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The hypersensitive Δg(σuδu) ← Σg transition of the uranyl ion
Volume
97. number 3
CHEMICAL
THE HYPERSENSITIVE Ag(eu6,)
PHYSICS
‘10 M.ly 1963
LETTERS
f Zg TRANSITION OF THE URANYL ION
Colin D_ FLINT Depar~n~cnr of Cbemisrry.
Received
Bkkbeck
College, hfaler Sweet. London
WCIE
7HX. Uh*
1 March 19S3: in final form 14 March 1983
to an electron tr.msfcr for 3 wwkly bondlns The Ag(uU6u) - \‘&(o,)’ t mnsition of the urmyl ion \\hich corrcsponds oxygen orbital IO a non-bonding uranium 5f+, -_ orbhal has characteristics siruikx to thcso-cailed hypcrscnsitivc Jf-4f trausitions of the I~nthmide ions.
The energies and intensities of most f +- f transitions of the Irtnthanide ionsare little effected by the environment, reflecting the weakness of the coupling of the f electrons to the ligands. It has been recognised for ~nany years [1,2] that the intensities (but not the energies) of a few transitions are remarkably dependent on the surroundings of the metal ion. Jgrgensen and Judd [3] termed such transitions hypersensitive and discussed mechanisms which might produce this phenomenon_ They noted that the free-ion transitions obey quadrupolar selection rules and suggested then an environment of ligands with inhomogeneous polarizability could enhance the transition probability_ hlore recently Mason et al. [4] have argued convincingly that hypersensitivity arose by a dynamic coupling mechanism in which the quadrupolar free-ion transition induces correlated dipoles in the ligands and hence a static or vibronic non-centrosymmetric ligandfield component may cause the f-f transition to aquire electric dipole intensity_ Judd [5] has discussed the close similarity between the “inhomogeneous dielectric” and “ligand polarization” viewpoints. Denning et al. [6] have shown that the low-lying excited electronic states of the uranyl ion are derived from a uuSu configuration in which an electron in a bonding
ou orbital
has been
excited
to a non-bonding
6,
orbital with essentially uranium 5f character_ This configuration generates II,, A,,
-0000/S
03.00 0 1983 North-Holland
electronic origins E, *A,,. (designated 1.11) occur near 20000 cm-l whilst tl’;e split components of the transitions to the A0 state are BZ., - Alz (designared 111) and B1,, + At2 (designated 13) at ~300 cm-’ and =l;OOzcm- l to higher energy [7.S]. Denning et al_ 191 have shown esperimentally that the we& pure electronic origin of the B,, p A,,, transition obeys quadrupolar selection ru& in crfstslline CslU0,C14 and have observed that the electric dipole vibronk ori gin based on the BZz + A,,, transition are mucl~ more
intense than those based 0; the E, - A,, transition. Hypersensitive transitions in tlie lanthanide ions obey quadrupolar selection rules. so the pure electronic origins are expected to have near zero intensity in centrosymmetric enviromnentsand hale never been convincingly identified. We have failed to confirm the claim [lo] that the directly observed ?Fjtr - 6Hs,2 electronic origin exhibits quadrupolsr selection rules. Electric dipole vibronic contributions may however be substantial [IO--121. Origin 111of the uranyl ion in DAI, symmetry therefore sharessome charscteristics of the hypersensitive lanthanide transitions_ There are two other characteristics which distinguish hypersensitive transitions in lanthanides. [ l,?] _ Firstly small static deviations from centrosymmetry can cause the electric dipole intensity of the electronic origins of hypersrnsi tive transitions to increase markedly whilst other trsnsitions are little effected. Secondly the intensity induced by a given structure for a given transition increases rzpidly as the polarizability of the ligands increase. We now cite esamples of these characteristics for the uranyl ion. 317
97.
\‘dUllK!
CIIEMICXL PHYSICS LETTERS
3
Ilu!lllw
For the idealised centrosymmetric (Dth) UOzCl~ion in [(Cl13 14Nj 1U02C14 all four origins I-IV are wcah bur rho cryst~lsrcsdil?~sbsorb water into crystal mterstkcs to generate mmority sites in which the urrrriyl ioris arc subject to minute non-centrosymmetric pa turbations [ 13 1. Origins III and IV increase nixl&i> in ~ntcns~ty both absolute11 and relative to O~I~IIIS I .mci II. The s.A [(cX,)~N] 1 UO-, Br4 is isoSKIc!c!ural wirh Ibe
20 hlnv 19s:
The concept of hypersensitivity is of necessity imprecisely defied. It isapparent however that the analoguts between the familiar hypersensitive lanthanide transitions and the Ag + C, transitions of the uranyl ion are so close as to warrant the application of the same term to both phenomena. This also indicates the importance of the quadrupole-induced l&and dipole contribution to the intensity mechanisms in uranyl spectroscopy. a result which is not surprising in view of the relatively large quadrupole transition moment of the S, + 2, transition_ As far as we are aware there is no previouskperimental evidence for hypersensitivity outside the f-f transitions of lanthanides. This work was carried out during the tenure of a NATO travel grant at the Utiivcrsity of Virginia. Charlottesville, USA. 1 wish to thank NATO for financial support mid the University of Virginia for hospit3lity.
wJtcr These are rc,hl~ observed on origins 111and IV hi11 c3111io1bc Jctcctcd on origiiis 1 and ii [ 171. II! !hC JlIJ~L~~~~llS
h!~~ll!~~-~0l!i~~~~~
the water IIW~L!S vc11
~11011~
i
LJo2
References
Br4-.yflz0
very wc~h 011 o!$!:s I and II but ha11 lli and IV. sometin~es showing
on
.!rc
c~~tplctc .ibsorptkm in the perpsndicuktr spectrum 1IS 1. III IIUIIL‘ 0f thcsr co11!p~1tmis is tI!c!c cvidcncc tht Ilic wdto IS wordin~tcd to the urdnum although I! cw~11pc~ ~vcll-dd~~~ecl Llrticc sires. Iii tbcse c~scs the pa ~~II~.I~IOII 01‘ the !lrJ!!yl 1011is vibronic but the high
111tci151tyof Ilie mblonic oiigins due to the wdter III tlic bromide comple.~ cledrl> IIiiplicatcs the phr iabtitt) of lhc .mion in the nicch.tnisni. Prcsum.lbI> :I c0up1111g lw1~w1i the’ dipole of the water 1110dr 111dc5
~iid
1lic
th!
!!llC!isir~
11icii iliockl ~lt~l.ltllUl.ll
1~’ [lx
Jlpolc
iiidiid
Ihiiiiiig
ct
.iI.
[b]
01 rbs halide
111cch~111s1!1s
ion is involved.
given a cdrdtil
I1.1vc
hi
the
uranyl
amlysis
of
ion within
Iii p.irricul,ir Ihe? argue IhJl 12, in -p]mr Ill~d~\
h~cII?’
perpcndiwf.~i
swic~
s19rc
1\‘ll(c7, 6;). In
Kb,UO,Brj-xli70 the in perpendick11~rspw-rriiiii, iiidicalfiig tlic lxxtI1rbatioIi of the 5, wbu.tl~ by the I!IO~I~S. Denninget al. consider only rbc pitrup-tbcL)lstlc implications of the in-plane Geld hut 11 IS &Jr Iii,u ;1 SII!!@ cryst.&field inte1pretrttion 1~~1th K,liO,( n.11ci
1,-~11~O
the A,.(U”(s”)
co11ple
~~!tc~i~!t~ mid
11kk~JppcJr
c‘duwt c\pLuti ur~iiyi
linr the pk\eb.
1011 ~,(u,h,,)
h1o11iiih2
the
iiiudi
state cornpisxs
g!wcr
sensitivity
of
the
to extcrnd perturbations tlian for tlic chloride com-
[ 11 D.E. licnric, K.L. I‘cllous and G.R. Clioppin. Coord. C~ICIII.Rev. 16 (1976) 199. 121 R.D. Peacock. SIrtIc!. uondinr 22 (1975) 83. i3] CA. J~rtxxwn mid R.R. Judd. hloi. Pbyi. S (1964) 281. LJ1 S.1‘. Mx.on. R.D. Peacock and lL S!cw.m. !dol. Pbys. 30 (1975) 1619. ISI U.K. Judd, J. Clxm. I’hys. 70 (1979) 4830. L’3l R.G. Dcnning, T.R. Snell~gove JII~ D-R. Woodwxk. Mol. l’hp. 37 (1979) 1109. 171 KG. tknninp. T.R. Snellprove and I>.R. Wood\\ark. X101. I’hy~. 32 (1976) -119. Sot. liraduy ISI CD. I‘tinr and PA. Tanner. JKl1c1n. Trms. 11 76 (19SZ) 103. 191 R.G. Dcnning. T.R. Sncllgrovc and D-R. Woodmxk, Mol. Pbys. 30 (1975) 1819. 1101 A.K. Rancrjcc and R.W. Scl!w.Ir!z. Clic~n. Pbys. 58 (1961) 255. 1111C.D. I-X!!! and l--L_ Stewart-Dxlinp. hloL Pbys. 14 (1961). IlZl T.R. 1;lulkner and 1-X Richardson. Mol. Phys. 35 (1978) 111. 1131C.D. l-‘Iinr and PA. Tanner. J. Chem. SW. i‘x,idsy Tram I1 77 (1981) 1665. II-11 C.D. t‘linr and PA. Tmnrr. J. Clwn. Sot. i‘xaday Trms 11 76 (l9SZ) 103. 115 ] C.D. l-lint and l’.A. Tmncr. Polyhedron, to be published. ] 16) C.D. Flint and PATmncr, hlol. Pllys. 43 (1981) 933. 1171 C.D. Flint md P.A. Tmncr. hlol. P11ys.44 (1981) 411. [ 181 C.D. I‘lint and PA. Tanner, submitted for publication.
97. number 3
CHEMICAL
THE HYPERSENSITIVE Ag(eu6,)
PHYSICS
‘10 M.ly 1963
LETTERS
f Zg TRANSITION OF THE URANYL ION
Colin D_ FLINT Depar~n~cnr of Cbemisrry.
Received
Bkkbeck
College, hfaler Sweet. London
WCIE
7HX. Uh*
1 March 19S3: in final form 14 March 1983
to an electron tr.msfcr for 3 wwkly bondlns The Ag(uU6u) - \‘&(o,)’ t mnsition of the urmyl ion \\hich corrcsponds oxygen orbital IO a non-bonding uranium 5f+, -_ orbhal has characteristics siruikx to thcso-cailed hypcrscnsitivc Jf-4f trausitions of the I~nthmide ions.
The energies and intensities of most f +- f transitions of the Irtnthanide ionsare little effected by the environment, reflecting the weakness of the coupling of the f electrons to the ligands. It has been recognised for ~nany years [1,2] that the intensities (but not the energies) of a few transitions are remarkably dependent on the surroundings of the metal ion. Jgrgensen and Judd [3] termed such transitions hypersensitive and discussed mechanisms which might produce this phenomenon_ They noted that the free-ion transitions obey quadrupolar selection rules and suggested then an environment of ligands with inhomogeneous polarizability could enhance the transition probability_ hlore recently Mason et al. [4] have argued convincingly that hypersensitivity arose by a dynamic coupling mechanism in which the quadrupolar free-ion transition induces correlated dipoles in the ligands and hence a static or vibronic non-centrosymmetric ligandfield component may cause the f-f transition to aquire electric dipole intensity_ Judd [5] has discussed the close similarity between the “inhomogeneous dielectric” and “ligand polarization” viewpoints. Denning et al. [6] have shown that the low-lying excited electronic states of the uranyl ion are derived from a uuSu configuration in which an electron in a bonding
ou orbital
has been
excited
to a non-bonding
6,
orbital with essentially uranium 5f character_ This configuration generates II,, A,,
-0000/S
03.00 0 1983 North-Holland
electronic origins E, *A,,. (designated 1.11) occur near 20000 cm-l whilst tl’;e split components of the transitions to the A0 state are BZ., - Alz (designared 111) and B1,, + At2 (designated 13) at ~300 cm-’ and =l;OOzcm- l to higher energy [7.S]. Denning et al_ 191 have shown esperimentally that the we& pure electronic origin of the B,, p A,,, transition obeys quadrupolar selection ru& in crfstslline CslU0,C14 and have observed that the electric dipole vibronk ori gin based on the BZz + A,,, transition are mucl~ more
intense than those based 0; the E, - A,, transition. Hypersensitive transitions in tlie lanthanide ions obey quadrupolar selection rules. so the pure electronic origins are expected to have near zero intensity in centrosymmetric enviromnentsand hale never been convincingly identified. We have failed to confirm the claim [lo] that the directly observed ?Fjtr - 6Hs,2 electronic origin exhibits quadrupolsr selection rules. Electric dipole vibronic contributions may however be substantial [IO--121. Origin 111of the uranyl ion in DAI, symmetry therefore sharessome charscteristics of the hypersensitive lanthanide transitions_ There are two other characteristics which distinguish hypersensitive transitions in lanthanides. [ l,?] _ Firstly small static deviations from centrosymmetry can cause the electric dipole intensity of the electronic origins of hypersrnsi tive transitions to increase markedly whilst other trsnsitions are little effected. Secondly the intensity induced by a given structure for a given transition increases rzpidly as the polarizability of the ligands increase. We now cite esamples of these characteristics for the uranyl ion. 317
97.
\‘dUllK!
CIIEMICXL PHYSICS LETTERS
3
Ilu!lllw
For the idealised centrosymmetric (Dth) UOzCl~ion in [(Cl13 14Nj 1U02C14 all four origins I-IV are wcah bur rho cryst~lsrcsdil?~sbsorb water into crystal mterstkcs to generate mmority sites in which the urrrriyl ioris arc subject to minute non-centrosymmetric pa turbations [ 13 1. Origins III and IV increase nixl&i> in ~ntcns~ty both absolute11 and relative to O~I~IIIS I .mci II. The s.A [(cX,)~N] 1 UO-, Br4 is isoSKIc!c!ural wirh Ibe
20 hlnv 19s:
The concept of hypersensitivity is of necessity imprecisely defied. It isapparent however that the analoguts between the familiar hypersensitive lanthanide transitions and the Ag + C, transitions of the uranyl ion are so close as to warrant the application of the same term to both phenomena. This also indicates the importance of the quadrupole-induced l&and dipole contribution to the intensity mechanisms in uranyl spectroscopy. a result which is not surprising in view of the relatively large quadrupole transition moment of the S, + 2, transition_ As far as we are aware there is no previouskperimental evidence for hypersensitivity outside the f-f transitions of lanthanides. This work was carried out during the tenure of a NATO travel grant at the Utiivcrsity of Virginia. Charlottesville, USA. 1 wish to thank NATO for financial support mid the University of Virginia for hospit3lity.
wJtcr These are rc,hl~ observed on origins 111and IV hi11 c3111io1bc Jctcctcd on origiiis 1 and ii [ 171. II! !hC JlIJ~L~~~~llS
h!~~ll!~~-~0l!i~~~~~
the water IIW~L!S vc11
~11011~
i
LJo2
References
Br4-.yflz0
very wc~h 011 o!$!:s I and II but ha11 lli and IV. sometin~es showing
on
.!rc
c~~tplctc .ibsorptkm in the perpsndicuktr spectrum 1IS 1. III IIUIIL‘ 0f thcsr co11!p~1tmis is tI!c!c cvidcncc tht Ilic wdto IS wordin~tcd to the urdnum although I! cw~11pc~ ~vcll-dd~~~ecl Llrticc sires. Iii tbcse c~scs the pa ~~II~.I~IOII 01‘ the !lrJ!!yl 1011is vibronic but the high
111tci151tyof Ilie mblonic oiigins due to the wdter III tlic bromide comple.~ cledrl> IIiiplicatcs the phr iabtitt) of lhc .mion in the nicch.tnisni. Prcsum.lbI> :I c0up1111g lw1~w1i the’ dipole of the water 1110dr 111dc5
~iid
1lic
th!
!!llC!isir~
11icii iliockl ~lt~l.ltllUl.ll
1~’ [lx
Jlpolc
iiidiid
Ihiiiiiig
ct
.iI.
[b]
01 rbs halide
111cch~111s1!1s
ion is involved.
given a cdrdtil
I1.1vc
hi
the
uranyl
amlysis
of
ion within
Iii p.irricul,ir Ihe? argue IhJl 12, in -p]mr Ill~d~\
h~cII?’
perpcndiwf.~i
swic~
s19rc
1\‘ll(c7, 6;). In
Kb,UO,Brj-xli70 the in perpendick11~rspw-rriiiii, iiidicalfiig tlic lxxtI1rbatioIi of the 5, wbu.tl~ by the I!IO~I~S. Denninget al. consider only rbc pitrup-tbcL)lstlc implications of the in-plane Geld hut 11 IS &Jr Iii,u ;1 SII!!@ cryst.&field inte1pretrttion 1~~1th K,liO,( n.11ci
1,-~11~O
the A,.(U”(s”)
co11ple
~~!tc~i~!t~ mid
11kk~JppcJr
c‘duwt c\pLuti ur~iiyi
linr the pk\eb.
1011 ~,(u,h,,)
h1o11iiih2
the
iiiudi
state cornpisxs
g!wcr
sensitivity
of
the
to extcrnd perturbations tlian for tlic chloride com-
[ 11 D.E. licnric, K.L. I‘cllous and G.R. Clioppin. Coord. C~ICIII.Rev. 16 (1976) 199. 121 R.D. Peacock. SIrtIc!. uondinr 22 (1975) 83. i3] CA. J~rtxxwn mid R.R. Judd. hloi. Pbyi. S (1964) 281. LJ1 S.1‘. Mx.on. R.D. Peacock and lL S!cw.m. !dol. Pbys. 30 (1975) 1619. ISI U.K. Judd, J. Clxm. I’hys. 70 (1979) 4830. L’3l R.G. Dcnning, T.R. Snell~gove JII~ D-R. Woodwxk. Mol. l’hp. 37 (1979) 1109. 171 KG. tknninp. T.R. Snellprove and I>.R. Wood\\ark. X101. I’hy~. 32 (1976) -119. Sot. liraduy ISI CD. I‘tinr and PA. Tanner. JKl1c1n. Trms. 11 76 (19SZ) 103. 191 R.G. Dcnning. T.R. Sncllgrovc and D-R. Woodmxk, Mol. Pbys. 30 (1975) 1819. 1101 A.K. Rancrjcc and R.W. Scl!w.Ir!z. Clic~n. Pbys. 58 (1961) 255. 1111C.D. I-X!!! and l--L_ Stewart-Dxlinp. hloL Pbys. 14 (1961). IlZl T.R. 1;lulkner and 1-X Richardson. Mol. Phys. 35 (1978) 111. 1131C.D. l-‘Iinr and PA. Tanner. J. Chem. SW. i‘x,idsy Tram I1 77 (1981) 1665. II-11 C.D. t‘linr and PA. Tmnrr. J. Clwn. Sot. i‘xaday Trms 11 76 (l9SZ) 103. 115 ] C.D. l-lint and l’.A. Tmncr. Polyhedron, to be published. ] 16) C.D. Flint and PATmncr, hlol. Pllys. 43 (1981) 933. 1171 C.D. Flint md P.A. Tmncr. hlol. P11ys.44 (1981) 411. [ 181 C.D. I‘lint and PA. Tanner, submitted for publication.