Phenylacetylene: A Hydrogen Bonding Chameleon

Microwave spectroscopic and computational analyses of the phenylacetylene⋯methanol complex: insights into intermolecular interactions

The microwave spectra of five isotopologues of phenylacetylene⋯methanol complex, C6H5CCH⋯CH3OH, C6H5CCH⋯CH3OD, C6H5CCH⋯CD3OD, C6H5CCD⋯CH3OH and C6H5CCH⋯13CH3OH, have been observed through Fourier transform microwave spectroscopy. Rotational spectra unambiguously unveil a specific structural arrangement characterised by dual interactions between the phenylacetylene and methanol. CH3OH serves as a hydrogen bond donor to the acetylenic π-cloud while concurrently accepting a hydrogen bond from the ortho C-H group of the PhAc moiety. The fitted rotational constants align closely with the structural configuration computed at the B3LYP-D3/aug-cc-pVDZ level of theory. The transitions of all isotopologues exhibit doublets owing to the methyl group's internal rotation within the methanol molecule. Comprehensive computational analyses, including natural bond orbital (NBO) analysis, atoms in molecules (AIM) theory, and non-covalent interactions (NCI) index plots, reveal the coexistence of both O-H⋯π and C-H⋯O hydrogen bonds within the complex. Symmetry adapted perturbation theory with density functional theory (SAPT-DFT) calculations performed on the experimentally determined geometry provide an insight into the prominent role of electrostatic interactions in stabilising the overall structural arrangement.

Dipole moment enhanced π-π stacking in fluorophenylacetylenes is carried over from gas-phase dimers to crystal structures propagated through liquid like clusters

The aggregates of monofluorinated phenylacetylenes in the gas-phase, investigated using the IR-UV double resonance spectroscopic method in combination with extensive structural search and electronic structure calculations, reveal the formation of liquid-like clusters with a π-stacked dimeric core. The structural assignment based on the IR spectra in the acetylenic and aromatic C-H stretching regions suggests that, unlike the parent non-fluorinated phenylacetylene, the substitution of a F atom on the phenyl ring increases the dipole moment, leading to robustness in the formation of a ππ stacked dimer, which propagates incorporating C-Hπ_{Ar/Ac} and C-HF interactions involving both acetylenic and aromatic C-H groups. The structural evolution of fluorophenylacetylene aggregates in the gas phase shows marginal effects due to fluorine atom position on the phenyl ring, with substitution in the para-position tending towards phenylacetylene. The present study signifies that the ππ stacked dimers act as a nucleus for the growth of higher clusters to which other molecular units are added predominantly via the {Ar}_C-Hπ_{Ar} type of interaction and the dominant interactions present in the crystal structures gradually emerge with increasing cluster size. Based on these features, gas-phase clusters of fluorophenylacetylene are hypothesized as "liquid-like clusters" acting as intermediates in the generation of various polymorphic forms starting from a ππ stacked dimer as the core molecular unit.

Mechanistic Insights of Multiplexed Effects of Pressure, Physical States, and Laser on the Polymerization Kinetics of Phenylacetylene Probed by In Situ FTIR Spectroscopy

High-pressure tuned polymerization kinetics have been examined to elucidate the (photon-assisted) polymerization mechanism of phenylacetylene (PA, C₆H₅C≡CH₃) under different stimuli including high-pressure, physical state, and UV radiation effects by using in situ FTIR spectroscopy. The pressure-induced glass-forming and crystalline states of PA are found to be formed at different compression rates. The experimental results suggest that the polymerization is induced in the glass-forming state at a very low pressure in contrast to that in the crystal phase where much higher threshold pressure is required. The measured rate constants were found to strongly depend on the pressure, the physical state, and UV radiation. In particular, the rate constant reduced to different extents either by UV irradiation or upon phase transition. The derived activation volumes from the rate constants allow the direct comparison and elucidation of photoactivation and environmental effects on the polymerization. Finally, the diffusion-controlled 1D polymer growth processes were suggested for the glass-forming state or the crystal state at specific pressures. Overall, the mechanistic insights of this work provide guidance of optimizing the multiplexed reaction conditions for the production of the conducting poly(PA).

Phenylacetylene as a gas phase sliding balance for solvating alcohols

Phenylacetylene offers two similarly attractive π binding sites to OH containing solvent molecules, the phenyl ring and the acetylenic triple bond. By systematically varying the solvent molecule and by methylating aromatic or acetylenic CH groups, the docking preference can be controlled. It ranges from almost exclusive acetylene docking to predominant phenyl docking, depending on how electron density is deposited into the conjugated system and how large the London dispersion interaction is. FTIR spectroscopy of supersonic jet expansions is used to observe the competitive docking preferences in phenylacetylene and some of its methylated derivatives. A new data evaluation procedure that estimates band strength uncertainties based on a Monte Carlo approach is introduced. We test how well two density functionals (B3LYP-D3 and M06-2X) in combination with a def2-TZVP basis set are able to describe the docking switch. B3LYP-D3 is slightly biased towards acetylenic hydrogen bond docking and M06-2X is strongly biased towards phenyl hydrogen bond docking. More accurate theoretical predictions are invited and some previous experimental assignments are questioned.

Hydrogen bonding between ethynyl aromates and triethylamine: IR spectroscopic and computational study

Ethynylpyridines (EPs) and ethynylbenzene (EB) are multifunctional systems able to participate in hydrogen-bonded complexes as both donors and acceptors of the H-atom. Their structures and stabilities are mainly a function of the hydrogen-bonding properties of the partner in the complex and the surroundings in which the complexation occurs. In this paper, IR spectroscopy and quantum chemical calculations are employed to characterize hydrogen-bonded complexes of 2- and 3-EP and EB with triethylamine (TEA) in tetrachloroethene (C₂Cl₄) solution. The formation of CH⋯N hydrogen bonds is experimentally confirmed by the appearance of TEA concentration-dependent signals in the IR spectra of the EPs and EB. Along with the signals due to unassociated CH and CC oscillators (2-EP: 3308 cm^-1 and 2120 cm^-1; 3-EP: 3308 cm^-1 and 2116 cm^-1; EB: 3313 cm^-1 and 2113 cm^-1) weak, red-shifted signals arise at ~3215 ± 5 cm^-1 and ~2105 ± 5 cm^-1 which are assigned to the stretching vibrations of hydrogen-bonded CH⋯ and CC⋯ oscillators, respectively. This result is at variance with those of previous investigations of EB and TEA in the gas phase. In the 2-EP⋯TEA complex these bands remain at the same position with increasing TEA concentration. However, in the 3-EP⋯TEA and EB⋯TEA complexes the CH⋯ stretching band demonstrates a slightly reduced red-shift as the TEA concentration increases, whereas the CC⋯ stretching band absorbs at the same wavenumber in the investigated TEA concentration range. The results of B3LYP-D3 calculations indicate that complexes with more or less linear CH⋯N intermolecular hydrogen bonds are more stable than other, dispersion-driven complexes. Complexes with the C_s symmetrical TEA conformer are predicted to have larger binding energy than those formed with the C₃ and C₁ symmetrical conformers. The predicted IR spectral shifts are slightly different for complexes with the three different TEA conformers. Association constants of hydrogen-bonded complexes at 26 °C are estimated to be ~0.1 mol^-1 dm³.

2-Ethynylpyridine dimers: IR spectroscopic and computational study

2-ethynylpyridine (2-EP) presents a multifunctional system capable of participation in hydrogen-bonded complexes utilizing hydrogen bond donating (CH, Aryl-H) and hydrogen bond accepting functions (N-atom, CC and pyridine π-systems). In this work, IR spectroscopy and theoretical calculations are used to study possible 2-EP dimer structures as well as their distribution in an inert solvent such as tetrachloroethene. Experimentally, the CH stretching vibration of the 2-EP monomer absorbs close to 3300 cm^-1, whereas a broad band with maximum around 3215 cm^-1 emerges as the concentration rises, indicating the formation of hydrogen-bonded complexes involving the CH moiety. The CC stretching vibration of monomer 2-EP close to 2120 cm^-1 is, using derivative spectroscopy, resolved from the signals of the dimer complexes with maximum around 2112 cm^-1. Quantum chemical calculations using the B3LYP + D3 model with counterpoise correction predict that the two most stable dimers are of the π-stacked variety, closely followed by dimers with intermolecular CH⋯N hydrogen bonding; the predicted red shifts of the CH stretching wavenumbers due to hydrogen bonding are in the range 54-120 cm^-1. No species with obvious hydrogen bonding involving the CC or pyridine π-systems as acceptors are predicted. Dimerization constant at 25 °C is estimated to be K₂ = 0.13 ± 0.01 mol^-1 dm³.

Hydrogen bond induced enhancement of Fermi resonances in N–H⋯N hydrogen bonded complexes of anilines

Enhancement of Fermi resonance intensities due to the formation of N–H⋯N hydrogen bonding of anilines with alkyl amines is analyzed using a two-state deperturbation model.

Unraveling non-covalent interactions within flexible biomolecules: from electron density topology to gas phase spectroscopy

The NCI (Non-Covalent Interactions) method, a recently-developed theoretical strategy to visualize weak non-covalent interactions from the topological analysis of the electron density and of its reduced gradient, is applied in the present paper to document intra- and inter-molecular interactions in flexible molecules and systems of biological interest in combination with IR spectroscopy. We first describe the conditions of application of the NCI method to the specific case of intramolecular interactions. Then we apply it to a series of stable conformations of isolated molecules as an interpretative technique to decipher the different physical interactions at play in these systems. Examples are chosen among neutral molecular systems exhibiting a large diversity of interactions, for which an extensive spectroscopic characterization under gas-phase isolation conditions has been obtained using state-of-the-art conformer-specific experimental techniques. The interactions presently documented range from weak intra-molecular H-bonds in simple amino-alcohols, to more complex patterns, with interactions of various strengths in model peptides, as well as in chiral bimolecular systems, where invaluable hints for the understanding of chiral recognition are revealed. We also provide a detailed technical appendix, which discusses the choices of cut-offs as well as the applicability of the NCI analysis to specific constrained systems, where local effects require attention. Finally, the NCI technique provides IR spectroscopists with an elegant visualization of the interactions that potentially impact their vibrational probes, namely the OH and NH stretching motions. This contribution illustrates the power and the conditions of use of the NCI technique, with the aim of providing an easy tool for all chemists, experimentalists and theoreticians, for the visualization and characterization of the interactions shaping complex molecular systems.

The exploration of hydrogen bonding properties of 2,6- and 3,5-diethynylpyridine by IR spectroscopy

Hydrogen bonding properties of 2,6- and 3,5-diethynylpyridine were analyzed by exploring of their interactions with trimethylphosphate, as hydrogen bond acceptor, or phenol, as hydrogen bond donor, in tetrachloroethene C2Cl4. The employment of IR spectroscopy enabled unravelling of their interaction pattern as well as the determination of their association constants (Kc) and standard reaction enthalpies (ΔrH(⦵)). The association of diethynylpyridines with trimethylphosphate in stoichiometry 1:1 is established through CH⋯O hydrogen bond, accompanied by the secondary interaction between CC moiety and CH3 group of trimethylphosphate. In the complexes with phenol, along with the expected OH⋯N interaction, CC⋯HO interaction is revealed. In contrast to 2,6-diethynylpyridine where the spatial arrangement of hydrogen bond accepting groups enables the simultaneous involvement of phenol OH group in both OH⋯N and OH⋯CC hydrogen bond, in the complex between phenol and 3,5-diethynylpyridine this is not possible. It is postulated that cooperativity effects, arisen from the certain type of resonance-assisted hydrogen bonds, contribute the stability gain of the latter. Associations of diethynylpyridines with trimethylphosphate are characterized as weak (Kc≈0.8-0.9mol(-1)dm(3); -ΔrH(⦵)≈5-8kJmol(-1)), while their complexes with phenol as medium strong (Kc≈5mol(-1)dm(3); -ΔrH(⦵)≈15-35kJmol(-1)). Experimental findings on the studied complexes are supported with the calculations conducted at B3LYP/6-311++G(d,p) level of theory in the gas phase. Two conformers of diethynylpyridine⋯trimethylphosphate dimers are formed via CH⋯O interaction, whereas dimers between phenol and diethynylpyridines are established through OH⋯N interaction.

Spectroscopic and ab initio investigation of 2,6-difluorophenylacetylene–amine complexes: coexistence of C–H⋯N and lone-pair⋯π complexes and intermolecular coulombic decay

Hydrogen bond and lone-pair⋯π interactions can coexist.

The study of hydrogen bonding and π⋯π interactions in phenol⋯ethynylbenzene complex by IR spectroscopy

Weak hydrogen bonds between phenol and ethynylbenzene in tetrachloroethene were explored by using FTIR spectroscopy. Association constants (Kc) were determined by high dilution method at two temperatures, 20°C and 26°C, and they are, respectively, 0.54±0.09 mol(-1) dm3 and 0.36±0.08 mol(-1) dm3. The position of ethynylbenzene stretching band, when in hydrogen bonding complex with phenol (CC⋯), is proposed to be governed by the interplay of OH⋯π (CC moiety or phenyl ring of ethynylbenzene) and π⋯π (phenyl ring of phenol⋯CC moiety or phenyl ring of ethynylbenzene) interactions. This conclusion is supported by the findings on the complex between ethanol and ethynylbenzene; in the latter, CC⋯ stretching band is shifted to the higher wavenumbers, as expected when ethynylbenzene interacts with hydrogen bond donor. Geometries and energies of the presumed complexes, as well as their vibrational spectra, are predicted by using ab initio calculations. The spectroscopic and thermodynamic data obtained here offer the missing pieces in the present picture of migration of H-atom of phenol OH group between competing hydrogen bond accepting centers on ethynylbenzene.

Electrostatics determine vibrational frequency shifts in hydrogen bonded complexes

The shifts in the acetylenic C–H stretching vibration in the C–H⋯X hydrogen-bonded complexes correlate with the electrostatic component of the stabilization energy.