Comparative ab initio/GIAO-CCSD(T) study of protonated propionaldehyde, isobutyraldehyde and pivalaldehyde cations and their superelectrophilic diprotonated dications

Structures of superelectrophilic diprotonated propionaldehyde (CH3CH2CHOH22+), isobutyraldehyde ((CH3)2CHCHOH22+) and pivalaldehyde ((CH3)3CCHOH22+) dications and their parent monocations were calculated using ab initio methods at the MP2/6-31G∗∗ and MP2/cc-pVTZ levels. Proton affinities of the neutral as well as monoprotonated aldehydes were calculated. Compared to proton affinities of neutral aldehydes, relative proton affinities of monoprotonated aldehydes were found to increase much more rapidly with the increase of methyl substitution. 13C NMR chemical shifts were also calculated using the GIAO-CCSD(T), GIAO-MP2 and GIAO-SCF methods. 13C NMR chemical shifts of the related tert-butyl cation ((CH3)3C+) and protonated tert  -butyl dication ((CH3)2CCH42+) were computed at the same level for comparison and to explore the effect of the additional charge in dications.