Energy barriers, substituted

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...

Accounting for this effect, it was possible to apply dynamic NMR spectroscopy to measure energy barriers to the prototropic rearrangements of pyrazoles. Temperature-variable spectra of a series of 4-substituted pyra-zoles 5 and 6 have been studied in methanol-d4 solutions and the free energy barriers of the degenerate type 2a 2b tautomerization reported (93CJC1443). 

Unsymmetrically substituted pentadienyl anions populate six planar conformations, which are in equilibration13 a 18. The energy barrier for a torsion in the potassium compound (R = primary alkyl) was estimated to be approximately 35 keal/mol for the 1,2-bond and 15 keal/mol for the 2,3- and 3,4-bonds. The barriers are much lower in the lithium compound. Not only the rate, but also the position of the equilibrium is greatly influenced by the cation from trapping experiments18 it was concluded that the exo-VJ anion is most stable for lithium and the exo-U form for potassium. 

Table 1. Energy barriers to ring rotation in unsubstituted and substituted cyclopentadienyl transition metal complexes.

Tetramethylbisdialine 86 has been used to synthesize substituted pentaheli-cenes characterized by a relatively high energy barrier to racemization due to the marked steric interactions between the methyl groups at the terminal aromatic rings [68b] (Scheme 2.39). 

As reported in Table 5 and in other recent publications [399,491 ], polymers with very low Tg are expected when the inherent skeletal flexibihty of poly-phosphazenes is coupled with fluorinated alcohols of low dimensions and/or of high chain mobility. In fact, the Tg values for POPs substituted with fluorinated alcohols vary between -50 °C and -90 °C, confirming the extreme chain mobility of these polymers and the existence in them of very low torsional energy barriers. 

Ironically, the most recent calculational treatments of the rearrangement from 56 to 57 indicate that even at room temperature tunneling does, in fact, dominate the H-shift process. Phenyl substitution in 56 should lower the energy barrier for rearrangement compared with 54, not only increasing the rate of classical rearrangement... 

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 asymmetrically substituted furoxans 9a and 9c undergo interconversion under the action of heat [6, 10]. The dinitrosoethylene structure 9b has been proposed as an intermediate (Scheme 6.2). This equilibrium has been studied in depth, in particular in 1,1,2,2-tetrachloroethane solution. Electron-releasing groups consistently favor the 4-position, while a similar preference for the 3-position is not in general observed for electron-withdrawing groups. The energy barriers for interconversion are lower... 

Figure 8. Calculated energy barriers AE for epoxidation of substituted olefins by the base-free complex 5 as a function of the energy of the olefin HOMO ji(C-C). Also, the estimated barrier height of 4-methoxystryrene is marked based on the corresponding HOMO energy.

Photolysis of N-methylborazine with CH3OH (e = 100) yielded 67% ortho substituted product while the reaction with HN(CH3)2 yielded over 90% para product. These results are explained by sterically hindered attack of the CH3OH and HN(CH3)2 at the ortho site. A lower energy barrier is also postulated to explain the statistical distribution of products in the CH3OH + N-methylborazine reaction. 

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 )... 

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