Conformational energies torsional barrier

The structures, energies, torsional barriers and vibrational spectra of three rotamers of tetrasulfane, H2S4, have been examined by Drozdova, Miaskiewicz and Steudel at the MP2/6-311G level [34]. Surprisingly, the cis-trans conformation (motif -l-H— symmetry Ci) is found to be most stable, followed by the all-cfs form (h—t symmetry C2), while the helical all-... 

Generally, only a small number of scaling constants are needed. For example, ten scaling constants and reference values were derived for hydrocarbons from comparisons of gas phase structures, conformational energies, rotational barriers, and vibrational frequencies measured by experiment and calculated by the QMFF. For the bond and bond angle energy functions in equation (1) the same scale factor is multiplied by the QMFF quadratic, cubic, and quartic force constants. Similarly, the same scale factor is used for the one-, two-, and threefold torsion force constants, and a single scale factor is used for all cross terms. 

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

Table 5 Energy values obtained for the ground and first singlet and triplet excited states of trans-biacetyl in two extremal conformations of the methyl groups, as well as torsional barriers.

Certain physical properties show that rotation about the single bond is not quite free. For ethane there is an energy barrier of about 3 kcal mol-1 (12 kJ mol-1). The potential energy of the molecule is at a minimum for the staggered conformation, increases with rotation, and reaches a maximum at the eclipsed conformation. The energy required to rotate the atoms or groups about the carbon-carbon bond is called torsional energy. Torsional strain is the cause of the relative instability of the eclipsed conformation or any intermediate skew conformations. 

There remains an interpretation of ta to be found, ta exhibits an activation energy of about 0.43 0.1 eV, about three times as high as the C-C torsional barrier of 0.13 eV. The discrepancy must reflect the influence of the interactions with the environment and therefore ta appears to correspond to relaxation times most likely involving several correlated jumps. The experimental activation energy is in the range of that for the NMR correlation time associated with correlated conformational jumps in bulk PIB [136] (0.46 eV) and one could tentatively relate ta to the mechanism underlying this process (see later). 

Figure 10.1. Solid line—MOP AC AMI conformational energy curve for PFHD as a function of backbone torsion angle. Broken line—Scaled MOPAC AMI conformational energy curve for PFHD as a function of backbone torsion angle. The inset shows an expanded view of the minimum well and trans barrier.

Another feature deals with the trans, trans and trans, cis conformations of DPC, as shown in Fig. 55. The calculations indicate that the trans, trans conformation of DPC is preferred by 4.7 kj mol 1 over the trans, cis conformation, with a barrier of 4.7 kj mol 1 between these two conformations. The trans, cis energy minimum is at a torsion angle of 150° about the Cc - O" axis relative to the trans, trans geometry. 

Note Added in Proofs. There has been additional work on the calculation of conformational energies for cyclohexane, see Section II. Wiberg and Boyd 129) conclude that non-bonded interactions make little contribution to the barrier, the most important component of which is torsional strain, in agreement with earlier work. Quantum mechanical calculations of various conformations which do not by their nature allow a spUtting of energies into Bayer strain, Pitzer strain, and van der Waals strain, have also been made I30,i3i), it has been concluded 12D that transition state conformations 1 and 2 are of similar energies i. e. that there is pseudorotation in the transition state as proposed by Pickett and Strauss 29,30),... 

The appearance of crown forms in tetroxocane and other compounds with heteroatoms in the 1,3,5 and 7 positions probably results from two stabilizing effects (a) a slightly lower eclipsing barrier for the —O—CHg— versus the —CH2—CH2 fragment in the crown there are eight partially eclipsed bonds which would benefit from relief of torsional strain (b) distorted crowns, i.e. the chair-chair and twist-chair-chair may suffer from higher dipolar repulsions than does the crown. Unfortunately, there are no conformational energy calculations available on any of the heterocyclic systems. 

Cartledge and Profeta have used molecular mechanics in combination with ab initio calculations to study conformations and rotational potential energy functions.They fine-tuned silicon parameters for the MM2 force field. The goodness of fit was determined by the fit to experimental structures and ab initio torsional barriers. 

R. Stolevik and P. Bakken, /. Mol. Struct., 220, 269 (1990). Conformational Structures and Energies Rotational Barrier Heights and Torsional Oscillations in Tetrahalodiphosphmes, Tetrahalohydrazines and X2-PX2 Molecules. 

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