The gas-phase acidities of substituted carbonic acids

Abstract

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the structure, gas-phase acidities and vibrational spectra of carbonic acid, H 2CO 3, and several of its derivatives RCO 2H (R=F, Cl, NH 2, H and CH 3). Geometry optimizations were performed at the MP2(FC)/6-31G(†) level of theory. The syn forms always are more stable than the anti. The highest gas-phase acidity (1288.5 kJ mol −1) has been calculated for chloroformic acid. Carbonic acid is, according to calculated ionizations of the first hydrogen, about 24 kJ mol −1 more acidic than formic acid. The fundamental vibrational frequencies have been computed using the Becke3LYP/6-311+G(d,p) method. The computed vibrational frequencies for formic and acetic acids were on average 2% higher than the observed values.