High Resolution\nRovibrational Spectroscopy of the\nν<sub>6</sub> and ν<sub>3</sub> + ν<sub>7</sub> Bands\nof H<sub>2</sub>CCCH<sup>+</sup>

The molecular ion H<sub>2</sub>CCCH<sup>+</sup> has been\nfurther\ninvestigated in a 4 K cryogenic ion trap instrument employing the\nleak-out spectroscopy (LOS) method. Transitions within two perpendicular\nbands, the fundamental ν<sub>6</sub> (antisymmetric H–C–H\nstretch) and the combination band ν<sub>3</sub> + ν<sub>7</sub> (CC stretch and CH<sub>2</sub> rock), have been measured\nin high resolution for the first time. The spectral analysis was aided\nby high level quantum chemical calculations and by our previous study\nof the parallel ν<sub>1</sub> and ν<sub>3</sub> + ν<sub>5</sub> bands [Silva, W. G. D. P. ; Mol. Phys. 2023, e2296613]. The novel results presented\nhere have allowed for a seventy-fold improvement on the accuracy of\nthe value of the <i>A</i><sub>0</sub> rotational constant\nof this near-prolate asymmetric top.