Effect of packing and tilt on the rotational barriers of an amino, nitro-substituted phenylene ethynylene trimer

Rotational potentials are computed for heptamers and nonamers of an amino, nitro-substituted phenylene ethynylene trimer molecule. A herringbone and a parallel-slipped packing arrangement are considered. The effect of tilting the molecules with respect to the surface as well as the effect of the gold support are also taken into account. The herringbone structure with the molecules perpendicular to the surface has a low rotational barrier (2 kcal/mol). Tilting the molecules by 30 degrees increases the rotational barriers significantly (16 kcal/mol). The parallel-slipped structure has rotational barriers of 7 kcal/mol. Including the effect of the gold support increases the rotational barriers for tilted molecules but has little effect on perpendicular molecules.     

Theoretical study of the dipole moment of oxygen monofluoride (OF)

Theoretical X²II potentials and dipole moment functions for OF are presented at the CASSCF, externally contracted Cl, and MRCI(SD) levels using DZP and extended gaussian basis sets. The best theoretical results (with experiment in parentheses) are μ_ν =0 = ?0.0089 (?0.0043) and μ_ν=1 = ?0.0318 (?0.0267) D, where the minus sign implies O⁺F^?.     

Chemisorption and the properties of metal clusters

In an earlier [C.W. Bauschlicher, D.H. Liskow, C.F. Bender and H.F. Schaefer, J. Chem. Phys. 62 (1975) 4815] we reported model studies of the chemisorption of atomic hydrogen on the (0001) surface of metallic beryllium. Four distinct sites for chemisorption were considered and the surface was modeled by clusters as large as Be₁₀. In the present paper, this work has been extended in two directions. First, two distint Be₁₃ clusters have been studied in an analogous manner. Second, the predicted chemisorption characteristics have been correlated with several properties of the isolated metal clusters. Metal cluster properties investigated include the ionization potential, cohesive energy, and singlet—triplet separation. The various sites for chemisorption are compared and electronic structure considerations discussed.     

Theoretical evidence for multiple 3d bondig in the V2 and Cr2 molecules

Calculated CAS SCF potential curves are reported for the ³Σ_g^? state of V₂ and the ¹Σ_g⁺ state of Cr₂. At the CAS SCF level the ³Σ_g^? state of V₂ is calculated to be bound (R_c = 1.77 Å ω_c = 593.6 cm^?1, D_e 0.33 eV) and to involve a triple 3d bond; while the Cr₂ potential curve is not bound but shows a shoulder near the experimental R_e and the wave function shows multiple 3d bonding in this region.     

Differentiating between hydrogen and fluorine on a diamond surface

We have computed the interaction energies for pyridine (C₅H₅N) and (CH₃)₃PO with H and F atoms on the surface of diamond(111) as a function of the type of neighboring surface atom. The pyridine-H and -F interaction energies differ by about 5 kcal/mol, which is only about one-third of that found for a one-dimensional model for the surface. The difference in the interaction energies for (CH₃)₃PO is larger. However, the (CH₃)₃PO-neighbor interaction energy is larger than for pyridine, so that the (CH₃)₃PO-H interaction becomes repulsive for six neighboring F atoms. Substituting CN for F dramatically increases the repulsion between the surface atoms and molecules. The repulsion is sufficiently large that H/CN does not appear to be better than H/F as a possible way to store data on a surface. While pyridine shows some potential as a possible probe to differentiate between H and F on a diamond surface, it is not ideal. Received: 7 April 1997 / Accepted: 20 May 1997     

Infrared spectra of HC[triple bond]C-MH and M-eta2-(C2H2) from reactions of laser-ablated group- 4 transition-metal atoms with acetylene

Reactions of laser-ablated group 4 transition-metal atoms with acetylene have been carried out. The ethynyl metal hydrides (HC[triple bond]C-MH) and corresponding pi complexes (M-eta(2)-(C2H2)) are identified in the matrix infrared spectra. The observed M-H and C-M stretching absorptions show that oxidative C-H insertion readily occurs during codeposition and photolysis afterward. The absorptions from the pi complex, on the other hand, are relatively weak in the original deposition spectrum but increase dramatically in the process of annealing. The vinylidene complex, another plausible product, is not identified in this study. The observed spectra and DFT calculations both show that the back-donations from the group 4 metals to the antibonding pi* orbital of C2H2 are extensive such that the group 4 metals form unusually strong pi complexes. Thus, it is the formation of two Ti-C bonds in the group 4 systems than leads to the stronger bonding than that in the group 8 systems. While bonds form, the Ti atom is weakly bound to C2H2, and we still refer to it as a pi complex. Evidence of relativistic effects is also observed in frequency trends for the Ti, Zr, and Hf products.     

The structure and energetics of (GaAs)n, (GaAs)n(-), and (GaAs)n+ (n=2-15)

Electronic and geometrical structures of neutral, negatively, and positively charged (GaAs)n clusters are computed using density functional theory with generalized gradient approximation. All-electron computations are performed on (GaAs)2-(GaAs)9 while effective core potentials (ECPs) are used for (GaAs)9-(GaAs)15. Calibration calculations on GaAs and (GaAs)9 species support the use of the ECP for the larger clusters. The ground-state geometries of (GaAs)n(-) and/or (GaAs)n+ are different from the corresponding neutral ground-state geometry, except for n=7, 9, 12, 14, and 15, where the neutral and ions have similar structures. Beginning with n=6, all atoms are three coordinate, except for (GaAs)10+ and (GaAs)13+. For the larger species, there is a competition between fullerenes built from hexagons and rhombi and geometrical configurations where Ga-Ga and As-As bonds are formed, which results in the formation of pentagons. As expected, the static polarizability varies in the order of anion>neutral>cation, but the values are rather similar for all three charge states. The thermodynamic stability for the loss of GaAs is reported.