Hindered internal rotations

The example of partial double bond character hindering internal rotation in amides has already been discussed above for dimethylformamide. Other examples of hindered rotation are presented in Table 3.7. 

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Molecules larger than those considered so far are fonned by linking together several smaller components. A new kind of dynamics typical of these systems is already seen in a molecule such as C2Hg, in which there is hindered rotation of the two methyl groups. Systems with hindered internal rotation have been studied in great... 

Gutowsky H S and Holm C H 1956 Rate processes and nuclear magnetic resonance spectra. II. Hindered internal rotation of amides J. Chem. Phys. 25 1228-34... 

A considerable variety of experimental methods has been applied to the problem of determining numerical values for barriers hindering internal rotation. One of the oldest and most successful has been the comparison of calculated and observed thermodynamic quantities such as heat capacity and entropy.27 Statistical mechanics provides the theoretical framework for the calculation of thermodynamic quantities of gaseous molecules when the mass, principal moments of inertia, and vibration frequencies are known, at least for molecules showing no internal rotation. The theory has been extended to many cases in which hindered internal rotation is... 

The ESR spectrum of methanesulfinyl radical (CH3SO), identified in a y-irradiated single crystal of dimethyl sulfoxide , indicates that the unpaired electron resides essentially (72%) on the sulfur 3p orbital with modest population on the sulfur 3s (0.65%) A detailed analysis of the temperature dependence leads to 2.6 kcal mol barrier height for the hindered internal rotation of the methyl group. At low temperature (88 K) the radical adopts a fixed conformation in which one proton lies in the nodal plane of the sulfur 3p orbital however, it was not possible to distinguish either experimentally or by ab initio SCF-MO calculations between the two possible conformations, that is, 2 and 3. 

An important property of chain molecules is that a major contribution to the standard entropy is conformational in nature, i.e. is due to hindered internal rotations around single bonds. This property is most relevant to cyclisation phenomena, since a significant change of conformational entropy is expected to take place upon cyclisation. Pitzer (1940) has estimated that the entropy contribution on one C—C internal rotor amounts to 4.43 e.u, A slightly different estimate, namely, 4.52 e.u. has been reported by Person and Pimentel (1953). Thus, it appears that nearly one-half of the constant CH2 increment of 9.3 e.u. arises from the conformational contribution of the additional C—C internal rotor. 

The rate constants were determined at a series of pressures in the fall-off region, and the fall-off curve was very similar to that obtained for the structural isomerization to propylene. The similarity of the two sets of data suggests that both reactions may proceed through similar reaction paths. One obvious possibility is that once again the trimethylene biradical is formed, which can undergo internal rotation followed by recyclization. An alternative transition state has been suggested which involves, as an activated complex, a much expanded cyclopropane ring in which hindered internal rotation occurs (see also Smith, 1958). 

The ionization potentials of some of the bipyridines have been investigated. Solubility data for 2,2 -bipyridine in aqueous solution, in aqueous solvent mixtures, and in various aqueous salt solutions have been obtained, whereas the heat of solution, heat capacities, and related data for 2,2 - and 4,4 -bipyridines in water have been measured. The enthalpies of solution of 2,2 -bipyridine in water and aqueous solvent mixtures have also been obtained. Dielectric relaxation studies of 2,2 -bipyri-dine in carbon tetrachloride have been reported in connection with hindered internal rotation. Partition coefficients for 2,2 -bipyridine between water and various organic solvents have been measured. ... 

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

Aston, J.G., Wills, P.E., Zolki, T.P (1955) The heat capacities from 10.9 K., heats of transition, fusion and vaporization, vapor pressures and entropy of pentafluorochloroethane, the barrier hindering internal rotation. J. Am. Chem. Soc. 77, 3939-3341. 

Another interesting example of hindered internal rotation is related to the relative stability of the /3-furanose form of D-fructose (12a) in dimethyl sulfoxide solutions,83139-141 in marked contrast to the fact that the /3-pyranose form (13) is by far the predominant tautomer in water. From the, 3C 7, relaxation data83 (see Table HI) of the three major tautomers of D-fructose—a-furanose (12b), jS-furanose (12a), and 0-pyranose (13)—it can be concluded that these tautomers tumble isotropically in both D20 and (CD3 )2SO solutions. The T, values83 for the second-... 

Herschbach, D. R. (1962). Bibliography for Hindered Internal Rotation and Microwave Spectroscopy , Lawrence Radiation Laboratory, Univ. of California, Berkeley, California. 

Steric overcrowding in (7 a) is apparent from a broad a-methylene proton signal. Broadening, indicative of hindered internal rotation, varies with the nature of the arms shorter arms impede rotation less than long ones... 

Hindered Internal Rotation in Carbohydrates Containing Nitrogen in the Ring, W. A. Szarek, S. Wolfe, and J. K. N. Jones, Tetrahedron Lett., (1964) 2743-2750. 

Kirtman, B. Interactions between Ordinary Vibrations and Hindered Internal Rotation. I. Rotational Energies. J. chem. Physics 37, 2516 to 2539 (1962). 

Kivelson, D. Theory of the Interactions of Hindered Internal Rotation with Over-All Rotations. I. Symmetric Rotors Methyl Silane. J. chem. Physics 22, 1733-1739 (1954). 

The large amplitude mode corresponds to a bending vibration or even a hindered internal rotation. The energy surface is then characterized by a more or less... 

Short-range interactions, i.e. those interactions between atoms or groups separated by only a small number of valence bonds, result in an effective constancy of bond angles and in torques hindering internal rotations. 

The vibrational qv) and rotational (q r) partition functions may be calculated within the harmonic and rigid rotor approximations, respectively. In addition, in this work, the qy values were corrected by replacing some of the large amplitude vibrations by the corresponding hindered internal rotations, when necessary. 

A hindered internal rotation of dipyrrole 173 (Figure 3) was assumed to cause a high quantum yield of its fluorescence. Both rotational conformations of dipyrrole 173 cis and trans) are essentially nonplanar with the pyrrole rings being out of the benzene plane by ca. 44° (05T7756). 

Stone, E. W., and Maki, A. H., 1962, Hindered internal rotation and ESR spectroscopy. J. Chem. Phys. 37 1326nl333. 

Other estimated values that have been reported include 200 cm ( ) and 37 cm (5 ). The inactive torsional frequency is treated as a hindered Internal rotation. We use an estimated potential barrier of 8.0 kcal mol ( ) to calculate heat capacity contributions for hindered rotation from the table of Pitzer and Brewer (9). Contributions below 201 K could not be obtained by... 

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