Rotation, internal hydrogen

Torsional parameters and VdW parameters for internal hydrogen bonds in the N—C—N moiety were obtained by fitting the ab initio rotational profiles of methylenediamine (MDA, 15) and /V-methylmelliylenediamine (NMMDA, 16). A comparison of relative conformational energies between ab initio and MM2 results for 15 and 16 is provided in Table 6. Bond length correction terms for inner and outer C—N bonds (K, K2 and... 

Hunt, R.H., Leacock, R.A., Peters, C.W., and Hecht, K.T. (1965). Internal-rotation in hydrogen peroxide The far-infrared spectrum and the determination of the hindering potential, J. Chem. Phys. 42, 1931-1946. 

Internal hydrogen-bonding in the biradicals from 17 and 18 increases overall quantum yields for product formation by impeding disproportionation. Likewise most polar solvents enhance quantum yields but lower the cyclization/cleavage ratio, presumably because H-bonding of solvent to the hydroxyl group increases steric hindrance to bond rotations. 

There has been considerable interest in the factors that control the stereoselectivity of cyclobutanol formation. Three main factors were identified quite early pre-existing conformational preferences due to steric effects or to internal hydrogen bonding solvation of the OH group and variable rotational barriers for cyclization. More recently Griesbeck has proposed that orbital orientation favoring soc produces another form of conformational preference in triplet biradicals [55], These factors have different importance depending upon the molecule. 

The IR spectra of 29 in dilute dichloromethane solution are interpreted in terms of an equilibrium mixture of internally hydrogen bonded conformers and a population of conformers in which the carboxy group is rotated so that the internal hydrogen bond is not formed. Because the absorption strengths of the two bands are not known, it is not possible to estimate the value of the equilibrium constant from the IR spectra. It is assumed that these salient features of hydrogen bonding in 29 can be extrapolated to triad 1 and dyad 5. 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Thus, reduction of the bicyclic derivatives 25 (RR = CH2 RR = CH=C(Ph)) affords the corresponding 26a-type products, while hydrogenation of 2-ethoxy-3-acetylpyridine gives, along with the carbonyl group reduction product, the imine isomer 26b (R = Me, R = Et). These results were explained by the so-called internal strain effect, e.g., by steric repulsion between the nitrogen and oxygen lone pair in rotationally restricted bicyclic derivatives or between the 2 and 3 substituents. 

Hindered rotation, 33, 34 internal, 367 Homopolymer, 168, 183 Hot bands, 374 Hot lattice, 4, 11, 21 Hydrates, 7, 9, 21, 31, 41 crystallization, 44 Hydrochloric acid clathrates, 2 in hydroquinone, 7 Hydrogen, bound, 4, 175 bromine hydrate, 35 4- carbon dioxide system, 110 4 carbon monoxide system, 96, 108 chloride hydrate, 35 clathrates, 2 chloride, 30... 

Platinum-cobalt alloy, enthalpy of formation, 144 Polarizability, of carbon, 75 of hydrogen molecule, 65, 75 and ionization potential data, 70 Polyamide, 181 Poly butadiene, 170, 181 Polydispersed systems, 183 Polyfunctional polymer, 178 Polymerization, of butadiene, 163 of solid acetaldehyde, 163 of vinyl monomers, 154 Polymers, star-shaped, 183 Polymethyl methacrylate, 180 Polystyrene, 172 Polystyril carbanions, 154 Potential barriers of internal rotation, 368, 374... 

Hindered Rotation (kT to) With hindered rotation, the potential energy of the internal rotation is restricted by a potential barrier, Vq, whose magnitude varies as the two parts of the molecules rotate past each other in a cyclic fashion. For example, in the molecule H3C-CCI3, the potential varies as the hydrogen atoms on one carbon move past the chlorine atoms on the other. 

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

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