Infrared spectroscopy of the acetyl cation and its protonated ketene isomer

Abstract

[C2,H3,O](+) ions are generated with a pulsed discharge in a supersonic expansion containing methyl acetate or acetone. These ions are mass selected and their infrared spectra are recorded via laser photodissociation and the method of argon tagging. Computational chemistry is employed to investigate structural isomers and their spectra. The acetyl cation (CH3CO(+)) is the global minimum and protonated ketene (CH2COH(+)) is the next lowest energy isomer (+176.2 kJ/mol). When methyl acetate is employed as the precursor, the infrared spectrum reveals that only the acetyl cation is formed. Partially resolved rotational structure reveals rotation about the C3 axis. When acetone is used as the precursor, acetyl is still the most abundant cation, but there is also a minor component of protonated ketene. Computations reveal a significant barrier to interconversion between the two isomers (+221 kJ/mol), indicating that protonated ketene must be obtained via kinetic trapping. Both isomers may be present in interstellar environments, and their implications for astrochemistry are discussed.

Discovery of the acetyl cation, CH3CO+, in space and in the laboratory

“…The acetyl radical, CH 3 CO, has been observed in the laboratory by Hirota et al (2006), but it is asymmetric and its lines show a very complex hyperfine structure. However, CH 3 CO + is a symmetric rotor (Mosley et al 2014) and the lowest energy isomer of H 3 C 2 O + . Its possible precursor, if formed through protonation, is ketene, which is one of the most abundant O-bearing species in TMC-1 (see Cernicharo et al 2020c).…”

“…A solenoid coil wound on the cell can generate an axial magnetic field (up to 300 G) to magnetically extend the negative glow, the region known to produce the highest concentrations of cations (compared to the positive column discharge; De Lucia et al 1983). We also tried acetone and acetaldehyde as precursors (Mosley et al 2014), but without success.…”

Recent advances in ketene chemistry

“…178 The energies of six isomeric structures of protonated ketene are reported there (Figure 14). Protonation of the ketene complex 371 ([Mo2Cp2{μ-C(Ph)CO)}(μ-PCy2)(CO)2]) gave the metal complex 372 in 88% yield as a red solid, and was interpreted as involving protonation on oxygen (Scheme 131).…”

Descendant of the X-ogen carrier and a "mass of 69": Infrared action spectroscopic detection of HC$_3$O$^+$ and HC$_3$S$^+$

“…Thioacylium species appear to have only been studied using mass spectrometry while gasphase spectroscopy of acylium ions is limited to only a few species. The acetyl cation, CH 3 CO + , for example, has been studied in a free-jet expansion source by infrared (IR) photodissociation of its van der Waals complex CH 3 CO + −Ar [24]. An electronic spectrum of HC 7 O + in the gas-phase and trapped in an inert gas matrix has also been obtained [25].…”