Room-temperature millimeter-wave spectrum of i-PrCHO. (a) Portion of the spectrum showing intense a-type R-branch transitions of the gauche conformer. The final computed spectrum for this conformer, computed for 300 K, is given for comparison (a-type lines are depicted in red, b-type lines are depicted in green, and c-type lines are depicted in purple). (b) Zoomed-in view of the spectrum, showing several a-type R-branch lines belonging to trans i-PrCHO (in blue) together with some intense lines of gauche i-PrCHO (in red). Intensity is given in arbitrary units.