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CLS Next Gen: Accurate Frequency–Frequency Correlation Functions from Center Line Slope Analysis of 2D Correlation Spectra Using Artificial Neural Networks
journal contribution
posted on 2020-07-03, 15:33 authored by David
J. Hoffman, Michael D. FayerThe center line slope
(CLS) observable has become a popular method
for characterizing spectral diffusion dynamics in two-dimensional
(2D) correlation spectroscopy because of its ease of implementation,
robustness, and clear theoretical relationship to the frequency–frequency
correlation function (FFCF). The FFCF relates the frequency fluctuations
of an ensemble of chromophores to coupled bath modes of the chemical
system and is used for comparison to molecular dynamics simulations
and for calculating 2D spectra. While in the appropriate limits, the
CLS can be shown to be the normalized FFCF, from which the full FFCF
can be obtained, in practice the assumptions that relate the CLS to
the normalized FFCF are frequently violated. These violations are
due to the presence of homogeneous broadening and motional narrowing.
The generalized problem of relating the CLS to the FFCF is reanalyzed
by introducing a new set of dimensionless parameters for both the
CLS and FFCF. A large data set was generated of CLS parameters derived
from numerically modeled 2D line shapes with known FFCF parameters.
This data set was used to train feedforward artificial neural networks
that act as functions, which take the CLS parameters as inputs and
return FFCF parameters. These neural networks were deployed in an
algorithm that is able to quickly and accurately determine FFCF parameters
from experimental CLS parameters and the fwhm of the absorption line
shape. The method and necessary inputs to accurately obtain the FFCF
from the CLS are presented.