Matrix Raman spectroscopy; The lithium atom-oxygen molecule cocondensation reaction

JOURNdL

OF

MOLECULAR

SPECTROSCOPY

41, 595-597

(1972)

NOTES Matrix

Raman

Spectroscopy;

Molecule

The

Cocondensation

Lithium

Atom-Oxygen

Reaction

The recently developed matrix isolation laser &man technique is applied to the study of lithium superoxide which is the major product of the cocondensation reaction of an atomic beam of lit,hium with a jet of molecular osygen at 4.2-15°K. The structure and bonding in the lithium superoxide molecule has recently been the SII~ject of a detailed investigation by matrix isolation infrared spectroscopy (1). Three infrared bands could bc ascribed to the expected (al + 7~2)vLi-0 stretching and (al) ~04 stretching modes of a triangular Lion molecule with Cpl)symmetry. An extremely important feature of the matrix infrared study was the assignment of the 04 stretching mode to a band at 1097 cm-l. This frequency was virtually lunshifted with respect to the corresponding mode in the free superoxide ion, the Raman frequency of which had recently been observed in alkali halide lattices (2). This data was int,erpreted as an indication that the bonding in LiOz should not be considered t,o be t,hat of a covalently bound triangular molecule, but rather that of a Li+ cation bonded directly to an O,- anion by coulombic attractive forces. The infrared frequency of t#he 04 stretching mode in LiO?: implied the almost complete transference of the Li (2s) valence electron into all OS,V~C~I~ X* molecular orbital. Additional evidence to test the proposed structure and bonding in LiO, may be obtained from the I&man speet,rum of matrix isolated LiOz If the bonding of lithium to two eqtlivalent oxygen atoms can best be rat.ionalized as ionic bonding, then the matrix Raman spectrum should display only one strong band, corresponding to the 04 stretching mode of the 01 ion, t,he frequency of which should be coincident with thr band observed in t,he infrared at 1097 cm-l. For the electrost,atic model, the symmetric and asymmetric motions of the Li’ (*ation relative to the 01 anion in the proposed CZ, triangular molecule, bot,h involve a dipole change, and consequently finite and observable matrix infrared absorption int,ensit,irs. However, on the basis of the Woodwood intensity theory for ion-pairs (3) the corresponding Li-0 modes in the matrix Raman spectrum should be extremely weak, at least 100 t imcs less intense than the stretching mode of the covalent,ly bonded O?- anion and probably uot observable. We have recently been able to show that matrix isolat.ion laser l&nan spectrosr(,pg is a viable technique, which is capable of detecting low concentrations (O.l-l.O?g,) of highly roactive species, suspended in solid matrices at cryogenic temperatures (4, 5). In the present study we simultaneously cocondensed a molecular beam of lithium atoms from a stainless steel Knudsen cell, with a jet of oxygen gas, onto a cold tip at 4.2”K. Thr Li:O? ratios were calculated to be approximately 1: 1000 in A and 1: 100 in B A blank r,ln (using conditions of extremely high spectral sensitivit)-) with pure oxygen was also performed at 4.2”K and showed no Raman lines in the region 1500-300 cnr-l. The mat ris Raman 595

NOTES

596

TABLE

I

THE MATRIX RAMAN SPECTRUM OF THE COCONDENSATIONPRODUCTS OF ATOMIC LITHIUM AND MOLECULAR OXYGEN 4.2”K (Li: 02 E 1: 1000)

B

A 1551 vvs 1504 w

1097 s

The infrared

42°K (Li:02 Z 1:lOO)

1551 1504 1148 1134 1097

vvs w wsh\” mw { s

Warm-up to 30°K and retooled to 4.2”K C 1551 vvs 1504 w

‘002 ‘YIN)

1148 wn

1097 802 -517 W” -517 -464 wa -464 frequencies (I) of matrix isolated LiO, v1 1097; ~2 744; Ye 507 cm-l v1 1097; ~2 699; va 492 cm-l

(1These bands arp assigned

Assignment

t.o higher aggregates

vvw 5 wa wa are assigned

Y 0-O (LiOs) Y O-0 (Li?Oz)

t,o bands at (7Li02) (6Li02)

of LiOt.

I

IWO

1000 500 FIG. l.A. The matrix Raman spectrum (4.2”K) of the cocondensation products of a molecular beam of lithium atoms with a jet of oxygen gas (Li:02 E 1:lOOO). B. The same as 1A but Li:O:! E 1:lOO. C. The Raman spect,rum (4.2”K) of the matrix described in 1B but after controlled warm-up to 30°K.

NOTES

597

spectra together with diffusion controlled warm up experiments are shown in Table I and Fig. lA-C. Our matrix Raman spectra clearly show only one major feature at 1097 cm-1 in t,he very dilute matrix A. This Raman line is strong and sharp and is coincident with the 0-O stretching mode observed in the infrared spectrum of matrix isolated LiOZ . Using conditions of extremely high sensitivity and warm-up experiments in the range 4.2 + 35°K we were unable to observe or associate any bands in the range 800-300 cm-l to LiO stret,ching modes of LiOZ The 1097 cm-l band showed a spectacular decrease in intensity during diffusion controlled warm up experiments (Fig. 1). If Raman lines associated with Li-0 motion of LiOr were present in our matrix spectra, we estimate that they must be of t,he order or less t,han lyc of the intensity of the observed 04 stretching mode. We conclude from these data that the assignment of the 0 stretching mode and the previously proposed model for the bonding in matrix isolated LiO, are both correct,. The intense 0-O stretching mode and apparent absence (or extreme weakness) of Li-0 stretching modes in the matrix Raman spectrum of LiOp , together with the previous matrix infrared data prove that the bonding between the lithium and oxygen should be considered to be essentially electrostatic in nature. Most important however, is that our data show for the first time the feasibility of matrix isolat,ion H.aman studies of metal atom-molecule cocondensation reactions. ACKNOWLEDGMENTS We wish to thank the Nat.ional financial support.

Research

Council

of Canada

and Erindale

College

for

REFERENCES 1. L. ANDRE~S, 1. Chem. Phys. 60,4288 (1969). 2. J. ROLFE, W. HOLZER, W. F. MURPHY, AND H. J. BERNSTCIN, J. Chem. Phys.

3. 4.

49, 963

(1968). J. H. B. GEORGE, J. A. ROLFE, AND L. A. WOOD~OOD, Trans. Faraday Sot. 49,375 (1953). Il. BOAL, G. BRIGGS, H. HUBER, G. A. OZIN, E. A. ROBINSON, AND A. VENDER VOET, iValure 231, 174 (1971); H. HUBF,R, G. A. OZIN AND A. VANDEK VOF,T, Nature 333, 166 (1971).

5. I). 13.Bo.\L .\NDG. A. OZIN, Spectrosc. Let,?., 4, 43 (1971). H. Hiin~:n AND Lash Miller

(Ihemical

Laboratories

University of Toronto, I[‘oronto, Ontario, Canada Received dug?& 11, i97l

and Erindale

College,

G. A. Ozrs