A matrix ESR study of free radicals

Abstract

The matrix isolation technique and electron spin resonance (ESR) spectroscopy have been applied to study carbon molecules. The C$\sb4$ molecule produced by the vacuum UV photolysis of diacetylene, C$\sb4$H$\sb2$, and butadiene, C$\sb4$H$\sb5$, has been trapped in argon matrices at 12 K. Using highly dilute samples, ESR signals previously assigned to linear, triplet C$\sb4$ formed from graphite vapor, exhibit splitting into separate $x$ and $y$ components, which indicate that C$\sb4$ actually has a nonlinear geometry. Studies on the effects of annealing and varying mole concentrations argue against a role for multiple trapping sites. The absence of any change in the spectrum on deuteration of the precursor, eliminates the possibility of proton hfs (hyperfine splitting) caused by the presence of hydrogen atoms in the molecule. Further evidence suggesting nonlinear character for C$\sb4$ formed in these experiments, is provided by the carbon-13 hfs observed by using isotopomers of the butadiene parent. A slightly bent structure is concluded for the C$\sb4$ molecule with an approximate angle between adjacent carbon bonds of $\sim$175$\sp\circ$. These results are in agreement with a recent FTIR (Fourier transform infrared) study of C$\sb4$. In the second part of the dissertation, a method which has general application for matrix studies of radical cations has been successfully developed.