PROTON-TRANSFER IN MALONALDEHYDE - AN AB-INITIO PROJECTOR AUGMENTED WAVE MOLECULAR-DYNAMICS STUDY

Proton transfer in malonaldehyde was studied by molecular dynamics sim ulations with the projector augmented wave (PAW) method, which combine s classical dynamics with ab initio quantum mechanical forces. The PAW trajectories were calculated for several temperatures between 1 and 6 00 K, for evolution time periods up to 20 ps, and with a constant time interval of 0.12 fs. At elevated temperatures proton transfer is not associated with a well-defined C-2v-symmetric transition state, but ta kes place in widely differing geometric situations, Although a short O - O distance favors proton transfer, it is neither a sufficient nor a necessary condition. Analysis of the data by a discriminant method an d with a neural network yielded several relevant molecular parameters, and the resulting discrimination functions predicted the occurrence o f proton transfer with an accuracy greater than 95%. The energetics of the proton motion was modeled by calculating time evolutions of the p otential energy along a properly chosen reaction coordinate within a h eavy-light-heavy atom approximation At any instant the proton motion i s governed by this potential, but while the proton moves, the potentia l also changes due to the dynamics of the molecule. Three extremes can be distinguished: i) Normal periods, in which the proton is trapped a t one oxygen atom. The proton is stationary within an approximately co nstant, strongly asymmetric potential; the frequency of about 2850 cm( -1) is close to the experimentally observed v(OH) frequency. ii) Stati stical isolated proton-transfer transitions, in which the proton rapid ly moves from one oxygen atom to the other. The process starts and end s with strongly asymmetric potentials, but passes through (nearly) sym metric double- or single-minimum potentials. iii) Proton-shuttling per iods, which include several consecutive nonstatistical transitions. Th ese are not true proton transfers. The proton is (quasi)stationary wit hin a (nearly) symmetric single-minimum potential, which remains appro ximately constant for a longer time period; the motion corresponds to a v(OH) vibration with a frequency of about 2000 cm(-1).