Infrared Spectroscopy of the Mass 31 Cation: Protonated Formaldehyde vs Methoxy

Abstract

Pulsed discharges containing methanol or ethanol produce ions having the nominal formula [C,H(3),O](+), i.e. m/z = 31. Similar ions resulting from electron impact ionization in mass spectrometers are long recognized to have either the CH(2)OH(+) protonated formaldehyde or CH(3)O(+) methoxy cation structures. The H(2)OCH(+) oxonio-methylene structure has also been suggested by computational chemistry. To investigate these structures, ions are expanded in a supersonic beam, mass-selected in a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy. Sharp bands in the O-H and C-H stretching and fingerprint regions are compared to computational predictions for the three isomeric structures and their vibrational spectra. Protonated formaldehyde is the most abundant isomer, but methoxy is also formed with significant abundance. The branching ratio of these two ion species varies with precursors and formation conditions.

Infrared spectroscopy of the acetyl cation and its protonated ketene isomer

“…The resulting fit parameters are given in Figure 3. As indicated, the constants from theory provide an adequate description of the spectrum, and the resulting rotational temperature is about 100 K. This temperature is higher than that found typically for ions from this source, 46 but is comparable to previous values for hydride ions with large rotational constants (e.g., protonated acetylene). 35 The middle trace of Figure 3 shows the rotational structure expected if the resolution corresponds to the laser linewidth (1.2 cm −1 ).…”

“…This structure has the argon binding on the OH group, which should cause a significant shift in the O-H stretch. 35,37,[44][45][46] Consistent with this, theory without the Ar predicts the O-H stretch at 3379 cm −1 , corresponding to a 326 cm −1 red shift. This behavior is, therefore, consistent with assignment of the 3045 cm −1 band to the O-H stretch of protonated ketene.…”

“…73 In oxygen-containing ions, protonated formaldehyde (CH 2 OH + ) is nearly 400 kJ/mol more stable than methoxy cation (CH 3 O + ), but a small amount of the latter was detected. 46 In each of these cases, significant stability differences exist between isomers, but large barriers to rearrangement allow the less stable structure to survive via "kinetic trapping." In ordinary mass spectrometry, kinetic trapping often occurs when ions are produced from different precursors having pre-formed molecular connectivity that requires significant energy to re-assemble.…”

“…[28][29][30][31][32][33][34][35][36][37][38][39] Other studies have examined small organic ions containing oxygen, such as HCO + , protonated methanol, protonated ethanol, and protonated formaldehyde. [40][41][42][43][44][45][46] Throughout this work, infrared spectroscopy has been complemented by computational chemistry to investigate stable ion structures and the possible occurrence of isomers.…”

From the mobile proton to wandering hydride ion: mechanistic aspects of gas‐phase ion chemistry

“…It may be finally underlined that carbocations bearing a π‐electron donor substituent X (X=F, OH, NH 2 , C unsaturated …) are ‘onium structures’ stabilized by resonance [ + CH 2 –X ← → CH 2 =X + ] and are consequently excluded from the denomination ‘primary carbocation’! For example, oxonium [CH 2 OH] + or immonium [CH 2 NH 2 ] + structures are stable, whereas their bridged proton (non‐classical) forms have no existence, or their heteroatom charged forms, alkoxy [CH 3 O] + and alkylamino [CH 3 NH] + are highly unstable . Higher homologues of onium ions may involve primary carbocations as transition states connecting two stable structures namely oxonium or immonium and cyclized forms resulting from internal nucleophilic attack as illustrated in Scheme .…”