Optical heterodyne-detected optical Kerr effect ͑OHD-OKE͒ experiments are used to study the orientational dynamics of the ionic organic liquid 1-ethyl-3-methylimidazolium nitrate (EMIM ϩ NO 3 Ϫ) over time scales from ϳ1 ps to ϳ2 ns, and the temperatures range from 410 to 295 K. The temperatures cover the normal liquid state and the weakly supercooled state. The orientational dynamics exhibit characteristics typical of normal organic glass-forming liquids. The longest time scale portion of the data decays as a single exponential and obeys the Debye-Stokes-Einstein relation. The decay of the OHD-OKE signal begins (ϳ1 ps) with a temperature independent power law, t Ϫz , zϭ1.02Ϯ0.05, the ''intermediate power law.'' The power law decay is followed by a crossover region, modeled as a second power law, the von Schweidler power law. The longest time scale decay is the exponential ␣ relaxation. The intermediate power law decay has been observed in van der Waals supercooled liquids previously. These are the first such observations on an ionic organic liquid. The observation of the dynamical signatures observed in other liquids demonstrates that the orientational dynamics of ionic organic liquids are fundamentally the same as van der Waals liquids and supports the universality of the intermediate power law decay in the dynamics of complex liquids. Within the mode-coupling theory ͑MCT͒ framework, the MCT critical temperature T C is estimated to be T C Х255 K.

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In this work, four types of polarizable models have been developed for calculating interactions between atomic charges and induced point dipoles. These include the Applequist, Thole linear, Thole exponential model, and the Thole Tinker-like. The polarizability models have been optimized to reproduce the experimental static molecular polarizabilities obtained from the molecular refraction measurements on a set of 420 molecules reported by Bosque and Sales. We grouped the models into five sets depending on the interaction types, i.e. whether the interactions of two atoms that form bond, bond angle and dihedral angle are turned off or scaled down. When 1-2 (bonded), 1-3 (separated by two bonds) interactions are turned off and/or 1-4 (separated by three bonds) interactions are scaled down, all the models including the Applequist model achieved similar performance: the average percentage errors (APE) ranges from 1.15% to 1.23%, and The average unsigned errors (AUE) ranges from 0.143 to 0.158 Å3. When the short-range 1-2, 1-3 and full 1-4 terms are taken into account (Set D models), the APE ranges from 1.30% to 1.58% for the three Thole models whereas the Applequist model (DA) has a significantly larger APE (3.82%). The AUE ranges from 0.166 to 0.196 Å3 for the three Thole models, compared to 0.446 Å3 for the Applequist model. Further assessement using the 70-molecule van Duijnen and Swart data set clearly showed that the developed models are both accurate and highly transferable and are in fact more accurate than the models developed using this particular data set (Set E models). The fact that A, B, and C model sets are notably more accurate than both D and E model sets strongly suggests that the inclusion of 1-2 and 1-3 interactions reduces the transferability and accuracy.

In the companion paper, we presented a set of induced dipole interaction models using four types of screening functions, which include the Applequist (no screening), the Thole linear, the Thole exponential model, and the Thole Tinker-like (another form of exponential screening function) functions. In this work, we evaluate the performance of polarizability models using large set of amino acid analog pairs that are frequently observed in protein structures as a benchmark. For each amino acid pair we calculated quantum mechanical interaction energies at the MP2/aug-cc-pVTZ//MP2/6-311++G(d,p) level with the basis set superposition error (BSSE) correction and compared them with molecular mechanics results. Encouragingly, all the polarizable models significantly outperform the additive F94 and F03 models (mimicking AMBER ff94/ff99 and ff03 force fields, respectively) in reproducing the BSSE-corrected quantum mechanical interaction energies. Particularly, the root-mean-square errors (RMSE) for three Thole models in Set A (where the 1–2 and 1–3 interactions are turned off and all 1–4 interactions are included) are 1.456, 1.417 and 1.406 kcal/mol for Model AL (Thole Linear), Model AE (Thole exponential) and Model AT (Thole Tinker-like), respectively. In contrast, the RMSE are 3.729 and 3.433 kcal/mol for F94 and F03 models, respectively. A similar trend was observed for the average unsigned errors (AUE), which are 1.057, 1.025, 1.011, 2.219 and 2.070 kcal/mol for AL, AE, AT, F94/ff99 and F03, respectively. Analyses based on the trend line slopes indicate that the two fixed charge models substantially underestimate the relative strengths of non-charge-charge interactions by 24% (F03) and 35% (F94), respectively, whereas the four polarizable models over-estimate the relative strengths by 5% (AT), 3% (AL, AE) and 13% (AA), respectively. Agreement was further improved by adjusting the van der Waals parameters. Judging from the notably improved accuracy in comparison to the fixed charge models, the polarizable models are expected to form the foundation for the development of high quality polarizable force fields for protein and nucleic acid simulations.

A system has been created in which chiral information inherent in circularly polarized light can be used to influence the helical sense characteristics of a polymer. A racemic mixture of a photoresolvable ketone-containing group was appended through different linkage patterns to a polymer having dynamically interconverting equal populations of left- and right-handed helical backbone conformations. Irradiation with circularly polarized light in the ketone's chromophore gave rise to easily measurable circular dichroism signals in the polymer backbone helix, which changed sign with a change in the sense of the circularly polarized light. This demonstrates that the small enantiomeric excess produced by the irradiation, even diluted with large proportions of achiral pendants, is capable of enforcing a disproportionate excess of one helical sense in the polymer. The results, expressed as optical activity as a function of the degree of polymerization, could be analyzed using an approximate solution of a one-dimensional quenched random-field Ising model. The experimental data were fit to the theory and demonstrated a crossover between statistical and thermal randomness in the response of the helical sense to the chiral information generated by the light.