The quartic force field of ethylene, C 2 H 4 , has been calculated ab initio using augmented coupled cluster, CCSD͑T͒, methods and correlation consistent basis sets of spdf quality. For the 12 C isotopomers C

The geometry and harmonic frequencies of naphthalene, azulene, phenanthrene, and anthracene have been computed using the Becke three-parameter Lee−Yang−Parr functional (B3LYP) and basis sets of spd and spdf quality. A simple scaling procedure for the harmonic frequencies is proposed that brings computed harmonics to within 10−20 cm-1 or better of experimental fundamentals without resorting to internal coordinate transformations. A complete reassignment of the vibrational spectrum of phenanthrene is proposed, and several reassignments are due for anthracene as well. The assignments of Sellers, Pulay, and Boggs (J. Am. Chem. Soc. 1985, 107, 6487) for naphthalene, and of Kozlowski, Rauhut, and Pulay (J. Chem. Phys. 1995, 103, 5650) for azulene, are largely confirmed.

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In straightforward continuation of Green’s function studies of the ultraviolet photoelectron spectra of polycyclic aromatic compounds [Deleuze et al., J. Chem. Phys. 115, 5859 (2001); M. S. Deleuze, ibid. 116, 7012 (2002)], we present a benchmark theoretical determination of the ionization thresholds of benzene, naphthalene, anthracene, naphthacene (tetracene), pentacene, and hexacene, within chemical accuracy [0.02–0.07 eV]. The vertical ionization potentials of these compounds have been obtained from series of single-point calculations at the Hartree–Fock, second-, third-, and partial fourth-order Møller–Plesset (MP2, MP3, MP4SDQ) levels, and from coupled cluster calculations including single and double excitations (CCSD) as well as a perturbative estimate of connected triple excitations [CCSD(T)], using basis sets of improving quality, introducing up to 510, 790, 1070, 1350, 1630, and 1910 basis functions in the computations, respectively. A focal point analysis of the convergence of the calculated ionization potentials has been performed in order to extrapolate the CCSD(T) results to an asymptotically (cc-pV∞Z) complete basis set. The present results confirm the adequacy of the outer-valence Green’s function scheme for strongly correlated systems. Adiabatic ionization energies have been further determined by incorporating Beck-three-parameter Lee–Yang–Parr functional corrections for zero-point vibrational energies and for geometrical relaxations. Extension of the analysis to the CCSD(T)/cc-pV∞Z level shows that the energy minimum form of the benzene radical cation is an obtuse structure related to the 2B2g state. Isotopic shifts of the adiabatic ionization potentials, due to deuterium substitution of hydrogens, have also been discussed.

The relative energies of stationary points on the potential energy surface of trans-stilbene have been accurately determined using Hartree–Fock, second and third-order Møller–Plesset (MP2, MP3), as well as Coupled Clusters theories with single and double excitations (CCSD), together with a perturbative estimate of connected triple excitations [CCSD(T)], in conjunction with basis sets of increasing size, containing up to 1130 basis functions. A focal point analysis has been carried out in order to determine how the energy differences and rotational barriers approach convergence, enabling extrapolation of the CCSD(T) results to a near-complete basis set. The investigated saddle points pertain to independent rotations of the phenyl rings about the single C–C bond, and to pedalling motions described by a twofold rotation of the central ethylene bond about the longitudinal axis of the molecule. The benchmark calculations presented in this study lead to the conclusion that, in the nonrelativistic limit and within the frozen core approximation, trans-stilbene in vacuum is a strictly planar molecule in its absolute energy minimum form, in sharp contrast with many previous theoretical studies. This point has been ultimately confirmed by an MP2 geometry optimization using the aug-cc-pVDZ basis set. At last, the energy of cis-stilbene relative to the trans-isomer is accurately evaluated.

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The structures, rotational moments, vibrational normal modes, and infrared spectra of small to medium-size ionized carbon clusters C n + (n = 4−19) are investigated using density functional (DFT) and coupled cluster (CC) theories. Comparison is made with the neutral systems from which they derive. In contrast to previous restricted open-shell Hartree−Fock (ROHF) results by von Helden and co-workers, electron correlation is shown to strongly limit distortions of the structure upon an adiabatic ionization process. Nonetheless, for such a process, the C4 n +2 and to a lesser extent the C4 n +1 cyclic systems are found to evolve from an essentially regular (i.e., cumulenic) pattern to a more alternating (i.e., polyynic) structure in their ionized forms, whereas the opposite trend is observed for the C4 n and C4 n +3 rings. Similarly, linear carbon clusters, which can be regarded as mostly cumulenic in their neutral form, tend to become more polyynic after ionization. Rotational moments, IR spectra, and adiabatic ionization potentials as well should provide specific markers of these contrasted characters and behaviors. It has been found that the linear cations show a much more pronounced IR intensity than the cyclic ones. Many of the studied species show a strong absorption in certain regions of the spectrum (e.g., around 2036 cm-1).

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