%0 DATA
%A Andrew C., Simmonett
%A Nathan J., Stibrich
%A Brian N., Papas
%A Henry F., Schaefer
%A Wesley D., Allen
%D 2009
%T Barrier To Linearity and Anharmonic Force Field of the Ketenyl Radical
%U https://acs.figshare.com/articles/journal_contribution/Barrier_To_Linearity_and_Anharmonic_Force_Field_of_the_Ketenyl_Radical/2817559
%R 10.1021/jp9024365.s001
%2 https://acs.figshare.com/ndownloader/files/4515151
%K cm
%K DBOC
%K CCSDT
%K electron correlation treatments
%K frequency
%K HCCO
%K barrier
%K point approach
%K Anharmonic Force Field
%K CBS
%K vibrational frequencies
%K cluster theory
%K Ketenyl RadicalThe
%K geometry optimizations
%K vibrational perturbation theory
%K basis sets
%K linearity
%K VPT
%K 2Π
%K quartic force field
%K AE
%K spectroscopic constants
%X 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*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}.