J. Morcillo

A critical study of the application of ultraviolet spectroscopy to the self-association of adenine, adenosine and 5′-AMP in aqueous solution

Abstract UV spectra of adenine, adenosine and 5′-AMP in aqueous solution have been measured over the concentration range 1 × 10 −6 −5 × 10 −2 M. The apparent molar absorptivity of these compounds changes upon concentration, showing two hypochromic effects at c −3 M and c &>; 5 × 10 −3 M, respectively, which may be explained in terms of self-association. A method for calculating self-association constants from these experimental data is developed, based on an association model in which the first hypochromic effect is interpreted in terms of formation of dimers, with an equilibrium constant K 2 , and the second effect is interpreted in terms of formation of polymers, with an equilibrium constant K n . The value of K 2 is of the order of magnitude of 10 4 for the three compounds. The value of K n is dependent on the model chosen for the analysis of the second effect, having an order of magnitude of 10 2 . The features of the self-association model are discussed, as well as the method for calculating self-association parameters from experimental data.

Vibrational spectra of ethyl acetate and monohaloacetates

Abstract Vibrational spectra of ethyl acetate and ethyl monohaloacetates have been studied. Infrared spectra in vapour and liquid phases, solutions in different solvents, and also Raman spectra of those compounds have been recorded in order to assign the fundamental bands. The existence of different rotamers about the CC bond joining the halomethyl and carbonyl groups produces a splitting in some bands of the spectra. It yields a more complex profile but, to some extent, it allows an easier assignment of those bands which present that splitting.

Studies of the self-association of 5′-GMP in aqueous solutions by Fourier transform infrared spectroscopy

Abstract Fourier transform infrared spectra of 5′-GMP in D2O solutions have been obta i ned and studied in the concentration range from 5.10−3M to 5.10−1M, at room temperature. For high concentrations marked changes occur in the frequency, intensity and shape of some bands. The changes are most clearly observed in the 1700–1500 cm−1 region. More dilute solutions show similar effects in presence of KCl. These changes in the infrared spectrum when the concentration of 5′-GMP increases have been attributed to the self-association of this mononucleotide in solution, in agreement with previous NMR results.

The infrared carbonyl band of ethyl haloacetates and rotational isomerism

Abstract The integrated intensities of the carbonyl band components of the following ethyl haloacetates, R-COOFt (R = CH 2 Cl, CH 2 Br, CHCl 2 and CHBr 2 ), have been measured at different temperatures. From these intensities, values of ΔH° for the equilibrium between the cis and gauche rotamers have been estimated. Ethyl haloacetates have been studied in both CCl 4 solution and in gas phase. The absolute values of ΔH° in solution are lower than in gas phase. Results obtained in gas phase indicate that the stability of the gauche rotamer increases with the size of the halogen atom.

Solvent influence on infrared spectra of the carbonyl stretching band in methyl alkyl ketones.

Abstract Solvent induced changes in frequency and intensity of the carbonyl band of five methyl alkyl ketones have been studied. Infrared spectra of MeCOR ketones (RMe, Et, iBu, sBu, tBu) in solution have been measured. Solvents with different polarity properties have been used and the results compared with data obtained for vapor in a previous work. Equations proposed by different authors relating solution values to those measured in gas phase have been considered, but they show limited validity for the systems studied. However a relationship between empirical scales of solvent polarity and carbonyl band frequencies was established that allowed calculation of a ν gas value very close to the experimental value. Significant linear correlations between intensity of the studied band and G parameter were also found (they are worse if π* is used) and the calculated intensity value for the gas phase is an acceptable estimation of the experimental one.

Vibrational spectra, normal vibrations and rotational isomerism of ethyl acetate and three deuterated analogues

Abstract IR and Raman spectra of ethyl acetates, CH3COOC2H5, CD3COOC2H5, CH3COOC2D5 and CD3COOC2D5, in the liquid and crystalline states have been recorded in the 3500-200 cm−1 region. These spectra are analysed on the basis of spectral comparison both among themselves and with spectra of the trichloro analogues CCl3COOC2H5 and CCl3COOC2D5 previously studied. All the ethyl acetates in the crystalline state give two completely separated carbonyl stretching Raman bands, suggesting that two, or an even number of molecules, exist in the crystal unit. Liquid-state spectra exhibit many band pairs arising from the trans-trans and trans-gauche conformers around CX3COOCX2CX3 axes (X = H and D) and the bands of the trans-trans conformer persist in the crystalline state. Normal coordinate calculations for the four species have been carried out on the two isomeric models using an MVIB program. The results explain wavenumber shifts on deuteration and the nature and separation of band pairs satisfactorily, and suggest that a large amount of vibrational coupling exists among skeletal stretching vibrations.

Identification of ionic and tautomeric species of uracil by second derivative UV absorption spectroscopy.

Abstract The individual UV absorption spectra of ionic and tautomeric species of uracil and eight derivatives have been determined by second derivative spectroscopy. Second derivative bands have been ascribed to different species by taking into account the proportion of each molecular species at each pH value and the tautomer fixed for derivatives. Theoretical results in the literature are in good agreement with the observed λ Min (D 2nd ) positions of both N(1)- and N(3)- uracil monoanion and dianion. Neutral uracil species (diketo form) presents six second derivative bands (two in normal spectra). For the three higher electronic transition energies, the discrepancy between calculated and experimental values does not exceed 0.05 e V. Literature assignment from normal spectra for the N(3)- anion are ππ* transitions but our results show that n π* are possible too.

Vibrational spectra and rotational isomerism of ethyl trichloroacetate

Abstract The IR and Raman spectra of ethyl trichloroacetate (E-TCA) and its deuterate (E-TCA-d5) have been measured in the liquid, glassy and crystalline states. Vibrational assignment was made by referring to isotopic wavenumber-shift, characteristic group frequencies of related esters and with the aid of a normal coordinate calculation on E-TCA and E-TCA-d5. It is suggested that in the liquid and glassy states there exist two molecular forms (trans—trans—trans and trans—trans—gauche) with regard to the internal rotations about the ClCCOCH2CH3 axis, and that the former persists in the crystalline state. The band pairs of E-TCA and the other ethyl esters are discussed in relation to the nature and number of rotational axes, effect of the heavy trichloromethyl group, and enhancement of band intensity by vibrational coupling.

Fourier transform infrared studies of 5′-GMP in 2H2O and H2O solutions.

Abstract In a previous work (ref. 1) we observed important changes in the 1700–1400 cm −1 region of FTIR spectra in 2 H 2 O solutions when 5′-GMP concentration increases. These changes can be attributed to the self-association of this mononucleotide. Recently, study of this process has been extended to other regions of the spectrum and to H 2 O solution. Fourier deconvolution has been employed in order to resolve the broad band into component bands. Differences have been observed between spectra in H 2 O and 2 H 2 O for the same solute concentration. The possible causes of these differences are indicated.

Rotational isomerism and solvent effect on infrared intensities of ν(CO) stretching band in methyl alkyl ketones

Abstract A solvent-sensitive rotational equilibrium has been observed in several methyl alkyl keyones. This equilibrium arises from the rotational of the R group around thr R-(CO) bond. The variation in relative intensity of the two bands observed in methyl isobutyl ketone has been measured. Frequency and intensity of each band and the total intensity have been ralated to the solvent properties using the G parameter as a measure solvent polarity. From these relationships the stable conformer in gas phase has been determined.