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1. Unraveling the Vibrational Spectral Signatures of a Dislocated H Atom in Model Proton-Coupled Electron Transfer Dyad Systems

   https://doi.org/10.1021/acs.jpca.3c00524  

   Chen, Liangyi ; Sibert, Edwin L. ; Fournier, Joseph A. ( April 2023 , The Journal of Physical Chemistry A)

   Phenol–benzimidazole and phenol–pyridine proton-coupled electron transfer (PCET) dyad systems are computationally investigated to resolve the origins of the asymmetrically broadened H-bonded OH stretch transitions that have been previously reported using cryogenic ion vibrational spectroscopy in the ground electronic state. Two-dimensional (2D) potentials describing the strongly shared H atom are predicted to be very shallow along the H atom transfer coordinate, enabling dislocation of the H atom between the donor and acceptor groups upon excitation of the OH vibrational modes. These soft H atom potentials result in strong coupling between the OH modes, which exhibit significant bend-stretch mixing, and a large number of normal mode coordinates. Vibrational spectra are calculated using a Hamiltonian that linearly and quadratically couples the H atom potentials to over two dozen of the most strongly coupled normal modes treated at the harmonic level. The calculated vibrational spectra qualitatively reproduce the asymmetric shape and breadth of the experimentally observed bands in the 2300–3000 cm–1 range. Interestingly, these transitions fall well above the predicted OH stretch fundamentals, which are computed to be surprisingly red-shifted (<2000 cm–1). Time-dependent calculations predict rapid (<100 fs) relaxation of the excited OH modes and instant response from the lower-frequency normal modes, corroborating the strong coupling predicted by the model Hamiltonian. The results highlight a unique broadening mechanism and complicated anharmonic effects present within these biologically relevant PCET model systems. 

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2. Deconstructing water’s diffuse OH stretching vibrational spectrum with cold clusters

   https://doi.org/DOI: 10.1126/science.aaw4086  

   Yang, Nan ; Duong, Chinh H. ; Kelleher, Patrick J. ; McCoy, Anne B. ; Johnson, Mark A. ( April 2019 , Science)

   The diffuse vibrational envelope displayed by water precludes direct observation of how different hydrogen-bond topologies dictate the spectral response of individual hydroxy group (OH) oscillators. Using cold, isotopically labeled cluster ions, we report the spectral signatures of a single, intact water (H2O) molecule embedded at various sites in the clathrate-like cage structure adopted by the Cs+·(D2O)20 ion. These patterns reveal the site-dependent correlation between the frequencies of the two OH groups on the same water molecule and establish that the bound OH companion of the free OH group exclusively accounts for bands in the lower-energy region of the spectrum. The observed multiplet structures reveal the homogeneous linewidths of the fundamentals and quantify the anharmonic contributions arising from coupling to both the intramolecular bending and intermolecular soft modes. 

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3. Isomer-specific cryogenic ion vibrational spectroscopy of the D 2 tagged Cs + (HNO 3 )(H 2 O) n=0–2 complexes: ion-driven enhancement of the acidic H-bond to water

   https://doi.org/10.1039/c9cp06689f  

   Mitra, Sayoni ; Duong, Chinh H. ; McCaslin, Laura M. ; Gerber, R. Benny ; Johnson, Mark A. ( February 2020 , Physical Chemistry Chemical Physics)

   We report how the binary HNO 3 (H 2 O) interaction is modified upon complexation with a nearby Cs + ion. Isomer-selective IR photodissociation spectra of the D 2 -tagged, ternary Cs + (HNO 3 )H 2 O cation confirms that two structural isomers are generated in the cryogenic ion source. In one of these, both HNO 3 and H 2 O are directly coordinated to the ion, while in the other, the water molecule is attached to the OH group of the acid, which in turn binds to Cs + with its –NO 2 group. The acidic OH stretching fundamental in the latter isomer displays a ∼300 cm −1 red-shift relative to that in the neutral H-bonded van der Waals complex, HNO 3 (H 2 O). This behavior is analyzed with the aid of electronic structure calculations and discussed in the context of the increased effective acidity of HNO 3 in the presence of the cation. 

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4. Origins of the diffuse shared proton vibrational signatures in proton-coupled electron transfer model dyad complexes

   https://doi.org/10.1063/5.0122777  

   Chen, Liangyi ; Ma, Zifan ; Fournier, Joseph A. ( October 2022 , The Journal of Chemical Physics)

   Phenol-benzimidazole and phenol-pyridine dyad complexes have served as popular model systems for the study of proton-coupled electron transfer (PCET) kinetics in solution-phase experiments. Interpretation of measured PCET rates in terms of key structural parameters, such as the H-bond donor–acceptor distance, however, remains challenging. Herein, we report vibrational spectra in the electronic ground state for a series of phenol-benzimidazole and phenol-pyridine complexes isolated and cryogenically cooled in an ion trap. The four models studied each display highly red-shifted and broadened OH stretching transitions that arise from strong H-bonding interactions between the phenol OH group and the basic N site on benzimidazole/pyridine rings. The OH stretch transition in each model displays relatively strong absorption onsets near 2500 cm −1 with broad shoulders that extend asymmetrically to higher frequencies over hundreds of wavenumbers. In contrast, the deuterated isotopologues yield much weaker OD stretch transitions that appear symmetrically broadened. The spectral breadth and shape of the OD stretch transitions are ascribed to variations in OD stretch frequencies that arise from zero-point distributions in the proton donor–acceptor low-frequency soft mode vibration. The asymmetric structure of the OH stretch transitions is attributed to a set of combination bands between the OH stretch and a series of low-frequency H-bond soft modes. The spectra and modeling highlight the importance of OH stretch-soft mode couplings, which are thought to play important roles in PCET and proton transfer dynamics. 

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5. Overcoming the out-of-plane bending issue in an aromatic hydrocarbon: the anharmonic vibrational frequencies of c-(CH)C 3 H 2 +

   https://doi.org/10.1039/D0CP01889A  

   Westbrook, Brent R. ; Del Rio, Weston A. ; Lee, Timothy J. ; Fortenberry, Ryan C. ( June 2020 , Physical Chemistry Chemical Physics)

   null (Ed.)

   The challenges associated with the out-of-plane bending problem in multiply-bonded hydrocarbon molecules can be mitigated in quartic force field analyses by varying the step size in the out-of-plane coordinates. Carbon is a highly prevalent element in astronomical and terrestrial environments, but this major piece of its spectra has eluded theoretical examinations for decades. Earlier explanations for this problem focused on method and basis set issues, while this work seeks to corroborate the recent diagnosis as a numerical instability problem related to the generation of the potential energy surface. Explicit anharmonic frequencies for c-(CH)C 3 H 2 + are computed using a quartic force field and the CCSD(T)-F12b method with cc-pVDZ-F12, cc-pVTZ-F12, and aug-cc-pVTZ basis sets. The first of these is shown to offer accuracy comparable to that of the latter two with a substantial reduction in computational time. Additionally, c-(CH)C 3 H 2 + is shown to have two fundamental frequencies at the onset of the interstellar unidentified infrared bands, at 5.134 and 6.088 μm or 1947.9 and 1642.6 cm −1 , respectively. This suggests that the results in the present study should assist in the attribution of parts of these aromatic bands, as well as provide data in support of the laboratory or astronomical detection of c-(CH)C 3 H 2 + . 

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