Polyacetylenes and Cumulenes, Potential Elements for Molecular Machines and Precursors of Carbon Clusters: A Theoretical Study

Abstract

Dedicated to Professor Jack D. Dunitz, a pioneer in science, on the occasion of his 80th birthday A recently described very simple procedure for estimating Hartree ± Fock (H. F.) energy and accurate nonrelativistic energy has been used with simple hydrocarbons possessing CC or CC bonds and for the acetylene dimer. Experimental characteristics (heats of formation, ionization potentials, electron affinities), structural features, and reactivity have been discussed in terms of quantum-chemistry characteristics at the H. F. level and also at a level including a part of electron correlation. Deviations from linearity with derivatives of long polyacetylenes and cumulenes are ascribed to the lowest-energy deformation vibrations, which decrease rapidly when passing from short to long acetylenic and cumulenic chains. The role of derivatives and heteroanalogues of hydrocarbons under study in atmospheric and interstellar chemistry is briefly mentioned. Derivatives with enhanced stability represent potentially promising construction materials for molecular devices and also resources for the preparation of defined clusters of C-atoms.Introduction. ± The purpose of this study is fourfold. First, it is connected with our recent work dealing with a simple extrapolation scheme for estimating H. F. and accurate nonrelativistic energies of atoms and molecules [1]. Second, infinite chains of various sorts of organic polymers attracted our attention as conductors and semiconductors already long time ago [2]. The systems studied in this paper represent, on the one hand, the first members of a series limiting at infinitely conjugated sp C-atom chains, and, on the other hand, especially the analogues thereof, represent rods for the potential connection of building elements of molecular machines [3]. Third, the species under study may serve as starting materials for preparing various sorts of C-atom clusters. Finally, some of the above-named hydrocarbons and derivatives thereof play a significant role in atmospheric and interstellar chemistry [4].To keep the size of this literature survey reasonable, we quote papers assuming, in our opinion, a key position in the area. These papers and references cited therein offer a rather complete picture of the situation. It is remarkable how much attention the realm of acetylenes, cumulenes, and derivatives thereof has attracted in recent years [5].It was Kloster-Jensen in the Heilbronner group who synthesized [6] in the seventies dihalodiacetylenes and halomethylacetylenes, in connection with photoelectron and vibrational spectroscopy, and, moreover, di-through pentaacetylene and recorded their electronic and photoelectron spectra. They also included a warning concerning explosiveness of triacetylene and quoted in this connection a work [7a] in which remarkable violence of detonations of triacetylene is mentioned. (Risk of violent detonation is mentioned in several papers by Gladysz, vide infra.)

Electronic Structure and Chain‐Length Effects in Diplatinum Polyynediyl Complexes trans,trans‐[(X)(R3P)2Pt(CC)nPt(PR3)2(X)]: A Computational Investigation

“…Two recent computational investigations suggest that spcarbon chains should be intrinsically nonlinear due to lowfrequency vibrational modes of the chain. [20,28] Indeed, our DFT calculations confirm that the diplatinum compounds feature a number of low-frequency vibrations of the carbon chain, both in-and out-of-plane. The frequency of these vibrations monotonically decreases as the chain lengthens (e.g., 146 cm À1 in PtC 4 Pt to 4 cm À1 in PtC 26 Pt), which indicates progressively higher flexibility with longer chains.…”

“…This is significant for the first members of the series (31 % in PtC 4 Pt, for which 33 % of the atoms are platinum), but drops sharply for later members of the series (6 % in PtC 20 Pt, for which 9 % of the atoms are platinum). Consequently, the HOMOs of the longer-chain compounds are almost exclusively carbon- chain-based.…”

Chemistry of Energetically Activated Cumulenes—From Allene (H2CCCH2) to Hexapentaene (H2CCCCCCH2)

“…Due to their unique electronic structure, [31] cumulenes and their substituted counterparts have important technical potential as components of molecular machines. [32,33] Recent studies of these nanosystems also suggest that these molecules can be important building blocks in nano-electronics [34] and nanomechanics-a newly emerging field in structural chemistry.…”

Second-order perturbation theory with complete and restricted active space reference functions applied to oligomeric unsaturated hydrocarbons

“…8 When such p-conjugated systems are incorporated in solid-state devices, one important aspect from the electronic structure point of view is the nature of the coupling of the organic molecule to the electrode and the manner in which this affects charge injection into the wire. 9 In this context, properties like the ionization potential and band gap of the wire are important, and the ability to compare accurately calculated values and experimental values offers the potential to lead to a better understanding of the physical and chemical properties of conjugated polymers, [10][11][12][13] particularly in as much as ionization potentials and band gaps in the organic wire can be modified by chemical substitution in much the same fashion as Fermi levels in solids may be modified by doping. 14,15 The accurate modeling of such polymers requires a theoretical method capable of describing electron correlation effects over very large conjugation lengths.…”