Rotation energy barrier

Smith G D and R L Jaffe 1996. Quantum Chemistry Study of Conformational Energies and Rotational Energy Barriers in u-Alkanes. Journal of Physical Chemistry 100 18718-18724,... 

This IS an unusually high rotational energy barrier for a single bond and indicates that the carbon-nitrogen bond has significant double bond character as the reso nance picture suggests... 

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

Table 3.4. Rotational Energy Barriers of Compounds of the Type CH3 — X"...

Scheme 5.2. Rotational Energy Barriers for Allyl Cations (kcal/mol)"...

In addition, for two coaxial armchair tubules, estimates for the translational and rotational energy barriers (of 0.23 meV/atom and 0.52 meV/atom, respectively) vvere obtained, suggesting significant translational and rotational interlayer mobility of ideal tubules at room temperature[16,17]. Of course, constraints associated with the cap structure and with defects on the tubules would be expected to restrict these motions. The detailed band calculations for various interplanar geometries for the two coaxial armchair tubules basically confirm the tight binding results mentioned above[16,17]. 

An ab initio theoretical study was conducted on 1,2,5-oxadiazole and 3-phenyl-l,2,5-oxadiazole to determine the molecular structures of these heterocyclic compounds. The rotational energy barrier between Ph ring and diazole nucleus was also evaluated. No considerable change of bond lengths inside the diazole nucleus was observed in the Ph-substituted heterocyclic compounds as compared to the oxadiazole and thiadiazole alone <2001MI215>. 

The broad and nearly universal applicability of the cinchonan carbamate CSPs for chiral acid separations is further corroborated by successful enantiomer separations of acidic solutes having axial and planar chirality, respectively. For example, Tobler et al. [124] could separate the enantiomers of atropisomeric axially chiral 2 -dodecyloxy-6-nitrobiphenyl-2-carboxylic acid on an C-9-(tert-butylcarbamoyl)quinine-based CSP in the PO mode with a-value of 1.8 and Rs of 9.1. This compound is stereolabile and hence at elevated temperatures the two enantiomers were interconverted during the separation process on-column revealing characteristic plateau regions between the separated enantiomer peaks. A stopped-flow method was utilized to determine the kinetic rate constants and apparent rotational energy barriers for the interconversion process in the presence of the CSP. Apparent activation energies (i.e., energy barriers for interconversion) were found to be 93.0 and 94.6 kJ mol for the (-)- and (-l-)-enantiomers, respectively. 

The structure of amino-substituted allenylidene complexes merits special attention since those compounds derived from unsymmetrically substimted amines usually give rise to isomeric mixmres in solution (C-N bond rotamers) [19-24, 27, 43]. For complexes tra s-[RuCl =C=C=C(NR R )Me (dppm)2] the rotational energy barriers around the C-N bond could be experimentally determined by dynamic NMR spectroscopy, the high values observed (ca. 85 kJ moP )... 

NMR Determination of Internal Rotation Rates and Rotational Energy Barriers 59... 

In the course of the depolymerization primary and tertiary terminal macroradicals are formed. A difference in the reactivity these radicals is explained by assuming that the /3-scission depends on the rotational energy barrier the terminal C-C- bond in the primary and tertiary terminal radicals (52). 

Table 27 Coalescence Temperatures and Rotational Energy Barriers for N-Alkyl-2-fonnylpyrroles...

The more lipophilic thioamide unit is also characterized by the poor H-bond acceptor nature of the sulfur and by its larger covalent radius (1.04 vs 0.74 A for O). The thioamide unit mainly adopts a Z-planar conformation, with a rotation energy barrier averaging about 23 vs 18 kcal-mol-1 for amides. 11 A recent ab initio computational study 12 suggests that conformational perturbation in linear thioamides is more likely effected in the C-terminal side. 

This is also reflected in quite high rotational energy barriers, which surprisingly are greater for thioamide than for the amide analogues. 

Geometry, energy, rotational energy barriers, IGLO and GIAO-MP2 NMR chemical shifts... 

Further compounds, such as gossypol 3, normally exist as atropisomers due to steric crowding effects. Hence, it is possible to prepare the compound in a homochiral state, provided that the experimental temperature is not raised sufficiently to overcome the restricted rotation energy barrier. If this does occur, then the outcome is a racemic mixture of (+)- and (—)-isomers. 

Rotational energy barrier measurements reported in CHEC-II(1996) for the NMe2 group of 5-(AGV-di methyl amino)-1,4,2-dithiazolium salt and of 5-(Ar,.A-dimethylamino)-l,3,4-oxathiazole-3,3-dioxide indicate a restricted rotation around the exocyclic C-N bond <1996CHEC-II(4)505>. The conformations of several 1,4,2-dithiazolidine derivatives, obtained from X-ray data, show that this particular ring adopts an envelope conformation with the nitrogen atom as the flap (see Section 6.04.3.1). 

Thiopyranylidene)thioxanthene TRIPOS, MOPAC93 AMI Conformation, rotational energy barriers 1997JOC4943... 

For the case of adsorbed complex molecules, which generally have a preferred orientation with respect to the substrate atomic lattice in their energy minimum configuration, the possibility of 2-D molecular rotations needs to be considered. These rotations require thermal activation, analogous to lateral transport. In the simplest case they imply the overcoming of a unique rotation energy barrier Er, which may be higher, equal or lower than the... 

Fig. 3 Geometries of DDMD (Min-1 to Min-3) and the rotational energy barriers between them (Sad-1 to Sad-5). Relative energies (in kcal/mol) determined at the MP2/6-311G level are indicated. The C-N-C-N dihedrals (j> and

Fig. 4. Dihedral geometry of the low-energy conformers of DDMD and the rotational energy barriers between them. Also shown are the geometries for methylene-centered dihedral pairs in the low-energy conformers of HMX.

Explain the dramatic difference in rotational energy barriers of the following three alkenes. Hint Consider what the transition states must look like.)... 

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