Photoionization Spectroscopic and Theoretical Study on the Molecular Structures of cis- and trans-3-Chlorothioanisole.

Resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra are measured for the cis- and trans-3-chlorothioanisole (3ClTA). The first electronic excitation energy (E 1) and the adiabatic ionization energy (IE) of the cis-rotamer are determined to be 33 959±3 and 65 326±5 cm-1, respectively, and those of the trans-rotamer are determined to be 34102±3 and 65 471±5 cm-1, respectively. Density functional theory (DFT) calculations confirm that both the cis- and trans-rotamers of 3ClTA are stable and coexist in their respective S0, S1, and D0 states. Both rotamers adopt planar structures with cis- being slightly more stable than trans- in the respective S0, S1, and D0 states. The conformation, substitution, and isotope effects on the molecular structure, active vibrations, and electronic transition and ionization energies of 3ClTA are analyzed.

R2PI and MATI spectroscopy of 3-fluoro-5-methylanisole: Combined effect of meta-substituents on molecular conformation.

The resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra of 3-fluoro-5-methylanisole (3F5MA) were recorded to explore the conformers arising from the rotation of meta-methyl group (m-CH3) and methoxy group (OCH3), namely the staggered (s)/eclipsed (e)-cis/trans 3F5MA. The theoretical calculations predicted that the stable conformer of cis 3F5MA is staggered in the S0 and S1 states, but eclipsed in the D0 state. While for trans 3F5MA, the staggered conformer is stable only in the S1 state and the eclipsed one is stable in the S0 and D0 states. The first electronic excitation energies (E1s) of cis and trans 3F5MA were determined to be 36,709 ±â€¯3 and 36,615 ±â€¯3 cm-1 by the R2PI spectroscopy. Correspondingly, by the MATI spectroscopy, the adiabatic ionization energies (IEs) were measured to be 66,908 ±â€¯5 and 66,692 ±â€¯5 cm-1. Compared with the cis 3F5MA, more low-frequency vibronic bands assigned to m-CH3 torsions are observed for the trans conformer in the R2PI spectrum, supporting the theoretically predicated e-trans → s-trans isomerization upon excitation. The MATI spectra of the cis and trans conformers are similar. Most of the observed cationic bands are related to the m-CH3 torsion modes, revealing the s-cis → e-cis and s-trans → e-trans isomerizations upon the D0 ← S1 ionization. By comparing with several analogues, it is found that the s/e conformational preference in each electronic state is mainly influenced by OCH3 group instead of the F atom. The combined effects of meta-substituents on molecular conformation and transition energies are discussed in detail. A kind of additivity of meta-substituent effects on s/e preference is qualitatively true for the S0 and S1 states but false for the D0 state. This is different from that of ortho-substituent effect, which is previously reported that the additivity of ortho-substituent effects on methyl rotation barriers in 2-fluoro-6-chlorotoluene is applicable to all the three electronic states while the additivity of meta-substituent effect on cis/trans preference is found to be true in all three electronic states of 3F5MA.

Modulation of Conformational Preferences of Heteroaromatic Ethers and Amides through Protonation and Ionization: Charge Effect.

Multiple approaches reveal the strong effects of a positive charge introduced by protonation or ionization on the conformation of o-heteroaromatic ethers and amides. The ethers and amides containing an ortho-N heteroatom are syn-preferring while those containing an ortho-O or ortho-S heteroatom are mostly anti-preferring. However, for all the monocyclic o-heteroaromatic ethers and amides, the protonated ones are all anti-preferring while the ionized ones are all syn-preferring. Interestingly, although both the protonation and ionization introduce a positive charge, they have such different effects on molecular conformation, very informative for understanding the origin of conformational preferences. Detailed analysis shows that the population of the introduced positive charge dictates the conformational preferences via electrostatic and orbital interactions. Compared to ortho-heteroatoms, meta-heteroatoms have weaker effect on conformational preference. Achieved by complete inductive method, the regularity of conformational preferences and switching provides easy ways to modulate conformers (by pH or redox), and makes this kind of ether or amide bond a conformational hinge applicable to design of functional molecules (drugs and materials) and modulation of molecular biological processes.

Conformational preferences and isomerization upon excitation/ionization of 2-methoxypyridine and 2-N-methylaminopyridine.

Conformers from the rotations of the methyl group and the methoxy or methylamino group, namely staggered (s)/eclipsed (e)-cis/trans 2-methoxypyridine (2MOP) and 2-N-methylaminopyridine (2NMP), are studied using theoretical calculations in combination with one-color resonance-enhanced two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopies. The calculations predict that, for cis 2MOP, trans 2MOP and trans 2NMP, only the s conformers are stable in the S0, S1 and D0 states. However, for cis 2NMP, the stable conformer is staggered in the S0 state but eclipsed in the S1 and D0 states, indicating an isomerization upon the excitation or ionization from the S0 state. This is experimentally supported by the 1C-R2PI and MATI spectra of 2NMP. Due to the relative instability, the number density of trans 2MOP is too low in the sample to be detected. All the bands in the 1C-R2PI and MATI spectra of 2MOP are assigned to s-cis 2MOP. The energy differences between cis and trans conformers are derived from excitation and ionization energies, indicating another conformational isomerization: stable trans 2NMP in the S0 and S1 states but stable cis 2NMP in the D0 state. For 2MOP, the so-called syn preference previously found for the S0 state is also observed in the S1 and D0 states. The conformational preference and isomerization are discussed with natural bond orbital calculations and reduced density gradient analysis. For 2MOP, the syn preferences are mainly caused by the exchange repulsion among several σ-orbitals of the OCH3 group and the pyridine ring. While the relative stabilities of the s and e conformers of cis 2MOP and cis 2NMP are simultaneously influenced by steric repulsion and orbital interactions.

A mini-photofragment translational spectrometer with ion velocity map imaging using low voltage acceleration.

A mini time-sliced ion velocity map imaging photofragment translational spectrometer using low voltage acceleration has been constructed. The innovation of this apparatus adopts a relative low voltage (30-150 V) to substitute the traditional high voltage (650-4000 V) to accelerate and focus the fragment ions. The overall length of the flight path is merely 12 cm. There are many advantages for this instrument, such as compact structure, less interference, and easy to operate and control. Low voltage acceleration gives a longer turn-around time to the photofragment ions forming a thicker Newton sphere, which provides sufficient time for slicing. Ion trajectory simulation has been performed for determining the structure dimensions and the operating voltages. The photodissociation and multiphoton ionization of O2 at 224.999 nm is used to calibrate the ion images and examine the overall performance of the new spectrometer. The velocity resolution (Δν/ν) of this spectrometer from O2 photodissociation is about 0.8%, which is better than most previous results using high acceleration voltage. For the case of CF3I dissociation at 277.38 nm, many CF3 vibrational states have been resolved, and the anisotropy parameter has been measured. The application of low voltage acceleration has shown its advantages on the ion velocity map imaging (VMI) apparatus. The miniaturization of the VMI instruments can be realized on the premise of high resolution.

Ionization spectroscopies and theoretical calculations of cis and trans 3-fluoro-N-methylaniline-Arn(n=1,2) van der Waals clusters: Structures and binding energies.

The ab initio and dispersion-corrected density functional theory (DFT) calculations of the van der Waals (vdW) clusters of cis and trans 3-fluoro-N-methylaniline-Arn (3FNMA-Arn) (n=1,2) predict that, for cis and trans 3FNMA-Ar1 clusters, the π-bound geometry with the Ar atom sitting over the ring is the global minimum in both neutral and cationic states, while for cis and trans 3FNMA-Ar2 clusters, the [π|π]-bound sandwich structure with two Ar ligands occupying nearly equivalent π-bound positions above and below the ring plane is the global minimum. The vibronic spectra of cis and trans 3FNMA-Ar1 clusters in the S1 state were recorded by using one-color and two-color resonant two-photon ionization (R2PI) techniques, the comparison of which yields an estimate of the binding energy of cluster in the S1 state. It is found that the linear correlation between the redshift of the S1←S0 electronic transition energy (E1) of cluster and the E1 of the monomer also holds for the Ar clusters of hetero-di-substituted aromatics. By recording the photoionization efficiency (PIE) curves and mass-analyzed threshold ionization (MATI) dissociation spectra of cis and trans 3FNMA-Ar1 clusters, the ionization energies (IE) and binding energies of clusters in the D0 state are obtained. The MATI spectra of the cis and trans 3FNMA-Ar1 cations exhibit significant progressions of the vdW bending mode (bx), indicating the structural changes of the clusters in the bending coordinate upon ionization. The combination of the three kinds of spectroscopic methods provides the binding energies of cis and trans 3FNMA-Ar1 clusters in the S0, S1 and D0 states with good accuracy, and the values estimated by dispersion-corrected DFT calculations compare well with the experimental results. From our studies, the ωB97X-D and TD-ωB97X-D methods using high quality basis set are recommended for studying the intermolecular interaction of such vdW clusters in the ground and excited states.

Resolved (v1, v2 = 1) Combination Vibrational States of CF3 Fragments in the Photofragment Translational Spectra of CF3I.

The photodissociation of CF3I → CF3(v1,v2) + I*/I has been investigated at 248, 266, and 277 nm with our high resolution mini-TOF photofragment translational spectrometer. Based on the theoretical calculations of Clary and of Bowman et al., now in this manuscript, we assign 701 cm-1 to the CF symmetric stretch (breathing) ν1 mode, and 1086 cm-1 to the umbrella ν2 mode of the CF3 fragment. In the obtained TOF spectra of I+ from the I* channel, situated in the 701 cm-1 gaps between the original series of (v1, 0) vibrational peaks, a new series of weaker (v1, 1) vibrational peaks are partially resolved. These observed new peaks with 1086 cm-1 ν2 mode excitation have never been reported in previous literature. In the TOF spectra of I+ from the I channel, the new series of (v1, 1) peaks are also partially resolved. However, these spectra of I channel are less satisfactory, because for higher Eavl and higher ET, the higher resolution of PTS is required. The potential energy at the curve crossing point and the excitation of CF3 (v1, 2) and (v1, 3) vibrational states have been also analyzed.

Photodissociation dynamics of ICH2Cl → CH2Cl + I*/I: photofragment translational spectroscopy at 304 and 277 nm.

The photodissociation dynamics of ICH2Cl → CH2Cl + I*/I at 304 and 277 nm has been investigated with our mini-TOF photofragment translational spectrometer with a weak acceleration field of <1 V cm(-1). Many peaks are resolved or partially resolved in the TOF spectra and the photofragment translational spectra (PTS) of both the I*((2)P1/2) channel and the I((2)P3/2) channel. These resolved peaks are assigned to the C-Cl stretch vibrational states of the CH2Cl fragment. The rotational energy ER of the CH2Cl fragment is highly excited due to its asymmetric structure. The value of ER/ET is measured to be about 0.71. In the I* channel, the partitioning of the available energy Eavl into the translational energy ET, the rotational energy ER, and the vibrational energy EV for each resolved vibrational state has been calculated.

Vibrational Spectra and Theoretical Calculations of cis- and trans-3-Fluoro-N-methylaniline in the Neutral (S(0)) and Cationic (D(0)) Ground States.

The mass-analyzed threshold ionization spectra of jet-cooled cis- and trans-3-fluoro-N-methylaniline (3FNMA) were recorded by ionizing via the vibrationless 0(0) and various vibrational levels of the S1 state. The adiabatic ionization energies of cis- and trans-3FNMA are determined to be 61,742 ± 5 and 61,602 ± 5 cm(-1), respectively. In the 0-1800 cm(-1) region, most of the observed vibrations in the D0 state result from the in-plane ring deformation and substituent-sensitive modes. For the high-frequency vibration region, the infrared-ultraviolet double-resonance and autoionization-detected infrared spectroscopies were applied to investigate the N-H and C-H stretching vibrations of bare 3FNMA in the S0 and D0 states. The C-H stretching vibrational information, which we failed to obtain for the bare 3FNMA cation, is complemented by recording the infrared-photodissociation spectra of its Ar cluster cation. It is revealed that a red-shifted frequency and an enhanced intensity are observed for the N-H stretch, while blue-shifted frequencies and greatly decreased intensities are found for both aromatic and the methyl C-H stretches. The blue shift of the C-H stretches is first explained by the balance of two factors, namely, the hyperconjugative interaction and the rehybridization effect. Analysis of the vibrational frequencies reveals a correlation between the relative stability of two rotamers in different electronic states and the relative rigidity of aromatic ring, indicating a mechanism of the long-range interactions "through bond" between the substituents. The density functional theory calculations can well reproduce the vibrational spectra in both S0 and D0 states. With the experimental and theoretical data, the substitution and conformation effects on the properties of 3FNMA in the S0 and D0 states, including the molecular structures, the reactive sites of electrophilic attack, and the vibrational behaviors, were discussed in detail.

Vibrationally mediated photodissociation of CH3I [v1 = 1] at 277.5 nm: the vibrationally adiabatic process.

From the photofragment translational spectra of C-H symmetric stretch excited CH3I [v1 = 1, v2 = 0] photodissociatioin at 277.5 nm, the vibrational distribution of photofragments CH3 (v1 = 0, v2 = 0), (0,1), (1,0), (1,1) in the I* channel are measured to be 0.02, 0.02, 0.47, 0.25, and those of CH3 (1,0), (1,1) in the I channel are 0.04, 0.05, respectively. It shows that most of the dissociated CH3I [1,0] retain the C-H symmetric stretch vibration v1 = 1 in the photofragments CH3, and the vibrational distribution in umbrella bending mode is not seriously affected by the original C-H symmetric stretch excitation. The photodissociation of CH3I [1,0] mainly follows the vibrationally adiabatic process. The original vibrational excitation [v1 = 1] of CH3I is quite like a spectator, and the intramolecular vibrational-energy redistribution (IVR) does not play obvious part during photodissociation.

Rotamers and isotopomers of 3-chloro-5-fluoroanisole studied by resonant two-photon ionization spectroscopy and theoretical calculations.

The ab initio and density functional theory (DFT) calculations reveal that two rotamers, denoted by cis and trans 3-chloro-5-fluoroanisole (3C5FA), are stable for each of the S(0), S(1), and D(0) states. In the one-color resonant two-photon ionization (R2PI) spectra, the band origins of the S(1)←S(0) electronic transition (0(0) bands) of cis(35)Cl-3C5FA and cis(37)Cl-3C5FA are both located at 36,468 ± 3 cm(-1), while the 0(0) bands of trans(35)Cl-3C5FA and trans(37)Cl-3C5FA are found to be 36,351 ± 3 and 36,354 ± 3 cm(-1). The two rotamers display very similar vibrational frequencies in the S(1) state, and the observed active modes mainly involve the in-plane ring deformation vibrations. By the two-color R2PI spectroscopy, the adiabatic ionization energies (IEs) of both isotopomers of 3C5FA are determined to be 69,720 ± 15 cm(-1) for the cis rotamer and 69,636 ± 15 cm(-1) for the trans rotamer. The substitution, conformation, and isotope effects on the properties of 3C5FA, including the molecular structures, vibrational frequencies, and electronic transition and ionization energies, were also discussed in detail.

Photofragment translational spectroscopy of CH3I at 225 nm--with the high excitation of the symmetric stretch vibration of CH3 fragment.

The photodissociation dynamics of CH(3)I at 225 nm is studied on our high resolution mini-TOF photofragment translational spectrometer. The photofragment translational spectra of the I* and the I channels via parallel (∥) and perpendicular (⊥) transitions, i.e., of the four pathways (3)Q(0), (3)Q(0) ← (1)Q(1), (1)Q(1), and (1)Q(1) ← (3)Q(0), are obtained with both the symmetric stretch (ν(1)) and the umbrella (ν(2)) vibrational modes of the CH(3) fragments partially resolved. The strong excitation of the symmetric stretch mode (ν(1)) is revealed in both the I and the I* channels. The branching fractions for the four pathways (0.09 for (3)Q(0), 0.03 for (3)Q(0) ← (1)Q(1), 0.34 for (1)Q(1), and 0.54 for (1)Q(1) ← (3)Q(0)) show that the parallel transition ((3)Q(0) ← X) is the major and the I channel is dominant in the photodissociation of CH(3)I at 225 nm. The curve-crossing probability is found to be 0.86 for (1)Q(1) ← (3)Q(0) but 0.08 for (3)Q(0) ← (1)Q(1).

Resonance-enhanced two-photon ionization spectroscopy and theoretical calculations of 3,5-difluoroanisole and its Ar-containing complex.

The structure and vibrations of 3,5-difluoroanisole (3,5-DFA) in the first electronically excited (S(1)) state were studied by mass-analyzed resonant two-photon ionization (R2PI) technique as well as the quantum chemical calculations. The ab initio and density functional theory (DFT) calculations reveal that only one structure is stable for each of the S(0), S(1), and D(0) states. In the one color R2PI spectrum, the band origin of the S(1)←S(0) electronic transition (0(0) band) of 3,5-DFA is found to be 37,595±3 cm(-1). In the S(1) state, most of the bands observed are related to the in-plane ring deformation and out-of-plane bending vibrations. The adiabatic ionization energy (IE) of 3,5-DFA is determined to be 70,096±15 cm(-1) by the two color R2PI technique, in agreement with the values predicted by the DFT approaches. The dihalogen-substitution effects on the molecular structure, vibrational frequencies, and electronic transition and ionization energies were discussed in detail. The van der Waals complex of 3,5-DFA with argon (3,5-DFA···Ar) was also observed and studied. The 0(0) band of 3,5-DFA···Ar complex is red-shifted by about 9 cm(-1) with respect to that of 3,5-DFA. Both the experimental data and the calculated results indicate that the formation of 3,5-DFA···Ar complex gives only a weak influence on the properties of 3,5-DFA moiety.

Communication: probing the entrance channels of the X+CH4→HX+CH3 (X = F, Cl, Br, I) reactions via photodetachment of X(-)-CH4.

The entrance channel potentials of the prototypical polyatomic reaction family X + CH(4) → HX + CH(3) (X = F, Cl, Br, I) are investigated using anion photoelectron spectroscopy and high-level ab initio electronic structure computations. The pre-reactive van der Waals (vdW) wells of these reactions are probed for X = Cl, Br, I by photodetachment spectra of the corresponding X(-)-CH(4) anion complex. For F-CH(4), a spin-orbit splitting (∼1310 cm(-1)) much larger than that of the F atom (404 cm(-1)) was observed, in good agreement with theory. This showed that in the case of the F-CH(4) system the vertical transition from the anion ground state to the neutral potentials accesses a region between the vdW valley and transition state of the early-barrier F + CH(4) reaction. The doublet splittings observed in the other halogen complexes are close to the isolated atomic spin-orbit splittings, also in agreement with theory.

Vibrationally resolved photofragment translational spectroscopy of CH3I from 277 to 304 nm with increasing effect of the hot band.

The photodissociation dynamics of CH(3)I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH(3)I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH(3) fragment. There are some extra peaks showing the existence of the hot-band states of CH(3)I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH(3) fragment is determined. The fraction σ of photofragments produced from the hot-band CH(3)I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I* channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel . E(int)/E(avl) of photofragments from ground-state CH(3)I remains at about 0.03 in the I* channel for all four wavelengths, but E(int)/E(avl) decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel . From the ground-state CH(3)I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P(cc) from the hot-band CH(3)I is lower than that from the ground-state CH(3)I. The potential energy at the curve-crossing point is determined to be 32,740 cm(-1).

Ultraviolet photodissociation of C2F5I with a small and simple photofragment translational spectrometer.

Photodissociation dynamics of C(2)F(5)I near 280 and 304 nm has been investigated on a small and simple time-of-flight photofragment translational spectrometer (PTS). On this new PTS, the photolyzed and ionized fragments, not accelerated by electric field, travel freely for a short flight path (<50 mm) and are detected by microchannel plates. In the spectra of the I(*)((2)P(1/2)) channel at 281.73 and 304.02 nm, vibrational peaks with spacing of approximately 350 cm(-1) are partially resolved, indicating the preferential excitation of CF(2) wag mode (nu(11)=366 cm(-1)) of C(2)F(5) photofragment. The fraction of the available energy disposed into the internal energy is higher than 50% for both I(*) channel and I channel, showing the high excitation of vibration in the C(2)F(5) fragments. The fragment recoil anisotropy parameter beta(I(*)), determined to be 1.70 at 281.73 nm and 1.64 at 304.02 nm, reveals that I(*) atoms are produced predominantly from the parallel (3)Q(0) <-- N transition. The anisotropy parameter beta(I), determined to be 1.25 at 279.71 nm and 0.88 at 304.67 nm, implies that I atoms are produced from two excited states, i.e., direct dissociation via the perpendicular (3)Q(1) <-- N transition, and indirect dissociation via the parallel (3)Q(0) <-- N transition then curve crossing to the (1)Q(1) potential energy surface. Analysis on the recent studies with vibrational state resolution in the photodissociation of alkyl iodides in the A band reveals that the "symmetric bending" mode on alpha-carbon of alkyl iodides is the preferential vibrational excitation mode, which can be explained by the classic impulsive model.

Theoretical and REMPI spectroscopic study on phenylhydrazine and phenylhydrazine-(Ar)n (n=1, 2) van der Waals complexes.

Phenylhydrazine and its van der Waals complexes with one or two argon atoms were investigated with theoretical calculations and resonant two photon ionization (R2PI) spectroscopy. The ab initio and DFT calculations found a conversion of the orbital hybridization of the Nbeta atom from sp3-like in the S0 state to sp2-like in the S1 state, suggesting that the lone pair electrons of the Nbeta atom are involved in a super p-p-pi conjugation over the skeleton of phenylhydrazine in the S1 state. The structural change of the hydrazino group in the S1<--S0 electronic transition was reflected by the vibrational excitations of the hydrazino group observed in the 1C-R2PI spectrum. The band origin of the S1<--S0 transition is determined to be 33610 cm(-1) and the adiabatic ionization energy (IE) of phenylhydrazine, measured by 2C-R2PI spectroscopy, is 62829+/-15 cm(-1). The S1<--S0 electronic transitions of phenylhydrazine-Ar and phenylhydrazine-Ar2 complexes were also observed in the 1C-R2PI spectrum, and their band origins are, respectively, red-shifted by 39 and 80 cm(-1) from that of phenylhydrazine.

Photofragment translational spectroscopy of n-C3H7I and i-C3H7I near 280 and 304 nm.

The photodissociation dynamics of propyl iodides n-C3H7I and i-C3H7I near 280 and 304 nm has been investigated with our mini-TOF photofragment translational spectrometer. When a single laser is applied for both the photodissociation of parent molecules and the REMPI of I atom photofragments, the TOF spectra of photofragments I*(2P1/2) and I (2P3/2) are obtained at four different wavelengths for these two iodides. For n-C3H7I, some small vibrational peaks are partially resolved (with separation of approximately 522 cm-1, corresponding to the RCH2 deformation frequency of the fragment n-C3H7) at 281.73, 279.71, and 304.67 nm. These results show that the RCH2 deformation is mostly excited. For i-C3H7I, we obtain some partially resolved vibrational peaks (with separation of approximately 352 cm-1, corresponding to the HC(CH3)2 out-of-plane bending frequency of the fragment i-C3H7) at 281.73 nm only. For n-C3H7I, the partitioning values of the available energy Eint/Eavl are 0.48 at 281.73 nm and 0.49 at 304.02 nm for the I* channel, and 0.52 at both 279.71 and 304.67 nm for the I channel. These energy partitioning values are comparable with the previous results at different wavelengths in the literature. For i-C3H7I, the Eint/Eavl values are 0.61 at 281.73 nm, 0.65 at 304.02 nm for the I* channel, and 0.62 at 279.71 nm, 0.49 at 304.67 nm for the I channel. The potential-energy-surface crossing and the beta values have also been discussed.

Flocculation-induced homolysis of hydrogen peroxide in aqueous colloid solution of titanium dioxide nanoparticles.

Thermal generation of oxygen and hydroxylated aromatic compounds by hydrogen peroxide, catalyzed by flocculation of titanium dioxide nanocrystallites aqueous suspension upon addition of hydrogen peroxide, is reported. The oxidation involves catalytic cleavage of a peroxide molecule followed by hydroxyl reaction with the organic solutes. The catalytic hydroxylation is associated with formation of TiO(2)-H(2)O(2) aggregates, which occurs within a specific range of [TiO(2)]/[H(2)O(2)] ratio. Comparison of the activation energy to literature values in the absence of nanoparticles indicates that flocculation induces an increase of the rate without decreasing the activation energy. This is, to the best of our knowledge, a unique case of nanoparticles catalysis driven by formation of a three-dimensional structure of the suspended particles.