The Nesbitt Laboratory pursues research in four main areas:
1) High resolution laser spectroscopy of radicals, ions and molecular ion clusters
2) Chemical Reaction Dynamics: Gas Phase and Interfaces
3) Quantum nanostructures and photonic nanomaterials
4) Single Molecule Biophysics: Microscopy, Kinetics and Thermodynamics
About the Nesbitt Laboratory
Highlighted Publications
The Role of Counterion Valence and Size in GAAA Tetraloop–Receptor Docking/Undocking Kinetics
The Role of Counterion Valence and Size in GAAA Tetraloop–Receptor Docking/Undocking Kinetics
For RNA to fold into compact, ordered structures, it must overcome electrostatic repulsion between negatively charged phosphate groups by counterion recruitment. A physical understanding of the counterion-assisted folding process requires addressing how cations kinetically and thermodynamically control the folding equilibrium for each tertiary interaction in a full‐length RNA. In this work, single-molecule FRET (fluorescence resonance energy transfer) techniques are exploited to isolate and explore the cation-concentration‐dependent kinetics for formation of a ubiquitous RNA tertiary interaction, that is, the docking/undocking of a GAAA tetraloop with its 11‐nt receptor. Rate constants for docking (kdock) and undocking (kundock) are obtained as a function of cation concentration, size, and valence, specifically for the series Na+, K+, Mg2 +, Ca2 +, Co(NH3)63 +, and spermidine3 +. Increasing cation concentration accelerateskdockdramatically but achieves only a slight decrease in kundock. These results can be kinetically modeled using parallel cation-dependent and cation‐independent docking pathways, which allows for isolation of the folding kinetics from the interaction energetics of the cations with the undocked and docked states, respectively. This analysis reveals a preferential interaction of the cations with the transition state and docked state as compared to the undocked RNA, with the ion–RNA interaction strength growing with cation valence. However, the corresponding number of cations that are taken up by the RNA upon folding decreases with charge density of the cation. The only exception to these behaviors is spermidine3 +, whose weaker influence on the docking equilibria with respect to Co(NH3)63 + can be ascribed to steric effects preventing complete neutralization of the RNA phosphate groups.
Large-amplitude dynamics in vinyl radical: The role of quantum tunneling as an isomerization mechanism
DOI:
Large-amplitude dynamics in vinyl radical: The role of quantum tunneling as an isomerization mechanism
We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated “reaction paths.” The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm−1. Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical.
Plasmonic Near-Electric Field Enhancement Effects in Ultrafast Photoelectron Emission: Correlated Spatial and Laser Polarization Microscopy Studies of Individual Ag Nanocubes
DOI:
Plasmonic Near-Electric Field Enhancement Effects in Ultrafast Photoelectron Emission: Correlated Spatial and Laser Polarization Microscopy Studies of Individual Ag Nanocubes
Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general.
Opportunities
The Nesbitt Group routinely has openings for postdoctoral associates and graduate students. In addition, the Nesbitt group also has traditionally made undergraduate research opportunities available for especially motivated and independent students.
Find out more on the Opportunities Page.