Anomalous phase shift between the angular dependence of the β 5′-H hfs constants produced by the phosphate groups in C4′ deoxyribose radicals: an electronic rather than a structural effect
Abstract
The angular dependence of the β 5′-H hyperfine splitting (hfs) constants in the 2′-deoxyribose 1′-amino 3′,5′-biphosphate C4′-radical (1) was previously computed to follow individually the relation aH5i′
=
B0i
+
B1i cos θi
+
B2i cos2θi (i
= a, b) at the UHF/6-31G//ROHF/3-21G level. The phase shift (Δφ
= 141°) between the angular dependence of the two aH5′
constants was found to be much larger than the normal value (Δφ
= 120°). This apparently anomalous behavior was
attributed to a distortion of geometry around C5′ owing to the presence of the phosphate groups. By contrast, it is demonstrated that a strongly electronegative β-substituent such as oxygen can produce a large phase shift. In the present case the anomalous phase shift is due exclusively to the electronic effect of the β-oxygen of the 5′-phosphate, whereas the 3′-phosphate has little effect. The aH5i′
values depend strongly on the dihedral angle ω between the singly occupied C4′-2pz orbital and the C5′-O bond and a unique relation, aH5′
=
A
+
B1 cos θ
+
B cos2θ
+
C cos θ cos ω, accounts for the angular
dependence of both the aH5′ constants. The angular dependence does not change significantly upon replacing the