Vibrational spectrum of CO2− in an argon matrix

CIiEhlICiL

VIBRATIONAL-

SPEITRtiM

L5 September

PHYSICS LE’ITERS

OF CO,

e

AN ARGON

1974

MATRIg

Marilyn E. JACOX &d Doiphus E. MILLIGti, Institute for hla:etils Research, National Btieau of Standards, Washihgton, DC 20234. US.4

:

Received 17 May‘1974

at 14 K of kc02 mivtures with OI!&Ii met31 have Absorptions wkch appear near 1600 cm-l on codepasitian been assigned to vg of an hi?.,.CO; ion pti, with 2n OCO valence an$e nex 130’. Molecular awegates contribute signiticantly to ihe observed spectrum.

1. I3ackground The interaction of low energy electrons with CO2 has been the subject ?f many experimental studies, the

results of which have rec!ntIy been summarized by Claydon et al. [I 1. Four di&iative attachment processes occur at electron energies below 20 eV. Of these, only the lowest ensrw process,‘with an onset near 3.8 eV, can be attributed to a bound compound state. Decay from this state involves excitation of ali three vibrational modes of CO,. Very’recent studies by Spence

et al. [2] have yielded

an inelastic

cross section

for this 3.8 eV process an order of magnitude greater than that previously reported. Ln the high resolution ‘electron transmission spectroscopy experiments of Sanche and Schulz [3], a progression of eibteen viirational levels was observed starting at 3.142 0.04 eV, with a fust vibrational spacing of 138 meV (1113 cm-l). The estiinated lifetime of CO, formed in the electron capture studies is far too short to permit its detection in a ma5s spectrometer. However, CO, was identified in the double maSs spectrometer studies of Paulson [4], in which the anion was produced by collisions with heavy species >uch as NO: rather than ti’th el,ectrons. Cooper and Compton IS] have also .observed long-hved CO, ions in the gas phase following the collisiqn of electrons or cesium atoms with organic anhydrides, in which the CO, group is initially bent at : ‘Work supportedin paitbyNAS& * Posthumdus contribution. ‘. .. .’

an angle ne’& 120’. The long lifetime of CC): formed under these conditions su&ests +ht the potential curve of CO, should be no more than 0.5 eV above that of COZ at a valence angle near I34”, estimated for CO, in its ground state (61. The ab initio calctlations of tiauss and Neumann [7] suggest that the potential minimum for ground-state CO~‘m~y, in fact, lie beIow that for CO2 at a valence tigle of 134O. CO, is readily detected in the solid state;where ctiulombic interactions favor its stabilization. Ovenall :nd Whiffen [6] identified its ESRspectcvrn in Wradiated sodium formate crystals. Carbon-13 studios yielded an estimate of 13$” for the COT Wenca an@ Zlere was evidence for an appreciable interaction with an axial sodium cation. titer studies by Chantrq.. and Whiffen [8] led to the assignment of absorption bands at 340, 280, and 255 nm to CO,. Be.nnett et al. [9] also qbserved the ESR spectrum COT upon codepositing an atomic beam sodiumor potassium with CO, at 77 K. An ESR signal cfiaracteristic of hydrocarbon glasses which had been exposed to 7 ratiation and of vtiious solutions in hydrocarbon glasses after ultraviolet irradiation leading to photoionization of the s&ute molecules, previously ascribed to trapped electrons; was shown by Jai -tin and Nbiecht. [lO]‘to be contributed by CO: I This reidentification has been conf%med by Wtia?d atid coworkers [ 1 l] . In’sdies of.tie ESR and infrared spsctra:of sodium f&r&e pressed in alkali halide di&.s and nibjetted to $ radiatic;, Hart&n and Hisatsune 1121 assigned ‘m abgrption’at 1672.0 cm-l to L2C~~. Ob-

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Volume 28,,1hnber 3 >.

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CHEIMI~.~~p~ys;cs LET~RS ‘.

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servation &an absorptioir &ribuied.:o. 13C0.z. at .. 1626.5 cm-l peatted an estimate of ,127’ I 8’ fs; the valence angle of CO,. No absoiptidn due to 0~:. late was &tected. Later stu&es by Hisatsune and coworkecs [13] tilso provided Little evidence f& oxalate. l’hcy proposed that thy reaction of CO, with itself leads instead to the formation of CO and carbonate.. Itifrared studies of the products of the interaction of potassium and sodium atoms with solid COz !l:ive re: cently been reported by Bennett et @. [Id], who assigned a pair of absorptions at 16W and.1628 cm-i in the K + CO, system to CO:.-In the Na + CO2 expzrknents, an absorption at 1601~cm-~ and, tentatively, an absorption at 1633 cn~~. were assigned to CO,. Recent studies in our laboratory of the. reaction of pllotoe!ectrons with N,O in an argon m&ix at I4 K have demonstrated the production of 0-i which may undergo subsequent reaction in the maty& [IS 1. It ‘WOof interest to attempt’ to produce CO, in the inert, non-ionic argon matrix environment not orily by the reaction of O- with CO, bul also by chargeitransfer interaction between various alkali metals and CO,. These experiments would help to cIarify the role‘ played by cluster ions such as (CO,JCO,, possibly imporiant in the previous infrared studies.

1974 ..’

at the expense of ihe M,O_abidrption on prolonged hadiaiion. @ an expe&nent utilizirig N,O enriched to 89% h oxygen-l 8, the CO, ab: sorptions were .shifted to 659 and 2330 cm-l, appro-priate for their assignm&t to 12C160180. In the.sodiurn-atom experi&ts, a broad, moderately intense absor$i& appeared at 1607 cm-l after irradiation, and i.t~the potassium-atom experiments moderately intense nbsorptions appeared at 1578.0 and 1608.3 cm-1 aCter irradiation. Counterparts of’these two absorptions appeared at 1569 and at 1592 cm-l irr the study utiltig N2180. A weak to moderately intens:, broad absorption also appeared at 1666 cm-l after irradiation of the deposits. &n kC02 = 250 f Na deposit at 14 K showed a weak to moderately intense, broad absorption at 1608 cm-1 which grew in intensity on irradiation. After irradiation, there was a broad, unstructured absa-ption between 1580 and 1700 cm-l and a weak to moderately intense; broad absorption at 1756 cm-l. Ar:CO, -mples of mole ratio ranging from 250 to 600 co-deposited with.& atomic beam of potassium showe! a very prominent absk-ption at 1630.0 cm-l, a somewhat weaker absorption at 1610 cm-l, and a modehtely intense absorption at 1587 cm-l. Although absorp?ions due to traces of matrix-jsolated water also appeared in this spectral repion; their relative intensi2. Observations ties and behavior on subsequent mercury-arc irradiatjon clearly diskguished them from the three new abb Ar: CO:N,O samples of mole ratio 250: 1: 1 and ,sorptions. On mercury-arc irradiation, the 1587 cm-l 500: 1: 1 were co-deposited at 14 K v+$than atomic absorption disappeared readily,and the 1610 and. beam of sodium or potassium, ustig apparatus and 1630 cm-l absorptions somewhat more slowly. In a : sampling conditions which hzve previously been destudy using a sample of CO2 enriched to 90% in c’arscribed [15,16]. The infrared spectrum of the resultbon-13_new absorptions at 1586.5 and 1565.0cn1-~ ing deposit was identica! to that for CO and N,*3 isodiminis~lecl in intensity,on irradiation. In experiments latcd in ~JIargon matrix in the zbsence of alkali metal using a CO2 timple enriched to 92% in oxygen-18, a atoms, except for a new moderately intense; broad abnew absorption appeared at 1602 cm-l. Spectrometer balancing problems and relatively low producr yields sorption which appeared h some experiments at i 533 cm- l. No changes were obkved upon proprecludl:d the identification of other new peaks belonged expos+e,of an Ar:CO:NZO deposit which did tween 1550 and 1590 cm-l in the oxygen-18 enrich.. not contain sodium or potas&m atoms to the flJ!.l ment experiments. light of a mediurkpres&e mercury &c or to 228.8 run Experim&ts were also conducted on Ar:COz X&Icadmium resonance radiation. .k the presence of an pies of niole r&o between 250 and 1000 co-deposited ~tith~an~at~mk beam of cesium produced using the alkali metal,‘irradialion led to the disappearance jf the -1533 cm-l,hbsor@iion ani io the appearance not o;lly ‘, -procedure developed, by Spiker and Andrews [17]. ,.’These e?;perimknts utilizkd a closed-cycle helium reof the absorptions $reviously repotted fork-adiated ‘. ’ frigeration systerkbpkratecj ai 14 K k-14 a reflection &:N,O + alkali metal samplea [l?] but aMof the L &pJiny~ configtiration &n&r to that.described.by ‘. tie. fund&enfaJ ,absorptions of CO,; whkh continued ’ 164.: .‘. ; _ ,,: ,

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Volume 28, number 2 I

I

I5 September

CHFMICAL PHYSICS LETTERS 1

Table 1 Absorptionsa) which appear be&een 1.550 and 1650 cm-’ on co-deposition of Ar:COe &pIes with an alkali metal

I

(l?J

--,

(bl

,~T J

M

cm-’

Na K

1608

CS

(cl

w-m, bib)

1587 w-m 1609 m.br 1630.0 m-5 1596.2 s 1599.5 w 1601.2 w 1603.8 m-s 1607.0 s 1625.0 N--m

‘) vf weak; m medium; s strong; br broad.

Cd)

b, Grows on mercury-arc

re1

irradiation.

of the same Ar:C02 sample with an atomic beam of cesium, the spectrum shown in tiace (b) was observed. Several relatively weak, poorly defmed absorptions appeared near 1580 cm- r. The 1591 cm-l peak was

!

contributed by the water the remtiring absorptions

I

I 1640

I

I 1600

I

I 1560

I cm-

1974

Fig. 1. Isotopic dependence of absorptions which appear near 1600 cmW1 on co-deposition at 14 K of AI:CGa samples with an atomic beam of Cs. (a) A&O2 = 500.4.1 pmol CQ de+ posited without Cs (b) Same sample a-sin (a). 15.2 umol additional COa co-deposited over period of 193 min with Cs effusing from Li + CsC!l mixture at 301” * 1°C. (c) Ar:C@ (50% 13c) = 250 + Cs 36.8 pmol Cq co-deposited over period of 281 mm with Cs effusing from Li + CsCl (92% y”O) =4ClO +Cs. mixture at 292” k 2’C. (d) Ar:C02 25.1 rmol C& co-deposited over period of 280 min with Cs effusing ffom fi + CsCI mixture at 292” f 2°C (e) Deposit of experiment.(d) after w arming to 38 K ar.d retooling to 14 K:

Hastie and co-workers [18]. In trace (a)-of fig. 1 is shown the absorption spectrum between 1540 art4 1660 cm-l characteristic of an Ar: CO2 deposit in the absence of cesium atoms. The three familiar absorptions due to traces of matr&isolated water are reccg nized. On co-deposition of about four times as much

impurity. The positions of in this spectral region are

summarized, together with the new absorptions of the sodium- and potassium-atom studies, in table 1. The 1610 cm-l water absorption appeared as an unresolved high-frequency shoulder on the 1607 cm-l peak, and the 1625 cm-l absorption was contributed both by water and by the cesium-atom reaction product. The relative intensity of the product absorption after subtraction of the water ccmtribution is given in table 1. When the deposit was w’armed to 38 K to induce molecular diffusion and m-cooled to 14 K, the 1596 cm-l absorption diminished mkedly in intensity, while the absorptions between 1599 and 1607 cm-1 and at 1625 cm-r grew in intensity and broadened. The relatively weak peaks at 1654 and. 1658 cm-l disappeared. In trace (c) of fig. 1 .is shown the spectrum of an Ar:C02 sample enriched to approximately 50% in carbon-l 3 _and co-deposited with an atomic beam of c&urn. Of especial interest is the replication of the 1.595-1625 cm-l absorption pattern between 1.550 and 1585 cm-l. Wi’Lhallowance for the contributions of water to the higher frequency set of abssrptions, the contour and.relative intensities of the two groups of absorptions match, behavior appropriate for 2 t-hation involving the.motion of 2 single carbon atom. In

V&&28;,number ._’ ,’ ‘_. .. &topic :Species 1

‘iiCi60; 13,360; 12p(&

-L.

._

2.

,,

CHEMICAL PHYSICS LETTERS’

.‘.i._: i5 September’1974 :

. . . Table 2 ” dependence of 1596 cm-’ C& absorpncn characteristic of Ar:r2C16& Ohs. (cm-‘)

kale”) (cm-‘)

1596.2 1554.2. 1569.0

2a=J2S0, (1596.2) 1553.9 1567.2

2a=13C1~

.’

+ Cs deposits at 14 K

2a=13Sa

(!596.2:r

(1596.2) 1552.8 1568.8

1553.3 1568.1 .’

.

2a = 14p” (1596.2) 1552.4 1569.5

s)Ref;[19].

,a study of an A.r:CC2 (90% 13C02) = 800 + Cs deposit using an exceptionally low cesium vaporization temperature-the 1554 cm-l’ peak was much ;rore.

prominent than any of its higher frequency satellites. In trace (d) is shown the specklm of an Ar: CO, + Cs de_@& with ?2% oxygen1 8 enrichment. Once again the absorption contour is similar to that shown in trace (b). Trace (e> shows the spectrum of the deposit of trace (d> after the sample .bad been warmed to 38 K and re-cooled to 14 K. The behavior of this sample on yup. exactly par&Is that described, but

.not shown in the figure, for the unenrkhed sample. ’ Several other cesium reilction products contibuted to the observed spectrum in typical experiments. A weak to moderately intense absorption at 1017 cn-l was shifted to 997 cm-l on oxygen-18 substitution and diminished in intensity or disappeared when the sample was warmed to 38 K. A prominent peak at 1336 cm-1 with a shoulder at 1342 cm-!, shifted to 13.07 cm-l in the heavily oxygen-18 enriched sample and also diminished in intensity as the sample was warmed. Neither of +&esepeaks was present in the carbon-113 enrichment study of fig. lc, suggesting that the species which contributed the 1017 and 1336 cm-l~,absorptions cannot be the same as that which contributed the 1595 and 1625 cm-l absorptions. In the study of the 90% ?3C02 e?u-iched sample, a week to moderately intense peak appeared at 1306 cm-l. The counterparts of the rektively weak 1654 and 1658cm-1 peaks were not observed in the 50% carbon-13 enriched sam@e of Bg. lc, and in the oxyge::;18 study of fig. 1c they were replaced by absorptiok at 1630, 1636, and 16440cm--l* Ah of the product peaks in +&eAr:CO, + Cs experi,ments dikni-&ed in intensity upon mercuiy-arc irradiation of the sample, although at varying relative rates. ” ‘..

: :

3. L&c&on The proximity of the prominent absorptions observed near .1.600 cm-l to those previously attributed to u3 ofC0; in an alkali halide lattice [12] and in solid C1D2[14] suggests a similar assignment. The relative prominence of the 1596 cm’1 peak in experiments having a high dilution of both CO, and cesium atoms and its decrease when the sample was warmed suggest that this peak is most likely to be contributed by an isolated Cs’... COT ion pair. Using the relationship for the isotopic dependence of v3 of a bent x1’, molecule given by Herzberg [19] and assuming various values of the valence angle, 2o, the positions for this fundamental of 13C160F and 12C180z have been calculated. As shown in table 2, for a valence angle of 130’ the deviation between observed and calculated values for both isotopic species is 0.9 cm-l. For other valence angles, g-cater deviatidns from the observed values result. It is presumed that there is a residual cation interaction even when ce.sium, for which charge transfer should be most complete, is used. IYthe cesium cation interacts most strongly with the central carbon atom, a somewhat decreased carbon isotopic shift-should be observed, and the valence angle may,be near 135O;a value giving very good agreement between the observed and calculated positions of the 12ClaOz absorption. On the other k-id, if the cesium cation interacts more strongly with one of the end oxygen atoms, the valence angle of the anion may be smaller &n 130’. The complexity and warm-up behavior of the ab‘. .sorptions near 1600 cm-l support the hypothesis that aggregates of type (COi)nCOF contribute to the spectrum. Presumably the coupling of'tie nearby CC, molecules.& sufficiently weak til,.atmixed carbon ‘. :

Volume 28; number 2

CH&ICAL

isotopic splittings are not-seen in the spectrum of fig. lc. Different types of cation sitesmay also help to ac-’ count for the detaiIs of the observed absorption, Pres-. ent data do not suffice for a more complete consideration of these interactions. A definitive assignment of the other absorptions characteristic of these experiments is not possible. t The 1017,1654 and 1658 cm-l absorptions of the cesium-atom experiments were relatively weak, and isotopic data for them are incomplete. A counterpart of the prominent 1336 cm-L absorption of the cesiumatom experiments was not present in the potassiumatom studies. Since this absorption varied considerably in intensity xelative to the absorptions near 1600 cm-1 and disappeared more slowly on mercuryarc irradiation, it appears not to have been contributed by CO,. The appearance of an ab.sorption near 1610 cm-1 in irradiated Ar:CO:N2O + M samples is consistent with the stabilization of’a small concentration of CO5 in these experiments. Presumably, conditions are such that COT is not formed in the type of site which leads to the prominent 1630 cm-l peak of the A&O2 + K experiments. The,continued growth in the CO2 absorp tions on prolonged irkddiation of the sample may result from several different processes, including the photodetachment of initially formed CO,. A prominent absorption also appeared at 1609 cm- l, but none at 1630 cm-l, on mercury-arc irradiation of Ar:CQ:O, + K deposits [20]. in these experiments, prominent absorptions at 1307 and 1494 cin:L, which were present in the initial deposit and grew on subsequent irradiation, were assigned to CO,, with an apparent C2, symmetry. As in the present experiments, the carbon-isotopic shift calculated for CO, v&a valence angle of 135’ was signifkantiy greater than the observed value, but the c&bon- and oxygen-isotopic shifts could be reproduced -within experimental error for a valence angle of 130’. In experiments using mixed oxygen-isotopic substitution, the intensity pattern of the absorptions attributed to CO? was’close to that expected assuming complete random&&ion of the oxygen isotopes in ‘the sample, suggesting that at least part of the COz.resulted from photodecomposition of CO,. .Two peaks, at 1535 and 1598 cm-l , can be attributed to 12C16,0180-, indicating that a soniawhat asymmetric catio~~teract~on does occur. Since the.dependence of the position of ‘_

15 September 1974

PHYSIbS LlkfERS

the ant&mmetric srretching absorption of 12Cr60180- on the valence angle (near 130”), on the position of the bending fun~~n~~, and on the magnitude,ofthe stretching-interaction force constant is small, it’has been possible to estimate a position of i596 cm-f, closeto the meti of the two mixed okygen-isotopic absorptions, for this fundamental. Because all cjfthe absorptions attributed to CO, shift in going from _mtassium to cesium in the alkali metal atomic beam, the coulombic interaction between cation and anion may lead to ah of the observed stabilization of CO, iu the matrix, and the question whether CO: in its ground eiectronic and vi= brationai state is more stable than CO2 bent to the same valence angle remains unanswered.

Refeicnces _I.

-’

[l] C-R. Claydon, G.A. Se@ and H.S. Taylor, I.Chem. PhYS.52 (1970‘) 3387.

[2] D.Spen&, J.L..Mauer and GJ. Schulz, J. Chem. Fhys, 57 (1972) 5516. [3] L. Sanche and G.J. Schulz, J. Chem. Phys. SR (1973) 479. [4] J-F. Paubon, J. Chem.Phys. 52 (L970) 963.

IS] CD. Cooper and R.N. Compton, Ckm. Phys. Lctten 14 (1972) 29. [6] D.W.‘OvenaU and D.H. Whiffen, hiol. Phys. 4 (1961) 135. [7] M. Krauss and D. Neumann, Chem. Fhys. titters 14 (1972) 26. [ 81 G.W. Chantry and D.H. Whiffen, Mol. Phys. 2 (1962) 189. [9] J.E. Rennett, B. Mile arid A. Thomas, Trans. Fuaday Sot. 61.(1965) 2357. [la] P.hI. Johnson and A.C. A.lbiWht, I. Chem. Phys.44 (1966) 1845. [:I] M. Shirom, R.F.C. C&ridge and J.E. Kibrd. I. Chem. F’hys. 47 (1967) 286. [12] K.O. Hartman and I.C. Hisatsuoe, I. C&em.

Pktys.44

(1966) 1913. [13] I.C. Hkatsune, T. Adl, E.C. Beahm and RJ. Kimpf. J. Phys. Chem. 74 (1970) 3225. [i4] J.E. Bcnnctt, SZ. Graham and B. We, Spectrochim. Acta 29A (1973) 375. [15] D.E. hfiUigan and M.E. la&x, I. Chem. Phys. 55 (L971) ‘3404. (161 hl.E, Jacox and D.E. Milligan, 5. SW. Spectra. 43 ,(1972) 148. [ 171 R.C. Spiker Jr. and L, An&ews, I. C&hem.Phyg. 58 11933) 713.

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