Bent polyynes: Ring strain studied by Raman and Infrared Spectroscopies

The structure of sp-carbon chains (polyynes) in solid state has been extensively studied by means of X-ray crystallography. These investigations typically show that a linear polyyne is rarely found and, in some cases, quite dramatic deviations from linearity can be observed. Recently [1] we have shown that polyynes in solution are bent such that inversion symmetry is lost (series of polyynes 1, see Figure 1) and the mutual exclusion principle between IR and Raman spectroscopy is not upheld. In order to asses whether different polyyne curvatures give intensity variations for those bands which violate the mutual exclusion principle, we have synthesized bent polyynes coupling the acetylenic skeleton (in our case 8 carbon atoms in the polyynic chain) with C10, C11, C12 alkyl chains in order to obtain a “ring” system (see Figure 1, systems 3a-c). Different alkyl chain lengths produce different head-tail distances (R) and therefore different curvatures of the polyyne chain. The joint use of Raman and IR spectroscopies in solution allows to reliably probe the bending of the sp chain controlled by the imposed chemical structure. In particular we have proven that both the band positions and the relative intensities in the IR and Raman spectra are curvature sensitive. Furthermore, also the Raman depolarization ratio ρ is curvature sensitive and this fact can be exploited for a purely Raman quantification of chain bending. Density Functional Theory (DFT) calculations have been also carried out on model systems. This allows to nicely interpret and reproduce the experimental observations

Bent polyynes: Ring geometry studied by Raman and IR spectroscopies

IR and Raman spectroscopy have been used to examine the vibrational characteristics of a series of three macrocyclic tetraynes in comparison to an acyclic analog. By changing the length of the alkyl tether of the macrocycles, varying degrees of bending of the tetrayne moiety can be achieved, and the joint use of IR and Raman spectroscopies provides an avenue to probe the impact of bending on the sp-chain. The spectroscopic data show a general trend toward increasing activation of Raman bands in the IR spectra, and vice versa, as bending of the polyyne chain is increased. DFT calculations provide a detailed rationalization of the experimental observations

Chain bending of polyynes investigated with Raman and IR spectroscopies

Polyynes are molecular systems characterized by π‐conjugated chains of carbon atoms in sp hybridization state. Unlike sp2 π‐conjugated systems, polyynes are formed by carbon chains with alternating single and triple bonds with no hydrogen bonded to the conjugated carbons [1]. Under this regard, polyynes constitute a truly one-dimensional system, being essentially atomic wires made of carbon. Among sp2 π-conjugated carbon systems, graphene is usually considered a flat planar system, but studies have proven that its peculiar out‐of‐plane dynamics is responsible for the negative in‐plane thermal expansion coefficient of graphite [2]. Similarly, while polyynes could be naively considered to be essentially linear systems, a recent spectroscopic study has shown the presence of an out‐of‐line bending effect as the polyyne chain length is increased [3]. In this contribution, IR and Raman spectroscopy are used to examine the vibrational characteristics of a series of three macrocyclic tetraynes [4] in comparison to an acyclic analog. By changing the length of the alkyl tether of the macrocycles, varying degrees of bending of the tetrayne moiety can be achieved. The joint use of IR and Raman spectroscopy provides a quantitative probe of the bending of the sp‐chain. The spectroscopic data show a general trend toward increasing activation of Raman bands in the IR spectra, and vice versa, as bending of the polyyne chain is increased. It will be also shown how Density Functional Theory calculations can offer a detailed rationalization of the experimental observations

Dynamic properties of solid ammonium cyanate

Ammonium cyanate is well-known to undergo a solid-state reaction to form urea. Knowledge of fundamental physicochemical properties of solid ammonium cyanate is a prerequisite for understanding this solid-state chemical transformation, and this paper presents a comprehensive study of the dynamic properties of the ammonium cation in this material. The techniques usedincoherent quasielastic neutron scattering (QENS) and solid-state 2H NMR spectroscopyprovide insights into dynamic properties across a complementary range of time scales. The QENS investigations (carried out on a sample with natural isotopic abundances) employed two different spectrometers, allowing different experimental resolutions to be probed. The 2H NMR experiments (carried out on the deuterated material ND4+OCN−) involved both 2H NMR line shape analysis and 2H NMR spin−lattice relaxation time measurements. The results of both the QENS and 2H NMR studies demonstrate that the ammonium cation exhibits reorientational dynamics across a wide temperature range, and several dynamic models (based on knowledge of the crystal structure of ammonium cyanate) were considered in this work. It is found that a tetrahedral jump model for the dynamics of the ammonium cation provides the best description for both the QENS and 2H NMR data, and there is excellent agreement between the values of activation parameters established from these two experimental approaches, with estimated activation energies of 22.6 ± 2.1 kJ mol−1 from QENS and 21.9 ± 1.0 kJ mol−1 from 2H NMR spin−lattice relaxation time measurements. The wider implications of the results from this work are discussed

Large Raman intensity and vibrational second order hyperpolarizability of adamantly capped polyynes

poster presentato al convegno GNSR 2007 (Catania

Toward carbyne: Synthesis and stability of really long polyynes

Molecules composed of sp-hybridized carbon chains (polyynes) are the simplest of the known conjugated organic oligomers. In comparison to their counterparts such as polyacetylene and polydiacetylene, however, the formation of polyynes has traditionally posed a difficult synthetic challenge. In particular, there is no reliable method to form end-capped polyethynylene, and monodisperse polyynes have therefore been assembled. As a result, structure–property relationships for shorter polyynes have been relatively well established in recent years, while extension of these trends toward longer polyynes has remained a difficult task. Using the Fritsch–Buttenberg–Wiechell (FBW) rearrangement, the formation of diynes through decaynes has become possible and has provided a unique chance to explore the physical characteristics of conjugated polyyne chains. This paper highlights recent advances in the synthesis of extended polyynes, as well as interesting aspects of their NMR, Raman, and UV/vis spectroscopic analyses. These synthetic achievements offer the opportunity to predict some of the properties of the carbon allotrope carbyne. In particular, a set of X-ray crystallo - graphic analyses of t-Bu end-capped polyynes (tBu[n]) shows a definitive experimental trend in reduced bond-length alternation (BLA)