71
72
flexibility, which can influence the spectral characteristics of paramagnetic particles, are considered. The main objects are the RIs of various derivatives of highly symmetric cyclic molecules such as benzene, C6H6 (D6h, π-type radicals) and cyclohexane, C6H12
(D3d, σ-radicals).
RIs of the highly-symmetric cyclic organic molecules are the Jahn-Teller ions.
Their PESes are the surfaces of conical intersection with the point of electronic terms crossing corresponding to a highly-symmetric ion structure with a degenerate ground electronic state. The crossing avoidance occurs through pseudorotation. Pseudorotation, being a sequence of ring deformations, gives the impression of a rotation of the entire particle. The PES scheme common for both benzene radical ions and cyclohexane radical cation (RC) is shown in Fig. 2, together with the PES section along the totally symmetric coordinate (Qg) connecting elongated (A) and compressed (B) ion structures.
Fig. 2. PES scheme for C6H6±. and c-C6H12+. and its section along the totally symmetric Qg mode; CIP – the conical intersection point; ΔEAB is the height of the pseudo-rotation barrier, ΔEAB = │EA − EB│.
We show that the conical intersection - the main feature of the PESes of highly symmetric Jahn–Teller ions, may persist for their low-symmetric derivatives. Hence, PESes of the low-symmetric species have a pseudorotational shape resulting from the intersection avoidance.
In particular, PESes of the RAs of a complete series of fluorinated benzenes are pseudorotation surfaces formed by a different number of nonplanar stationary structures. The planar structure disturbance arises due to vibronic coupling of the ground π and low-lying excited σ states. The mirror symmetry of the out-of-plane distortions results in doubling the number of stationary structures. The majority of the fluorinated benzene RAs reveal structural flexibility with respect to pseudorotation.
That is also true when other substituents are present. Thus, the pentafluoroaniline RA PES was shown to be a pseudorotation surface too. It is very similar to that of C6F6ˉ˙. Taking the place of one of the fluorine atoms, the NH2 group slightly disturbs the surface structure.
The example of octafluoronaphthalene RA shows that extension of the conjugated system in going from mono- to bicyclic aromatic compounds may not destroy the conical intersection. The PES of this RA is a pseudorotation surface, but the height of the pseudorotation barrier is relatively large. So not pseudorotation, but non-planar
73
structures inversion determines the dynamical averaging of the hfc parameters in this particle.
The OD ESR spectrum of the RA of decafluorobiphenyl is also a result of the dynamic averaging of the hfc constant. The PES structure of this RA originates from combining three kinds of structural distortions, namely, conical intersection avoidance (an orthogonal D2d(2E) structure corresponds to a conical intersection point), out-of- plane deviations of fluorine atoms and breaking the pentafluorophenyl ring equivalence.
At conical intersection avoidance, the unpaired electron density is transferred from one pentafluorophenyl ring to another.
Preserving the attributes of a highly symmetric precursor in its low-symmetric derivatives is not a feature of IRs of only the aromatic compounds. Adiabatic PESes of the alkylcyclohexane RCs, c-C6H11R+• (R = Me, Eth, iso-Pr, tert-Bu) are pseudorotational surfaces too. The number of stationary structures and the height of barriers at the pseudorotation path vary in the series. The energy difference between the stationary structures of minimum and maximum energy is the measure of the RC structural flexibility. This value depends significantly on the substituent.
Term-crossing and pseudorotation preserve in the radical cations of low-symmetric bicycliс decalin molecules as well.
RIs of molecules with nongenerate electronic state can also reveal the structural flexibility with respect to pseudorotation. The adiabatic PES of the cyclopentane RC (Fig. 4) has a very intricate architecture and combines 60 stationary structures. The RC is structurally non-rigid with respect to inversion and pseudorotation, and the experimental proton hfc parameters can be well described only with taking into account
both types of motion.
Fig. 4. The complete scheme of the multidimensional PES of the cyclopentane CR depicted in the form of the complete graph with ten vertices: K10. The ten vertices relate to the PES global minima, conformational transitions between the minima occur along the inversion and pseudorotation coordinates, for each pair of minima the conformational transition occurring in one stage (through the only transition state). The edges of the graph represent these conformational transitions.
Results of combined theoretical and experimental investigations considered showed that structural distortions with symmetry lowering and correspondingly a complex multi-hole PES architecture resulting from the avoided term crossing are the common features of radical ions of a wide range of cyclic organic molecules including bicyclics.
74
Bifurcation transitions in chemical systems under low magnetic fields
Alexey A. Kipriyanov, Jr.1,♦ and Peter A. Purtov1,2,♠
1V.V. Voevodsky Institute of Chemical Kinetics and Combustion, Siberian Branch of the Russian Academy of Sciences, Institutskaya 3, 630090, Novosibirsk, Russia
2Novosibirsk State University, Pirogova 2, 630090, Novosibirsk, Russia
♦Email: akipriyanov@yahoo.com
♠Email: purtov@kinetics.ncs.ru
Introduction
The influence of magnetic fields on chemical processes has long been the subject of interest to researchers. For this time physically clear notions have been formed of the fact that though the energy of magnetic interactions is small, under certain conditions relatively weak magnetic fields can noticeably affect the rates of chemical reactions with the participation of paramagnetic particles. It has been established that the magnetic effect manifests itself in the competition of different channels of conversion in elementary reaction stages, and is determined by the dependence of chemical process effectiveness on the spin state of the pair of the reacting particles, as well as by magnetosensitivity of transitions between spin states (the model of radical pairs).
Numerous investigations show that commonly the effect of a magnetic field on chemical reactions is insignificant with impact less than 10 percent. However, there are some papers that point to the observation of external magnetic field effect on chemical and biochemical systems actually having a significant impact on the reactions.
On the other hand, it is well-known that in non-equilibrium processes even small perturbations can cause essential consequences in non-linear systems where feedbacks play an important role. The reason is the state stability disturbance, and therefore abrupt change of the process regime.One can believe that in some chemical or biochemical systems rather strong influence of weak magnetic fields is also determined by the disturbance of stationary state stability.
Photochemical system
The system under study describes dissociation reaction of cyclic ketones under the action of external radiation (laser), which results in biradicals and their subsequent recombination. The system can exchange energy with reservoir.
Thus, under the action of external radiation the molecule-precursor A produces biradical B that can subsequently recombine to give the initial molecule
A B A:
. (1)
The kinetic equation defining the concentration change of biradicals B is as follows
B A
abs
B K T n
N V
I dt
dn ( )
, (2)
75
where
n
B is biradical concentration, K(T) is monomolecular recombination rate constant of biradicals B depending on the temperature T of the reacting system, Iabs is the energy absorbed by the reacting system per unit time, is laser generation frequency, is the Planck constant, V is the volume of solution excited by laser radiation, NA is the Avogadro constant.The internal energy of the system changes due to radiation absorption and loss of heat to the reservoir kept at constant temperature T0 . The mean variation rate of external energy of the system can be written as
) (T T0 dt I
dE
abs
, (3)
where is the heat emission coefficient between the reacting system and reservoir,
T
is the mean temperature of the system.So, the evolution of the system in question will be described by differential equations (2) and (3), however, we are interested solely in stationary states of the system defined by the condition
0
dt
dT dt dn dt dn dt
dE A B . (4)
Below we give stationary dependence of the reacting system temperature
T
on external radiation intensityI
0 . Solid line denotes the dependence in the absence of external magnetic field, dotted line – in the presence of magnetic field.Figure 1. Stationary temperature dependence of the reacting system on external radiation intensity in the presence and in the absence of magnetic field.
180 200 220 240 260 280 300 320 340 360 380 400
0,0 0,2 0,4 0,6 0,8 1,0 1,2
I0, W
T, K
with magnetic field without magnetic field
1
2
3 4
76 Lipid peroxidation reaction
One of the feasible defense mechanisms of organism adaptation to varying conditions is the activation of lipid peroxidation reaction (LPR), which plays a key role in the life of cells (e.g., apoptosis or necrosis). It is known that the normal conditions of cell vital activity involve a certain level of LPR induced by the formation of active oxygen forms. Taking into account the necessity of keeping a certain LPR level under stationary conditions, it is assumed that any change of LPR produces the basic response of living system to an external action, which initiates other defense mechanisms of the organism as a whole.
The generally accepted mechanism of this reaction corresponds to the following combination of elementary stages
(1.1) 𝐿𝐻 + 𝑂2𝑘→ 𝐿1 ⦁ (1.2) 𝐿⦁+ 𝑂2→ 𝐿𝑂𝑘2 2⦁
(1.3) 𝐿𝑂2⦁+ 𝐿𝐻→ 𝐿𝑂𝑂𝐻 + 𝐿𝑅𝑘3 ⦁ (1.4) 𝐿𝑂𝑂𝐻→ 𝐿𝑂𝑘4 ⦁+ 𝑂𝐻⦁ (1.5) 𝐿𝑂⦁+ 𝐿𝐻𝑘→ 𝐿𝑂𝐻 + 𝐿5 ⦁ (1.6) 𝑂𝐻⦁+ 𝐿𝐻→ 𝐻𝑘6 2𝑂 + 𝐿⦁ (1.7) 𝐿⦁+ 𝐿⦁𝑘→ 𝑃7 1
(1.8) 𝐿⦁+ 𝐿𝑂2⦁𝑘→ 𝑃8 2 (1.9) 𝐿𝑂2⦁+ 𝐿𝑂2⦁→ 𝑃𝑘9 3
(1)
where 𝐿𝐻 is the lipid molecule, 𝑂2 is oxygen, 𝑃1, 𝑃2 and 𝑃3 are the stable reaction products that take no part in further process. The elementary stages of reaction (1) are usually divided into the reaction of chain initiation (1.1), reactions of chain continuation (1.2), (1.3), (1.5), (1.6), the reaction of chain branching (1.4), and reactions of chain termination (1.7), (1.8) and (1.9).
Basing on the fact that a living cell permanently exchanges substances with the environment used as a building material, and the final metabolism products are removed from the cell, a simplified model of the living cell could be thought of as a flow reactor of volume 𝑉 . A fresh mixture of lipids at concentration [𝐿𝐻]0 with inhibitor at concentration [𝐼]0 under the oxygen saturation conditions is constantly delivered into the system with rate 𝜔; the reaction mixture is constantly removed from the vessel at the same rate. It is assumed that both the reaction mixture delivered into the system and the reaction system itself are in thermal equilibrium with the environment. In this case, the concentration of reacting substances reaches its stationary state called the cellular homeostasis.
77
Below a bifurcation diagram is presented. It represents the dependence of stationary peroxide radical concentration on inhibitor concentration in the presence and in the absence of magnetic field.
The values [𝐼]0 = 1.7 ∙ 10−7 mol/L and [𝐼]0 = 1.01 ∙ 10−5 mol/L are the bifurcation values, i.e., near these values, a minor change in [𝐼]0 leads to a jump-like transition of the system into another stable state 𝑥2∗.
Figure 3. Bifurcation diagram representing the dependence of stationary peroxide radical concentration on inhibitor concentration in the presence and in the absence of magnetic field.
Figure 2. Schematic picture of LPR proceeding in a living cell.
78
EPR spectroscopy for the study of catalyst active sites
Alexander M. Volodin
Boreskov Institute of Catalysis SB RAS, Pr. Acad. Lavrentieva, 5, 630090, Novosibirsk, Russia
E-mail:volodin@catalysis.ru
EPR spectroscopy is a convenient and informative tool for investigation of active sites of heterogeneous catalysts and intermediates of chemical reactions taking place on their surface. Experimental methods and the most interesting results obtained at Boreskov Institute of Catalysis during investigation of different types of catalytic systems will be presented as examples in this report. This will include the following results and methods:
1. ‘In situ’ experiments. They are informative but very complicated and usually require long experiments.
A typical scheme and description of the installation used for such experiments are presented in [1]. The studied sample is placed into a quartz EPR tube, which is placed into the spectrometer resonator and connected to the high-vacuum installation. The thermal control system makes it possible to regulate the sample temperature in the range of 90-800 K. The sample can be also subjected to illumination with monochromatic light in the wavelength range from 248 to 1024 nm.
a) Light-induced processes initiated by surface absorption by oxide materials.
The formation of oxygen radical anions and reactions with their participation will be discussed.
Illumination of many nanocrystalline oxides (MgO, CaO, TiO2, ZnO, etc.) in the absorption range of surface complexes including low-coordinated surface ions can be accompanied by the formation of oxygen radical anions on their surface [2, 3].
Of special interest are atomic radical anions O- with high reactivity that could be key intermediates in catalytic and photocatalytic oxidation reactions. Possible mechanism of processes leading to the formation of such radical anions and reactions with their participation resulting in the formation of radical and ions-radical species will be discussed.
b) Investigation of active sites in Fe-ZSM-5 zeolites. Spin design of iron complexes in FeZSM-5 zeolites will be demonstrated.
ZSM-5 zeolites are widely used catalysts with high acidity and active sites capable of ionizing aromatic molecules with high ionization potential (even benzene) with the formation of radical cations [1, 4]. The possibility of light-induced processes in such systems and their mechanism will be discussed.
79
Paramagnetic NO molecule is a convenient spin probe for obtaining information on Lewis acid sites on oxide surfaces [5]. It can also be used for modification of the spin state in iron complexes in Fe-ZSM-5 zeolites [6]. The information gained about the state of Fe (II) in such zeolites using this method will be discussed.
2. ‘Ex situ’ experiments (spin probe method). They yield information on various types of diamagnetic sites on the surface of catalysts. In many cases they allow the researcher to predict the activity of catalysts in selected catalytic reactions.
Such experiments allow us to perform express analysis of a large number of samples and establish correlations between the studied active sites and the catalytic activity.
a) Radical cations of adsorbed aromatic compounds can be used as spin probes for investigation of active sites on the surface of acid catalysts. Correlations with the catalytic activity will be presented.
Radical cations formed after adsorption of aromatic compounds with different ionization potentials on the surface of acid catalysts (sulfated ZrO2, Al2O3) proved to be a convenient tool for investigation of acid sites of various catalysts and determining their concentrations [4, 7, 8]. In many cases this approach could be used for predicting activity of the studied catalysts.
b) Radical anions of aromatic nitrocompounds can be used for characterization of electron-donor sites on the surface of oxide catalysts. The role of donor sites on the surface of oxide supports in stabilization of supported noble metals will be discussed.
Radical anions of aromatic nitrocompounds are convenient spin probes for determining the concentrations of donor sites on the surface of oxide materials and studying their properties [9]. Experiments performed by us recently showed that such sites are responsible for stabilization of supported noble metals in atomically dispersed form [10]. In many cases such ionic clusters of supported metals have high activity in catalytic oxidation reactions.
The concentration of active sites on the surface of catalysts is usually low.
Therefore, EPR in many cases can yield information about active sites of catalytic systems that cannot be studied by other physicochemical or spectroscopic methods.
References
1. V.A. Bolshov, A.M. Volodin, G.M. Zhidomirov, A.A. Shubin, A.F. Bedilo "Radical Intermediates in Photoinduced Formation of Benzene Cation-Radicals over H-ZSM-5 Zeolites." J.Phys.Chem., 98(29) (1994) 7551-7554.
2. A.M. Volodin “Photoinduced Phenomena on the Surface of Wide-Band Gap Oxide Catalysts.” Catalysis Today, 58(2-3) (2000) 103-114.
80
3. S.E. Malykhin, A.M. Volodin, A.F. Bedilo, G.M. Zhidomirov “Generation of O- Radical Anions on MgO Surface: Long-Distance Charge Separation or Homolytic Dissociation of Chemisorbed Water?” J.Phys.Chem.C,, 113(24) (2009) 10350–10353.
4. A.F. Bedilo and A.M. Volodin “Radical Cations of Aromatic Molecules with High Ionization Potentials on the Surfaces of Oxide Catalysts: Formation, Properties, and Reactivity.” Kinetics and Catalysis, 50(2) (2009) 314–324.
5. A. Volodin, D. Biglino, Y. Itakagi, M. Shiotani, A. Lund “ESR study of monomer and triplet state dimer NO adsorbed on sulfated zirconia.” Chem. Phys. Lett., 327, (2000) 165-170.
6. A.M. Volodin, G.M. Zhidomirov, K.A. Dubkov, E.J.M. Hensen, R.A. van Santeen
“Spin design of iron complexes on FeZSM-5 zeolites.” Catalysis Today, 110(3-4) (2005) 247-254.
7. A.F. Bedilo, A.S. Ivanova, N.A. Pahomov, A.M. Volodin “Development of an ESR Technique for Testing Sulfated Zirconia Catalysts.” J.Molec. Catal.A, 158(1) (2000) 409-412.
8. R.A. Zotov, V.V. Molchanov, A.M. Volodin and A.F. Bedilo “Characterization of the Active Sites on the Surface of Al2O3 Ethanol Dehydration Catalysts by EPR using Spin Probes.” J.Catal., 278 (1) (2011) 71-77.
9. D.A. Medvedev, A.A. Rybinskaya, R.M. Kenzhin, A.M. Volodin and A.F. Bedilo
“Characterization of electron donor sites on Al2O3 surface.” Phys. Chem. Chem. Phys., 14(8) (2012), 2587–2598.
10. A.A. Vedyagin, A.M. Volodin, V.O. Stoyanovskii, I.V. Mishakov, D.A. Medvedev and A.S. Noskov “ Characterization of active sites of Pd/Al2O3 model catalysts with low Pd content by luminescence, EPR and ethane hydrogenolysis.” Appl.Catal.B, 103(3-4) (2011) 397-403.
81