Jean-Pierre Francois

THE ANHARMONIC FORCE FIELD OF ETHYLENE, C2H4, BY MEANS OF ACCURATE AB INITIO CALCULATIONS

UNIV INSTELLING ANTWERP,INST MAT SCI,DEPT CHEM,B-2610 WILRIJK,BELGIUM. NASA,AMES RES CTR,MOFFETT FIELD,CA 94035. SAN DIEGO SUPERCOMP CTR,SAN DIEGO,CA 92186.MARTIN, JML, LIMBURGS UNIV CENTRUM,DEPT SBG,UNIV CAMPUS,B-3590 DIEPENBEEK,BELGIUM.

Structure and vibrational spectra of carbon clusters Cn (n = 2–10, 12, 14, 16, 18) using density functional theory including exact exchange contributions

Abstract Geometries and harmonic frequencies of linear C n and cyclic C 2 n ( n = 2–9) clusters have been studied using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional method and compared with ab initio coupled cluster calculations. For C 2 through C 10 , results are of nearly the same quality as those obtained at the CCSD(T)/[3s2pld] (coupled cluster with all single, double, and quasiperturbative triple substitutions) level, except for relative energies. C 4 nn clusters ( n = 2–4) are polyacetylenic rings with C 2 n h symmetry, and C 4 n +2 ( n = 1–4) clusters are cumulenic rings with C (2 n +1)h symmetry. They have intense infrared absorptions in the 1800–2000 cm −1 region, in addition to intense vibrations around 500 cm −1 . Tentative assignments of some bands in the 1900–1950 cm −1 region to cyclic C 14 and C 18 are proposed.

High level theoretical study of the structure and rotational barriers of trans-stilbene

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 Moller–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 with...

Benchmark theoretical study of the ionization threshold of benzene and oligoacenes

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 Moller–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 ion...

ON THE STRUCTURE AND VIBRATIONAL FREQUENCIES OF C20

Abstract The bowl, ring, and cage structures of C 20 have been studied using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) method. Fingerprint-quality infrared spectra have been computed for all species. The computed geometries differ drastically from those obtained using the local density approximation (LDA). Combining coupled cluster results at LDA and SCF geometries with the present B3LYP results, and including zero-point energies, we find the bowl structure to be the lowest in energy, two distinct cage structures with C i and C 2h symmetry both about 11 kcal/mol higher, and a C 10h ring about 47 kcal/mol higher. Statistical thermodynamical calculations reveal that the bowl will dominate up to about 1200 K, and the ring above 1900 K. Despite being much lower in energy than the ring, the cage structure is not present in appreciable proportion due to the extra vibrational entropy the ring derives from many extremely low-lying vibrations.

Structure and relative energetics of C2n+1 (n = 2−7) carbon clusters using coupled cluster and hybrid density functional methods

Abstract The geometry and harmonic frequencies of linear and cyclic structures of C 5 , C 7 , C 9 , C 11 , C 13 , and C 15 have been calculated using a hybrid density functional method (B3LYP), and the linear-cyclic isomerism considered using coupled cluster methods. It was established unambiguously that, from C 11 onwards, these molecules prefer cyclic structures. Structures with a blunt apical angle are considerably more stable than the elongated structures found at the MP2 level, which gives a qualitatively incorrect description of the potential surface. The B3PW91 (Becke 3-parameter-Perdew-Wang) functional appears to reproduce the CCSD(T) isomerization energies better than its B3LYP (Becke 3-parameter-Lee-Yang-Parr) counterpart. No indication appears to exist for the presence of either cyclic C 7 and C 9 , or linear C 11 and C 13 , in matrix-trapped graphite vapor.

The structure, stability, and infrared spectrum of B2N, B2N+, B2N−, BO, B2O and B2N2.

Abstract The structure, infrared spectrum, and heat of formation of B 2 N, B 2 N − , BO, and B 2 O have been studied ab initio. B 2 N is very stable; B 2 O even more so. B 2 N, B 2 N − , B 2 O, and probably B 2 N + have symmetric linear ground-state structures; for B 2 O, an asymmetric linear structure lies about 12 kcal/mol above the ground state. B 2 N + , B 2 N − and B 2 O have intense asymmetric stretching frequencies, predicted near 870, 1590 and 1400 cm −1 , respectively. Our predicted harmonic frequencies and isotopic shifts for B 2 O confirm the recent experimental identification by Andrews and Burkholder. Absorptions at 1889.5 and 1998.5 cm −1 in noble-gas trapped boron nitride vapor belong the BNB and BNBN ( 3 Π), respectively; a tentative assignment of 882.5 cm −1 to BNB + is proposed. Total atomization energies Σ D e (Σ D 0 ) are computed (accuracy ±2 kcal/mol) as: BO 193.1 (190.4), B 2 O 292.5 (288.7), B 2 N 225.0 (250.3) kcal/mol. The ionization potential and electron affinity of B 2 N are predicted to be 8.62±0.1 and 3.34±0.1 eV. The MP4-level additivity approximations involved in G1 theory results in errors on the order of 1 kcal/mol in the Σ D e values.

Potential energy surface of B4 and total atomization energies of B2, B3, and B4☆

Abstract The potential energy surface of B 4 has been studied ab initio using extended basis sets and coupled-cluster methods. The ground state ( 1 A 1g ) is a singlet square that undergoes pseudo-Jahn-Teller distortion to a rhombic structure ( 1 A g ), but the energy gain is so small that the effective structure will be square at all practical temperatures. Total atomization energies Σ D O (kcal/mol) for B 2 (62.2±2), B 3 (189.1±2–192.6±2), and B 4 (312.2±2) are computed by a modification of GAUSSIAN-1 (G1) theory. The validity of scaling and bond-additivity schemes for computing binding energies of higher clusters is discussed.

Valence One-Electron and Shake-Up Ionization Bands of Carbon Clusters. III. The Cn (n = 5,7,9,11) Rings

The 1h (one-hole) and 2h-1p (two-hole; one-particle) shake-up bands in the valence ionization spectrum of odd-membered carbon rings (C5, C7, C9, C11) are investigated by means of the third-order algebraic diagrammatic construction [ADC(3)] scheme for the one-particle Greens function. Despite a severe dispersion of the σ- and π- ionization intensity over intricately dense sets of satellites, the present study undoubtedly confirms that structural fingerprints in ionization spectra could be usefully exploited to discriminate the cyclic C5, C7, C9, and C11 species from their linear counterparts in plasma conditions. Such spectra could also be used to indirectly trace very fine details of the molecular structure, such as bond length alternations, out-of-plane distortions, or the strength of cyclic strains. Both structurally and electronically, the cyclic isomers of the C5 and C9 clusters must be described as even-twisted cumulenic tori, whereas the C7 and C11 cyclic species are simply planar polyynic rings. I...

Optical properties of trans‐stilbene using semiempirical and time‐dependent density functional theory: A comparative study

The present study focuses on the low-lying valence electronically excited states of trans-stilbene. A comparison is made between semiempirical CIS-ZINDO/S and ab initio time-dependent density functional theory (TD-DFT) results obtained in conjunction with a cc-pVDZ basis set and using three different functionals, namely SVWN, B3LYP, and B3P86. Geometry optimization with the B3LYP functional and the cc-pVDZ basis set provides as most stable form an almost planar structure. With the exception of the TD-SVWN approach, and despite the limitation of the cc-pVDZ basis set, these methods provide overall a rather satisfactory insight into the main valence transitions identified experimentally from one-photon and two-photon absorption measurements. In particular, our CIS-ZINDO/S and TD-DFT results for excitation energies and symmetry assignments of lines are in rather close agreement with a recent CASPT2 study. On the other hand, only qualitative agreement can be claimed for the oscillator strengths and line composition, in particular at the highest excitation energies, where strong configurational mixing between excited states occurs. As a by-product of the present study, our TD-DFT results for the energy excitation threshold, λmax, indirectly confirms the planar nature of trans-stilbene in the vacuum, even at room temperature. c