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

A QM + MM direct dynamics simulation, using MP2/6-3 IG* theory as a model for the intramolecular potential of protonated glycine (gly-H+), is used to study gly-H+ + diamond {111} SID. The simulations are performed for collisions normal (theta = 0 degrees) and oblique (theta = 45 degrees) to the surface and at a collision energy of 70eV (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 gly(2)-H+, in QM + MM direct dynamics simulations in which the AM I 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+, H-2 + NH2CHCOOH+, and NH2CH2+ + C(OH)(2) for theta = 0 degrees, and NH3 + CH2COOH+, NH2CH2+ + C(OH)(2), NH2CH2+ + HCOOH, and NH + C(OH)(2)CH3+ for theta = 45 degrees. SID at theta = 45 degrees was studied previously with AM1 and the percentage of gly-H+ trajectories which shatter with MP2/6-31G* is the same as found for AMI. (c) 2007 Elsevier B.V. All rights reserved.