Charge transfer transitions of the O2+-Ar and O2+-N2 complexes.

Electronic transitions are observed for the O2+-Ar and O2+-N2 complexes over the 225-350 nm range. The transitions are not associated with recognized electronic band systems of the respective atomic and diatomic constituents (Ar+, Ar, O2+, O2, N2+, and N2) but rather are due to charge transfer transitions. Onsets of the O2+-Ar and O2+-N2 band systems occur at 3.68 and 3.62 eV, respectively, corresponding to the difference in the ionization potentials of Ar and O2 (3.69 eV), and N2 and of O2 (3.51 eV), suggesting the band systems arise from intramolecular charge transfer transitions to states correlating with O2(X3Σg-) + Ar+ (2Pu) and O2(X3Σg-) + N2+(X2Σg+) limits, respectively. The dominant vibronic progressions have ωe values of 1565 cm-1 for O2+-Ar and 1532 cm-1 for O2+-N2, reasonably close to the value for the neutral O2 molecule in its X3Σg- state (1580 cm-1). Higher energy band systems for O2+-Ar and O2+-N2 are assigned to transitions to states correlating with the O2 (a1Δg) + Ar+ (2Pu) and O2 (a1Δg) + N2+(X2Σg+) limits, respectively.

Using isotopologues to probe the potential energy surface of reactions of C2H2 ++C3H4.

Investigations into bimolecular reaction kinetics probe the details of the underlying potential energy surface (PES), which can help to validate high-level quantum chemical calculations. We utilize a combined linear Paul ion trap with a time-of-flight mass spectrometer to study isotopologue reactions between acetylene cations (C2H2 +) and two isomers of C3H4: propyne (HC3H3) and allene (H2C3H2). In a previous study [Schmid et al., Phys. Chem. Chem. Phys. 22, 20303 (2020)],1 we showed that the two isomers of C3H4 have fundamentally different reaction mechanisms. Here, we further explore the calculated PES by isotope substitution. While isotopic substitution of reactants is a standard experimental tool in the investigation of molecular reaction kinetics, the controlled environment of co-trapped, laser-cooled Ca+ ions allows the different isotopic reaction pathways to be followed in greater detail. We report branching ratios for all of the primary products of the different isotopic species. The results validate the previously proposed mechanism: propyne forms a bound reaction complex with C2H2 +, while allene and C2H2 + perform long-range charge exchange only.

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

The B̃1A' ← X̃1A' electronic spectra of the 1-butyn-3-yl cation (H3CCHCCH+) and the H3CCHCCH+-Ne and H3CCHCCH+-Ar complexes are measured using resonance enhanced photodissociation over the 245-285 nm range, with origin transitions occurring at 35936, 35930, and 35928 cm-1, respectively. Vibronic bands are assigned based on quantum chemical calculations and comparison of the spectra with those of the related linear methyl propargyl (H3C4H2+) and propargyl (H2C3H+) cations. The photofragment ions are C2H3+ (major) and C4H3+ (minor), with the preference for C2H3+ consistent with master equation simulations for a mechanism that involves rapid electronic deactivation and dissociation on the ground state potential energy surface.

Electronic Spectra of Diacetylene Cations (HC4H+) Tagged with Ar and N2.

Electronic spectra of mass-selected HC4H+-Arn (n = 1-3) and HC4H+-(N2)n (n = 1-2) complexes are measured over the 290-530 nm range using resonance-enhanced photodissociation spectroscopy in a tandem mass spectrometer. Vibronic transitions in the visible region are compared with previous experimental and theoretical results for the Ã2Πu ← X̃2Πg band system of HC4H+. Hole burning experiments confirm that transitions over the 290-340 nm range involve the diacetylene cation (HC4H+). On the basis of previous experiments and comparison with spectra of isoelectronic molecules the peaks are assigned to the 22Πu ← X̃2Πg band system, with the origin transition for HC4H+-Ar occurring at 29723 cm-1. The main progression has a spacing of 906 cm-1 and is assigned to the symmetric C-C stretch vibrational mode (ν3). The assignment of additional bands is complicated by spectral congestion, the possible presence of energetically close-lying electronic states, vibronic coupling effects, and by the fact that HC4H+ possibly becomes nonlinear in the 22Πu state.

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

The electronic spectrum of the methyl propargyl cation (2-butyn-1-yl cation, H2C4H3+) is measured over the 230-270 nm range by photodissociating the bare cation and its Ar and N2 tagged complexes in a tandem mass spectrometer. The observed A'1←A'1 band system has an origin at 37 753 cm-1 for H2C4H3+, 37738 cm-1 for H2C4H3+-Ar, and 37 658 cm-1 for H2C4H3+-N2. The methyl propargyl cation photodissociates to produce either C2H3++C2H2 (protonated acetylene + acetylene) or H2C4H++H2 (protonated diacetylene + dihydrogen). Photodissociation spectra of H2C4H3+, H2C4H3+-Ar, and H2C4H3+-N2 exhibit similar vibronic structure, with a strong progression of spacing 630 cm-1 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 H2C4H3+ 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 H2 elimination.

Electronic spectrum of the protonated diacetylene cation (H2C4H+).

The B̃1A1←X̃1A1 electronic band system of the protonated diacetylene cation (H2C4H+) is measured over the 230-295 nm range by photodissociating H2C4H+ ions stored in a cryogenic ion trap and by photodissociating H2C4H+ tagged with Ar and N2 in a tandem mass spectrometer. The B̃1A1←X̃1A1 band system has an origin at 34 941 cm-1 for H2C4H+, 34 934 cm-1 for H2C4H+-Ar, and 34 920 cm-1 for H2C4H+-N2. The spectra of H2C4H+, H2C4H+-Ar, and H2C4H+-N2 display similar vibronic structure, which is assigned using ab initio calculations to progressions in two symmetric a1 C-C stretch vibrational modes (ν6 and ν4), with band spacings of 860 and 1481 cm-1, respectively.

Electronic spectrum of the propargyl cation (H2C3H(+)) tagged with Ne and N2.

The Ã(1)A1 ← X̃(1)A1 band system of the propargyl cation (H2C3H(+)) is measured over the 230-270 nm range by photodissociation of mass-selected H2C3H(+)-Ne and H2C3H(+)-N2 complexes in a tandem mass spectrometer. The band origin occurs at 37 618 cm(-1) for H2C3H(+)-Ne and 37 703 cm(-1) for H2C3H(+)-N2. Ground and excited state ab initio calculations for H2C3H(+) using the MCSCF and coupled-cluster (CC) response methods show that the ion has C2v symmetry in the ground X̃(1)A1 and excited Ã(1)A1 states and that the strong vibronic progression with a spacing of 630 cm(-1) is due to the C-C stretch vibrational mode, ν 5.