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1. How the nature and charge of metal cations affect vibrations in acetone solvent molecules

   https://doi.org/10.1039/D3CP05204D  

   Phasuk, Apakorn; Lemaire, Joel; Steinmetz, Vincent; Maître, Philippe; Metz, Ricardo B (January 2024, Physical Chemistry Chemical Physics)

   Vibrational spectra of a series of gas-phase metal 1+ and 2+ ions solvated by acetone molecules are collected to investigate how the metal charge, number of solvent molecules and nature of the metal affect the acetone. The spectra of Cu+(Ace)(N2)2, Cu+(Ace)4, and M2+(Ace)4, where M = Co, Ni, Cu, and Zn are measured via photodissociation by monitoring fragment ion signal as a function of IR wavenumber. The spectra show a red shift of the C=O stretch and a blue shift of the C–C antisymmetric stretch. DFT calculations are carried out to provide the simulated spectra of possible isomers to be compared with the observed vibrational spectra, and specific structures are proposed. The red shift of the C=O stretch increases as the number of acetone molecules decreases. Higher charge on the metal leads to a larger red shift in the C=O stretch. Although all of the M2+ complexes have very similar red shifts, they are predicted to have different geometries due to their different electron configurations. Unexpectedly, we find that the calculated red shift in the C=O stretch in M+/2+(Ace) is highly linearly correlated with the ionization energy of the metal for a wide range of metal cations and dications. 

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2. Reactions of Aluminum Oxide Cluster Cations with Ethane: A Mass‐Spectrometric and Vibrational Spectroscopy Study

   https://doi.org/10.1002/cphc.202400427  

   Phasuk, Apakorn; Metz, Ricardo_B (November 2024, ChemPhysChem)

   Abstract The pathways for the reactions of aluminum oxide cluster ions with ethane have been measured. For selected ions (Al₂O⁺, Al₃O₂⁺, Al₃O₄⁺, Al₄O₇⁺) the structure of the collisionally‐stabilized reaction intermediates were explored by measuring the photodissociation vibrational spectra from 2600 cm⁻¹–3100 cm⁻¹. Density functional theory was used to calculate features of the potential energy surfaces for the reactions and the vibrational spectra of intermediates. Generally, more than one isomer contributes to the observed spectrum. The oxygen‐deficient clusters Al₂O⁺and Al₃O₂⁺have large C−H activation barriers, so only the entrance channel complexes in which intact C₂H₆binds to aluminum are observed. This interaction leads to a substantial (~200 cm⁻¹) red shift of the C−H symmetric stretch in ethane, indicating significant weakening of the proximal C−H bonds. In Al₃O₄⁺, the complex formed by interactions with three C₂H₆is investigated and, in addition to entrance channel complexes, the C−H activation intermediate Al₃O₄H⁺(C₂H₅)(C₂H₆)₂is observed. For oxygen‐rich Al₄O₇⁺, the C₂H₆is favored to bind at an aluminum site far from the reactive superoxide group, reducing the reactivity. As expected, oxygen‐rich species and open‐shell cluster ions have smaller barriers for C−H bond activation, except for Al₃O₄⁺which is predicted and observed to be reactive. 

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3. The missing NH stretch fundamental in S ₁ methyl anthranilate: IR-UV double resonance experiments and local mode theory

   https://doi.org/10.1039/D0CP01916J  

   Blodgett, Karl N.; Fischer, Joshua L.; Zwier, Timothy S.; Sibert, Edwin L. (July 2020, Physical Chemistry Chemical Physics)

   The infrared spectra of jet-cooled methyl anthranilate (MA) and the MA–H 2 O complex are reported in both S 0 and S 1 states, recorded using fluorescence-dip infrared (FDIR) spectroscopy under jet-cooled conditions. Using a combination of local mode CH stretch modeling and scaled harmonic vibrational character, a near-complete assignment of the infrared spectra is possible over the 1400–3700 cm −1 region. While the NH stretch fundamentals are easily observed in the S 0 spectrum, in the S 1 state, the hydrogen bonded NH stretch shift is not readily apparent. Scaled harmonic calculations predict this fundamental at just below 2900 cm −1 with an intensity around 400 km mol −1 . However, the experimental spectrum shows no evidence of this transition. A local mode theory is developed in which the NH stretch vibration is treated adiabatically. Minimizing the energy of the corresponding stretch state with one quantum of excitation leads to a dislocation of the H atom where there is equal sharing between N and O atoms. The sharing occurs as a result of significant molecular arrangement due to strong coupling of this NH stretch to other internal degrees of freedom and in particular to the contiguous HNC bend. A two-dimensional model of the coupling between the NH stretch and this bend highlights important nonlinear effects that are not captured by low order vibrational perturbation theory. In particular, the model predicts a dramatic dilution of the NH stretch oscillator strength over many transitions spread over more than 1000 cm −1 , making it difficult to observe experimentally. 

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4. Infrared Laser Stark Spectroscopy of Methyl Fluoride in ⁴ He Nanodroplets

   https://doi.org/10.1002/cphc.202400224  

   Raston, Paul L; Douberly, Gary E (June 2024, ChemPhysChem)

   Abstract We measured the rotationally resolved infrared spectra of helium solvated methyl fluoride at 3 μm and 10 μm, wherein lies C−H and C−F stretching bands, respectively. The linewidths (FWHM) were found to increase with increasing vibrational energy and range from 0.002 cm⁻¹in thev₃band (C−F stretch) at ~1047 cm⁻¹, to 0.65 cm⁻¹in thev₄band (asymmetric C−H stretch) at ~2997 cm⁻¹. In between these two bands we observed the lower and upper components of the Fermi triad bands (ν₁/2ν₂/2ν₅) at ~2859 and ~2961 cm⁻¹. We carried out Stark spectroscopy on the lower band on account of its narrower linewidths (0.04 vs. 0.14 cm⁻¹, respectively). The objective of performing Stark spectroscopy was to see if there is any evidence for a rotational linewidth dependence on the external field strength, due to a reduced difference in between methyl fluorides rotational energy gap and the roton‐gap of superfluid helium. We did not find any evidence for such an effect, which we largely attribute to the rotational energy gap not increasing significantly enough by the external field. We point to another molecule (formaldehyde) whose energy levels are predicted to show a more promising response to application of an external field. 

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5. Probing adsorption of methane onto vanadium cluster cations via vibrational spectroscopy

   https://doi.org/10.1063/5.0169118  

   Kozubal, Justine; Heck, Tristan; Metz, Ricardo B (November 2023, The Journal of Chemical Physics)

   Photofragment spectroscopy is used to measure the vibrational spectra of V2(+)(CH4)n (n = 1–4), V3(+)(CH4)n (n = 1–3), and Vx(+)(CH4) (x = 4–8) in the C–H stretching region (2550–3100 cm−1). Spectra are measured by monitoring loss of CH4. The experimental spectra are compared to simulations at the B3LYP+D3/6-311++G(3df,3pd) level of theory to identify the geometry of the ions. Multi-reference configuration interaction with Davidson correction (MRCI+Q) calculations are also carried out on V2(+) and V3(+). The methane binding orientation in V2(+)(CH4)n (n = 1–4) evolves from η3 to η2 as more methane molecules are added. The IR spectra of metal-methane clusters can give information on the structure of metal clusters that may otherwise be hard to obtain from isolated clusters. For example, the V3(+)(CH4)n (n = 1–3) experimental spectra show an additional peak as the second and third methane molecules are added to V3(+), which indicates that the metal atoms are not equivalent. The Vx(+)(CH4) show a larger red shift in the symmetric C–H stretch for larger clusters with x = 5–8 than for the small clusters with x = 2, 3, indicating increased covalency in the interaction of larger vanadium clusters with methane. 

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