The adsorption of formic acid and sodium formate on the stoichiometric anatase (101) surface has been
studied by means of density functional calculations with a slab geometry. On the clean surface, the most
stable adsorption structure for HCOOH is a molecular monodentate configuration, hydrogen bonded to a
surface bridging oxygen, while for HCOONa a dissociated bridging bidentate geometry is preferred. The
bidentate chelating structure is energetically unstable for both the acid and the salt. On the hydrated surface,
both HCOOH and HCOONa preferentially form an inner-sphere adsorption complex. HCOOH maintains a
monodentate coordination, but, due to the interaction with a nearby water molecule, it becomes dissociated,
while HCOONa again prefers a bridging bidentate structure. The energies for adsorption from an aqueous
solution are estimated to be 0.30 and 0.79 eV for HCOOH and HCOONa, respectively.
The adsorption of formate and acetate from aqueous solutions at pH 3-9 onto the TiO 2 rutile (110) surface was studied by ATR-FTIR spectroscopy. The spectra indicated that there was only one type of adsorbed species, and that formate and acetate were adsorbed in a similar manner. On the basis of the measured ν as -(COO)ν s (COO) splitting (∆ν as-s ) of 191 and 87 cm -1 , for formate and acetate, respectively, the monodentate (ester type) binding mode could be excluded. Ab initio calculations at the Hartree-Fock level showed that for pentacoordinated Ti IV , present in the (110) surface, the chelating bidentate binding mode is unstable with respect to the rearrangement to the monodentate or the bridging bidentate mode. The computed vibrational frequencies of formate and acetate adsorbed in a bridging bidentate mode onto Ti clusters with 2-5 Ti centers, representing the (110) surface, agreed with experiment and thus showed that this methodology can be used for the determination of the structures of adsorbates on, for example, metal oxide surfaces in contact with aqueous solutions.
We report the results of an investigation on the preparation, spectral, and photoelectrochemical properties of Ru(II)-polypyridyl complexes containing a new phosphonated terpyridine (P-terpy) ligand: [Ru(H(2)P-terpy)(2)] and [Ru(HP-terpy)(Me(2)bpy)(NCS)]. Resonance Raman spectral and luminescence studies allow probing into the nature of the low-energy MLCT transitions observed in these complexes. The crystal and molecular structure of the mixed-ligand complex [Ru(HP-terpy)(Me(2)bpy)(NCS)] based on X-ray diffraction study is reported. This complex appears to be a promising candidate as a photosensitizer in dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO(2).
The mechanism for the water-exchange reaction with the transition metal aqua ions from Sc III through Zn II has been investigated. The exchange mechanisms were analyzed on the previously reported model (Rotzinger, F. P. J. Am. Chem. Soc. 1996, 118, 6760) that involves the metal ion with six or seven water molecules. The structures of the reactants/products, transition states, and penta-or heptacoordinated intermediates have been computed with Hartree-Fock or CAS-SCF methods. Each type of mechanism, associative, concerted or dissociative, proceeds via a characteristic transition state. The calculated activation energies agree with the experimental ∆G q 298 or ∆H q 298 values, and the computed structural changes indicate whether an expansion or compression takes place during the transformation of the reactant into the transition state. These changes are in perfect agreement with the changes deduced from the experimental volumes of activation (∆V q 298 ). The motions of the ligands involved in the exchange reaction are described by the imaginary vibrational mode. All these computed quantities allow the attribution of the water-exchange reactions to the A, I a , or D mechanisms with use of the terminology of Merbach (Merbach, A. E. Pure Appl. Chem. 1982Chem. , 54, 1479. Within the present model, no transition state has been found for the I d mechanism. It remains to be verified, using an improved model, whether it really does not exist. The dissociative mechanism is always feasible, but it is the only possible pathway for high-spin d 8 , d 9 , and d 10 systems. In contrast, the associative mechanism requires that the transition metal ion does not have more than seven 3d electrons. Thus, Sc III , Ti III , and V III react via the A, Ni II , Cu II , and Zn II via the D (or I d ) mechanism, whereas for the elements in the middle of the periodic table, the high-spin 3d 3 -3d 7 systems, both associative (I a /A) and dissociative (D) pathways are feasible. The present results suggest that for Sc III hexa-and heptacoordinated species could coexist in aqueous solution.
The structures of the transition states and
intermediates formed in the water-exchange of hexaaqua
of the first row transition elements have been computed with ab initio
methods at the Hartree−Fock or CAS-SCF
level. As an approximation, water molecules in the second
coordination sphere except one, bulk water, and anions
have been neglected. For each of the three types of activation,
namely associative, concerted, and dissociative
mechanism, a representative transition metal complex has been studied,
Each type of mechanism proceeds via a characteristic transition
state. For the A and D mechanisms, respectively,
hepta- or pentacoordinated intermediates are formed, and their
lifetimes were estimated based on the energy difference
between that of the transition state and the corresponding
intermediate. The computed activation energies are in
agreement with the experimental values and are independent of the
mechanism or the charge on the metal center.
The bond length changes occurring during the activation agree with
the corresponding experimental ΔV
a recent article, Åkesson et al. (J.
1994, 116, 8705) proposed an
interpretation of the experimental
⧧ values that differs from that commonly
applied (Merbach, A. E. Pure Appl. Chem.
59, 161). In particular,
they claimed a dissociative activation for the water-exchange of the
hexaaqua ions of VII and MnII in spite of
negative volumes of activation. The present computational results
on VII are in perfect agreement with the
mechanism attributed on the basis of its ΔV
value. It should be noted that in principle, the D mechanism is
for all the hexaaqua ions of the first transition series, but in many
cases, the associative or concerted pathway is
preferred. For a given complex, all the possible mechanisms must
be analyzed, before the most favorable pathway
can be determined. The presently studied case of VII,
where equal energies of activation have been computed for
the Ia and D mechanism, illustrates this point. The
attribution of the mechanism was only possible by
with the experimental volume of activation. Computed energies of
activation alone may not suffice to identify the
mechanism; a safe attribution can only be made if the structural
changes agree with the volume of activation.
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