where and are NBO orbital energies, and F is the Fock operator [52].
If the stabilization energy E(2) is large, then
there is a strong interaction between electron donors and electron
acceptors. Formation of H-bond in the conjugate bases of Deoxy-hexose
sugars implies that a certain amount of electronic charge is transferred
from the proton donor to the proton acceptor as well as a rearrangement
of electronic density within each atom occurs. Tables 7 and 8 list
charge transfer and stabilization energies,E(2) , for conventional and unconventional
intra molecular H-bonds in the conjugate bases of Deoxy-hexoses sugars
optimized at B3LYP/6-311++G(d,p) level. These H-bonds involve the
electronic charge transfer from LP to BD*; where the LP denotes the lone
pairs of the oxygen donor atoms, n (O), and BD* denotes the
anti-bonding (H-O) acceptors, δ*,.
The highest charge transfers are related to bifurcated intramolecular
H-bonds in the conjugate bases of aldo-pentose sugars. The following
stabilization energy values, E(2) , were found
for various bifurcated H-bonds in the conjugate bases of L-arabinose:
for O3…HO1-O1 and
O3…HO4-O4 (formed
from deprotonation of L-arabinose at
C3-HO3), ∑ E(2)= 61.13 kcal.mol-1; for
O4…HO2-O2 and
O4…HO3-O3 (formed
from deprotonation of D-ribose at
C4-HO4), ∑ E(2)= 38.65 kcal.mol-1; for
O3…HO2-O2 and
O3…HO4-O4 (formed
from deprotonation of D-ribose at
C3-HO3), ∑ E(2)= 24.86 kcal.mol-1; and for
O2…HO1-O1 and
O2…HO3-O3 (formed
from deprotonation of L-lyxose protonated at
C2-HO2), ∑ E(2)=28.20 kcal.mol-1. There are good correlations between
the delocalization interaction energy and the other parameters obtained
by AIM analysis for describing the strength of hydrogen bonding (Figures
4 and 5).
The orbital interactions For L-arabinose deprotonated at
C3-HO3 aren O3→σ*(HO1-O1)
andn O3→σ*(HO4-O4)
with ∑ E(2) =61.13
kcal.mol-1; and for D-ribose deprotonated at
C4-HO4, they aren O4→σ*(HO2-O2),n O4→σ*(HO3-O3)
with ∑ E(2) =38.65
kcal.mol-1. The highest stabilization energyE(2) corresponds to the orbital interaction ofn O4→σ*(HO3-O3) in
L-fucose deprotonated at C4-HO4 withE(2)= 17.46 kcal.mol-1; andn O2→σ*(HO3-O3) in
L-rhamnose deprotonated C2-HO2 withE(2)= 14.42 kcal.mol-1.
The NBO and AIM results account for stabilization of the conjugate bases
of CH3-pentose and aldo-pentose sugars via
intramolecular H-bonds, which make them more acidic than normal alcohols
containing just one OH group.