TY - JOUR
T1 - How Acidic Is Carbonic Acid? Graphical abstract HHS Public Access
AU - Pines, Dina
AU - Ditkovich, Julia
AU - Mukra, Tzach
AU - Miller, Yifat
AU - Kiefer, Philip M.
AU - Daschakraborty, Snehasis
AU - Hynes, James T.
AU - Pines, Ehud
N1 - Publisher Copyright:
© 2016 American Chemical Society.
PY - 2016/3/17
Y1 - 2016/3/17
N2 - Carbonic, lactic, and pyruvic acids have been generated in aqueous solution by the transient protonation of their corresponding conjugate bases by a tailor-made photoacid, the 6-hydroxy-1-sulfonate pyrene sodium salt molecule. A particular goal is to establish the pK a of carbonic acid H 2 CO 3. The on-contact proton transfer (PT) reaction rate from the optically excited photoacid to the carboxylic bases was derived, with unprecedented precision, from time-correlated single-photon-counting measurements of the fluorescence lifetime of the photoacid in the presence of the proton acceptors. The time-dependent diffusion-assisted PT rate was analyzed using the Szabo-Collins-Kimball equation with a radiation boundary condition. The on-contact PT rates were found to follow the acidity order of the carboxylic acids: the stronger was the acid, the slower was the PT reaction to its conjugate base. The pK a of carbonic acid was found to be 3.49 ± 0.05 using both the Marcus and Kiefer-Hynes free energy correlations. This establishes H 2 CO 3 as being 0.37 pK a units stronger and about 1 pK a unit weaker, respectively, than the physiologically important lactic and pyruvic acids. The considerable acid strength of intact carbonic acid indicates that it is an important protonation agent under physiological conditions.
AB - Carbonic, lactic, and pyruvic acids have been generated in aqueous solution by the transient protonation of their corresponding conjugate bases by a tailor-made photoacid, the 6-hydroxy-1-sulfonate pyrene sodium salt molecule. A particular goal is to establish the pK a of carbonic acid H 2 CO 3. The on-contact proton transfer (PT) reaction rate from the optically excited photoacid to the carboxylic bases was derived, with unprecedented precision, from time-correlated single-photon-counting measurements of the fluorescence lifetime of the photoacid in the presence of the proton acceptors. The time-dependent diffusion-assisted PT rate was analyzed using the Szabo-Collins-Kimball equation with a radiation boundary condition. The on-contact PT rates were found to follow the acidity order of the carboxylic acids: the stronger was the acid, the slower was the PT reaction to its conjugate base. The pK a of carbonic acid was found to be 3.49 ± 0.05 using both the Marcus and Kiefer-Hynes free energy correlations. This establishes H 2 CO 3 as being 0.37 pK a units stronger and about 1 pK a unit weaker, respectively, than the physiologically important lactic and pyruvic acids. The considerable acid strength of intact carbonic acid indicates that it is an important protonation agent under physiological conditions.
UR - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5747581/pdf/nihms929639.pdf
UR - https://www.mendeley.com/catalogue/26ccb2e9-508e-3205-b035-c2a3952d2869/
U2 - 10.1021/acs.jpcb.5b12428
DO - 10.1021/acs.jpcb.5b12428
M3 - Article
AN - SCOPUS:84961194538
SN - 1520-6106
VL - 120
SP - 2440
EP - 2451
JO - Journal of Physical Chemistry B
JF - Journal of Physical Chemistry B
IS - 9
ER -