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-pV<i>X</i>Z (<i>X</i> = 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∼) 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<sup>−1</sup> is obtained for reaching the <sup>2</sup>Π Renner−Teller configuration of HCCO from the <sup>2</sup>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 (ν<sub>1</sub>, ν<sub>2</sub>, ν<sub>5</sub>) fundamental frequencies of (3212, 2025, 483) cm<sup>−1</sup> agree satisfactorily with the experimental values (3232, 2023, 494) cm<sup>−1</sup>.