Research ArticleChemistry

Acid solvation versus dissociation at “stardust conditions”: Reaction sequence matters

See allHide authors and affiliations

Science Advances  07 Jun 2019:
Vol. 5, no. 6, eaav8179
DOI: 10.1126/sciadv.aav8179
  • Fig. 1 IR depletion spectra of pure water and mixed HCl-water clusters, resulting from pickup sequence cell 1 (HCl) and cell 2 (water), measured at different mass channels.

    (A) IR spectra of pure H218O clusters, measured at m/z = 41. (B to D) Spectra in the presence of a small amount of HCl gas, measured at m/z = 41, 61, and 81, respectively. These mass channels correspond to H+(H218O)2 (m/z = 41), H+(H218O)3 (m/z = 61), and H+(H218O)4 (m/z = 81) fragments. The bands corresponding to the bending vibration of the pure water (H2O)n (n = 4 and 5) clusters are highlighted in the light blue shade. The broad band highlighted in yellow is assigned to the umbrella mode of the H3O+ core of the dissociated species. HCl partial pressure of 0.14 mPa and water partial pressure of 0.53 mPa were used in all the measurements. The proton originating from HCl is shown in dark gray. a.u., arbitrary units.

  • Fig. 2 Two different pathways for the aggregation of HCl(H2O)4 clusters in helium droplets, depicting that acid formation at ultracold temperatures follows a unique pathway.

    (Left) The first HCl and, subsequently, four water molecules are picked up, yielding aggregation-induced dissociation of HCl and the formation of the smallest droplet of acid, H3O+(H2O)3Cl. (Right) The first four water molecules aggregate to form a cyclic water tetramer, and upon subsequent addition of one HCl molecule, the HCl is found to remain undissociated.

  • Fig. 3 Comparison of the IR spectra of mixed HCl-H218O clusters, measured at m/z = 41, for two distinct experimental scenarios.

    (A) HCl pickup precedes the pickup of water molecules. (B) Water pickup precedes the pickup of HCl.

  • Fig. 4 Schematics of the helium droplet machine at the FELIX Laboratory (BoHeNDI@FELIX).

    QMS, quadrupole mass spectrometer.

  • Fig. 5 m/z = 41 ion current, for pure H218O clusters, as a function of time, is shown as a representative example.

    The ion current was integrated over several measurements in the frequency range of 1000 to 1700 cm−1. Ion current before 10 ms was used as a reference. The ion current in the ~10- to 13-ms time window contains depletion signal.

  • Table 1 Number of water molecules (n) contributing to the observed bands, determined from the fit of the pressure-dependent spectroscopic signal to Poisson distributions.

    The pickup curves were recorded at a particular on-resonance frequency. See also the “Ionization mechanism of pure water and mixed HCl-water aggregates” and “The umbrella motion of hydronium: Parent cluster size determination” sections in the Supplementary Materials.

    Resonance frequency
    (cm−1)
    Mass channel
    (atomic mass units)
    n
    1320414
    1320614
    1400614
    1250815

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/6/eaav8179/DC1

    Section S1. Experimental design and additional results

    Section S2. Simulation methods and additional results

    Fig. S1. Details of the second vacuum chamber.

    Fig. S2. Variation of ion current at m/z = 36 with HCl partial pressure in cell 1.

    Fig. S3. Dependence of the ion current upon water partial pressure as detected on m/z = 41, 61, and 81.

    Fig. S4. Reference ion current, as a function of H218O partial pressure, detected at mass channels m/z = 41, 61, and 81.

    Fig. S5. Variation of the spectroscopic signal (red dots) as a function of H218O partial pressure for different mass channels and frequencies.

    Fig. S6. Poisson fit assuming a superposition of two Poisson distributions for pickup of (H2O)4 and (H2O)5.

    Fig. S7. Relevant structures of the HCl(H2O)4 cluster as well as that of the Eigen complex, (H3O)+(H2O)3, all optimized using BLYP.

    Fig. S8. Relevant structures of the HCl(H2O)5 cluster all optimized using BLYP.

    Fig. S9. Quantum O─H distance distribution functions of the three protons and the oxygen within the hydronium moiety of the CIP5 (top) and SIP5 (bottom) conformers at 1.25 K.

    Fig. S10. Mechanism leading to the formation of CIP5 after the aggregation of the fifth water molecule with the HCl(H2O)4 SIP C3 conformer in terms of representative snapshots sampled from the ab initio aggregation simulations.

    Fig. S11. Mechanism leading to the formation of SIP5 (right) via attachment of the fifth water molecule to the Cl site of the SIP C1 conformer (left) in terms of representative snapshots sampled from the ab initio aggregation simulations.

    Fig. S12. Mechanism leading to the formation of the cyclic UD structure (bottom left) after the aggregation of the HCl molecule with the cyclic water tetramer (H2O)4 has generated the AG structure (top left) in terms of representative snapshots sampled from the ab initio aggregation simulations.

    Fig. S13. Alternative mechanism leading to the formation of the cyclic UD structure (bottom left) after the aggregation of the HCl molecule with the cyclic water tetramer (H2O)4 has generated the AG structure (top left) in terms of representative snapshots sampled from the ab initio aggregation simulations.

    Table S1. Relative electronic energy (without inclusion of zero-point energy correction), dipole moment, and (unscaled harmonic) frequency of the hindered umbrella mode of the hydronium moiety, H3O+, in case of ion pair structures for the most relevant HCl(H2O)n isomers with n = 4 and 5, based on optimized MP2 and BLYP structures.

    Table S2. Aggregation simulations of SIP + H2O with initial NVT electrostatic steering alignment of the water molecule.

    Table S3. Aggregation simulations of SIP + H2O with the water molecule in random orientations and positions around the center of the SIP cluster.

    Table S4. Aggregation simulations of the cyclic (H2O)4 cluster with an HCl molecule using three different classes of initial conditions, using different cluster-molecule distances (d), relative velocities (v), and harmonic constants (k), as specified.

    References (53, 54)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Experimental design and additional results
    • Section S2. Simulation methods and additional results
    • Fig. S1. Details of the second vacuum chamber.
    • Fig. S2. Variation of ion current at m/z = 36 with HCl partial pressure in cell 1.
    • Fig. S3. Dependence of the ion current upon water partial pressure as detected on m/z = 41, 61, and 81.
    • Fig. S4. Reference ion current, as a function of H218O partial pressure, detected at mass channels m/z = 41, 61, and 81.
    • Fig. S5. Variation of the spectroscopic signal (red dots) as a function of H218O partial pressure for different mass channels and frequencies.
    • Fig. S6. Poisson fit assuming a superposition of two Poisson distributions for pickup of (H2O)4 and (H2O)5.
    • Fig. S7. Relevant structures of the HCl(H2O)4 cluster as well as that of the Eigen complex, (H3O)+(H2O)3, all optimized using BLYP.
    • Fig. S8. Relevant structures of the HCl(H2O)5 cluster all optimized using BLYP.
    • Fig. S9. Quantum O─H distance distribution functions of the three protons and the oxygen within the hydronium moiety of the CIP5 (top) and SIP5 (bottom) conformers at 1.25 K.
    • Fig. S10. Mechanism leading to the formation of CIP5 after the aggregation of the fifth water molecule with the HCl(H2O)4 SIP C3 conformer in terms of representative snapshots sampled from the ab initio aggregation simulations.
    • Fig. S11. Mechanism leading to the formation of SIP5 (right) via attachment of the fifth water molecule to the Cl site of the SIP C1 conformer (left) in terms of representative snapshots sampled from the ab initio aggregation simulations.
    • Fig. S12. Mechanism leading to the formation of the cyclic UD structure (bottom left) after the aggregation of the HCl molecule with the cyclic water tetramer (H2O)4 has generated the AG structure (top left) in terms of representative snapshots sampled from the ab initio aggregation simulations.
    • Fig. S13. Alternative mechanism leading to the formation of the cyclic UD structure (bottom left) after the aggregation of the HCl molecule with the cyclic water tetramer (H2O)4 has generated the AG structure (top left) in terms of representative snapshots sampled from the ab initio aggregation simulations.
    • Table S1. Relative electronic energy (without inclusion of zero-point energy correction), dipole moment, and (unscaled harmonic) frequency of the hindered umbrella mode of the hydronium moiety, H3O+, in case of ion pair structures for the most relevant HCl(H2O)n isomers with n = 4 and 5, based on optimized MP2 and BLYP structures.
    • Table S2. Aggregation simulations of SIP + H2O with initial NVT electrostatic steering alignment of the water molecule.
    • Table S3. Aggregation simulations of SIP + H2O with the water molecule in random orientations and positions around the center of the SIP cluster.
    • Table S4. Aggregation simulations of the cyclic (H2O)4 cluster with an HCl molecule using three different classes of initial conditions, using different cluster-molecule distances (d), relative velocities (v), and harmonic constants (k), as specified.
    • References (53, 54)

    Download PDF

    Files in this Data Supplement:

Navigate This Article