Internal Rotational Barriers

Potential barriers versus torsion angle for internal rotation are calculated for the following hydroperoxides and their corresponding methylperoxides  

T/K 5000). The Rotator program takes into account the entropy of mixing and the optical isomer corrections. 

Data on the rotational barriers in the Cd—OOC, CdO—OC, CdOO—C and Cd—C bonds for the peroxides are summarized in Table 3.12, along with data on the corresponding barriers in hydroperoxides for comparison purposes. 

Rotation barriers for Cd—CH3 bonds for all peroxides (hydro and methyl-peroxides) have the same 3-fold symmetry and have all relatively low barriers, 1.3 to 2.1 kcal mof  

The Cd—C rotation barrier in allyl hydroperoxide (CH2=CH—CH2OOH) is higher (6.4 kcal mol ) which is 3 times the corresponding rotational barrier for allyl peroxide (CH2=CH— CH2OOCH3). Both have one-fold symmetry. 

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The strikingly different characteristics of transition-metal hyperconjugative interactions are particularly apparent in their influence on internal rotation barriers. To illustrate, let us first consider ethane-like Os2H6, whose optimized staggered and eclipsed conformations (displaying conspicuous deviations from those of ethane) are shown in Fig. 4.81. 

Formaldoxime (2) has been shown to have a planar structure with Cs symmetry (Figures 1 and 4). Experimental and theoretical studies found the anti conformer (anti-2) to have the lower energy (with Asyn-mtiE = ca 24.2 kJmol ), which may reflect the lone-pair-lone-pair repulsion between oxygen and nitrogen atoms. The anti-syn internal rotational barrier is about 38-42 kJmol depending on the level of theory applied " . 

The energy calculations were performed without fixing the fiber identity period, the only assumption being that the chain forms a helical structure, that is, the set of internal rotation angles repeats along the subsequent monomeric units of the chain. For the calculation the internal rotation barriers, van der Waals interactions [mainly after Scheraga (31)], and dipole-dipole interactions were taken into account. 

Preliminary approximate calculations of the transition state in C2H6 + IIO reaction, also executed by the Partial Reservation of Double-Centered Differential Overlap (PRDDO) method [29, 30] indicate the interaction between masked ethane conformation (with a calculated internal rotation barrier equal 1.3kJ/mol) and the HO radical approaching it in the C—C plane. The distance between C—C and O—O bond sites was taken as the reaction coordinate. It is found that planar structure (II) corresponds to the transition state in the C2H6 + H02 reaction. 

This method, as well as the internal rotation barrier technique, has been used by Dobbs and Hehre to yield C=X n bond energies for X = C, Si, Ge, and Sn (142). Schleyer and Kost (143) compared an X=Y double bond to two single bonds via isodesmic reactions and used standard single bond energies to obtain an estimate for the it bond energies of first and second row systems. The Ge—Ge and Ge=Si n bond energies were... 

It is an assumption that the n bond eneigy, thus defined, is equal to the internal rotation barrier. For ethylene, the thermochemical n bond energy and the barrier to internal rotation are within experimental error of one another. Note that the isomerization enthalpy is strictly valid only if the DSSE(H3CCH) is determined from H3CCH3. 

On the basis of NMR spectroscopy data the internal rotation barriers for N(CH3)2 groups in twenty I -(/V, /V-di methylthiocarbam< >y I)pyra/oIe derivatives were determined [294],... 

The internal rotation barriers of the dimethylamino groups in substituted azoles including 5-nitro-2-dimethylamino-l,3,4-oxadiazole (AG =9 kcal/mol, 133°C) were also defined by NMR spectroscopy [523],... 

R. A. Scott and H. A. Scheraga,/. Chem. Phys., 42, 2209 (1965). Method for Calculating Internal Rotation Barriers. 

The infrared and especially microwave spectra of methylamine and its deuterated species have been studied in considerable detail [see paper for further references]. The potential barriers to internal rotation and inversion are both relatively high [Table 6 internal rotation barrier is 684 cm in the ground state of CH3NH2] but the splittings of the energy levels are measurable. 

In these calculations the electronic contributions have been assumed to cancel, and vibrational assignments and internal rotation barriers were calculated according to Pitzer. Other assumptions have been discussed by the authcjrs. The vibrational contributions (not shown in table) nearly cancel each other near 300°K and can be neglected below 500°K in the calculation of AiS° for the reactions, so that the AaS° shown in the last two lines of Table XII.3, aside from symmetry changes, can be equated to the standard entropy of activation. 

Guo, H. and Karplus, M., Ab initio studies of hydrogen bonding in N-methylacetamide Structure, cooperativity, and internal rotation barriers, J. Phys. Chem. 96,7273-7287 (1992). 

Bone, R. G. A., and Handy, N. C., Ab initio studies of internal rotation barriers and vibrational... 

They have done calculations for C2H6, Si2H6 and Gc2H6 geometries and internal rotation barriers. They also used the model potential in eqn (2-L-22) for calculation of equilibrium geometries of some homonuclear diatomics," such as O2, Bc2, B2, C2, N2, O2, F2, P2 and CI2. Molecular anisotropy in some diatoms such as H2, O2, C2, N2, O2 (singlet), F2, HF and CO have been evaluated with same model potential. A nonlocal pseudopotential in the FSGO model ... 

L. Goodman and V. Pophristic, Where does the dimethyl ether internal rotation barrier come from , Chem Phys. Lett. 259, 287-295 1996. 

Structural parameters for the planar and orthogonal forms of H2BNH2 have been computed from ab initio calculations.151 The best estimate of the internal rotation barrier is 33.3 kcal mol-1. 

Frankosky M, Astron JG (1965) The heat capacity and entropy of hexamethylbenzene from 13 to 340 K. An estimate of the internal rotation barrier. J. Phys. Chem. 69 3126... 

Electron-diffraction data in the gas phase [43] suggest that ferrocene prefers to adapt an eclipsed conformation, with an internal rotational barrier of 0.9 0.3 kcal/mol. The calculated barrier derived from the vibrational frequency of the internal rotational mode is 0.72 kcal/mol [44]. Our LDA/NL calculation finds the eclipsed conformation to be the most stable, with a calculated rotational barrier of 0.69 kcal/mol, in good agreement with experiment. The structure of ferrocene has been studied by several theoretical methods. Our optimized geometrical parameters are similar to those that had been obtained previously by Fan and Ziegler [25] employing the same 1.DA/NL scheme (Table 8). The LDA/NL geometry represents a better fit to experiment than... 

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