Transient organic species such as simple localized hydrocarbon biradicals have long been a subject of intense research effort. While some have been detected in low-temperature matrices, direct detection of 1,4-cyclohexadiyl and related biradicals has been unsuccessful due to their low stability and fast intersystem crossing rates. The goal of this research is to detect and characterize these biradicals in single crystals by EPR spectroscopy. The success of this undertaking relies on the combination of low-temperature detection, rigidity of the crystal environment and rational design of precursors and of the crystal lattice packing. This research will also contribute the understanding of factors that influence stability and decomposition products of energetic materials in the solid state.

Triplet localized hydrocarbon bioradicals have been extensively studied as intermediates and end-products of azoalkane denitrogenation, and as test cases for the predictive ability of various ab-initio and semi-empirical computational methods. An example of the latter is the lively debate surrounding possible participation of 1,4-cyclohexadiyl (2) biradicals as intermediates in Cope rearrangement. They are also of great practical interest as possible building blocks of high-spin organic molecules. Therefore, various aspects of their energetics and structure such as "through bond" and "through space" coupling, structural and substituent effects on singlet-triplet energy gaps and on intersystem crossing (ISC) rates are of particular interest. This program focuses on direct detection and characterization of triplet 1,3-cyclopentadiyl (1) and related biradicals, and biradicals derived from 1,4-cyclohexadiyl (2), by low-temperature EPR spectroscopy in single crystals. These species are produced by direct photolysis of the respective azoalkanes DBH (5) and DBO (6) via intermediate diazenyl biradicals 3 and 4.

This research will involve: 1. Preparation of single crystals of 5 and derivatives such as 1-phenyl-, 7,7-dimethyl- and 1,4-dimethyl-5, and 1-phenyl- and 1-methyl-6, and photolysis of these crystals between 4K and 77K. Since the spectra of 1 was observed in frozen matrices (at 4-77K), photolysis of appropriate derivatives of 5 will be carried out first. Experimental conditions will be optimized in order to get clean EPR spectra of biradicals discussed here. Least-squares analysis will be employed to obtain the eigenvalues and eigenvectors of the relevant biradical tensors from the dependence of the spectra on crystal orientation relative to the magnetic field of the spectrometer. 2. Preparation of the following cyclic azoalkane precursors of type 5 and 6 equipped with alkyl chain substituents. Numerous published procedures are available for the preparation of these molecules. Only minor modifications will be required. Alkyl chains with a terminal ionic auxiliary X, an organic carboxylate/amine salt functionality, will be used to manipulate the packing of the lattice and hence the conformational flexibility of the resulting hydrocarbon biradicals. This project will introduce REU participants to a number of concepts and techniques not routinely encountered in the undergraduate research setting. These include: exposure to EPR spectroscopy and to the concepts of organic solid-state reactivity, and familiarity with techniques of preparation of optical quality organic crystals. Working knowledge of basic EPR principles will reinforce the familiarity and understanding of magnetic resonance while working with single crystals will better prepare a student for encounter with the real world where increasing emphasis is placed on the applications of solid-state chemistry.

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