Kinetics of Exchange between Zero-, One-, and Two-Hydrogen-Bonded States of Methyl and Ethyl Acetate in Methanol

Abstract

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 non-linear 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, 0.12 ps−1 for exchange between 1hb and 2hb states.

Enhanced vibrational solvatochromism and spectral diffusion by electron rich substituents on small molecule silanes

“…Consequently, we attribute the slower component to the hydrogen bonding dynamics of isopropanol with itself since the time scale is similar to what has been reported. [78][79][80][81] To confirm this hypothesis, we computed the FFCFs from QM/MM simulations, which were fitted to a combination of three exponentials plus a constant offset. Overall, the parameters obtained from the computed FFCFs (Table II) are in accord with those determined from experiment (Table I).…”

“…Additionally, the time constants for the second exponential are in reasonable accord with the lifetime of hydrogen bonds. [78][79][80][81] The third exponential contains dynamics that are motionally narrowed and would contribute to the homogeneous line width (Γ). The fast inhomogeneous spectral diffusion component (∆ 1 and τ 1 ) is ascribed to collective motions of bulk solvent molecules approximately resembling a dielectric continuum.…”

Effects of neutral and charged substituents on the infrared carbonyl stretching frequencies in phenyl and alkyl benzoates in DMSO

“…Infrared stretching frequency of the carbonyl group, ν CO , has been perhaps the most widely used infrared spectral feature for elucidating interactions within and between molecules. Numerous electronic, steric and solvation effects in a myriad of compounds – ketones, carboxylic acids, esters, metal carbonyl complexes, etc. – have been studied via the carbonyl group stretching frequency.…”

Ester-Derivatized indoles as fluorescent and infrared probes for hydration environments

“…The ester carbonyl group is a strong H-bonding acceptor, whereas 2-propanol (β=0.95, α=0.76) is both a strong H-bonding acceptor and a strong H-bonding donor. Therefore, the ester carbonyl group can form zero, one, or two H-bonds with 2-propanol [29]. We can assign the higher frequency peak as the ester carbonyl stretching vibration which does not form any H-bonds with 2-propanol [26,29].…”

“…Therefore, the ester carbonyl group can form zero, one, or two H-bonds with 2-propanol [29]. We can assign the higher frequency peak as the ester carbonyl stretching vibration which does not form any H-bonds with 2-propanol [26,29]. However, DMSO (β=0.76, α=0) is a strong H-bonding acceptor and a very weak H-bonding donor.…”