Rotational freedom

Calculate the rotational contribution to the entropy of adsorption of benzene on carbon at 35°C, assuming that the adsorbed benzene has one degree of rotational freedom. 

Calculate the rotational contribution to the entropy of adsorption of ammonia on silica at -30°C, assuming (n) that the adsorbed ammonia retains one degree of rotational freedom and (b) that it retains none. In case (n) assume that the nitrogen is bonded to the surface. 

Hermans, J., Wang, L. Inclusion of loss of translational and rotational freedom in theoretical estimates of free energies of binding. Application to a complex of benzene and mutant T4-lysozyme. J. Am. Chem. Soc. 119 (1997) 2707-2714... 

Below a temperature of Toi 260 K, the Ceo molecules completely lose two of their three degrees of rotational freedom, and the residual degree of freedom is a ratcheting rotational motion for each of the four molecules within the unit cell about a different (111) axis [43, 45, 46, 47]. The structure of solid Ceo below Tqi becomes simple cubic (space group Tji or PaS) with a lattice constant ao = 14.17A and four Ceo molecules per unit cell, as the four oriented molecules within the fee structure become inequivalent [see Fig. 2(a)] [43, 45]. Supporting evidence for the phase transition at Tqi 260 K is... 

Larger cycloalkanes have increasing rotational freedom, and the very large rings (C25 and up) are so floppy that they are nearly indistinguishable from open-chain alkanes. The common ring sizes (C3-C7L however, are severely restricted in their molecular motions. 

Nuclear magnetic resonance spectroscopy of the solutes in clathrates and low temperature specific heat measurements are thought to be particularly promising methods for providing more detailed information on the rotational freedom of the solute molecules and their interaction with the host lattice. The absence of electron paramagnetic resonance of the oxygen molecule in a hydroquinone clathrate has already been explained on the basis of weak orientational effects by Meyer, O Brien, and van Vleck.18... 

If the rotational freedom of the aryloxy group is restricted, as in 4-[(arylsulfonyl)methyl]-2H-l-benzopyranes (55), the remote group interaction is not possible anymore and these derivatives fragment mainly by ejecting ArS02H and S0238 (equation 26). 

Smectic phases are more highly ordered than nematic phases, and with an ordering of the molecules into layers. There are a number of different smectic phases which reflect differing degree of ordering. Crystal smectic phases are characterised by the appearance of inter-layer structural correlations and may in some cases be accompanied by a loss of molecular rotational freedom. 

Molecules in the gas phase have rotational freedom, and the vibrational transitions are accompanied by rotational transitions. For a rigid rotor that vibrates as a harmonic oscillator the expression for the available energy levels is ... 

Upon adsorption, the molecule loses its rotational freedom and so only the vibrational transition is observed (but at a different frequency, see Fig. 4.22). For CO, three factors contribute to this shift ... 

In the usual chemical formulas written for chain polymers the sue-cessive units are projected as a co-linear sequence on the surface of the sheet of paper. This form of representation fails to convey what is perhaps the most significant structural characteristic of a long polymer chain, namely, its capacity to assume an enormous array of configurations. This configurational versatility is a consequence of the considerable degree of rotational freedom about single bonds of the chain. In the simple polymethylene chain, for example, the conventional formula... 

IR and Raman spectroscopic studies on films and powders of PDHS indicate that the hexyl side chains are crystallizing into a hydrocarbon type matrix (40). This is indicated by the presence of a number of sharp characteristic alkane bands which become dramatically broadened above the transition temperature. Similar changes are observed for n-hexane below and above the melting point. CPMAS 29Si NMR studies on PDHS also show that the rotational freedom of the side chains increases markedly above the transition temperature (41,42). All of the spectral evidence... 

A new photoactive monoclinic polymorph of 6-(2, 4 -dinitrobenzyl)-2,2 -bipyri-dine was obtained from an acetone/methanol solution, and the structure compared to the previously known photoactive orthorhombic and photoinactive monoclinic forms [94]. Correlation of these structures with those of related nitrobenzylpyridines was used to understand the relationships existing between structure and photochro-mism. The comparison of the reaction cavities around the reactive pyridyl-benzyl-nitro fragment indicated that photochromic activity required rotational freedom of the ortho-nitro group in the crystal and its accessibility from the proton-donor and proton-acceptor sites. 

---