A. M. Amorim da Costa

Reorientational molecular motion in liquids: a comparison of Raman and Rayleigh scattering

The spectral distribution of Rayleigh and Raman anisotropy scattering has been measured for ten simple organic liquids at 20°C and the interpretation in terms of reorientational motions is discussed. All spectra approximated closely to a Lorentzian form. In most cases the spectral linewidths derived from Raman bands were greater (by up to 150 per cent) than those derived from Rayleigh measurements. Furthermore, the linewidths of Raman anisotropy scattering from different totally symmetric vibrations of the same molecule were found to differ by as much as 100 per cent. The Rayleigh-Raman linewidth differences correlate strongly with the Raman vibrational linewidths and it is suggested that there are significant interactions between rotational and vibrational energies as a result of molecular interactions in the liquids. These observations obscure any interpretation of linewidth differences in terms of cooperative reorientational motions in these liquids. Anisotropy linewidths were also studied as a functio...

Mesophase formation in lead(II) decanoate

Abstract Structures of the thermotropic mesophases of lead(II) decanoate are reassigned following optical and X-ray diffraction studies. These results, and those of D.S.C., Raman and 207Pb N.M.R. spectroscopy, indicate formation of a lower temperature mesophase involving mainly increased lateral disorder, and a higher temperature Lα (smectic A) phase resulting from chain disordering and decreased lead-carboxylate interaction. Comparison of experimental thermodynamic data for the phase transitions with theoretical data in the literature indicates that the entropy change for the lower to higher mesophase transition is dominated by the increase in chain disorder.

Thermotropic Polymorphism in Liquid-Crystalline Lead(II) Alkanoates

Abstract Lead(II) carboxylates with even chain lengths from octanoate to octadecanoate have been investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and dilatometry, together with Raman, infrared, and 207Pb N.M.R. spectroscopy. The reassignment of the high temperature mesophase as smectic A (Lx) for the octanoate to dodecanoate is confirmed. This phase is most stable for the decanoate. The intermediate temperature mesophase is a highly ordered three dimensional lamellar structure based on ionic layers. The alkyl chains in this phase are partially disordered (melted) through the introduction of gauche conformations and show decreased lateral interactions; however, they remain largely in an extended conformation due to the constraints imposed by the ionic network. Melting of the alkyl chains occurs therefore in more than one step. Re-cooling to the crystalline phase appears to result in a different polymorphic crystalline form (albeit still lamellar) from the...

A comparative ab initio MO study of internal rotations in ethylamine and n-propylamine

Abstract A comparative ab initio study of the internal rotations in ethylamine and n-propylamine using a 3-21G(N * ) basis set and fully optimized geometries is presented. For n-propylamine, the NH 2 rotation about the :N-C bond, for a trans skeleton, and the rotations about the C1-C2 bond, for trans and gauche orientations of the nitrogen lone pair, are considered.

Conformational studies by Raman spectroscopy and statistical analysis of gauche interactions in n-butylamine

Abstract Raman spectra of n -butylamine recorded at different temperatures show pairs of bands whose temperature-dependent intensities clearly suggest their assignment to different conformers in simultaneous equilibria. Normal coordinate analysis and i.r. spectra of n -butylamine are also used to assign the spectra. These vibrational data are interpreted and correlated with structural information obtained from a statistical analysis of gauche skeletal arrangements in n -butylamine at different temperatures.

Interactions of Copper (II) Chloride with β‐Cyclodextrin in Aqueous Solutions

The interaction between copper (II) chloride and the carbohydrate β‐cyclodextrin (β‐CD) has been studied in aqueous solutions (298.15 K and 310.15 K) using measurements of diffusion coefficients and electrical conductivity. Significant effects on the electrical conductivity were observed in the presence of the β‐CD, suggesting interactions between this carbohydrate and copper chloride. Support for this came from diffusion coefficient measurements. These studies have been complemented by molecular mechanics calculations.

Structures and vibrational spectra of CH3OCH2CH2OH: the hydrogen-bonded conformers

Ab initio calculations at the MP2/6-31G* and MP2/6-31G*//6-31G* levels have been carried out for the monomer of 2-methoxyethanol (CH3OCH2CH2OH). The MP2/6-31G* results indicate that the two more stable conformers (tGg′ and gGg′) display intramolecular hydrogen bonds directed from the hydroxy H atom to one of the lone pairs of the ether O atom, and that the tGg′ conformer is 6.3 kJ mol–1 more stable than the gGg′ conformer. As the MP2/6-31G* and MP2/6-31G*//6-31G* calculations do not yield results differing by more than a few tenths of a kJ mol–1, it is concluded that the structure-sensitive and the dynamic correlation corrections are far from being additive. While the optimization of geometry for the correlated wavefunction generally leads to increase of bond lengths and reduction of bond angles, on the whole the geometrical parameters undergo similar changes in different conformers. Ab initio harmonic 6-31G* derived force fields were used to perform normal mode analyses for the more stable conformers. The calculated ν(CH) frequencies are found to correlate linearly with some of the ab initio calculated CH bond lengths. An interpretation of the FTIR and Raman spectra for the liquid phase consonant with the structural and vibrational ab initio results is presented. Two spectral features observed both in Raman and in FTIR spectra and associated with ν(OH) in monomeric species are ascribed to conformers, in accord with the theoretical and experimental results. On the whole, both the structural and the vibrational results presented point to a distinction between the hydrogen-bonded G-type conformers (tGg′ and gGg′) and the higher energy T-type conformers (tTg and tTt).

Conformational studies of n-propylamine by combined ab initio MO calculations and Raman spectroscopy

Abstract The results of ab initio SCF-MO calculations performed with a 3-21G(N*) basis set, for fully optimized geometries of five conformations of n -propylamine, are presented. The calculated relative order of total energies for these conformers is TT≈GG′>TG>GT>GG. At 300 K, the Boltzmann distribution of populations is 18, 37, 20, 19 and 7%, respectively. Raman spectra of n -propylamine and n -propylamine-N- d 2 in the liquid phase exhibit a number of bands whose temperature-dependent intensities clearly suggest the occurrence of different conformers in simultaneous equilibria. Deuteration of the amine group originates pairs of Raman bands at 428 and 440 cm −1 and at 863 and 885 cm −1 . The bands at 428 and 885 cm −1 are favoured by reduction of temperature. Normal coordinate calculations permit the assignment of the Raman and i.r. spectra in good agreement with experimental evidence. Among the five possible conformers of n -propylamine, it is possible to detect the presence of at least three conformations in the liquid phase, corresponding to the skeletal trans (TT and GT) and at least one of the skeletal gauche (TG, GG or GG′) forms. In the solid phase, only the bands ascribed to the TT form were observed. The ab initio results for the isolated molecule show that the all- trans conformation, TT, and the conformation GG′ have the smallest energies. On the other hand, the vibrational results for the liquid and solid phases indicate that the all- trans conformation, TT, is the more populated form. In addition, this conformer presents the highest calculated dipole moment, in good agreement with the liquid phase Raman spectroscopic results which point out that this conformation is favoured by polar solvents. Intermolecular interactions operating in the liquid n -propylamine, possibly of the hydrogen bonding type, are responsible for altering the relative order of conformational stability as predicted by the ab initio SCF-MO results for the isolated molecule.

Micellar aggregation of CTAB in water and chloroform solutions—a study by laser raman spectroscopy

Abstract Changes in the C-H stretching region of the Raman spectra of CTAB (hexadecyltrimethylammonium bromide) in aqueous and chloroform solutions with varying concentration, temperature, and electrolyte additives are interpreted in terms of structural and environmental effects. The results are in agreement with the presence of several gauche rotational isomers of CTAB in the aqueous and chloroform solutions, and with CTAB micellar association of the ordinary type in water and “inverted” micellar association in chloroform.

The carbonhydrogen stretching region of the Raman spectra of 1,6-hexanediamine: N-deuteration, ionisation and temperature effects

Abstract The Fermi resonance interaction between the methylene symmetric CH stretching mode and appropriate binary combinations involving the methylene bending mode in the Raman spectra of polyamines, namely in 1,6-hexanediamine derivatives, under different conditions of deuteration and ionisation of the head amino groups and varying temperature has been analysed. The tentative assignment of the observed bands in the CH stretching region of the registered Raman spectra is reported and the ratio of the Raman intensities of the symmetric stretching to the antisymmetric fundamentals is considered in its possible relation to the order/disorder of the skeletal hydrocarbon chains, from the all-trans prevailing geometry in the solid state at lower temperatures to the increasing amount of gauche arrangements on heating and melting.