Cyclobutadiene matrix isolation study

Fraga, S., Possible interactions of cyclobutadiene with by-products in inert matrix isolation studies. Tetrahedron Lett., 22, 3343,1981. 

During all these studies on cyclobutadienes and tetrahedranes formed via carbenes as transient species we wondered whether matrix isolation IR spectroscopy might be a good tool for the direct observation of cyclopropenylidene (2) and trimethylenemethane (3). This is indeed the case. 

Later, the experimental evidence for square cyclobutadiene was called into question. Krantz reported the photolysis of bicyclopyranone in which the carbon atom eliminated as CO2 was labeled with C. One important infrared band that had been assigned to a vibration of square planar cyclobutadiene in earlier studies was altered by the isotopic change, suggesting that this band was due to CO2 trapped with the cyclobutadiene in the rigid rare gas matrix. Thus, the experimental data did not answer the question of the structure of cyclobutadiene. Later work on the theoretical determination of the infrared spectrum of cyclobutadiene ° and further matrix isolation spectroscopy experiments, including the use of polarized IR spectroscopy... 

Many photochemical reactions are carried out at low temperatures as low as 4 K to slow down the reaction rate for the study of the lifetimes of the reactive intermediates. The most useful matrix materials are solid argon, solid neon and solid nitrogen. The initial photoproduct is trapped within a rigid matrix that inhibits the decay of the reactive species in diffusion process. For example, 5-hydroxy-a, P, y, 5-unsaturated valerolactone 1 on photochemical decomposition gives cyclobutadiene 2 and carbon dioxide. The intermediate and the products of this reaction are characterized in low-temperature matrix isolation process. 

Cyclobutadiene is a key compound in the study of antiaromaticity since it is the smallest neutral example and it is planar. Its chemistry has been the subject of several reviews. It was first observed in an argon matrix, being formed by the photolysis of a-pyrone. Subsequently, it was prepared from a variety of other precursors. It is highly reactive, and it readily dimerizes when the matrix softens and molecular diffusion becomes important. The dimerization process has been studied theoretically. Although cyclobutadiene cannot be isolated in the pure form, it can be stabilized by the formation of metal complexes. 

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