Intramolecular Dispersion Attraction in Tetraalkylammonium Cations Revealed by Cryogenic Ion Mobility Mass Spectrometry.

We performed cryogenic ion mobility mass spectrometry and quantum chemical calculations of tetraalkylammonium (TAA) cations, (CnH2n+1)4N+ (n = 4-8), to study geometrical structures of TAA cations. We measured collision cross sections (CCSs) of TAA cations with He buffer gas atoms at 86 K. In addition, CCSs were calculated for optimized structures of TAA ions to compare with experimental CCSs. For n = 4 and 5, calculated CCSs of nearly planar conformers with all-trans four alkyl chains agree well with experimental CCSs. On the other hand, for n = 8, calculated CCSs of a conformer with two gauche alkyl chains reproduce experimental CCSs. The structural transition from all-trans to gauche conformers occurs around n = 6-8. The dispersion attraction between alkyl chains is a major interaction to stabilize the gauche conformer of n = 8.