Effect of pressure on the proton transfer rate from a photoacid to a solvent. 4. Photoacids in methanol.
The pressure dependence of the excited-state proton dissociation rate constant of four photoacids, 2-naphthol-6,8-disulfonate (2N68DS), 10-hydroxycamptothecin (10-CPT), 5-cyano-2-naphthol (5CN2), and 5,8-dicyano-2-naphthol (DCN2), are studied in methanol. The results are compared with the results of the pressure dependence study we recently conducted for several photoacids in water, ethanol, and propanol. The pressure dependence is explained using an approximate stepwise two-coordinate proton transfer model. The increase in rate, as a function of pressure, manifests a strong dependence of proton tunneling on the distance which decreases with an increase of pressure between the two oxygen atoms involved in the process. The decrease in the proton transfer rate with increasing pressure reflects the dependence of the reaction on the solvent relaxation rate. We found that, for the relatively weak photoacids 2N68DS, 10-CPT, and 5CN2, the proton transfer rate constant increases by a factor of about 5-8 at a pressure of about 1.5 GPa. For a strong photoacid like DCN2, the rate increase was only by a factor of 2.
Genosar
Liat
L
Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel.
Lasitza
Tatiana
T
Gepshtein
Rinat
R
Leiderman
Pavel
P
Koifman
Nahum
N
Huppert
Dan
D
D000143:Acids / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D000432:Methanol / Q000737:chemistry*; D015394:Molecular Structure; D010777:Photochemistry; D011312:Pressure; D011522:Protons; D012997:Solvents / Q000737:chemistry*; D013997:Time Factors; D014783:Viscosity
D000143:Acids; D011522:Protons; D012997:Solvents; D000432:Methanol