Spectroscopic Detection and Characterization of Cyanooxomethylium, NCCO$^+$

TL;DR

This work delivers the first spectroscopic detection and comprehensive characterization of the linear acylium NCCO$^+$, combining low-resolution IRPD of NCCO$^+$–Ne, high-resolution leak-out IR spectroscopy of the bare ion’s ν$_2$ band, and IR/mmw double-resonance rotational spectroscopy, all guided by high-level CCSD(T) calculations. The study provides detailed structural parameters, identifies the NCCO$^+$ ν$_2$ fundamental near 2150 cm$^{-1}$, and measures 16 pure rotational transitions up to about 246 GHz, achieving excellent agreement with theory. These results yield precise rotational constants and quadrupole coupling values, enabling reliable predictions for future astronomical searches. The work also demonstrates the efficacy of action spectroscopy in cold ion traps for rapid, high-resolution spectroscopy of astrochemically relevant ions and outlines pathways to investigate related isomers and related species.

Abstract

Cyanooxomethylium, NCCO$^+$, a fundamental linear acylium ion, has been observed spectroscopically for the first time using action spectroscopy in ion trap apparatuses. A first low-resolution infrared spectrum was obtained between 500 to 1400 cm$^{-1}$ and 2000 to 2500 cm$^{-1}$ using the Free Electron Laser for Infrared eXperiments (FELIX) and the FELion apparatus, employing infrared predissociation of the weakly bound NCCO$^+$-Ne complex. Subsequently, high-resolution studies of the bare ion were performed with the COLtrap II setup, one targeted at the CN-stretching mode $ν_2$ around 2150 cm$^{-1}$ using leak-out spectroscopy and one at the pure rotational spectrum employing a leak-out infrared/millimeter-wave double resonance approach covering transition frequencies as high as 246 GHz. Spectroscopic detection and analysis were guided by high-level quantum-chemical calculations performed at the CCSD(T) level of theory. The collected data permit accurate frequency predictions to support future astronomical searches with sensitive radio telescopes.

Figures (5)

• Figure 1: Schematic view of the 10 K 22-pole ion trap apparatus COLtrap II (adapted from Figure 1 in Ref.bast_JMS_398_111840_2023 ; reproduced with permission from Elsevier). The setup is extended here by a millimeter-wave (mmw) setup for IR/mmw double resonance experiments, consisting of a synthesizer, an amplifier-multiplier chain and a horn antenna. The mmw beam (shown in pale red) is focused by an elliptical mirror and enters the vacuum chamber through a diamond window. A small hole in this mirror also allows the IR laser (shown in yellow) to pass along the axis. See the text for further details.
• Figure 2: Contour plot of NCCO+ --Ne showing the fc-CCSD(T)/aug-cc-pVTZ potential energy as a function of the Ne atom position covering the interval [0.075, 0.975] kcal/mol in steps of 0.075 kcal/mol above the global minimum. Atom color code: nitrogen (blue), carbon (black), and oxygen (red).
• Figure 3: FELIX IRPD spectrum of NCCO+ --Ne (blue trace, top) obtained using the FELion ion trap instrument at a nominal trap temperature of 7 K. Location of the fundamental vibrational bands calculated at the fc-CCSD(T)/ANO2 level of theory is shown as inverted red sticks for comparison.
• Figure 4: LO spectrum of the $\nu_2$ stretching mode of bare NCCO+ (blue trace) recorded at a nominal trap temperature of 42(2) K using 10$^{12}$ cm$^{-3}$ of N$_2$ as a neutral collision partner. Normalized signal is plotted against the wavenumber. A simulation of the rotational-vibrational lines assuming a rotational temperature of 50 K is shown for comparison as inverted red sticks. An enlarged view of the $P$(11) transition is shown in the top left corner.
• Figure 5: Pure rotational transition $J = 9 \leftarrow 8$ in the ground vibrational state of NCCO+, recorded using a LO IR/mmw double resonance scheme. For this recording, the IR laser was stabilized on the $P(9)$ transition of the $\nu_2$ vibrational mode (orange arrow) while tuning the mmw source in steps of 10 kHz around the predicted frequency of 82.146 GHz. Here, resonant mmw excitation (red arrow) results in an increase of the LO signal of about 8 %. Normalized counts (red dots) are determined from the ratio of the ion counts in the mmw search window and the counts determined at an off-resonant reference frequency. Calculated quadrupole hyperfine structure from nitrogen is indicated as black sticks.