publications

2023

1. Exploring the Origins of the Intensity of the OH Stretch-HOH Bend Combination Band in Water

   Rachel M. Huchmala, and Anne B. McCoy

   J. Phys. Chem. A, Aug 2023

   DOI
2. Isotope Effects in the Zundel–Eigen Isomerization of H ^\textrm+ (H _\textrm2 O) _\textrm6

   Jacob M. Finney, Tae Hoon Choi, Rachel M. Huchmala, and 4 more authors

   J. Phys. Chem. Lett., May 2023

   DOI

2022

1. Exploring the Origins of Spectral Signatures of Strong Hydrogen Bonding in Protonated Water Clusters

   Rachel M. Huchmala, and Anne B. McCoy

   J. Phys. Chem. A, Mar 2022

   Abs DOI

   The effects of anharmonicity on the spectral features of strong ionic hydrogen bonds are explored through reduced dimensional studies of the couplings between the hydrogen bonding OH and the donor−acceptor OO stretching vibrations in protonated water clusters with 2−4 water molecules. Specifically, this study focuses on how the anharmonicities and couplings in these ions are reflected in the vibrational spectra by exploring the intensities of the transitions to states with excitation in both the OH and the OO stretching vibrations and changes in the frequency of the OO stretching vibration when the OH stretching vibration is excited. These questions are addressed through the application of several approximate treatments that are based on an adiabatic separation of the high-frequency OH and lowfrequency OO stretching vibrations as well as low-order expansions of the potential and dipole surfaces. While an adiabatic approximation captures most of the trends found in the spectra and from an analysis of the two-dimensional model, a vibrational Franck−Condon approach fails to capture the intensities of these transitions. Of the terms in the expansion of the dipole moment function, those that are proportional to ΔrOH and ΔrO2 H are found to provide the largest contributions to the calculated intensities of the transitions involving excitation of both the OH and the OO stretches. This leads to the conclusion that the intensities of these transitions encode information about the frequency and anharmonicity of the OH stretching vibration and how they are affected by changes in the OO distance. The anharmonicity of the potential also leads to changes in the OO stretching frequency with excitation of the OH stretching vibration. The direction of this change in frequency encodes additional information about the strength of the ionic hydrogen bond.

2. Infrared spectroscopic signature of a hydroperoxyalkyl radical (•QOOH)

   Anne S. Hansen, Trisha Bhagde, Yujie Qian, and 7 more authors

   J. Chem. Phys., Jan 2022

   Abs DOI

   Infrared (IR) action spectroscopy is utilized to characterize a prototypical carbon-centered hydroperoxyalkyl radical (●QOOH) transiently formed in the oxidation of volatile organic compounds. The ●QOOH radical formed in isobutane oxidation, 2-hydroperoxy-2-methylprop1-yl, ●CH2(CH3)2COOH, is generated in the laboratory by H-atom abstraction from tert-butyl hydroperoxide (TBHP). IR spectral features of jet-cooled and stabilized ●QOOH radicals are observed from 2950 to 7050 cm−1 at energies that lie below and above the transition state barrier leading to OH radical and cyclic ether products. The observed ●QOOH features include overtone OH and CH stretch transitions, combination bands involving OH or CH stretch and a lower frequency mode, and fundamental OH and CH stretch transitions. Most features arise from a single vibrational transition with band contours well simulated at a rotational temperature of 10 K. In each case, the OH products resulting from unimolecular decay of vibrationally activated ●QOOH are detected by UV laser-induced fluorescence. Assignments of observed ●QOOH IR transitions are guided by anharmonic frequencies computed using second order vibrational perturbation theory, a 2 + 1 model that focuses on the coupling of the OH stretch with two low-frequency torsions, as well as recently predicted statistical ●QOOH unimolecular decay rates that include heavy-atom tunneling. Most of the observed vibrational transitions of ●QOOH are readily distinguished from those of the TBHP precursor. The distinctive IR transitions of ●QOOH, including the strong fundamental OH stretch, provide a general means for detection of ●QOOH under controlled laboratory and real-world conditions.

3. Character of the OH Bend–Stretch Combination Band in the Vibrational Spectra of the “Magic” Number H _\textrm3 O ^\textrm+ (H _\textrm2 O) _\textrm20 and D _\textrm3 O ^\textrm+ (D _\textrm2 O) _\textrm20 Cluster Ions

   Nan Yang, Rachel M. Huchmala, Anne B. McCoy, and 1 more author

   J. Phys. Chem. Lett., Sep 2022

   DOI
4. Vibrational Signatures of HNO _\textrm3 Acidity When Complexed with Microhydrated Alkali Metal Ions, M ^\textrm+ ·(HNO _\textrm3 )(H _\textrm2 O) _\textrm \textitn =5 (M = Li, K, Na, Rb, Cs), at 20 K

   Sayoni Mitra, Thien Khuu, Tae Hoon Choi, and 4 more authors

   J. Phys. Chem. A, Mar 2022

   DOI

2021

1. Coupling of torsion and OH-stretching in \textittert -butyl hydroperoxide. I. The cold and warm first OH-stretching overtone spectrum

   Anne S. Hansen, Rachel M. Huchmala, Emil Vogt, and 8 more authors

   J. Chem. Phys., Apr 2021

   Abs DOI

   The infrared (IR) spectrum of tert-butyl hydroperoxide (TBHP) in the region of the first OH-stretching overtone has been observed under jet-cooled and thermal (300 K, 3 Torr) conditions at ∼7017 cm−1. The jet-cooled spectrum is recorded by IR multiphoton excitation with UV laser-induced fluorescence detection of OH radical products, while direct IR absorption is utilized under thermal conditions. Prior spectroscopic studies of TBHP and other hydroperoxides have shown that the OH-stretch and XOOH (X = H or C) torsion vibrations are strongly coupled, resulting in a double well potential associated with the torsional motion about the OO bond that is different for each of the OH-stretching vibrational states. A low barrier between the wells on the torsional potential results in tunneling split energy levels, which leads to four distinct transitions associated with excitation of the coupled OH-stretch-torsion states. In order to interpret the experimental results, two theoretical models are used that include the OH-stretch-torsion coupling in TBHP. Both methods are utilized to compute the vibrational transitions associated with the coupled OH-stretch-torsion states of TBHP, revealing the underlying transitions that compose the experimentally observed features. A comparison between theory and experiment illustrates the necessity for treatments that include OH-stretch and COOH torsion in order to unravel the spectral features observed in the first OH-stretching overtone region of TBHP.

2. Coupling of torsion and OH-stretching in \textittert -butyl hydroperoxide. II. The OH-stretching fundamental and overtone spectra

   Emil Vogt, Rachel M. Huchmala, Casper V. Jensen, and 7 more authors

   J. Chem. Phys., Apr 2021

   Abs DOI

   The vibrational spectra of gas phase tert-butyl hydroperoxide have been recorded in the OH-stretching fundamental and overtone regions (ΔvOH = 1–5) at room temperature using conventional Fourier transform infrared (ΔvOH = 1–3) and cavity ring-down (ΔvOH = 4–5) spectroscopy. In hydroperoxides, the OH-stretching and COOH torsion vibrations are strongly coupled. The double-well nature of the COOH torsion potential leads to tunneling splitting of the energy levels and, combined with the low frequency of the torsional vibration, results in spectra in the OH-stretching regions with multiple vibrational transitions. In each of the OH-stretching regions, both an OH-stretching and a stretch–torsion combination feature are observed, and we show direct evidence for the tunneling splitting in the OH-stretching fundamental region. We have developed two complementary vibrational models to describe the spectra of the OH-stretching regions, a reaction path model and a reduced dimensional local mode model, both of which describe the features of the vibrational spectra well. We also explore the torsional dependence of the OH-stretching transition dipole moment and show that a Franck–Condon treatment fails to capture the intensity in the region of the stretch–torsion combination features. The accuracy of the Franck–Condon treatment of these features improves with increasing ΔvOH.