# Barrier To Linearity and Anharmonic Force Field of the Ketenyl Radical

journal contribution

posted on 29.10.2009, 00:00 by Andrew C. Simmonett, Nathan J. Stibrich, Brian N. Papas, Henry F. Schaefer, Wesley D. AllenThe 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

*X*Z (*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∼) 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}Π 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 (ν_{1}, ν_{2}, ν_{5}) fundamental frequencies of (3212, 2025, 483) cm^{−1}agree satisfactorily with the experimental values (3232, 2023, 494) cm^{−1}.