An ab initio direct dynamics simulation of protonated glycine surface-induced dissociation

A QM + MM direct dynamics simulation, using MP2/6-31G* theory as a model for the intramolecular potential of protonated glycine (gly-H+), is used to study gly-H+ + diamond {1 1 1} SID. The simulations are performed for collisions normal (θi = 0°) and oblique (θi = 45°) to the surface and at a collision energy of 70 eV (1614 kcal/mol). The gly-H+ energy-transfer dynamics, observed in this study, are in accord with previous studies in which AMBER and AM1 were used for the ion's intramolecular potential [S.O. Meroueh, Y. Wang, W.L. Hase, J. Phys. Chem. A 106 (2002) 9983]. A particularly important finding is that a significant fraction of the gly-H+ ions fragment by a shattering mechanism as they collide with the surface. This result supports earlier studies in which shattering fragmentation was also observed for both gly-H+ and gly2-H+, in QM + MM direct dynamics simulations in which the AM1 semiempirical QM model was used for the ion's intramolecular potential, instead of the MP2/6-31G* model. Using MP2/6-31G* the predominant shattering fragmentation channels, in decreasing order of importance, are NH3 + CH2COOH+, NH3 + CO + CH2OH+, H2 + NH2CHCOOH+, and NH2CH2+ + C(OH)2 for θi = 0°, and NH3 + CH2COOH+, NH2CH2+ + C(OH)2, NH2CH2+ + HCOOH, and NH + C(OH)2CH3+ for θi = 45°. SID at θi = 45° was studied previously with AM1 and the percentage of gly-H+ trajectories which shatter with MP2/6-31G* is the same as found for AM1.