Heavy atom nitroxyl radicals. I: An ab initio study of the ground and lower electronic excited states of the H(2)As=O free radical.

A series of ab initio calculations have been undertaken to predict the spectroscopic properties of the ground and first two excited states of the recently discovered arsenyl (H(2)AsO) free radical. This 13 valence electron species can be viewed as similar to the formaldehyde radical anion with a ground state electron configuration of cdots, three dots, centered(pi)(2)(n)(2)(pi( *))(1). The arsenyl radical is nonplanar (pyramidal) in the ground state with a 59 degrees out-of-plane angle and a 1.67 A AsO bond length. It has a low-lying n-pi( *)(A (2)A(")) excited state (T(e) approximately 5000 cm(-1)) which has a much larger out-of-plane angle (86 degrees ) and longer AsO bond length (1.81 A). The pi-pi( *)(B (2)A(')) excited state at approximately 20 500 cm(-1) is less pyramidal (out-of-plane angle=70 degrees ) and has a somewhat shorter AsO bond (1.77 A). Similar trends are found for the H(2)PO and H(2)NO free radicals, although the latter has a planar ground state, due to sp(2) hybridization of the N atom, and a very long B state AsO bond length. The geometric variations of the ground and excited states of the H(2)EO (E=N, P, As) radicals, as well as the ground states of the corresponding anions and cations, can be readily rationalized from the Walsh diagram of the anion. The variations in the E-O bond length are a result of changes in both the orbital occupancy and pyramidalization of the molecule. The results of the present work have been employed in the analysis of the B (2)A(')-X (2)A(') electronic band system of the H(2)AsO free radical as reported in the companion paper.