Propene internal rotational barrier

Figure 8 Depiction of major factors contributing to propene internal rotation barrier, Cme-Csec bond weakening, designated in red Cme-Hip bond and Csec-H antibond interaction, designated in green. Contour diagrams are as in Figure 5...

Methyl rotors pose relatively simple, fundamental questions about the nature of noncovalent interactions within molecules. The discovery in the late 1930s1 of the 1025 cm-1 potential energy barrier to internal rotation in ethane was surprising, since no covalent chemical bonds are formed or broken as methyl rotates. By now it is clear that the methyl torsional potential depends sensitively on the local chemical environment. The barrier is 690 cm-1 in propene,2 comparable to ethane,... 

Most barriers to internal rotation turn out to be repulsive dominant. Such is the case for methanol, methylamine, propane, propene, hydrazine, and, as has been seen, ethane and ethyl fluoride. Attractive dominant barriers are indicated for acetaldehyde, hydroxylamine, and hydrogen peroxide. 

The tables may be used to calculate barrier heights in cases where appropriate spectroscopic data are not available, but where experimental values of heat capacity or entropy are known at one or more temperatures. The calorimetrically determined value of the barrier height may then be used in conjunction with the tables to calculate internal rotation contributions to thermodynamic properties over an extended temperature range. Examples of this procedure include calculations for ethane," propene, acetaldehyde, buta-1,2-diene, acetic acid, hexafluoro-ethane, 3-methylthiophen, and 2-methylthiophen. Where spectroscopic values of the barrier height have subsequently been determined, satisfactory agreement has been obtained with the earlier calorimetric values. The agreement between calorimetric (8.16 kJ mol ) and subsequent micro-wave [(8.28 0.07) kJ mol ] values of the barrier height in propene... 

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