Reactions in low temperature solid co-condensates and size effects

Article abstract of DOI:10.1080/15421400590957873

The interaction of magnesium atoms, clusters and nanoparticles with different organic and inorganic substances were studied in low temperature solid cocondensates. A combination of matrix isolation techniques and preparative cryochemistry was applied to distinguish the activity of metal species of different sizes.

Full text of DOI:10.1080/15421400590957873

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Reactions in Low Temperature
Solid Co-Condensates and Size
Effects
a
Gleb B. Sergeev
a
Department of Chemistry , Moscow State
University , Leninskie gory, Moscow, Russia
Published online: 31 Aug 2006.
To cite this article: Gleb B. Sergeev (2005) Reactions in Low Temperature Solid Co-
Condensates and Size Effects, Molecular Crystals and Liquid Crystals, 440:1, 85-92,
DOI: 10.1080/15421400590957873
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Mol. Cryst. Liq. Cryst., Vol. 440, pp. 85–92, 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590957873
Reactions in Low Temperature Solid Co-Condensates and
Size Effects
Gleb B. Sergeev
Department of Chemistry, Moscow State University, Leninskie gory,
Moscow, Russia
The interaction of magnesium atoms, clusters and nanoparticles with different
organic and inorganic substances were studied in low temperature solid co-
condensates. A combination of matrix isolation techniques and preparative cryo-
chemistry was applied to distinguish the activity of metal species of different sizes.
Keywords: clusters; cryochemistry; magnesium atoms; matrix isolation; nanoparticles;
solid co-condensates
INTRODUCTION
At the present time, the attention of many scientists in the fields of
physics, chemistry, materials science and biology is devoted to nano-
particles; their synthesis, properties and different reactions. The rea-
son for this lies in the fact that particles of nanometer size show new
properties.
The most interesting subject is the connection of chemical proper-
ties of metallic particles with their size [1]. Small metal particles with
sizes in range from 1 to 10 nm exhibit high and sometimes unusual
chemical reactivity, and show a strong variation in their activity
depending on cluster size. These are size effects in nanochemistry.
Currently, the most successful areas are the study of such effects in
gas phase reactions and chemisorption [2].
Low and super-low temperatures may be also used for distinguish-
ing the activity of metal atoms and nanoparticles. This method is
based on a combination of matrix isolation techniques and preparative
cryochemistry.
The work is supported by RFBR grant 05-03-32293.
Address correspondence to Gleb B. Sergeev, Department of Chemistry, Moscow State
University, Leninskie gory, 1=3, Moscow, 119992, Russia. E-mail: gbs@kinet.chem.msu.ru
85

8
6
G. B. Sergeev
GENERAL REMARKS
In our investigations, we used the condensation of reagent vapors on a
cooled surface in special cryostats under conditions which exclude
interaction in the gas phase. The scheme in Figure 1 illustrates the
fundamental possibility of using low temperature in order to obtain
metal clusters, mono- and polynuclear metal complexes and ligand-
stabilized nanoparticles. It is important that every possible line in this
scheme should be considered as a kind of nanoreactor.
Chemical interactions in low-temperature co-condensates begin
with metal atoms. Addition of ligand into the systems may cause the
formation of metal particles of different size and their stabilization
or reaction with ligand molecules. The aggregation of metal atoms
and interaction with ligands occurs practically without an activation
barrier.
The high reactivity of small metal species is the main difficulty in
establishing the relation between the size of the particles and their
chemical activity. There are also problems in producing and isolating
the compounds with definite compositions. The reactions shown in
scheme Figure 1 are complex multifactor processes taking place under
non-equilibrium conditions. Low temperature co-condensates possess
internal accumulated energy.
FIGURE 1 Dependence of chemical activity on size of particles.

Reactions at Low Temperatures and Size Effects
87
The size of metal particles and their reactivity are determined by a
combination of different experimental conditions. The main exper-
imentally controllable factors are the substrate temperature,
metal=ligand ratio, reagent condensation rate, and the rate of sample
annealing. Together with the chemical nature of the reagents all fac-
tors mentioned determine the pathway of processes.
The general scheme of our cryochemical synthesis and reactions
with metals, encapsulated in organic or inert polymer matrices, are
presented in Figure 2. The first step is the co-condensation of metal
FIGURE 2 Cryochemical synthesis of nanosize metals encapsulated in
organic and polymer matrices.

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8
G. B. Sergeev
FIGURE 3 Size effects in chemistry.
and ligand vapors on a surface at low or very low temperature. Thus
we produce a solid co-condensate film. It is possible to observe the
stabilization or reactions of metal species in such films using IR,
UV-vis and ESR spectroscopy and electrical measurements. During
the annealing to room temperature, the dispersion of metal nanopar-
ticles or metal-containing polymer films are formed. Here we used
TEM to determine the size of particles. Finally, we produce nanosized
metal-containing organosols or solid films and use these materials to
study various chemical transformations, catalytic activity, and gas
sensor properties.
The lifetime of highly active species such as metal atoms, their
dimers or trimers, during the co-condensation on a cold surface is
inversely proportional to the condensation rate and depends on the
nature of relaxation and diffusion processes in the system. The inten-
sity of the particle beam determines the number of collisions of the
atoms and molecules with the surface and with each other.
Working with low-temperature co-condensates we can face several
size effects. Corresponding definitions are presented in the table
shown in Figure 3 [3]. In low temperature co-condensates the follow-
ing size dependencies are possible: film thickness, size (number of
atoms) and combinations of both.
REACTIONS OF MAGNESIUM PARTICLES
In the organic chemistry of polyhalogen compounds it is known that at
room temperature in solution carbon tetrachloride does not react with
bulk magnesium. The situation changes dramatically in Mg-CCl4

Reactions at Low Temperatures and Size Effects
89
co-condensates on a cold surface at temperatures close to the that of
liquid nitrogen (77 K). In the case of carbon tetrachloride we have com-
petition between chemical reactions and formation of different chemi-
cal intermediates and products: Grignard reagent (CCl MgCl),
3
trichloromethyl radical (CCl ), dichlorocarbene (CCl ). The reaction
3
2
mechanism has been studied in detail [4].
Magnesium clusters of different nuclearity can be obtained in low
temperature matrices. The process of cluster formation is simply
controlled by changing the reagent ratio. The particle size and the
CꢀHal bond energy are important. We have studied the interaction
of magnesium species with halogenated butanes and compared the
yields of octane (product of recombination) with the CꢀHal bond
energy. The results are correlated with the CꢀHal bond energy and
our scheme of reaction [5].
New data were obtained for reactions of magnesium particles with
polyhalomethanes in the temperature range 12–70 K [6]. The system
magnesium-carbon tetrachloride was studied in detail. UV-vis spectra
of magnesium particles in argon are shown here in Figure 4. The
magnesium particles were identified using the literature. Atoms,
dimers, trimers and tetramers are all stable at 12 K and may be found
in the matrix immediately after the condensation. Annealing of the
condensate, leads to simultaneous linear decrease of all absorption
peaks with increasing temperature. The spectrum totally disappears
at 35 K.
Addition of 10% of CCl changes the annealing behavior of the sys-
4
tem. The data, presented in the next figure (Fig. 5) clearly shows that
dimers and trimers are consumed first. Magnesium tetramers and
FIGURE 4 Temperature dependence of UV-Vis spectra of Mg=Ar ¼ 1=1000
condensate. Shown are traces in the range (from top to bottom 12–34 K).

9
0
G. B. Sergeev
FIGURE 5 Temperature dependence of UV-Vis spectra of Mg=CCl
=100=1000 co-condensate. Variation of relative peak intensity of several
absorption lines.
4
=Ar ¼
1
Mgx oligomers are a bit more stable. The adsorption of atoms was
nearly constant and does not change up to 27 K, while the absorption
of all clusters decreases. Only when less than 20% of the initial cluster
concentration remained did the atomic peak also start to decrease.
One can see that atoms start to react after clusters and the reactivity
changes in the sequence Mg ꢁ Mg > Mg ꢁ Mg.
2
3
4
IR-spectroscopic studies have shown that the only stable products of
the reaction are C Cl and C Cl . The formation of Grignard reagent
2
4
2
6
was not detected (Fig. 6). We suppose that the reaction proceeds by
the sequential abstraction of chlorine atoms accompanied by metal
cluster decomposition. The intermediate product is probably the com-
plex between magnesium chloride and dichlorocarbene. Further trans-
formation of this complex leads to formation of stable products. This
scheme of the reaction agrees with theoretical calculations [7].
FIGURE 6 Proposed scheme of the reactions in Mg=CCl
2–40 K.
4
co-condensate at
1

Reactions at Low Temperatures and Size Effects
91
We also studied reaction of magnesium particles with other poly-
halomethanes. The interaction with CHCl in the temperature range
3
1
2–70 K was not observed. Reaction with CFCl3 occurs only at
temperature T ¼ 70 K. We consider these observed facts to be a result
of the higher bond energy in these compounds.
Size effects have been also found in the reaction of Mg with alkyl
and aryl halides. In this case we have seen the influence of the film
thickness on reaction rate. Under some critical conditions, explosions
are observed [8].
At the present time, we suppose that in the reactions with metal
particles the combination of two size effects takes place – the film
thickness and particle size. Thus, low temperature co-condensates pos-
sess redundant energy. This energy can be released in various chemi-
cal reactions. The main forms of the accumulated energy are energy of
chemical transformation, excess energy of non-equilibrium phase
state, energy of defects in amorphous and crystalline substances,
and energy of mechanical stresses.
Explosive reactions at low temperatures take place also in systems
which do not contain metal particles. Such a reaction was found for
acetyl chloride and diethylamine [9].
We have also shown size effects in water condensate films. The
experiments were carried out using an original low temperature thin
film setup for differential calorimetry. Spontaneous fast release of
heat was observed at 80 K, when the water film thickness increased
to a critical value [10]. In our opinion, the fast process is connected
to the crystallization of the amorphous water. The decrease of stresses
was observed when film thickness reached its critical value. The for-
mation of a crack network in the film was detected at the same time.
A general scheme for the processes that took place in the growing
solid co-condensate film during its formation are presented in the
literature [11]. There are different forms of the accumulated energy
and competition between different chemical transformations (fast
and slow).
CONCLUSIONS
Low and super-low temperatures allow the study of reactions of active
metal species: atoms, small clusters and their nanosized aggregates.
Such processes can be accompanied by unusual chemical reactions.
The formation of hybrid metal-organic supramolecular structures,
metal clusters, and nanosized metal particles can take place during
the low-temperature co-condensation and during further thermal
treatment of the samples.

9
2
G. B. Sergeev
The most interesting findings in cryochemistry of metal nanoparticles
are the size effects of various sorts [12]. As such effects we consider the
influence of metal species size (the number of atoms in the structure)
or film thickness on their reactivity.
REFERENCES
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[2] Sergeev, G. B. (2001). Nanochemistry of metals. Rus. Chem. Rev., 70, 808–825.
[3] Sergeev, G. B. & Bochenkov, V. E. (2003). Physical chemistry of ultradispersed
(nano) systems, Moscow, 24–29, (Russian).
[
[
4] Zagorsky, V. V. & Sergeev, G. B. (1990). Mol. Cryst. Liq. Cryst., 186, 81–86.
5] Ivashko, S. V., Zagorsky, V. V., & Sergeev, G. B. (2001). Solid states transforma-
tions of butyl halides with magnesium. Mol. Cryst. Liq. Cryst., 356, 443–448.
6] Mikhalev, S. P., Solov’ev, V. N., & Sergeev, G. B. (2002). Cryoreactions of magnes-
[
[
ium atoms and clusters with neat and argon diluted CCl
Conf. on Low Temperature chem., Keuruu. 100–101.
4 3 3
, CHCl and CFCl Intern.
7] Granovskiy, A. A., Vanyushin, A. V., Polikarpov, E. V., Kovba, V. M., & Nemukhin,
A. V. (2001). Calculations of magnesium with tetracarbonchloride reaction products
by perturbation theory methods. Vestn. Mosk. Univ. Ser. Khim., 42, 371–373
(in Russian).
[
8] Sergeev, G. B. & Efremov, M. Yu. (1996). Size effects in reactions of solid organic
compounds with metal particles. Mol. Cryst. Liq. Cryst., 278, 17–25.
9] Sergeev, G. B. (1998). Mol. Reactions in solid low temperature co-condensates. Mol.
Cryst. Liq. Cryst., 313, 155–166.
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nanostructures. Surf. Sci., 500, 628–655.
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of nanosystems. Rus. Chem. Rev., 72, 643–663.