HIGH RESOLUTION SPECTROSCOPY OF HEXAMETHYLENETETRAMINE (HMT) C$_{6}$N$_4$H$_{12}$

Abstract

Hexamethylenetetramine, or HMT (C$_{6}$N$_4$H$_{12}$) is a N-substituted derivative of adamantane C$_{10}$H$_{16}$ which is the smallest sample of the diamondoid molecules family. Thanks to their high stability, diamond-like molecules have long been suspected to be present in space, 160 (1969)} (note that diamond nanocrystals are extracted from Murchinson meteorites, 160 (1987)}), and HMT is known to be an abundant residue of UV irradiated ice analogs, 327 (1995)} and might be present in Titan's atmosphere. Using the Bruker IFS 125 coupled to a multipass cell (absorption path length of 150 m) of the AILES beamline at SOLEIL, we recorded the IR spectrum of gas phase HMT in the 300--3000 cm$^{-1}$ spectral region with an unapodized resolution 0.001 cm$^{-1}$. HMT is a solid powder with about 0.008 mbar vapour pressure at room temperature, it is a $T_d$ molecule (as adamantane) and has 25 vibrational modes from which only 9 are infrared active. Over the 9 IR active modes, we were able to rotationlly resolved the spectra of 6 of them. The analysis of all the resolved bands has been performed thanks to the XTDS and SPVIEW softwares developed in Dijon for such molecules, {\bf 251} 102--113 (2008).}. Each band can be considered as isolated and we get very good fits of line positions, with a root mean square deviation better than $5\times 10^{-4}$ cm$^{-1}$ for $J$ values up to 80 or more in each case. As for our recent study concerning adamantane, 024310 (2012)}, the resulting synthetic spectra will permit an active search of this very stable specie in different sources of the interstellar medium.