Matrix Isolation of Novel Reactive Intermediates - Quantum Mechanical Tunneling in Atmospherically and Astrochemically Relevant Compounds -

Abstract

Small, reactive molecules are intermediates postulated in atmospheric and astrochemical processes. However, little to nothing is known about many such species. With the help of the matrix isolation technique the lifetime of reactive molecules can be increased allowing for their direct spectroscopic investigation. Quantum mechanical tunneling leads to the depletion of some matrix-isolated species. Investigating this effect eventually led to a novel chemical principle, namely tunneling control. During this work, two novel hydroxycarbenes, among others, were generated and their tunneling behaviors were studied. The focus lies on the effect of substitution on tunneling half-lives, aiming to create a better and more intuitive understanding of quantum mechanical tunneling in chemistry. This might eventually enable exploiting this effect, e.g., in chemical synthesis. The species generated herein were discussed regarding their role in atmospheric and astrochemical processes in the literature and their direct spectroscopic characterization provides the chemical basis for models used in these fields. In the first publication, the hitherto unknown thiolimine tautomer HC(NH)SH of thioformamide (HC(S)NH2) was generated photochemically in cryogenic argon and dinitrogen matrices. One of the four observed conformers of this species reacts in a tunneling-enabled C–S rotamerization. The second publication reports the first generation and spectroscopic characterization of aminohydroxymethylene (H2N–C̈–OH). Aminohydroxymethylene is persistent in solid argon at 3 K and decomposes to NH3 + CO as well as HNCO + H2 upon UV excitation. Another novel hydroxycarbene, namely ethynylhydroxycarbene (HC≡C–C̈–OH), is the subject of the third publication. The compound reacts in a conformer-specific quantum mechanical tunneling process, which is typical for hydroxycarbenes, to propynal. The fourth publication describes the formation of the hitherto unknown cis-cis-conformer of dihydroxycarbene (HO–C̈–OH) from energetically lower-lying conformers by irradiation with near-infrared light in solid dinitrogen. Besides conformational tunneling, the measured kinetic profiles of the decay of this new compound hint towards a side reaction, namely its decomposition to CO2 and H2.