Knowledge Bank

The $B~-X~$ laser-induced-fluorescence (LIF) spectrum of jet-cooled iso-propoxy radical ($i$-C$3$H$7$O$\cdot$) has been recorded. The observed rotational and fine structure of the origin band has been well simulated with rotational constants for both the $X~$ and $B~$ states and the electron spin-rotation constants of the $X~$ state. The line intensities are well simulated with a parallel transition type, requiring the same symmetry for the $X~$ as for the $B~$ state (A$ ^\prime$),\; which\; confirms\; the\; previous\; suggestion,\; 5986\; (1989).\}\; that\; going\; from\; ethoxy\; (C$_2$H$_5$O$\cdot$) to iso-propoxy, the energy ordering of the in- and the out-of-plane half-filled $p$-orbitals is reversed and the ground state symmetry changes from A to A. The fit rotational constants are consistent with those obtained from the quantum chemistry calculations. However, the fit spin-rotation constants, particularly the nonzero $ϵaa$ with $a$-axis perpendicular to the $Cs$ plane, can not be explained by second-order perturbation theory. Nevertheless they can be semi-quantitatively explained based on the transferability of the electron spin-molecular rotation tensor, 276 (2003).} in the axis system defined by the half-filled $p$-orbital and the $p\pi$-orbital using ethoxy as the reference molecule. The failure of the second-order perturbation theory, as well as that of the pure precession approximation, is due to the strong vibronic interaction between the near-degenerate $X~$ and $A~$ states. The vibrationless levels of these two states are separated by only 68 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ as determined in previous disperse-fluorescence study., 11780 (2004).} The near degeneracy of the $X~$ and $A~$ states of iso-propoxy and reversal of the energy ordering going from ethoxy to iso-propoxy have also been demonstrated in quantum chemistry calculations. Based on the result of iso-propoxy, spin-rotation constants of other secondary alkoxy radicals can be predicted and used for the simulation of the $B~-X~$ rovibronic spectra of these radicals. Specifically, analysis of the high-resolution LIF spectrum of cyclohexoxy ($c$-C$6$H$\mathrm{<mrow\; data-mjx-texclass="ORD">11</mrow>}$O$\cdot$), 10579 (2004).} is ongoing.