Theoretical Investigation of Diradicals and Carbenes

This thesis focuses on a variety of computational work on a range of different organic diradical and carbene species. Chapter 1 gives an introduction to using computational chemistry to predict rate constants for a tunneling reaction. The installation and use of Polyrate and Gaussrate software is discussed from the perspective of teaching a new graduate student the use of the software. Though the number of publications using this software is increasing every year, there remains a lack of easily obtainable resources for the novice user. The chapter also provides an overview of transition state theory (TST) and the many different tunneling approximations available in the software.;Chapters 2 and 3 detail the two main investigations into tunneling. Chapter 2 outlines the study of a hydrogen migration reaction of triplet o-tolylcarbene and several naphthylcarbene species. Tunneling rates and the potential energy surface (PES) of these species were explored computationally in comparison to experimental matrix and spectroscopy work. This work explores the differences in the PES of the carbenes in an attempt to rationalize the observed experimental differences in tunneling rates. Chapter 3 explores the heavy-atom tunneling present in the ring expansion of benzazirine to ketenimine, an important process along the long studied phenylnitrene pathway. The role of heavy-atom tunneling will be discussed in comparison to other substituted benzazirine species, and previously studied systems.;Chapter 4 introduces the study of two families of (sigma,sigma) diradical species on the C8H4 PES; didehydropentalene (DDP) and didehydrobenzocyclobutadiene (DDB). CASSCF and CASMP2 calculations were performed to evaluate the differences in through-bond and through-space coupling of the two spins in the singlet state.;Chapter 5 discusses the use of computational chemistry to better understand and explain the observed spectroscopy of three matrix isolated species; HC 7H, MeC7H, and MeC3Me.