Journal of Photochemistry and Photobiology A: Chemistry 277 (2014) 37–44 Contents lists available at ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal h om epa ge: www.elsevier.com/locate/jphotochem Photochemical transformations of 5-methyltetrazole. Matrix isolation FTIR and DFT studies M. Pagacz-Kostrzewa, J. Krupa, M. Wierzejewska ∗ Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14, 50-383 Wroclaw, Poland a r t i c l e i n f o Article history: Received 22 October 2013 Received in revised form 6 December 2013 Accepted 15 December 2013 Available online 21 December 2013 Keywords: Tetrazole ring cleavage Tunable laser photochemistry Methylcarbodiimide Methylnitrilimine Methylcyanamide Hydrogen cyanide a b s t r a c t In situ photolysis of 5-methyltetrazole (MT) isolated in low temperature argon matrices was induced by tunable UV laser radiation. The progress of the reactions was followed by FTIR spectroscopy that allowed for spectroscopic identification of three photoproducts resulting from the MT ring cleavage, namely C-methylnitrilimine CH 3 CNNH, N-methylcarbodiimide CH 3 NCNH, methylcyanamide CH 3 NHCN. The kinetic profiles obtained for these products show different behavior of the species in the course of irradiation. The N-methylcarbodiimide molecules were found to decompose further into two HCN or HNC molecules that interact in the matrix cages to form different hydrogen bonded dimers. Interpretation of the observed photoprocesses was supported by quantum chemical calculations (DFT, TD-DFT). © 2013 Elsevier B.V. All rights reserved. 1. Introduction Tetrazoles and other nitrogen-rich heterocyclic derivatives exhibit a large number of practical applications such as environ- mentally friendly energetic materials [1–5], stabilizing substituents in drug design [6–9], metal coordination compounds including spin-crossover materials [10,11] and peptides components in metal chelating agents [12]. One of the most interesting aspects of the properties of tetra- zole derivatives is their photolytic behavior. This subject has been extensively studied within last several decades and systems includ- ing both the simplest tetrazole and different C- or N-substituted species were studied [13–22]. As it was shown by Maier et al. [13] the unsubstituted tetrazole easily eliminates molecular nitro- gen upon photolysis leading to formation of such products as nitrilimine, carbodiimide and cyanamide. Later on, photochem- ically induced ring opening reactions have been reported for a number of tetrazole derivatives and were found to be common processes for this group of compounds. A type of substituent attached to the ring determines the most preferred fragmenta- tion pathways. For simple tetrazole derivatives, carbodiimides are final products that are the most often reported for both C- and N-substituted tetrazoles. In addition, cyanamides and nitril- imines sometimes appear. More exotic photoproducts such as ∗ Corresponding author. Tel.: +48 713757332. E-mail address: maria.wierzejewska@chem.uni.wroc.pl (M. Wierzejewska). 1-cyclopenta-2,4-dienylketenimine, in the case of 2-(tetrazol-5- yl)pyridine photolysis were also reported [19,21]. Although the 5-methyltetrazole molecule is one of the simplest N H containing tetrazoles its photochemistry was not reported so far. It has however received increased attention in recent years. Pinto et al. studied its tautomerism [23], electronic structure and thermal decomposition [24] by UV photoelectron spectroscopy and theoretical calculations. DFT study of the photoelectron spectra of 5-methyltetrazole has been recently performed as well [25]. The main goal of the present work is to find out what are the UV photolysis pathways of 5-methyltetrazole isolated in solid argon using tunable narrow-band UV radiation. We used the matrix isola- tion technique coupled with FTIR spectroscopy since such approach proved to be appropriate to gain our knowledge of both structural preferences and photochemistry of stable and unstable species [26–28]. Identification and IR characteristics of the photoproducts formed during the 5-methyltetrazole photolysis were supported by extensive DFT and TD-DFT calculations. 2. Experimental 2.1. Matrix isolation studies 5-Methyltetrazole (MT) (purity >98%) was purchased from TCI and used without further purification. In order to obtain matrices containing 5-methyltetrazole, the crystalline sample was allowed to sublimate at ca. 305 K from a small electric oven located inside the vacuum vessel of the cryostat near the cold window. The vapour 1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2013.12.011