Static light scattering from aqueous solutions of polyethylene oxide) has been recorded over the temperature range 20-90 °C and at eight different concentrations varying from 0.25% to 2% for a polymer of molecular weight 20000. In spite of the low molecular weight, strong angular dependence of the scattering and a very high average particle weight Mw were found, indicating the presence of large aggregates of polyethylene oxide) in the aqueous solutions. The aggregation increases markedly upon heating; above 60 °C, however, a decrease in Mw occurs. At 40 °C minima and maxima in the angular distribution were observed that are characteristic of globular, fairly monodisperse structures. The dimensions of these globular particles were found to decrease with rising temperature, in contrast to the mean square radius of gyration (S2)*, which increases up to 70 °C. The second osmotic virial coefficient A2 decreases with rising temperature as the system approaches its temperature of 102 °C. Finally, the intrinsic viscosity in water is shown to be only slightly influenced by the presence of globular aggregates. These findings indicate the coexistence of high-density spherulites with low-density microgel aggregates and are discussed in terms of the hydrophobic interactions between the polymer and solvent.
VPT2+K spectroscopic constants and matrix elements of the transformed vibrational Hamiltonian of a polyatomic molecule with resonances using Van Vleck perturbation theory
Dispersed fluorescence spectra from the 000 rotational level of 40, 41, 51, and 3141 S1 formaldehyde (H2CO) have been recorded. From these spectra, 198 new vibrational states have been assigned with energies up to 12 500 cm−1, and their positions have been determined to within an uncertainty of 1 cm−1. The assignment of vibrational lines to specific vibrational states becomes increasingly difficult at the higher energy regions of the spectra (≳9000 cm−1) due to extensive state mixing. Harmonic and first-order anharmonic vibrational constants were extracted from fits to these vibrational states. For states with highest zero-order coefficient squared greater than 35%, the standard deviation of the spectroscopic fit is 6.9 cm−1. For states which are lower energy (<9500 cm−1) and relatively pure (zero-order coefficient squared greater than 0.75 or largest in a given normal mode combination), the standard deviation is 1.7 cm−1. Good agreement with ab initio vibrational constants calculated by Martin et al. [J. Mol. Spectrosc. 160, 105 (1993)] is achieved, except in cases where all observed states contributing to the determination of a particular constant are significantly mixed. These deviations are readily explained by a consideration of anharmonic vibrational interactions that occur among specific combinations of normal modes. The average mean deviation between all experimentally determined energies and a recent theoretical calculation by Burleigh et al. [J. Chem. Phys. 104, 480 (1996)] is 2.6 cm−1.
Spectra of S0 D2CO rovibrational eigenstates with 28 300 cm−1 of vibrational excitation are measured by Stark level-crossing spectroscopy. In this new method, the lifetime of a single J, K, M-resolved S1 state is monitored as a function of electric field. Enhanced nonradiative decay causes the S1 lifetime to decrease as S0 states are Stark tuned into resonance. Analysis of the resulting resonance lineshapes yields complete distributions of S0 decay rates (linewidths) and S1-S0 coupling matrix elements. The S0 decay rates represent the first measurements of unimolecular dissociation rates of a polyatomic molecule at the eigenstate-resolved level. S0 decay widths vary from 6.4×10−5 to 3.8×10−3 cm−1 and S1-S0 coupling matrix elements vary from 3.5×10−7 to 4.7×10−5 cm−1, demonstrating that chemical properties of neighboring eigenstates fluctuate by over two orders of magnitude. The observed density of S0 vibrational states is ∼400 per cm−1, six times greater than an estimate including first-order anharmonic corrections. The small increase of level density with J indicates that Ka is nearly a good quantum number for J≤4. The barrier height to unimolecular dissociation on the S0D2CO surface is determined to be 80.6±0.8 kcal/mol, corresponding to 79.2±0.8 kcal/mol for H2CO, in good agreement with ab initio predictions. Quantitative agreement between the magnitude of experimentally determined decay rates and an RRKM rate calculation with all parameters set by ab initio calculation is found.
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