Chromium dihydride (CrH2): theoretical evidence for a bent 5B2 ground state

Ab initio molecular electronic structure theory has been used to study two likely candidates for the quintet ground state of CrH₂ at the self-consistent field, the single and double excitation configuration interaction (CISD), the single and double excitation coupled cluster (CCSD), and the single, double, and perturbative triple excitation coupled cluster (CCSD(T)) levels of theory. The largest basis sets utilized for geometry optimizations, designated two-f TZ2P, have a contraction scheme of Cr(14s11p6d2f/10s8p3d2f), H(5s2p/3s2p). At the CCSD(T) level of theory using this two-f TZ2P basis, a bent (C_2v symmetry) ⁵B₂ state is predicted to lie a mere 4·2 kcal mol^-1 (4·5 kcal mol^-1 including zero-point vibrational energy correction) lower in energy than linear (D_∞h symmetry) ⁵Σ_g ⁺ HCrH. Theoretical predictions for the equilibrium geometry, harmonic vibrational frequencies, and isotopic frequency shifts of the ⁵B₂ state compare favourably with the results of recent matrix isolation FT-IR work by Xiao, Hauge, and Margrave. The two-f TZ2P CCSD(T) optimized geometry of ⁵B₂ CrH₂ is r _e = 1·658 Å and ?_e = 114·4° (118° ± 5° is the experimentally estimated bond angle), while the harmonic vibrational frequencies are ω₁(a₁) = 1710, ω₂(a₁) = 582, and ω₃(b₂) = 1683 cm^-1 (experimentally assigned fundamentals in an argon matrix are the symmetric stretch 1651 cm^-1 and asymmetric stretch 1615 cm^-1). Isotopic frequency shifts of CrD₂ relative to CrH₂ are Δω₁(a₁) = -492 and Δω₃(b₂) = -477 cm^-1 (compared with symmetric stretch -462 cm^-1 and asymmetric stretch -448-cm^-1 from experiment).