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We report a full self-consistent ab initio calculation of the current-voltage curve and the conductance of thiolate capped polyynes in contact with gold electrodes. We find the conductance of polyynes an order of magnitude larger compared with other conjugated oligomers. The reason lies in the position of the Fermi level deep in the HOMO related resonance. With the conductance weakly dependent on the applied bias and almost independent of the length of the molecular chain, polyynes appear as nearly perfect molecular wires. The study of transport properties of single molecules have attracted a significant attention because of their potential use in molecular electronic devices.One of the major classes of molecules considered in conductivity studies, primarily for their molecular wire behavior, 1,2,3,4,5 is conjugated oligomers. They have shown a number of useful nonlinear properties such as conductance switching and negative differential resistance.6,7 However, inspite of a number of interesting experiments 4,8 a molecule with good molecular wire properties has not yet been spotted.A useful molecular wire should provide a high and stable conductance over a wide bias region and for various lengths of the molecules. A linear chain of carbon atoms with double bonds between neighboring atoms, usually referred to as cumulene, was proposed as an ideal molecular wire 1 and the calculations of the conductance of cumulene connected to gold electrodes were reported. 9,10Lang and Avouris 9 showed that the conductance of cumulene did not stay constant in the ballistic regime, but rather oscillated between the constant values characteristics of the odd and even number of atoms in the chain.In this paper we show an entirely different behavior of polyynes, another form of the carbon atom chain. Polyynes are simple and yet most intriguing of conjugated organic oligomers. Only recently, have they been assembled up to decayne.11 Formed as a linear chain of carbon atom pairs (CC) n , with alternating single and triple bonds, they are a unique, truly one-dimensional, molecular system. Two π-electron systems of the sphybridized structure provide polyyne with approximately cylindrical electronic delocalization along the conjugated backbone. The electronic transport is therefore independent of the rotation around the single bond, which is a limitation often present in other organic oligomers. 7We have obtained the electronic structure and transport properties of a series of polyynes up to octayne, connected to gold electrodes. The stability of polyyne with respect to single-and triple-bond alternations was achieved by fixing the molecule at the ends with thiol bonds. In addition, the thiol capped polyynes make a strong chemisorption bond onto the metallic electrodes. We found that they had more than an order of magnitude higher conductance when compared with other conjugated oligomers. In contrast to the cumulenes they were not prone to oscillations in conductance with the length of the molecule. We also found that their conductance wa...

A b s t r a c t Although beeswax adulteration represents one of the main beeswax quality issues, there are still no internationally standardised analytical methods for routine quality control. The objective of this study was to establish an analytical procedure suitable for routine detection of beeswax adulteration using FTIR-ATR spectroscopy. For the purpose of this study, reference IR spectra of virgin beeswax, paraffin, and their mixtures containing different proportions of paraffin (5 -95%), were obtained. Mixtures were used for the establishment of calibration curves. To determine the prediction strength of IR spectral data for the share of paraffin in mixtures, the Partial Least Squares Regression method was used. The same procedure was conducted on beeswax-beef tallow mixtures. The model was validated using comb foundation samples of an unknown chemical background which had been collected from the international market (n = 56). Selected physico-chemical parameters were determined for comparison purposes. Results revealed a strong predictive power (R 2 = 0.999) of IR spectra for the paraffin and beef tallow share in beeswax. The results also revealed that the majority of the analysed samples (89%) were adulterated with paraffin; only 6 out of 56 (11%) samples were identified as virgin beeswax, 28% of the samples exhibited a higher level of paraffin adulteration (>46% of paraffin), while the majority of the analysed samples (50%) were found to be adulterated with 5 -20% of paraffin. These results indicate an urgent need for routine beeswax authenticity control.In this study, we demonstrated that the analytical approach defining the standard curves for particular adulteration levels in beeswax, based on chemometric modelling of specific IR spectral region indicative for adulteration, enables reliable determination of the adulterant proportions in beeswax.

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