G. Fini

Local order and vibrational coupling in solutions of polar molecules

The separation between the isotropic and the anisotropic component of some Raman bands of polar molecules is studied as a function of concentration in different solvents. It is found that it depends linearly on the ratio between the volume fraction and the static dielectric constant of the solution, and becomes zero at a finite concentration. It is suggested that this concentration threshold is related to the relaxation time of the vibrational energy. An explanation is proposed for the observed dependence on the dielectric constant.

Evidence for short-range orientation effects in dipolar aprotic liquids from vibrational spectroscopy. Part 1.—Ethylene and propylene carbonates

The CO stretching frequency of liquid ethylene and propylene carbonates is higher in the infrared than in the Raman spectrum. The difference amounts to 13 cm–1 for ethylene carbonate at 313 K. This effect is explained as the consequence of a coupling between the transition dipoles of neighbouring molecules, which is made possible by some degree of alignment of molecular dipoles due to the high dipole moment of these molecules (about 16 × 10–30 C m). A study of dilution and temperature effects confirms this interpretation.

Short-range orientation effects in dipolar aprotic liquids—III. Intermolecular coupling of vibrations in sulfoxides, sulfones, nitriles and other compounds

Abstract In the Raman spectrum of dipolar aprotic liquids the anisotropic component of bands arising from totally symmetric vibrations falls at a higher frequency than the isotropic component. The corresponding i.r. band maximum appears at about the same frequency as the Raman anisotropic component. These effects disappear with dilution in an inert solvent. This behaviour can be explained by assuming that these liquids are composed of clusters inside which the molecules are oriented in a partially ordered way.

Evidence for short-range orientation effects in dipolar aprotic liquids from vibrational spectroscopy. Part 2.—Carbonyl compounds

In the Raman spectrum of pure liquids, the anisotropic component of the CO stretching band falls at a higher frequency than the isotropic component. In the same liquids the infra-red band maximum appears at almost the same frequency as the Raman anisotropic component. In some cases the infra-red band may be resolved into two components, the stronger coinciding with the anisotropic, the weaker with the isotropic Raman band. These effects disappear with dilution and are reduced with increasing temperature.This behaviour is explained in terms of a model which assumes that dipolar aprotic liquids are composed of clusters of molecules oriented in an (at least partially) ordered way.

The effect of composition on the noncoincidence of the isotropic and anisotropic Raman frequencies and its intrepretation by means of a simple dielectric model

The difference between the anisotropic and the isotropic frequencies of the C=O stretching bands of acetone, acetophenone, methylbenzoate, and dimethylformamide was measured in several mixtures of known dielectric constant. Its change with concentration can be explained by making the assumption that the interaction energy of the dissolved dipoles is described by the dielectric model of Onsager–Frohlich over the whole composition range. The same model accounts for the change of the above difference in neat liquids with temperature.

Solvent effects on Raman band intensities

Abstract The intensities of polarized Raman bands of carbon tetrachloride, tetrachloroethylene, benzene, cyclohexane, carbon disulfide, chloroform, acetone, and acetonitrile have been measured in binary mixtures with a wide variety of solvents, at concentrations ranging from 20 to 80% in volume. The scattering coefficient increases, in most cases linearly, with the refractive index of the mixture; furthermore, its value extrapolated to zero concentration exhibits a close correlation with the refractive index of the pure solvent. The results are in fair agreement with a theoretical formula, derived from Onsagers theory of dielectric polarization.

Binding of copper(II) to carnosine: Raman and IR spectroscopic study

A comparative Raman and FTIR study of carnosine, a dipeptide present in several mammalian tissues, and its complexes with copper(II) at different pH values was carried out. The neutral imidazole ring gives rise to some bands that appear at different wavenumbers, depending on whether the imidazole ring is in the tautomeric form II or I. At pH 7 and 9 the molecule exists in equilibrium between the two tautomeric forms; tautomer I is predominant. Metal coordination is a factor that affects the tautomeric equilibrium, and the copper(II) coordination site can be monitored by using some Raman marker bands such as the vC(4)=C(5) band. On the basis of the vibrational results, conclusions can be drawn on the functional groups involved in the Cu(II) chelation and on the species existing in the Cu(II)-carnosine system. At neutral and basic pH the most relevant species formed when the Cu(II)/carnosine molar ratio is not very different from unity is a dimer, [Cu(2)L(2)H(-2)](0). In this complex the ligand coordinates the metal via the N (amino), O (carboxylate), and N (amide) donor atoms while the N(tau) nitrogen atoms of the imidazole rings (tautomer II) bridge the copper(II) ions. At a slightly acidic pH the two monomeric complexes [CuLH](2+) and [CuL](+) were present. In the former the imidazole ring takes part in the Cu(II) coordination in the tautomeric I form whereas in the latter it is protonated and not bound to Cu(II).

Raman and IR spectroscopic investigation of zinc(II)-carnosine complexes.

The zinc(II)-L-carnosine system was investigated at different pH and metal/ligand ratios by Raman and IR spectroscopy. The Raman and IR spectra present some marker bands useful to identify the sites involved in metal chelation at a specific pH value. In particular, the neutral imidazole group gives rise to some Raman bands, such as the nu C(4)===C(5) band, that change in wave number, depending on whether the imidazole ring takes the tautomeric form I or II. Even if tautomer I is predominant in the free ligand, metal coordination can upset tautomeric preference and N(tau)- and N(pi)-ligated complexes can be identified. Although weak compared to those of aromatic residues, these Raman marker bands may be useful in analyzing metal-histidine interaction in peptides and proteins. On the basis of the vibrational results, conclusions can be drawn on the species existing in the system. Depending on the available nitrogen atoms, various complexes can be formed and the prevalent form of the species depends mainly on the pH. At basic pH carnosine gives rise to two different neutral complexes: a water-insoluble polymeric species, [ZnH(-1)L](0)(n), and a dimer, [Zn(2)H(-2)L(2)](0). The first is predominant and involves the tautomeric I form of the imidazole ring in metal chelation; the second contains tautomer II and increases its percentage by going from a 2 to 0.25 metal/ligand ratio. Conversely, the dimeric species dominates at pH 7, whereas two charged species, [ZnHL](2+) and [ZnL](+), are formed under slightly acidic conditions. In the [ZnHL](2+) complex the imidazole ring takes part in the Zn(II) coordination in the tautomeric I form, whereas in [ZnL](+) the ring is protonated and not bound to the Zn(II) ion. In addition, the curve fitting analysis of the 1700-1530 cm(-1) Raman region was helpful in indicating the predominant species at each pH.

DSC and Raman study on the interaction between polychlorinated biphenyls (PCB) and phospholipid liposomes

Polychlorinated biphenyls (PCB) are very toxic lipophilic substances widely used in the past as non flammable dielectric fluids. PCB accumulation in the environment appears to be a great risk for human health. Dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylethanolamine (DMPE) liposomes with increasing amounts of Aroclor 1254 have been studied by DSC and Raman spectroscopy. Noticeable changes take place in thermograms and Raman spectra even in the presence of small amounts of Aroclor, suggesting the existence of strong interactions due to the insertion of the PCB molecules into the hydrophobic core of liposomes. In DPPC liposomes, a ‘solution like’ system is observed and the main effects are the decrease of both the melting temperature and ΔH of transition, with a simultaneous increase in the half width and asymmetry of the peak. On the contrary, in DMPE liposomes, a complex structure of the thermograms, which comes from the coexistence of different phases, is observed in most of the analysed systems. The behaviour is explained on the basis of a different penetration depth into the bilayer due to the polar interactions involving the polar head of the lipid. The existence of an interdigitate phase in DMPE–PCB liposomes is also evident in the experimental results.

On the secondary structure of some vibrational bands of acetonitrile

Abstract The occurrence of shoulders on some Ramani bands of acetonitrile in relation to the formation of molecular clusters is discussed.