- Raman spectroscopy of glyoxal oligomers in aqueous solutions
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Raman microscopy and Attenuated Total Reflection infrared spectroscopy were utilized to facilitate investigations of equilibria between various hydrated and oligomeric forms of glyoxal in aqueous glyoxal solution droplets. The assignment of spectra is obtained with the assistance of B3LYP density functional quantum chemical calculations of vibrational wavenumbers, Raman activities, and infrared intensities. Several forms of glyoxal derivatives with similar functional groups, e.g., hydroxyl and dioxolane rings, are found to be present. The absence of a Raman spectral peak corresponding to the vibrational carbonyl stretch provides evidence that both carbonyl groups of a glyoxal molecule become hydrated in solutions of a broad concentration range. The presence of bands corresponding to deformation vibrations of the dioxolane ring indicates that dihydrated glyoxal oligomers are formed in glyoxal solutions with concentrations of 1 M and higher. Under typical ambient temperature and humidity conditions, concentrated glyoxal solution droplets undergo evaporation with incomplete water loss. Our results suggest that formation of crystalline glyoxal trimer dihydrate from concentrated solutions droplets is hindered by the high viscosity of the amorphous trimer and requires dry conditions that could rarely be achieved in the atmosphere. However, crystallization may be possible for droplets of low initial glyoxal concentrations, such as those produced by evaporating cloud droplets.
- Avzianova, Elena,Brooks, Sarah D.
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- Investigation of the mechanism of dissociation of glycolaldehyde dimer (2,5-dihydroxy-1,4-dioxane) by FTIR spectroscopy
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Glycolaldehyde represents the simplest α-hydroxycarbonyl moiety - a common structural feature of reducing sugars. It exists in solid state, only in crystalline dimeric form as 2,5-dihydroxy-1,4-dioxane. However, in solution phase or during heating, it dissociates into different dimeric and monomeric forms. FTIR spectroscopy was used to study the effect of temperature, pH and solvent on the dissociation and chemical transformations of glycolaldehyde. The infrared spectra were recorded in different solvents as a function of time and temperature (both during heating and cooling cycles) between 30 and 85°C. During heating, glycolaldehyde cyclic dimer generated two bands in the carbonyl region, one at 1744 cm-1 and the other at 1728 cm-1. These bands increased during the heating cycle and decreased during the cooling cycle. The data indicated that the glycolaldehyde cyclic dimer (2,5-dihydroxy-1,4-dioxane) undergoes a ring opening to form the acyclic dimer (1728 cm-1) that can recyclize into the 2-hydroxymethyl-4-hydroxy-1,3-dioxolane structure. The acyclic dimer can also dissociate into monomeric glycoladehyde (1744 cm-1) in equilibrium with the enediol form (1703 cm-1). There is evidence to indicate oxidation of glycolaldehyde into glycolic acid during heating, in either neutral or basic aqueous solutions.
- Yaylayan, Varoujan A.,Harty-Majors, Susan,Ismail, Ashraf A.
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- Kinetics, Mechanism, and Thermodynamics of Glyoxal-S(IV) Adduct Formation
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The reversible addition of glyoxal (ethanedial) and S(IV) to form glyoxal monobisulfite (GMBS) was studied spectrophotometrically over the pH range of 0.7-3.3.Far from equilibrium, the rate of GMBS formation is given by d/dt = (k1,appα1) + k2,appα2), where = + + , = + -> + 2->, α1 = ->/, and α2 = 2->/.The apparent rate constants, k1,app = 0.13 M-1 s-1 and k2,app = 2.08E3 M-1 s-1, are pH independent functions of the dehydration equilibrium constants of (CH(OH)2)2 and CHOCH(OH)2, and intrinsic rate constants for the reaction of HSO3- and SO32- with unhydrated and singly hydrated glyoxal.Glyoxal dibisulfite (GDBS) and GMBS were shown to dissociate with a rate given by d/dt = -1 + k"-1KD3 + (k'-2K'a2 + k"-2K"a2KD3)/+>>t + -3 + k-4Ka3/+>>t, where k-1 and k-2 correspond to the release of bisulfite and sulfite, respectively, from unhydrated and hydrated GMBS species; k-3 and k-4 correspond to the release of bisulfite and sulfite from GDBS; KD3 is the dehydration constant for GMBS; K'a2, K"a2, and Ka3 are acid dissociation constants.Stability constants for the formation of GMBS and GDBS were determined to be cK1 = ->/(-) = 2.81E4 M-1 and cK2 = -)2>/(->->) = 1.45E4 M-1 at 25 deg C and μ = 0.2 M.
- Olson Terese M.,Hoffmann, Michael R.
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p. 533 - 540
(2007/10/02)
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