close

Вход

Забыли?

вход по аккаунту

?

Cu-H2 Photochemistry in the Matrix; ESR FTIR UVVIS Spectroscopic and Kinetic Studies.

код для вставкиСкачать
a square pyramid (150O). The difference in the P201
(169.8(3) pm) and P202 (173.5(2) pm) bond lengths, however, indicate that P2 exhibits some residual trigonal-bipyramidal character. Analysis of the interplanar dihedral angles for the normalized bond lengths shows that in 2 P2
The UV/VIS spectrum of Cu atoms isolated in a Kr/H2
matrix at 12 K (Cu : Kr :H 2 = 1 : lo4 :lo3) closely resembled
that observed for Cu atoms in a pure Kr matrix"], although
the effects caused by 310 nm photoexcitation of the entrapped Cu atoms ( ~ ' P c ~ ~ Swere
)
markedly different.
Whereas in the case of the pure Kr matrix loss of Cu atoms
was characterized by approximately second order kinetics,
consistent with the bimolecular dimerization 2 Cu-Cu?,
the photoinduced decay of Cu atoms in the Kr/H2 matrix
was much faster, and was characterized by first order kinetics; such decay kinetics would be expected for a reaction
of the type Cu H2-products, where Hz is in great excess.
Only minimal growth of Cu2 was observed under the conditions mentioned. Furthermore, the photodecay rates of
Cu atoms in Kr/H2 and Kr/Dz matrices were found to be
identical.
Direct evidence for a chemical reaction involving photoexcited copper atoms and molecular hydrogen was obtained by means of in situ ESR and FTIR studies of C u /
Kr/H2 matrices['].
+
CY
d03
Fig. I . Molecular structure of 2 in the crystal. Details of the crystal structure
can be obtained from the Fachinformationszentrum Energie Physik Mathematik, D-7514 Eggenstein-Leopoldshafen (F.R.G.) by quoting the number
CSD 50 175, the name of the author, and the complete citation of the article.
has more than 80% square pyramidal character. In contrast, 3 exhibits only cu. 56% C2" distortion towards this
geometry[91.The SPIP2 angle in 3 (1 12.8(1)") is considerably smaller than the FePl P2 angle in 2 (l20.3( 1 ) O ) . Possibly the different coordination at P2 in 2 and 3 has its origin in steric interactions e.g. between C12 and atoms of
the Fe(CO), group. The PI P2 distance (224.q 1) pm) in 2 is
in the expected range (218-225 pm). In both 2 and 3 the
two methyl groups C11 and C12 have trans-orientations; a
cis arrangement would be sterically much less favorable.
A
-
Received: July 15, 1981 [Z 75 IE]
German version: Angew. Chem. 94 (1982) 393
The complete manuscript of this communication appears in:
Angew. Chem. Suppl. 1982, 907-912
H
I
[3] G. Bergerhoff, 0. Hammes, D. Hass, Acro Crystatiogr. 835 (1979) 181.
141 N. Weferling, W. S . Sheldrick, R. Schmutzler, Liebigs Ann. Chem. 1982.
167.
[S] H. W. Roesky, K. Amhrosius, W. S. Sheldrick, Chem. Ber. 112 (1979)
1365; H. W. Roesky, K. Ambrosius, M. Banek, W. S. Sheldrick, ibid. 113
(1980) 1847.
191 D. Schomburg, N. Weferling, R. Schmutzler, J. Chem. SOC.Chem. Commun. 1981. 609.
D
D
D
I
I
~
t
5122
.
I
,
I
I
\
I
5800
> .
5880
1
1
"
3100 3200 3300 3400 3500 3600
[GI
Cu-H2 Photochemistry in the Matrix; ESR, FTIR,
UV/VIS Spectroscopic and Kinetic Studies**
By Geoffy A . Ozin*, Steven A . Mitchell, and
Jamie Garcia Prieto
In this communication we wish to report that photoexcited copper atoms react with molecular hydrogen in a
krypton matrix to yield C u H and H atoms.
[*] Professor G. A. Ozin, Dr. S. A. Mitchell
[**I
3x0
Lash Miller Chemistry Laboratories, University of Toronto
80 St. George St., Toronto, Ontario M5S 1Al (Canada)
Dr. J. Garcia Prieto
lnstituto Mexicano del Petroleo
Av. Cien Metros, No. 152, Mexico, 14. D. F. Mexico (Mexico)
This work was supported by the Natural Sciences and Engineering Research Council (NSERC). S. A . M . thanks the NSERC and Noranda for
graduate scholarships, and J. G. P. the lnstituto Mexicano del Petroleo
for financial support.
0 Verlag Chemie GmbH. 6940 Weinheim. 1982
c
-
Fig. I . Left: Part of the ESR spectra (Varian E4, 9.278 GHz) showing the
high field doublet associated with "CU and "CU atoms isolated in freshly
deposited Cu-containing Kr/HI (A), Kr/DZ(B),and Kr/HD (C) matrices (ratios 1 : 10': lo') at 12 K. Right: Spectra showing the presence of H and D
atoms in the various matrices after 2 I h photolysis at 310 nm. No H or D
atoms were detected prior to photolysis.
In each case the ESR spectra of the Cu/Kr/H2, Cu/Kr/
D2, and Cu/Kr/HD matrices showed only the known19"l
spectrum of Cu atoms, and no H or D atoms. The high
field doublet associated with 63Cu and 65Cu atoms is reproduced in Figure 1. Photoexcitation (3 10 nm) of the entrapped Cu atoms caused decay of their resonance lines
with concurrent appearance and rapid growth of resonance lines due to H and/or D atoms (Fig. 1, right). No
lines attributable to Cu" species, e.g. CuHz were observed
in these experiments in contrast to similar studies with
CH4[']and C2H6151
in which the primary products H3CCuH
and H5C2CuH, respectively, were detected in addition to
0570-0833/82/0505-0380 $ 02.50/0
Angew. Chem. In(. Ed. Engl. 21 (1982) No. 5
+
the major secondary photolysis channel ‘CH3 C U H and
‘C2H5 + CuH, respectively. Interestingly, in K r / H D matrices there was a pronounced intramolecular isotope effect and H atoms were approximately twice as favored
over D atoms. This isotope effect and the identical copper
atom photodecay rates in Kr/H2 and Kr/D2 matrices are
similar in both direction and magnitude to those found for
the reactions of 3Po,l-Cdand -Hg atoms with HD, HZ, and
DZin the gas phase. These reactions are known to produce
the metal hydride, H, and D atoms, respectively, and have
been postulated to occur by a direct insertion-dissociation mechanism involving side-on attack of H2 or D2 by
the excited metal atom. However, to make definitive statements on the Cu/H2 system, further experiments are necessary.
The formation of C u H and CUD, respectively, as products of the photochemical reaction of C u atoms in a Kr/
H2, Kr/D2 or Kr/HD matrix was detected by FTIR studies
[intense bands in the IR spectra at 1852 (Cu/Kr/H,) and
i336 c m - ’ (Cu/Kr/D2)][”!
An interesting possibility is that the much less intense
band at 1907 (Cu/Kr/H2) or 1388 cm-I (Cu/Kr/D,), respectively, is associated with a dicopper monohydride species, formed by a reaction of the type C u + C U H CUZH.
In summary, the photochemical reaction of copper
atoms with molecular hydrogen in K r matrices can be written as follows:
cu H Z
+
uv/310,,m
Kr matrix. I Z K
Evidence for this photochemical reaction stem from a
number of sources, including UV/VIS absorption, fluorescence emission/excitation and ESR spectroscopy. In a typical UV/VIS absorption trace of a Cu/CH4 matrix ( I : lo3,
12 K) the freshly desposited sample shows the presence of
C u atoms which coexist with small amounts of CuZ. Prolonged resonance excitation of the Cu atoms [ 15 minutes,
320 nm intensity at the sample: 80 ywatt c m P 2 ;
CU(~P)+CU(*S)]
causes rapid bleaching of all the C u atom
absorptions, and formation of ‘CH3 and C u H as the major
photoproducts (absorbing at R = 200-250 nm) with very
little alteration of the concentration of Cu, (Fig. I)“].
tI
2
’ CuH + H
We found no evidence for formation of a copper dihydride
HCuH intermediate.
Received: Januarv~.7. 1982 [Z 76a [El
German version: Angew. Chem. 92 (1982) 380
The complete manuscript of this communication appears in:
Angew. Chem. Suppl. 1982. 785-797
CAS Registry numbers:
Cu, 7440-50-8; H:, 1333-74-0
3 0 2 5 0
[ I ] G. A. Ozin, D. F. Mclntosh, S. A. Mitchell, J. Garcia-Prieto, J. Am.
Chem. SOC.103 (1981) 1574.
151 G. A. Ozin, S. A. Mitchell, J. Garcia-Prieto, Angew. Chem. 94 (1982) 218;
Angew. Chem. I n ( . Ed. Engl. 21 (1982) 211.
171 G. A. Ozin, S. A. Mitchell, J. Garcia-Prieto, J. Phys. Chem. 86 (1982)
473: G. A. Ozin, H. Huber, D. F. Mclntosh, S. A. Mitchell, J. G. Norman
Jr., L. Noodleman, J. Am. Chem. Soc. 101 (1979) 3504.
[8] Experimental details of our in siru fiber optics matrix photochemical assembly, suitable for ESR, FTIR and UV/VIS absorption spectroscopic
and kinetic studies: G. A. Ozin, S. A. Mitchell, J. Garcia-Prieto, to be
published.
[9a] P. H. Kasai, D. McLeod, Jr., J. Chem. Phys. 55(1971) 1566.
1131 R. B. Wright, J. K. Bates, D. M. Gruen, Inorg. Chem. 17 (1978) 2275.
CH4 Activation by Photoexcited Cu2 Molecules**
By Geoflrey A. Ozin*, Steven A . Mitcheli, and
Jamie Garcia-Prieto
We report here our observations concerning the first example of alkane activation using a selectively photoexcited
M2 species: the matrix reaction of Cu2(B) with CH,.
S. A. Mitchell
Lash Miller Chemical Laboratories, University of Toronto
80 St. George St., Toronto, Ontario M5S IAI (Canada)
Dr. 1. Garcia-Prieto
lnstituto Mexicano del Petroleo
Av. Cien Metros, No. 152, 14. D. F. Mexico (Mexico)
(*] Prof. Dr. G. A. Ozin, Dr.
[**I
This work was supported by the Natural Sciences and Engineering Research Council (NESRC). S . A . M. thanks NSERC and Noranda for
graduate scholarships, and J. G.-P. the lnstituto Mexicano del Petroleo
for financial support.
Angew. Chem. Int. Ed. Engl. 21 (1982) NO. 5
MO
350 400 450
h[nrn]
-
Fig. 1. UV/VIS absorption spectrum of Cu atoms in a CH, matrix
(Cu :CH,= I : 10’) at 12K. A) Directly after deposition, B) after irradiation
for 15 min at 320 nm, and C) after irradiation for 5 min at 378 nm (cf.
text).
Subsequent photoexcitation of Cu2 (5 min, d = 378 nm)
results in rapid bleaching of the CuZabsorptions, with concurrent ten-fold growth in the intensity of the Cu atom resonance absorption bands. This observation should be
sharply contrasted with the minor net photodissociative
yield (10%after 30 min) following similar photoexcitation
of Cu, in rare gas matrices[’]. Furthermore, the
Cu(’D)-Cu(’S)
emission observed in rare gas matrices
after photoexcitation of Cu2 is completely quenched in the
corresponding spectra of CH, matrices[51. ESR spectroscopy clearly revealed significant production of “CH3 radicals (quartet centered around g = 2) following excitation of
Cu2 in CH,. Trace amounts of H atoms (doublet centered
around g = 2) were observed in these experiments; however, no evidence for the insertion product H3CCuH was
obtained, although because of the low Cu, concentration
employed in these experiments the amount of H,CCuH
could have been below the detection limit of the ESR
measurements. This observation should be contrasted with
the reaction of Cu(’P) formed by directed resonance excitation of Cu(*S) with CH, in the matrix, which leads to efficient generation of H3CCuH in the primary step‘’].
0 Verlag Chemie GmbH. 6940 Weinheim. 1982
0570-0833/82/0505-0381 $02.50/0
38 1
Документ
Категория
Без категории
Просмотров
1
Размер файла
220 Кб
Теги
photochemistry, spectroscopy, matrix, esr, ftir, kinetics, studies, uvvis
1/--страниц
Пожаловаться на содержимое документа