Rotational transition state

Next, examine the equilibrium structure of acetamide (see also Chapter 16, Problem 8). Are the two NH protons in different chemical environments If so, would you expect interconversion to be easy or difficult Calculate the barrier to interconversion (via acetamide rotation transition state). Rationalize your result. Hint Examine the highest-occupied molecular orbital (HOMO) for both acetamide and its rotation transition state. Does the molecule incorporate a n bond. If so, is it disrupted upon rotation ... 

Some derivatives of triafulvene undergo rotation about the carbon-carbon double bond even at room temperature. Given that cis-trans isomerization about double bonds is normally very difficult (see Chapter 7, Problem 1), how would you rationalize this Examine the electrostatic potential map for perpendicular hexaphenyltriafulvene (the rotational transition state).Would polar solvents tend to lower or raise the rotation barrier Explain. 

As expected, the C-C bond lengA widens significantly for the rotational transition state. Here, the agreement between the semi-empirical MNDO results and the first-principles LDF results is remarkable. The discrepancy in the C-H bond length remains, but the trend of a small bond shortening from the ground state to the rotational transition state can be found for both the MNDO and the LDF calculation. 

Fig. 5. Selected geometric parameters (A) of the optimized rotational transition-state structures for allylic isomerization via the r(3-1s y ,ri1(C3)-octadienediyl-Ni11 TSiSo[3a] and TSiSo[3b],...

Experiments have demonstrated that the stoichiometric cyclotrimeriza-tion becomes accelerated by the presence of donor phosphines (i.e., PMe3, PEt3, PPh3) and also by excess butadiene.93 However, the rotational transition-state structure TS SO[6b] is found to be not stabilized in enthalpy by coordination of butadiene. Therefore, incoming butadiene does not serve to facilitate allylic isomerization and will not assist this process. Accordingly, reductive elimination is indicated to be accelerated by excess butadiene, which will be examined in the next section. 

Variations in interatomic distances for the complexes and transition states on going from E = Si to Ge are rather small and of the same magnitude as those found in the products. Energies of stationary points for reactions 9 and 10 are presented in Table 24. An additional feature of the complex Cl is its C symmetry, and the existence as left (Cll) or right (Clr) handed forms, which are separated by the very low ( ia = 0.4 Kcalmol-1) rotational transition state TSO. A similar situation was found for transition state TS2. It also has a C symmetry, and possesses left (TS21) and right (TS2r) handed forms divided by a low rotational maximum. 

The barriers for methyl groups bonded to N-sp2 atoms are smaller than the respective barriers for methyl bonded to C-sp2 atoms in the corresponding carbon systems, suggesting facile bending of the N-sp2-CH3 groups in the rotational transition state. The authors also compare the steric size of the lone pair of the nitrogen in 84c and 84i and, curiously, different values are obtained in terms of van der Waals radii. 

Comparison of available solution data with the gas-phase values obtained show that the destabilization of the larger freely rotating transition state by solvent packing forces must be important in these substituted nitrites therefore, the unexpectedly small phase dependence may be attributed to dielectric effects which are opposite in direction but similar (or slightly lower) in magnitude to these steric forces. For methyl nitrite AG 298 >s lower in solution than in the gas phase (AAG 298 = 0.8 kJ mol-1) while for the other primary alkyl nitrites is slightly higher (AAG+298 0.0 to 2.5 kJ mol-1). 

In this connection, a very recent paper by Allinger (8) should be mentioned. A force field has been developed to permit molecular mechanics calculations on various molecular structures of elemental sulfur. The conformational characteristics of sulfur rings containing five to 12, 14, 16, 18, or 20 sulfur atoms have been examined. Comparison with experimental data is made in all cases where such data exist, and predictions are made for other cases. Ab initio molecular orbital calculations using an STO-3G basis set were carried out for cyclohexasulfur and are consistent with the molecular mechanics calculations in indicating that the chair and the twist forms are two stable conformations, with the chair about 15 kcal/mol more stable while the boat (C2t,) is a twist rotational transition state. Calculation of possible conformations of the pro-... 

Our attempts at obtaining gas-phase ESCA data for the two 2-quinuclidone derivatives 7 and 10 were unsuccessful possibly due to thermal decomposition upon heating the sample to improve volatility [61], (It should be noted that 6,6,7,7-tetramethyl-2-quinuclidone (10) is fairly unreactive with nucleophiles due to steric hindrance [66]. More recently, we [61b] have obtained results supporting these studies by examining the core potentials of the planar ground states and rotational transition states of formamide and dimethylacetamide. The simplest explanation of the data is loss of the contribution of 4C with concommitant increased contributions from 4A and 4B [61b]. 

The calculation revealed that about two thirds of the excess of spin is located on the terminal CH2 group, whereas 0.31 lei is localized on the chromium atom. In the rotational transition state, however, the unpaired electron was predicted to be completely localized at the carbon atom of the terminal CH2 group. The computed energy barrier for the CH2 rotation is 12.3 kcal/mol. 

The barriers obtained for N-bonded methyl groups are smaller than the corresponding C-methyl barriers, suggesting facile bending of N(sp )—CHj groups in the rotational transition state, and peri and ortho substitutions lead to increase of the rotational barriers. 

The calculated energies (kcalmoP ) for TBP intermediates and SP pseudo-rotational transition states for exocyclic cleavage pathways in hydrolysis of MEP are given in Scheme 14. The pseudorotational barrier for exocyclic cleavage in dilute acid (pathway a), which accounts for <50% of total product, is 1.2kcalmol corresponding to a rate of ca. 10 s . As expected, pseudorotation is a rapid process in this system. In dilute base, in which minimal exocyclic cleavage is observed (pathway c), the pseudorotational barrier is doubled at 2.3 kcal mol . ... 

The status of the eclipsed conformation as the rotational transition state needs considering. When and in a fragment rotate past each other,... 

TSl (with O-C-C-H eclipsed) and TS2 (with O-C-C-O eclipsed) are rotational transition states (see Fig. 9.4). 

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