Ching-Hsing Yu, Lothar Schäfer,
Michael Ramek, David M. Miller, and Brian J. Teppen,
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(X-C1-C2-C3),
(C1-C2-C3-Y)-torsional
spaces. The results make it possible to
construct parameter surfaces and their gradients in
,-space. The magnitude of the gradient,
|
P|,
of a structural parameter P (a bond length, bond angle, or non-bonded distance)
in ,-torsional space
is a measure of torsional sensitivity (TS); i.e., a measure of
the extent to which bond lengths, bond angles, and non-bonded distances
change at a point in ,-space with backbone torsional angles.
It is found that TS is not constant throughout the conformational space of
a molecule, but varies in a characteristic way. It seems that, regardless
of the nature of X or Y, extended forms are typically in regions of low TS;
puckered conformations, of high TS. Conformations with two sequential
gauche torsional angles (GG sequences) are characterized by high TS of
1,5-non bonded distances concomitant with relatively low TS of other
internal coordinates. This property of GG sequences is the source of
a stabilizing and cooperative energy increment that is not afforded by
other torsional sequences, such as trans-trans or trans-gauche. A structural
data base, consisting of thousands of HF/4-21G structures of X-C-C-Y and
X-C-C-C-Y systems, has been assembled and is available on a CD.