Rotation, barrier

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

VViberg K B and M A Murcko 1988. Rotational Barriers. 2. Energies of Alkane Rotamers. An Examination of Gauche Interactions. Journal of the American Chemical Society 110 8029-8038. 

Fhe van der Waals and electrostatic interactions between atoms separated by three bonds (i.c. the 1,4 atoms) are often treated differently from other non-bonded interactions. The interaction between such atoms contributes to the rotational barrier about the central bond, in conjunction with the torsional potential. These 1,4 non-bonded interactions are often scaled down by an empirical factor for example, a factor of 2.0 is suggested for both the electrostatic and van der Waals terms in the 1984 AMBER force field (a scale factor of 1/1.2 is used for the electrostatic terms in the 1995 AMBER force field). There are several reasons why one would wish to scale the 1,4 interactions. The error associated wilh the use of an repulsion term (which is too steep compared with the more correct exponential term) would be most significant for 1,4 atoms. In addition, when two 1,4... 

Recall (Section 3 1) that the rotational barrier in ethane IS only 12 kJ/mol (3 kcal/mol)... 

Ionization energies can be computed to about 0.2 eV rotational barriers to about 0.5 kcal/mol dipole moments to about 0.5 D barriers to inversion to about 2.5 kcal/mol infrared frequencies can be computed with about a 15% error (usuaHy too high) and protonation energies are accurate to about 1 piCunit. 

Butadiene, the simplest conjugated diene, has been the subject of intensive theoretical and experimental studies to understand its physical and chemical properties. The conjugation of the double bonds makes it 15 kJ/mole (3.6 kcal/mol) (13) more thermodynamically stable than a molecule with two isolated single bonds. The r-trans isomer, often called the trans form, is more stable than the s-cis form at room temperature. Although there is a 20 kJ/mole (4.8 kcal/mol) rotational barrier (14,15), rapid equiUbrium allows reactions to take place with either the s-cis or r-trans form (16,17). 

Thiirane, 2-phenyl-conformation rotational barriers, 7, 138 polymerization, 7, 144 Thiirane, tetraaryl-synthesis, 7, 175 Thiirane, tetrafluoro-halogenation, 7, 148 polymerization, 7, 144 reactions... 

Although the rotation barrier is chiefly created by the high-frequency modes, it is necessary to consider coupling to low-frequency vibrations in order to account for subtler effects such as temperature shift and broadening of tunneling lines. The interaction with the vibrations q (with masses and frequencies m , tu ) has the form... 

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. ... 

The rotational barrier in methylsilane (Table 3.4, entry 5) is significantly smaller than that in ethane (1.7 versus 2.88 kcal/mol). This reflects the decreased electron-electron rqjulsions in the eclipsed conformation resulting from the longer carbon-silicon bond length (1.87 A) compared to the carbon-carbon bond length (1.54 A) in ethane. 

The haloethanes all have similar rotational barriers of 3.2-3.7 kcal/mol. The increase in the barrier height relative to ethane is probably due to a van der Waals rqjulsive efiect. The heavier halogens have larger van der Waals radii, but this is ofiset by the longer bond lengths, so that the net efiect is a relatively constant rotational barrier for each of the ethyl halides. 

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. 

There is another mechanism for equilibration of the cation pairs A, Aj and B, Bj, namely, inversion at oxygen. However, the observed barrier represents at least the minimum for the C=0 rotational barrier and therefore demonstrates that the C-O bond has double-bond character. 

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... 

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