An ultrafast vibrational study of dynamical heterogeneity in the protic ionic liquid ethyl-ammonium nitrate. I. Room temperature dynamics

Johnson, Clinton; Parker, Anthony W.; Donaldson, Paul M.; Garrett-Roe, Sean

doi:10.1063/5.0044822

Cited by 16 publications

(35 citation statements)

References 65 publications

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“…Usually, 1/3 T or –1 does not contribute much to T Hom –1 , so we think this is a reasonable approximation for convenience, though model fitting to polarization-dependent spectra to fit T or has been done. 32 …”

Section: Resultsmentioning

confidence: 99%

Least-Squares Fitting of Multidimensional Spectra to Kubo Line-Shape Models

Robben

Cheatum

2021

J. Phys. Chem. B

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We report a comprehensive study of the efficacy of least-squares fitting of multidimensional spectra to generalized Kubo line-shape models and introduce a novel least-squares fitting metric, termed the scale invariant gradient norm (SIGN), that enables a highly reliable and versatile algorithm. The precision of dephasing parameters is between 8× and 50× better for nonlinear model fitting compared to that for the centerline-slope (CLS) method, which effectively increases data acquisition efficiency by 1–2 orders of magnitude. Whereas the CLS method requires sequential fitting of both the nonlinear and linear spectra, our model fitting algorithm only requires nonlinear spectra but accurately predicts the linear spectrum. We show an experimental example in which the CLS time constants differ by 60% for independent measurements of the same system, while the Kubo time constants differ by only 10% for model fitting. This suggests that model fitting is a far more robust method of measuring spectral diffusion than the CLS method, which is more susceptible to structured residual signals that are not removable by pure solvent subtraction. Statistical analysis of the CLS method reveals a fundamental oversight in accounting for the propagation of uncertainty by Kubo time constants in the process of fitting to the linear absorption spectrum. A standalone desktop app and source code for the least-squares fitting algorithm are freely available, with example line-shape models and data. We have written the MATLAB source code in a generic framework where users may supply custom line-shape models. Using this application, a standard desktop fits a 12-parameter generalized Kubo model to a 106 data-point spectrum in a few minutes.

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Section: Resultsmentioning

confidence: 99%

Least-Squares Fitting of Multidimensional Spectra to Kubo Line-Shape Models

Robben

Cheatum

2021

J. Phys. Chem. B

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“…T or −1 doesn't contribute much to T hom −1 , so we think this is a reasonable approximation for convenience, though model fitting to polarization dependent spectra to fit T or has been done. 17 For comparison, we also analyze the simulated data using the center-line-slope (CLS) method. 12,13 One feature of 2D Kubo lineshapes is that they are asymmetric in frequency and the asymmetry is, itself, frequency dependent (see Figure S1).…”

Section: A Fitting Simulated Datamentioning

confidence: 99%

“…This manuscript provides a comprehensive study of how fitting generalized Kubo lineshapes to multidimensional spectra compares to the CLS method in terms of the accuracy, precision and reliability of the resulting parameters. While Garret-Roe and coworkers have shown several examples of model fitting 2D IR waiting-time series using the fmincon function in MATLAB, [17][18][19][20] a comparison of accuracy between model fitting and the CLS method by fitting to simulated spectra, where true values of parameters are known, has been missing. We find our model fitting routine improves precision over the CLS method by 8× to 15× on average for Kubo time constants and 8× to 50× for Kubo amplitudes and homogeneous dephasing, which is due in part to a novel figure of merit used in our fitting algorithm that we refer to as the scale invariant gradient norm (SIGN).…”

Section: Introductionmentioning

confidence: 99%

Least-Squares Fitting of Multidimensional Spectra to Kubo Lineshape Models

Robben

Cheatum

2021

Preprint

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We report a comprehensive study of the efficacy of least-squares fitting of multidimensional spectra to generalized Kubo lineshape models and introduce a novel least-squares fitting metric, termed the Scale Invariant Gradient Norm (SIGN), that enables a highly reliable and versatile algorithm. The precision of dephasing parameters is between 8× to 50× better for nonlinear model fitting compared to the CLS method, which effectively increases data acquisition efficiency by one to two orders of magnitude. Whereas the center-line-slope (CLS) method requires sequential fitting of both the nonlinear and linear spectra, our model fitting algorithm only requires nonlinear spectra, but accurately predicts the linear spectrum. We show an experimental example in which the CLS time constants differ by 60% for independent measurements of the same system, while the Kubo time constants differ by only 10% for model fitting. This suggests that model fitting is a far more robust method of measuring spectral diffusion than the CLS method, which is more susceptible to structured residual signals that are not removable by pure solvent subtraction. Statistical analysis of the CLS method reveals a fundamental oversight in accounting for the propagation of uncertainty by Kubo time constants in the process of fitting to the linear absorption spectrum. A standalone desktop app and source code for the least-squares fitting algorithm are freely available with example lineshape models and data. We have written the MATLAB source code in a generic framework where users may supply custom lineshape models. Using this application, a standard desktop fits a 12-parameter generalized Kubo model to a 106 data-point spectrum in a few minutes.

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“…In other work, Maroncelli's group showed that the C153 solvation times in several IL classes (imidazolium, ammonium, and phosphonium) follows a well-defined linear correlation with viscosity that was a linear extension of that observed in conventional dipolar solvents [24,25]. To date, the role that IL-IL and/or IL + cosolvent interactions play in modified IL solutions remains a topic interest [2,7,15,16,18,[40][41][42][43][44].…”

Section: Introductionmentioning

confidence: 98%

“…Dynamical measurements from extrinsic probes provides a useful perspective for determining the range of effects of the molecular interactions that govern probe-solvent interactions. In a very recent report, Garrett-Roe and coworkers identified two sub-ensembles in EAN from SCN − vibrational and rotational relaxation using 2-D IR spectroscopy, giving yet further evidence of the dynamical heterogeneity that is observed in ILs [43]. Given the inherent nanoheterogeneity within ILs and IL solutions, we set upon the task of evaluating whether hydrodynamic theory provided a good description against which we could assess probe rotational motion in the EAN/MeOH system.…”

Section: Introductionmentioning

confidence: 99%

Coumarin 153 Dynamics in Ethylammonium Nitrate: The Effects of Dilution with Methanol

Heitz

Sabo

Robillard

2021

Sustainable Chemistry

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Magic angle intensity decay and dynamic fluorescence anisotropy measurements were made on the binary solvent system composed of ethylammonium nitrate ([N2,0,0,0+][NO3−], EAN) + methanol (MeOH) across the complete EAN mole fraction range (xIL = 0–1) using the neutral dipolar solute coumarin 153 (C153) at 295 K. Stokes–Einstein–Debye (SED) hydrodynamic theory was used as a model framework to assess the C153 rotational reorientation dynamics. Departure from stick SED prediction was observed (in contrast to literature reports that used cationic or anionic dyes) and indicated a significant influence of domain nanoheterogeneity on probe dynamics. Steady-state spectroscopy indicated minimal changes in spectral peak and width with mole fraction, except at xIL = 0.3 where absorption widths decreased by ~170 cm−1, signaling that C153 sensed a change in solution heterogeneity. Magic angle intensity decays corroborated the steady-state observation and the excited-state lifetimes showed a marked change from xIL = 0.2–0.4 where EAN-EAN interactions became notably more significant. C153 average rotation times (⟨τrot⟩) showed significant solvent decoupling with increased EAN. The rotational data were fit to a power law dependence, ⟨τrot⟩ ∝ (ηT)p, where p = 0.82, demonstrating the presence of dynamic heterogeneity in the EAN/MeOH solutions. With increased EAN, rotation times showed that the heterogeneity became increasingly more significant since the rotation times systematically decreased away from the hydrodynamic stick limit.

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An ultrafast vibrational study of dynamical heterogeneity in the protic ionic liquid ethyl-ammonium nitrate. I. Room temperature dynamics

Abstract: This is a copy of the published version, or version of record, available on the publisher's website. This version does not track changes, errata, or withdrawals on the publisher's site.

Cited by 16 publications

References 65 publications

Least-Squares Fitting of Multidimensional Spectra to Kubo Line-Shape Models

Least-Squares Fitting of Multidimensional Spectra to Kubo Line-Shape Models

Least-Squares Fitting of Multidimensional Spectra to Kubo Lineshape Models

Coumarin 153 Dynamics in Ethylammonium Nitrate: The Effects of Dilution with Methanol

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