Methanol rotational barriers

Changing the atom bound to a methyl group from carbon to nitrogen to oxygen, as in going from ethane to methylamine to methanol, produces a decrease in the rotational barrier from 2.88 to 1.98 to 1.07kcal/mol. This closely approximates the 3 2 1 ratio of the number of H—H eclipsing interactions in these three molecules. 

Semi-empirical models are markedly inferior to all other models dealt with (except the SYBYL molecular mechanics model) for barrier calculations. Major trends in rotation barriers are often not reproduced, for example, the nearly uniform decrement in rotation barrier from ethane to methylamine to methanol. None of the semi-empirical models is better than the others in this regard. One the other hand, AMI is clearly superior to MNDO and PM3 in accounting for nitrogen inversion barriers. All in all, semi-empirical models are not recommended for barrier calculations. 

Calculations have shown that the rotational barrier of the C-O bond in methanol (1.1 kcal/mol) is significantly lower than the corresponding rotational barrier of methyl hypofluorite (MeOF, 3.7 kcal/mol) or methyl hypochlorite (MeOCl, 3.5 kcal/mol), in which a strong [Pg.19]

Wu, Y.-D. Houk, K. N. Theoretical studies of rotational barriers of heteroatom derivatives of methanol./. Phys. Chem. 1990, 94, 4856-4861. 

Considerably higher values (by more than lOkJ/mol) of the rotational barriers for heteroatom derivatives of methanol, HCHj—OX (X = F, Cl, O", OH, and OHj), than for methanol itself were rationalized by Wu and Houk (334) in terms of the hyperconjugation and rt-type orbital... 

Looking more globally at the potential surface, we encounter phenomena such as rotational barriers and inversion barriers. The barriers to rotation in ethane and many other compounds are described well by any of the basis sets so far considered (STO-3G, 3-21G, 6-31G ), even at the HF level. However, when the barriers are small, as in the case of methanol ( 1.1 kcal/mol), the HF method tends to overestimate A rot. 

Figure 7 Depiction of major factors contributing to methanol internal rotation barrier. Oxygen a lone pair reorganization, designated in red C-Hop bond energy change, designated in green...

Eclipsed X-Y bonds are most likely to be encountered for C-N and particularly C-O bonds, for these are shorter than C-C bon(Js, so interactions between the R groups along the bond are enhanced. Furthermore, the inherent preference for bond-staggering as measured by the rotational barrier in ethane, methyl amine, and methanol, is reduced for C-N and C-O bonds, so with suitable, not very extensive, substitution, eclipsing is once more likely for these bonds. Since eclipsing in such circumstances is usually a steric effect, it is not surprising that. MM3 calculations agree satisfactorily with the experimental evidence when the latter is available, and such calculations have widely useful predictive value. 

Methanol clathrates, 2, 20 in hydroquinone, 7 Methyl alcohol, barrier force of internal rotation, 381... 

With accurate calculated barriers in hand, we return to the question of the underlying causes of methyl barriers in substituted toluenes. For simpler acyclic cases such as ethane and methanol, ab initio quantum mechanics yields the correct ground state conformer and remarkably accurate barrier heights as well.34-36 Analysis of the wavefunctions in terms of natural bond orbitals (NBOs)33 explains barriers to internal rotation in terms of attractive donor-acceptor (hyperconjuga-tive) interactions between doubly occupied aCH-bond orbitals or lone pairs and unoccupied vicinal antibonding orbitals. 

R. H. Hunt, W. N. Shelton, F. A. Flaherty, and W. B. Cook, Torsion rotation energy levels and the hindering potential barrier for the excited vibrational state of the OH stretch fundamental band V of methanol. J. Mol. Spectrosc. 192, 277 293 (1998). 

Up to six nitric oxides readily add to ketoenolates when they are treated with strong base in methanol. This sequential addition is stereospecific, generating Z-configured products. The tris(diazene-iV-oxide-iV -hydroxylates) are an intriguing type of molecular propeller (Figure 21) with relatively low energy barriers for rotation of the three N2O2 blades around the C—N bond. 

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