Effects of protonation on acetone: nuclear magnetic resonance and ab initio studies

Abstract

The electronic and geometric structural changes of acetone upon protonation are studied by ¹³C NMR spectroscopy and by molecular orbital (MO) calculations using the LORG (local orbital/local origin) method to calculate chemical shieldings and the CLOPPA (contributions for localized orbitals within the polarised propagator approach) analysis of the J coupling constants. In protonated acetone the ¹³C NMR spectrum has been resolved and two different methyl resonances (1.2 ppm apart) have been assigned. The one-bond C–C coupling constants between the carbonyl carbon and the methyl carbons differ by 4.5 Hz. The CLOPPA method was employed to study the orientational effect of the unprotonated oxygen lone pair on the coupling constants. Full ab initio optimized geometries at the 6-31G**/MP2 level are reported for acetone, its mono- and di-protonated species, the relative stabilities are discussed and geometries and Mulliken charge distributions are compared.