Barrier to Rotation of

This hindered rotation of is governed by various forces, which can be divided into bonded (electronic) and nonbonded interactions ( steric effects). The direct electronic interaction between M and Hj results from overlap of the appropriate molecular orbitals. Nonbonded interactions such as van der Waals forces between the atoms and the other atoms on the molecule may vary as rotates. Intermolecular interactions should not contribute much to the barrier to rotation of as the metals are far apart. However, they may have a minor effect on the coordination geometry about M, which could in turn affect M-H2 binding. 

This section desoibes how information on M-H2 binding can be extracted from series of measurements of the barrier to H2 rotation. Background is given in Section 6.2.1 on the model for hindered rotation of a dumbbell molecule and the measurement of transitions of this hindered rotator by B S. Section 6.2.3 discusses the relationship of the rotational barrier to the various factors that give rise to it and the conclusions that can he drawn concerning M-H2 binding. 

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The calculations of the optimum geometry show a slight lenghthening of the C—H bonds because of the electron release to the tc system. These calculations also reveal a barrier to rotation of the methyl group of about 1.5-2.0 kcal/mol. Interaction between the hydrogens and the n system favors the eclipsed conformation to fliis extent. Let us examine the... 

Torsional barrier the energy barrier to rotation of a single bond. 

Preparation of Phosphoranes from Phosphorus(in) Compounds.—Benzoylphosphor-anes have been made from xenon difluoride, as shown for (80).76 The barrier to rotation of the benzoyl group in (80) is found to be below 8 kcal mol-1, although... 

From Stull, Westmm and Sinke we find the barrier to rotation of the nitro group in nitrobenzene is 25.1 kJ mol 1, to be compared with the rotational barrier of the amino group in aniline of 14.2 kJ mol-1. 

Barriers to rotation about the C—N bond of /V,/V-dimethylformamide are known to be affected by concentration and the nature of the solvent. As expected, polar solvents tend to increase the barrier by stabilizing the polar structure (2). Therefore, it is not surprising that, whereas the barrier to rotation of N,N-dimethylformamide is about 21 kcal/mol in solution, the barrier becomes as low as 15.6 kcal/mol in the gas phase (32). In the practical question of isolating atropisomers, it is the magnitude of the barrier in solution that matters. 

The effects of the substituents on nitrogen on rotational barriers were discussed by Yoder and Gardner (34) for formamides and acetamides. The pertinent data, given in Table 5, suggest that the barriers to rotation of formamides are not affected by the bulkiness of the alkyl group on nitrogen, but such a conclusion... 

Effect of Substituents on Nitrogen on the Barrier to Rotation of Formamides and Acetamides (RCONR2) ... 

Having obtained stable rotamers of compound 6, Staab and Lauer (45) extended the work to see whether rotamers of amides that normally have lower barriers as a result of a disfavored canonical structure 2 due to electronic effects are also isolable. They found that the rotamers of 2,4,6-trwm-butylbenzoben-zimidazolide (7) were isolable, but those of the corresponding imidazolide (8) were not. The barrier to rotation of the former in hexachlorobutadiene solution was 28.7 kcal/mol for the E Z process at 80°C. The barrier to rotation of the latter was estimated at less than 23 kcal/mol. It is possible to attribute this result to electronic effects that raise the ground state energy, because the aromatic... 

Siddall and his co-workers (46) have examined the barriers to rotation of a series of 2,6-disubstituted anilides. Af-Ethyl-A/-(2,6-xylyl)formamide (9) was recrystallized as a uranyl nitrate complex, and one isomer, which at equilibrium was favored by a factor of 3 1, was enriched up to a 30 1 ratio. The kinetics of rotation were examined at 0 to 29°C. The Arrhenius activation energy was 26 3 kcal/mol and log A was 18.5 2.4 hr-1. Siddall and Gamer (47) were able to obtain an almost pure isomer (which also predominated at equilibrium 1.3 1 for the ethyl compound and 1.1 1 for the methyl compound) of Ar-alkyl- V-(2-methyl-4,6-dibromophenyl)-l-naphthamide (10). The half-lives of... 

Siddall (48) also reported that the barriers to rotation in N-substituted N-(2-chloro-6-methylphenyl)formamides (11) were high, but not high enough for the isolation of atropisomers. The exact barriers were not reported but, if one compares them with those in compound 9, the barriers to rotation of these compounds are lowered by the substitution of the chloro group for the methyl on the aromatic ring. 

Ito and his co-workers (51) noticed that an adduct (14) of tropone with iV-ethoxycarbonylazepine appeared to undergo slow internal rotation by H NMR, the barrier at 83°C being 18.3 kcal/mol. As was discussed earlier, the ethoxy-carbonyl group gives a lower barrier than those of acetyl and formyl derivatives. Indeed, by changing the /V-substitutent from ethoxycarbonyl to acetyl, the barrier was raised to 20.0 kcal/mol. The formyl derivative showed a barrier to rotation of 23.0 kcal/mol at 20°C. It was possible to isolate a pure Z isomer and a nearly pure E isomer of the formyl derivative (15) by TLC. The free energy of activation... 

Walter and Becker (64) have investigated the barrier to rotation of /V-isopropyl-N-(2,6-dichlorophenyl)thioformamide (25). Its Z form crystallized, but on dis-... 

Comparison of the Barriers to Rotation of Amides, Thioamides, and Selenoamides... 

Hydrazones are analogs of enamines. Their barriers to rotation about the N—N bond are expected to be analogous to those discussed for the enamines. However, the barriers to rotation of hydrazones 35 and 36, which are analogs of enamines that afforded stable atropisomers, were found to be lower. The barrier in 35 was only 16.7 kcal/mol. The barrier is again higher for the 5-membered compound 35 than for the 6-membered 36. 

The barrier to rotation of the cyclopentadienone hydrazone 37 is reported to be less than 11 kcal/mol (71). Introduction of a formyl group into the 2-position of the cyclopentadienylidene ring raised the barrier to 11.8 kcal/mol (71). 

Mannschreck (80) extended this work, and the barrier to rotation of /V-benzyl-A/-neopentylnitrosamine (41) was measured as 22.1 kcal/mol at 100°C by equilibration. At equilibrium, the E form is slightly favored. 

Barriers to rotation of nitrosamines in which the amino part is embedded in a cyclic system seem generally to be smaller. However, Harris and associates (82) reported that the barrier of /V-nitroso-2,2,5,5-tetramethylpyrrolidine (43) was over 22.6 kcal/mol. This must be higher than the barrier required for isolation of rotamers at room temperature, and is even higher than that in /V-nitroso-2,2,6,6-tetramethylpiperidine (44). Harris and Pryce-Jones attribute the high barrier of 43 relative to 44 to the more stable ground state of the former. If the pyrrolidine derivative is properly substituted, the atropisomers are expected to be isolable at room temperature. 

The barriers to rotation of esters deserve mention here, especially in comparison to amide barriers. The H NMR spectra of some nitrites (45) were measured in 1957 (83). The temperature had to be lowered to - 58°C at 30 MHz to see the separate signals of propyl nitrite. The barriers to rotation were ca. 10 kcal/mol. This result may be rationalized by considering the lesser electron-donating ability of the alkoxy relative to the dialkylamino group. The dipolar canonical form (46) of nitrite esters is not as stable as that of nitrosamines. 

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