L. F. C. de Oliveira

Raman spectra of some transition metal squarate and croconate complexes

Abstract The Raman spectra of a series of transition metal squarate and croconate polymeric complexes have been obtained. The data suggest that in the case of the squarate complexes the D4h symmetry of the oxocarbon is conserved in the complexes, while in the case of the croconate complexes the symmetry is reduced to C2v. In both series the frequency values indicate that in the polymeric chains the interaction of the oxocarbons with the metal ions is to a great extent delocalized. In contrast, the Raman spectra of the sodium salts of the oxocarbons, reported here for the first time, suggest a substantial reduction in symmetry in their unit cells. In the case of the croconate, the Raman spectrum of the solid has enabled the identification of one of the fundamentals not reported in previous works.

Chromophore selective resonance Raman enhancement in copper(II) squarate complexes with nitrogenous counterligands

Abstract A resonance Raman investigation of the mixed ligand complexes Cu(bipy)Sq and Cu(phen)Sq (where Sqsquarate, bipy=2,2′ -bipyridine and phen=1,10-phenanthroline) was undertaken. The results clearly indicate that the transitions observed at ca. 425 nm in the electronic spectra of such complexes can be assigned to a metal—oxocarbon charge transfer transition. In particular the carbonyl stretching mode is substantially enhanced, suggesting that the charge transfer is mainly located in that moiety of the oxocarbon. The drastic change observed in the enhancement pattern of the free oxocarbon can be ascribed to a significant reduction of its local symmetry in the mixed ligand complexes.

The interaction of the squarate ion with trivalent transition metal ions as revealed by resonance Raman spectroscopy

Abstract Resonance Raman spectroscopy was used to reveal the nature of the metal-ligand interaction in a series of metallo-squarate complexes (metal = FE3+, V3+, Cr3+ and Cu2+; squarate is the anion of 1,2-dihydroxycyclobutenedione). From the observed data it is clear that vibrational spectroscopy is not a reliable technique for deciding between the several coordination modes proposed in the literature for the ligand. However, the outstanding selectivity of the resonance enhancement of the carbonyl stretching mode in the Fe(III) and Cu(II) complexes allows the assignment of the absorption bands in the corresponding electronic spectra to ligand-to-metal charge transfer transitions.

Raman, resonance Raman and infrared spectra of potassium uranyl croconate

Abstract The Raman, resonance Raman and IR spectra of potassium uranyl croconate, UO 2 (H 2 O)K 2 (C 5 O 5 ) 2 were obtained and interpreted. Several croconate modes are split indicating a substantial decrease in the oxocarbon symmetry, as is to be expected from a recent crystallographic investigation, revealing the coordination of the oxocarbon to be two non-equivalent UO 2+ 2 moieties in a monodentate fashion. In terms of vibrational frequency shifts it can be concluded that the UO 2+ 2 moiety behaves as an isolated oscillator. The resonance Raman results suggest that the strong band centered around 450 nm in the UV—vis spectrum should be assigned to a charge transfer transition from the oxocarbon to the uranyl ion. In fact, as resonance is approached, both uranyl and croconate modes are enhanced. It can also be inferred that the chromophore is rather delocalized into the oxocarbon ring, rather than localized in the carbonyl groups as previously observed for other croconate complexes.

A Raman Spectroscopic Investigation of Sulphadiazine and of Its Dirhodium Tetracarboxylate Adducts

Abstract A tentative assignment of the vibrational modes of sulphadiazine is made, based on the infrared and Raman data. In addition, dirhodium tetracarboxylates and their adducts with sulphadiazine are investigated by Raman spectroscopy, with emphasis in the identify the metal-metal and metalligand stretching modes.

A RAMAN Spectroscopic Investigation of Alloxan and Alloxan Monohydrate

Abstract The present Raman investigation of anhydrous alloxan and the misnamed alloxan monohydrate suggests that some of the assignments previously made on basis of the IR spectrum of the anhydrous compound must be changed, i.e., the ones related to the NH stretching and ring breathing modes. The hydrogen bonding network in the crystal of the so called alloxan monohydrate, although extensive, is not strong enough to cause substantial frequency shifts in the NH and CO stretching modes.

Reply to Areta et al.: Time to withdraw and let the myth rest

to the editor: It is undisputable that caffeine variation in coffee—which is not as dramatic in Brazil as reported elsewhere ([2][1])—is a potential bias pertaining to our, and any, study assessing its habitual dietary intake. In fact, such variability exists in any natural food due to

Evidences of Electronic Effects in the Vibrational Spectra of Some Rhodium(II) Carboxylate Complexes

Abstract Abstract - A series of dirhodium tetracarboxylate complexes, containing triphenylphosphine as axial ligand was investigated by electronic and Raman spectroscopy, aiming to detect the influence of electronic effects of the carboxylate substituents on the v1[v(RhRh)] and v2[v(RhO)] Modes. When pairs of complexes with similar molecular weights are compared, as in the case of the cyclopentanecarboxylate and trifluoroacetate complexes, the striking difference in the inductive character of the two species results in substantial shifts of the v1 and v2 modes as a consequence of variations in the corresponding force constants rather than a simple mass effect as previously observed for non-substituted rhodium-carboxylates.