Biomedical Engineering Reference
In-Depth Information
Fig. 5.4
Models of the sugar
moiety in the nucleic acid
backbone: tetrahydrofuran
(top left),
3-hydroxytetrahydrofuran
(bottom left), and
2-deoxyribose (right). The
molecules are not drawn to
the same relative scale
of calculated results to gas-phase experiments would require a conformational
average. Accordingly, we began our study of THF [
95
] with preliminary SMC
calculations at the static-exchange level for two different puckered geometries,
one having C
2
symmetry and one having C
s
symmetry, as well as for the C
2
v
(planar ring) geometry that had been imposed, for computational efficiency, in
previous calculations [
96
,
97
]. The SMC calculations produced two broad resonance
peaks whose energies were quite similar in the two puckered geometries; however,
in C
2
v
the splitting between the peaks increases by about 3 eV, indicating that
forcing the ring to be planar introduces significant error. The full calculation with
polarization effects included was therefore carried out on the C
2
structure. The
resulting integral cross section, Fig.
5.5
, exhibits a broad resonance maximum at
8.3 eV and a still broader peak or shoulder centered between 13 and 14 eV. Each
peak contains both
2
A
and
2
B
symmetry components, indicating that it arises from
more than one resonance. Notably absent, however, are any narrow resonances, or
indeed any evident resonances below those at about 8 eV, that could temporarily
trap electrons and promote dissociative attachment. Indeed, experiment shows only
very weak dissociative-attachment peaks, one near 1.25 and the other near 7.5 eV
[
42
]. Several other calculations [
96
-
98
] and measurements [
99
-
101
]oflow-energy
elastic scattering by THF have been carried out, and these are in general agreement
with our results. As an example, Fig.
5.6
shows differential cross sections (that is,
scattering as a function of the angle between the incident beam and the direction of
detection) at selected energies as determined by various groups. Although there are
differences in detail, overall agreement is satisfactory.
