Low-Stakes, Growth-Oriented Testing in Large-Enrollment General Chemistry 1: Formulation, Implementation, and Statistical Analysis.

We formulate an alternative to high-stakes examinations that is designed to help students grow, and we describe its implementation in a large-enrollment General Chemistry 1 class. In our alternative grading approach, students complete weekly assessments. Each assessment has four items that are aligned to explicit learning objectives and a level in Marzano's taxonomy, , , , and , which can be used by students and instructors to gauge the progression of student learning.

Rotationally-Resolved Two-Dimensional Infrared Spectroscopy of CO(g): Rotational Wavepackets and Angular Momentum Transfer.

Angular momentum transfer and wavepacket dynamics of CO(g) were measured on the picosecond time scale using polarization-resolved two-dimensional infrared (2D-IR) spectroscopy. The dynamics of rotational levels up to ≈ 50 are observed simultaneously at room temperature. Rotational wavepackets launched by the pump pulses cause oscillations in the intensity of individual peaks and beating patterns in the 2D-IR spectra.

pH Jumps in a Protic Ionic Liquid Proceed by Vehicular Proton Transport.

The dynamics of excess protons in the protic ionic liquid (PIL) ethylammonium formate (EAF) have been investigated from femtoseconds to microseconds using visible pump mid-infrared probe spectroscopy. The pH jump following the visible photoexcitation of a photoacid (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt, HPTS) results in proton transfer to the formate of the EAF. The proton transfer predominantly (∼70%) occurs over picoseconds through a preformed hydrogen-bonded tight complex between HPTS and EAF.

CH Mode Mixing Determines the Band Shape of the Carboxylate Symmetric Stretch in Apo-EDTA, Ca-EDTA, and Mg-EDTA.

The infrared spectra of EDTA complexed with Ca and Mg contain, to date, unidentified vibrational bands. This study assigns the peaks in the linear and two-dimensional infrared spectra of EDTA, with and without either Ca or Mg ions. Two-dimensional infrared spectroscopy and DFT calculations reveal that, in both the presence and absence of ions, the carboxylate symmetric stretch and the terminal CH bending vibrations mix.

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

Using ultrafast two-dimensional infrared spectroscopy (2D-IR), a vibrational probe (thiocyanate, SCN) was used to investigate the hydrogen bonding network of the protic ionic liquid ethyl-ammonium nitrate (EAN) in comparison to HO. The 2D-IR experiments were performed in both parallel (⟨ZZZZ⟩) and perpendicular (⟨ZZXX⟩) polarizations at room temperature. In EAN, the non-Gaussian lineshape in the FTIR spectrum of SCN suggests two sub-ensembles.

Intramolecular Vibrational Energy Relaxation of CO in Cross-Linked Poly(ethylene glycol) Diacrylate-Based Ion Gels.

Ultrafast two-dimensional infrared spectroscopy (2D-IR) and Fourier transform infrared spectroscopy (FTIR) were used to measure carbon dioxide (CO) in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][TfN]), cross-linked low-molecular-weight poly(ethylene glycol) diacrylate (PEGDA), and an ion gel composed of a 50 vol % blend of the two. The center frequency of the antisymmetric stretch, ν, of CO shifts monotonically to lower wavenumbers with increasing polymer content, with the largest line width in the ion gel (6 cm). Increasing polymer content slows both spectral diffusion and vibrational energy relaxation (VER) rates.

Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction.

Vibrational spectroscopy is an essential tool in chemical analyses, biological assays, and studies of functional materials. Over the past decade, various coherent nonlinear vibrational spectroscopic techniques have been developed and enabled researchers to study time-correlations of the fluctuating frequencies that are directly related to solute-solvent dynamics, dynamical changes in molecular conformations and local electrostatic environments, chemical and biochemical reactions, protein structural dynamics and functions, characteristic processes of functional materials, and so on. In order to gain incisive and quantitative information on the local electrostatic environment, molecular conformation, protein structure and interprotein contacts, ligand binding kinetics, and electric and optical properties of functional materials, a variety of vibrational probes have been developed and site-specifically incorporated into molecular, biological, and material systems for time-resolved vibrational spectroscopic investigation.

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: III. Dynamics and Spectroscopy.

In recent years, interest in carbon capture and sequestration has led to numerous investigations of the ability of ionic liquids to act as recyclable CO-sorbent materials. Herein, we investigate the structure and dynamics of a model physisorbing ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([CCIm][PF]), from the perspective of CO using two-dimensional (2D) IR spectroscopy and molecular dynamics simulations. A direct comparison of experimentally measured and calculated 2D IR line shapes confirms the validity of the simulations and spectroscopic calculations.

Enthalpic Driving Force for the Selective Absorption of CO by an Ionic Liquid.

Molecular dynamics (MD) simulations validated against two-dimensional infrared (2D-IR) measurements of CO in an imidazolium-based ionic liquid have revealed new insights into the mechanism of CO solvation. The first solvation shell around CO has a distinctly quadrupolar structure, with strong negative charge density around the CO carbon atom and positive charge density near the CO oxygen atoms. When CO is modeled without atomic charges (thus removing its strong quadrupole moment), its solvation shell weakens and changes significantly into a structure that is similar to that of N in the same liquid.

Ultrafast dynamics of ionic liquids in colloidal dispersion.

Ionic liquid (IL)-surfactant complexes have significance both in applications and fundamental research, but their underlying dynamics are not well understood. We apply polarization-controlled two-dimensional infrared spectroscopy (2D-IR) to study the dynamics of [BMIM][SCN]/surfactant/solvent model systems. We examine the effect of the choice of surfactants and solvent, and the IL-to-surfactant ratio (W-value), with a detailed analysis of the orientation and structural dynamics of each system.

Temperature and chain length dependence of ultrafast vibrational dynamics of thiocyanate in alkylimidazolium ionic liquids: A random walk on a rugged energy landscape.

Ultrafast two-dimensional infrared spectroscopy of a thiocyanate vibrational probe (SCN) was used to investigate local dynamics in alkylimidazolium bis-[trifluoromethylsulfonyl]imide ionic liquids ([Im][TfN], n = 2, 4, 6) at temperatures from 5 to 80 °C. The rate of frequency fluctuations reported by SCN increases with increasing temperature and decreasing alkyl chain length. Temperature-dependent correlation times scale proportionally to temperature-dependent bulk viscosities of each ionic liquid studied.

Reorientation-induced spectral diffusion of non-isotropic orientation distributions.

When reorientation of a vibrational chromophore is faster than the relaxation of its local environment, the frequency fluctuation correlation function (FFCF) measured by two-dimensional infrared spectroscopy (2D-IR) spectroscopy is an interplay of scalar structural spectral diffusion and vectorial reorientation-induced spectral diffusion (RISD). Theory has been established to calculate the RISD component of different polarization configurations with the assumption that the molecule orients randomly in a local electric field. We show here that in the [BMIM][SCN]/AOT/chlorobenzene system, where the local electric field is strong, this assumption is incapable of reproducing the experimental results.

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: II. Spectroscopic Map.

The primary challenge for connecting molecular dynamics (MD) simulations to linear and two-dimensional infrared measurements is the calculation of the vibrational frequency for the chromophore of interest. Computing the vibrational frequency at each time step of the simulation with a quantum mechanical method like density functional theory (DFT) is generally prohibitively expensive. One approach to circumnavigate this problem is the use of spectroscopic maps.

Two-dimensional ultrafast vibrational spectroscopy of azides in ionic liquids reveals solute-specific solvation.

The stereochemistry and the reaction rates of bimolecular nucleophilic substitution reactions involving azides in ionic liquids are governed by solute-solvent interactions. Two-dimensional ultrafast vibrational spectroscopy (2D-IR) shows that the picosecond dynamics of inorganic azides are substantially slower than organic azides in a series of homologous imidazolium ionic liquids. In water, both organic and inorganic azides spectrally diffuse with a ∼2 ps time constant.

Thiocyanate as a local probe of ultrafast structure and dynamics in imidazolium-based ionic liquids: water-induced heterogeneity and cation-induced ion pairing.

Ultrafast two-dimensional infrared spectroscopy (2D-IR) of thiocyanate ([SCN]−) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1C1(2)im][NTf2]) ionic liquids probes local structure and dynamics as a function of the water content, solute counterion, and solute concentration. The 2D-IR spectra of the water-saturated ionic liquids resolve two distinct kinds of dynamics. This dynamical heterogeneity is explained as two subensembles, one with and one without a water molecule in the first solvation shell.

Azide-water intermolecular coupling measured by two-color two-dimensional infrared spectroscopy.

We utilize two-color two-dimensional infrared spectroscopy to measure the intermolecular coupling between azide ions and their surrounding water molecules in order to gain information about the nature of hydrogen bonding of water to ions. Our findings indicate that the main spectral contribution to the intermolecular cross-peak comes from population transfer between the asymmetric stretch vibration of azide and the OD-stretch vibration of D(2)O. The azide-bound D(2)O bleach/stimulated emission signal, which is spectrally much narrower than its linear absorption spectrum, shows that the experiment is selective to solvation shell water molecules for population times up to ~500 fs.

Three-dimensional infrared spectroscopy of isotope-substituted liquid water reveals heterogeneous dynamics.

The dynamics of the hydrogen bond network of isotopically substituted liquid water are investigated with a new ultrafast nonlinear vibrational spectroscopy, three-dimensional infrared spectroscopy (3D-IR). The 3D-IR spectroscopy is sensitive to three-point frequency fluctuation correlation functions, and the measurements reveal heterogeneous structural relaxation dynamics. We interpret these results as subensembles of water which do not interconvert on a half picosecond time scale.

Enhancing signal detection and completely eliminating scattering using quasi-phase-cycling in 2D IR experiments.

We demonstrate how quasi-phase-cycling achieved by sub-cycle delay modulation can be used to replace optical chopping in a box-CARS 2D IR experiment in order to enhance the signal size, and, at the same time, completely eliminate any scattering contamination. Two optical devices are described that can be used for this purpose, a wobbling Brewster window and a photoelastic modulator. They are simple to construct, easy to incorporate into any existing 2D IR setup, and have attractive features such as a high optical throughput and a fast modulation frequency needed to phase cycle on a shot-to-shot basis.

Structural inhomogeneity of water by complex network analysis.

There is still an open debate regarding the structure forming capabilities of water at ambient conditions. To probe the presence of such inhomogeneities, we apply complex network analysis methods to a molecular dynamics simulation at room temperature. This study provides both a structural and quantitative characterization of kinetically homogeneous substates present in bulk water.

The OH stretch vibration of liquid water reveals hydrogen-bond clusters.

Using molecular dynamics simulations, we investigate the fluctuations of the hydrogen-bond network in liquid water and its relation to vibrational spectroscopy. We show that the high-frequency shoulder, which is most evidently found in Raman spectra of the OH stretch vibration of isotope diluted water, is (at least to a certain extent) related to a three-fold hydrogen-bonded ring. This suggests that it is not always sufficient to classify individual water molecules when studying some aspects of hydrogen-bond dynamics, rather, one should consider the topology of the local structure around a given water molecule.