High Resolution Millimeter Wave Absorption Spectroscopy of Flexi- ble Complex Organic Molecules: Laboratory Spectrum of 1, 2-butanediol

The enhancing sensibility of radioastronomical observations allows for detec- tion of complex organic molecules (COMs) with increasing size. Observations performed by the Atacama Large Millimeter Array (ALMA) open up new oppor- tunities to reveal the COMs, at the same time, the huge amount of data collected and the extremely rich surveys represent a challenge for the astrochemistry community. Among all the detected molecules, the diols are object of chemical interest, because of their similarity with important biological building block molecules such as sugar alcohols. The simplest of them, ethylene glycol (EG), is one of the largest COMs detected in space thus far. Lines attributable to the most stable conformer of EG were detected in different environments and recently also the higher energy conformer has been observed both towards IRAS 16293-2422 and the Orion KL. Observations of 1, 2- and 1, 3-propanediol toward Sgr B2 (N-LMH) were attempted as part of the GBT Prebiotic Interstellar Molecule Sur- vey Legacy Project, but no transitions were detected. Although up to now, due the fact that the column densities of molecules tend to decrease with increasing molecular weight, no large diols have been observed in interstellar space, owing to the raising sensitivity of the radioastronomy observations, their future detection can not be excluded. In this context we report, for the first time, the laboratory millimeter spectrum of 1, 2-butanediol (BD) recorded in the 59.6-103.6 GHz frequency region (5.03-2.89 mm). BD (the ethylated form of EG) is a flexible molecule characterized by a great conformational complexity, thus at room condi- tions the population is distributed in a large number of species, leading to a very congested spectrum. This problem has been overcome exploiting the rotational and conformational cooling produced by the supersonic expansion technique. Six conformers of BD, including the global minimum, have been assigned yielding the rotational constants and centrifugal distortion constants up to the forth- or sixth-order. The experimental spectroscopic constants and theoretical electric dipole moment components were used to predict the rotational spectrum of each of the observed conformers up to 163 GHz. Making use of the training received by the ALMA Regional Center in Bologna, selected lines of the most stable conformer of BD were searched toward the IRAS 16293-2422 A source exploiting Band 3 observations performed during the ALMA-project 2012.1.00712.S. No match was found, however further observations, aimed specifically to its search, could be more successful. Moreover considering that the maximum spectral signal of heavy molecules is predicted at higher frequencies, additional laboratory measurements at sub-millimeter wavelengths can be performed, starting from the present assignments. The new data could be used to attempt new observations in COMs-reach sources