High energy conformers of M+(APE)(H2O)0–1Ar0–1 clusters revealed by combined IR-PD and DFT-MD anharmonic vibrational spectroscopy

Abstract

IR-PD vibrational spectroscopy and DFT-based molecular dynamics simulations are combined in order to unravel the structures of M(+)(APE)(H2O)0-1 ionic clusters (M = Na, K), where APE (2-amino-1-phenyl ethanol) is commonly used as an analogue for the noradrenaline neurotransmitter. The strength of the synergy between experiments and simulations presented here is that DFT-MD provides anharmonic vibrational spectra that unambiguously help assign the ionic clusters structures. Depending on the interacting cation, we have found that the lowest energy conformers of K(+)(APE)(H2O)0-1 clusters are formed, while the lowest energy conformers of Na(+)(APE)(H2O)0-1 clusters can only be observed through water loss channel (i.e. without argon tagged to the clusters). Trapping of higher energy conformers is observed when the argon loss channel is recorded in the experiment. This has been rationalized by transition state energies. The dynamical anharmonic vibrational spectra unambiguously provide the prominent OH stretch due to the OH···NH2 H-bond, within 10 cm(-1) of the experiment, hence reproducing the 240-300 cm(-1) red-shift (depending on the interacting cation) from bare neutral APE. When this H-bond is not present, the dynamical anharmonic spectra provide the water O-H stretches as well as the rotational motion of the water molecule at finite temperature, as observed in the experiment.

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

“…low and higher energy conformers. Experimental and dynamical theoretical spectra including anharmonic vibrational effects 18,52 can furthermore be compared at that stage. This spectroscopic comparison is presented in ref.…”

Influence of hydration on ion–biomolecule interactions: M+(indole)(H2O)n(M = Na, K; n = 3–6)

“…The effective temperatures of cluster ions can be deduced using RRKM-EE formulism. 53,74 Previous studies have shown that the effective temperatures of cation-biomolecule-water complexes, where water loss is the predominant evaporative cooling mechanism, are B300 K. 24,[45][46][47] Isotopic (H/D) analyses were applied to M + (indole)(H 2 O) 5,6 clusters to facilitate the identification of specific vibrational features and will be discussed in detail in the corresponding sections.…”

“…Following the absorption of a single photon, fragmented species are mass selected by a second quadrupole mass filter, tuned to the H 2 O loss channel, and detected by a Channeltron electron multiplier detector in pulse mode. The OPO/OPA laser scans through the OH/NH (3300 cm À1 to 3800 cm À1 ) and OD (2350 cm À1 to 2850 cm À1 ) stretching frequency regions at a step of 3 cm À1 (the typical Full Width Half Maximum, FWHM, of the vibrational spectral features are more than 15 cm À1 in our experimental set-up 24,46,47,75 ). Absolute frequencies are determined by calibration with known water absorption lines that are simultaneously detected photoacoustically and recorded with the IRPD spectrum.…”

“…24 Another challenge to the IRPD spectral analysis of the indole-cation-water complex is the intrinsic overlap of H-bonded OH stretches and H-bonded NH stretches that have been encountered in a wide range of systems. 25,[41][42][43][44][45][46][47] A more recent theoretical study of cationindole-water complexes with up to six H 2 O suggested more possible configurations, in which H 2 O molecules fill the first hydration shell of the cation and the water-cation complex interacts with indole via hydrogen bonding to the p system. 48 Monovalent cations-water interactions have been systematically studied using IRPD spectroscopy over the past decade.…”

Gas-Phase Infrared Spectroscopy of Neutral Peptides: Insights from the Far-IR and THz Domain

“…Such strategies can be found in e.g. refs , , , and − for various gas-phase molecular systems. Exploring and localizing minima on the PES are nowadays a routine performed by all well-known classical MD and quantum chemistry packages (e.g., GROMACS, LAMMPS, Schrodinger suite, Gaussian, TURBOMOLE, ADF, Gamess, NWChem, ORCA, and for most other popular packages used in the gas-phase community).…”