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A 2019, 123, 8053−8062 pubs.acs.org/JPCA Gas-Phase Vibrational Spectroscopy of the Hydrocarbon Cations ‑ + + ‑ + fl l C3H ,HC3H , and c C3H2 : Structures, Isomers, and the In uence of Ne-Tagging † ‡ § ∥ ‡ † ‡ ⊥ † Sandra Brünken,*, , Filippo Lipparini, , Alexander Stoffels, , Pavol Jusko, , Britta Redlich, § ‡ Jürgen Gauss, and Stephan Schlemmer † FELIX Laboratory, Institute for Molecules and Materials, Radboud University, Toernooiveld 7c, NL-6525ED Nijmegen, The Netherlands ‡ I. Physikalisches Institut, UniversitatzuKö ̈ln, Zülpicher Str. 77, D-50937 Köln, Germany § Institut für Physikalische Chemie, Johannes Gutenberg-Universitaẗ Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany ∥ Dipartimento di Chimica e Chimica Industriale, Universitàdi Pisa, Via G. Moruzzi 13, I-56124 Pisa, Italy *S Supporting Information ABSTRACT: We report the first gas-phase vibrational + + spectra of the hydrocarbon ions C3H and C3H2 . The ions were produced by electron impact ionization of allene. Vibrational spectra of the mass-selected ions tagged with Ne were recorded using infrared predissociation spectroscopy in a cryogenic ion trap instrument using the intense and widely tunable radiation of a free electron laser. Comparison of high- level quantum chemical calculations and resonant depletion + measurements revealed that the C3H ion is exclusively formed in its most stable linear isomeric form, whereas two + isomers were observed for C3H2 . Bands of the energetically + favored cyclic c-C3H2 are in excellent agreement with calculated anharmonic frequencies, whereas for the linear + 2Π − open-shell HCCCH ( g) a detailed theoretical description of the spectrum remains challenging because of Renner Teller and spin−orbit interactions. Good agreement between theory and experiment, however, is observed for the frequencies of the + ff stretching modes for which an anharmonic treatment was possible. In the case of linear l-C3H , small but non-negligible e ects of the attached Ne on the ion fundamental band positions and the overall spectrum were found. 1. INTRODUCTION key to understand the large variation in the cyclic-to-linear ff The hydrocarbon ions C H+ and C H + have long been isomer ratio of neutral C3H2 observed in di erent astronomical 3 3 2 environments,17,18 which is difficult to be reproduced in Downloaded via RADBOUD UNIV NIJMEGEN on September 19, 2019 at 08:31:21 (UTC). recognized as important intermediates in the carbon chemistry 19,20 1,2 astrochemical model calculations. Vibrational spectroscopy See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. in interstellar molecular clouds, leading to the formation of 3 is a standard tool for structural determination. The IR data ever more complex hydrocarbon chain molecules and + + aromatic species such as benzene.4 They are directly presented here on isomeric forms of C3H and C3H2 might, chemically related to the formation of the corresponding therefore, in future studies be used for probing the isomeric outcome of, for example, the astronomically relevant C H+ + neutral species C3H and C3H2, which are widespread in 3 5−11 different regions in our Galaxy. The dominant pathway H2 reaction. + leading to the neutrals is via dissociative electron recombina- The recent detection of the linear l-C3H isomer via its tion of C H + and C H+, respectively, which in turn are formed rotational transitions in several regions of the interstellar 3 2 3 3 11,21,22 via a competing reaction and association mechanism from medium has triggered several laboratory and theoretical + 12 C3H with H2. The latter reactions have been extensively spectroscopic studies on this closed-shell hydrocarbon ion. In studied in the laboratory at varying temperatures in selected order to identify the astronomical lines, high-resolution ion flow tube (SIFT) and ion trap experiments, but still many rotational spectra were recorded using rotational action − 23 open questions remain.13 16 One of them is the role of spectroscopy in a cryogenic ion trap and Fourier transform isomeric variants of the reactant and product ions, all having stable cyclic and noncyclic forms, which can at best be Received: June 28, 2019 indirectly probed by mass spectroscopic methods alone.13 Revised: August 18, 2019 Isomer-dependent ion molecule reaction kinetics might be the Published: August 19, 2019 © 2019 American Chemical Society 8053 DOI: 10.1021/acs.jpca.9b06176 J. Phys. Chem. A 2019, 123, 8053−8062 The Journal of Physical Chemistry A Article microwave spectroscopy,24 providing accurate spectroscopic mass selector. The ions are guided to the 22-pole ion trap parameters for its vibrational ground state. The observed large which is kept at a fixed temperature in the range 6.5−11.8 K, centrifugal distortion parameter prompted Huang et al.25 to where they are cooled to the ambient temperature by collisions question the original identification of the astronomical lines as in a bath of He and Ne, with a mixture ratio of 3:1 and number + ∼ 15 −3 belonging to l-C3H based on quantum chemical calculations, density of 10 cm , provided via a pulsed piezo valve. The resulting in a much smaller value. Later, further theoretical buffer gas pulse is triggered typically 10 ms before the ions investigations resolved this issue and attributed the large enter the trap and continues until around 20−50 ms after the centrifugal distortion effect to a shallow C−C−C bending ion pulse. Under these conditions, around 10% of the primary + + potential in this linear ion, which is not accurately treated by ions (C3H or C3H2 ) form weakly bound complexes with Ne the standard second-order vibrational perturbation theory (see Figures S1 and S2 in the Supporting Information for (VPT2) approach.26,27 However, to date, only indirect corresponding mass spectra and time evolution). + information on vibrational band positions of l-C3H is available For the spectroscopic experiments, the ions are stored for up from recent photoelectron spectroscopic studies.28 In the same to 7 s in the ion trap where they are exposed to several laser + study, the cyclic c-C3H isomer was also observed, calculated pulses before they are extracted from the trap. An IR-PD to lie around 70 kJ/mol higher in energy than the linear spectrum is recorded by mass-selecting and counting the 29 + + isomeric ground state. C3H -Ne (or C3H2 -Ne) complex ions and varying the laser + The open-shell C3H2 ion has three stable isomers, with the wavelength. The absorption of a resonant IR photon leads to a + 2 cyclic (c-C3H2 , A1) variant being the ground state, followed depletion in the number of complex ions N(ν) compared to + 2Π C N ff by a linear HCCCH ( g, 28 kJ/mol higher) and a linear 2v the baseline number 0 observed o -resonant. To account for 2 ( A1, 182 kJ/mol higher) form. The relative energies are saturation, varying baseline, laser pulse energy E, and pulse theoretical values, including zero-point energy corrections from numbers n, the signal is normalized prior to averaging as 30 31 ν Wong and Radom, agreeing well with later calculations. ln(NN ( ) /0 ) I + − I = , yielding the intensity in units of relative cross Experimental spectroscopic data on C3H2 are sparse. Franck nE· /( hν ) Condon analyses of the vibrationally resolved photoelectron section per photon. spectra of neutral cyclic c-C3H2 provided estimates of the The IR radiation for the IR-PD measurements was provided + ν ν equilibrium geometry of cationic c-C3H2 and of its 2 and 3 by FEL-2 of the FELIX Laboratory (operated at 5 or 10 Hz) in stretching band positions.32,33 In a more recent study, in the frequency region 500−1950 cm−1.Themacropulse addition, the threshold photoelectron spectrum of linear energies in the interaction region were varied between 1 and HCCCH was investigated for the first time.28 Because this 20 mJ, and the FEL optimized for narrow bandwidths of study aimed at determining adiabatic ionization energies of the typically full width at half-maximum (fwhm) of 0.5−1 %. For − ff + neutrals, no information on the Renner Teller-a ected C3H , FEL-2 was also used in its third harmonic mode to cover vibrational modes of HCCCH+ was provided. the region 1950−2500 cm−1. The CH stretching region The combination of a cryogenic ion trap instrument with the −1 a between 2800 and 3900 cm was covered with a tabletop widely tunable free electron lasers at the FELIX Laboratory34 OPO/OPA system (Laservision), providing 10 Hz pulsed IR + −1 enabled us now to measure the vibrational spectra of C3H and radiation with up to 13 mJ/pulse and a fwhm of 3.5 cm . + fi C3H2 for the rst time. Broadband IR spectra covering most 2.2. Quantum Chemical Calculations. Quantum chem- of the fundamental bands were recorded using infrared- ical calculations were performed to determine the structure predissociation (IR-PD) action spectroscopy of the isolated and vibrational spectra of the species studied experimentally in + ions tagged with a Ne atom. The vibrational band positions are this work. The structures of the ground states of C3H and + assigned and interpreted with the help of high-level quantum cyclic c-C3H2 were optimized with analytical gradients at the chemical calculations with an emphasis on the ions’ isomeric coupled-cluster (CC) level of theory using the CC singles and composition and the influence of the rare gas tag.