Time-resolved molecular dynamics of single and double hydrogen migration in ethanol

Being the lightest, most mobile atom that exists, hydrogen plays an important role in the chemistry of hydrocarbons, proteins and peptides and most biomolecules. Hydrogen can undergo transfer, exchange and migration processes, having considerable impact on the chemical behavior of these molecules. Although much has been learned about reaction dynamics involving one hydrogen atom, less is known about those processes where two or more hydrogen atoms participate. Here we show that single and double hydrogen migrations occurring in ethanol cations and dications take place within a few hundred fs to ps, using a 3D imaging and laser pump-probe technique. For double hydrogen migration, the hydrogens are not correlated, with the second hydrogen migration promoting the breakup of the C–O bond. The probability of double hydrogen migration is quite significant, suggesting that double hydrogen migration plays a more important role than generally assumed. The conclusions are supported by state-of-the-art molecular dynamics calculations.

A liquid ethanol sample is prepared in a sealed container, which is connected to the gas inlet for the COLTRIMS jet. The headspace is evacuated several times to remove the air. The ethanol vapor above the liquid is then expanded into the jet first through a 30 μm diameter nozzle, then a skimmer, providing a ~1 mm diameter, thin jet of ethanol molecules. A static, homogeneous electric field provided by the COLTRIMS spectrometer directs ions to a z-stack microchannel plate detector with a delay line (hex) anode, 27 cm from the interaction region. To collect the highest energy H + fragments with full angular acceptance, the spectrometer field was set to about 90 V/cm. This high field and a short extraction region (5 cm) made it impossible to measure electrons with any resolution, although they were also recorded during the experimental run time. The 3D momentum vectors are reconstructed from the time of flight (TOF) and position for each ion [1,2].
Coincidence channels are identified by plotting the TOF of the first ion that hits the detector versus the TOF of the second ion (subsequent analysis considers all ion pairs that satisfy momentum conservation, not just when the two coincidence ions come as TOF1 and TOF2). The three channels presented here, OH + + C2H5 + , H2O + + C2H4 + , and H3O + + C2H3 + are slightly overlapped due to the broad recoil peaks and the difference in mass of the fragments by only a hydrogen. For each channel, constraints are applied which require momentum conservation in all three dimensions to clean the channels from false coincidences.
The Ab Initio Molecular Dynamics simulations were carried out with the Atom Centered Density Matrix Propagation [3][4][5] method, ADMP, as implemented in the Gaussian09 Package [6]. The ADMP method 3 is an extended Lagrangian approach in which the classical trajectories are performed by propagating the density matrix. The electronic structure was described in the framework of the density functional theory -DFT, in particular using the B3LYP functional [7,8] in combination with the atomic centered gaussians basis set 6-31++G(d,p). In order to ensure the adiabaticity of the dynamics we imposed a time step of ∆t=0.1 fs and a fictitious electron mass of 0.1 amu.
The simulations were carried out assuming vertical ionization, following the Franck-Condon principle, taking as starting points the two conformers of neutral ethanol (whose geometries were optimized at the same level of theory B3LYP/6-31++G(d,p)). The excitation energy was randomly distributed in each trajectory among the nuclear degrees of freedom, performing the propagation in the electronic ground state of the corresponding ion. For each isomer and each value of excitation energy 500 trajectories were computed. The maximum propagation time was 3 ps; at this point two main analyses were carried out: -Two atoms have been considered to be bonded if the distance between them is smaller than 3 Å; at larger distances we assume that the atoms belong to separate fragments. The charge in the corresponding fragment is obtained as the sum of the atomic charges in the fragment adopting the Mulliken population scheme [9].
-Migration times for trajectories leading to H2O + or H3O + have been assigned by identifying the time at which the distance between the migrating H atom and O goes below 1 Å for the first time. Times for 4 OH + , H2O + or H3O + formation have been assigned by identifying when the distance between the O atom and the C atom is larger than 3 Å.
The computational strategy adopted here was successfully employed to describe the fragmentation dynamics of positively-charged molecules in the gas phase induced in collisions with highly-charged ionsglycine [10], beta-alanine [11], and gamma aminobutyric acid [12] and in X-ray ionizationthiophene [13]. An example of a full trajectory analysis is shown in supplementary figure 2. H2O + + C2H4 + and c) H3O + + C2H3 + for the triple coincidence channel H + + H2O + + C2H3 + .

Supplementary Discussion:
Here we discuss the cation and dication contributions to double coincidence and triple coincidence channels. To determine the main ionic state of the molecule following interaction with the pump pulse, we conducted a single pulse study with an intensity ~4×10 14 W/cm 2    To extract the rate constants from this model, which is the quantity that can be most likened to a H migration time, the three equations (2) to the delay dependence of the channel yields for the KER ranges indicated. For positive delays, the probe is more intense than the pump. The role of the pump and probe pulses are reversed for negative 12 delays. Error bars are estimated from propagating the error of the k values given by the fitting procedure with Origin [15].
Here we discuss the effect of the initial internal energy.