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Orientational and site selectivity in low temperature infrared laser-induced photochemistry
Volume 52, number 3
CHEMICAL PHYSICS LETTERS
15 December 1977
QRLEN-CATKWAL AND SlTE SWTNITY IN LOW TEMPERATURE
INFRARED
LASER-INDUCED
Brian DAVIES, Allister McNEISH, Martyn POLIAKOFF, Department
of Inorganic Chemistry,
The Universiry.
Newcastle
PHOTOCHEMISTRY Michel TRANQUILLE*
and James J. TURNER
upon Tyne. NE1 7RLJ. UK
Received 22 August 1977
IR laser irradiation of ‘3C’80-enriched orientations,
Or isolated
in particular
matrix
Fc(CO)4 in N2 and 4r matrices at 20 K, shows that molecules in particular sites, can be induced to react preferentially.
1. Introduction We have recently examined the IR laser-induced photochemistry of the fragment Fe(CO)d in several low-temperature matrices [ 1,2]. The’reactions are highly selective. They display both isotopic selectivity and steredchemical selectivity, and have enabled us to identify [2] the intramolecular rearrangement mode of Fe(CO)4, and provide evidence [2] for the inhomogeneous broadening of the IR absorption bands of Fe(CO)4. In this paper we describe further IR laser eXpeiimentS
with
Fe(CO)a
which
ilh.Eitrate
orienta-
tional and matrix site selectivity.
2. Orientational
selectivity
In this section we consider the behaviour of Fe(CO)4 in an N2 matrix. IR laser irradiation into the “C-O stretching” modes of Fe(CO)4 promotes intramolecular ligand exchange [3]. When the Fe(C0)4 is partially
enriched with 13C180, the ligand exchange can be foilowed by changes in intensity of IR absorption bands of the different Fe(12C160)4_,(13C180)X species in the region 2100-1870 cm-l. For example, laser irradiation interconverts the two isomers, A and B, of Fe(12C160)(13C180) 3-
* Permanent address: Laboratoirc de Spectroscopic Infrarouge, UniversiJ de Bordeaux, 33405 Talence, France.
x : ‘3c’80
An interesting problem is whether or not this laserinduced rearrangement involves local melting of the matrix and free rotation of the rearranging molecule_ By using a plane polarized laser for irradiation, we show that free rotation does not occur. If a sample of randomly oriented non-rotating noncubic molecules is irradiated with plane-polarized light, those molecules whose absorption vector is perpendicular to the electric vector of the photolysing radiation cannot absorb the light; for other orientations the probability of absorption depends on the usual cos20 term. Thus, molecules in some orientations will be destroyed faster than others and the spectrum of the unphotolysed molecules will show increasing linear dichroism as the reaction proceeds**. On the other hand the spectrum of the photolysis product can only show linear dichroism if free rotation of the molecules does not occur during the photochemical act. In these experiments the “reactant” and “product” are diffcr-
** The principles of photoselection have been discussed e.utensively elsewhere in connection with the polarized photolysis of matrix isolated molecules [4].
UV/visible
477
CHEMICAL PHYSKS
Votume 52, number 3
-
0
ic cm-1
Fig. 1. Polarized IR absorption spectra of the iowest frequency C-O stretching mode of isomer B of Fe(‘aC*60)(‘3C’80)~, produced by UV photolysis of Fe(CO)s, enriched 165% with s3Cr80, in an N2 matrix (Fe(CO)s : N2. 1: 3000). Spectra were taken with light &me polarized parallcl (solid line) and perpendicular (broken line) to the polarization of the CO laser (a) before laser irradiation, (b) after 1 min plane polarized Iaser irradiation, 500 mW at 1880.5 cm-‘. ent isomers ofFe(~2C~60)4_,(13C180)x,e_g_
We first consider
Aand
B.
cm -1 fig. 2. Polarized IR absorption spectra of bands due to isomers A and 3 of pe(*2C160)(13C’aO) in an Nz matrix. The spectra were recorded after irradiation of the arrowed band of A using the CO faser at 1 E97.5 cm+ , Q 700 mW, with light poiarized parallel (solid line) and perpendicular (broken line) to the polarization of the CO laser. (The same sample was used in fig. 1, but at a different stage of the experiment.)
the reaction
1880.5 cm-’ B-A. Polarized
the pofarized spectra of one band of B? before and after irradiation. It is clear from the spectra that there is no dichroism at the start of the experiment (fig. la) but that substantial dichroism develops during phptolysis (fig. 1b). This immediately shows that the molecules of Fe&O)4 are “non-rotating” on the timescale of the experiment (e-g_ several hours). More importantly, the dichroism means that there is preferential reaction of molecules in particular orientations. This is the first example of otienr”atin& selectivity in an IR laser-induced reaction. We now examine absorption bands of both reactant Fig. 1 iltustrate:
t The band is, in reality, the co-incident absorption of three
different isomers of Fe(12C160)+.X(13C180)X [J = 2,3,4). see refs. [2,5]. but under the conditions used, the strongest component is due to B. Furthermore, the interpretation of the results reported here does not in any way depend on the detailed assignment of these bands to particularisomers.
473
15 December 1977
LETTERS
and product
during the reaction
1897.5 cm-’ A-B. polarized
Initially neither A nor B showed any linear dichroism. The polarized spectra of A and B, recorded after laser irradiation, are illustrated in fig. 2. These spectra show that dichroism develops not only in the irradiated band of the “reactant”, A (fig. 2A) but also in the bands of the “product”, B (fig. 2B). Furthermore, these bands of A and E display opposite dichroism. Thus, the moIecules of B are oriented relative to the irradiated molecules of A. This implies that random rotation of the molecule does not form part of the rearrangement process. It shows, moreover, that the mechanism is unlikely to involve localized melting of the matrix which would be expected to allow free rotation.
b
-v
15 December 1977
CHEMICAL PHYSICS LETTERS
Volume 52, number 3
a
A
Fig. 3. IR absorption spectrum of the split bz “C-O stretching” mode of Fe(‘3C180)4 in an Ar matrb, produced by brief UV photolysis of Fe(CO)s, (Fe(CO)s : Ar, 1: 3000), (a) (centre spectrum) after UV photo&is. (b) after 30 min irradiation with the continuously tunable SFRL at 1882 cm-* (15 mW total power), co-incident with the highest frequency bands, (c) after 1 min irradiation with the CO laser at 1880.5 cm-’ , 10 mW, co-incident with the middle band - note that the matrb was briefly exposed to UV irradiation between spectra (b) and (c), to regenerate the destroyed Fe(CO)+
two bands remain unaffected_ Thus, the three bands must all be due to different molecules, and the splitting is therefore almost certainly caused by different sites. After Iaser irradiation the spectra remain unchanged for several hours, which means that there is no rapid thermal exchange between the different types of Fe(CO)4. This is the first time that matrix site selectivityhas been observed in an IR laser-induced reaction. It is also the first photochemical application of the spin flip Raman laser [7] (SFRL) and illustrates the advantage of using a continuously tunable laser, which can be used to irradiate any band. The cw CO laser, which we have used for previous reactions, produces output
at discrete
wavelengths
separated
by -4
cm-l central
and in this case can orzZy be used to irradiate the band of the Fe(C0)4 triplet, fig. 3c 3. Site effects have been a particularly badly understood aspect of matrix isolation and these experiments show that IR lasers have great promise as a tool for probing the environment of trapped molecules.
4. Experimental 3. Site selectivity The interpretation matrix so-called
isolated
of the vibrational
molecules
is frequently
spectra hampered
Experiments were performed using an Air Products Displex CS-202 cooler and model APD-B automatic
of by
“matrix
splittings” [6]. Thus, when Fe(C0)4 is generated by brief UV (e.g. = 15 s) photolysis of Fe(CO)5 in an Ar matrix, the b, “C-O stretching” mode gives rise, under moderate resolution (0.9 cm-‘) to three IR bands separated by ~2 cm-l _ These bands have been attributed to otherwise identical Fe(C0)4 molecules isolated in different matrix sites, possibly involving weak interaction with the photo-ejected CO molecules [5]. Until now, however, it has been impossible to prove that the three bands are indeed due to different types of molecules. Fig. 3a shows these three bands, and the results of tunable IR laser-irradiation of the different components, figs. 3b and 3c. From examination of the whole C-O stretching region of the spectrum (not illustrated) it is clear that the laser is promoting the reaction: IR laser
Fe(CO)a + CO -
Fe(CO)5 .
In the part of the spectrum illustrated, only the irradiated band is reduced in intensity, while the other
temperature
controller,
CsBr substrate
and pulsed de-
position of the matrix. IR spectra were recorded on a Perkin-Elmer 580 spectrophotometer fitted with a Perkin-Elmer common beam wire grid polarizer on an AgBr substrate, and a common beam coated Ce filter (OCLI) to prevent light of wavelength shorter than 3 pm from reaching the matrix. The spin flip Raman laser and model PL3 cw CO laser were built by Edinburgh Instruments. The principles
of the
SFRL
[7] and the technical
details
[9] of
the CO laser have been described
elsewhere. The SFRL used an n-type InSb crystal (N, = 8 X 1014 cme3) with anti-reflection coatings, cooled by liquid helium. The crystal was pumped using the 1893 cm-l output of the CO laser, which consists of two closely spaced f Under very
h&h resolution (0.01 cm-‘, using the SFRL as a spectrometer) the three bands of fii_ 3 show some tine
structure; SFRL photochemistry shows that there are intensity changes within this fiie structure. The detailed interpretation of this behaviour and its relevance to the Fe(CO)4 photochemistry and site selectivity will be presented elsewhere [ 8 ] -
479
Yolume 52, number 3
CHEMKAL PHYSlCS LETTERS
lines, chopped at 200 Hz. The tuned Stokes output
was not separated from the residual pump radiation. The power, pump + Stokes, at the matrix was = 15 mW (1882 cm-l). For the polarization experiments a wire grid polarizer was placed in the laser beam close to the matrix. In all experiments the laser beam was focussed to beam diameter of x2 cm, a size similar to that of the matrix.
We thank the SRC for supporting this research, for a reseaich assistantship (to A.McN.), and for a studentship (to B.D.). We are grateful to the European Science Exchange Programme for a grant (to M.T. for visiting Newcastle). We thank Dr. R-L. Allwood for his assistance, and Professor SD. Smith and members of the Physics Department of Heriot-Watt University for their help and advice.
480
15 December 1977
References A. McNeish, M. Poliakoff, K.P. Smith and J.J. Turner, Chem. Commun. (19763 859. B. Davies, A. McNcish, M. Poliakoff and J.J. Turner, J. Am. Chem. Sot. (Dec. 1977). to be published. B. Davies, A. McNeish, M. Poliakoff, M. Tranquilie and J-J. Turner, Chem, ~omrnu~, submitted for publication. J-K- Burdett, R.N. Peru& M. Poliakoff and J.J. Turner, Chem. Commun. (1975) 157; J. Pure Appl. Chem. 49 (1977) 271. M. Pofiakoff and J.J. Turner, J. Chem. Sot. Dalton Trans. (1973) 1351; (1974) 2276. H.E. Hallam, ed., Vibrational spectroscopy of trapped species (Wiley, New York, 1973). J.K. Burdett and M, Poliakoff, Chem. Sot. Rev. 3 (1974) 293, and references thereiu. B. Davies, A. McNeish, M. Poliakoff and J.J. Turner, to be published. M.J. Colles, R.B. Dennis, J-W. Smith, J-S. Webb and RL. Allwood, Opt. FraserTechnol. 7 (1975) 73.
CHEMICAL PHYSICS LETTERS
15 December 1977
QRLEN-CATKWAL AND SlTE SWTNITY IN LOW TEMPERATURE
INFRARED
LASER-INDUCED
Brian DAVIES, Allister McNEISH, Martyn POLIAKOFF, Department
of Inorganic Chemistry,
The Universiry.
Newcastle
PHOTOCHEMISTRY Michel TRANQUILLE*
and James J. TURNER
upon Tyne. NE1 7RLJ. UK
Received 22 August 1977
IR laser irradiation of ‘3C’80-enriched orientations,
Or isolated
in particular
matrix
Fc(CO)4 in N2 and 4r matrices at 20 K, shows that molecules in particular sites, can be induced to react preferentially.
1. Introduction We have recently examined the IR laser-induced photochemistry of the fragment Fe(CO)d in several low-temperature matrices [ 1,2]. The’reactions are highly selective. They display both isotopic selectivity and steredchemical selectivity, and have enabled us to identify [2] the intramolecular rearrangement mode of Fe(CO)4, and provide evidence [2] for the inhomogeneous broadening of the IR absorption bands of Fe(CO)4. In this paper we describe further IR laser eXpeiimentS
with
Fe(CO)a
which
ilh.Eitrate
orienta-
tional and matrix site selectivity.
2. Orientational
selectivity
In this section we consider the behaviour of Fe(CO)4 in an N2 matrix. IR laser irradiation into the “C-O stretching” modes of Fe(CO)4 promotes intramolecular ligand exchange [3]. When the Fe(C0)4 is partially
enriched with 13C180, the ligand exchange can be foilowed by changes in intensity of IR absorption bands of the different Fe(12C160)4_,(13C180)X species in the region 2100-1870 cm-l. For example, laser irradiation interconverts the two isomers, A and B, of Fe(12C160)(13C180) 3-
* Permanent address: Laboratoirc de Spectroscopic Infrarouge, UniversiJ de Bordeaux, 33405 Talence, France.
x : ‘3c’80
An interesting problem is whether or not this laserinduced rearrangement involves local melting of the matrix and free rotation of the rearranging molecule_ By using a plane polarized laser for irradiation, we show that free rotation does not occur. If a sample of randomly oriented non-rotating noncubic molecules is irradiated with plane-polarized light, those molecules whose absorption vector is perpendicular to the electric vector of the photolysing radiation cannot absorb the light; for other orientations the probability of absorption depends on the usual cos20 term. Thus, molecules in some orientations will be destroyed faster than others and the spectrum of the unphotolysed molecules will show increasing linear dichroism as the reaction proceeds**. On the other hand the spectrum of the photolysis product can only show linear dichroism if free rotation of the molecules does not occur during the photochemical act. In these experiments the “reactant” and “product” are diffcr-
** The principles of photoselection have been discussed e.utensively elsewhere in connection with the polarized photolysis of matrix isolated molecules [4].
UV/visible
477
CHEMICAL PHYSKS
Votume 52, number 3
-
0
ic cm-1
Fig. 1. Polarized IR absorption spectra of the iowest frequency C-O stretching mode of isomer B of Fe(‘aC*60)(‘3C’80)~, produced by UV photolysis of Fe(CO)s, enriched 165% with s3Cr80, in an N2 matrix (Fe(CO)s : N2. 1: 3000). Spectra were taken with light &me polarized parallcl (solid line) and perpendicular (broken line) to the polarization of the CO laser (a) before laser irradiation, (b) after 1 min plane polarized Iaser irradiation, 500 mW at 1880.5 cm-‘. ent isomers ofFe(~2C~60)4_,(13C180)x,e_g_
We first consider
Aand
B.
cm -1 fig. 2. Polarized IR absorption spectra of bands due to isomers A and 3 of pe(*2C160)(13C’aO) in an Nz matrix. The spectra were recorded after irradiation of the arrowed band of A using the CO faser at 1 E97.5 cm+ , Q 700 mW, with light poiarized parallel (solid line) and perpendicular (broken line) to the polarization of the CO laser. (The same sample was used in fig. 1, but at a different stage of the experiment.)
the reaction
1880.5 cm-’ B-A. Polarized
the pofarized spectra of one band of B? before and after irradiation. It is clear from the spectra that there is no dichroism at the start of the experiment (fig. la) but that substantial dichroism develops during phptolysis (fig. 1b). This immediately shows that the molecules of Fe&O)4 are “non-rotating” on the timescale of the experiment (e-g_ several hours). More importantly, the dichroism means that there is preferential reaction of molecules in particular orientations. This is the first example of otienr”atin& selectivity in an IR laser-induced reaction. We now examine absorption bands of both reactant Fig. 1 iltustrate:
t The band is, in reality, the co-incident absorption of three
different isomers of Fe(12C160)+.X(13C180)X [J = 2,3,4). see refs. [2,5]. but under the conditions used, the strongest component is due to B. Furthermore, the interpretation of the results reported here does not in any way depend on the detailed assignment of these bands to particularisomers.
473
15 December 1977
LETTERS
and product
during the reaction
1897.5 cm-’ A-B. polarized
Initially neither A nor B showed any linear dichroism. The polarized spectra of A and B, recorded after laser irradiation, are illustrated in fig. 2. These spectra show that dichroism develops not only in the irradiated band of the “reactant”, A (fig. 2A) but also in the bands of the “product”, B (fig. 2B). Furthermore, these bands of A and E display opposite dichroism. Thus, the moIecules of B are oriented relative to the irradiated molecules of A. This implies that random rotation of the molecule does not form part of the rearrangement process. It shows, moreover, that the mechanism is unlikely to involve localized melting of the matrix which would be expected to allow free rotation.
b
-v
15 December 1977
CHEMICAL PHYSICS LETTERS
Volume 52, number 3
a
A
Fig. 3. IR absorption spectrum of the split bz “C-O stretching” mode of Fe(‘3C180)4 in an Ar matrb, produced by brief UV photolysis of Fe(CO)s, (Fe(CO)s : Ar, 1: 3000), (a) (centre spectrum) after UV photo&is. (b) after 30 min irradiation with the continuously tunable SFRL at 1882 cm-* (15 mW total power), co-incident with the highest frequency bands, (c) after 1 min irradiation with the CO laser at 1880.5 cm-’ , 10 mW, co-incident with the middle band - note that the matrb was briefly exposed to UV irradiation between spectra (b) and (c), to regenerate the destroyed Fe(CO)+
two bands remain unaffected_ Thus, the three bands must all be due to different molecules, and the splitting is therefore almost certainly caused by different sites. After Iaser irradiation the spectra remain unchanged for several hours, which means that there is no rapid thermal exchange between the different types of Fe(CO)4. This is the first time that matrix site selectivityhas been observed in an IR laser-induced reaction. It is also the first photochemical application of the spin flip Raman laser [7] (SFRL) and illustrates the advantage of using a continuously tunable laser, which can be used to irradiate any band. The cw CO laser, which we have used for previous reactions, produces output
at discrete
wavelengths
separated
by -4
cm-l central
and in this case can orzZy be used to irradiate the band of the Fe(C0)4 triplet, fig. 3c 3. Site effects have been a particularly badly understood aspect of matrix isolation and these experiments show that IR lasers have great promise as a tool for probing the environment of trapped molecules.
4. Experimental 3. Site selectivity The interpretation matrix so-called
isolated
of the vibrational
molecules
is frequently
spectra hampered
Experiments were performed using an Air Products Displex CS-202 cooler and model APD-B automatic
of by
“matrix
splittings” [6]. Thus, when Fe(C0)4 is generated by brief UV (e.g. = 15 s) photolysis of Fe(CO)5 in an Ar matrix, the b, “C-O stretching” mode gives rise, under moderate resolution (0.9 cm-‘) to three IR bands separated by ~2 cm-l _ These bands have been attributed to otherwise identical Fe(C0)4 molecules isolated in different matrix sites, possibly involving weak interaction with the photo-ejected CO molecules [5]. Until now, however, it has been impossible to prove that the three bands are indeed due to different types of molecules. Fig. 3a shows these three bands, and the results of tunable IR laser-irradiation of the different components, figs. 3b and 3c. From examination of the whole C-O stretching region of the spectrum (not illustrated) it is clear that the laser is promoting the reaction: IR laser
Fe(CO)a + CO -
Fe(CO)5 .
In the part of the spectrum illustrated, only the irradiated band is reduced in intensity, while the other
temperature
controller,
CsBr substrate
and pulsed de-
position of the matrix. IR spectra were recorded on a Perkin-Elmer 580 spectrophotometer fitted with a Perkin-Elmer common beam wire grid polarizer on an AgBr substrate, and a common beam coated Ce filter (OCLI) to prevent light of wavelength shorter than 3 pm from reaching the matrix. The spin flip Raman laser and model PL3 cw CO laser were built by Edinburgh Instruments. The principles
of the
SFRL
[7] and the technical
details
[9] of
the CO laser have been described
elsewhere. The SFRL used an n-type InSb crystal (N, = 8 X 1014 cme3) with anti-reflection coatings, cooled by liquid helium. The crystal was pumped using the 1893 cm-l output of the CO laser, which consists of two closely spaced f Under very
h&h resolution (0.01 cm-‘, using the SFRL as a spectrometer) the three bands of fii_ 3 show some tine
structure; SFRL photochemistry shows that there are intensity changes within this fiie structure. The detailed interpretation of this behaviour and its relevance to the Fe(CO)4 photochemistry and site selectivity will be presented elsewhere [ 8 ] -
479
Yolume 52, number 3
CHEMKAL PHYSlCS LETTERS
lines, chopped at 200 Hz. The tuned Stokes output
was not separated from the residual pump radiation. The power, pump + Stokes, at the matrix was = 15 mW (1882 cm-l). For the polarization experiments a wire grid polarizer was placed in the laser beam close to the matrix. In all experiments the laser beam was focussed to beam diameter of x2 cm, a size similar to that of the matrix.
We thank the SRC for supporting this research, for a reseaich assistantship (to A.McN.), and for a studentship (to B.D.). We are grateful to the European Science Exchange Programme for a grant (to M.T. for visiting Newcastle). We thank Dr. R-L. Allwood for his assistance, and Professor SD. Smith and members of the Physics Department of Heriot-Watt University for their help and advice.
480
15 December 1977
References A. McNeish, M. Poliakoff, K.P. Smith and J.J. Turner, Chem. Commun. (19763 859. B. Davies, A. McNcish, M. Poliakoff and J.J. Turner, J. Am. Chem. Sot. (Dec. 1977). to be published. B. Davies, A. McNeish, M. Poliakoff, M. Tranquilie and J-J. Turner, Chem, ~omrnu~, submitted for publication. J-K- Burdett, R.N. Peru& M. Poliakoff and J.J. Turner, Chem. Commun. (1975) 157; J. Pure Appl. Chem. 49 (1977) 271. M. Pofiakoff and J.J. Turner, J. Chem. Sot. Dalton Trans. (1973) 1351; (1974) 2276. H.E. Hallam, ed., Vibrational spectroscopy of trapped species (Wiley, New York, 1973). J.K. Burdett and M, Poliakoff, Chem. Sot. Rev. 3 (1974) 293, and references thereiu. B. Davies, A. McNeish, M. Poliakoff and J.J. Turner, to be published. M.J. Colles, R.B. Dennis, J-W. Smith, J-S. Webb and RL. Allwood, Opt. FraserTechnol. 7 (1975) 73.