Barrier to linearity and anharmonic force field of the ketenyl radical

The troublesome barrier to linearity of the ketenyl radical (HCCO) is precisely determined using state-of-the-art computations within the focal point approach, by combining complete basis set extrapolation, utilizing the aug-cc-pVXZ (X = D, T, Q, 5, 6) family of basis sets, with electron correlation treatments as extensive as coupled cluster theory with single, double, triple, and perturbative quadruple excitations [CCSDT(Q)]. Auxiliary terms such as diagonal Born-Oppenheimer corrections (DBOCs) and relativistic contributions are included. To gain a definitive theoretical treatment and to assess the effect of higher-order correlation on the structure of HCCO, we employ a composite approximation (c approximately ) to all-electron (AE) CCSDT(Q) theory at the complete basis set (CBS) limit for geometry optimizations. A final classical barrier to linearity of 630 +/- 30 cm(-1) is obtained for reaching the (2)Pi Renner-Teller configuration of HCCO from the (2)A'' ground state. Additionally, we compute fundamental vibrational frequencies and other spectroscopic constants by application of second-order vibrational perturbation theory (VPT2) to the full quartic force field at the AE-CCSD(T)/aug-cc-pCVQZ level. The resulting (nu(1), nu(2), nu(5)) fundamental frequencies of (3212, 2025, 483) cm(-1) agree satisfactorily with the experimental values (3232, 2023, 494) cm(-1).