Infrared Spectroscopy and 193 nm Photochemistry of Methylamine Isolated in Solid Parahydrogen

Abstract

The in situ UV photolysis of a precursor molecule trapped in a parahydrogen (pH2) matrix is a simple method used to generate isolated radical photofragments that are well suited for infrared spectroscopic studies. However, for molecules that can dissociate via multiple pathways, little is known about how the pH2 matrix influences the branching among these open pathways. We report FTIR spectroscopic studies of the 193 nm photodecomposition of methylamine (MA, CH3NH2) isolated in pH2 quantum matrixes at 1.8 K. We observe single exponential decay of the MA precursor upon irradiation and the quantum yield for MA photodissociation is measured to be Φ = 0.26(2) consistent with a weak pH2 cage effect. By comparing to gas-phase results, we show the in situ photolysis results in greater production of molecular products (CH2═NH + H2) compared to radical products (CH3NH + H) consistent with the idea of partial caging of the H atom photofragments. The information gained in this work can be used to guide future photolysis studies in pH2 matrixes.

The Study of Reactive Intermediates in Condensed Phases

“…Recent breakthroughs in single-molecule imaging based on atomic force microscopy (AFM) and scanning tunneling microscopy (STM) are enabling the mapping of structures of reactive intermediates frozen on solid surfaces under ultra-high vacuum; for example, Pavlicek et al recently reported AFM observation of arynes produced by elimination of two iodine atoms from a polycyclic aromatic ortho-diiodoarene molecule. 53 Rare-gas, 54,55 and more recently para-hydrogen, 56 matrices have been extensively used in combination with infra-red absorption spectroscopy to characterize vibrational modes of the intermediates, and the development of superfluid helium nanodroplets as inert hosts now affords rotationally resolved spectra of exotic species. 57,58 However, these experimental studies of isolated and trapped molecules do not generally reveal the chemical lifetimes of the intermediates or their reaction dynamics.…”

Hydrogen‐atom tunneling reactions in solid para‐hydrogen and their applications to astrochemistry

“…[20][21][22] 3.2 | Direct photochemical production at 193 nm Anderson's group reported the direct generation of H atoms via photolysis of precursor molecules in situ at 193 nm in a p-H 2 matrix. Studies on HCOOH, [41] CH 3 NH 2, [42] and NC(O)NH(CH 3 ) [43] were conducted to explore H-atom reactions, in which the H-abstraction channel was predominantly observed. This method provides important insight to understand H-atom tunneling reactions occurring in a p-H 2 matrix, but the photolysis at 193 nm typically induces photo-fragmentation of a parent species or the products and interferes in the isolation of radicals of interest.…”

“…This study demonstrated the dual-cyclic mechanism of H-abstraction and H-addition reactions (depicted in Figure 6i) and a chemical connection between CH 3 NH 2 and CH 2 NH in the ISM. Mutunga and Anderson [42] performed an experiment in which a CH 3 NH might not be an optimal approach for the identification of reaction intermediates. We investigated glycine [NH 2 CH 2 C(O)OH] because it is both the simplest amino acid and a key building block in the synthesis of protein.…”

“…This study demonstrated the dual‐cyclic mechanism of H‐abstraction and H‐addition reactions (depicted in Figure 6i) and a chemical connection between CH 3 NH 2 and CH 2 NH in the ISM. Mutunga and Anderson [ 42 ] performed an experiment in which a CH 3 NH 2 / p ‐H 2 matrix was irradiated at 193 nm at 1.7 K; they recorded IR spectra of CH 4 and NH 3 species that were formed from photoproducts •CH 3 and •NH 2 . Additionally, strong enhancement of closed‐shell molecule CH 2 NH rather than isolation of either •CH 2 NH 2 or CH 3 NH• was reported.…”