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THE INTERACTION OF NAKED IRON ATOMS WITH SMALL MOLECULES
M. Pasternak, P. Barrett
To cite this version:
M. Pasternak, P. Barrett. THE INTERACTION OF NAKED IRON ATOMS WITH SMALL MOLECULES. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-79-C1-84.
�10.1051/jphyscol:1980114�. �jpa-00219624�
THE
INTERACTION OF NAKED IRON ATOMS
WIMSMALL
MOLECULESM. Pasternak and P.H. ~ a r r e t t ~
Department of Physics and Astronomy, [Pel Aviv University, Ramat Aviv, IsraeZ.
x Department of Physics, University of CaZifornia, Santa Barbara, CA. 93106, U.S.A.
Abstract
-
The Mossbauer Effect in 5 7 ~ e is used to discover and characterize chemical species formed between bare iron atoms or molecules with molecules of quasi-inert gases at low temperatures. Experiments were conducted with N 2, CH4, C2H49 C2H6, C6H6 and NH3. The later product: was U.V. irradiated and its photoproducts were analyzed. It was found that cryochemical species with N2, CH4 and C H are formed2 4
with Fez or higher polymer, solely. Monomers bind to a single molecule of MI and 3 both monomers and dimers form a distinct bonding with ethane. The experimental part is described with emphasis in new developments.
1. INTRODUCTION In 1977 Barrett and Montano [6] proposed the form- The technique of matrix isolation, orig-
inally devised to trap chemical species for spectroscopic observations, had been introduced by Porter [I] and Pimentel [2] during the 50's.
In the past years there has been a renaissance in matrix isolation research emphasizing the aspects of synthesis, structure and reactivity [3]. The Mossbauer Effect spectroscopy is a relatively newcomer in this field. Following the natural path of Rare Gas Matrix Isolation (RGMI) pioneered by works of I2 [4], Fe, and Sn [5] in solid argon, it is now being shifted towards cryochemistry.
The disperse medium at low temperatures (4
-
20 K)provides excellent conditions for trapping isol- ated atoms, molecules and exotic chemical react- ants and study the nature of their chemical bond.
The RGMI technique is now being extended to non- rare gases, mixtures of rare and normal gases and studies combined with photolysis. Naturally, the most prominent isotope studied so far was 57~e.
ation of Iron-Nitrogen species formed by Fe with 2 the nitrogen host.
The subject of this work is to present recent results which unequivocally suggest the formation of chemical bond of atoms and molecules of iron with basic simple molecules. The studies are based on the observation of the isomer shift, quadrupole interaction and sometimes of the temp- erature dependence of -£-. In what follows, the experimental part will be described and then examples will be given of cryospecies formation of iron with N2, NH3, CH4, C2H4, and C2H6.
2. EXPERIMENTAL
The experimental aspects of RGMI tech- niques [71 and in particular of the application of Mossbauer Effect has been described elsewhere.
[a].
Here we describe an experimental set-up used at our laboratory in recent studies, which has some advantages over others published so far [9]. A picture of the system is shown in Fig. 1.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980114
C1-80 JOURNAL DE PHYSIQUE
f u r n a c e s . (See F i g . 2 ) .
FIG.1.-A p i c t u r e o f t h e e x p e r i m e n t a l s e t - u p . ( 1 )
-
a commercial e v a p o r a t o r ; (2)-
c o l l a r where ( a ) d e t e c t o r and ( b ) d r i v e r i s mounted, and ( 3 ) a r o t a t a b l e He c r y o s t a t .It c o n s i s t s of a commercial e v a p o r a t o r w i t h custom-made s t a i n l e s s s t e e l c o l l a r and v a r i a b l e t e m p e r a t u r e c r y o s t a t . The c o l l a r was d e s i g n e d h a v i n g i n mind o p t i m a l ( 1 ) g e o m e t r i c a l c o n d i t i o n s f o r c o u n t i n g ( 2 ) h i g h e v a p o r a t i o n e f f i c i e n c y and a l s o t o e n a b l e t h e u s e o f s p a r e e n t r a n c e windows f o r o t h e r p u r p o s e s . The c r y o s t a t , which c a n b e r o t a t e d , h a s i t s a b s o r b e r t a i l made of p u r e copper i n t o which a ' b e r y l l i u m s u b s t r a t e i s t h e r m a l l y anchored. The t a i l i s c o o l e d by a f o r c e d s t r e a m o f l i q u i d h e l i u m and t h e t e m p e r a t u r e may b e v a r i e d e i t h e r by h e a t i n g o r by c o n t r o l l i n g t h e helium s t r e a m . The m a t r i x g a s e n t e r s t h e cryo- s t a t t h r o u g h a n e e d l e v a l v e and i t i s mixed w i t h one o r two m e t a l vapour beams emanating from two alumina c r u c i b l e s . The c r u c i b l e s a r e c o n t a i n e d i n two i n d e p e n d e n t r e s i s t a n t h e a t e d t a n t a l u m
FIG.2.-A c r o s s s e c t i o n view of t h e c o l l a r ; (1) Be s u b s t r a t e t h e r m a l l y anchored t o ( 2 ) copper t a i l , ( 3 ) and (4) LH and LN r a d i a t i o n s h i e l d s
2
r e s p e c t i v e l y ; (5) water-cooled r a d i a t i o n s h i e l d o f f u r n a c e s ; (6) c r u c i b l e s i n s i d e t a n t a l u m furnace's;
(7) mylar windows; (8) s o u r c e ; ( 9 ) d r i v e r ; (10) s c i n t i l l a t i o n d e t e c t o r ; (11) g a s o u t l e t . The h i g h c o o l i n g power of t h e t a i l a l l o w s u s t o a c h i e v e a h i g h e v a p o r a t i o n e f f i c i e n c y (12-15%/cm 2 ) which i s a n i m p o r t a n t f a c t o r c o n s i d e r i n g t h e p r i c e s of e n r i c h e d i s o t o p e s . The s o u r c e used was e i t h e r
5 7 57
Pd( Co) o r Rh( Co) a t room t e m p e r a t u r e w i t h a c o n v e n t i o n a l Mossbauer system i n c o r p o r a t i n g i n on- l i n e PDP-8A computer.
3 . THE CRYOCHEMISTRY OF RARE IRON ATOMS AND MOLECULES
The h y p e r f i n e c o n s t a n t s of RGMI Feo and Fe2 (monomer and dimer) a r e w e l l e s t a b l i s h e d [S]
and may b e u s e d a s r e f e r e n c e f o r t h e e x t e n t of f o r m a t i o n o f chemical bond w i t h t h e h o s t m o l e c u l e s . I n t a b l e 1 we summarize t h e d a t a o f i.s. and quad- r u p o l e s p l i t t i n g of c r y o c h e m i c a l s p e c i e s r e p o r t e d s o f a r . A common e x p e r i m e n t a l f e a t u r e i s t h a t upon e l e v a t i o n of t e m p e r a t u r e of t h e m a t r i c e s , when t h e m o b i l i t y o f t h e m o l e c u l e s i n c r e a s e , t h e s p e c t r u m u s u a l l y b r o a d e n s , , s m e a r s , and t h e o r i g i n a l
(63' = e2qQ/2) i n mm.s -1 for uarious cryochemica2 species
one, cannot b e reproduced by r e t u r n i n g t o low t e m p e r a t u r e s . Another i n t e r e s t i n g f e a t u r e i s t h e tendency of Fez o r h i g h e r polymers, i n s t e a d of
M a t r i x i . ~ . QS Remarks
R e f s .
monomers, t o form chemical bond w i t h t h e h o s t mol-
C2H4
e c u l e s .
* - t r i m e r (?) * r e a c t i n g monomer
p r e s e n t w o r k
Feo -.73, 3.392
NH3 C2H6
Fe-N2
-
With i r o n c o n c e n t r a t i o n of 0.6% atomic,C6H6
.25 1.05
1 6 .64
1 . 9 3
*-phc
Feo
-
.822.73 RGMI
B a r r e t t and Montano [6] r e p o r t e d t h e o b s e r v a t i o n
Fe2 . 5 1 .81 pe
-.75 0
5
CH4
Feo Fe2
of a n a b s o r p t i o n spectrum c l e a r l y p o i n t i n g t o a
Fe* X .37 1.59
Fe:,
-
. 4 12.60 Fez
-.I1 4.05
5
FeNH3FeNH2FeNHFeo -.67
0
1 0
N2
Fez Fe2
f o r m a t i o n of a n Fe2-(N2) molecule. Evidences
X
t h a t Fe i s r e q u i r e d t o form a chemical bond w i t h 2
F e z ' - 8 4 1.57 .73
1.64 1.42, .58
0,O
1 2
-.78 2.70
6
N were found by c o n t r o l l i n g t h e c o n c e n t r a t i o n of 2
.98 2.15
.71 1.17
Fe. A t v e r y low c o n c e n t r a t i o n , o n l y t h e monomer t o l y z e d
1 3
was o b s e r v e d w i t h i.s. s i m i l a r t o t h a t o f Fe-Rare- g a s .
Fe-C2H4
-
The spectrum o f F e ( l % ) i n e t h y l e n e-
(C2H4) a t 4.2 K i s shown i n F i g . 3 .FIG .3 .-Absorption spectrum of 5 7 ~ e ( l % ) i n e t h y l e n e a t 4.2 K. Doublets: c o r r e s p o n d t o ( 1 ) n o n - r e a c t i n g monomer; (2) Fe2-(C H ) and (3) r e a c t i n g t r i m e r s
2 4 x o r h i g h e r polymers.
The spectrum c o n s i s t s of t h r e e components. One component [ I ] c o r r e s p o n d s t o t h e n o n - r e a c t i n g mon- omer w i t h i.s. similar t o t h a t of RGMI and quadru- p o l e s p l i t t i n g (QS) somewhat l a r g e r t h a n t h a t of Fe i n N2. The prominent component i s t h a t of Fez- (C 2 4 x H ) c o r r e s p o n d i n g t o a c h e m i c a l l y bound i r o n molecule t o one o r more e t h y l e n e m o l e c u l e s . The l a r g e QS, t h e p o s i t i v e v a l u e of t h e isomer s h i f t and t h e r e l a t i v e l y narrow l i n e w i d t h ( 2 P 0 . 3 5 -+
0.01 m . s - l ) p o i n t ' t o a s p e c i f i c s p e c i e s formed by Fe2+ nC2H4 -t Fe (C H ) A t h i r d weak component
2 2 4 x '
is a l s o observed y e t w i t h lower v a l u e s of i.s. and 9.9. T h i s component may b e t h e r e s u l t of a t r i m e r o r h i g h e r polymer i n t e r a c t i o n w i t h t h e h o s t . Pe-C2Hg
-
U n l i k e t h e d o u b l e bonded c a r b o n e t h y l e n e(d(C=C) = 1.33A), e t h a n e w i t h s i n g l e bonded c a r b o n (d(C-C) = 1.54A) form c h e m i c a l bonds w i t h b o t h t h e monomer'and t h e dimer of Fe. The spectrum i n F i g . 4 is composed o f t h r e e components. The f i r s t c o r r e s p o n d s t o a n o n - r e a c t i n g i r o n monomer. The second d o u b l e t h a s a similar QS y e t a l e s s negat- i v e isomer s h i f t ( i . s . = -0.41 mm.s -1 ) . We i n t e r - p r e t t h i s r e s u l t a s a monomer and chemical r e a c t - i o n w i t h t h e h o s t . The t h i r d component h a s a pos- i t i v e isomer s h i f t and a QS v e r y s i m i l a r t o t h o s e of Fe2(N2)x. We i n t e r p r e t t h i s component a s a Fe2-(C H ) s p e c i e s . Both monomers have narrow
2 6 x
l i n e w i d t h b u t t h e dimer r e a c t a n t h a s a b r o a d l i n e
C1-82 JOURNAL DE PHYSIQUE
s u g g e s t i n g t h e p o s s i b i l i t y of a d i s t r i b u t i o n of Fe2 chemical r e a c t i o n . S i m i l a r t r e n d s were obs-
s i t e s . erved w i t h CD4.
1
Fe-Amonia and its photofragments-
With t h e excep-230L count$ (xl(r6)
t i o n of a l k a l i and a l k a l i n e e a r t h s , ammonia i n i t s v a r i o u s phases does n o t r e a c t w i t h b u l k m e t a l s . However, almost a l l m e t a l s w i l l adsorb ammonia s t r o n g l y on t h e i r s u r f a c e s . With t r a n s i t i o n m e t a l s such a s i r o n , s u r f a c e chemical r e a c t i o n s proceed 225
-
even a t room temperature, forming Fe-NH2, Fe-NH and
J t
4 -2 0 2 4 Fe-H 1131. Molecules c o n t a i n i n g NH 3 bind t o zero-
FIG -4 .-Absorption spectrum of 5 7 ~ e (1%) i n e t h a n e a t 4.2 K. Doublet No.1 w i t h t h e more n e g a t i v e i.s.
corresponds t o t h e n o n - i n t e r a c t i n g monomer. Second d o u b l e t 1 corresponds t o t h e i n t e r a c t i n g monomer and d o u b l e t 2 cotresponds t o t h e i n t e r a c t i n g i r o n molecule w i t h C2H6.
Fe-CHq
-
The spectrum of i s o l a t e d i r o n atoms i n CH4 has been r e p o r t e d t o have t h e same isomer s h i f t a s i n RGMI [ l o ] . We have confirmed t h i s r e s u l t and a l s o shown t h a t t h e Fe atoms f o l l o w t h e l a t t i c e-4 -2 0 2 4
dynamics of s o l i d methane a s manifested i n t h e VELOCITY ( m m - i ' ) order-disorder phase t r a n s i t i o n of 22 KO [ l l ]
.
Oni n c r e a s i n g t h e i r o n c o n c e n t r a t i o n t o form a dimer, FIG.5.-Absorption spectrum of 57Fe (O.lZ) i n q u i t e unexpected r e s u l t s were found. Lines a t ammonia a t 4.2 K. The lower p o r t i o n d e p i c t s t h e .
-1 spectrum of a Fern3 s p e c i e s ; t h e upper p o r t i o n - I - 0 . 5 m m . ~ - ~ a n d a t + 1 . 4 2 mm.s were found, b u t
d e p i c t s t h e photolyzed spectrum c o n s i s t i n g of two no l i n e s c h a r a c t e r i s t i c t o Fe2 i n RGMI d o u b l e t s belonging t o Fern2 and FeNH ( s e e t e x t ) . (-0.11 mm.s -1 ) were observed. T h i s i s a s t r o n g The inset r e p r e s e n t s t h e u n i t c e l l of s o l i d ammonia evidence t h a t chemical r e a c t i o n between Fe2 and v a l e n t m e t a l s , w h i l e n o t numerous, a r e known, For CH4 h a s occured. A p l a u s i b l e r e a c t i o n i s t h e oxi- example, Ir(NH3)5, Pd(W ) [14] and Fe(C0I4 NH3.
3 4
d a t i v e cleavage of t h e CH bond, namely: Experiments done w i t h 0.1% Fe a t 5K r e s u l t e d i n a Fe2+CH4&-Fe-Fe-CH3 o r Fe- e-CH3.
W
s h a r p d o u b l e t a b s o r p t i o n spectrum ( s e e F i g . 5 ) . Evidences f o r a Fe-H bond were found from The i r o n low c o n c e n t r a t i o n and presence of one com- I R s t u d i e s [12]. By i n c r e a s i n g t h e concentration ponent s u g g e s t t h a t t h e i r o n s i t e is w e l l defined of Fe i n a methane m a t r i x , weak l i n e s n o t d e t e c t e d and unique i n a non-cubic symmetry. From t h e l a c k i n pure methane were found a t 2041, 2038 and of temperature dependence of t h e QS ahd from t h e 1891 cm-l. T h e i r i n t e n s i t y v a r i e d a s t h e s q u a r e of v a l u e s of t h e 1.8. we concluded t h a t t h i s i s a t h e i r o n c o n c e n t r a t i o n confirming t h e concept of "covalent diamagnetic" compound [15]. The samplespectrum r e v e a l e d no remnants of t h e o r i g i n a l doub- l e t . The spectrum i s composed of two s i t e s ( s e e F i g . 5) which were a t t r i b u t e d t o Fe-NH2 and Fe-NH.
The o b s e r v a t i o n of only two d o u b l e t s invokes t h e model where a n i r o n atom i s bonded t o one s i n g l e ammonia molecule.
4. DISCUSSION AND CONCLUSIONS
Our s t u d i e s and t h e s t u d i e s u s i n g o t h e r methods [3] t o c h a r a c t e r i z e chemical r e a c t i o n s of m e t a l atoms i n low temperatures m a t r i c e s , s u g g e s t a new and e x c i t i n g a p p l i c a t i o n of t h e Mossbauer Spectroscopy. This a p p l i c a t i o n i s of paramount i n t e r e s t . The i n f o r m a t i o n o b t a i n e d from t h e hyp- e r f i n e i n t e r a c t i o n g i v e s s t r a i g h t evidence f o r p o s s i b l e bond formation. How and when i s t h i s chemical bond formed? We have evidence t h a t once t h e m a t r i x i s formed and kept a t low temperature, t h e s p e c i e s formed i s s t a b l e . During t h e evapor- a t i o n , t h e p r o b a b i l i t y t h a t a metal atom w i l l i n t - e r a c t i n t h e beam w i t h a gas molecule i s extremely low. W e propose a model where a " r e a c t i o n zone"
i s formed immediately upon condensation and b e f o r e s o l i d i f i c a t i o n . Within a s h o r t time, b e f o r e t h e
mobile enough t o form dimers and/or chemically r e a c t w i t h t h e h o s t m a t r i x . Once t h e l a y e r s ' r i g i d i t y i n c r e a s e s t h e f r o z e n s p e c i e s s t a b i l i z e . The h e a t of r e a c t i o n c a n b e e a s i l y absorbed by t h e m a t r i x . Few q u e s t i o n s though remain. Why dimers o r h i g h e r polymers of Fe a r e needed t o form a bond w i t h m a t r i c e s l i k e CH4, C2H4 and N2. T h i s i s un- expected s i n c e Fe2 i s 30 kcal/mole more s t a b l e t h a n 2 Fe. Are t h e new s p e c i e s formed an i n t e r - mediate chemical s t a t e , o r do t h e y r e p r e s e n t a n o x i d a t i v e c l e a v a g e of t h e h o s t molecules? I n t h e c a s e of NH we have evidence t h a t indeed an FeNH
3 3
molecule is formed, b u t is i t a l s o t h e c a s e w i t h CH4, C2H6 C2H4, e t c . ? P h o t o l y s i s s t u d i e s may h i n t t o a mechanism of chemical r e a c t i o n s a t t h i s low temperature. Undoubtedly more experiments and sys- t e m a t i c a c q u i s i t i o n of d a t a a r e needed. The r e - wards of t h e s e s t u d i e s a r e numerous. They may l e a d t o t h e understanding of m i c r o s c o p i c mechanism of c a t a l y s i s and chemisorption, and t o t h e e l l u c i d - a t i o n of t h e b a s i c i n t e r m e d i a t e s t e p s i n t h e chem- i c a l bond formation.
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*
Work supported i n p a r t by The I s r a e l Corn- [4] BUKSHPAN,s.,
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C1-84 JOURNAL DE PHYSIQUE
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