Ligand Vibrations Research Articles

A Raman and infrared spectroscopic study of the uranyl silicates—Weeksite, soddyite and haiweeite: Part 2

Raman spectroscopy has been used to study the molecular structure of a series of selected uranyl silicate minerals including weeksite K 2[(UO 2) 2(Si 5O 13)]·H 2O, soddyite [(UO 2) 2SiO 4·2H 2O] and haiweeite Ca[(UO 2) 2(Si 5O 12(OH) 2](H 2O) 3 with UO 2 2+/SiO 2 molar ratio 2:1 or 2:5. Raman spectra clearly show well resolved bands in the 750–800 cm −1 region and in the 950–1000 cm −1 region assigned to the ν 1 modes of the (UO 2) 2+ units and to the (SiO 4) 4− tetrahedra. Soddyite is characterized by Raman bands at 828.0, 808.6 and 801.8 cm −1, 909.6 and 898.0 cm −1, and 268.2, 257.8 and 246.9 cm −1, attributed to the ν 1, ν 3, and ν 2 (δ) (UO 2) 2+, respectively. Coincidences of the ν 1 (UO 2) 2+ and the ν 1 (SiO 4) 4− is expected. Bands at 1082.2, 1071.2, 1036.3, 995.1 and 966.3 cm −1 are attributed to the ν 3 (SiO 4) 4−. Sets of Raman bands in the 200–300 cm −1 region are assigned to ν 2 (δ) (UO 2) 2+ and UO ligand vibrations. Multiple bands indicate the non-equivalence of the UO bonds and the lifting of the degeneracy of ν 2 (δ) (UO 2) 2+ vibrations. The (SiO 4) 4− tetrahedral are characterized by bands in the 470–550 cm −1 and in the 390–420 cm −1 region. These bands are attributed to the ν 4 and ν 2 (SiO 4) 4− bending modes. The minerals show characteristic OH stretching bands in the 2900–3500 and 3600–3700 cm −1.

Ligand, Cofactor, and Residue Vibrations in the Catalytic Site of Endothelial Nitric Oxide Synthase

A study of bovine endothelial nitric oxide synthase by Fourier transform infrared (FTIR) spectroscopy in the 1000-2500 cm(-)(1) range is reported. Binding of CO to the reduced enzyme gives two heme(II)-CO nu(C)(-)(O) stretches (1927 and 1904 cm(-)(1)) which appear to be in rapid equilibrium. Photolysis of this heme(II)-CO compound is accompanied by perturbation of the local fine structure around the catalytic site giving vibrational changes of protein backbone, substrate, amino acid residues, and cofactors, to which heme, substrate arginine, and catalytic site residues contribute. Possible assignments of vibrations to heme, substrate arginine, and catalytic site residues are discussed. The discussion of assignments is informed by known structures, absorbance frequencies, and extinction coefficients of residues and cofactors, analysis of H(2)O-D(2)O exchange effects, analysis of substrate (14)N-(15)N (guanidinium)-arginine exchange effects, and comparison with the nNOS isoform (which differs in the replacement of asparagine 368 with an aspartate within the substrate binding site). The FTIR data can be modeled on the known structure of the catalytic site and indicate the extent of modulation of vibrational modes upon photolysis of the CO compound.

The prediction of Raman spectra of platinum(II) anticancer drugs by density functional theory

We present the method of theoretical calculations of the Raman intensities and the simulated Raman spectra of platinum(II) complexes. Theoretical Raman spectra of the anticancer agents: cisplatin ( 1), carboplatin ( 2), cis-[Pt(orotato)(NH 3) 2] ( 3), cis-[PtCl 2(NH 3)(2-picoline)], ZD0473 ( 4), and the two transient species of 4 (the hydrolysis products) were calculated by density functional mPW1PW method with several basis sets. For comparison, the experimental Raman spectra of compounds 1– 3 were measured. The clear-cut assignment of the Pt–ligand vibrations in the Raman spectra of the investigated compounds has been made on the basis of the calculated potential energy distribution.

Molecular Spectroscopy of Uranium(IV) Bis(ketimido) Complexes. Rare Observation of Resonance-Enhanced Raman Scattering from Organoactinide Complexes and Evidence for Broken-Symmetry Excited States

Electronic absorption and resonance-enhanced Raman spectra for ketimido (azavinylidene) complexes of tetravalent uranium, (C(5)Me(5))(2)U[-N=C(Ph)(R)](2) (R = Ph, Me, and CH(2)Ph), have been recorded. The absorption spectra exhibit four broad bands between 13 000 and 24 000 cm(-1). The highest-energy band is assigned to the ketimido-localized p( perpendicular)(N)-->pi(N=C) transition based on comparison to the spectra of (C(5)H(5))(2)Zr[-N=CPh(2)](2) and (C(5)Me(5))(2)Th[-N=CPh(2)](2). Upon excitation into any of these four absorption bands, the (C(5)Me(5))(2)U[-N=C(Ph)(R)](2) complexes exhibit resonance enhancement for several Raman bands attributable to vibrations of the ketimido ligands. Raman bands for both the symmetric and nominally asymmetric N=C stretching bands are resonantly enhanced upon excitation into the p( perpendicular)(N)-->pi(N=C) absorption bands, indicating that the excited state is localized on a single ketimido ligand. Raman excitation profiles for (C(5)Me(5))(2)U[-N=CPh(2)](2) confirm that at least one of the lower-energy electronic absorption bands (E(max) approximately 16300 cm(-1)) is a charge-transfer transition between the U(IV) center and the ketimido ligand(s). The observations of both charge-transfer transitions and resonance enhancement of Raman vibrational bands are exceedingly rare for tetravalent actinide complexes and reflect the strong bonding interactions between the uranium 5f/6d orbitals and those on the ketimido ligands.

Vibrational spectra of charge transfer complexes of lead phthalocyanine

Infrared spectra of six charge transfer complexes of lead phthalocyanine (PbPc), namely, PbPc-I 2, PbPc-TCNQ, PbPc-DDQ, PbPc-chloranil, PbPc-TCNE and PbPc-TNF, where TCNQ=7,7,8,8-tetracyano-1,4-quinodimethane, DDQ=2,3-dichloro-5,6-dicyano- p-benzoquinone, TCNE=tetracyano- p-ethylene and TNF=2,4,5,7-tetranitro-9-fluorenone have been studied in the range of 400–4000 cm −1. The analysis of featureless absorption is carried out for studying transition across the Peierls gap of 0.225 eV. The electronic absorption envelopes at 1500, 1100 and 3400 cm −1 are found to have Gaussian shapes and not the degenerate oscillators, as found in purely organic conductors. There is a pairing of two electrons on phthalocyanine ligand as required in Little's model, and consequently, the electronic absorption envelope is a doublet. Electronic absorption envelope is a doublet showing two peaks at 1500 and 1100 cm −1, indicating a two-electron problem in PbPc. Metal–ligand vibrations between 400 and 700 cm −1 lead to indirect transition between the valence and conduction bands and phonon-mediated coupling between metal chains and the side chains.

Application of a universal force field to mixed Fe/Mo-S/Se cubane and heterocubane clusters. 2. Substitution of iron by molybdenum in Fe4(S/Se)4 clusters with terminal halide and thiolate ligands.

Infrared and Raman spectra of Fe(4)(S/Se)(4) clusters with terminal halide ligands and MoFe(3)S(4) clusters with terminal thiolate and halide ligands are presented and interpreted on the basis of the force fields determined in the accompanying paper. The Raman spectra of halide coordinated Fe(4)(S/Se)(4) clusters are characterized by the fact that vibrations of the terminal ligands appear with little or vanishing intensity. Infrared and Raman spectra of MoFe(3)S(4) clusters with terminal thiolates are correlated to those of corresponding Fe(4)S(4) systems, which were investigated in part 1 of this study and interpreted with normal coordinate analysis. Band assignments are checked by employing MoFeS(4) clusters with terminal halide ligands. Spectra of these systems are in turn compared to those of their Fe(4)S(4) counterparts, i.e., Fe-S cubane clusters with chloro, bromo, and iodo ligands. A consistent interpretation of all spectra is presented. General implications of these results are discussed.

Coordination sphere vibrations in copper(II), nickel(II) and cobalt(II) complexes with 4-imidazoleacetic acid; metal isotope, deuteration, and density functional study

Low-frequency vibrational spectra of Cu(II), Ni(II) and Co(II) complexes with 4-imidazoleacetic acid (HIA) are discussed. Metal isotope and deuteration effects were measured and used to assign the metal–ligand vibrations. These effects were included in density functional calculations carried out for the [Cu(IA) 2 ] complex.

Synthesis and vibrational spectroscopy of new palladium(II) complexes with 2-hydrazino-2-imidazoline. Band assignment based on isotope substitution and density functional calculations

The novel complexes of formula [Pd(hi)X 2] (hi—2-hydrazino-2-imidazoline; X—Cl, Br) were investigated by vibrational spectroscopy and quantum-mechanical methods. The experimental band assignment was supported by measured metal isotope shifts and deuteration effects. Both experiments were reproduced in DFT calculations and used as additional criteria verifying the computed data. All normal vibrations were characterized in terms of potential energy distribution. The stretching vibration of Pd(II)–imidazoline bond was observed at about 220 cm −1. The vibrational coupling of most metal coordination sphere modes and ligand vibrations was found.

Vibrational spectra, structure and hydrogen bonding of 5-tert-butylpyrazole and its zinc complexes

New zinc complexes [L 2ZnX 2] with X=Cl ( I), Br ( II), I ( III) and NO 3 ( IV), [L 3Zn(OClO 3)]ClO 4 ( V) and [L 4Zn](ClO 4) 2 ( VI) with 5-tert-butylpyrazole as ligand L were synthesized and characterized by infrared-, Raman-, mass- and NMR-spectroscopy. The assignment of the vibrational frequencies for the complexes in the range of 4000–80 cm −1 is proposed. Analysis of the IR spectra in the range of ν NH frequencies shows that 5-tert-butylpyrazole forms cyclic associates in the solid using intermolecular NH⋯N hydrogen bonds. This was proven by a crystal structure determination, which showed that L exists as tetramers in the solid state. In solution there is an equilibrium between tetramers and monomers and, probably, dimers or trimers. In the complexes, the NH-groups of L form intramolecular H-bonds with the X-group. The intramolecular H-bond in VI has interionic character and partially dissociates in solution at high dilution. Ligand vibrations that are sensitive to the coordination and the dependence of their frequencies on the anionic group X has been revealed.

Synthesis and vibrational spectroscopy of new palladium(II) complexes with 2-hydrazino-2-imidazoline. Band assignment based on isotope substitution and density functional calculations

The novel complexes of formula [Pd(hi)X2] (hi—2-hydrazino-2-imidazoline; X—Cl, Br) were investigated by vibrational spectroscopy and quantum-mechanical methods. The experimental band assignment was supported by measured metal isotope shifts and deuteration effects. Both experiments were reproduced in DFT calculations and used as additional criteria verifying the computed data. All normal vibrations were characterized in terms of potential energy distribution. The stretching vibration of Pd(II)–imidazoline bond was observed at about 220 cm−1. The vibrational coupling of most metal coordination sphere modes and ligand vibrations was found.

Coordination sphere vibrations in copper(II), nickel(II) and cobalt(II) complexes with 4-imidazoleacetic acid; metal isotope, deuteration, and density functional study

Low-frequency vibrational spectra of Cu(II), Ni(II) and Co(II) complexes with 4-imidazoleacetic acid (HIA) are discussed. Metal isotope and deuteration effects were measured and used to assign the metal–ligand vibrations. These effects were included in density functional calculations carried out for the [Cu(IA) 2] complex.

Properties of molecular lanthanide crystals spectra in the near-IR region

Electronic absorption spectra of isomorphic Gd 3+, Tb 3+ and Yb 3+ acetate crystals with the [Ln 2(CH 3COO) 6(H 2O) 4] formula were measured in the 800–2700 nm range at room temperature. For comparison purposes crystal spectra of the [Y 2(CH 3COO) 6(H 2O) 4]·4H 2O and Na 3[Yb(dpa) 3]·13H 2O (dpa=2,6-pyridinedicarboxylate) were also recorded. IR spectra and a luminescence spectrum of the Tb 3+ acetate crystal were taken as well. The analysis of the IR and near-IR (NIR) spectra of the crystals has shown that the f–f transitions in the NIR range can be dominated by overtone vibrations of ligands.

Spectroscopic properties and quantum chemistry-based normal coordinate analysis (QCB-NCA) of a dinuclear tantalum complex exhibiting the novel side-on end-on bridging geometry of N2: correlations to electronic structure and reactivity.

The vibrational properties and the electronic structure of the side-on end-on N(2)-bridged Ta complex ([NPN]Ta(micro-H))(2)(micro-eta(1):eta(2)-N(2)) (1) (where [NPN] = (PhNSiMe(2)CH(2))(2)PPh) are analyzed. Vibrational characterization of the Ta(2)(micro-N(2))(micro-H)(2) core is based on resonance Raman and infrared spectroscopies evaluated with a novel quantum chemistry-based normal coordinate analysis (QCB-NCA). The N-N stretching frequency is found at 1165 cm(-)(1) exhibiting a (15)N(2) isotope shift of -37 cm(-)(1). Four other modes of the Ta(2)N(2)H(2) core are observed between 430 and 660 cm(-)(1). Two vibrations of the bridging hydrido ligands are also identified in the spectra. On the basis of experimental frequencies and the QCB-NCA procedure, the N-N force constant is determined to be 2.430 mdyn A(-)(1). The Ta-N force constants are calculated to be 2.517 mdyn A(-)(1) for the Ta-eta(1)-N(2) bond and 1.291 and 0.917 mdyn A(-)(1) for the Ta-eta(2)-N(2) bonds, respectively. DFT calculations on 1 suggest that the bridging dinitrogen ligand carries a charge of -1.1, which is equally distributed over the two nitrogen atoms. However, orbital analysis reveals that the terminal nitrogen makes lower contributions to the pi orbitals and much higher contributions to the pi orbitals of the N(2) ligand than the bridging nitrogen. This suggests that reactions of the dinitrogen ligand with electrophiles should preferentially occur at the terminal N atom, in agreement with experimental results.

Room temperature vibrational photoluminescence and field emission of nanoscaled tris-(8-hydroxyquinoline) aluminum crystalline film

The nanoscaled tris-(8-hydroxyquinoline) aluminum (AlQ3) crystalline film was synthesized by vapor condensation. It was stacked with nanometer-sized rods, approximately 100 nm wide and 1 μm long, and had a surface roughness of about 100 nm. The vibronic progression with several separated peaks was observed in the photoluminescence spectrum at room temperature. It is attributed to the crystallinity of AlQ3 and the coupling of vibrations of the individual ligands to the fluorescence transition. The emission current was also observed with a turn-on field of 12.0 V/μm, and a current density of about 0.8 mA/cm2 at 22 V/μm. Therefore, the AlQ3 crystalline film provides a choice for field emission.

Intervalence involvement of bridging ligand vibrations in hexaruthenium mixed-valence clusters probed by resonance Raman spectroscopy.

Resonance Raman spectroscopy, performed using spectroelectrochemistry and with excitation in the intervalence bands of three pyrazine-bridged, mixed-valence dimers of trinuclear ruthenium clusters, shows resonant enhancement of symmetric bridging ligand modes. The resonant enhancements and frequency shifts of these bridging ligand modes are observed as a function of varying electronic communication between charge sites, and they show that a three-state vibronic model which explicitly includes the participation of the bridging ligand is needed to explain the spectroscopic behavior of these near-delocalized complexes.

Density functional modelling of metal–ligand vibrations in polymeric zinc(II) complex with 4-imidazoleacetic acid

Metal–ligand vibrations were localised in the far-infrared spectrum of polymeric [ZnCl(IA)(HIA)]·H 2O complex (HIA denotes the 4-imidazoleacetic acid) with help of metal isotope and ClBr substitution effects. Detailed assignment of the observed bands has been proposed on the basis of the density functional theory calculations, carried out for three models representing the metal surrounding in various extents. The observed isotope shifts and halogen substitution effects were reproduced in calculations and used for verification of the theoretical spectra.

The power and limits of the Judd—Ofelt theory: a case of Pr3+ and Tm3+ acetates and dipicolinates

Electronic absorption spectra of crystals and solutions of praseodymium and thulium acetates and dipicolinates are reported. The influence of spectral properties of both ligands on intensities of the f—f transitions in the crystals studied is discussed. In the near-IR region, overtone vibrations of ligands are strongly coupled with the f—f transitions, particularly with the hypersensitive ones. In the UV spectral range, absorption bands of the π → π*, n → π* and possible charge transfer of metal ⇔ ligand transitions affect f—f transitions and should also be taken into account. The phenomenological Judd—Ofelt parameters evaluated for praseodymium complexes in solution and crystals are meaningless, because no set of intensities unbiased by ligand transitions and/or neighbouring f-d configurations is available. For Tm3+ compounds the Judd-Ofelt analysis has given quite good results, however no significant influence of the metal ion environment changes on the Ωλ parameters can be observed.

Resonance Raman studies on xanthine oxidase: observation of Mo(VI)-ligand vibrations.

Resonance Raman spectra were investigated for the sulfo and desulfo forms of cow's milk xanthine oxidase, with various visible excitation lines between 400 and 650 nm, and Mo(VI)-ligand vibrations were observed for the first time. The Mo(VI)=S stretch was identified at 474 and 462 cm(-1 )for the (32)S- and (34)S-sulfo forms, respectively, but was absent in the reduced state and in the desulfo form. The Mo(VI)=O stretch was weakly observed at 899 cm(-1 )for the sulfo form and shifted to 892 cm(-1) with very weak intensity for the dioxo desulfo form. In measurements of an excitation profile, the two bands at 474 and 899 cm(-1) showed maximum intensity at similar excitation wavelengths, suggesting that the Raman intensity of the metal-ligand modes is due to the Mo(VI)<--S charge transfer transition, and that this is the origin of the intrinsically weak features of the Mo(VI)-ligand Raman bands. When the sulfo form was regenerated from the desulfo form, the 899 cm(-1) band reappeared. However, the band at 899 cm(-1) showed no frequency shift when regeneration was conducted in H(2)(18)O, or after several turnovers in the presence of xanthine in H(2)(18)O. When the sulfo form was reduced and reoxidized in H(2)(18)O buffer, the 899 cm(-1) band reappeared without any frequency shift. These observations suggest that the oxo oxygen in the Mo center of xanthine oxidase is not labile. Low-frequency vibrations of the Mo center were observed together with those of the Fe(2)S(2) center with some overlaps, while FAD modes were observed clearly. The absence of dithiolene modes in XO is in contrast to the Mo(VI) centers of DMSO reductase and sulfite oxidase.

N-isotope effects on the Raman spectra of Fe(2)S(2) ferredoxin and Rieske ferredoxin: evidence for structural rigidity of metal sites.

The diiron ferredoxins have a common diamond-core structure with two bridging sulfides, but differ in the nature of their terminal ligands: either four cysteine thiolates in the Fe(2)S(2) ferredoxins or two cysteine thiolates and two histidine imidazoles in the Rieske ferredoxins. Contributions of the bridging (b) and terminal (t) ligands to the resonance Raman spectra of the Fe(2)S(2) ferredoxins have been distinguished previously by isotopic substitution of the bridging sulfides. We now find that uniform (15)N-labeling of Anabaena Fe(2)S(2) ferredoxin results in shifts of -1 cm(-1) in the Fe-S(t) stretching modes at 282, 340, and 357 cm(-1). The (15)N dependence is ascribed to kinematic coupling of the Fe-S(Cys) stretch with deformations of the cysteine backbone, including the amide nitrogen. No (15)N dependence occurs for the nu(Fe-S(b)) modes at 395 and 426 cm(-1). Similar effects are observed for the Rieske center in T4MOC ferredoxin from the toluene-4-monooxygenase system of Pseudomonas mendocina. Upon selective (15)N-labeling of the alpha-amino group of cysteine, the vibrational modes at 321, 332, 350, and 362 cm(-1) all undergo shifts of -1 to -2 cm(-1), thereby identifying them as combinations of nu(Fe-S(t)) and delta(Cys). These same four modes undergo similar isotope shifts when T4MOC ferredoxin is selectively labeled with (15)N-histidine ((15)N in either the alpha1,delta1 or delta1,epsilon2 positions). Thus, the Fe-S(Cys) stretch must also be undergoing kinematic coupling with vibrations of the Fe-His moiety. The extensive kinematic coupling of iron ligand vibrations observed in both the Fe(2)S(2) and Rieske ferredoxins presumably arises from the rigidity of the protein framework and is reminiscent of the behavior of cupredoxins. In both cases, the structural rigidity is likely to play a role in minimizing the reorganization energy for electron transfer.

Vibrational Spectroscopy of Cyanide-Bridged, Iron(II) Spin-Crossover Coordination Polymers: Estimation of Vibrational Contributions to the Entropy Change Associated with the Spin Transition

A series of cyanide-bridged, iron(II) spin-crossover coordination polymers of formula Fe(py)2[M(CN)4] (M = Pd or Pt; py = pyridine) and Fe(pz)[M(CN)4]·2H2O (M = Ni, Pd, or Pt; pz = pyrazine) have been studied by recording the Raman and infrared (IR) spectra of their low-spin (LS) and high-spin (HS) forms in the solid state. Spectral changes upon metal and ligand substitution were used to guide the assignment of vibrational modes. Vibrational entropy changes upon spin transition were analyzed within the harmonic approximation. The results confirm the major contribution of the Fe−ligand vibrations to the overall entropy change associated with the spin transition (∼60%), butcontrary to expectationsreveal significant changes in other intra- and intermolecular vibrations, of possible relevance to the high cooperativity of the spin transition.