Near-UV Spectrum of the Hexamethylenetetramine Cation

Abstract Hexamethylenetetramine (HMT; (CH 2 ) 6 N 4 ) is a complex cage-like molecule that forms by the reaction of ammonia with formaldehyde, two of the most prevalent components of ice mantles on interstellar dust grains. Its presence in carbonaceous chondrites is confirmed, and laboratory experiments simulating astrophysical ices reveal that HMT can form in high abundance when certain ice mixtures are exposed to ionizing radiation. Nevertheless, HMT has not yet been detected in the interstellar medium and laboratory measurements are still needed for spectroscopic assignments. Here, we report the electronic spectrum of the HMT + radical cation recorded between 305 and 350 nm (28,570–32,790 cm –1 ) in the gas phase. The vibronic spectrum of cold HMT + is obtained by the photodissociation of mass-selected ions in a tandem mass spectrometer coupled to a cryogenic ion trap held at 5 K. The results are interpreted with the aid of complementary time-dependent density functional theory calculations and compared to the photoelectron spectrum of neutral HMT. The resolved vibronic structure attributed to the D 3 ( 2 A 1 )←D 0 ( 2 A 1 ) electronic transition appears above the band origin at 29,990 cm −1 (333.4 nm) and grows increasingly more congested toward higher photon energy, suggesting considerable geometric distortion upon the electronic excitation of the Jahn–Teller distorted ground electronic state. The combined spectroscopic and computational results are discussed within the astrophysical context.