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Barrier To Linearity and Anharmonic Force Field of the Ketenyl Radical

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journal contribution
posted on 29.10.2009 by Andrew C. Simmonett, Nathan J. Stibrich, Brian N. Papas, Henry F. Schaefer, Wesley D. Allen
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∼) 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 2Π Renner−Teller configuration of HCCO from the 2A′′ 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 (ν1, ν2, ν5) fundamental frequencies of (3212, 2025, 483) cm−1 agree satisfactorily with the experimental values (3232, 2023, 494) cm−1.