The electronic structure, spectra, and magnetic properties of actinyl ions

JOURNAL

OF MOLECULAR

The Electronic

SPECTROSCOPY

Structure,

6, 164-187 (1961)

Spectra,

of Actinyl

and Magnetic Ions.

Part 1. The Uranyl S. I-‘. MCGLYNK Coates

Chemical

Laboratories,

Louisiana

Properties

Ion*?

AND J. I(.

SMITH

State University,

Baton Rouge,

Louisiana

A calculation of the l-electron MO energy levels of UOz++ has been carried out using rough estimat,es of overlap integrals of a set of a Slater orbitals presumed to correspond to the relevant oxygen and uranium atomic orbit&. It is shown that the ground state of a weak uranyl ion is most probably I&+ and that the U-O bond order is three. It is suggested that the variation of the U-O bond length in the UO*++ entity, and the decrease of the antisymmetric and symmetric vibrational stretching frequencies should condit.ion a ligand spectrochemical series. A discussion of the spectrum of the uranyl ion is presented in t,erms of a Hund’s coupling case (a) molecule. It is shown that the visible absorption band of the UOz++ ion most probably contains three distinct electronic transitions, which are the components of a T + S process. The lifetime of the emission process as calculated from the reverse absorption is 4.76 X 1OP set; as ralculated using a spin-orbital perturbation operat.or it is of the order of 1OP 1P set; the experimental value ranges from 1 X 1OP to 7 X 1OP sec. It is also shown that t,he transition is of an approximate charge-transfer type, and for the emission process is advocated. the use of the term “phosphorescence” A discussion of magnetic susceptibility and its anistropy is also Bresented I. INTRODUCTION The optical spectroscopy of urnnyl salts has had a long and colorful history. The light-emission and absorption properties were apparently first, investigated systematically by Stokes (1) in 1852 and further study wit.nessed t.he development of phosphoroscopy (2, S) and the discovery of natural radioactivity (4). The spectral properties of uranyl salts have since been studied in quite great detail, making the spectrum of the uranyl ion one of the most ext,ensively investigated of all molecular spect,ra. Indeed, since the isolation of the transuranium elements, the spectra of the X0%+ and X02++ ions, where X is Kp, Pu, or Am, have also become well known, Nonetheless, most work has been empirical,

*This research was supported by a Research Corporation Cottrell Grant and by a National Science Foundation Grant G-7390. t Presented at the Symposium on Molecular Electronic Spectroscopy, American Chemical Society Meeting, Cleveland, Ohio, April 11, 1960. 164

SPECTRA and very litt’lc published

OF ACTINYL

work is extant

lti5

IONS. I

or the intcrprc-

on either the corrclat.ion

t ation of the spectra or the uranyl ion and its actinyl The

reason

for

this

apparent

analogs. of intcrprct~at~ivc inter&

lack

is due not so

much to apathet)ic effort, as it is to t)hc magnitude of the difficulties such study involves. The obstruent, fack which confront an,yone who might, at tempt t hroretical

consideration

of t)hese ions are det#ailed below,

made here in order to surmolmt,

The free uranium figuration

(:j,

atom is presumed

. . .!$%d’7.~~.This

6)

. . 5_f’b?‘is’ proposed ferred

by Dawson

for reasons

fact, that

advanced l’a( )?++ and I’aOzt

reverse

atom

the 5j orbital

so import,ant

to cat~se instability. of t lw estcrnal

(7 ); howr\er

It is to be emphasized elements

tonium, electrons

unstablr.

designation

reasons whereas

is pre-

arc based on the I:(),-‘. and lr( &‘-

The reason advanced

is that ilr

stability

( r~‘dr ir~jra 1, is sufficiently

of Mnrrus

is also a sensitive that’ the transient

weak

function nature

of

( l(1) for the g round states of the trans-

they are: n-p,

. . . rif” fid’ 7s”

Pll,

. . . 5f” BdO7s’

Am, 0..

5f 7 &lo 7s”

Cm,

5Ji tirl’ 7x2.

...

with the elect ran configurations

one; it is assumed,

however,

of t,hese

t#hat*the 5f orbitals

are

bound in uranium than arc the Gtl levels and perhaps even more the 7s orbital (11 I and that this is also t,rue of nrptunium, plu-

and americium.

Othcrmisr,

it is presumrd

that, the numhrrs

are 6, 7, 8, and 9 for IT, ?;p? 1’11, and Am, respectivrly.

shrlle of the metal binding

in the (‘OIIconfiguratiorl

in energy than the (id, whereas the metaland where, c,ollsccluc.lltly, t.hc 5f,-2p,

that our concern

is not a primary

more tightly tightly than

the former

(9 1, it seems probable

are adopted;

wit)h the

is lowtr

to actinyl

ionic charge

clcmellt

concur

Since the energy of the 5j’ orbital

I’( )?+ has a similar basis. The elcctroti configurations ttrallitnn

not

by Connick (8). Thcsr arc nonexistent, species,

is true in prot,ouctinium

oxygtw txd,

assumptions

to have six valcnc~c electrons dots

arc known, even though UOzt is rather the uranium

as are thr

them:

rnrrgy

of valrncy The valrnw

are considered

in these metals

IO br t,he 5.f, (M, 7s, and 7p orbitnls and t,hcir to bc 5.f G 7s > &Z > 7p. Only the above ceil-

tral-at om atomic orbitals ( AO’s ) will br used in forming the complrte orbitals (MO’s) of the S02++ or X’02~’ ions.

molreuhu

13. THE: GEOMETRIC STRIJCTCJIW OF ACTINYI,10~s It w:w deduced acelak,

?;a(,1T02)

(12j

from s-ray

(CH3COO),

diffraction

, was cubic

data that, cryst#alline sodium many1 with

space

group

~2~3, which

by

166

McGLYNN

AND

TABLE

SMITH I

VIBRATIONAL FREQUENCIESOF THE URANYLION Geometric type

Symmetry species

8 (cm-‘) a

Symmetric stretch Asymmetric stretch Bending a Values quoted are from Ref. 16. imposes linearity on the UOZ‘+ ion. This conclusion is supported by the work of Zachariasen (13) and of Samson and Sill& (14). Unfortunately, the x-ray data do not position unambiguously the light oxygen atoms and the linearity is still open to question. Infrared and Raman work are no more conclusive. If the uranyl ion is linear, only one frequency, the symmetric &etch (see Table I), should appear in the Raman, whereas two frequencies, t,he asymmetric stretch and the bending mode should appear in the infrared. On t,he other hand if the uranyl ion lacks a cent,er of symmetry (i.e., is bent,) t,he Raman and infrared are no longer expected to be mut,ually exclusive, and three frequencies should appear in both. Arguing in the above manner, infrared studies of crystalline uranyl salts (15, 16)) and Raman spectra of saturated solutjions of uranyl salts (1’7, 18), indicate a bent structure. The selection rules quot,ed are based however on the presence of an inversion center, and are strictly valid only in a completely isolated ion. The cryst’al field (with the exception of a highly symmetric crystal at a very low temperature) is usually sufficient to destroy the cent,er of symmetry in the crystal, while the strongly asymmetric fields to which t,he ion is subjected in solution will surely bring about a similar occurrence, and thus lead one to some doubt of the CtV structure of the uranyl ion. The complet,e antithesis of the result,s of t’he two methods used (i.e. x-ray and vibrational spectra) led to some confusion. Thus, critical reviews of the literature by Sutton (19) and by Dieke and Duncan (20) led the former to conclude that the uranyl ion was bent,, whereas the latter authors assumed it to be linear. Fortunately, recent, work has been more unequivocal. Intensity considerations of diffract,ed x-rays have allowed considerable accuracy (21) in positioning t’he oxygens with respect to the uranium and linearity seems well established; and intensity considerations in both the infrared (22) and Raman (23) also accord with linearity. Infrared (24) and x-ray (65, 26) diffraction data for the other actinyl ions, U02+ excepted, show t,hese t,o be linear also. h-o st’ructural data on symmetry

UOz+ has appeared, no doubt, because t,his is the most unstable of the pentavalent, species (,%‘, 28). Neither are any spectral data on this ion available, but, despite

this lack of informat,ion t,here seems no reason why U02+ should not be linear also, and it is presumed to hr so in this work.

SPECTRA

FIG. 1. Presumed showr

lying

OF

ACTINYL

IONS.

structure of a trinitr:lto-ur:tnyl complex. in a plane perpendicular to the collinear O--I!-0

167

1

The three entity.

nitrate

groups

:trc

Anot.her consideration and one which is of some importance t.o any discwssirtn crystal and of solution spectra is the extent to which the ion is solvat,ed. For example in the readily extractable t’ri-nitrate many1 ion, I’( )?( N03)3--, there seems little doubt but that, the three nitrate ions are coordinated with uranium in the manner shown (29) in Fig. 1, where t#hc six Ii-0 coordinate bonds lie in a plane perpendicular (to within I”) of the two axial IT-0 bonds of uranyl. In the many1 nitrate hexahydratc it’ is probable that six wat,cr oxygrns are similarl\ equatorially coordinat,ed to uranium ($0). Of the several hundred known (wordination wmpounds of t.hc many1 iou, the rcluatorial coordination nun1 her varies from four t’o six, and may he always six. The rcsult,ant ecluatorially-applied field mill undouhtcdly affect the many1 ion and modify its spectrum, hut in the atwnw of evidence of any wry great modifiwtion and from the fwt that thcl axial I--- -0 distance is much shorter (0.8 A in sodium many1 acetate (31 ) I than t,he equatorial C-0 lcngt’h, it is presumed that t)he spectrum is primarily influenced by the axial field, t’hr equatorial field exerting only a small perturbing influenre. In so far as t.his discussion is wnwrned the relevant uranyl entity is presumed to he the D,t, ion depicted in Fig . 2; t,hc equatorial field will he NMsidcred only when experiment dictates the nwcssit,y of such. The last and greakst, difliculty, and t’hc one which has owasinned all of t h(s prrwding preamble, is t,he fact that these ions are molecular, and that one must resort to MC) calculations which involve electrons from at,omic shells of principal Of

McGLYNN

168

AND

SMITH

px2

PXI PYI

PY2

*z

P42

91

FIG. 2. Axes defined for a collinear

O-U-O

entity.

quantum number greater than four (n > 4). Such calculations are very unreliable (Sd), since they would presumably use Slater A0 wave functions; it is for this reason that we have not required theory to predict shape, as any good theory should. The D,h point group, with a sufficient number of representations shown to define the necessary molecular eleckronic terminology is shown in Table II. The manner in which the central atom AO’s transform under the operations of t,his point group may readily be found by subjecting the orbital part of t,he A0 wave functions to a sufficient number of these operations. The AO’s considered were the Sf, Gd, 7s and 7p AO’s, and their transformation propert’ies are given in Table III. The atomic orhitals used are defined with respect to the central axes of Fig. 2, and in particular the f-orbitals used are exactly those given by Shirmazan and Dyatkins (3.9, 34 ) who have discussed1 their hybridization properties. C.

QUANTUM

CHEMICAL

CALCULATIONS

The oxygen group orbitals (GO’s) are readily formed by simple addition and subtraction, followed by multiplication by an appropriate normalizing factor. They are tabulated in Table III. The oxygen GO’s were formed from the p, , p, , and p, AO’s; the oxygen 2s AO’s are considered to be so tightly bound (35) as t,o be but negligibly involved in bonding, and their transformation properties are not tabulated in the table. On this premise the 2s electrons remain lone pairs throughout. On the other hand, the p,, AO’s may be considered as sp hybrids directed t,oward t,he central met,al atom. The final effect of t#his will be a lowering in energy of the final CS~+and gfLf MO’s over that which would have occurred in the absence of hybridization, and that now the oxygen nonbonding (or lone pair) AO’s are the sp hybrids which are directed away from the central atom. The former case would seem to be the more probable in view of t,he large valence state excitation energy involved. 1These three papers, translated in conjunction with the present work, appear in a report submitted to the Physics Branch, Office of Naval Research, U. S. Navy and to the Chemistry Division, Office of Scientific Research, A.R.D.C., U. S. Air Force in December. 1955 by the Florida State University.

SPECTRA

OF ACTINYL TABLE

IONS.

I

II

THE I&, POINT GROUPDEFINED E

IC,

C’?’

i

r” + !,” , 2

1

1 1 1 1

1 1

-1

(il;

1 1 1 2

2 cos $0

0

2 2 2 2 2 2 2 2 2 2 2

2 2 2 2 2 2 2 2 2 2 2

0 0 0 0 0 0 0 0 0 0 0

ILb

:’

cos $0 cos 2p CO8 2q cos 3q COR3p cos 4cp cos 49 cos 5p cos 5$0 cos 6~ cos 6q

-1 -1

1I Q

Lit',

1

1 -1

1

1

-1

-1

-2 2 -2 2 -2 2 -2 2 -2

1 0

cos $0 2 cos 29 -2 COB2p 2 cos 3‘p -2 cos 3l,c 2 COY4p -2 cos -lp 2 cos 5p -2 COB5’F 2 cos 6p -2 cos 6p

0 0 0 0 0 0 0 0 0 0 0

-2

2

1 -1 -1

2 cos $0

2 -2

$1I’

/

:t tll denotes :I Rotation Operator TABLE

III

SYMMETRYORBITALSFOR XOe”~2)+ IONS IN D-ah

The nt‘xt st,op involves the familiar secular equation technique, the solution of a cubic being necessary in the case of t’he a-MO ‘s, and a quartic for the ?r-MO%. For this purpose Slater WF’s (36) were used but, rather than do involved calculabions with bad initial funct’ions, rough graphical estimates of overlap integrals were made and t,hen resort was had to the criterion of maximum overlap (5’7’).

170

McGLYNN

AND

SMITH

The “energies” of the isolated GO’s and AO’s which comprise a given MO, and the nodal properties of the final MO itself were considered. By these means the MO energy scheme depicted in Fig. 3 was set up. This set of one-electron energies is hardly noteworthy for its basis in theory. However, it does seem to be a reasonable scheme, the final validity of which will depend on the ext,ent to which it yields accord with experiment. It will also serve another important purpose: namely, that it’ will make possible a discussion of the electronic properties of actinyl ions on a superficially theoretical level. The four lowest energy MO’s, all of which are metal-oxygen bonding, are shown schematically in Fig. 3. It is rather hard to say which of the two orbitals, (T%+or qo+, is lower in energy. The overlapping criterion favors (T,+, but the fact that it is antibonding in the oxygens mitigat’es against it. However, these two orbitals are both strongly L-0 bonding, are always completely filled with electrons and are not involved in any electronic transitions; their relative energy consequently has litt’le effect on any further discussion. We have however depict,ed the gU+t,o be the lower energy MO. The a-orbitals are considerably higher in energy than the a-orbitals, and of the a-orbit’als, it is expected because of overlap, nodal and isolated A0 “energy” reasons that the ?T~is of lower energy, and it, is shown t’o be so in Figure 3. The r-orbitals are t’he highest energy filled MO’s of the actinyl ions and t’hey are consequently of importance in t,he electronic spectroscopy of these species; because of this no firm assumptions as to the relative energy of the T-MO’S will be made at this point. The two atomic-like f-orbitals, 6, and (blL, are posit’ioned lower in energy than the atomic-like d-orbital, 6, , for previously cit,ed reasons. The &-MO is positioned lower than the c$,, because of the higher angular momentum of the latter around the internuclear (2) axis. It must be remembered, however, that these are one-elect,ron levels and that t#heir positions may invert, for ligand-field reasons, in the actual actinyl ion. The relative energies of the next seven MO’s are hard to estimate. It is probable t,hat they occur in two groups: three MO’s which are slightly antibonding shown on the upper right of Fig. 3, and four which are strongly antibonding on the upper left of the same figure, with considerable, but unestimable energy overlap between the two groups. It is for this reason that they are shown on a sliding scale in Fig. 3. Again, and happily so, these orbitals are so high in energy that they appear not to be appreciably involved in the spectroscopy of the uranyl ion, and further remarks on them, at this time seem unnecessary. However, discussion of the spectrum of h’pOz++ will require consideration of the 2a,+ and 2~, orbitals, and of the relative energy of 5f and 6d orbitals. The fortunate situation then exists, that most of the relevant spectral and magnetic properties of the actinyl ions are to be interpreted using the MO’S lx,, la,, 6,) and & . It is this fact alone which allows further elaboration. However, one other factor must be considered, namely, t’he extent to which

SPECTRA

OF ACTINYL

10%.

I

171

FIG. 8. One electron MO’s of a collinear O-U-O entky. The MO’s are shoq-n separated into their component AO’s for reasons of clarity, and are approximately scaled in energy. In some cases more than one A0 of uranium contributes to the MO; in such instances the A0 which is mixed least into the MO is shown hy dotted lines: The MO’s shown in the cross-hatched region at top of the figure are of higher energy, but’ considerable uncertaint,y exists as to their relative position. The uranium AO’s depicted are hydrogenic and are drawn to scale. Further discussion of this diagram, particularly of the relative energies of da I 2a,,+,and 2~,, will he found in Paper 11 of this series (Ref. 47 of text).

McGLYNN AND SMITH

172

equatorial coordination may affect the energies of the MO’s and t,he energies of the resultant states. The representation generated by six equivalent equatorial bonds contains the irreducible representation al, , bl, , el, , and eZgof t#he Deb point group. Since, of all valence orbitals considered, only fi(Z~_y~jtransforms as blu 7 and only the degenerate pair d,, , dzz_yz transforms as eZs (all in D6h), this immediately requires the participation of these orbitals in t,he appropriat,e equatorial six-fold coordination of actinyl ions. The &MO and thef,!,+z) component of the +,-MO are unaffected by the equat’orial ligation and will remain atomic in character t’hroughout. According to Coulson and Lester (38) who also use an overlap criterion of bonding, the f-orbitals involved in the six-fold ligation are 6f rather than 5j, which if true would imply that’ the fi(Z~_l/~jcomponent of +u is also virtually atomic. The furt,her effects of coordination in the equatorial plane will be discussed where such seems relevant to logical discourse. II. THE URANYL ION A. THE GROUND STATES There are twelve electrons involved in binding, six from the uranium, four from each of t,he t,wo oxygens, and minus two to account for the doubly positive charge of the ion. Filling of these electrons into the molecular orbit’als, with due account being taken of energy and the Pauli principle yields the lowest energy electron configuration (l&+)2(

1cQ7+)“(17rX)4(17r0)4,

l&+

(1)

and leads to a t,otally symmetric singlet ground state. On this basis the metaloxygen axial bond is a triple bond consisting of one u- and t#wor-linkages, and is to be contrasted with the localized MO picture which supposes t,his bond to be double, and made up of one (T-and one r-linkage. This triple bond picture is essential to spectroscopic interpretat’ion, and it explains very naturally t,he special st,ability (39) which seems associated with eutectic (40) at 400°C; the the UO?++ ion. It is stable in molten KCl-NaCl uranate ion, UOf+, is more properly written ( UOz++)Oz since it contains the linear uranyl grouping (13 j , as does the oxide (39) UO, ; the U-O axial bond length may be extremely short, the lowest observed value being about l.BA, and this has led already to inferences (26, 31) of triple bonding. However, t,he U-O bond lengt,h is st,rongly dependent on the coordinating ligand, a variation between 2.08 and 1.6A being observed (26)) or a total contraction of 20% . This is somewhat greater than the normal contraction of 14% obtained by superposing a a-bond on a a-bond (i.e., in going from a single to a double bond), and would suggest tha.t in the uranyl ion the actual variation is between a single and a triple bond. This may he understood as follows: the nucleophilic properties of ligands are given by the Tsuchida spectrochemical series as in Table IV, where it is understood that the equatorial ligation should increase in strength as one

SPECTRA OF ACTINYL

IONS. I

I 7:-1

goes from SOa- to F- ligands. It is to be noted from Table IV that as the COvxlcncy of equatorial ligand to uranium bonding increases, t’hc uranyl axial bond length increases and the y1 vihrut,ional frequency decreases in a manner which, but, for the acetate behavior, accords completely with variation along the sptctro~hemicnl series. Cnfortm~ately no other data are available, and it would seem that further studies of the type exemplified in Table IV arc to be desired. The reasons for this “representative atom” behavior of the uranyl ion art six-fold ligation does not probably to he interpreted as follows: Equatorial directly affect- t,he la,,’ or la, orbit&. However, group theoret.ically, hot,h the 1C!/ALa11d 1T,, will be delocalized to some extent over the whole complexcd entity, and this delowlizat~ion will increase as one proceeds down tho series of Table IV; wncomitant wakening of the axial bond will thereby result’. Furthermore. tho involvcmellt of the c#I~( and 6,, RIO’s in the equat’orial ligation, mill lead to SOIIll' t,lcc,troll-o~~~~l)allcy of these orhitals, and will cause some decrease in t’he strength of the axial bond because of increased clect.rostatic repulsion wit,h the highly Ilcgati\.c asial oxygens. It is, presumably a somewhat similar effect which condit ions the regular decrease (,“J ) of v1 as oue proceeds along the series VOZf*. SpO,-‘, 1’r1()?+A, and _4mOCtf; the extra valence electrons in the last three spec+s are fed into the & or 6,, orbitals of the X02 +’ ion and one expects the clwtron rtp~&ion bet,ween t.hcsr extra electrons and those on the axial oxygctts to dewease the bond strength in a regular fashion, much as is observed. ‘I’hc prqwscd bonding of the urauyl ion is certainly novel in that, it invol\-es both ,f and tl electrons in the axial and equatorial bonds. The participation of rl-orhitals in bonding is quite well known, but the bonding pr(JpertiW (Jf j-orhitals Irave ouly lately been recognized. Gluwkauf and ~IcBay (,,/tf ) first suggestrd that j-orbit:& take part in the ecluatorial linkage s, and even though disputed ( .$S1 th participation of ,f-orbitals seems required by group theory. The possibility of j-orbital involvement in the axial bouds was considered by C’ounick and Hl~gus (.L9). The latter author considered (,$S) that the st#ability of the parapcriodute ion, 104-, as contrasted to the Iwncsistent, Br04- ion was due 10 ,l-orhit,aI participation in the iodine-~ oxygen bonds, an idea which may \-et’! wc~ll lw true. in any case, in t.hc linear uranyl clltity one (hannot, have a F bond t)etweetl ench oxygen and lu-anium without the part ieipat,ion of either the 7~: or ;5.f13;1O’s of lwanium. The considerably lower energy of the latt,er orbit,al alld its grrater overlap with p of oxygen, caouplrd with the unusual strength of the ~lralli~~n~~~os!-gcl1 bond, are in themselves sufficient proof of ,f-orbital involvt>ment. I”ortun:~ttly, magnetic properties of uranyl salts (see 1I.C) yield almost, iudisputahle proof of ,f-electron involvement It has bee11 stated that the jormal bond order of I--PO is expected to he three. OIIC of these bonds, a” , may he so weak as to be negligible, but nonetheless the I- -() bond should be very strong. It is somewhatj diwoncerting then that the freqiicnc.y 6, , which does not involve motiolt of the nra~nium atom, has a \-ulue

174

McGLYNN

AND

TABLE

SMITH

IV

EFFECT OF LIGAND ON THE SYMMETRICSTRETCHINGFREQUENCYOF UOs++ Species

Tsuchida series

RbUOz(NO&

1.58

873

NO,A

cs2uo2c1,

1.73

830

CIA

NaUOz(Ac)a

1.71

850

ACA

KUOzFs

1.76

789

F-

as small as 860 cm-‘; this is especially so when contrasted with the value of this frequency in tetrahedral species such as 0~04 , Re04-, W04=, etc., where an average is about 950 cm-‘, and where the formal bond-order (with inclusion of d-orbitals) is 2x. The reason might lie in the weakness of binding of the + CIA, rQ , and 7rUorbitals. Thus neglect of d-orbitals in tetrahedral species drops the formal bond order to lx, whereas neglect of these higher energy orbitals in uo*++ drops the order to somewhat less than 19;. B. SPECTROSCOPIC PROPERTIES Any discussion of the spectra of act,inyl ions must needs take cognizance the values of br , the spin-orbital

coupling parameter

of

of a 5f electron, Ex , the

linear ligand field splitting parameter and the various electrostatic repulsion parameters. It is to be noted that, the value of & will doubtless be dependent on the particular orbital (& , 6, , etc.) in which a 5f electron finds it,self, and thus that a rigorous theory of these molecules would require knowledge of &fo , &I , tsr2 , and &f3 where the new subscript indicates the component of moment’um of the Eif-electron along the uranyl axis. Such knowledge is at present unavailable, and even if it, were, would not, be of much help since we should also have to know the extent to which covalent spreading (44) of a given f-electron into the viciniby of oxygen reduced spin-orbital coupling. Because of these difficulties an average & is t,o be preferred, and some est,imate of its magnitude may be obtained using

lnz= n31(1

(2 - a>4 +

l)(l

+

-1

l/2)

52343 cm 7

(2)

where Z is the atomic number, c the extent to which the orbital in question is shielded from the nuclear charge, and where the assumption of a central-field charge-gradient is made. If (Tis evaluated as 78 by use of the Coulson-Duncanson rules (Sb), ,& is found equal to 42.6 cm-‘, which is unrealistically small. However, if G = 58, as found from magnetic suscept’ibility (45) of neptunyl compounds a value of 1481.9 cm-l is obtained, and this value seems t,o give

SPECTRA

OF ACTINYL

IONS.

I

17,s

reasonable agreement with experiment. Values ranging from 500-2400 cm-’ may he found in the literat#urc. The axial oxygens are negatively charged and for this reason, as also their field proximit,y to the uranium at,om, will give rise to a st,rong electrostatic along the uranyl axis in directions I* + 0. The effect of this linear field will be to invert the normal one-electron f-orhit,al energies. Since & with angular IMJmrntum f:Sh,Bn about t,he uranyl axis avoids the negatively charged axial osygens much more effectively than 6, , t,he energy of the former will hc lo\vercd rrlati1.c to that of the lat,ter, and the order of increasing energy of the .f-orbit& should be +,, < 6,‘ < ST,, < 2cr,,+ 9 value of -17,000 rm-’ for the stability of & relative to 6,‘ has been deduced from experiment by Eisenstein and l’ry(*(s (_iS), hut this value, based on incomplet,e spectral evidence, is much too lurg~. Indeed, one of the effects of ligation may br to rc-invert +, and 6,, ; conaidcration of the spectrum of NpO?‘+ using two parameters, &, and h’h , where the latter is the energy difference between orbitals of angular moment,a Xh ‘%a and i/\ + 1 ih, 2~ about the X0?+’ axis and is presumed independent of A, Icads to it value of stjout 8000 cn-’ (47). A1 flIrther effctk of this fairly large axial clrctrk field is that it will strongly couple the individual angular momenta to the internuclear axis, such that the total resultnllt of angular momentum, -1, along this axis becomes a reasonably “good” cluantum number. The orbitally generated magnetic field in turn couples the spins alo~lg t.he internuclear axis such that the t.otal spin result,ant, 2, along this ask also becomes a “good” quantum number (except for states for whicah A = 0 ). The similarity of the model chosen to Hund’s coupling case (a ), :rncl the facat that molecular RussrllManders notation (48) may he used is to t)(b uotcd.

Thr effects of electro,static repulsion integrals arc more difficult to e\-al\latc, but should be generally smaller than the previous energy quantities. Their cff&s will only he discussed yuant,itatively where unavoidable. The lowe.st energy configurational transition is Iv+*. “0

. . ( 17r,~3(4u)1;

which is orbitally forbiddrn (hut Laporte about 8000 cm-’ higher should lie 1, + + AD

1s3A1,, ) 1331,LG )

allowed)

... (1Ku)"(& )'. II ,

while with energy

%,, ,'%*,,

(3) baricentri ill

the component X + n of which is allowed and _cy polarized. It is impossible to say which of these many transitions is lowest’ in energy, but it, will probably be T ++ S in character. It now behooves one to consider experiment to see if any assignments may be made. The emission and absorption spect’ra of uranyl nitrate hexahydrat,e obtained 2 This is also the conclusion

of Eisenstein

and Pryce

(unpublished)

176

McGLYNN

AND

SMITH

Absorption

30 WAVE

NUMBER

x I$

an-’

FIG. 4. The emission spectrum and low-energy absorption spectrum of UOz++. The scale on the left refers to molar extinction coefficient. The regions 1, 2 and 3 are somewhat arbitrarily defined: The height of the emission spectrum and the boundary of region 1 are drawn to emphasize a mirror image relationship. Regions 2 and 3 are drawn in the manner indicated to show their relationship to the M and D series of Duncan and Dieke (SO). The arbitrariness in drawing in these boundaries does not appreciably affect the results of Table V. There may be more than three transitions in the absorption region indicated, but complete resolution awaits further work. in aqueous solution at 23°C are displayed in Figs. 4 and 5. These spectra are most probably charact’eristic (49) of the UOz(Hz0)6++ ion. In Fig. 4 the emission is shown as the reverse of region 1 while the absorption is somewhat arbitrarily divided into regions 1, 2, and 3. The reasons for the mirroring are as follows: ( 1) The emission shows no obvious mirror-image relation to the first absorption band (region 1, 2, and 3) of uranyl and in fact is of much less spectral width-5000 cm-’ compared to 11,000 cm-l. axes are aligned (2) In single crystals of RbUOz( NO 3) 3 in which the O-U-O along the optic axis of the crystal, no Zeeman splitting is observed (20) for t’he emission, or for region 1. By contrast region 2 does split into two components

(20). (3) The oscillator strengths and lifetimes of the various designated absorption regions, 1 to 5, are tabulated in Table V. The lifetime of an emission which is the reverse of region 1 is calculated to be 4.76 X 10e4 set, whereas that associated with an emission mirroring the first absorption band (region 1, 2, and 3) should be 3.43 X lop5 set, or less than the former by a factor of 1-L It is to be emphasized that these are intrinsic lifetimes which would be obtained in the absence of any nonradiative dissipative processes, and are thus maximal values. These predicted values do however contain the effect of the medium in relaxing forbiddenness of a transition, and as such are to be compared with the experimentally measured values of 7meanfor liquid or glassy solutions from room temperature to ones as low possibly as 77°K. The emission has in fact been shown

SPECTRA

OF ACTINYL

4-

IONS.

177

I -2000

.-

2 w u

5 -1500

k $

.I000

f, I= 2 5 w

.500

50 WAVE

NUMBER

8

0

- cd

FIG. 5. The absorption spectrum of UO z++ with the high-energy regions 4 and 5 defined. Arrows refer curve to appropriate scale. Region 4 probably corresponds to the UV series of Dieke and Duncan (20). The dotted line has been drawn in by exponentially extrapolating the boundary of region 5. Thef-number of region 5 quoted in Table V has been obtained by doubling the area under the depicted boundary of this region, and is subject to some error, since the maximum observed may be spurious and due to the large amount of scattered light coming through a wide slit.

decay (50, 51) exponentially with a lifetime varying from 1 X lo-” to i X 1O-4 SW; 7 decreases with increasing temperature, increasing concentrat’ion of solution and decreasing viscosity (or equivalently with increasing quenching), and in glassy solution at 77°K has the latt,er value above. The agreement of the quot,ed values with that calculated using region 1 is rather excellent. The experiments of Hall and Dieke on single crystals at 4°K are to be compared with the absorption spectra of these crystals at, that temperature, and are not relevant to the present discussion (59). Since region 1 apparent#ly corresponds to a single electronic excitation it seems reasonable t.o suppose that in t#he low-energy absorption band of UOz++ there are occzluded at least’ three distinct) electronic transitions, which correspond to regions 1, 2, and 3, and which are the three component,s of a triplet’. The fairly regular separation of these bands: ii,,, = 22,050, 24,125, and 27,000 cm-‘, giving a multiplet separation of ca 2500 cm-’ would indicate that these are the t,hree components of either 3Au or 3~,, ; the observed multiplet separation is too to

178

McGLYNN

AND

TABLE

SMITH

V

QUANTITIES CALCULATED FROM THE ABSORPTION SPECTRUM OF UO,++ ION Region a 1 2 3 1+2+3 4 5 4+5 8 Definition

1.29 1.18 3.45 1.41 7.71 2.65 7.98

x x x X X X x

7*

Ymarc

fb 10-s 10-t 10-h 10-d 10-S 1OP 10-T

22,050 24,125 27,000 25,000 34,000 48,000 48,000

cm-l cm-l cm-l cm-l cm-r cm-r cm-r

4.76 4.39 1.21 3.43 9.30 1.69 1.63

X X X X X X X

10d4 set 10m6set 1OV set lo-& see 10-7sec 10e8 set 10e8 set

of region is given in Figs. 4 and 5.

b Oscillator strength defined by f = 4.32 X 1OP 1 e d Y. c Wave number of maximum extinction of the appropriate region. * The mean lifetime as calculated from the absorption by 1 7 = 3.47 X 108 -_2 %d?

1 gu * - * __ g1 se di;’

where 9, the refractive index, is set equal to unity, and where the degeneracies of upper are taken as gU = 2 for all except 1~ states, and lower energy states, gUand gl , respectively, where g = 1.

small to permit ready interpretation on the basis of either a “+,, or 31’Ustate, and possibly sufficiently large to enable elimination of 3112t. However, t,his latter discussion neglects effects of intermediate coupling, and hence the selection of the 3A~U, 3A2Uand 3A3Ucomponents as those occluded in the low-energy band of the uranyl ion is somewhat arbitrary. The effect of spin-orbital coupling is t,o give each of these levels an energy [(A. x), which corresponds in the case of the selection made to energies -2& 0, and +2& which for E = 1500 cm-’ produces a multiplet splitting of 3000 cm-‘, corresponding to that observed of 2500 cm-‘. The spin-orbital coupling operators, which transform as simple rot,ations, generat.e the irreducible representations ug- and 1~~of D-h . ug- corresponds to axial spin-component Z = 0, such that the total symmetry of 3A2u.is A, ; 7rgcorresponds to axial spin-component S = fl, such that the total symmetry of 3A11L and 3A3Uis either lI, or & . This knowledge enables one to ident,ify group-theoretically the appropriate perturbing singlets, and from an experimental knowledge of the energy and lifetimes of these states one may then calculate lifetimes of the three triplet components. The appropriate first-order pex%urbation theory expression is (53) :

where the contribution from only one perturbing signlet state, S*, which is of lowest energy is considered. rs*+s is t’he lifetime of the perturbing singlet and

SPECTRA

OF ACTINYL

179

IONS. I

TABLE VI LIFETIMER OF 2’ + S EMISSIONS FROM TRIPLET STATE COMPONENTS LOCATED IN REGIONS 1, 2, AND 3 CALCULATED AS DUE TO MIXING WITH SINGLETS LOCATED IN RE~IOKS 4 AND 5 Triplet region. T 1 2 3 1 2 3 B

Perturbing singlet, S* Region Region Region Region Region Region

AE = %,,x(s*)

~~ and

-

4 4 4 5 5 5

AE

(a~-‘)~

11,950 9875 7000 >25,950 >23,875 >21,000

.g.cm-‘)

T[ 2’ + S] (set)

1481.9 1481.9 1481.9 1481.9 1481.9 1481.9

1.17x 6.09 X 4.2 X 5.34 x 3.46 X 1.98 X

10-4 1IF 1OV 10-5 10m5 1O-4

~.

i%,,,(T).

F,~ arc> t,he wave llumbers of perturbed triplet and perturbing singlet, states, respect,i\-ely. HI3 is the matrix element of the spin-orbital coupling operator between T and S*, and is a product of two parts: a radial part for which ( = 1500 cm-’ and an angular part which is approximated as unity; HI3 is then eclual to < and AE = ~~~ - i+. The only two spectral regions in which t,he perturbing singlets might, lie are I and 5 of Fig. 5, and the lifetimes evaluated, using the above equation, for the presumed triplet tran&ions of regions 1, 2, aud 3 perturbed by the presumed singlets of regions 1 and 5, wit’h .$ = 1481.9 cn--’ are given in Table j71. Comparison of Tables VI and V leaves very little doubt hut that the low-energy absorption band of uranyl ion is a T + A’absorpIn particular if regions tion, and that regions 1 and 5 are S* + S absorptions. 1, 2, and 13arc components of a “regular” mult,iplet, in view of the group-theoretic: discussion above, regions 1 and 3 should be perturbed by the same singlet. and region 2 should he perturbed by another different singlet stat’e. In accord with this if region 4 perturbs the triplet components of region 1 and 3, the ratio of lifetimes (a more theoretically significant quantit,y 1 calculated is 0.35 whereas the experimental rat’io (from Table T-1 is 0.39; region 3 is then automatically selected as being the pert,urbing singlet for the triplet of region 2 and the calculat,cd value 3.-M X 10P5 is to be compared with the experimental of -!.3Y X 10. ’ sec. The above conclusions are independent8 of presumed symmetry species of the triplet state, and depend for \-alidit)y only on the “regular” nature of the multiplet. However, if the triplet state is selected ar having an orbital part, of symmetry A, region 5 is ~mamhig~~ol~sly ‘A 1Land region 1 is either ‘@‘I< or III,, . If the triplet stat,e has symmetry II, then region 5 is ‘n, and region -l is ‘2, or ‘A(, Siner region ,5 has a very high nbsorhtivity, it would seem to he best, described as due to the transition ‘n,, + lx,+ which is the only one of the enumerated S* * S t,ransitions which is allowed. This would fixate the low-energy liranyl absorption as %I?&+ I&+, and would of course necessit#ate some incrcasr in the vs111e of .$; this will not matcriall_y affect t,he rcsult,s of Table VI as long as

180

McGLYNN

AND SMITH

E is not allowed to be greater t’han 2000 cm-‘. The conclusions just arrived at are shown in the energy diagram of Fig. 6, where the states in ascending order might also be I&+, 3A1,L , 3A~u. , 3A~21 , ‘IIlu or 1@32c,and ‘Azu , but where the depicted ordering is the more probable. No firm preference for either set of states exists. Further comment on Fig. 6 seems necessary. It seems reasonably certain that the ground state is correctly identified as l&+, and that the lowest lying excited level is a triplet multiplet. However, the symmetry species of this triplet state is still in some doubt. The attribution 3Au gives a reasonable multiplet splitting and it is to be expected that the component 3A1u.will not show a Zeeman effect (reduction of symmetry from Dmh to C,,). The only states which will not show a Zeeman effect are ‘ZO , eIIl,2 , 3Al , “Q , 5Hz,etc., which in view of the fact that none of these except 3A1 can reasonably occur as a low-lying energy state of uoz++, would select 3A1 as the terminal level of region 1 absorption and the genesis level for the emission. On the other hand, region 2, has been shown to split into two components when a magnetic field is applied along the optic axis of RbU02(N03)2 ; the observed separation is 3& per unit field, where /30is the Bohr magneton. This would accord with species 3n1 , but not with 3A~which would exhibit a splitting of 400 . Zeeman evidence is thus rather inconclusive, and it may be that the expected double axial degeneracy of region 1 is already split in the absence of a magnetic field by equatorial ligand field, or Renner effects, or if the correct state designation is 3~Io, into components 3110+u and 3IIo-u . In any case, whatever the correct symmetry species of the excited triplet state, the absorption is due to excitation of an electron from a bonding orbital, la, , to a non-bonding 5f-A0 of uranium. This is similar to the transition type proposed by Jorgenson (54, 9) who calls it an “electron transfer spectrum.” This is a correct name inasmuch as the orbital is weakly bonding, and in it most of the electron density is centered on the oxygens, whereas in the t,erminal orbital of the absorption process all of the electron density of the optical electron is centered on the uranium; absorption thus causes transfer of charge from the two oxygens to the uranium. Other authors have not, specified, however, from whence it is that the ele&on kansfers, whet,her from equatorial ligands, axial oxygens, or from some orbital which embraces both oxygens and equatorial ligands. This is to be contrasted with the interpretation given here of transfer from la, . It has been shown that the orbital lvr, is group-t,heoretically independent of hexagonal equatorial ligation, which accords wit,h the observed relative insensit,ivity of the spectral location of absorption regions 1, 2, and 3 to the type of ligand. Regions 4 and 5 are likewise ascribed here to transfer from the axial oxygens, but substantiation of t#hisview at present is impossible. The transition 311,(or “A,) + ‘Z9+ causes a decrease in U-O binding, and it is expected that vibrational frequencies in the excited state should be considerably less than those in absorption. That this is so is readily seen from Table VII where are recorded the vibronic peaks in both absorption and emission, and where the mean frequency has decreased from 848 cm-’ in emission to 739 cm-’

SPECTRA

OF ACTIXYL

IONS.

181

I

Tentative AsGgnments >48000

T

J/ - ‘n,u

cm“

34000

cm-’

27000

cm-’

24125

cm-’

22050

cm-’

4

n=e-,

’

0

I 3n”

a=o,c I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I I

I I

1

I

I

I

I

I

I

I

I

1

-

5;

FIG;. 6. Tentative

energy scheme of the UO, ++ ion spectrum. The set of states symbols quoted is not the only possible set (see text), and are given here so that some individual may have t,he pleasure of verifying them, or the extreme satisfaction of showing them in error. The latter of course implies subst,itution of the correct set of designations.

in absorptjion. This drastic change in the vibrat’ional interval has previously been observed by TCasha (55) and was adduced by him as evidence bot,h for considerable decrease in the binding in the excikd state and for the molecular nature of the excitat,ion involved; bot)h points accord with t,he present8 interpretation.

182

McGLYNN

AND SMITH

TABLE

VII

VIBRATIONAL SPACING IN THE ABSORPTION AND EMISSION SPECTRA Band number

Molar extinction coefficient, e

Wave number s-(cm+)

A (Wave number) Aa (cm-‘)

Absorption 1

2 3 4 5 6 7 8 9 10 11 12

0.41 1.08 1.70 3.98 7.90 9.81 8.62 5.75 3.40 3.31 3.65 4.10

20,554 21,344 22,027 22,727 23,529 24,213 24,813 25,600 26,300 27,050 27,825 28,600

790 763 700 802 684 600 787 700 750 775 775 Average

739

Emission 1 2 3 4 5

20,105 19,232 18,370 17,594 16,712

873 862 776 882 Average

848

The most prominent vibration observed coupled to the elect,ronic process of regions 1, 2, and 3 is go+, and would seem to indicate orbital-allowedness of the electronic transition involved. This would indicate the excit.ed states %, . Indeed, if these states were 3AU the most prominent, vibrational intervals should be 930 cm-’ and 200 cm-‘, since only the combination ?T~X v,+ can make A, + is the vibration most likely to be excited by vibronically allowed. However, rsQ the geometrical change (extension) accompanying the transitions 311, (or “AU)+?z,+. Finally, the excited state is expected to be linear, and there seems no basis or cause for attributing the long lifetime of the emission to a Fmnck-Condon forbidden transition from a bent excited state as has been done by Stepanov (56’). If this were the case long progressions in the bending mode would be expected in both absorption and emission, but no more than one or two quanta of this vibration have ever been observed coupled to the electronic transition. At any rate the necessity for such an explanation seems now obviated. A number of general comments are now in order. Emission occurs only from the lowest excit,ed state, the higher levels degrading (52) radiationlessly even

SPECTRA

OF ACTINYL

IONS. I

183

at 4°K. This result is substantiated by the fact that under suit,able circumstances the quantum efficiency of t,he emissive process is unity (57). That there should be only one emitting state is not, an uncommon occurrence in molecular spectroscopy (58)) where energy levels are functions of internuclear spacing and geometry, and where accordingly crossing of t#hc usually hyperdimensional potential energy surfaces is quite expected. In part,icular, for the uranyl ion where [ is sufficiently large (-1700 cm-’ ) t)o promote a high quantum yield of intersystem crossing (by iucreasing the frequency factor of t’hc appropriate Arrhenius expression), the observed results are just those to be expected. Indeed, for molecwlcs in which t’hc optkal electrons have 5 2 1000 cm-‘, there may exist a rule that the onl!/ emitting level is the excited one of lolvest’ energy regardless of its multiplicit!j.

The term “fluorescence” is generally applied to the uranyl ion emission,this despit,e the fact that it’ was originally (2) krmed ‘iphosphorescence.” Since the uranyl ion is a molecular ent’ity, and since t,hr emission is also inherently molecular and T 3 S in nature the molecular spectroscopic name of “phosphoresctnw” would seem in order. This advocated readopt,ion of the original name (2 ) of this emission must, beg apology of those solid-state physicists who apparently use this t,erm (i.e,, phosphorcsrence) for emissive proresxes due to depopulation of excited kapping sites which are usually unspecified, but generally suppowd t’o be extrinsic. The differentiation in nomrnclat,ure of 7’ 3 S and S* -+ S processes by the terms “fluorescence” and “phosphorescence” is extremely convenient, and because of chemical phcnomcnology also newssary. Such usage for UOt++ is hereby strongly advocated.

The magnetic aaalog of the Langevin-Dehye

equation is

where x is the molar magnetic: susceptibility, pz the mean square magnetic moment and a the average polarizahility. In Hund’s coupling case (a), the sit,uation presumed to exist in the ac%inyl ions, the anisotropic g-factors are given by 911 = a(A + 2X),

!?* = 0,

(7)

where /Lz= Po2(ga2 + &P2)/4 and where POis the Bohr magneton.

Since the ground state of U02+’

(8) is ‘zO+,

184

McGLYNN

AND

SMITH

A + 22: = 0, and the magnetic susceptibility Since

will be given entirely by Nn.

(9) where i;i is the radius vector of the jth electron and where L and S are angular and spin momentum vectors, respectively. #a is the ground state of energy Ea and the summation i extends over all excited states $; of energies Ei . The first term in Eq. (9) is that of diamagnetism while the second is that of a temperature independent paramagnetism and will not, generally, be zero, even for a totally symmet,ric ‘2,+ state. The reason for this latter is that the perpendicular component of orbital angular momentum is not well quantized, but fluctuates because of a continuous transfer of momentum back and forth between elect,rons and nuclei. Such paramagnetism may outweigh the first term of Eq. (9)) and will do so, subject to consideration of Pascal’s law, for many compounds in which d or f electrons are involved in bonding. The uranyl ion has just such a small intrinsic temperature-independent paramagnetism (5931)) the molar suscept’ibility being 57 X lo@. Qualitative calculations by Eisenstein and Pryce (46), who presume that two 5f electrons are involved in the u-bonding of UOz++ indicate that the second term of Eq. (9) is sufficiently large to account for the observed susceptibility; this paramagnetic susceptibility is expected to be markedly anisotropic, being (for reasons cited above) zero parallel t’o the O-U-O axis and maximal perpendicular to this axis. These conclusions are in accord (46) with experiment,, and the inclusion of some 5f-electron character in the r-bonds of uranyl ion will not materially affect the calculations of susceptibility either as to its magnitude or anisotropy. Paramagnetism attributable to similar causes is to be observed (54) in the ions CrOd- and MnOl-, which also have totally symmetric ground states, and in which there is considerable admixture of 3d WF’s (6Y). It is tempting to speculate on the possibility of using such paramagnetism in conjunction with Pascal’s law as a determinant, of whether or nob d or f electrons part’icipate in bonding. Two further experiments suggest themselves. If the emitting state of UOz++ is 3~0uit should be paramagnetic with g,, = 2 and gl = 0. Since the emission lifetime is sufficiently long to ensure a reasonable steady state population of the presumed 3~0ulevel under conditions of intense illumination, it should be possible to observe paramagnetism of the excited state and this observation would be facilitated by using a complex of uranyl, UOn(N03)2.6Ht0, in which the diamagnetism of the ligands swamps the inherent temperat)ure independent paramagnetism of the ground state UO2++ entit’y. Qualitative observations of this effect have been made in this laboratory, but the experiments are as yet quantitatively incomplete. It should be noted also that the anisotropy of the paramagnetic susceptibility is expected to be different in the two states *ZQ+ and 3&U :

SPECTRA

OF ACTKYL

IONS. I

185

the pammagnetism is maximal perpendicular to t,he O--I:-0 axis in the ‘Zy+ st,at,e and maximal parallel to t#his axis in a ‘~0~ state. This implies that a single crystal of a paramagnetic uranyl compound, RbU02(N03), , may be made to oscillate in a uniform magnetic field by exposure to an intense chopped light source, the chopping being slow. Again, this effect has been observed in these lahoratork, hut sufficient chance seems to he assoriated with the observat,ions to render the experiment somewhat inconclusive. In any case, a 3A1Ustate should not, exhibit, either of these effects, and the qualitative observations thus srrw to distinguish between the two possibilities ko,~ and “A,, ; however, the observation:: do not otherwise verify, and will not do so until made quantitat’ive, t#hr (~orrwtnws of t,he designation %IilC,. III.

CONCLUSIOK

It swrns

obvious that many deductions made in t)his discussion of the uranyl ion require substantiation; it also seems obvious t’hat many details will be changed in order to attain conformity to future experiment,. Yet there remains very little doubt’ in the minds of t’he authors that the simple model presented cont,ains much beauty and perhaps some trut.h, and that, future work need only put the model on n truly quant,itative basis to produce a complete understanding of these ions. ACKNOWLEDGMENTS ilny discussion such as this owes much to other \vrit,ers, and the authors are in this respect much indeht.ed to the works of Dieke and Duncan (20) and of Rabinowitch (&9, 65) which served to introduce and orient them in this area of molecular spectroscopy. Particularly must the authors acknowledge their indebtedness to Professor M. Kasha; it was he who first brought the xctinyl ions to our attention, and his consultation in the interim between then and now has been extremely valuable. Our gratitude is also tendered to the agencies, Research Corporation and The National Science Foundation which supported this work.

RWEIVEI) : September

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6, 195 (1952); Phil. May. [4] 4, 388 (1852). IS. HECQLEREL, Ann. chim et ph~~s. 27, 539 (1871). H. E. ROSWE, “Spectrum Analysis,” 3rd ed., p. 188 fl’. Macmillan, London, 1873. H. BECWEREL, Conlpt. rend. 122, 120, 501, 559, 689, 762, 1086 (1896). C;. T. SEABORG, “The Actinide Elements,” p. 733. McCrawHill, New York, 1954. (+. I<. VILLAR, Bol. fuc. ing. y agr. Montevideo 6, 191 (1955). J. K. Dawso~, Nucleonics 10, 39 (1952). II. E. CDNS-ICK, J. Chew Sot. Suppl. 235 (1949). C. Ii. J$RGENSON, “Energy Levels of Complexes and Gaseous Ions.” J. Gjellerups Forlag, Copenhagen, 1957. 10. R. MARRUS, U. S. Atomic Energy Commission Report UCRL-8547 (1958). ii. J. R. Mc37.4~~~ AND P. M. GRIFFIN, J. Opt. Sot. Bm. 49, 162 (1959). 12. I. FASKVCHEN, Phys. Rez,. 43, 1048 (1933). (:.

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W. H. ZACHARIASEN,Acta Cryst. 1,265,277,281 (1948). S. SAMSONAND L. G. SILL~N, Arkiv. Kemi Min. Geol. 26, No. 21 (1947). G. K. T. CONN AND C. K. Wu, Trans. Faraday Sot. 34, 1483 (1938). J. LECOMPTEAND R. FREYMANN, Bull. sot. chim. France 8, 622 (1941). H. W. CRANDALL,U. S. Atomic Energy Commission Report MDDC-1294 (1947). B. S. SATYANARAYANA,Proc. Indian Acad. Sci. 16A, 414 (1942). J. SUTTON, J. Chem. Sot. Suppl. 275 (1949). G. H. DIEKE ANDA. B. F. DUNCAN, “Spectroscopic Properties of Uranium Compounds.” McGraw-Hill, New York, 1949. bf. W. H. ZACHARIASEN,Acta Cryst. 7. 783, 788 (1954). 22. A. N. SEVCHENKO,Izvest. Akad. Nauk U.S.S.R., ser. Fiz. 16, 613, 624 (1951). 23. J. SUTTON, Nature 169, 235 (1952). 24. L. H. JONES AND R. A. PENNEMAN, J. Chem. Phys. 21, 542 (1953); L. H. JONES, Spectrochim. Acta 16, 409 (1959); J. Chem. Phys. 23, 2105 (1955). 25. F. H. ELLINGERAND W. H. ZACHARIASEN,J. Phys. Chem. 68,405 (1954). 26. W. H. ZACHARIASEN,Acta Cryst. 7, 795 (1954). 27. K. A. KRAUS AND F. NELSON, J. Am. Chem. Sot. 71,2517 (1949); K. A. Klaus, F. NELSON, AND G. L. JOHNSON,J. Am Chem. Sot. 71,251O (1949). 28. J. R. NIGON, R. A. PENNEMAN, E. STARITZKY, T. K. KEENAN, AND L. B. ASPREY, J. Phys. Chem. 68, 403 (1954). 29. L. KAPLAN, R. A. HILDEBRAND,AND M. ADER, J. Inorg. Nucl. Chem. 2, 153 (1956). SO. A. E. COMYNS, Atomic Energy Research Establishment (Harwell, England) C/R 2320 (1957); Chem. Revs. 60, 115 (1960). 31. W. H. ZACHARIASENAND H. A. PLETTIXGER,Acta Cryst. 12, 526 (1959). 32. W. E. DUNCANSONAND C. A. COULSON,Proc. Roy. Sot. Edinburgh 62,37 (1944). 33. J. C. EISENSTEIN, J. Chem. Phys. 26, 142 (1956). 34.M. G. SHIRMAZANANDM. E. DYATKINA, Doklady Akad. Nauk. S.S.S.R. 77,75 (1951); 82, 755 (1955) ; J. Chem. Phys. U.S.S.R. 27,491 (1953). 35. J. F. MULLIGAN, J. Chem. Phys. 19, 347 (1951). 36. J. C. SLATER, Phys. Rev. 38, 1109 (1931). 37. L. PAULING, J. Am. Chem. Sot. 63, 3226 (1931). 38. C. A. COULSONAND G. R. LESTER, J. Chem. Sot. p. 3650 (1956). 39. R. E. CONNICK, AND Z. Z. HIGHS, J. Am. Cham. Sot. 74, 6012 (1952). 40. D. M. GRUEN AND R. L. MCBETH, J. Inorg. Nucl. Chem. 9, 290 (1959). 41. E. GLUECI~AUFAND H. A. C. MCKAY, Nature 166, 594 (1950). 42. L. I. KATZIN, Nature 166, 605 (1950). 4s. Z. Z. HUGUS, J. Am. Chem. Sot. 74, 1076 (1952). 44. J. OWEN, Proc. Roy. Sot. A227, 183 (1955). 45. D. M. GRUEN, J. Chem. Phys. 20, 1818 (1952). 46. J. C. EISENSTEIN AND M. H. L. PRYCE, Proc. Roy. Sot. A229, 20 (1955). 47. S. P. MCGLYNN AND J. K. SMITH, J. Mol. Spectroscopy 6, 164, 188 (1961). 48. G. HERZBERG, “Molecular Spectra and Molecular Structure. I. Diatomic Molecules.” Prentice-Hall, New York, 1939. 49. E. RABINOWITCH,U. S. Atomic Energy Commission Report ANL-5173 (1953). 50. R. DELORMEAND F. PERRIN, J. phys. radium 10, 177 (1929). 51. J. T. RANDALL AND M. H. F. WILKINS, Proc. Roy. Sot. 184, 347 (1945). 52. L. A. HALL AND G. H. DIEKE, J. Opt. Sot. Am. 47, 1092 (1967). 53. D. S. MCCLURE, J. Chem. Phys. 17, 905 (1949). 54. C. K. JORGENSON,Acta Chem. Stand. 9,405 (1955). 55. M. KASHA, J. Chem. Phys. 17. 349 (1949). 56. B. I. STEPANOV,Zhur. Eksptl. i Teort. Fiz. 21, 1153 (1951). 1s. 14. 15. 16. 27. 18. 19. 20.

SPECTRA

OF ACTISYL

IONS. I

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