Publications by authors named "Shawn Sandhu"

The formation of following the double ionization of small organic compounds via a roaming mechanism, which involves the generation of H and subsequent proton abstraction, has recently garnered significant attention. Nonetheless, a cohesive model explaining trends in the yield of characterizing these unimolecular reactions is yet to be established. We report yield and femtosecond time-resolved measurements following the strong-field double ionization of CHX molecules, where X = OD, Cl, NCS, CN, SCN, and I.

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  • * Understanding RNA ensembles involves analyzing free energy landscapes, which show the relative stability of different conformations and the barriers between them.
  • * Recent research indicates that while RNA behavior varies in cellular contexts compared to in vitro, integrating quantitative measurements with computational methods can enhance predictions of RNA function and lead to new biological insights.
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Identifying the short-lived intermediates and reaction mechanisms of multi-channel radical cation fragmentation processes remains a current and important challenge to understanding and predicting mass spectra. We find that coherent oscillations in the femtosecond time-dependent yields of several product ions following ultrafast strong-field ionization represent spectroscopic signatures that elucidate their mechanism of formation and identify the intermediate(s) they originate from. Experiments on endo-dicyclopentadiene show that vibrational frequencies from various intermediates are mapped onto their resulting products.

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We report femtosecond time-resolved measurements of the McLafferty rearrangement following the strong-field tunnel ionization of 2-pentanone, 4-methyl-2-pentanone, and 4,4-dimethyl-2-pentanone. The pump-probe-dependent yields of the McLafferty product ion are fit to a biexponential function with fast (∼100 fs) and slow (∼10 ps) time constants, the latter of which is faster for the latter two compounds. Following nearly instantaneous ionization, the fast time scale is associated with rotation of the molecule to a six-membered cyclic intermediate that facilitates transfer of the γ-hydrogen, while the ∼50-100 times longer time scale is associated with a π-bond rearrangement and bond cleavage between the α- and β-carbons to produce the enol cation.

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  • The study investigates how three hydrogen atoms in cyclopropane dication can form hydrogen cations (H) through a reaction mechanism, which may be significant in interstellar chemistry.
  • Measurements reveal that the time taken to form H is about 249 ± 16 femtoseconds, suggesting a process that begins with ring-opening rather than a direct concerted mechanism.
  • Further analysis shows that both cyclopropane and its isomer, propene, share similar H formation times and support a reaction sequence involving ring opening, H-migration, and the roaming of neutral H before forming H, contributing to our understanding of gas-phase chemical reactions.
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Cyanine molecules are important phototheranostic compounds given their high fluorescence yield in the near-infrared region of the spectrum. We report on the frequency and time-resolved spectroscopy of the S state of IR806, which demonstrates enhanced emission upon binding to the hydrophobic pocket of human serum albumin (HSA). From excitation-emission matrix spectra and electronic structure calculations, we identify the emission as one associated with a state having the polymethine chain twisted out of plane by 103°.

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Background: Although there are studies investigating the pathologic origins of mild cognitive impairment (MCI), they have revolved around comparisons to normal elderly individuals or those with Alzheimer's disease (AD) or other dementias. There are few studies directly comparing the comprehensive neuropathology of amnestic (aMCI) and nonamnestic (naMCI) MCI.

Methods: The database of the Brain and Body Donation Program ( www.

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Objective: To determine whether Apolipoprotein E4 (Apo E4) gene status or ApoE gene dose affect the lipid profile in AD.

Background: Links between hypercholesterolemia and AD development continue to grow. Presently, limited information exists about the influence of the Apo E genotype on the lipid profile characteristics in AD.

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