Pt⁺(C₂H₂)_n Complexes Studied with Selected-Ion Infrared Spectroscopy

Abstract

Platinum cation complexes with multiple acetylene molecules are studied with mass spectrometry and infrared laser spectroscopy. Complexes of the form Pt + (C 2 H 2 ) n are produced in a molecular beam by laser vaporization, analyzed with a time-of-flight mass spectrometer, and selected by mass for studies of their vibrational spectroscopy. Photodissociation action spectra in the C−H stretching region are compared to the spectra predicted for different structural isomers using density functional theory. The comparison between experiment and theory demonstrates that platinum forms cation−π complexes with up to three acetylene molecules, producing an unanticipated asymmetric structure for the three-ligand complex. Additional acetylenes form solvation structures around this three-ligand core. Reacted structures that couple acetylene molecules (e.g., to form benzene) are found by theory to be energetically favorable, but their formation is inhibited under the conditions of these experiments by large activation barriers.

The Near-Sightedness of Many-Body Interactions in Anharmonic Vibrational Couplings

“…The characteristic vibrational motions of atoms serve as signatures for the identification of molecules and functional groups, as well as drivers of thermodynamic functions during chemical transformations . The nuanced shifts in these frequencies reflect essential electronic-structure characteristics deriving from both covalent bonding and noncovalent interactions. − Infrared and Raman spectroscopies probe this vibrational behavior and, albeit indirectly, the potential energy and dipole/polarizability surfaces that govern vibrational dynamics. Couplings between vibrational motions serve as the physical mechanism by which heat and vibrational energy transfer occur, − and the same couplingsnow probed by multidimensional spectroscopy experiments − also serve as the dominant contributor to deviations from a canonical harmonic reference picture …”