Observation and spectroscopy of high-lying states of the CaOH radical: Evidence for a bent, covalent state

Abstract

The CaOH radical has been produced in a supersonic molecular beam by the reaction of water vapor with laser desorbed calcium. Three new electronic states, the D 2Σ+, E 2Σ+, and F states have been observed by laser induced fluorescence and resonance enhanced multiphoton ionization spectroscopy. The D and E states are linear, but the F state is bent, the first bent and strongly covalent state of CaOH to be observed. Vibrational constants for the D state have been determined and a partially rotationally resolved spectrum has provided the rotational constant and Ca–OH bond length in this state. The D and E states are more strongly bound than the ground state, perhaps indicating some covalent contribution to the bonding. Additional vibrational constants of the ground electronic state including the CaO–H vibrational frequency have been determined from the emission spectra.

Ab initio calculations on electronic states of CaOH

“…7͒. The present findings are in agreement with the experimental evidence of mixing between the Ẽ and F states, 8 2 AЉ ͑see above͒, have linear minimum energy geometries. It is rather tenuous to insist on the existence of the above off-linear minima found for some of the 2 AЈ states, given the small energy differences involved and the accuracy of the present calculations.…”

“…9,11 Accordingly, the ground and these Rydberg excited states have similar ͑linear͒ geometry, which is consistent with theoretical work 11 and experimental findings for most of the observed excited states. A bent excited state has been proposed correlating with the ͑3d͒ F 2 ⌸ linear state, as deduced from extensive activity in the bending vibration in the observed spectra; 8 indicating considerable covalent character for this state. Configuration interaction calculations on the lighter members of this series, BeOH and MgOH, where covalent bonding is expected to contribute, did find the first excited state, 2 2 AЈ, correlating with the A 2 ⌸ of linear geometry, to have a bent equilibrium geometry in both systems.…”

Accurate ab initio potential energy surface and vibration‐rotation energy levels of hydrogen peroxide

“…Therefore, its equilibrium structure can be expected to be linear. The observed electronic-vibrationalrotational spectra of the CaOH and CaOD species [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] are indeed consistent with this structure and a nearly harmonic CaOH bending potential energy function. In particular, the dependence of the effective rotational constant B V on the quantum number V 2 of the CaOH bending mode was found to be nearly linear, 11-14 thus indicating low anharmonicity of the CaOH bending vibration.…”

A Critical Appraisal of the Experimental Data on the Molecular Structure and Spectra of Halides, Oxides, and Hydrides of the s-, d-, and f-Block Elements

“…In particular density functional theory (which is nearly as old as quantum mechanics) is establishing a reputation for speed and reliability for compounds of the transition elements. [127] It is also salutory to recall that although the ground and several excited electronic states of CaOH are linear, [128] ªthe F state is bent, the first bent and strongly covalent state of CaOH to be observedº.…”