M. A. Morelli

Raman and infrared spectra on silica gel evolving toward glass

Abstract Infrared and Raman spectra of gels obtained from specially prepared solutions of Si(OC2H5)4 (TEOS) which have been thermally treated in the 40–800°C temperature range, are reported and discussed with reference to the spectra of fused quartz. The results show that the gel to glass transformation is an hydrolytic polycondensation process, which takes plase gradually and is practically completed in the samples treated at 800°C. Particularly revealing is the behaviour of the bands due to OH (associated and/or unassociated H-bonded) stretching modes, including those of water, which are weakened by increasing temperature; the behaviour of the band due to the SiOH stretching mode; the behaviour of the network bands, some of which become more intense with increasing temperature, due to asymmetric and symmetric stretching and the bending SiOSi modes. In the Raman spectra of gels heated at different temperatures two peaks also appear at ≈600 and ≈490 cm−1, which are also present in the Raman spectrum of fused quartz, they are related to network defects of the glass structure, for which no definite interpretation has yet been given in the literature.

Further investigations on Raman spectra of silica gel evolving toward glass

Abstract In a previous investigation a continuous evolution of silica gel structure toward that of v-SiO 2 was verified by Raman spectroscopy. Modifications on the intensity of some Raman bands usually associated in the literature to “defects” of the structure were observed. In the present work the study has been extended to the transformation with temperature of silica gels obtained from tetraethoxysilane (TEOS) with different H 2 O/TEOS molar ratios. Raman spectra as well as thermogravimetric data are discussed in terms of the effect of this ratio on the local structure and the sintering process of silica gel.

Raman defect peaks in the spectra of Na2O silica gels evolving toward glass

Abstract In previous works the continuous thermal evolution toward v-SiO2 of gels obtained from tetraethoxysilane (TEOS) and with different H2O/TEOS molar ratios was studied by Raman spectroscopy; Raman defect peaks at ∼ 600 and ∼ 490 cm−1 were observed both in gels and in the v-SiO2. In this work the structural effects of small amounts of Na2O on silica gels evolving toward glass are studied in order to give a further experimental contribution to the problem of “Raman defect peaks”. Some considerations on the nature of these defect peaks are now presented considering all the results so far obtained.

Interaction of bipyridilium herbicides with model membranes

Abstract Divalent cationic bipyridilium derivatives (Paraquat) have been widely used in agriculture; the primary target of these chemicals has been considered to be the phospholipids, and their effects could be modulated by the presence of bivalent cations. A study of paraquat (PQ)-phospholipid (PL), with different charge at pH 7, has been carried out by means of some different chemico-physical techniques. The systems PQ-dipalmitoylphosphatidic acid (DPPA) and PQ-dipalmitoylphosphatidylcholine (DPPC) water suspensions at pH 7 have been investigated. The experimental results demonstrate that the interaction between PQ and PL is mainly electrostatic; PQ strongly binds DPPA, which bears one negative charge at pH 7, but only weakly DPPC. The small changes in the DSC curves of DPPC can be explained by a small penetration into PL bilayer of the hydrophobic tail of PQ molecules. The presence of calcium ions, in an amount sufficient to bind completely the lipid, strongly alters the DPPA-PQ interactions, thus inhibiting the binding of PQ to lipid. On the contrary, the DPPC-PQ interactions are not modified by the presence of calcium ions. The behaviour of PQ is different from that of other polycations as polyamines, due to the steric effects present in the PQ molecule. Our results confirm that the negatively charged PL could be the possible molecular target of PQ. The electrostatic interactions are strongly modulated by the presence of cations with a great affinity for the phosphate group, such as calcium ions.

A Raman and infrared spectroscopic study of compounds characterized by strong hydrogen bonds

Abstract Raman and IR spectra of some components of the sodium carbonate—sodium hydrogen-carbonate system and deuterated analogues are presented and discussed. The behaviour of the asymmetric stretching vibration vOHO shows the presence of strong asymmetric and strong symmetric hydrogen bonds in sodium hydrogencarbonate and sodium sesquicarbonate, respectively, in agreement with X-ray diffraction measurements. In the case of the Wegscheider salt (3NaHCO3.Na2CO3) the spectra show that hydrogen bonds of different “strengths” are present and lead to a hypothesis for the probable structure of this salt.

Effects of the herbicide prometryn on the physicochemical properties of artificial model membranes

Several derivatives of 1,3,5-triazine (eg atrazine, prometryn) are effective herbicides and have been extensively used in agriculture. A study of prometryn (Prom)-phospholipid (PL), with different charges, was carried out by means of calorimetric and spectroscopic techniques. The Prom-dipalmitoylphosphatidylcholine (DPPC) and Prom-dipalmitoylphosphatidic acid (DPPA) water suspensions systems at pH values ranging from 5 to 9 were investigated. The results show that Prom does not significantly perturb the hydrophobic core of the lipid bilayer and suggest that the herbicide localizes near the glycerol backbone of the lipid, perturbing the environment of the carbonyls of the two ester groups. At pH 5 the vibrational band attributable to N-H stretch of Prom disappears thus suggesting that the absorption into the bilayer modifies the physicochemical properties of the herbicide. The two PL considered behave similarly and the effect of the charge is not noticeable. The strength of the Prom-PL interaction decreases with decreasing pH.

The role of water in biological systems

Abstract A general model of the liquid water structure consistent with its physical, chemical, structural, and also biological properties has not been described in the literature so far. Therefore, different structure models of liquid water have been taken into account in relation to its OH stretching Raman modes. Some factors (structure factor, H-bond cooperativity) typical of liquid and biological water have been considered. Also, the different roles played by water in biological systems are elucidated and discussed by means of a few general structure models, suitable for explaining the perturbations induced on liquid water by solutes (ionic, acids and bases, apolar) and surfaces. In particular, the possibility of strong acid—base H-bond interactions between different groups has been explored together with the biological role of water. Furthermore, the modulating effect of water on the H-bond strength and the consequent properties have been hypothesized. Finally, the behaviour of water structure in tissues and its possible correlation with normal and pathological conditions are reported.

Anomalous physical properties and Raman spectra of glassy B2O3BaTiO3Na2O materials

Abstract Anomalous physical properties (refractive index and density) of B2O3BaTiO3Na2O ternay glasses are determined and discussed on the basis of the structure present in the glasses and evidenced by vibrational Raman spectroscopy. These glasses behave in a manner analogous to the alkali B2O3X2O binary glasses for molar ratio R = basic oxide/B2O3 up to 0.3, with oxygen binding by means of bridging bonds while boron coordination changes from the trigonal to tetrahedral type. The phenomenon is indicated by a progressive weakening of the 806 cm−1 peak (attributable to a breathing vibration of the boroxol unit) and by a concomitant strengthening of the ∼775 cm−1 peak (attributable to a vibrational mode of boroxol units, or derived units, containing at least one 4-coordinate boron atom). For higher R values the Raman spectra bring to light the progressive demolition of the structural units responsible for the 775 cm−1 Raman peak, which gives rise (the transformation is complete for R ∼ 1) to two new main structural units, orthoborate [BTi4O10]−1 (peak at 845 cm−1) and metaborate BO2− (peak at 715 cm−1).

SiO2 gel-glasses prepared from alkoxide, colloidal and amine-silicate solutions, a comparison

Abstract Silica gels were prepared by the alkoxide, collidal and amine-silicate methods. The obtained gels and glasses were characterized by density and Vickers microhardness measurements, thermal analysis and Raman spectroscopy. Differences in physical properties and in the behavior during thermal treatment are discussed and related to the process chemistry and the structure of the products.

Raman and NMR Studies on the Molecular Interactions between Electrophilic Reagents and Nucleic Acids Constituents

The molecular interactions between nucleic acids bases (using nucleosides as a model molecule) and the methylating carcinogen dimethyl sulfate (DMS) as well as those between their methylation products and anticarcinogen cis-Pt(NH3)2Cl2 (cisplatin) have been studied by Raman, IR and NMR spectroscopies. The aim of this work is to contribute to the study of the molecular mechanism of the cancerogenesis and of the pharmacological action of cisplatin. Localized and delocalized (mispairing) molecular reactivities of 1-methyladenosine (1-MeAdo) and 7-methylguanosine (7-MeGuo) have been related to biological activity, while the interactions of the same molecules with cisplatin have been related to the anticarcinogenic activity of the drug.