Electronic spectroscopy of small carbocations

© 2020 Giel MullerThis work focuses on the measurement and analysis of gas-phase electronic spectra of C4H2+, C4H4+, C4H5+, and C6H4+ cations. The spectra are recorded by photodissociating the cations, or their messenger tagged complexes, in a tandem mass spectrometer and monitoring the photofragment signal as a function of laser wavelength. Ultimately, structural and energetic information extracted from these spectra, complemented by quantum chemical calculations, may provide insights into the roles of cations in the chemistry of flames, plasmas, and extraterrestrial environments. Understanding the vibronic structures of the cations is also fundamental for facilitating their possible detection in remote environments. Electronic spectra of C4H2+-Ar_n (n = 1-3) and C4H2+-(N2)_n (n = 1-2) complexes are recorded over the 290-530 nm range by monitoring C4H2+ photofragments. Intense narrow bands in the visible region arise from the A2(Pi)u<--X2(Pi)g electronic transition of the diacetylene cation, HC4H+. Vibronic transitions are compared with previous results and theoretical predictions for the A2(Pi)u<--X2(Pi)g band system based on calculations at the CCSD/cc-pCVTZ and EOM-CCSD/cc-pCVTZ levels of theory. Hole burning experiments confirm that the weak, broad bands in the ultraviolet region also correspond to excitations of HC4H+. These bands are assigned to the 22(Pi)u<--X2(Pi)g electronic transition based on calculated energies and oscillator strengths, previous experiments, and spectra of isoelectronic molecules. The origin transition is observed at 29723 cm-1 (336.44 nm) for HC4H+-Ar, followed by a progression in the symmetric C-C stretch vibration, spaced by 906 cm-1. Spectral congestion observed toward shorter wavelengths is possibly due to the presence of close-lying electronic states, vibronic coupling effects, and HC4H+ being bent in the 22(Pi)u electronic state. Electronic spectra of Ar- and N2-tagged C4H4+ cations, generated from acetylene ion-molecule reactions, are recorded over the 296-550 nm range by monitoring C4H4+ and C4H3+/C4H2+ photofragments. The spectra exhibit a clearly resolved band system commencing at 511 nm, a set of weak bands around 410 nm, and an intense broad band in the UV. Band assignments are based on previous spectroscopic studies and calculations for the four lowest energy C4H4+ isomers. The visible region of the spectrum is dominated by the B2A’’<--X2A’’ electronic transition of the vinylacetylene cation (VA+) whose origin transition occurs 19558 cm-1 (511.30 nm) for VA+-Ar and at 19551 cm-1 (511.48 nm) for VA+-N2. The spectrum features a strong progression in the central C-C stretching vibration spaced by approximately 806 cm-1 and well defined vibronic transitions associated with two bending vibrational modes. The weaker 410 nm system is assigned to the D2B3<--X2B2 electronic transition of the butatriene cation (BT+). Two broad bands, following the origin transition at 24399 cm-1 (409.85 nm) in the C4H4+-Ar spectrum, arise from excitation of a symmetric C=C stretching vibration and possibly the CH2 twisting vibration. The observation of C4H4+ and C4H3+ /C4H2+ photofragments following excitation of BT+-Ar complexes over the 380-410 nm range accords with calculated dissociation pathways on the ground state C4H4+ potential energy surface. The broad band appearing at wavelengths below 320 nm is predicted to be due to electronic transitions of all four of the lowest energy C4H4 + isomers, including the VA+ and BT+ cations. The B2A’<--X2A’ electronic spectrum of the 1-butyn-3-yl cation (H3CCHCCH+) is recorded over the 245-285 nm range by photodissociating the bare cation and its Ne and Ar-tagged complexes. Origin transitions for H3CCHCCH+ and H3CCHCCH+-Ne are observed at 35936 cm-1 (278.27 nm) and 35930 cm-1 (278.32 nm), respectively. Additional bands in the spectra are assigned to vibronic transitions of H3CCHCCH+ through quantum chemical calculations and comparison of the spectra with those of similar molecules, the propargyl (C3H3+) and methyl propargyl (H3C4H2+) cations. Excitation of H3CCHCCH+ produces C2H3+ +C2H2 (protonated acetylene + acetylene) and C4H3+ +H2 (protonated diacetylene + hydrogen molecule) photofragments. The preferred formation of C2H3+ +C2H2 photofragments is correctly predicted by master equation simulations, generated from calculated dissociation pathways on the ground state C4H5+ potential energy surface. Electronic spectra of Ar- and N2-tagged C6H4+ cations, generated from acetylene ion-molecule reactions, are recorded over the 265-700 nm range by monitoring C6H4+ and C4H2+ photofragments. Bands are assigned to transitions of five isomers based on previous spectroscopic studies, calculated stationary points on the ground state C6H4+ potential energy surface, hole burning experiments, and spectral simulations generated from TD-DFT calculations. In the C6H4+-Ar spectrum, bands over the 526-700 nm range arise from the B2A’’<-- X2A’’ electronic transition of the 1-hexene-1,3-diyne cation (CH2CHCCCCH+) and the C2Bg <-- X2Au electronic transition of the trans-3-hexene-1,5-diyne cation (HCCCHCHCCH+), with origin transitions occurring at 604 and 581 nm, respectively. Over the 375-460 nm range, broad bands at 440, 425, and 423 nm are assigned to vibronic transitions within the C2A’’<-- X2A’’ band system of the 1-ethynyl-3-methylene-cyclopropa-1,2-diene cation (H2C[c –C3H]CCH+). Sharper bands at 416, 407, and 400 nm are ascribed to the B2A’’<-- X2A’’ electronic transition of the propargyl cyclopropene cation ([c –C3H2]CHCCH+). Between 265 and 375 nm, several bands appear, superimposed on a broad band that extends for several thousand wavenumbers. The 371 nm band is assigned to the B2A’’<-- X2A’’ origin transition of the 1-hexene-1,3-diyne cation (CH2CHCCCCH+), agreeing with previous assignments. In light of the TD-DFT calculations, the bands at 365 and 357 nm are tentatively reassigned to the C2B<--X2B electronic transition of the propadienylidene cyclopropene ([c –C3H2]CCCH2+) isomer. The underlying band possibly arises from the C2Bg <-- X2Au electronic transition of the trans-3-hexene-1,5-diyne (HCCCHCHCCH+) cation, which undergoes a reduction in symmetry upon photoexcitation. Formation of C4H2+ photofragments at wavelengths below 426 nm (2.91 eV) agrees with calculated C6H4+ dissociation pathways on the ground state C6H4+ potential energy surface