A Careful Consideration of the Influence of Structure, Partial charges and Basis Sets on Collision Cross Sections of Monosaccharides when Comparing Values from DFT Calculated Conformers to those Obtained Experimentally
Molecular modelling is routinely employed to assign 3D structures to collision cross sections (CCSs) derived from ion mobility mass spectrometry experiments (IM-MS). The assignment of model structures to the experimental CCSs remains an ambiguous task, where one of several methods may be used to obtain a CCS from a given set of coordinates. The most reliable of the commonly used techniques, the Trajectory Method, starts with atomic coordinates which can be accompanied by partial atomic charges, obtained using ab initio methods. Here, we use lithiated - and {beta}-glucose ions as exemplar molecules to detect the effect conformational modification and changes to the partial charge distribution have on computed collision cross sections. Six popular charge schemes (Mulliken, APT, CHelpG, MK, HLY and NPA) were examined in combination with three functionals (Hartree-Fock, B3LYP and M05) and five basis sets (STO-3G, 3-21G, 6-31G, 6-31+G and 6-31G*) on twenty unique structures. Our findings indicate that molecular conformation makes a significant contribution to fluctuations of partial charges in Electrostatic Potential (ESP) and Mulliken charge scheme; Partial charges derived using Natural Population Analysis (NPA) and ESP methods are largely independent of functional and basis set selection; and both selection of the charge scheme and functional/basis set combination play a large role in the resultant CCS, often causing few percent fluctuations in the computed values.