Barrier height methods

Fig. 55. The potential of hindered rotation of the CH3 group in nitromethane (CH3NO2) crystal, (a) calculated from INS data, Vi = 0.586 kcal/mol, V = 0.356 kcal/mol, S = 30°, and (b) calculated with the atom-atom potential method [Cavagnat and Pesquer 1986]. The barrier height is 0.768 kcal/mol.

Comparison of Barrier Heights (BH) in kcal mol for the Proton Transfer in Malonaldehyde Computed by Different Quantum Chemical Methods... 

One of the simplest chemical reactions involving a barrier, H2 + H —> [H—H—H] —> II + H2, has been investigated in some detail in a number of publications. The theoretical description of this hydrogen abstraction sequence turns out to be quite involved for post-Hartree-Fock methods and is anything but a trivial task for density functional theory approaches. Table 13-7 shows results reported by Johnson et al., 1994, and Csonka and Johnson, 1998, for computed classical barrier heights (without consideration of zero-point vibrational corrections or tunneling effects) obtained with various methods. The CCSD(T) result of 9.9 kcal/mol is probably very accurate and serves as a reference (the experimental barrier, which of course includes zero-point energy contributions, amounts to 9.7 kcal/mol). 

With the introduction of two parameters in Eq. (4-15), the EH-MOVB method can be calibrated to reproduce exactly the experimental barrier height and the desired reaction energy. 

DFT method combined with a cluster model approach was compared regarding its suitability for describing both structures and energy profiles. This study shows that the relative stability and geometry depend on the cluster sizes in agreement with previous studies [15] but shows that the energy barrier heights of the reaction processes are not affected. 

Some results are shown in Table 3 together with experimental values where available and also some barrier heights calculated by Pople et al using an ab initio SCF method. 

Method Geometry (A) Barrier height (kcal/mol) AH298 (kcal/mol)... 

Several attempts have been made to analyse the captodative effect through rotational barriers in free radicals. This approach seems to be well suited as it is concerned directly with the radical, i.e. peculiarities associated with bond-breaking processes do not apply. However, in these cases also one has to be aware that any influence of a substituent on the barrier height for rotation is the result of its action in the ground state of the molecule and in the transition structure for rotation. Stabilization as well as destabilization of the two states could be involved. Each case has to be looked at individually and it is clear that this will provide a trend analysis rather than an absolute determination of the magnitude of substituent effects. In this respect the analysis of rotational barriers bears similar drawbacks to all of the other methods. 

Unimolecular pyrolysis of the tautomers of monothioformic acid (two conformers of thiol- and two conformers of thiono-) have been studied by ab initio methods with STO-3G and 6-31 G basis sets. The barrier heights for dehydrogenation (via a four-centre transition state) and dehydrogensulfldation (via a three-centre transition state) of thiol formic acid are 67.47 and 67.09 kcalmol" respectively. Dehydration of 5-cw-HCSOH occurs via a three-centre transition state with an activation energy of 81.18 kcalmoG this is much greater than for dehydration of the s-trans form, which occurs via a four-centre transition state with a barrier of only 68.83 kcalmol" ... 

---