EPR study of the NO2+3 radical in irradiated K(UO2)(NO3)3 and K2(UO2)(NO3)4 single crystals

Volume

92,

number

CHEhllCALPHYSlCSLETTLRS

4

29 October

1982

EPR STUDY OF THE NOS+ RADICAL IN IRRADIATED K(U02)(N03)3

AND KZ(UOZ)(N03)4 SINGLE CRYSTALS

TX. CUNDU RAO R S IL’.. Indm Insrrrure o/ Techology.

Bombay400

076.hdw

and K.V. LINGAMand B.N. BHATTACHARYA Physrcs Deppmrmenr. huhan Instuure of Tec!rrlolon. Bombay-l00 076.Indw

A new radul obswrd 31 low tempcrclrurc m I-uradrxcd K2(L102)(N03), sm$ cr)suls 1~1sb~c‘n ICIIKIIIW~) ~.sr~n~d to B lutherto unknown okymion radral. NO’*3 . The asrrgnmcnt and lhc hch of r4N hypcrimc slructurc. rofcrhcr \r~th the g factors which xc lower than the frcc-spm value, xc dficusscd m terms of an orbrtsl level scheme

1. Introduction

1. Experimental and results

ESR studies on -y-irradiated uranyl nitrate systems showed that stable NO, radicals are formed at the bidentate and umdentate nitrate sites available in these crystals [ I,?]. The systems investigated were unnyl mtrates of rubidium (RUN), potassium (PUN), dlpotassium @PUN) and diammoruum (DAUN). In these crystals the rutrate 1011shave planar geometry with in-plane distortion. In RUN, the uranium atom of the anion forms bidentate bonds with three rutrate Ions lying in the equatorial plane of the linear uranyl ion. The detaded crystal structure of PUN has not been reported. ESR analysis of NO, ra&cals showed that the anion is similar to that in RUN but the three bldentate nitrates are not Identical. In DAUN, the anion has pairs of bldentate and umdentate nitrate ions coordinated to uranium atom. DPUN is found to be lsostructural to DAUN. The bldentate nitrate ates are of special interest as NO, radicals from these sites showed a small increase in the g factors. Lnthe present study a new radical, also located at the brdentate sites, observed UI PUN and DPUN is reported_ It has no observablehyperfiie structure and hasg factors that are belowthe freespm value. The assignment of th radical is discussed.

vant crystal structures, etc. have been described [a]. The low-temperature studies needed in the present work were done at the lowest temperatures obtamable with the Varian varnble-temperature accessory. The space groups of PUN and DPUN are P,, and P21,c respectively and the new radial showed no apprecnble decay m either crystal. The results of PUN are considered fiist. The ESR spectra recorded at room and low temperature (77 K) in PUN with the magnetic field along the crystallographic u a,us are shown in tigs. la and lb. For this orientation the four symmerry-related anions are equivalent and the triplet spectrum seen at low fields is assoclatcd with the three bidentate NO, radical sites of an anion. The triplet seen (at IIIcreased gain) at higher fields could simhrly be associated with the three sites of a new radical. This conclusion is based on the detaded anisotropy studies as described earlier [?I_ The unequal intensities attributed to the differences m the three radical sites, noted earlier for NO,, are also present III the three high-fieldlines. III fact a comparison ofg WIIsotropy curves in one of the nitrate planes (fig. 2)

0 009-2614/82/0000-0000/S

02.75 0 1982 North-Holland

The experunental methods employed and the rele-

333

Volume

CHEMICAL PHYSICS LETTERS

91 number .l

29 October 1982

The anisotropy studies were carried out at low

and marked linewidth reduction with temperature was real&d. A typical low-temperature spectrum for an arbitrary orientation is shown in fig. lc. The orientation is chosen so that all the four anions are inequivalent and all twelve lines of the high-field spectrum can be seen. A careful e.xamination of such derivative tracings for a number of orientations at low modulation amplitudes showed peak height variations for each of the twelve lines. No detaded analysis of these peak height variations could be made. Such variations were also noted for NO, radicals and in expanded scale additional triplet structure associated with 13N hype&me interaction temperatures,

was seen [?I. Expanded-scale I I$ I I SR spectra01 lk rddlc& III K(IJO~)(NOJ)J For (a) end (b). 111s m.~t.nmc licld IS along rhc cr)st~UqT;lphlc n .I\IS ml tar (c) .md (d), It 1sm an arbitrary duccuon. An c\panded rc~rd usmg low modubrlon (0 I G) of the msctt (c) ISshoan Ifl(dJ

shownin fg. Id. It is seenthat the linesare narrowwith no observable

hyperfme effects.

2

with the corresponding

curves of the NO3 radicals III this crystal [3_] clrarly established that the new radical is also located at the bldentate sites.

2 0001

I

.

EXPERIMEN;I

recordings of the kt

four high-field lines of fg. lc are

0028

xl@00

a

300 K

I

-+

I

AL

0 H//b

30

60

90

120

150

ia0 H/lb

rip. 3. ESR spcctn recorded in K~(UO~)(NO~)J for the magnetic iicld along the b ati are shown VI (a) and @). The p amotropy curves in the bldcnrare plane for NO3 (top) and NOi’(bottom) are shown rn (c).

Direction cosines of the prmcipxl aws of NO3 and NO:+ radnls in Ii2(U02)(N03)4 wnh respect IOG~, 6, and c* axes (o l:normal to b 3x1s m 203 plane and c*. normal to

203 phne) DUCCIIOII cosines

Prinopal

atis a-

-_____ x

NO,

Y z NO;+

x ! z

b

c*

0.749 -0 663 0

0 663 0 749 0

0 0 I

0.743 -0 669 0

0 669 0 743 0

0 0 I

r

= up

infomatlon

dlfficult. Further, the observed g shifts can only be dn-

In DPUN no high-field lines were observed at room temperature. At low temperatures, the spectrum was found to be complex in freshly irradiated crystals. However, many of the imtial radicals dec=iyed with time and the lures of interest could readily be rdentified from the anticrpated anaotropy together with the known g factors. The spectra from an aged crystal are shown in figs. 3a and 3b. A comparison of the g anisotropy curves of the new radical in the brdentate

cussed qualitatively as the opticrldsta is not avsrlable. However, a systematic comparison of the relativeg shifts with the corresponding NO, radicals is found to be of considerable help in the possrble assignment, as drscussed below. The amsotropy studies clearly cstabhshcd that the new radrcals are also located at the bidentatc mtrate sites in both the crystal systems. The good agreement with the crystallograpluc duection cosines [a] indiwtes that the radrcal sites are nearly identical to those of the undamaged molecule. Hence we start

2

NO3

RWJOz)(N03)3

1015

~2(U02)(NO3)4

2 012

2 032 2 032

2017 2013

2 013 ? 012 2013

2 0’8 ? 030 2 029

2.016 2016 2016

I .998

1 985

l 998

I.998 1.998

1.981 1 980 I981

I .997 I .997 I .997

WJO,l(NO3)3

l 1 3

K,W’~)(N03)~ NO;+

WJO3)(NO3)3

of the new

radical makes its LI1 tmbiguous assignment rather

0, NO2 : un~quc snglc

Table

sites with the observed orientatrons of theg tensors. In table 1 the direction cosines of the principal axes ofboth the radrcals with respect to u*bc* axesare given. The principal g factors of the new radrcal III both the crystal systems are given in table 2. The labeling of the principal axes 1sidentical to the corresponding ones of NO, radicals [ 1,?I_ The -7axis IS perpendrcular to the mtrate plane and the x axes is the bisector of the unique (smallest ONO) angle. Theg factors of NO, radrcals are also mcluded for later comparison.

The lunited expermental

1 x

1

1982

3. Discussion

Y

Y3 o/“‘o I

-----_

Oc1obcr

NO3 radicals m this crystal are shown in rip. 3c. The known structural data permits a comparison of the relevant direction cosines of the bidentate nitrate

Table I

Radlul

29

CHEMICAL PHYSICS LETTERS

Volume 91. number 4

l 2 3

1.999

~‘olumr 92. number 4

CHCMICAL PHYSICS LETTERS

29 October

1982

with a reasonable hypotheas thrtt It is derived from a

mtrogen-containing ohyanion. Such species with no resolvable t-l N hyperfinc structure and low g factors hale not been reported to our kmwledge.

to the conslderatlon of

hitherto

This

lads

unknown odd-elec-

tron systems derived from the tutrate ion. A cursory comparison of theg shifts of the new radical and that of NO, shows that the predommantg sftifts are along the prmclpal~ axes. To understand g slufts the rehtlve posrtlon of the orbital levels IS necessary. 7Ihr actual level scheme, which IS somewhat uncertain [3-91, depends on the geometry assumed by these radicals in the crystal. As pomted out earlier, the fact that both these radicals are found to be located at the bidentate sites wlthout much reorientation unplies that the level schemes are essenttally unaltered. These consideratlons III turn unply that the level scheme of the oddelectron species of interest is obtamed by additmn or removal of the slectrons from the uppermost occupied

oj.

cz.

CZ.

‘c’s‘

TIN 4 Orbnal level scheme showmg coneWon berwecn &h end C,., The rclcvant exatatmns ior thcg shlits ol” NO3 NOi+ arc shoan sepxatcly

systems the in-plane closing of the unique angle is 3p

For NOi+ radical the unpaired electron is in the a2 orbital, which ISessentially a non-bonding orbital, consisting of oxygen out-of-plane p orbitals devoid of any nitrogen character. TUB explains one of the mztmobservedfeatures, viz. the absenceof t3N hyperfine structure. Regardmgthe dominant g, tits for both NO,‘and NOi+ radicals, it is seen Tom the level diagram that the relevant excitation (3b, + la,) and (I a-, + 3b7) for both the radicals involve the same levels. Hence the magnitudes of theg,, shifts are expected to be large and comparable. (For NO;+, an additional smaller contribution from Ia2 +2b2 excnatlon should also be included which wtll be considered later). The opposite nature of theg shifts for NO, and NO;’ radicals follows readily from the abovementioned excitations. The reversal 1sa consequence of the fact that in the case of NO;’ an electron from a half-filled orbital is excited to an empty one whereas for NO, an electron from a lower filled orbital is promoted to a half-ftied orbltal. It may be noted that the uncertainty of the position of the levels does not affect these arguments in which the experimental fiidmgs of NO, are used, to check for Internal consistency m the assignment of NO:+, assuming that both the radxals are sun&r to the un-

prOpthtc

damaged nitrate ion. Thus,

kvels of rhe corresponding known scheme of NO,. A careful esammation of such alternatives showed that the twenty one valence electron radical viz NOi+ appears to account for most of the observed features. In D,, symmetry, the Walsh scheme prerhcts an orbltally degenerate ground state for NO;+ radicals. FoUowmg Walsh. the shell-fing scheme 1s

(la;)n(Za

,)O(3e’)O,

2E”

Due to the Jahn-Teiler effect a lower symmetry configuration is anticipated It 1s interesting to note that a planar geometry with m-plane distortion IS predicted to be a favourable site for the Isoelectronic CO; radtal [IO]. Thr bldentate sites are known to have this lower symmetry and consequently the degeneracy of the e levels will be removed. Furthermore it is known

from exLer

studies that u1 the uranyl rutrate

for the m-plane dIstortion

[ 1 ,?I.

The krel

scheme which takes into account these features and also esplams the observed g shifts of NO, radicals is shown m fig. 4. For simphctty G, symmetry is assumed. This is justified from the fact that the g factors of NO, radicak at btdentate sites are found to be msensitive to ddferences m the geometry in these systems [ii?]. Only relevant orbttals and the excitations needed for the qualitative discussion of theg shrfts are shown. 356

there appears to be no

serious conflict with the main observed features. It is interesttng to examine in some detail the consequences of the assignment, in particular the differences in the magnitudes of g shifts for both the radicals. It is seen that these are larger for NO, radicals (table 2). It was noted in our earlier studies [I,21 that theg sMts for bidentate NO3 radicalsare slightly Iarger as compared to NO, radicals reported in other

systems. These were mterpreted in terms of the spmorbrt effects from the uramum atom due to the possibility of 5f orbltak participating in the bidentate

bonding. In this context a sbnllar analysis for NOi+ radical is relevant. The Sf orbit& that conform to the C?, point group were given earlier [?I. The bonding of uranyl systems in the molecular orbrtal scheme has also been drscussed [ 11,12]. According to the these authors, the mjxing of the at, a2 and b, orbitals are less favourable (~5116) than for the 63 orbital. The relevant exctted states for the x, y and z g tits of NOi+ radical are A,, B, and B, respectively. Hence the contribution from the spin-orbit effects of uraruum are expected to be unaltered for g,, and neghgrble for g_v and gc. On the other hand, for NO,, it was found that all the three principalg factors have additronal contrtbutions as observed. Even though both the radtcals are associated wrth the bidentate nitrates, the mati

cause for the above differences

may be traced to

the

fact that the ground states are different. In any case for NO:+, the gc shift IS expected to be small as the p orbitals constituting the a2 orbttal have no component of angular momentum along the J a.+.and hence cannot couple with the external magne’rc field. For theg, sluft, the state wnh A, symmetry k espetted to be low m energy (as a consequence of inplane closing of ti!e unique angle). As mentioned earher, theg,, shifts are expected to be simdar for NO, and NO? without the ccntnbution from Ia2 - 2h, excitation. Tlus contrrbution is expected IO reducerheg,. shrft by an amount which dependson thepositlon of the 7b, level and thus the level scheme predicts I &,.W03)

I > I&j

29 October 1982

CHEhKAL PHYSICSLETTERS

Volume92. number3

OUO: +)I .

From table 2 it is seen that all the rehtiveg shrfts arein qualitativeagreement with the above arguments. It IS believed that the Ievel sheme ss plaunble and BC-

counts simulraneously for [he observed fexures of both radicals. Even though the asslgnmrnt is not based on quantitative estimates, the collective evidence presented should be consdered asstrong. Nevertheless, there are some features, not readgy understood, thar must be pointed out. Firstly, even though the a2 orbital has no nitrogen character, a small IJN mteraction may not be ruled out from induect mechanisms. It is difficult to estimate the magnitude of these ef-

fects. A more serious difficulty is m understanding the observed temperature dependence of NO, and NO:’ radical. In the absence of reorientation, the temperature dependence of NO$’ can only be attrtbuted to spm-lattice rehxation mechanisms. If these are also caused by uramum spin-orbit effects [13j, one would expect that they should be reflected in the g shifts of NO;+. But the arguments based on bonding schemes of uranql systems imply a smaller Sf orbital contrrbution to the ground state of NO:‘. Ftnally, the mechanisms of formation of these radicals are not understood. Further experimental and theoretical studies of this radical would be useful, and at thts stage the assignment may be consrdrred as tentative.

4. Conclusion Addttronsl week lutes obscrvcd rn -y-irradiated angle cry~als of PUN and DPIJN were sssigned to NOf+ radicals. These ire best studied at low temper-

atures. Bidentate nitrate sites with planar geometry wrth in-plane drstortion appear to be favourable for this radical. PIausibibty arguments were given to espbrn the relatweg shifts of NO3 and NO:+ radicals on the basis of a common orbital level diagram. This procedure is constdered to be more reasonable in view of the uncertainty of the positton of these levels as UIdicated by various electtomc SlNCtUre calculations [3-g]_ Further experimental and theorettcal studies are desirable to confiim the assignment.

Acknowkdgement We wuh to thank Dr. hl.V.V.S. Reddy for hts help.

RCfSXCIlCCS [ 11 T K Gundu

MO,

ii k’ Llnpm

and E N Bhattachuya.

J. htsgn Rcson 16 (197-t) 369 (2( T K;.Gundu Rao, K.V. Lmgamand B N BhJtrachsr)J, J. hlqm Rsson 45 (1981)422 131 J.F. Okcn and L. BurncUc.1. Am Chsm. Sot. 92

(1970) 3659. 141 R Lcfcrre Jnd (1970) [Sl T.lI.H 939.

L Rcsuyre.

Thcorcr.

Chum. Ac~r I8

391.

Walkurand J.A Norsky.

hlol. Phbs 21 (1971)

357

Volume 92, number 4

CHEMICAL PHYSICS LEl’TERS

161 A A hlcddnnsla and N.N BulgAov, J. Srrwx. Chem. 10 (1969) 278. 17 I S.P. Dolm and h!.E Dyalkma. J. Struct. Chem. 13 (1972) 906. [S] D J. Vaughan and J.A. TosselI. Am. Mmeral

765. [9] D. Couticr and L A BurneUc.Chcm (1973) 460

3%

58 (1973)

Phys. Letters 18

29 October

1982

[JO] B M. Glmarc and T.S. Chou. J. Chrm. Phys. 49 (1968)

4043. [ 111 S P. hlcClynn. J_K. Smith and W.C. Neely, J Chem. Phys. 35 (1961) 105. [l?] C. Gorller-Walrand and L.C. Vanquickenbome, J. Chem. Phys. 54 (1971) 4178. [ 131 D.A. Hutchmson. K S. Ulen. J Russell and J.K.S. Wan, J. Chem Phys. 73 (1980) 1862