More Like this

1. Sub-Doppler infrared spectroscopy of resonance-stabilized hydrocarbon intermediates: ν ₃ / ν ₄ CH stretch modes and CH ₂ internal rotor dynamics of benzyl radical

   https://doi.org/10.1039/c7cp05776h  

   Kortyna, A.; Samin, A. J.; Miller, T. A.; Nesbitt, D. J. (January 2017, Physical Chemistry Chemical Physics)

   Highly reactive benzyl radicals are generated by electron dissociative attachment to benzyl chloride doped into a neon–hydrogen–helium discharge and immediately cooled to T rot = 15 K in a high density, supersonic slit expansion environment. The sub-Doppler spectra are fit to an asymmetric-top rotational Hamiltonian, thereby yielding spectroscopic constants for the ground ( v = 0) and first excited ( v = 1, ν 3 , ν 4 ) vibrational levels of the ground electronic state. The rotational constants obtained for the ground state are in good agreement with previous laser induced fluorescence measurements (LIF), with vibrational band origins ( ν 3 = 3073.2350 ± 0.0006 cm −1 , ν 4 = 3067.0576 ± 0.0006 cm −1 ) in agreement with anharmonically corrected density functional theory calculations. To assist in detection of benzyl radical in the interstellar medium, we have also significantly improved the precision of the ground state rotational constants through combined analysis of the ground state IR and LIF combination differences. Of dynamical interest, there is no evidence in the sub-Doppler spectra for tunneling splittings due to internal rotation of the CH 2 methylene subunit, which implies a significant rotational barrier consistent with partial double bond character in the CC bond. This is further confirmed with high level ab initio calculations at the CCSD(T)-f12b/ccpVdZ-f12 level, which predict a zero-point energy corrected barrier to internal rotation of Δ E tun ≈ 11.45 kcal mol −1 or 4005 cm −1 . In summary, the high-resolution infrared spectra are in excellent agreement with simple physical organic chemistry pictures of a strongly resonance-stabilized benzyl radical with a nearly rigid planar structure due to electron delocalization around the aromatic ring. 

   more » « less
2. Reinvestigation of the ν ₃ – ν ₆ Coriolis interaction in trifluoroiodomethane

   https://doi.org/10.1039/D4CP02640C  

   Bhujel, Arun; Akter, Salma; Ali, Muhammad Qasim; Park, G Barratt (November 2024, Physical Chemistry Chemical Physics)

   An updated fit to the interacting levelsν₃andν₆of CF₃I has been evaluated in this work. 

   more » « less
3. Mid-infrared Doppler-free saturation absorption spectroscopy of the Q branch of CH ₄ ν ₃ = 1 band using a rapid-scanning continuous-wave optical parametric oscillator

   https://doi.org/10.1364/OL.530567  

   Shah_Riyadh, S_M; Telfah, Hamzeh; Jones, Ian_W; Bersson, Jonathan_S; Cheng, Cun-Feng; Hu, Shui-Ming; Foote, David_B; Liu, Jinjun (July 2024, Optics Letters)

   We have developed a mid-infrared Doppler-free saturation absorption spectroscopy apparatus that employs a commercial continuous-wave optical parametric oscillator (CW OPO), complemented by a home-built automation and wavelength scanning system. Here, we report a comprehensive spectral scan of the Q branch transitions of theν₃= 1 band of methane (CH₄) with an average linewidth (FWHM) of 4.5 MHz. The absolute frequency calibration was achieved using previously reported transition frequencies determined using optical frequency combs, while a Fabry–Perot etalon was used for the relative frequency calibration. We report 15 transitions with improved accuracies of 1.13 MHz (3.76 × 10⁻⁵cm⁻¹). 

   more » « less
4. VT‐NMR Analysis of Rotation‐Inversion of N ‐(4‐hydroxybutyl)‐ N ‐(2,2,2‐trifluoroethyl) tert ‐butyl Carbamate: Utilizing the −CH ₂ CF ₃ Appendage as a Reporter on E / Z ‐Isomerization

   https://doi.org/10.1002/slct.202401323  

   Jameson, Brian; Glaser, Rainer (April 2024, ChemistrySelect)

   Abstract The rotational barrier about the CN carbamate bond ofN‐(4‐hydroxybutyl)‐N‐(2,2,2‐trifluoroethyl)tert‐butyl carbamate1was determined by variable temperature (VT)¹³C and¹⁹F NMR spectroscopy. The −CH₂CF₃ appendage reports on rotational isomerism and allows for the observation of separate signals for the E‐ and Z‐ensembles at low temperature. The activation barrier for E/Z‐isomerization was quantified using Eyring‐Polanyi theory which requires the measurements of the maximum difference in Larmor frequency Δν_max and the convergence temperature T_c. Both Δν_max and T_c were interpolated by analyzing sigmoidal functions fitted to data describing signal separation and the quality of the superposition of the E‐ and Z‐signals, respectively. Methods for generating the quality‐of‐fit parameters for Lorentzian line shape analysis are discussed. Our best experimental value for the rotational barrier ΔG_c^≠(1)=15.65±0.13 kcal/mol is compared to results of a higher level ab initio study of the modelN‐ethyl‐N‐(2,2,2‐trifluoroethyl) methyl carbamate. 

   more » « less
5. Computer‐Assisted 3D Structure Elucidation (CASE‐3D): The Structural Value of ² J _CH in Addition to ³ J _CH Coupling Constants

   https://doi.org/10.1002/ange.201915103  

   Koos, Martin R. M.; Navarro‐Vázquez, Armando; Anklin, Clemens; Gil, Roberto R. (January 2020, Angewandte Chemie)

   Abstract We present a method to use long‐range CH coupling constants to derive the correct diastereoisomer from the molecular constitution of small molecules. A set of 79²J_CHand³J_CHvalues collected from a single HSQMBC experiment on a sample of strychnine were used in the CASE‐3D (computer‐assisted 3D structure elucidation) protocol. In addition to the most commonly used³J_CHcoupling constants, the subset of 32²J_CHvalues alone showed an excellent degree of configuration selection. The study is mainly based on comparison of DFT‐calculated^2,3J_CHvalues with experimental ones, critical for the case of²J_CH. But the configuration selection also works well using³J_CHvalues predicted from a semi‐empirical Karplus‐based equation limited to H−C−C−C fragments. The robustness, shown using strychnine as a proof of concept, makes theJ‐based CASE‐3D analysis a viable option for the application in fields such as peptide and carbohydrate research, organic synthesis, natural‐product identification and analysis, as well as medicinal chemistry. 

   more » « less