Chem3D Users Manual - CambridgeSoft

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The V1 torsional constant creates a torsional energy
difference between the conformations represented
by the two torsional energy minima of the V2
constant. As discussed previously, a negative value
of V1 means that a torsional energy minimum
occurs at 0° and a torsional energy maximum
occurs at 180°. The value of V1=-0.100 means that
cis-2-butene is a torsional energy minimum that is
0.100 kcal/mole lower in energy than the torsional
energy maximum represented by trans-2-butene.
The counterintuitive fact that the V1 field is
negative can be understood by remembering that
only the total energy can be compared to
experimental results. In fact, the total energy of
trans-2-butene is computed to be 1.423 kcal/mole
lower than the total energy of cis-2-butene. This
corresponds closely with experimental results. The
negative V1 term has been introduced to
compensate for an overestimation of the energy
difference based solely on van der Waals repulsion
between the methyl groups and hydrogens on
opposite ends of the double bond. This example
illustrates an important lesson:
There is not necessarily any correspondence
between the value of a particular parameter used in
MM2 calculations and value of a particular physical
property of a molecule.
V3
The V3, or 120° Periodicity Torsional constant,
field contains the third of three principal torsional
constants used to compute the total torsional
energy in a molecule. V3 derives its name from the
fact that a torsional constant of 120° periodicity can
have three torsional energy minima and three
torsional energy maxima within a 360° period. A
positive value of V3 indicates there are minima at -
60°, +60° and +180° and there are maxima at -
120°, 0°, and +120° in a 360° period. A negative
value of V3 causes the position of the maxima and
minima to be reversed, as in the case of V1 and V2
above. The significance of V3 is explained in the
following example.
The 1-1-1-1 torsional parameter of n-butane is an
example of the V3 torsional constant. The values of
V1, V2 and V3 in the Torsional Parameters table
are 0.200, 0.270 and 0.093 respectively. Because a
positive value of V3 indicates that there are minima
at -60°, +60° and +180° and there are maxima at -
120°, 0°, and +120°, the minima at ±60° signify the
two conformations of n-butane in which the methyl
groups are gauche to one another. The +180°
minimum represents the conformation in which the
methyl groups are anti to one another. The
maximum at 0° represents the conformation in
which the methyl groups are eclipsed. The maxima
at ±120° conform n-butane in which a methyl
group and a hydrogen are eclipsed.
The V1 and V2 torsional constants in this example
affect the torsional energy in a similar way to the V1
torsional constant for torsions about a carboncarbon
double bond (see previous example).
NOTE: The results of MM2 calculations on hydrocarbons
do not correspond well with the experimental data on
hydrocarbons when only the V3 torsional constant is used
(when V1 and V2 are set to zero). However, including small
values for the V1 and V2 torsional constants in the MM2
calculations for hydrocarbons dramatically improve the
correspondence of the MM2 results with experimental results.
This use of V1 and V2 provides little correspondence to any
particular physical property of hydrocarbons.
286• Parameter Tables CambridgeSoft
Torsional Parameters