- Email: [email protected]
Site-selective laser photochemistry and the simplification of IR spectra in low-temperature matrices
Volume
78. number
CHEMICAL
I
SITE-SELECTIVE
LASER
IN LOW-TEMPERATURE hlartyn
POLIAKOFF
Recetbed
5 Janur)
IR hs+r-mduced Fe(CO)a molecules
PHOTOCHEMISTRY
AND THE SIhlPLIFICATION
chemlsrry has been used to euracr, from a comphcated rsohted rn a specltic stte m xn Ar nurrtx at 12 K
Ar matriu, from a spectrum fuU of matrix sphttings.
2. Results Fe(CO), 1s a coordmatively unsaturated molecule, generated by UV photolyss of Fe(CO), and destroyed by irradtatlon with an IR laser in the C-O stretching repon of the spectrum [5] UV.ArrlXGl-L~12K “a
15 February
19&I
OF IR SPECTRA
1931
The mterpretatlon of the IR spectrum of almost any marrL\-isolated molecule IS comphcatcd by socalled “matrL\ sphttmgs” [ 11. These cause a stngle wbrattonal mode to @ve rise to a number of closely spaced bands, typically between 0 1 and 5 cm- 1 apart. Sometimes, rhey are caused by the matrLx enlxonment lowenng molecular symmerq and sphrtmg degenerate vlbratlons. More often, they are thought to be the result of othtrwwe ldentlcal molecules bemg trapped in different sites m the matrLx_ However, the nature of these sites and how they affect the spectrum remam a mystery. Recent lugh-resolution specrra obtamed wth tunable lasers of FTCR mterferometers have been shown that these splittmgs are even more comphcated than was previously supposed [Z-4] Here, we show how an IR laser can be used to extract the spectrum of Fe(CO),. isolated m a smgle site m an
-
LE-MERS
MATRICES
1 Introduction
WWS
PHYSICS
IR laser, “c-0
0 009-2614/81/OOOO-OOOO/S
’
Fe(CO),
+ CO.
C2V
02.50
0 North-Holland
IR spccrrum
the bxnds assoazrcd
\\~th
Fz(CO), has a distorted structure with C1, s> r-,1met9 161, and. for a nurture of FIZZ and Fe(CO), , one would expect five IR bands UI the region 2030-1960 cm-‘. Fig. la shows that, in an Ar mztnk, the spectrum IS conslderably more compllcared. With a resolution of 0.12 cm-l _the spectrum contains dt least 19 bands with several further unresolved shoulders. Groups of bands corresponding to rhree of the expected IR modes (a; and e’ of Fe(CO), and bz of Fe(C0)4), are quite easily ldentlfiably but those banas due to the al and bl modes of Fe(C0)4 are extenstrely overlapped Using tunable IR lasers and 13C180 enriched Fe(CO)S, we have alread) shown that rhe mdlvldual bands wlthm rhe b, group are due to molecules trapped m different sires, bur unfortunately the presense of several Fe(lzC160),_, (13C180)x species, each with dtfferent IR absorptlons, made the rest of the spectrum too complex for detailed analysis [3.7] _ The present evperunent differs from the earlier ones in that here the CO laser IS cooled with hquld mtrogen and IS tunable to the absorption frequencies of lsotopically unenriched Fe(C0)4, so circumventing the problem posed by CO isotopes. When the matru was irralated with the CO laser at a frequency corresponding to the strongest band in the b2 group of Fe(CO)l, z hole (0.3 cm- * fwhm) was “burnt” in the spectrum at the laser frequency, mdicatmg that Fe(C0)4 molecules had been removed from a specific site (fig. 2). At the same fune there was some growth in inrensity of the adJacent band, assigned
PubMung
Company
1
L’olumz
7s. number
CHEhlICL\L
1
IS February
PH\ SICS LETTERS
1981
to the b, mod2 of an Fe(CO), molecule occupymg a &ff2ren; matrL\ slt2 Sinular slt2 mterconverston was 322” m our earher e\penmcnts [3] More slgniticanrly there were mtznslty changes throughout the C-O strerchmg regron of rhe spectrum These changes are best s22n tn the dlff2rence spectrum, shown m fig lb N tuch 1%~ produced by subtractmg the spectrum recorded after laser trradlatton from the scarrmg spectrum. fig la The postttve peaks (pomtmg upwards) are those which decrease on laser trradtatlon. whtle rh2 n2gat1~2 peaks are those which grow I” Intenslry The follo\~xng pomrs arc clear from fig 1b (1) The !aser causes bands of Fz(CO)~ to dzcressz and bands of Fe(CO), to go\\ sonfirmmg that the laser IS Indeed lnducmg
Fe(CO14
-t CO
the reaction
1973 3 cm-’
* WCO)s
(2) The positive spectrum
oi F2(CO)4 m tig lb 1s the orIginal spectrum m fig la It Is. in fact. reduced to the three bands. a1 + b, + b,. wtuch are expecrcd m this rsglon for Cl,, Fc(CO)~. and presumably rzpresents th2 spectrum of F2(CO)4 m a smgle matrtl ~112. The frequcnctes and assignment of these bands are shoan m rabk 1. (3) The spectrum of F2(CO), ;s also simpler m fig lb chsn m tig la Smce rhere are still three bands tn the 2’ regton of fig 1 b, the spectrum cannoL be that of Fe(COJ5 m a smgle site Neverrhelsss, the slmphtied spectrum does sho\v that laser trradtatton of Fs(CO), only regenerates Fe(CO),- m a Itmtted number of the possible matw. sites (The matnl sphrtmgs much
FIN 1 (a) IR spectrum oi FefCO)s
m an Xr rnxtrl\ (FefCO)s Ar, I 5000) at 12 IC atier 20 mm photo11 SIS \Itth an untiltercd mcdlum pressurz Hg arc Bxnds are asbrgned to FefCO)s and Fe(CO)4 3s mdtcatcd @) IR ddfercncc spectrum LUusrrxmg the mtrn~it!. chlngrs mused b> IR Ixscr urx-
dutton at the xro~cd frequency of the UrnpIe The dtiierence spectrum \\xs gcncrated b\ computer subtractton of the spectrum r&en
aiter ia~cr uradtsrlon
!
‘21
irom spectrum
(a)
r
simpler
than
of Fe(CO), and the mechamsm of these reactIons be dIscussed m more dctrul elxwhere.)
Table 1 Wsrenumbers
tcm-‘)
Asaenmenr
of Fc(CO)a
wll
rn an Ar matnx
This \%or2,
Ret- 161=)
31
1995 8
1988 5
bl
1991.9
1996.3
1994 1 b, Fm 7 (a) Expanded trace of the pzrt of f~ 1z assgncd to the b-mode of Fe(CO)I (b) The sxme regton after 35 nun trrzdmtlon at the arrowed frequrnw, 1973 3 cm-l with the 6 - 5 P(J0) bne of the CO laser, 150 mW
1973 3
19735 1973 0
a) Frequenaes oi so-called “trre~ersiile” Fe(CO)+ m the absence of mreracrlon wth the photoeJected CO group.
Volume
78, number
1
CHEhlICAL
PHYSICS LETTERS
3 Discussion We have pre\~ously shown that th2 band spllttmgs of Fe(CO)4 m an Ar marrlx are due. at least partially,
to mteractlon
with the photoejected
CO group 181.
and ha\2 already asslgned the bands of F2(CO)4 m c,rcumsrances where this mteraction IS apparently absent [6]. The present assignment IS for Fe(CO)4 which 1s almost certamly mteracting with the CO. because the two molecules e\enruaily react The two assignments differ m the relatltc positions of the al and bl modes, 322 table 1. Usmg a C-O factored force tield. the frequency of the al mode IS pdrticularly sensitive to the value of one Interaction force constant, kl’, and calculat!ons show that, evzn If our ongmal assignment were wrong. the general conclusions of our prekqous work [5-S] \\ould be unchanged Although the 2xxt nature of the sites remams as mysterious s e\er. it is clear that. given a contmuously tunable laser. most of the bands In the C-O stxtchmg region of the spectrum of Fc(CO)~ could be assigned to specific sites Fe(CO), IS one of the few molecules which has been found to undergo IR laserInduced rcactlons m matrIces [9], and so the expenment reported here IS not, as yet. widely applicable. Nevertheless. It does open the prospect of understandmg the detaded matrL\ sphctlngs of at 12ast on2 molecule m a way that has been lmposslble unttl now
4 Exp+rimental The expermlents were performed at the University ot Cahfomtx Berkeley. m the laboratory of Professor G C Puncntel and Dr. H. Frei. The equipment. Au Products CS-202 Dlsple\ refrlg2rator, Nlcolet 7 199 mterferometer and CO laser are described elsewhere [lo]. Redlstllled Fe(CO)5 (Alpha) and Ar (hlatheson) were premtxed and depouted slowly 0 5 mmol h-l on to a CsI wmdow at 13 K Since the splittings m the IR spectra wer2 almost Identical to those observed m matrices formed by pulsed deposition at 20 K. the przcise deposItIon contitlons do not seem to be cntxal m these experunents.
IS February
1981
Fe(CO), 1s e\tr2me!) photo-ssnsmve and IS rapIdI: destroyed by near IR radlatlon [5] Al1 spectra were recordzd at 0 12 cm-l resoiutlon. with a pohshed G2 disc to ehmmate \xlble and near-IR radIanon from ths mterferometer beam. Apart from tigher resolution, the mterferometer gives an important advantage over a dlsperslve spectrometer whch measures the mtensltlzs ot the various bands sequentially Because Fc(CO)~ 1s so sensitive to extraneous near-IR radiation. this sequenteal samphng means that there IS some uncertamty over small changes m the relative mtensltles of widely sepdrated bands. The frequ2ncy multlple_ung of the mt2rferometsr cllmmates this uncenamt>
Acknowledgement I am mdebtcd rhls work I thank hllils and Dr. P H nar?. ekprnments. G C. Pmientel for a trsvzl grant and
to Dr H Frel for his contnbutlon to Xlr. S P Church, Professor IAl Turner for their help with preluruProf2ssor .I J Turner and Professor their adk-tce. the Royal Socletj far rhe SRC for support
References
111 H E
Hallzm, ed , VIbratIonal spectroscop] of trapped species (X’u’llc), New York, 1973) I21 Rf Dubs and H H Gunthud. Chcm. Phls Lerrers 61 (1979) 105. [31 Xl Pohakoif, N Bresdon, B Dabxs, A hfcNelsh and J J Turner, Chem Phys Letters 36 (1976) 373 I-11 L H Jones, B I Swanson and S E Ekberg, Chum Phys. Lerrers 68 (1979) 499 [jl 31 Pohakoif, Chum Sot. Re\ 7 (1978) 527. 161 hl Pollakoif and JJ Turner. J. Chem Sot Dalton Trans (1974) 27-76 [71 B Dabxes, A hlcNelsh, hf Pobakoii. hf. Tranqudle and J-l. Turner. Chem. Phys. Letters 52 (1977) 477 181 hf. Pohakoff and J.J. Turner, J Cbem. Sot Dalton Tram (1973) 1351. [91 hl Pohakoff and J.J Turner, m Chenucal and bloloacal apphcarlons of hsers, \‘ol. 5, cd C B. hloore (Acadermc Press, Nea York. 1980) tto1 H Frel, L Fredln and C C. F’unenrel, J Chem. Ph?. s 73 (Jan. 1981), to bz pubhshrd
3
78. number
CHEMICAL
I
SITE-SELECTIVE
LASER
IN LOW-TEMPERATURE hlartyn
POLIAKOFF
Recetbed
5 Janur)
IR hs+r-mduced Fe(CO)a molecules
PHOTOCHEMISTRY
AND THE SIhlPLIFICATION
chemlsrry has been used to euracr, from a comphcated rsohted rn a specltic stte m xn Ar nurrtx at 12 K
Ar matriu, from a spectrum fuU of matrix sphttings.
2. Results Fe(CO), 1s a coordmatively unsaturated molecule, generated by UV photolyss of Fe(CO), and destroyed by irradtatlon with an IR laser in the C-O stretching repon of the spectrum [5] UV.ArrlXGl-L~12K “a
15 February
19&I
OF IR SPECTRA
1931
The mterpretatlon of the IR spectrum of almost any marrL\-isolated molecule IS comphcatcd by socalled “matrL\ sphttmgs” [ 11. These cause a stngle wbrattonal mode to @ve rise to a number of closely spaced bands, typically between 0 1 and 5 cm- 1 apart. Sometimes, rhey are caused by the matrLx enlxonment lowenng molecular symmerq and sphrtmg degenerate vlbratlons. More often, they are thought to be the result of othtrwwe ldentlcal molecules bemg trapped in different sites m the matrLx_ However, the nature of these sites and how they affect the spectrum remam a mystery. Recent lugh-resolution specrra obtamed wth tunable lasers of FTCR mterferometers have been shown that these splittmgs are even more comphcated than was previously supposed [Z-4] Here, we show how an IR laser can be used to extract the spectrum of Fe(CO),. isolated m a smgle site m an
-
LE-MERS
MATRICES
1 Introduction
WWS
PHYSICS
IR laser, “c-0
0 009-2614/81/OOOO-OOOO/S
’
Fe(CO),
+ CO.
C2V
02.50
0 North-Holland
IR spccrrum
the bxnds assoazrcd
\\~th
Fz(CO), has a distorted structure with C1, s> r-,1met9 161, and. for a nurture of FIZZ and Fe(CO), , one would expect five IR bands UI the region 2030-1960 cm-‘. Fig. la shows that, in an Ar mztnk, the spectrum IS conslderably more compllcared. With a resolution of 0.12 cm-l _the spectrum contains dt least 19 bands with several further unresolved shoulders. Groups of bands corresponding to rhree of the expected IR modes (a; and e’ of Fe(CO), and bz of Fe(C0)4), are quite easily ldentlfiably but those banas due to the al and bl modes of Fe(C0)4 are extenstrely overlapped Using tunable IR lasers and 13C180 enriched Fe(CO)S, we have alread) shown that rhe mdlvldual bands wlthm rhe b, group are due to molecules trapped m different sires, bur unfortunately the presense of several Fe(lzC160),_, (13C180)x species, each with dtfferent IR absorptlons, made the rest of the spectrum too complex for detailed analysis [3.7] _ The present evperunent differs from the earlier ones in that here the CO laser IS cooled with hquld mtrogen and IS tunable to the absorption frequencies of lsotopically unenriched Fe(C0)4, so circumventing the problem posed by CO isotopes. When the matru was irralated with the CO laser at a frequency corresponding to the strongest band in the b2 group of Fe(CO)l, z hole (0.3 cm- * fwhm) was “burnt” in the spectrum at the laser frequency, mdicatmg that Fe(C0)4 molecules had been removed from a specific site (fig. 2). At the same fune there was some growth in inrensity of the adJacent band, assigned
PubMung
Company
1
L’olumz
7s. number
CHEhlICL\L
1
IS February
PH\ SICS LETTERS
1981
to the b, mod2 of an Fe(CO), molecule occupymg a &ff2ren; matrL\ slt2 Sinular slt2 mterconverston was 322” m our earher e\penmcnts [3] More slgniticanrly there were mtznslty changes throughout the C-O strerchmg regron of rhe spectrum These changes are best s22n tn the dlff2rence spectrum, shown m fig lb N tuch 1%~ produced by subtractmg the spectrum recorded after laser trradlatton from the scarrmg spectrum. fig la The postttve peaks (pomtmg upwards) are those which decrease on laser trradtatlon. whtle rh2 n2gat1~2 peaks are those which grow I” Intenslry The follo\~xng pomrs arc clear from fig 1b (1) The !aser causes bands of Fz(CO)~ to dzcressz and bands of Fe(CO), to go\\ sonfirmmg that the laser IS Indeed lnducmg
Fe(CO14
-t CO
the reaction
1973 3 cm-’
* WCO)s
(2) The positive spectrum
oi F2(CO)4 m tig lb 1s the orIginal spectrum m fig la It Is. in fact. reduced to the three bands. a1 + b, + b,. wtuch are expecrcd m this rsglon for Cl,, Fc(CO)~. and presumably rzpresents th2 spectrum of F2(CO)4 m a smgle matrtl ~112. The frequcnctes and assignment of these bands are shoan m rabk 1. (3) The spectrum of F2(CO), ;s also simpler m fig lb chsn m tig la Smce rhere are still three bands tn the 2’ regton of fig 1 b, the spectrum cannoL be that of Fe(COJ5 m a smgle site Neverrhelsss, the slmphtied spectrum does sho\v that laser trradtatton of Fs(CO), only regenerates Fe(CO),- m a Itmtted number of the possible matw. sites (The matnl sphrtmgs much
FIN 1 (a) IR spectrum oi FefCO)s
m an Xr rnxtrl\ (FefCO)s Ar, I 5000) at 12 IC atier 20 mm photo11 SIS \Itth an untiltercd mcdlum pressurz Hg arc Bxnds are asbrgned to FefCO)s and Fe(CO)4 3s mdtcatcd @) IR ddfercncc spectrum LUusrrxmg the mtrn~it!. chlngrs mused b> IR Ixscr urx-
dutton at the xro~cd frequency of the UrnpIe The dtiierence spectrum \\xs gcncrated b\ computer subtractton of the spectrum r&en
aiter ia~cr uradtsrlon
!
‘21
irom spectrum
(a)
r
simpler
than
of Fe(CO), and the mechamsm of these reactIons be dIscussed m more dctrul elxwhere.)
Table 1 Wsrenumbers
tcm-‘)
Asaenmenr
of Fc(CO)a
wll
rn an Ar matnx
This \%or2,
Ret- 161=)
31
1995 8
1988 5
bl
1991.9
1996.3
1994 1 b, Fm 7 (a) Expanded trace of the pzrt of f~ 1z assgncd to the b-mode of Fe(CO)I (b) The sxme regton after 35 nun trrzdmtlon at the arrowed frequrnw, 1973 3 cm-l with the 6 - 5 P(J0) bne of the CO laser, 150 mW
1973 3
19735 1973 0
a) Frequenaes oi so-called “trre~ersiile” Fe(CO)+ m the absence of mreracrlon wth the photoeJected CO group.
Volume
78, number
1
CHEhlICAL
PHYSICS LETTERS
3 Discussion We have pre\~ously shown that th2 band spllttmgs of Fe(CO)4 m an Ar marrlx are due. at least partially,
to mteractlon
with the photoejected
CO group 181.
and ha\2 already asslgned the bands of F2(CO)4 m c,rcumsrances where this mteraction IS apparently absent [6]. The present assignment IS for Fe(CO)4 which 1s almost certamly mteracting with the CO. because the two molecules e\enruaily react The two assignments differ m the relatltc positions of the al and bl modes, 322 table 1. Usmg a C-O factored force tield. the frequency of the al mode IS pdrticularly sensitive to the value of one Interaction force constant, kl’, and calculat!ons show that, evzn If our ongmal assignment were wrong. the general conclusions of our prekqous work [5-S] \\ould be unchanged Although the 2xxt nature of the sites remams as mysterious s e\er. it is clear that. given a contmuously tunable laser. most of the bands In the C-O stxtchmg region of the spectrum of Fc(CO)~ could be assigned to specific sites Fe(CO), IS one of the few molecules which has been found to undergo IR laserInduced rcactlons m matrIces [9], and so the expenment reported here IS not, as yet. widely applicable. Nevertheless. It does open the prospect of understandmg the detaded matrL\ sphctlngs of at 12ast on2 molecule m a way that has been lmposslble unttl now
4 Exp+rimental The expermlents were performed at the University ot Cahfomtx Berkeley. m the laboratory of Professor G C Puncntel and Dr. H. Frei. The equipment. Au Products CS-202 Dlsple\ refrlg2rator, Nlcolet 7 199 mterferometer and CO laser are described elsewhere [lo]. Redlstllled Fe(CO)5 (Alpha) and Ar (hlatheson) were premtxed and depouted slowly 0 5 mmol h-l on to a CsI wmdow at 13 K Since the splittings m the IR spectra wer2 almost Identical to those observed m matrices formed by pulsed deposition at 20 K. the przcise deposItIon contitlons do not seem to be cntxal m these experunents.
IS February
1981
Fe(CO), 1s e\tr2me!) photo-ssnsmve and IS rapIdI: destroyed by near IR radlatlon [5] Al1 spectra were recordzd at 0 12 cm-l resoiutlon. with a pohshed G2 disc to ehmmate \xlble and near-IR radIanon from ths mterferometer beam. Apart from tigher resolution, the mterferometer gives an important advantage over a dlsperslve spectrometer whch measures the mtensltlzs ot the various bands sequentially Because Fc(CO)~ 1s so sensitive to extraneous near-IR radiation. this sequenteal samphng means that there IS some uncertamty over small changes m the relative mtensltles of widely sepdrated bands. The frequ2ncy multlple_ung of the mt2rferometsr cllmmates this uncenamt>
Acknowledgement I am mdebtcd rhls work I thank hllils and Dr. P H nar?. ekprnments. G C. Pmientel for a trsvzl grant and
to Dr H Frel for his contnbutlon to Xlr. S P Church, Professor IAl Turner for their help with preluruProf2ssor .I J Turner and Professor their adk-tce. the Royal Socletj far rhe SRC for support
References
111 H E
Hallzm, ed , VIbratIonal spectroscop] of trapped species (X’u’llc), New York, 1973) I21 Rf Dubs and H H Gunthud. Chcm. Phls Lerrers 61 (1979) 105. [31 Xl Pohakoif, N Bresdon, B Dabxs, A hfcNelsh and J J Turner, Chem Phys Letters 36 (1976) 373 I-11 L H Jones, B I Swanson and S E Ekberg, Chum Phys. Lerrers 68 (1979) 499 [jl 31 Pohakoif, Chum Sot. Re\ 7 (1978) 527. 161 hl Pollakoif and JJ Turner. J. Chem Sot Dalton Trans (1974) 27-76 [71 B Dabxes, A hlcNelsh, hf Pobakoii. hf. Tranqudle and J-l. Turner. Chem. Phys. Letters 52 (1977) 477 181 hf. Pohakoff and J.J. Turner, J Cbem. Sot Dalton Tram (1973) 1351. [91 hl Pohakoff and J.J Turner, m Chenucal and bloloacal apphcarlons of hsers, \‘ol. 5, cd C B. hloore (Acadermc Press, Nea York. 1980) tto1 H Frel, L Fredln and C C. F’unenrel, J Chem. Ph?. s 73 (Jan. 1981), to bz pubhshrd
3