Phase transitions and hindered rotation in dimethylacetylene at high pressures probed by Raman spectroscopy

Pressure-Induced Hydrogen Transfer in 2-Butyne via a Double CH···π Aromatic Transition State

Hydrogen transfer (H-transfer) is an important elementary reaction in chemistry and bioscience. It is often facilitated by the hydrogen bonds between the H-donor and acceptor. Here, at room temperature and high pressure, we found that solid 2-butyne experienced a concerted two-in-two-out intermolecular CH···π H-transfer, which initiated the subsequent polymerization. Such double H-transfer goes through an aromatic Hückel six-membered ring intermediate state via intermolecular CH···π interactions enhanced by external pressure. Our work shows that H-transfer can occur via the CH···π route in appropriate conformations under high pressure, which gives important insights into the H-transfer in solid-state hydrocarbons.

Investigations on non-classical silylium ions leading to a cyclobutenyl cation

The formation of simple non-classical silylium ions from [Me₃Si]⁺ sources and alkenes or alkynes was investigated, but mainly oligomerization was observed. Yet, the reaction with MeC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CMe led to a room temperature stable cyclobutenyl cation. DFT calculations suggest that a non-classical silylium ion intermediate was formed on the way to this product.

Instead of yielding the desired non-classical silylium ions, the reactions of different alkenes/alkynes with several [Me₃Si]⁺ sources mostly led to oligomerization, or – in the presence of Me₃SiH – hydrosilylation of the alkenes/alkynes. Yet, from the reaction of 2-butyne with ion-like Me₃Si–F–Al(OR^F)₃ (R^F = C(CF₃)₃) the salt of the silylated tetramethyl cyclobutenyl cation [Me₄C₄–SiMe₃]⁺[al–f–al]^–1 ([al–f–al]^– = [(R^FO)₃Al–F–Al(OR^F)₃]^–) was obtained in good yield (NMR, scXRD, Raman, and IR). All the experimental and calculated evidence suggest a mechanism in which 1 was formed via a non-classical silylium ion as an intermediate. The removal of the [Me₃Si]⁺ moiety from the cation in 1 was investigated as a means to provide free tetramethyl cyclobutadiene (CBD). However, the addition of [NMe₄]F, in order to release Me₃SiF and form CBD, led to the unexpected deprotonation of the cation. The addition of 4-dimethylaminopyridine to remove the [Me₃Si]⁺ cation as a Lewis acid/base adduct, led to an adduct with the four-membered ring in the direct neighborhood of the Me₃Si group. By the addition of Et₂O to a solution of 1, the [F–Al(OR^F)₃]^– anion (and Et₂O–Al(OR^F)₃) was generated from the [al–f–al]^– counterion. Subsequently, the [F–Al(OR^F)₃]^– anion abstracted the [Me₃Si]⁺ moiety from [Me₄C₄–SiMe₃]⁺, probably releasing CBD. However, due to the immediate reaction of CBD with [Me₄C₄–SiMe₃]⁺ and subsequent oligomerization, it was not possible to use CBD in follow-up chemistry.

Pressure-induced amorphization and reactivity of solid dimethyl acetylene probed by in situ FTIR and Raman spectroscopy

Conjugated polymers are prominent semiconductors that have unique electric conductivity and photoluminescence. Synthesis of conjugated polymers under high pressure is extremely appealing because it does not require a catalyst or solvent used in conventional chemical methods. Transformation of acetylene and many of its derivatives to conjugated polymers using high pressure has been successfully achieved, but not with dimethyl acetylene (DMA). In this work, we present a high-pressure study on solid DMA using a diamond anvil cell up to 24.4 GPa at room temperature characterized by in situ Fourier transform infrared and Raman spectroscopy. Our results show that solid DMA exists in a phase II crystal structure and is stable up to 12 GPa. Above this pressure, amorphization was initiated and the process was completed at 24.4 GPa. The expected polymeric transformation was not evident upon compression, but only observed upon decompression from a threshold compression pressure (e.g. 14.4 GPa). In situ florescence measurements suggest excimer formation via crystal defects, which induces the chemical reactions. The vibrational spectral analysis suggests the products contain the amorphous poly(DMA) and possibly additional amorphous hydrogenated carbon material.

Pressure-selected reactivity between 2-butyne and water induced by two-photon excitation

High-pressure photochemistry between 2-butyne (H3CC≡CCH3) and trace amount of H2O was investigated at room temperature using multiline UV radiation at λ ≈ 350 nm and monitored by FTIR spectroscopy. Instead of the expected polymerization of 2-butyne, the IR spectral analysis suggests the formation of cis- and trans-2-butene, as well as 2-butanone, as the primary products. The possible reaction mechanisms and production pathways of these products were examined, where the dissociation of water molecule as the other reactant is believed as the essential step of the photochemical reaction. We further found that initial loading pressure of the mixture can not only substantially influence the reaction kinetics, but also regulate the accessibilities to some reaction channels, which was evidenced by quantitative analysis of the characteristic IR bands of 2-butene and 2-butanone. The relative abundance of two products is found to be highly dependent on pressure and radiation time. This study provides attractive physical routes in the absence of solvents, catalysts, and radical initiators, to synthesis the relevant products with a great selectivity and feasibility.

Pressure tuning of the Fermi resonance in liquid methanol: Implications for the analysis of high-pressure vibrational spectroscopy experiments

It has been argued that pressure tuning allows for unambiguous assignment of the nonperturbed bands involved in the Fermi coupling of molecular systems in the condensed phase. Here we study the pressure evolution of the Fermi resonance occurring in liquid methanol between the symmetric methyl-stretch fundamental and the methyl-bending overtones. Our analysis is based on Raman experiments in both stretching and bending fundamental regions, which are used to evaluate the effect of pressure on accidental degeneracies occurring in the vibrational spectra of liquid methanol. We emphasize that the difference in frequency of the Fermi doublet constitutes the governing quantity to determine the condition at which the exact degeneracy of the unperturbed modes occurs. Analysis based on the intensity ratio of the Fermi doublet must be disregarded. We confirm the necessity of measuring the full vibrational spectrum under pressure in order to obtain the Fermi coupling parameters unambiguously and to give a correct assignment of the bands involved in the resonance phenomenon. We also analyze the possible occurrence of several simultaneous resonance effects using a multilevel perturbation model. This model provides an appropriate description of the frequencies observed in the experiments over the whole pressure range if we consider that the main resonance occurs between nu3 and 2nu10, in contrast to previous assignments. Our global analysis leads to some general rules concerning measurement and interpretation of high-pressure vibrational spectroscopy experiments.