Proton Transfer in Malonaldehyde: An Ab Initio Projector Augmented Wave Molecular Dynamics Study

Abstract

Proton transfer in malonaldehyde was studied by molecular dynamics simulations with the projector augmented wave (PAW) method, which combines classical dynamics with ab initio quantum mechanical forces. The PAW trajectories were calculated for several temperatures between 1 and 600 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 takes 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 and with a neural network yielded several relevant molecular parameters, and the resulting discrimination functions predicted the occurrence of proton transfer with an accuracy greater than 95 %. The energetics of the proton motion was modeled by calculating time evolutions of the potential energy along a properly chosen reaction coordinate within a heavy ± light ± heavy atom approximation. At any instant the proton motion is governed by this potential, but while the proton moves, the potential also changes due to the dynamics of the molecule. Three extremes can be distinguished: i) Normal periods, in which the proton is trapped at one oxygen atom. The proton is stationary within an approximately constant, strongly asymmetric potential; the frequency of about 2850 cm À1 is close to the experimentally observed n(OH) frequency. ii) Statistical isolated proton-transfer transitions, in which the proton rapidly moves from one oxygen atom to the other. The process starts and ends with strongly asymmetric potentials, but passes through (nearly) symmetric double-or single-minimum potentials. iii) Proton-shuttling periods, which include several consecutive nonstatistical transitions. These are not true proton transfers. The proton is (quasi)-stationary within a (nearly) symmetric single-minimum potential, which remains approximately constant for a longer time period; the motion corresponds to a n(OH) vibration with a frequency of about 2000 cm À1

Tautomeric Equilibria in Relation to Pi-Electron Delocalization

“…205a,b Molecular dynamics simulations with the projector augmented wave method, which combines classical dynamics with ab initio quantum mechanical forces, showed in addition that the proton transfer between two identical cisenolone forms of malondialdehyde occurs via the tunneling effect. 213 The quantum mechanical tunneling between the C s structures apparently occurs even at room temperature and thus through a relatively low potential energy barrier. 214 In fact, this barrier was found to be equal to 6-7 kcal mol -1 .…”

Time-resolved photoelectron spectroscopy of proton transfer in the ground state of chloromalonaldehyde: Wave-packet dynamics on effective potential surfaces of reduced dimensionality

“…For example, classical approximations have been used in simulations of PT in malonaldehyde 62,63 and in studies of double PT in formic acid dimer. 25 Sharafeddin et al have also used a classical description of skeletal modes coupled to a proton wave packet.…”

Arnoldi Algorithm for the Simulation of Multidimensional Infrared Spectroscopy

“…This number increases exponentially with n and m, and only a very limited number of normal modes can be handled by standard diagonalization methods. However, we note that the Hamiltonian matrix is very sparse because all the matrix elements between two basis states which differ by more than 4 quanta vanish, if (10) where represents the number of vibrational quanta on the n'th normal mode in the i'th basis state. To save memory, the coordinate storage format was therefore used for the Hamiltonian matrix.…”

“…An ab Initio molecular dynamics study at 500 K implied a sub ps proton transfer. 10 The ab Initio potential energy surface (PES) for intramolecular proton transfer has been reported. 11,12 In the present work we compute the cross peaks in infrared four-wave mixing, induced by anharmonic couplings between the C=O stretch, H-O-C bend and O-H stretch vibrations related to the intramolecular hydrogen bonding.…”