A pedagogical overview of intramolecular vibrational redistribution (IVR) phenomena in vibrationally excited molecules is presented. In the interest of focus and simplicity, the topics covered deal primarily with IVR in the ground electronic state, relying on examples from the literature to illustrate key points. The experimental topics discussed attempt to sample systematically three different energy regimes on the full potential surface corresponding to (i) “low”, e.g., moderate- to high-resolution vibrational spectroscopies, (ii) “intermediate”, e.g., stimulated emission pumping and high overtone spectroscopies, and (iii) “high”, e.g., photofragment/predissociation dynamical spectroscopies. The interplay between experiment and theory is highlighted here because it has facilitated enormous advances in the field over the past decade.

Vibrational Energy Flow in Highly Excited Molecules: Role of Intramolecular Vibrational Redistribution

The structure of microsolvated benzene derivatives and the role of aromatic substituents.

When confined to the nanoscale, the glass transition temperature (Tg) of polymer films can deviate substantially from the bulk, i.e., the Tg-confinement effect. Due to ease of processing most studies have focused on the thickness-dependent Tg of thin films, while few have focused on extending investigations beyond thin films to other geometries. As polymers confined to higher geometrical dimensionalities become the enabling material in technologies ranging from drug delivery to plastic electronics to ultrafiltration, a greater understanding of size effects on the Tg is warranted. Here, we investigate the effects of three-dimensional confinement on the Tg of polymer nanoparticles under soft and hard confinement and quantitatively compare our results to those of thin films to explore commonalities or differences between the Tg-confinement effect for polymers confined to different geometries. Via modulated differential scanning calorimetry, we show that Tg decreases with size for polystyrene (PS) nanoparticl...

Glass Transition Temperature of Polymer Nanoparticles under Soft and Hard Confinement

It is generally believed that the strength of the polymer–nanoparticle interaction controls the modification of near-interface segmental mobility in polymer nanocomposites (PNCs). However, little is known about the effect of covalent bonding on the segmental dynamics and glass transition of matrix-free polymer-grafted nanoparticles (PGNs), especially when compared to PNCs. In this article, we directly compare the static and dynamic properties of poly(2-vinylpyridine)/silica-based nanocomposites with polymer chains either physically adsorbed (PNCs) or covalently bonded (PGNs) to identical silica nanoparticles (RNP = 12.5 nm) for three different molecular weight (MW) systems. Interestingly, when the MW of the matrix is as low as 6 kg/mol (RNP/Rg = 5.4) or as high as 140 kg/mol (RNP/Rg= 1.13), both small-angle X-ray scattering and broadband dielectric spectroscopy show similar static and dynamic properties for PNCs and PGNs. However, for the intermediate MW of 18 kg/mol (RNP/Rg = 3.16), the difference betwee...

Controlling Interfacial Dynamics: Covalent Bonding versus Physical Adsorption in Polymer Nanocomposites

Recent experimental results on the dynamics of glass-forming materials, particularly polymers, are surveyed. The focus is on aspects of the behavior that are connected to or correlated with structural relaxation. These results include the invariance to thermodynamic conditions (temperature, pressure, volume) of a number of properties—breadth of the relaxation dispersion, number of dynamically correlating molecules, Johari−Goldstein secondary relaxation time, onset of the dynamic crossover, and the product of temperature and specific volume with the latter raised to a material constant—provided the structural relaxation time is maintained constant. Additional salient experimental findings include the correlation of various high-frequency processes, usually measured in the glassy state, with properties of the equilibrium material above Tg. These correlations indicate that the glass transition, although conventionally defined by the relaxation time becoming larger than experimental time scales (>100 s), has ...

Relaxation Phenomena in Vitrifying Polymers and Molecular Liquids

The torsional tunneling splittings of the asymmetric C–H stretches (ν2 and ν9) in methanol are inverted with the E level lower in energy than the A level, whereas the symmetric C–H stretch (ν3) is normal with A below E. An internal coordinate model, which treats the torsion and the three C–H stretches simultaneously, accounts for the observed tunneling splittings. The model parameters are the local stretching frequency ω=2934.0 cm−1, the direct local–local coupling λ=−42.2 cm−1, and a single stretch-torsion coupling parameter μ=12.9 cm−1. The torsion-vibration coupling is nonadiabatic in the sense that it is not consistent with a Born–Oppenheimer separation of the torsion from the other vibrations. The fact that the model is based largely on the G6 molecular symmetry suggests that tunneling inversion may be common in torsional molecules. The torsionally mediated couplings among the C–H stretches do not conserve symmetry in the Cs point group and are strong enough to contribute to rapid intramolecular vibrational redistribution (IVR).

An internal coordinate model of coupling between the torsion and C–H vibrations in methanol

Sub-Doppler Infrared Spectra and Torsion–Rotation Energy Manifold of Methanol in the CH-Stretch Fundamental Region

Previous work by the present authors (Can. J. Chem. 72, 652 (1994)) pointed out the acceleration of IVR in flexible molecules when the prepared vibration is close to the centre of flexibility (COF). A COF was defined as a bond about which hindered internal rotation can occur thereby giving rise to molecular flexibility. Here, it is shown that the rate of IVR is inversely correlated with the height of the barrier to internal rotation for systems in which the prepared vibration is adjacent to the COF. In ethanol and hydrogen peroxide the barriers are low ( 380 cm −1 ) and the relaxation of the adjacent O-H vibration is fast (4 to 26 ps). On the other hand, higher barrier torsions (1100 to 1700 cm −1 ) adjacent to the chromophore in 1-butyne, 2-fluoroethanol, and 1,2-difluoroethane give rise to much longer IVR lifetimes (270 to 565 ps). Most of the IVR lifetimes used in this paper were derived from discretely resolved spectra in which a bright state transition is fragmented into a clump of molecular eigenstates; the remainder are from rotationally selected double resonance spectra. An algorithm is described for the derivation of consistent IVR lifetimes for coupling cases ranging from intermediate down to the very sparse limit where only a few perturbing states are explicitly observed in the spectrum

The Effect of the Torsional Barrier Height on the Acceleration of Intramolecular Vibrational Relaxation (IVR) by Molecular Flexibility

The understanding of size-dependent properties is key to the implementation of nanotechnology. One controversial and unresolved topic is the influence of characteristic size on the glass transition temperature (Tg) for ultrathin films and other nanoscale geometries. We show that Tg does depend on size for polystyrene spherical domains with diameters from 20 to 70 nm which are formed from phase separation of diblock copolymers containing a poly(styrene-co-butadiene) soft block and a polystyrene hard block. A comparison of our data with published results on other block copolymer systems indicates that the size dependence of Tg is a consequence of diffuse interfaces and does not reflect an intrinsic size effect. This is supported by our measurements on 27 nm polystyrene domains in a styrene-isobutylene-styrene triblock copolymer which indicate only a small Tg depression (3 K) compared to bulk behavior. We expect no effect of size on Tg in the limit as the solubility parameters of the hard and soft blocks div...

/pdf/effect-of-nanoscale-confinement-on-glass-transition-of-2ddk12l0hj.pdf

Effect of nanoscale confinement on glass transition of polystyrene domains from self-assembly of block copolymers.

High-Resolution Infrared Spectra in the C–H Region of CH2F2: The ν6and 2ν2Bands☆

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