lengths of the bonds between the cation and the oxygen atoms of the ether bridges were not significantly different from those of other complexes whose ligands are crown ethers. The same parameter for Gd-N bonds was lower than those reported for both complexes whose ligands are cyclic or acyclic [11Berg, T.; Simeonov, A.; Janda, K.D. A combined parallel synthesis and screening of macrocyclic lanthanide complexes for the cleavage of phospho di- and triesters and double-stranded DNA. Comb. Chem., 1999, 1(1), 96-100., 27a(a) Caravan, P.; Ellison, J.J. McMurry, linium (III) chelates as MRI contrast agents: structure, dynamics, and applications. Chem. Rev., 1999, 99, 2293.(b) Aime, S.; Botta, M.; Terreno, E. Gd (III)-based contrast agents for MRI in advances in inorganic chemistry; R. van Eldik and I. Bertini, Eds.; Elsevier: San Diego, 2005; Vol. 57, pp. 173–237.]. Regarding the Gd-O water distance, it was practically the same as that found experimentally for different Gd complexes with q = 1 [38(a) Parker, D.; Dickins, R.S.; Puschmann, H.; Crossland, C.; Howard, J.A.K. Being excited by lanthanide coordination complexes: aqua species, chirality, excited-state chemistry, and exchange dynamics. Chem. Rev., 2002, 102(6), 1977-2010.].

The torsion angles presented in the table confirm the pro-posed configuration shown in Fig. (8a).

Table 2
Distances and bond/torsion angles, involving the central nine-coordinated Gd (III) atom.

When evaluating the information obtained from computational calculations, it must be taken into account that the structure obtained and proposed as the most probable was estimated in gaseous state. This is important to consider since there is experimental evidence that, in solution, the Gd (III) complexes derived from cyclic ligands suffer from structural changes due to the inversion of the ring and rota-tion of the arms [27a, 38(a) Parker, D.; Dickins, R.S.; Puschmann, H.; Crossland, C.; Howard, J.A.K. Being excited by lanthanide coordination complexes: aqua species, chirality, excited-state chemistry, and exchange dynamics. Chem. Rev., 2002, 102(6), 1977-2010.].