CHROMIUM DIHYDRIDE (CRH2) - THEORETICAL EVIDENCE FO A BENT B-5(2) 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 (C2v symmetry) B-5(2) 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 infinity h symmetry) 5-Sigma(g)+ HCrH. Theoretical predictions for the equilibrium geometry, harmonic vibrational frequencies, and isotopic frequency shifts of the B-5(2) 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 B-5(2) CrH2 is r(e) = 1.658 Anstrong and theta(e) = 114.4-degrees (118-degrees +/- 5-degrees is the experimentally estimated bond angle), while the harmonic vibrational frequencies are omega1(a1) = 1710, omega2(a1) = 582, and omega3(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 Delta-omega1(a1) = - 492 and Delta-omega3(b2) = -477 cm-1 (compared with symmetric stretch - 462 cm-1 and asymmetric stretch -448-cm-1 from experiment).