Stalking Higher Energy Conformers on the Potential Energy Surface of Charged Species

Abstract

Combined theoretical DFT-MD and RRKM methodologies and experimental spectroscopic infrared predissociation (IRPD) strategies to map potential energy surfaces (PES) of complex ionic clusters are presented, providing lowest and high energy conformers, thresholds to isomerization, and cluster formation pathways. We believe this association not only represents a significant advance in the field of mapping minima and transition states on the PES but also directly measures dynamical pathways for the formation of structural conformers and isomers. Pathways are unraveled over picosecond (DFT-MD) and microsecond (RRKM) time scales while changing the amount of internal energy is experimentally achieved by changing the loss channel for the IRPD measurements, thus directly probing different kinetic and isomerization pathways. Demonstration is provided for Li(+)(H2O)3,4 ionic clusters. Nonstatistical formation of these ionic clusters by both direct and cascade processes, involving isomerization processes that can lead to trapping of high energy conformers along the paths due to evaporative cooling, has been unraveled.

Infrared Predissociation Vibrational Spectroscopy of Li+(H2O)3–4Ar0,1 Reanalyzed Using Density Functional Theory Molecular Dynamics

“…29 The IR-PD spectrum of Li + (H 2 O) 3 recorded through the H 2 O loss channel (Figure 4, bottom) still displays the symmetric OH stretch observed at lower temperature for isomer 3 + 0 ( Figure 3, bottom), whereas the well-resolved antisymmetric band is now replaced by a rotational sub-band structure due to free rotation of the water molecules around their symmetry axes. The perpendicular vibrational transition moment gives rise to ΔK = ±1 free rotor transitions.…”

“…This illustrates very well that the lowest energy conformer 4 + 0 of Li + (H 2 O) 4 cannot be completely ruled out from the distribution of conformers formed in the experiment, as nicely shown by the collisions. 29 Figure 3 now presents the IR-PD spectrum of Li + (H 2 O) 3 Ar recorded through the Ar loss channel (bottom) together with the dynamical IR spectra computed at 50 K for the three lower energy conformers of Li + (H 2 O) 3 , namely 3 + 0, 2 + 2, and 2 + 1, respectively, second, third, and fourth traces in Figure 3. The 2 + 2 and 2 + 1 conformers were found to be quasi isoenergetic (Table S1, Supporting Information), separated by a barrier of about 4.0 kJ/mol (free energy barrier from metadynamics simulations, see Figure S2 in the Supporting Information), and about 30 kJ/mol higher in energy than the 3 + 0 lowest energy conformer (Table S1, Supporting Information).…”

“…28 Recently, we have applied DFT-MD simulations to directly model the formation processes of these ionic clusters. 29 The formation of highenergy conformers was confirmed and discussed with respect to temperature.…”

“…In our previous investigation of the collisional processes leading to the formation of Li + (H 2 O) 3−4 clusters by DFT-MD, combined with RRKM statistical calculations, 29 we have estimated that the Li + (H 2 O) 3−4 Ar argonated species probed through the Ar loss channel in IR-PD have a temperature around 60 K. We have consequently chosen this temperature for the DFT-MD trajectories to get the theoretical spectroscopic features of the low-temperature clusters. For the theoretical spectroscopic features of non-argonated species Li + (H 2 O) 3−4 , corresponding to a water loss channel in the IR-PD experiment, we have run different temperatures and searched for the best match to the experimental features, so that we could provide an estimate of the clusters temperature probed in IR-PD.…”

“…These two conformers are separated by about ∼2−7 kJ/mol (electronic energies), depending on the level of theory employed (Table S2, Supporting Information), with the 4 + 0 being the lowest energy conformer. The direct dynamics of the collisions for the formation of Li + (H 2 O) 4 coupled with RRKM statistical calculations 29 have shown, at low temperature (100 K), that the higher energy conformer 3 + 2 is predominantly formed together with a smaller population of the lowest energy isomer 4 + 0.…”