Rotational barriers calculation

Some more general remarks, however, remain to be added concerning the accuracy of semiempirical calculations and the internal dynamics of the molecule investigated. A closer look at the energy scale of Figure 1 reveals that the minimum for the structure with almost perpendicular CS bonds is a rather shallow one - partly due to the assumed constant geometry for the H CS subunits. The rotational barrier calculated, A 0.04 ev =... 

As with 2-methylbutane, the gauche form of isobutyl chloride predominates in the gas, liquid and solid phases119. The gauche conformer is the only one present in solid isobutyl bromide, but the anti form is slightly more stable in the vapor phase119. With the iodide,the anti form is not only more stable in the gas120 but also in the annealed solid121. Terms for the torsion potentials of the bromide and iodide are shown in Table 17. From these potentials one can obtain the enthalpy difference of conformers and barriers to rotation. Barriers calculated from Table 15 are shown in boldface. 

Evidence for the conjugative interaction of cyclopropyl with adjacent vinyl cations comes from a number of sources. Theoretical calculations for a variety of substituted vinyl cations (109), including the case where R = c-Pr, indicated that substituent effects in 109 were similar to those of the corresponding ethyl cations (110) . The rotational barrier calculated for cyclopropylvinyl cation was half that of cyclopropylethyl cation, a result tentatively attributed to conjugation of cyclopropyl with the 7r-bond of the vinyl cation when the cyclopropyl was twisted perpendicular to the vacant p orbitaP . ... 

Some SCF MO results for other rotational barriers calculated using the 6-31G basis set and geometry optimization of all structures are (values in kcal/mol) CH3OH,... 

Some HF/6-31G results for other rotational barriers calculated with geometry optimization of all structures are (values in kcal/mol) CH3OH, 1.36 calculated versus 1.07 experimental CH3CHO, 1.03 calculated versus 1.17 experimental CH3NH2, 2.39 calculated versus 1.98 experimental CHsSiHs, 1.40 calculated versus 1.70 experimental (CCCBDB). 

These structural effects are also found by MO calculations. Calculations at die MP4/6-311++G level have been performed on the ally cation and indicate a rotation barrier of 36-38 kcal /mol. ... 

STO-3G and 3-2IG MO calculations indicate a rotational barrier that is substantially reduced relative to the corresponding barrier in ethylene. The transition state for the rotation is calculated to have a charge separation of the type suggested by the dipolar... 

Calculate the rotational barrier between the anti and anticlinal forms of N-butane using the AMI (or PM3 if you prefer) and HF/6-31G(d) model chemistries. Use the results for the anti form that you obtained in Exercise 6.1. Note that the anticlinal form is a transition structure you will find the Opt TS,CalcFC] keyword helpful in optimizing this structure. 

Rotational barriers for bonds which have partly double bond character are significantly too low. This is especially a problem for the rotation around the C-N bond in amides, where values of 5-10 kcal/mol are obtained. A purely ad hoc fix has been made for amides by adding a force field rotational term to the C-N bond which raises the value to 20-25 kcal/mol, and brings it in line with experimental data. Similarly, the barrier for rotation around the central bond in butadiene is calculated to be only 0.5-2.0 kcal/mol, in contrast to the experimental value of 5.9 kcal/mol. 

For 6, the activation energy for rotation about the MSi bond has been measured as AG = 40.3 (+ 5) kJ/mol [143]. According to MO calculations, a genuine Cr = Si double bond has no rotational barrier worth mentioning. This applies also, with some restrictions, to the discussed base adducts. 

Table A4.6 gives the internal rotation contributions to the heat capacity, enthalpy and Gibbs free energy as a function of the rotational barrier V. It is convenient to tabulate the contributions in terms of VjRTagainst 1/rf, where f is the partition function for free rotation [see equation (10.141)]. For details of the calculation, see Section 10.7c.

Rotational Barrier in Ethylene. It is well known that the rotational barrier of the ethylene molecule cannot be adequately described by a single reference Hartree-Fock calculation SCF calculations on this level resulted in values of 126 kcal/mole (30) and 129 kcal/mole (31) whereas the experimental value is 65 kcal/mole (32). Open-shell ab initio calculations of double zeta+polarization quality give the more acceptable value of 48 kcal/mole (33). Inclusion of correlation such as in CEPA calculations yield theoretical results within the experimental error bar (34), albeit at a considerable computational cost. 

Using local spin density functional (LSDF) theory, we obtain 70 kcal/mole for the rotational barrier of the ethylene molecule (35). In these calculations, we use the equivalent of a double-zeta+polarization basis set, i.e. for C two 2s functions. 

There is ample evidence [9,17,44] that the INDO SCF procedure transformed according to this scheme (C INDO) can provide predictions comparable to those of minimal-basis-set ab initio SCF calculations for conformations and rotational barriers of conjugated molecules in the ground state. 

The inversion barrier for syn/anti isomerization of H2Si=NH is only 5.6 kcal mol-1, whereas the internal rotation energy is 37.9 kcal mor1 (SOCI level of calculation). The rotation barrier can be equated to the ir-bond strength. The inversion transition state has an even shorter SiN bond length of 153.2 pm. The symmetry is C2V.9,10... 

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