Dissociative Photoionization of Chloro-, Bromo-, and Iodocyclohexane: Thermochemistry and the Weak C–Br Bond in the Cation

Abstract

Double-imaging photoelectron photoion coincidence spectroscopy (i 2PEPICO) with tunable synchrotron vacuum ultraviolet radiation was used to record threshold ionization mass spectra of the halocyclohexanes C6H11X (X = Cl, Br, and I). Calculations show that experimental dissociative ionization thresholds correspond to thermochemical limits. Among the processes observed (X loss, followed by C2H4 or C3H6 loss; C2H3Cl loss; HCl loss, followed by CH3 or C2H4 loss), halogen atom loss can be used to derive enthalpies of formation and C–X bond energies in the cation. As an ancillary value, we propose a new proton affinity for cyclohexene at PA298K(c-C6H10) = 771.5 ± 1.7 kJ mol–1. The halogen loss onsets 10.74 ± 0.06 eV, 10.125 ± 0.005, and 9.474 ± 0.005 eV thus yield Δf H o 298K(C6H11X (g)) = −164.4 ± 6.2, −114.4 ± 2.3, and −56.3 ± 2.3 kJ mol–1 for X = Cl, Br, and I, respectively. The last two agree with DFT-calculated isodesmic reaction energies very well, as opposed to G4 theory for X = Br. The C–X bond energy in the cation is the lowest for X = Br. This is the sum result of the weakening C–X bond in the neutral and the increasing stabilization of the parent ion with increasing halogen size.

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

“…We used G4 and CBS-QB3 theories to calculate the 0 K energy and the thermal enthalpy of four low-energy conformers of m -hydroxybenzaldehyde (1.4) and m -anisaldehyde (1.5) as well as two conformers of p -hydroxybenzaldehyde (0.2), p -anisaldehyde (0.1), and vanillin (0.6), within 5 kJ mol –1 of the global minimum. The conformational zero-point and thermal enthalpies were used to estimate the effect of the room temperature conformational equilibrium on the thermal enthalpy of each species, similar to the approach of Almeida et al and our work on halocyclohexanes . These corrections are given here in kJ mol –1 in parentheses for each of these samples.…”

“…The conformational zero-point and thermal enthalpies were used to estimate the effect of the room temperature conformational equilibrium on the thermal enthalpy of each species, similar to the approach of Almeida et al 83 and our work on halocyclohexanes. 88 These corrections are given here in kJ mol −1 in parentheses for each of these samples. The effect of large-amplitude motions, such as internal hindered rotation, on the thermal enthalpy is also a legitimate concern.…”

Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde

“…The pathway branches at [8], and the system may ring contract over a transition state [12] ‡ at a 0.38 eV barrier to form a five-membered ring intermediate [13] at 12.95 eV. This goes on to lose CH 3 and forms the m/z 92 In the first step, the methylene attaches to the benzene ring over a 0.97 eV barrier and forms bicyclic [16] at 11.85 eV, which then ring contracts over [17] ‡ to yield [18]. The three-membered ring in [18] opens over the transition state [19] ‡ to form the ketene-loss precursor [20] at 11.23 eV, which yields m/z 65 [21] at 13.67 eV above the neutral.…”

“…Nonetheless, the mass spectral assignment is greatly facilitated if reference mass spectra are known. Cations of similar size as anisaldehyde, for example, of halocyclohexanes or methyl butyrate, often exhibit a varied and quickly changing fragmentation pattern as a function of the internal energy, which is difficult to predict theoretically. Therefore, a detailed investigation of anisaldehyde valence photoionization is desirable to lay the groundwork for using photoionization mass spectrometric approaches to study the oxidative depolymerization of lignin.…”