Hydrogen-Bonded Complexes of Phenylacetylene–Acetylene: Who is the Proton Donor?

Abstract

Hydrogen-bonded complexes of C2H2 and phenylacetylene (PhAc) were studied using matrix isolation infrared spectroscopy and quantum chemical computations. Both C2H2 and PhAc, being potential proton donors, the question arises as to which of the two species would be the proton donor in the PhAc-C2H2 complex; a question that this work primarily addresses. The molecular structures, vibrational frequencies, and interaction energies of the PhAc-C2H2 complexes were calculated at the M06-2X and MP2 levels of theory, employing both 6-311++G(d,p) and aug-cc-pVDZ basis sets. At the M06-2X/aug-cc-pVDZ level, two nearly isoenergetic complexes (BSSE corrected) were indicated to be the global minima; one a C-H···π complex, where C2H2 served as a proton donor to the phenyl π-system in PhAc, and the other a C-H···π complex, where C2H2 served as a proton donor to the acetylene π-system in PhAc. Of the two, only the second complex was identified in the matrix, evidenced by a characteristic large shift in the ≡C-H stretch of C2H2. Experiments were also performed using PhAc deuterated at the acetylene hydrogen (PhAcD) to study the isotopic effects on the vibrational spectra of complexes. The isotopic studies further confirmed the structure of the complex trapped in the matrix, thereby presenting unambiguous evidence that C2H2 served as the proton donor to the acetylene π-system of PhAc. The theory of atoms-in-molecules (AIM), energy decomposition (EDA), and natural bond orbital (NBO) analysis were performed to understand the nature of the interactions involved in the complexes.

An aromatic noble-gas hydride: C6H5CCXeH

“…Other characteristic absorptions in a xenon matrix appear at 755, 688, 642, and 607 cm −1 (756, 688, 644, and 608 cm −1 in a krypton matrix; 756, 689, 647, and 610 cm −1 in an argon matrix), corresponding to the phenyl CH out-of-plane, ≡C–H in-plane, phenyl CH out-of-plane, and ≡C–H out-of-plane bending modes, respectively. These frequencies are in good agreement with the values previously reported for the ≡C–H stretching band (3323.2 and 3310.8/3309.2 cm −1 ) and the phenyl CH out-of-plane bending band (758.7 cm −1 ) in a nitrogen matrix 23, 24 . The splitting of the ≡C–H stretching band was explained by a Fermi resonance between the acetylenic ≡C–H stretching vibration and a combination of one quantum of the C≡C stretching vibration and two quanta of the C≡C–H out-of-plane bending vibration 25–27 .…”

“…However, some splitting still occurs in the ≡C–D stretching region (2601.5/2595.0/2592.5 cm −1 ) probably due to different matrix sites. A similar situation occurs in a nitrogen matrix 23, 24 .

Figure 2FTIR spectra of PhAc (upper trace) and PhAc-d 6 (lower trace) in a xenon matrix ( a ). Calculated spectra at the M06-2X/aug-cc-pVTZ level of theory and scaled by a factor of 0.97 ( b ).

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Does a hydrogen bonded complex with dual contacts show synergism? A matrix isolation infrared and ab-initio study of propargyl alcohol–water complex

“…Matrix isolation experiments were performed using a Sumitomo closed cycle helium compressor-cooled cryostat (HC-4E1) to attain a temperature of 12 K. The experimental details have been described elsewhere [17][18][19][20][21].…”

Intermolecular Complexes and Molecular Conformations Directed by Hydrogen Bonds: Matrix Isolation and Ab Initio Studies

“…6. 19 As is evident from the figure, three of these minima show ≡C-H···π contacts, while one structure manifests a π···π interaction. The energy difference between the various isomers is rather minimal.…”

“…Details as to how the scaling factors were computed are discussed in detail in many of our earlier publications. [18][19][20][21] The computed wavenumbers together with the intensities of the various vibrational modes were used to simulate a vibrational spectrum using the SYNSPEC program, 22 assuming a Lorentzian line profile with a full-width-half-maximum (FWHM) of 1.0 cm −1 .…”