2 research outputs found
Beyond Local Group Modes in Vibrational Sum Frequency Generation
We combine deuterium labeling, density
functional theory calculations,
and experimental vibrational sum frequency generation spectroscopy
into a form of “counterfactual-enabled molecular spectroscopy”
for producing reliable vibrational mode assignments in situations
where local group mode approximations are insufficient for spectral
interpretation and vibrational mode assignments. We demonstrate the
method using <i>trans</i>-β-isoprene epoxydiol (<i>trans</i>-β-IEPOX), a first-generation product of isoprene
relevant to atmospheric aerosol formation, and one of its deuterium-labeled
isotopologues at the vapor/silica interface. We use our method to
determine that the SFG responses that we obtain from <i>trans</i>-β-IEPOX are almost exclusively due to nonlocal modes involving
multiple C–H groups oscillating at the same frequency as one
vibrational mode. We verify our assignments using deuterium labeling
and use DFT calculations to predict SFG spectra of additional isotopologues
that have not yet been synthesized. Finally, we use our new insight
to provide a viable alternative to molecular orientation analysis
methods that rely on local mode approximations in cases where the
local mode approximation is not applicable
Accurate Line Shapes from Sub‑1 cm<sup>–1</sup> Resolution Sum Frequency Generation Vibrational Spectroscopy of α‑Pinene at Room Temperature
Despite the importance of terpenes
in biology, the environment,
and catalysis, their vibrational spectra remain unassigned. Here,
we present subwavenumber high-resolution broad-band sum frequency
generation (HR-BB-SFG) spectra of the common terpene (+)-α-pinene
that reveal 10 peaks in the C–H stretching region at room temperature.
The high spectral resolution resulted in spectra with more and better
resolved spectral features than those of the Fourier transform infrared,
femtosecond stimulated Raman spectra in the bulk condensed phase and
those of the conventional BB-SFG and scanning SFG spectroscopy of
the same molecule on a surface. Experiment and simulation show the
spectral line shapes with HR-BB-SFG to be accurate. Homogeneous vibrational
decoherence lifetimes of up to 1.7 ps are assigned to specific oscillators
and compare favorably to lifetimes computed from density functional
tight binding molecular dynamics calculations. Phase-resolved spectra
provided their orientational information. We propose the new spectroscopy
as an attractive alternative to time domain vibrational spectroscopy
or heterodyne detection schemes for studying vibrational energy relaxation
and vibrational coherences in molecules at molecular surfaces or interfaces