Linear C_n Clusters:  Are They Acetylenic or Cumulenic?

Abstract

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.

Carbon-atom wires: 1-D systems with tunable properties

“…[9][10][11]77 CAWs of increasing length show a decreasing BLA as a result of increasing π-conjugation. 54,73,[78][79][80][81] However, the BLA = 0 limit (cumulene) is not reached due to Peierls distortion. In this framework, the alternate ( polyyne-like) structure found in most of the synthesized sp-carbon wires 56 has been usually interpreted as the result of Peierls distortion, as also reported for other classes of π-conjugated materials.…”

“…81 As a result of the balance between π-electron delocalization and Peierls distortion, C n wires possess a cumulene-like structure (determined by the endgroup effects) for n < 52. Longer C n wires exhibit an alternated, polyyne-like structure due to the onset of Peierls distortion.…”

Polarons and Charge Carrier Solvation on Conjugated Carbon Chains: A Comparative Ab Initio Study

“…A very convenient and well-known [5][6][7]22,23 way to characterize the geometry of dimerized polymeric structures is via bond-length alternation (BLA) patterns…”

“…would be uniform, that is, independent of the spatial bond position n, we denote the magnitude of this equilibrium ground-state pattern as δ 0 (this is not to be confused with a specific site value of δ n=0 ). and in published 16,23 tabulated numerical data.…”

“…17,[19][20][21] Quite different level-of-theory-dependent magnitudes of the effective electron-phonon coupling have also been found for the dimerized ground states of polymeric systems. 22,23 These observations thus bear on an important general issue of a choice of appropriate ab initio methods 18,24 to faithfully describe properties of 1D semiconductors. In our computations for this paper, we compare results derived with the GAUSSIAN 03 implemented 25 HF and two popular hybrid-functional DFT meth-ods (BHandHLYP and B3LYP) in vacuum and solvent environments, the latter represented within the framework of the polarizable continuum model.…”