Acid–base interactions between formamide and tetrahydrofuran molecules: Raman investigation of speciation in a binary system of biological interest

2008

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Raman experiments of formamide (FA) and p-dioxane (DX) mixtures at different compositions were carried out. A red shift of the C-O stretching band of DX was observed upon dilution, while blue shifts were observed for the C-H stretching and C-O-C bending bands. In this latter region, the new band at ∼441 cm −1 , whose intensity shows large dependence on the FA concentration, has been assigned to an FA-DX adduct and it is reported for the first time in the literature. The spectral changes observed in the C-O-C bending region allowed to determine a proportion of 4 : 1 FA-DX and this experimental evidence is also presented for the first time by Raman spectroscopy. The present work shows an excellent agreement with our previous investigation, where the 2 : 1 FA -THF (tetrahydrofuran) adduct was characterized.

Section: Introductionsupporting

confidence: 93%

Investigating the formation of an adduct of formamide with dioxane by means of Raman spectroscopy

Norbert

2008

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“…The vibrational assignment for each band has already been performed in two recent works. [9,10] In the present study, when the FA concentration increases from 2.5 to 12.6 M, the band at 898 cm −1 (assigned to an FA-THF adduct) has its intensity increased at the expense of the 915 cm −1 band (ascribed to the C-O stretching vibration of THF). The 910 cm −1 band also suffers a reduction of intensity, but it is less sensitive to an acid-base-type interaction.…”

Section: Resultsmentioning

confidence: 91%

“…For the FA-THF adduct, it was necessary to use the integrated intensity, I A , and the specific intensity of the band at 898 cm −1 (J A = 0.715). [9] The FA concentration such as dimer and higher aggregate, c [FA]n , was calculated subtracting from the total FA concentration, c T , the contribution of the FA-THF adduct, c A . On this way, the extent of association, 1 − α, can be determined as [8,18] 1…”

Section: Resultsmentioning

confidence: 99%

Preferential interactions in the formamide/ tetrahydrofuran/dioxane system: a study by Raman spectroscopy

2008

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Significant changes observed in the Raman spectra of formamide (FA)-tetrahydrofuran (THF)-dioxane (DX) mixtures have been interpreted in terms of preferential interactions. The Gutmann's donor (DN) and acceptor (AN) number values of these solvents give a good interpretation for the differences observed. In the ternary system, THF behaves as a stronger base than DX and the band at ∼442 cm −1 , recently assigned to the FA-DX adduct is only observed at the highest FA concentration. Quantitative analyses performed at the C O (FA) and C-O (THF) stretching regions show that the extent of the association for the [FA]n adduct is significantly larger than for the FA-THF adduct. Electrostatic parameters, such as dipole moment and dielectric constant, are also regarded as a better interpretation of these associations. The good correlation between DN and electrostatic parameters indicates that the basic strength increases in the order DX < THF < FA.

“…[1] Several hydrogen-bonded formamide (FA) complexes have been investigated in our laboratory and the results are important for a better understanding on the nature of the hydrogen bond in systems exhibiting the peptide type of bonding and DNA structures. [2] In those works, FA essentially acts as a Lewis acid where its N-H site causes wavenumber shifts of the characteristics bands of acetonitrile (ACN), [3] dimethylsulfoxide (DMSO), [4] tetrahydrofuran (THF), [5] and dioxane (DX). [6] In addition, interactions between FA molecules lead to wavenumber shifts of the C O stretching mode (basic site), [7,8] and both these shifts are in good agreement with its amphoteric character.…”

Section: Introductionmentioning

confidence: 99%

FT‐Raman, FTIR and density functional theory studies of a hydrogen‐bonded formamide:pyridine complex

Jacinto

Siqueira

2009

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Raman and IR experiments have been carried out on formamide (FA) and pyridine (Py) mixtures at different compositions. The appearance of a new Raman band at 996 cm −1 (ν 1 region of Py), whose intensity depends on the FA concentration, is assigned to an FA : Py adduct and this result is in excellent agreement with those of other authors who employed noisy light-based coherent Raman scattering spectroscopy (I (2) CARS). Another band at 1587 cm −1 (ν 8 region of Py) has been observed for the first time by using Raman and IR spectroscopies. Its intensity shows the same dependence on the FA concentration and this fact allows us to also attribute it to an FA : Py adduct. The good relationship between the Raman and IR data demonstrates the potential of the vibrational spectroscopy for this kind of study. Owing to higher absolute Raman scattering cross section, the ν 1 region of Py has been chosen for the quantitative analysis and a stoichiometry of 1 : 1 FA : Py is reported. The experimental data are very well supported by the density functional theory (DFT) calculation, which was employed for the first time to the present system. Furthermore, the actual investigation shows an excellent agreement with those reported from computational calculations for similar systems. A comparison with our previous studies confirms that the solvent dielectric constant determines the stoichiometry of a given Lewis acid-base adduct in the infinite dilution limit.