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Abstract:
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CCSD (T ) has been used in the past to accurately predict compute the spectra and structures of small molecules . However , the large execution times required for these calculations has limited their use in larger molecules such as benzene and [10]annulene . The parallelization of analytic second derivatives of post Hartree -Fock methods , including CCSD (T ) , has enabled the VPT2+D treatment of the vibrational states of benzene . The fundamental frequencies and infrared active two quantum transitions that result are within 20 cm⁻¹ of the experimental values when treated for Fermi and Darling -Dennison resonances and empirical estimates for the harmonic frequencies and equilibrium bond lengths are determined to be within 12 cm⁻¹ and 0 .004 Å of the values at the CCSD (T ) /ANO2 (fc ) level of theory . The parallelization also facilitated the identification of two candidates for the structures of two isomers of [10]annulene . The harmonic frequencies of several conformations proposed in the literature were computed at the CCSD /DZP level of theory with five of the conformations being ground states . The NMR shifts of four of these structures were computed using CCSD (T ) /tzp and conformation 6 (C₂ "twist" ) was identified as the likeliest structure for one of the isomers isolated . The remaining compound was identified as conformation 2b (C₂ "boat" ) due to a low conformation barrier and the proximity of its average NMR shifts to experiment . The identification of both compounds rely on properties computed using analytic derivatives not entirely on the relative energies of optimized geometries . |