Problematic p-benzyne: Orbital instabilities, biradical character, and broken symmetry

Abstract

Articles you may be interested inTheoretical prediction of nuclear magnetic shieldings and indirect spin-spin coupling constants in 1,1-, cis-, and trans-1,2-difluoroethylenes A benchmark theoretical study of the electronic ground state and of the singlet-triplet split of benzene and linear acenes J. Chem. Phys. 131, 224321 (2009); 10.1063/1.3270190 CCSD(T) study of the far-infrared spectrum of ethyl methyl etherThe equilibrium geometry, harmonic vibrational frequencies, and infrared transition intensities of p-benzyne were calculated at the MBPT͑2͒, SDQ-MBPT͑4͒, CCSD, and CCSD͑T͒ levels of theory using different reference wave functions obtained from restricted and unrestricted Hartree-Fock ͑RHF and UHF͒, restricted Brueckner ͑RB͒ orbital, and Generalized Valence Bond ͑GVB͒ theory. RHF erroneously describes p-benzyne as a closed-shell singlet rather than a singlet biradical, which leads to orbital near-instabilities in connection with the mixing of orbital pairs b 1u -a g ͑HOMO-LUMO͒, b 2g -a g ͑HOMO-1-LUMO͒, and b 1g -a g ͑HOMO-2-LUMO͒. Vibrational modes of the corresponding symmetries cause method-dependent anomalous increases ͑unreasonable force constants and infrared intensities͒ or decreases in the energy ͑breaking of the D 2h symmetry of the molecular framework of p-benzyne͒. This basic failure of the RHF starting function is reduced by adding dynamic electron correlation. However RHF-MBPT͑2͒, RHF-SDQ-MBPT͑4͒, RHF-CCSD, RB-CCD, and RHF-CCSD͑T͒ descriptions of p-benzyne are still unreliable as best documented by the properties of the b 1u -, b 2g -, and b 1g -symmetrical vibrational modes. The first reliable spin-restricted description is provided when using Brueckner orbitals at the RB-CCD͑T͒ level. GVB leads to exaggerated biradical character that is reduced at the GVB-MP2 level of theory. The best results are obtained with a UHF reference wave function, provided a sufficient account of dynamic electron correlation is included. At the UHF-CCSD level, the triplet contaminant is completely annihilated. UHF-CCSD͑T͒ gives a reliable account of the infrared spectrum apart from a CCH bending vibrational mode, which is still in disagreement with experiment.

The bergman cyclizations of the enediyne and its N‐substituted analogs using multiconfigurational second‐order perturbation theory

“…The C 1 AC 6 (C 3 AC 4 ) and C 1 AC 2 (C 5 AC 6 , C 2 AC 3 , and C 4 AC 5 ) bonds are predicted to be 1.438 and 1.366 Å , respectively, which are very close to the CASSCF(12,12)/ TZ2P values of Lindh and Persson [18] (the discrepancies being 0.004 Å for both) and similar to the results of the CCSD(T), MBPT, BPW91, and B3LYP calculations (see Table 1 and Ref. [48]) (the discrepancies being within 0.018 Å ). The 1TS s (connecting 1R s and 1P s ) has the C 2v symmetry.…”

Carbon‐rich Compounds: Computational Considerations

“…8.6, the quality of the results is further improved by single-point energy computations on the BLYP/6-31G* structures utilizing the Brückner-doubles coupled cluster approach including triple excitations perturbatively {BCCD(T), also called BD(T)} based on BS-UHF reference wavefunctions. In contrast, coupled-cluster computations with unmodified UHF molecular orbi- tals {CCSD(T)} give poor results although they often deal well with multireference problems when the effect of triple excitations is included; this is due to the fact that in some open-shell cases CCSD(T) overestimates the single cluster-amplitudes [97]. The use of Brückner orbitals, in which the singles contribution is zero due to orbital rotation, leads to a significant improvement [98].…”

Thermal Rearrangements of Heteroatom‐Bridged Diallenes

“…Intrinsic reaction coordinate [32] (IRC) computations were utilized to confirm the TSs. Single-point energies, using the same function but with a larger basis set (6-311+G*), were computed for all species; Brueckner doubles coupled-cluster energies [33,34] utilizing a cc-pVDZ basis set [BD(T)/cc-pVDZ] were additionally computed for comparison in some critical cases. The validity of the use of these computational schemes for these types of molecules has been demonstrated in our previous studies and were confirmed by other groups.…”