pH Dependence of the Imidazole-2-carboxaldehyde Hydration Equilibrium: Implications for Atmospheric Light Absorbance

Imidazole-2-carboxaldehyde (IC) has been identified as an aqueous “brown carbon” absorber and a possible atmospheric photosensitizer. IC exists in a pH-dependent equilibrium between its aldehyde and geminal diol form; the diol form is the dominant species in solution at pH <5. Calculated molar absorptivity coefficients are 13700 ± 200 cm^–1 M^–1 at 287 nm for the aldehyde and 7800 ± 100 cm^–1 M^–1 at 212 nm for the diol. This shift from aldehyde to diol changes the peak light absorption of the aqueous solution from 287 to 212 nm, which is beyond the actinic range and may have implications for radiative forcing. The observed pH-dependent shift in the hydration equilibrium of IC is driven by the interaction between its hydration and protonation equilibria. Calculated p<i>K</i>_a values are 2.5 ± 0.4 and 5.94 ± 0.05 for the aldehyde and diol, respectively, and are consistent with the trend toward increasing diol as the pH decreases. The acid–base equilibrium affects both the solubility and the major species in solution depending on the protonation state of IC and may affect atmospheric light absorption, brown carbon, and photosensitization under acidic conditions. These findings indicate a need for a greater level of attention to the effect of the matrix in aqueous atmospheric systems, particularly concerning species affected by multiple equilibria

Free Energy Map for the Co-Oligomerization of Formaldehyde and Ammonia

Density functional theory calculations, including Poisson–Boltzmann implicit solvent and free energy corrections, are applied to construct a free energy map of formaldehyde and ammonia co-oligomerization in aqueous solution at pH 7. The stepwise route to forming hexamethylenetetramine (HMTA), the one clearly identified major product in a complex mixture, involves a series of addition reactions of formaldehyde and ammonia coupled with dehydration and cyclization reactions at key steps in the pathway. The free energy map also allows us to propose the possible identity of some major peaks observed by mass spectroscopy in the reaction mixture being the result of stable species not along the pathway to HMTA, in particular those formed by intramolecular condensation of hydroxyl groups to form six-membered rings with ether linkages. Our study complements a baseline free energy map previously worked out for the self-oligomerization of formaldehyde in solution at pH 7 using the same computational protocol and published in this journal (<i>J. Phys. Chem. A</i> <b>2013</b>, <i>117</i>, 12658)

Glycolaldehyde Monomer and Oligomer Equilibria in Aqueous Solution: Comparing Computational Chemistry and NMR Data

A computational protocol utilizing density functional theory calculations, including Poisson–Boltzmann implicit solvent and free energy corrections, is applied to study the thermodynamic and kinetic energy landscape of glycolaldehyde in solution. Comparison is made to NMR measurements of dissolved glycolaldehyde, where the initial dimeric ring structure interconverts among several species before reaching equilibrium where the hydrated monomer is dominant. There is good agreement between computation and experiment for the concentrations of all species in solution at equilibrium, that is, the calculated relative free energies represent the system well. There is also relatively good agreement between the calculated activation barriers and the estimated rate constants for the hydration reaction. The computational approach also predicted that two of the trimers would have a small but appreciable equilibrium concentration (>0.005 M), and this was confirmed by NMR measurements. Our results suggest that while our computational protocol is reasonable and may be applied to quickly map the energy landscape of more complex reactions, knowledge of the caveats and potential errors in this approach need to be taken into account