Electronic spectrum and photodissociation chemistry of the linear methyl propargyl cation H2C4H3+

Abstract

The electronic spectrum of the methyl propargyl cation (2-butyn-1-yl cation, HCH) is measured over the 230-270 nm range by photodissociating the bare cation and its Ar and N tagged complexes in a tandem mass spectrometer. The observed A'1←A'1 band system has an origin at 37 753 cm for HCH, 37738 cm for HCH-Ar, and 37 658 cm for HCH-N. The methyl propargyl cation photodissociates to produce either CH+CH (protonated acetylene + acetylene) or HCH+H (protonated diacetylene + dihydrogen). Photodissociation spectra of HCH, HCH-Ar, and HCH-N exhibit similar vibronic structure, with a strong progression of spacing 630 cm corresponding to excitation of the C-C stretch mode. Interpretation of the spectra is aided by ground and excited state calculations using time dependent density functional theory at the ωB97X-D/aug-cc-pVDZ level of theory. Ab initio calculations and master equation simulations were used to interpret the dissociation of HCH on the ground state manifold. These calculations support the experimentally observed product branching ratios in which acetylene elimination dominates and also suggests that channel switching occurs at higher energies to favor H elimination.

Electronic Spectrum and Photodissociation Chemistry of the 1-Butyn-3-yl Cation, H3CCHCCH+

“…On the basis of the calculated molecular orbitals shown in Figure , the HOMO–1 → LUMO electronic transitions should be similar for P , MP , and BT . In accordance with these expectations, the B̃ 1 A′ ← X̃ 1 A′ electronic transitions of MP and P occur at similar wavelengths and exhibit comparable progressions in the C–C stretching vibration, demonstrating that the on-axis methyl group has little influence on the propargyl chromophore.…”

“…The low scaling factor necessary to reconcile the calculated and the observed excited state C−C frequency indicates that the EOM-CCSD/cc-pVTZ calculations overestimate the frequency, as also found for P and MP. 13,23 For P, much better agreement with experiment was found for calculations using the restricted active space self-consistent field (RASSCF) method with the cc-pVTZ basis set. The computational cost of the RASSCF/cc-pVTZ level calculations prevents a similar approach being used for the larger C 4 H 5 + systems (MP and BT).…”

“…To explore the dissociation mechanism of BT, stationary points on the ground state C 4 H 5 + potential energy surface were calculated at the MP2/aug-cc-pVDZ level of theory, building on a previous study of MP. 13 Transition states and intermediates were located by scanning along internal coordinates. Intrinsic reaction coordinate (IRC) calculations were used to verify the transition states.…”

“…Small carbocations are important in plasmas, combustion processes, , and extraterrestrial environments. − There is a growing body of spectroscopic data for small fundamental carbocations in the gas phase, including C 4 H 5 + , − the subject of this paper. C 4 H 5 + cations have been detected in Titan’s atmosphere using the ion neutral mass spectrometer and are suspected to lead to formation of polycyclic aromatic hydrocarbons and tholins that populate the haze layers, − although it is unclear which isomers are present.…”

Isotope-specific reactions of acetonitrile (CH3CN) with trapped, translationally cold CCl+

“…These calculations were carried out using a custom version of the MultiWell2020 suite of programs, [48][49][50] modified to treat bath-gas collisions using the Langevin model. Simulations followed a general approach that we have used extensively to investigate ion reaction dynamics in a diverse range of instruments, including ion trap, 51 tandem, 52 and ion mobility 53 mass spectrometers. Electronic energies, vibrational frequencies, and moments of inertia were from the CCSD(T)/CBS//MP2/aug-cc-pVTZ model chemistry calculations.…”