Millimeter wave spectroscopy of propynal isotopologues and structure determination

Rotational transitions of propynal (HCCCHO) have been measured in the 150–900 GHz region by millimeter wave spectroscopy and in the far infrared region by high resolution FTIR spectroscopy using a synchrotron source. For the parent isotopologue, assignment of MMW transitions up to very high quantum numbers (J = 100, K_a = 25) reveals evidence of extensive perturbations in the ground vibrational state due to Fermi-asymmetry resonance with an excited vibrational state. A fit to nearly 3000 ground state transitions yields effective constants that are suitable for describing relatively unperturbed rotational levels up to K_a = 13. Over 1000 transitions were assigned and fitted for each singly substituted ¹³C species and nearly 400 transitions for the ¹⁸O variant. Re-analysis of literature data on deuterated species, aided by centrifugal distortion constants from hybrid density functional theory calculations at the B2PLYP/aug-cc-pVTZ level, provides a further set of rotational constants. This allows determination of a R_m⁽²⁾ structure for propynal with the following geometry: r(C≡C) = 1.2066(15), r(CC) = 1.4486(14), r(CO) = 1.2087(10), r(C­H ald) = 1.1069(8), r(C­H acet) = 1.0578(13), θ(CCC) = 176.71(22), θ(OCC) = 123.23(7), θ(HCC ald) = 114.43(31), θ(HCC acet) = 178.45(16). A new R_s structure was also derived.