The composition and structures of the two protonated species formed
from uncharged molybdic acid, MoO2(OH)2(OH2)2
0, in strongly acidic solutions have
been investigated using a combination of density functional theory
calculations, first-principles molecular dynamics simulations, and
Raman spectroscopy. The calculations show that both protonated species
maintain the original octahedral structure of molybdic acid. Computed
pK
a values indicated that the O
moieties are the proton acceptor sites and, therefore, that MoO(OH)3(OH2)2
+ and Mo(OH)4(OH2)2
2+ are the probable protonated
forms of Mo(VI) in strong acid solutions, rather than the previously
accepted MoO2(OH)2–x
(OH2)2+x
x+ (x = 1, 2) species. This finding is shown
to be broadly consistent with the observed Raman spectra. Structural
details of MoO(OH)3(OH2)2
+ and Mo(OH)4(OH2)2
2+ are
reported.