Knowledge Bank

In the course of a search for ESE spectrum of $C2-1$ isolated in its $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma g+$ ground state the ion pairs $M+C2-(M+=li+$, $Na+$, $K+$, $Cs+$) have been formed at $4\circ K$ by the photolysis of a matrix consisting of 1% M and 1% $C2H2$ in $Ar.2$ Isotopic substitutions iondicate a triangular conformation with equivalent C atoms. An estimate of spin densities in $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C2-$ has been derived from the observed hyperfine splittings. Infrared and electronic spectra of the ion pairs demonstrate the variation of the $C2-$ vibrational frequency in the ground state and in the excited $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma u+$ state as a function of the alkali $cation.3$ $C4$ has been prepared by the photolytic dissociation of 1% $C4H2$ (diacetylene) in an argon matrix. The ESR spectrum consisting of the $\Delta$M = 1 (xy, and $xy2$) and $\Delta$M = 2 transitions confirms that $C4$ is linear and has a $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma g4-$ ground state in agreement with the MO calculations of Pitzer and Clement. A value for the zero-field splitting constant $D=0.224cm-1$ has been determined.