Kinetics of exchange between zero-, one-, and two-hydrogen-bonded states of methyl and ethyl acetate in methanol.
It has recently been shown that the ester carbonyl stretching vibration can be used as a sensitive probe of local electrostatic field in molecular systems. To further characterize this vibrational probe and extend its potential applications, we studied the kinetics of chemical exchange between differently hydrogen-bonded (H-bonded) ester carbonyl groups of methyl acetate (MA) and ethyl acetate (EA) in methanol. We found that, while both MA and EA can form zero, one, or two H-bonds with the solvent, the population of the 2hb state in MA is significantly smaller than that in EA. Using a combination of linear and nonlinear infrared measurements and numerical simulations, we further determined the rate constants for the exchange between these differently H-bonded states. We found that for MA the chemical exchange reaction between the two dominant states (i.e., 0hb and 1hb states) has a relaxation rate constant of 0.14 ps(-1), whereas for EA the three-state chemical exchange reaction occurs in a predominantly sequential manner with the following relaxation rate constants: 0.11 ps(-1) for exchange between 0hb and 1hb states and 0.12 ps(-1) for exchange between 1hb and 2hb states.
†Ultrafast Optical Processes Laboratory and ‡Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.
Lev
L
Chuntonov
†Ultrafast Optical Processes Laboratory and ‡Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.
Ileana M
IM
Pazos
†Ultrafast Optical Processes Laboratory and ‡Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.
Jianqiang
J
Ma
†Ultrafast Optical Processes Laboratory and ‡Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States.
Feng
F
Gai
D000085:Acetates / Q000737:chemistry*; D003198:Computer Simulation; D006860:Hydrogen Bonding; D007700:Kinetics; D000432:Methanol / Q000737:chemistry*; D008956:Models, Chemical; D012997:Solvents; D013055:Spectrophotometry, Infrared
D000085:Acetates; D012997:Solvents; C007650:ethyl acetate; C046923:methyl acetate; D000432:Methanol