Lennard-Jones interaction parameters of Mo and W in He and N-2 from collision cross-sections of Lindqvist and Keggin polyoxometalate anions

Drift tube ion mobility spectrometry (DTIMS) coupled with mass spectrometry was used to determine the collision cross-sections ((CCS)-C-DT) of polyoxometalate anions in helium and nitrogen. As the geometry of the ion, more than its mass, determines the collision cross-section with a given drift gas molecule, we found that both Lindqvist ions Mo6O192- and W6O192- had a (CCSHe)-C-DT value of 103 +/- 2 angstrom(2), and both Keggin ions PMo12O403- and PW12O403- had a (CCSHe)-C-DT value of 170 +/- 2 angstrom(2). Similarly, ion mobility experiments in N-2 led to (CCSN2)-C-DT values of 223 +/- 2 angstrom(2) and 339 +/- 4 angstrom(2) for Lindqvist and Keggin anions, respectively. Using optimized structures and partial charges determined from density functional theory calculations, followed by CCS calculations via the trajectory method, we determined Lennard-Jones 6-12 potential parameters epsilon, sigma of 5.60 meV, 3.50 angstrom and 3.75 meV, 4.40 angstrom for both Mo and W atoms interacting with He and N-2, respectively. These parameters reproduced the CCS of polyoxometalates within 2% accuracy.