Polar Group Enhanced Gas-Phase Acidities of Carboxylic Acids: An Investigation of Intramolecular Electrostatic Interaction’ Kinetic Method

We studied the effects of polar groups on the gas-phase acidities of carboxylic acids experimentally and computationally. In this connection, the gas-phase acidities (ΔHacid, the enthalpy of deprotonation, and ΔGacid, the deprotonation free energy) of borane-complexed methylaminoacetic acid ((CH3)2N(BH3)CH2CO2H) and methylthioacetic acid (CH3S(BH3)CH2CO2H) were measured using the kinetic method in a flowing afterglow-triple quadrupole mass spectrometer. The values of ΔHacid and ΔGacid of (CH3)2N(BH3)CH2CO2H were determined to be 328.8 ± 1.9 and 322.1 ± 1.9 kcal/mol, and those of CH3S(BH3)CH2CO2H were determined to be 325.8 ± 1.9 and 319.2 ± 1.9 kcal/mol, respectively. The theoretical enthalpies of deprotonation of (CH3)2N(BH3)CH2CO2H (329.2 kcal/mol) and CH3S(BH3)CH2CO2H (325.5 kcal/mol) were calculated at the B3LYP/6-31+G(d) level of theory. The calculated enthalpies of deprotonation of N-oxide−acetic acid (CH3NOCH2CO2H, 329.4 kcal/mol) and S-oxide−acetic acid (CH3SOCH2CO2H, 328.6 kcal/mol) are comparable to the experimental results for borane-complexed methylamino- and methylthioacetic acids. The enthalpy of deprotonation of sulfone−acetic acid (CH3SO2CH2CO2H, 326.1 kcal/mol) is about 2 kcal/mol lower than the S-oxide−acetic acid. The calculated enthalpy of deprotonation of sulfoniumacetic acid, (CH3)2S+CH2CO2H, is 243.0 kcal/mol. Compared to the corresponding reference molecules, CH3NHCH2CO2H and CH3SCH2CO2H, the dipolar group and the monopolar group substituted carboxylic acids are stronger acids by 11−14 and 97 kcal/mol, respectively. We correlated the changes of the acidity upon a polar group substitution to the electrostatic free energy within the carboxylate anion. The acidity enhancements in polar group substituted carboxylic acids are the results of the favorable electrostatic interactions between the polar group and the developing charge at the carboxyl group