Pulse radiolysis of alkali metal cations in isopropylamine: Correlation of optical absorption spectra with electron spin resonance data

Chemical Physics 1.5(1976) 377-382 0 North-Holland Publishing Company

PULSE RADIOLYSIS CORRELATION

OF ALKALI METAL CATIONS IN ISOPROPYLAMINE:

OF OPTICAL ABSORPTION SPECTRA WITH

ELECTRON SPIN RESONANCE DATA*

W.A.SEDDON,J.W. FLETCHER and F.C. SOF’CHYSHYN P/iysicalChenzisfty Branch. Afomic Enerp of Canada Limited. Chalk River Nuclear Laboratories. Chalk Rivet. Ontario. KOJ IJO. Canada

Received 26 March 1976

Pulse radiolysis of alkali metal cations in isopropylamineindic=ltesthe formation of three distinct optical bands attributed to solvatedelectrons,ei. ion-pairs(M’, ea and alkali anions M-. It is found that the ion-pair spectia exhibit a distinct blue shift from that of e;. Comparisons with results obtained in ethylamine, tetrahydrofuran and other solvents demonstrate that the position of the ion-pair band can be correlated with the percent atomic character observed by ESR for the “monomer” species in alkali metal solutions. Results are presented for the alkali metal series, Li, Na, K, Rb and Cs.

1. Introduction Pulse radio!ysis and flash photolysis studies of alkali metal-ion solutions in ethylamine (EA) [I ,2] and tetrahydrofuran (THF) [3-S], have demonstrated the existence of three distinct optical bands attributed to the formation of solvated electrons, e,, ion-pairs Mt,e, (or “monomer” species M) and alkali anions M-. e; + M’ *(M’, e;) or M,

(1)

(M+.e;) f e; * M-.

(2)

The parent e, absorption extends well into the infrared with a maximum,A,, 2 1800 nm for EA [ 1,2] and 2100 * 50 nm for THF [4,7,9,10], whereas the metaldependent M- band has a maximum close to the visible

coveringthe region 700 to 1100 nm from Na- to Cs[2,7,11-161. On the other hand the ion-pair or M species absorbs at a wavelength intermediate to that of ei and M’. For EAh,,= 1400 MI and is Independent of M’ , whereas for THF a pronounced blue shift occurs to 890,

1125,118O and 1400 nm for the correspondingNa, K, Li and Cs ion-pairs [3,7,8] . In a recent review [ 171 we noted a correlation be* AECL No. 5454.

tween the shift in the (Na+, e;) ion-pair band maximum with THFjEA solvent composition and the ESR “monomer” hyperfine splitting observed by Catterall in KITHFIEA mixtures. In such solutions the percent

atomic character, deduced from the hyperfine coupling constant for the “monomer”, decreasessignificantly from THF to EA and parallelsthe shift in optical frequency for (Na+, ea with solvent composition [18]. The above trend can be extended to other solvents. For example, in methylamine (MA) [ i9 ] and ethylenediamine (EDA) [20-231, little or no change in the optical spectrum, from that of e;, is observed on ionpair formation. In both these cases the percent atomic character observed by ESR in alkali metal solutions is small and much less than observed in EA or THF [ 15, 24-271.

Consideringalternative solvents it has been .established that, for a given alkali metal, isopropylamine (IPA) solutions exhiiit hyperfine coupling constants and atomic character >EA but aHF [ 15,24,26] _This suggests that the blue shift in the optical spectrum of @l+, e;) might therefore occur between that observed in EA and THF and possibly exhibit some M’ deper.dence on $,g_ This paper describes our observations on the pulse radiolysis of IPA solutions with particular reference to

the formation of transient ion-pair spectra and their

,378 .:

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Wi.4.Seddon et al./Fzdse radiolysis of alkali metal cations in isbpropylamine

correlation with ESR data. Detaikofall experimental techniques and solution preparation have been described elsewhere [2,28-301. using an 0.5 ns pulse, doses ranged from -3 X 10ly to .2X 10ZoeV L-I.

2. R&&s

6

i

n-

and discussion

2.1. Optical spectra

Fig. 1 shows the optical spectra observed at room temperature in pure IPA and in -1 X 10e3 M potassiumisopropyiamide (KIPA) so!utions. In both solutions the e; absorption, observed immediately after the pulse, is relatively weak and shows no evidence of a maximum below 1550 run, the experimental limit due to solvent absorption. In IPA at doses per pulse -2 X 102O eV P1 the eh absorption decays completely with a half-life t l/2 = 1 /JSwhereas in KIPA solu-

tions a further growth is observed which reaches a maximum intensity -7 ns after the pulse. During this growth the spectrum exhibits a distinct change with a shift in xma to 1250-1300 nm. This intermediate absorption fmally decays to form a long-lived species with a characteristic band, X,, = 875 nm, assigned to the species K-. The changes outlined above parallel those observed previously in EA and MA [2,19]. The important dif-

Fig. 1. OpiicaI absorption spectra observed in IPA and -1 X 10m3M KIPA solutions. (X) Immediately after the puke; (c) KIPA after 7 MISS; (A) KIPA after 400 ps. The intensity of the initial spktrum (X) at 1500 nm is about ten times less than (0) at 1250 nm but is shown on the same scale for &uity.

.-

1 6

-LI

2

\

0

i

01 500

600

IO00

1200

1qoo

I

1600

h(nnl

Fig. 2. Optical absorption spectra observed in Na/KiIPAand -1 X to4 M KB& solutions. (X) KB(b4solutions immediately after the pulse (right-hand scale); (0) Na/KlPA after 2 ps: (A)after 400 ~5.

ference is that the intermediate spectrum, assigned to the ion-pair (Kf , e;), shows a significant blue shift from that recorded for t$e same species in EA and MA. Fig. 2 shows the optical spectra obtained in solutions prepared via the addition of Na/K a!loy (solubility unknown) or saturated with (-IO4 M) potassium tetraphenylboron (KB@&. In the Na/KIPA solution the intermediate spectrum observed -2 ns after the pulse is very similar to that noted above for KIPA solutions and is consistent with the formation of (K+, es). This absorption again subsequently decays but in this case, forms a long-lived band due to Na- with X,, = 675 nm. Sodium alone is insoluble in IPA but in common with studies of mixed metal systems in EA [ 17,3 1] , is present in sufficient concentration from the Na/K aUoy to produce Na- as the final product, rather than K-. In the KB’JJ4solution the initial absorption decays completely after the pulse, but its spectrum is virtually identical to the intermediate spectrum obtained in KIPA solutions or solutions prepared via Na/K alloy. Since it has been shown previously f3,7,8,17] that the cation in salts of B04 acts as an efficient scavenger for ei this then provides strong confirmatory evidence that the spectrum with h,, = I300 nm is due to (K’, 21). The slight, -50 run, shift to the blue apparent in the basic solutions can be explained by the presence of an under-

319

WA Seddon et al.fRdse radiolysis of alhli metal cations in, isopropylamine

Fig. 3. Optical absorptionspectra observed immediately after the pulse in -7.5 X 10e2 hi NaB04 and 1.4 X 10-l &INaAIM4. For comparison both spectra are normalized to the same peak height at 1100 pm. (-) NaB04 solutions, (X) NaAIb soIutions.

lying contribution from K’ or Na’. Fig. 3 shows the optical spectrum observed immediately after the pulse in -7.5 X 10m2M NaB@d and -1.4 X 10-l M NaAIH4. In both cases the absorption peaks at 1100 MI and decays with the formation of some Na- (spectra not shown). The shoulder observed at 650-700 nm in NaBo4 solutions shows that in this case a small amount of Na- is present at the end of the pulse. Although the spectra exhibit considerable overlap it can be shown that in solutions containing 2 X lo-’ M NaBo4, the decay’of e; morQtored at 1650 MI, is associated with a corresponding increase’in absorption monitored at 950 nm. Both are complete in G.5 I.ls. We conclude that the principal absorption corresponds to the formation of the intermediate species @Ja+,ei) and that /c(ei + Na+) > 3 X 1Otl M’l s-l, a value comparable to that observed in THF [3,7]. In NaAlH4 solutions the absorption is skewed more toward the infrared. This may be due to the underlying presence of e; evidence for which can be seen as a very rapid initial component to the decay at wavelengths 21200 nm. Fig. 4 shows the spectra obtained in saturated RbmA (-2 X IO? M) and 1A X IO’3 M CsIPA solutions. In both cases the long time scale spectra correspond to those expected for Rb- and Cs- with A,, at 920 and

01

I

600

BOO

ICOC

1200

11100

1600

Fig. 4. U~JYSfigure: Opticalabsorption spectraobserved in -2 X 10 M RbIPA solutions. (X) 12 ps after the pulse, (o) after 160 JLS.Broken tine represents normalized spectrum of Rb- observed in alkali metal solutions in EA [40 J. Lower figure: Spectra observed in 1.4 x 10” M CsIPA solutions. (X) I JGafter the pulse, (*) after 40 ps. Broken line represents norma&ed spectrum ofCs_observed in alkali metal sol~~tions in THF j14,ISJ.

1100 run, respectively. The initial spectrum co’rresponding to e; is not shown. The intermediate short time scale spectra have A,, in the region 1300-1400 nm with the precise location obscured by an underlying contribution from Rb- or Cs-. This band is consistent wi!h the formation of (Rb+, e;} or (Cs’, ei) ion-pairs. These spectra are slightly, but significantly, red shifted with respect to &theintermediate bands observed in Na and K solutions. It is worth noting that in Rb, and particularly Cs

380

\?A. Seddon et k/Rtlse mdioljsis ofalkali metal cations in isopropylomin~

-

II’

I

I

I

I

I

I

I

600

800

1000

I2OC

1900

1600

1800

X (nm) Fig. 5. Optical absorption spectra observed in saturated LiAl& solutions. (a) Immediately after the pulse, (*) after 4 ps. Inset: Spectra observed in -10” M LiIPA solutions. (A) Immediately after tbe pulse (right-hand scale), (*) after 150 JJS.

solutions, the long-lived M- spectra contain a significant contribution from the corresponding ion-pair indicating, as in EA [2,16], that the dissociation constant of Mis in &order Cs- > Rb’ > K- > Na-. Similarly, the overall equilibria appear to be more analogous to those in EA than for MA [2,19]. Fig. 5 shows the spectra observed in saturated (-low2 M) LiIpA and LiAlH4 (<0.1 M) solutions. At doses of .-4 X IO” e!J L-’ the initial absorption in LiiPA solutians is followed by a slow growth which reaches its maximum intensity --I 50 w after the puise and then remains.s!able for a period of 21 ms. In LiAIH4 solu-tions the absorption decays immediately and completely within -SO m. Id each case the initial spectrum corresponds.to that attributed to e;,whereas the spectrum observed a finite time after the pulse shows a defjnite shift to x ,.,.,= 7 JqO nm. Again this can be attributed to ion-pair formation. This process seems to be very rapid since in both cases a sharp “spike”, due to the

decay of e;, can be observed immediately after the pulse at wavelengths >I600 run. 2.2. CorrelationwithESR The discovery of metal hype&e splitting in solutions of alkali metals [l&24-27,32-26] demonstrated unequivocally the formation of a ‘*monomer” species of stoichiometry M. It is well established that this hyperfine splitting shows a marked increase with temperature and with a decrease in solvent polarity [l&24,26] . If such splittings are compared with values for the free atom the atomic character approaches 36% at 2S°C in THF whereas in EA the value is -12%. The conesponding value in IPA is 22% [24]. Our optical resu!ts demonstrate a correlation between the blue shift in Amax (hl+, e;) from h,, ei an! the trends in atomic character observed by ESR. The magnitude of the optical shift decreases with in-

W.A.Seddon et al./Pulseiadio!vsisof alkalimetalcationsin isopmpylam!ne

creasing solvent polarity along the series THF > IPA > EA > MA x EDA z NH,. Likewise, for a given aikali metal, the hyperfme coupling constants or percent atomic character decrease in the same order [ 15,24,26] . The nature of the “monomer” species in alkali metal solutions has been rationalized in terms of the rapid equilibria between free atoms M and ion-pairs 1371. M + (M+, ei) or (&,

e&gJ,t

+ (Mf,

e,),oose_

(3)

As such the coupling constants are expected to decrease in the order M 1 (M*, ei)wt > (M+, e&,,wlth the transition from tight to loose ion-pairs favoured by increased polarity or soliating power of the medium. It seems reasonable that a loose association or solventseparated ion-pair should exhibit an optical spectrum closer to e;, whereas a tighter ion-pair would tend toward that of M-. For a given solvent the shift to the red from (Na+, e3 to (Cs+, es3 is therefore predictable in relation to the corresponding M- spectra. Although comparable ESR data are not available general trends indicate an iricrease in atomic character from K + Cs [25,27,38,39]. Therefore one might expect the frequency difference in X,, between M’ and (Mf,ei) to be b the order Cs < Rb < K < Na. This is observed in both THF [7] and IPA. In both solvents lithium is anomalous [7,8]. In concIusion, it is worth noting that the high rate constani for the formation of (Na+, ei) in NaB@, solutions appears to be inconsistent with the slow buildup which gives rise to the same species in basic solution. Whether this is due to markedly different dissociation constants for the rtspective salts or alternative rate controlling processes is not yet understood. Studies on this aspect are continuing.

References

i >i

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.‘. - ...

...

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