Light induced reactions in cryogenic matrices

Rui Fausto and Andrea Gómez-Zavaglia

DOI: 10.1039/9781849732826-00001

In this chapter light induced reactions in cryogenic matrices are addressed, ranging from conformational isomerizations to complex bond-breaking/ bond-forming processes. These include reactions induced by radiation in both UV-visible and IR regions, and also photochemical processes where noble gas atoms participate directly, leading to formation of covalently bound noble gas containing molecules.

1 Introduction

In the previous issues of this series (vol. 37 and 38), we provided an extensive review on the literature dealing with light induced reactions in cryogenic matrices published during the period July 2004 – December 2009. The present chapter focuses on the reports on the same subject appearing in the specialized literature during 2010.

We will concentrate in studies dealing mainly with organic compounds, since application of matrix isolation to study photoinduced processes in inorganic/organometallic systems is reviewed shortly elsewhere in this book. Section 2 considers UV-driven photochemical processes, while section 3 deals with IR-induced ground state hot vibrational chemistry. A short section (2.4) is dedicated to photochemical processes where the matrix noble gas atoms participate directly, leading to formation of covalently bound noble gas containing molecules.

The fundamentals and technical descriptions of the matrix isolation method can be found in the books by Meyer, Andrews and Moskovits, Barnes et al., Dunkin or Fausto. More specific and recent reviews on matrix isolation and its application to the study of ligh induced processes can also be consulted by those which are less familiar with the subject, including a recently published special issue of the prestigious Annual Reports on the Progress of Chemistry, the fifth report in that series dedicated to matrix isolation (after those published in 1985, 1991, 1997 and 2001). Light-induced noble gas chemistry in cryogenic matrices has been addressed by Nemukhin et al. and by Khriachtchev, Räsänen and Gerber, who also present therein their perspectives on future developments of this field of research.

Very interesting reviews by Wentrup, and Winkler and Sander, focusing on rather recent developments in aryne chemistry, with a special emphasis on the matrix isolation of benzynes, tridehydrobenzenes and related systems were published in 2010. Other particularly relevant studies reported in this year were those by Olbert-Majkut et al., where Raman spectroscopy was used to investigate the product ratios of the H2O + CO and H2 + CO2 photodecomposition channels of formic acid in different matrices, concluding that in argon matrix the dominating process is the dehydration (H2O + CO) channel, whereas the decarboxylation (H2 + CO2) channel is the most prominent one in solid xenon, and by Lapinski et al., on the infrared induced selective rotamerization of the aminohydroxy conformers of cytosine. In this latter work, the authors were able to control in an efficient way the relative populations of two aminohydroxy conformers of cytosine, differing in rotation of the OH group by ~180°, using narrowband, near-infrared laser light. For cytosine monomers isolated in a low-temperature argon matrix, laser irradiations at 7013 cm-1 and at 7034 cm-1 were found to induce effective transformations of the two conformers into each other in a reversible way. It was also demonstrated that other forms of cytosine (amino-oxo and imino-oxo) are not affected by near-IR irradiation.

2 UV-visible-induced reactions in cryomatrices

Most of the studies on the topics addressed by the present review that were reported during 2010 refer to photochemical processes induced by UV-visible light. In this section, some of the most relevant of those studies are shortly presented. Firstly, photoinduced conformational isomerization processes will be addressed. Then, the more complex bond-breaking/bond-forming reactions will be considered, including tautomerizations and other structural isomerizations, fragmentation reactions, and formation of complex and weakly bound species. The last part of this section addresses the subject of noble gas chemistry, where the matrix noble gas atoms take an active role in photochemical reactions promoting formation of covalently bound noble gas containing molecules.

2.1 Conformational isomerizations

Kalume et al. reported experimental and computational studies of the photolysis of atmospherically important 1,2-dibromoethanes of general formula 1,2-C2X4Br2 (X = H, F) in Ar matrices at 5 K. They found that a significant conformational relaxation occurs for 1,2-C2H4Br2 (observed anti/gauche ratio = 30:1) but not for 1,2-C2F4Br2 (anti/gauche = 3:1), which was explained taking into account the larger barrier to rotation about the C-C bond in the latter compound. It was suggested that the matrix pulsed deposition method, as compared with conventional continuous deposition methods, can lead to an increased conformational relaxation for systems with low barriers to internal rotation. Photolysis of 1,2-C2H4Br2 at λ = 220 nm reveals the growth of infrared bands assigned to the gauche conformer (Fig. 1), which occurs concomitantly to production of the C2H4-Br2 charge transfer and C2H3Br-HBr complexes and the C2H4Br radical. In the case of 1,2-C2F4Br2, irradiation at 220 nm was found to lead mainly to the anti and gauche conformers of the C2F4Br radical, whose vibrational and electronic spectra were characterized for the first time. The increase in yield of radical for 1,2-C2F4Br2 was attributed to the stronger C–Br bond in the fluoro-substituted radical species.

A series of studies on UV-induced conformational isomerizations in aromatic aldehydes has been reported by Fausto and co-workers. The conformational space of monomeric pyrrole-2-carbaldehyde (P2C) was investigated theoretically at the MP2 and DFT (B3LYP) levels, with the 6-311++G(d,p) basis set. The compound can assume two conformations, cis and trans, regarding the orientation of the N–C–C=O dihedral angle. The cis form was found to be the conformational ground state,...