Beschreibung:
<jats:p>The high-resolution infrared emission spectrum of BeF2 vapor at 1000°C was rotationally analyzed with the assistance of large-scale ab initio calculations using the coupled-cluster method including single and double excitations and perturbative inclusion of triple excitations, in conjunction with correlation-consistent basis sets up to quintuple-zeta quality. The ν3 fundamental band, the ν1+ν2, ν1+ν3, and 2ν2+ν3 combination bands, and 18 hot bands were assigned. The symmetric stretching (ν1), bending (ν2), and antisymmetric stretching (ν3) mode frequencies were determined to be 769.0943(2), 342.6145(3), and 1555.0480(1)cm−1, respectively, from the band origins of the ν3, ν1+ν3, and ν1+ν2 bands. The observed vibrational term values and B rotational constants were fitted simultaneously to an effective Hamiltonian model with Fermi resonance taken into account, and deperturbed equilibrium vibrational and rotational constants were obtained for BeF2. The equilibrium rotational constant (Be) was determined to be 0.235354(41)cm−1, and the associated equilibrium bond distance (re) is 1.3730(1)Å. The results of our ab initio calculations are in remarkably good agreement with those of our experiment, and the calculated value was 1.374Å for the equilibrium bond distance (re). As in the isoelectronic CO2 molecule, the Fermi resonance in BeF2 is very strong, and the interaction constant k122 was found to be 90.20(4)cm−1.</jats:p>