Ground-state and excited-state structures of tungsten-benzylidyne complexes.

The molecular structure of the tungsten-benzylidyne complex trans-W(≡CPh)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) in the singlet (d(xy))(2) ground state and luminescent triplet (d(xy))(1)(π*(WCPh))(1) excited state (1*) has been studied using X-ray transient absorption spectroscopy, X-ray crystallography, and density functional theory (DFT) calculations. Molecular-orbital considerations suggest that the W-C and W-P bond lengths should increase in the excited state because of the reduction of the formal W-C bond order and decrease in W→P π-backbonding, respectively, between 1 and 1*. This latter conclusion is supported by comparisons among the W-P bond lengths obtained from the X-ray crystal structures of 1, (d(xy))(1)-configured 1(+), and (d(xy))(2) [W(CPh)(dppe)(2)(NCMe)](+) (2(+)). X-ray transient absorption spectroscopic measurements of the excited-state structure of 1* reveal that the W-C bond length is the same (within experimental error) as that determined by X-ray crystallography for the ground state 1, while the average W-P/W-Cl distance increases by 0.04 Å in the excited state. The small excited-state elongation of the W-C bond relative to the M-E distortions found for M(≡E)L(n) (E = O, N) compounds with analogous (d(xy))(1)(π*(ME))(1) excited states is due to the π conjugation within the WCPh unit, which lessens the local W-C π-antibonding character of the π*(WCPh) lowest unoccupied molecular orbital (LUMO). These conclusions are supported by DFT calculations on 1 and 1*. The similar core bond distances of 1, 1(+), and 1* indicates that the inner-sphere reorganization energy associated with ground- and excited-state electron-transfer reactions is small.

Energetics of linear carbon chains in one-dimensional restricted environment.

The energetics of even and odd linear C(n) carbon chain clusters are investigated by hybrid density functional theory (DFT) calculations. These molecular species are especially interesting due to their recent observation inside carbon nanotubes by polarized resonant Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) by different research groups. Neutral, anionic and dianionic carbon chains were studied with sizes up to n=75, although most presented calculations are limited to nC(2n) is of the order of 40-20 kcal mol(-1), which gradually decreases with increasing chain size, n. These barriers can be overcome during the high temperature synthesis or annealing conditions, but not when cooled down for the HRTEM and Raman experiments. Therefore, in addition to the already observed long chains also shorter chains should be observable under appropriate conditions inside carbon nanotubes.

Linear C(n) clusters: are they acetylenic or cumulenic?

Uncapped linear Cn clusters have been studied with hybrid density functional theory focusing on the geometry, HOMO-LUMO gap, and the longitudinal optical (LO) vibrational mode. The latter two correlate well with the bond length alternation (BLA) of the optimized geometry. Due to end effects, the BLA is not constant along the chains. The degree of BLA changes continuously with increasing n: starting with essentially nonalternating structures (cumulenic), then turning into strongly alternating (acetylenic) structures. This transition has not yet been described or characterized and occurs at relatively large values of n. The implications for the widely observed characteristic LO vibrational bands of linear carbon clusters are discussed.

Application of a novel linear/exponential hybrid force field scaling scheme to the longitudinal Raman active mode of polyyne.

The properties of an infinite carbon chain (polyyne), an allotropic form of elemental carbon, are of importance in materials science as well as astronomy. The Raman active longitudinal optical (LO) frequencies are calculated with first-principles methods for oligoynes and polyyne and compared with experiments. Since traditional force constant scaling schemes fail in this case, we introduced a linear/exponential scaling scheme based on the exponential behavior of the carbon-carbon bond stretching force constant couplings in quasi-one-dimensional conjugated chains. The LO Raman active frequency is predicted at 1870-1877 cm-1. Our results provides further evidence for the assignment of the characteristic Raman peaks near 1850 cm-1 of the recently discovered long linear carbon chains encapsulated inside multiwalled or double-walled carbon nanotubes.

Bond length alternation and energy band gap of polyyne.

The bond length alternation (BLA) and energy band gap of polyyne are investigated by various first-principles theories, including Hartree-Fock, MP2, hybrid, and nonhybrid density functional theories. Both solid-state calculations utilizing periodic boundary conditions on polymers and molecular quantum mechanical calculations on extra-long oligomers were performed with consistent results. By validation on similar linear conjugated polymers, polyacetylene and polydiacetylene, the combination of hybrid-DFT schemes, B3LYP//BHandHLYP or B3LYP//KMLYP, is shown to give the best predictions for both geometry and band gap of polyyne based on available experimental data. We conclude that the best estimate of the BLA of polyyne is about 0.13 A and that of the band gap is about 2.2 eV.