High resolution laser spectroscopy of the C̃ 2Δ–X̃ 2Σ+ transition of CaOH and CaOD: Vibronic coupling and the Renner–Teller effect

Theoretical rovibronic spectroscopy of the calcium monohydroxide radical (CaOH)

The rovibronic (rotation-vibration-electronic) spectrum of the calcium monohydroxide radical (CaOH) is of interest to studies of exoplanet atmospheres and ultracold molecules. Here, we theoretically investigate the òΠ-X̃²Σ⁺ band system of CaOH using high-level ab initio theory and variational nuclear motion calculations. New potential energy surfaces (PESs) are constructed for the X̃²Σ⁺ and Ã²Π electronic states along with Ã-X̃ transition dipole moment surfaces (DMSs). For the ground X̃²Σ⁺ state, a published high-level ab initio PES is empirically refined to all available experimental rovibrational energy levels up to J = 15.5, reproducing the observed term values with a root-mean-square error of 0.06 cm^-1. Large-scale multireference configuration interaction calculations using quintuple-zeta quality basis sets are employed to generate the Ã²Π state PESs and Ã-X̃ DMSs. Variational calculations consider both Renner-Teller and spin-orbit coupling effects, which are essential for a correct description of the spectrum of CaOH. Computed rovibronic energy levels of the Ã²Π state, line list calculations up to J = 125.5, and an analysis of Renner-Teller splittings in the ν₂ bending mode of CaOH are discussed.

Infrared Spectra and Electronic Structure Calculations for the Group 2 Metal M(OH)2 Dihydroxide Molecules

Reactions of laser-ablated Mg, Ca, Sr, and Ba atoms with O2 and H2 in excess argon give new absorptions in the O-H and O-M-O stretching regions, which increase together upon UV photolysis and are due to the M(OH)2 molecules (M = Mg, Ca, Sr, and Ba). The same product absorptions are observed in the metal atom reactions with H2O2. The M(OH)2 identifications are supported by isotopic substitution and theoretical calculations (B3LYP and MP2). The O-H stretching frequencies of the alkaline earth metal dihydroxide molecules decrease from 3829.8 to 3784.6 to 3760.6 to 3724.2 cm(-1) in the family series in solid argon, while the base strength of the solid compounds increases. Calculations show that Sr(OH)2 and Ba(OH)2 are bent at the metal center, owing to d orbital involvement in the bonding. Although these molecules are predominantly ionic, the O-H stretching frequencies do not reach the ionic limit of gaseous OH- going down the family group because of cation-anion polarization and p(pi) --> d(pi) interactions.

Electronic structure of the calcium monohydroxide radical

Effective valence shell Hamiltonian H(v) calculations are used to map out three-dimensional potential energy surfaces for the 12 lowest electronic states of the CaOH radical. Excitation energies and spectroscopic constants are compared with experiment and prior computations where available, but many previously unavailable data are provided, including excited state dipole moments and oscillator strengths. Particular attention is paid to clarify the nature of nonlinear and quasilinear excited states, Renner-Teller couplings, and state mixings. The F (2)Pi and G (2)Pi (6 (2)A(') and 8 (2)A(')) states are both found to possess nonlinear local minima, due to an avoided crossing. Attention is also focused on the characteristics of basis sets necessary in high-accuracy calculations for the CaOH radical.

Infrared Spectra of the Group 2 Metal Dihydroxide Molecules

Group 2 metal atoms (Mg, Ca, Sr, and Ba) react on ultraviolet photoexcitation with O(2), H(2) mixtures in solid argon at 10 K to produce new absorptions in the O-H and O-M-O stretching regions. The effect of detailed isotopic substitution on these two absorptions identifies the M(OH)(2) molecules. The stepwise decrease in the O-H stretching modes in this chemical family demonstrates an increase in ionic character, which parallels the increase in base strength for the analogous solid compounds.

Laser Excitation Spectroscopy of the &Btilde;2A1-&Xtilde;2A1 Transition of the CaOCH3 Radical

The (0-0) band of the &Btilde;2A1- &Xtilde;2A1 transition of CaOCH3 at 565 nm has been recorded and analyzed at Doppler limited resolution (FWHM = 0.005 cm-1). The molecule was generated using a laser ablation/molecular beam source in which a solid calcium rod was ablated and a mixture of argon seeded with a few percent of methanol was used as the carrier gas. The rotational analysis of the band is consistent with a linear Ca-O-C geometry in both electronic states. Values for the spin-rotation parameters are determined for the excited &Btilde;2A1 state. Copyright 1998 Academic Press.