The homodimers of singly fluorine-substituted phenylacetylenes were investigated using electronic and vibrational spectroscopic methods in combination with density functional theory calculations. The IR spectra in the acetylenic C-H stretching region show a marginal red shift for the dimers relative to the monomers. Further, the marginal red shifts indicate that the acetylenic group in all the dimers is minimally perturbed relative to the corresponding monomer. The observed spectra were assigned to a set of π-stacked structures within an energy range of 1.5 kJ mol, which differ in the relative orientation of the two monomers on the basis of M06-2X/aug-cc-pVTZ level calculation. The observed red shift in the acetylenic C-H stretching vibration of the dimers suggests that the antiparallel structures contribute predominantly based on a simple coupled dipole model. Energy decomposition analysis using symmetry-adapted perturbation theory indicates that dispersion plays a pivotal role in π-π stacking with appreciable contribution of electrostatics. The stabilization energies of fluorophenylacetylene dimers follow the same ordering as their dipole moments, which suggests that dipole moment enhances the ability to form π-stacked structures.

The red-shifts in the acetylenic C-H stretching vibration of C-H∙∙∙X (X = O, N) hydrogen-bonded complexes increase with an increase in the basicity of the Lewis base. Analysis of various components of stabilization energy suggests that the observed red-shifts are correlated with the electrostatic component of the stabilization energy, while the dispersion modulates the stabilization energy.

The title reactions were studied at two collisional energies (E c) in a crossed-beam product-imaging experiment. We found that all three initial CH stretching excitations suppress the reactivity toward the abstraction of the unexcited D atom. In terms of vibrational suppression factor, σs/σg, the product channels of CH3(00/41) + DCl and CH3(11/31) + DCl show opposite mode-specific trends. However, the angular distributions of both channels are nearly identical to that of the ground-state reaction at the same E c, regardless of the initial reactant states. Tentatively, we ascribed these two observations to a vibrationally induced narrowing effect of the attack angle near the barrier to reaction. As for the DCl coproduct state distributions, the two channels are distinct but show little mode-specific difference. When CH3(00) is probed, the DCl coproduct peaks at v = 1 are accompanied by substantial rotational excitation, whereas the DCl products associated with CH3(11/31) are both vibrationally and rotationally cold. We attributed the different (correlated) energy disposals to a manifestation of trajectory bifurcations in the postbarrier region, with a vibrationally nonadiabatic pathway leading to CH3(00) + DCl­(v = 1) and the other adiabatically to the CH3(11/31) + DCl­(v = 0) channel. For both pathways the initial CH stretching excitation acts as a conserved mode by preferentially retaining one quantum of vibrational excitation in the reaction.

Binary complexes of 2,6-difluorophenylacetylene with methylamine, dimethylamine, trimethylamine and triethylamine were investigated using one colour resonant two photon ionization and infrared-optical double resonance spectroscopic techniques combined with high level ab initio calculations. All four amines form CAc-H···N hydrogen-bonded complexes. Additionally trimethylamine and triethylamine form complexes characterized by Lp···π interactions, due to the electron deficient nature of the phenyl ring of 2,6-difluorophenylacetylene. The Lp···π interacting structure of the 2,6-difluorophenylacetylene-trimethylamine complex is about 1.5 kJ mol(-1) higher in energy than the CAc-H···N hydrogen-bonded structure, which is the global minimum. Energy decomposition analysis indicates that the electrostatics and dispersion interactions favour the formation of CAc-H···N and Lp···π complexes, respectively. Interestingly the CAc-H···N hydrogen-bonded complex of 2,6-difluorophenylacetylene-triethylamine showed a smaller shift in the acetylenic C-H stretching frequency than the 2,6-difluorophenylacetylene-trimethylamine complex. The observed fragmentation of the binary complexes of 2,6-difluorophenylacetylene with the four amines following resonant two-photon ionization can be explained on the basis of the intermolecular coulombic decay process.

The beneficial impact of the initial rotational |jK〉-mode selection on the vibrational-specific reactivity is discussed.

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We report a crossed-beam imaging experiment on the title reactions at two collisional energies (E c ) of 5.3 and 10 kcal mol −1 . Both the integral cross sections relative to the ground-state reactivity and the differential cross sections were measured and compared. We found that one-quantum excitations of the CH 3 -stretching vibrations of the CH 3 D reagent exerted profound mode-specificity in forming the umbrella-modeexcited CH 2 D(4 1 ) products with the vibrational efficacy of v 4 > v 1 -I > v 1 -II at both E c values. The concomitantly formed HCl(v) coproducts were vibrationally cold. Interestingly, the branching ratios of (v = 1)/(v = 0) appeared invariant to the initial stretch-modes of excitation at E c = 5.3 kcal mol −1 , yet exhibited a pronounced mode-specific dependency in the order of v 1 -II > v 1 -I > v 4 at E c = 10.3 kcal mol −1 . This large and E c -dependent disparity between the two Fermi-coupled reagents, v 1 -I and v 1 -II, is particularly significant and could be another facetin addition to that in the recently reported vibrational enhancement factorsof the Fermi-phase-induced interference effect manifested in the product vibrational branching ratio. The pair-correlated angular distributions (v CH2D , v HCl ) s = (4 1 , 0) s in the three stretch-excited reactions were globally alike and resembled that of the ground-state reaction pair (0 0 , 0) g , suggestive of a direct abstraction mechanism of the peripheral type. This is in sharp contrast to all other vibrationally excited pairs of (1 1 , 0) s , (3 1 , 0) s , and (6 1 , 0) s previously reported in the CH 2 D + HCl isotopic channel, for which both the direct abstraction and a time-delayed resonance pathway partake.

The title reaction was studied in a crossed-beam scattering experiment at the collisional energy ( E) ranging from 0.46 to 4.53 kcal mol. Using a time-sliced velocity-imaging technique, both the pair-correlated integral and differential cross sections were measured. On the basis of the observed structures in state-specific excitation functions and the patterns in the E evolution of product angular distributions, we inferred that the title reaction proceeds predominantly via a resonance-mediated pathway, in contrast to the previous findings in the isotopically analogous reactions where the alternative direct abstraction pathway often dominates the reactivity. Despite the complexity of numerous scattering resonances involved in this six-atom reaction, extending our understanding of the isolated resonance in the analogous benchmark F + HD (H) reaction enables us to propose plausible mechanistic origins for the formation as well as the decay of the complicated overlapped resonances.