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. Formic Acid–Ammonia Heterodimer: A New Δ-Machine Learning CCSD(T)-Level Potential Energy Surface Allows Investigation of the Double Proton Transfer

   https://doi.org/10.1021/acs.jctc.3c01273  

   Houston, Paul L; Qu, Chen; Yu, Qi; Pandey, Priyanka; Conte, Riccardo; Nandi, Apurba; Bowman, Joel M; Kukolich, Stephen G (March 2024, Journal of Chemical Theory and Computation)

   Gagliardi, Laura (Ed.)

   The formic acid-ammonia dimer is an important example of a hydrogen-bonded complex in which a double proton transfer can occur. Its microwave spectrum has recently been reported and rotational constants and quadrupole coupling constants were determined. Calculated estimates of the double-well barrier and the internal barriers to rotation were also reported. Here we report a full-dimensional potential energy surface (PES) for this complex, using two closely related Δ-machine learning methods to bring it to the CCSD(T) level of accuracy. The PES dissociates smoothly and accurately. Using a 2d quantum model the ground vibrational-state tunneling splitting is estimted to be less than 10−4 cm−1. The dipole moment along the intrinsic reaction coordinate is calculated along with a Mullikan charge analysis and supports mildly ionic character of the minimum and strongly ionic character at the double-well barrier. 

   more » « less
3. Microwave measurements and structure calculations for a glyoxylic acid – Water complex

   https://doi.org/10.1016/j.jms.2023.111862  

   Daly, Adam M; Hill, Rhett P; Gonzalez, Myla G; Kukolich, Stephen G (January 2024, Journal of Molecular Spectroscopy)

   Heaven, Michael (Ed.)

   The microwave spectra for a hydrogen-bonded trans-2 glyoxylic acid–water complex were measured in the 6–16 GHz frequency range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The rotational constants for the dimer were determined to have the following values: A = 9384.2354(31), B = 1707.63973(73), and C = 1447.44879(56) MHz. The hydrogen bonded structures and rotational constants were calculated for the lowest energy dimglyoxylic acid - water using DFT, MP2 and CCSD calculations with various basis sets. The B3LYP/aug-cc-PVQZ-DG3 calculations yielded rotational constants of A = 9393.59, B = 1713.76, and C = 1453.23 MHz, in very good agreement with experimental values. The calculations show two feasible tunneling motions involving hydrogen atoms in this complex. 

   more » « less
4. Formation and detection of metastable formic acid in a supersonic expansion: High resolution infrared spectroscopy of the jet-cooled cis -HCOOH conformer

   https://doi.org/10.1063/5.0093401  

   Doney, Kirstin D.; Kortyna, Andrew; Chan, Ya-Chu; Nesbitt, David J. (May 2022, The Journal of Chemical Physics)

   High-resolution direct absorption infrared spectra of metastable cis-formic acid (HCOOH) trapped in a cis-well resonance behind a 15 kcal/mol barrier are reported for the first time, with the energetically unstable conformer produced in a supersonic slit plasma expansion of trans-formic acid/H 2 mixtures. We present a detailed high-resolution rovibrational analysis for cis-formic acid species in the OH stretch ( ν 1 ) fundamental, providing first precision vibrational band origin, rotational constants, and term values, which in conjunction with ab initio calculations at the couple-cluster with single, double, and perturbative triple [CCSD(T)]/ANOn (n = 0, 1, 2) level support the experimental assignments and establish critical points on the potential energy surface for internal rotor trans-to-cis isomerization. Relative intensities for a- and b-type transitions observed in the spectra permit the transition dipole moment components to be determined in the body fixed frame and prove to be in good agreement with ab initio CCSD(T) theoretical estimates but in poor agreement with simple bond-dipole predictions. The observed signal dependence on H 2 in the discharge suggests the presence of a novel H atom radical chemical mechanism for strongly endothermic “up-hill” internal rotor isomerization between trans- and cis-formic acid conformers. 

   more » « less
5. A computational and spectroscopic study of MgCCH (X ² Σ ⁺ ): towards characterizing MgCCH ⁺

   https://doi.org/10.1080/00268976.2023.2267135  

   Burns, Joseph E.; Cheng, Qianyi; Fortenberry, Ryan C.; Sun, Ming; Zack, Lindsay N.; Zaveri, Trishal; DeYonker, Nathan J.; Ziurys, Lucy M. (October 2023, Molecular Physics)

   New computational and experimental studies have been carried out for the MgCCH radical in its X2Σ+ state. Coupled cluster theory with single, double, and perturbative triples, CCSD(T), was used in conjunction with post-CCSD(T) and scalar relativistic additive corrections to compute vibrational quartic force fields for this molecule. From the quartic force fields, higher-order spectroscopic properties, including rotational constants, were obtained. In tandem, the five lowest energy rotational transitions for MgCCH, N = 1→0 through N = 5→4, were measured for the first time using Fourier transform microwave/millimeter wave methods in the frequency range 9 -50 GHz. The radical was created in the Discharge Assisted Laser Ablation Source (DALAS) developed in the Ziurys group. A combined fit of these data with previous millimeter direct absorption measurements have yielded the most accurate rotational constants for MgCCH to date. The computed principle rotational constant lies within 1.51-1.65 MHz of the experimental one, validating the computational approach. High-level theory was then applied to produce accurate rovibrational spectroscopic constants for MgCCH+, including a rotational constant of B0 = 5354.5–5359.5 MHz.. These new predictions will further the experimental study of MgCCH+, and aid in the low-temperature characterization of MgCCH, detected towards the circumstellar shell of IRC+10216, a carbon-rich star. 

   more » « less