Repulsive Potential between Nonbonded Methyl Groups

The vibrational analysis of many molecules depends critically on our being able to supply values for some of the force constants. In particular, for many chemically interesting molecules it is important to know the repulsive force constants between two methyl groups bonded to the same atom, and we have used the known frequencies of tetramethyl carbon, silicon, germanium, tin, and lead to determine this force constant as a function of the carbon-carbon internuclear distance. Our results indicate that FCC=∂2V(rCC)/∂(rCC)2=196.5 rCC−6.8 md/A.