特別講演

特別講演 1P.K. Kuroda

特別講演 2D.H. Hendrickson

"Radioge:Q̲c̲h̲emisτry: 1936・1
986"
byくuroda，Departmentof Chernistry，University ofArkansas，P.K.IFayettev
ille，Arkansas 72
70
1 U.
S.
A.

ABSTRACTThe historyof radiochemistry is anaiogous t
o that oforgani
consideredchemistry. At the beginning ofthe19th century，itwas cthat inorganic andorganic substanceswere sharply differentiatedbyrti‑the cir
cur
nst
ance that，whereas theformermightbe prepared bya
e formed in the bodies ofani
mals
fi
ci
a
l means，thelatter couldonly band plants as the result of v
italforce. In 1928，Freiderich Wdhler(1800‑1882) showed，however，that urea，preeminent1y a product ofe prepared synthetica11y fr
orn inorganic
ani
r
nalr
netabolisr
n，cou1d bmaterials. A spectacular adv
ancer
nen
toforganicchernistrythen follow
田ed throughout the19th century.
herni
sts believed t
hat
At thebeginning ofthe 20th century，cthenatural radioactivitywas created sor
new
herei
n the Universe 10ng
before theearth carneint
o existence andthe radioactiveelements
rene Joliot・
could not be prepared byartifi
cialrneans. In 1934，ICurie (1896‑
1956) and Frederic Joliot (1900・1958) showed，however，
that radioactive isotopes could beprepared artifica11y. A spec‑
tacular advancement of radiochemistrythen fo11owed the discoveryo
fartificia1 radioactivity.
ut twoyears 1ater，I hadthe good
1was a student in 1934，b
fortuneofattending a speci
al lecture given byDr.F.W. Aston onqL

the subjecτof isotopes which was he1dョτthe Imperia
1 辺niversity
n June 12，1936. Ibecame verymuch interested in someof了okyooofthe ideas expressed byDr
. Aston and the fact that 1spentthenext50years ofmy 1ife doing research in the field of studies whichsno doubt dueto mymay best be described as Radio・Geo‑Chemistryi
encounterwith Dr. Aston in1
936.
I1earned from Dr. AstonIsbook "r~ass-Spectra and Isotopes"
o.，London，1933) that the historyof.Radio‑(Edward Arno1d &Co thetime when Sir.
chemistry can be traced back another50years，tnhis Presidentia1 Address to the Chemi‑Willi
am Crook己s (1832~1919) ica1 Section ofthe British Associationwhichwas he1d at Birmingham，Eng1and，in Septe~ber 1886，expressed his concept of isotopes. Hestated:Isit not possib1e，oreven feasible，~hat these heavierand 1ighter atoms may have been in some cases subsequent1y sorted outby a process resembling chemical fractionation? This sorting out mayhave taken p1ace in partwhi1e atomicmatterwas condensing fromthetmayhave been part1yprimal stateof intense ignition，but a1so ieffected in geologica1 ages by successive solutions and reprecipita‑
tions ofthe various earths.1IIt seems thatwhat Crookes had inmind

exact1yone hundredyears ago was a 10ng‑range prediction ofnotonlyetero‑the discoveryof isotopes，but also ofthe conceptof isotopic hogeneityor isotopic anoma1ies in theear1y solarsystem，whichwas t
become the focus of intense investigation bymany researchers intheecond
fie1d of astronomy，physics，chemistry andgeologyduring the shalfofthe 20th century.

︒
︒

Dur
in
gm
ye
ar
lyd
aysa
tt
heU
niv
e
r
sityo
fTo
kyo，1spen
tmuc
hofm
yt
i
med
oin
gr
esear
c
ho
nr
adio
ac
tiv
ed
isequ
i
1ibr
i
aa
mon
gn
atur
al
1y‑
in
gr
ad
ionu
c
1ide
si
ntheh
ydr
osphere(forexamp1e，1くuroda，Bull.
occurroc.Jaoar
12
1，52，58，1948;34，43，149，203，1949). TheseCh四 . Sstudie
sev
entu
a1
1y1edtothet
heor
yconcer
nin
gt
heO
k
10phenomenon
く1(uroda，J.C
hem.P
hys.25，781，1956). 1a1sobecameinterestedint
heb
e
hav
ior
so
ff
i
ssionp
rodu
ct
sfr
omnu
c
1earw
ea
po
n
stest
si
nt
heat
mo
sp
her
ea
n
dt
he
s
es
t
u
di
e
s1e
dt
ot
her
e
s
ear
c
hc
o
ncer
nin
gt
hee
xi
s
田

t
e
nc
e of24斗P
Ui
nt
hee
ar
l
ysola
rs
ys
t
em(1くuroda，Nature187，36，ow
eandKuroda，J.G
eophys.Res:.1.
Q.
， 709，立笠). Dur
ingthe立包; Rpen
tm
os
to
fmyt
i
m
ea
ndener
gya
ttem
pti
n
gt
odev
e
10pa
1
9
70's，1su
n
i
fie
dt
heor
yo
ft
hei
soto
pi
ca
nom
a
1ie
si
nt
hesolarsystem(seeくuroda，"TheOrigin oftheChemica1E
1ementsandtheforexamp1e，1Ok
1oPhenomenon"，Springer‑
Ver
1ag，Ber
1in，He
i
de
1ber
g，Ne
wYor
k，1982). Duringthepastfe
wyear
s，w
ehav
ebeenconcentr
atin
gont
hehen
ga
ndstudyofa
1pha‑recoi
1effectsofuranium(forexam
p1e，Sく￨uroda，虫主主主三，535，198士;Radiochim. Act~ E
̲， 93，旦笠; inpress

15，1985;Geochem.J.，in1986;J.Radioana
1yt.Nuc
1.C
hem.92，3press，1986)
. R
esu
1tsf
romt
hesestu
diesw
i
11be.
rev
iewedint
hisogetherw
iththe1atestresu
1t
sfromthestudieswhic
harer
eport，tcurrent1ybein
gcarr
寸edoutinour1abor
ator
寸es.

‑4‑

コ7FeMdssbauer Soectr。、sCODicStudies of
E工ectronicallyLabile 7ransition Metal Compounds

by David N. HendricKsor1，School of Chemical Sciences，
University of Illinois，Urbana，
工 llinois 61801，U.S.A.

ABSTRACT57Fe Mδssbauer spectroscopy is useful in studying the fluctua‑
tions of valence electron density in electronically labile compounds̲̲̲̲，
̲̲
̲̲IFe
工1
1 complexes a__~ ~_11 __~ ~_1 工 Isuch as mixed valenceFeIndFe andFespi
n‑申.
J.
.
.
J.
.

.
J.
.
.
J.
.
.
J
.
.

.
J
.
.
.
J.
.

.
J.
.
.
J.
.
.
J
.
.

crossover complexes. Electronically labile complexes have an elec‑
七r
onicdegeneracy or near degeneracy，which results in a strong司

coupling of the electronic coordinates to certain vibrational coordi‑
nates. A double‑well potential‑energy diagram results. In the caseof the mixed‑valence complex themolecule can interconver、tbetween
工工 工I
I __~ ̲1II 工
工the two different vibronic states FeA FeE andFeA FeE ・ Thet:'.
J.
.

.
J.
.
.
J
.
.

.
J
.
.
.
J
.
.

spin‑crossover complex can interconvert betweenlow‑ and high‑spinstates. As a result of this vibronic coupling，electronically labile
complexes are very sensitive to environmental effects. The presenceof a solvate molecule can dramaticallyrate of inter、
conversion.
工ntermolecular interactions in the solid state lead to cooperativityphuin the dynamics and phase transitions are seen.
Mðssbaue~spect~oscopic

data form an importan七 part of the database on沼 ixea‑γalence iron compo~~ds such as the two types whichare
pict辺、 ed below:

‑
‑
<Sミ>

x

《ふ〈会+金>‑x

Iよ1n the case of the mixed‑valence biferrocenes (on 1eft side) the工1̲̲.
.
.
. r.o̲
III

molecule has present both Fe.. and Fe.~. ions. Mδssbauer spectros‑
copy can be used to study the intramolecular electron transfer from1t~_ ~，__̲I
1.
工the Fe工..
o the Fe.
.
ion in the cation. 工n the solid state there is"，a dramatic dependence of the rate of electron transfer on the bifer‑rocene substituent X as well as in changes in the counteranion. The
7 M
X =C1 triiodide compound is valence‑1ocalized on the 5~IFedssbauer

IItime scale at 340K，as indicated by the appearance of both Fe.
.andI
I工 quadrupole sp1it doublets in the Mδssbauer、spectrum. This slow

Fe"'~~四rate of electron transfer is rationa1ized by the resu1ts of the X‑raystructure which show the 工3Z工工(Fe~~"')

counteranionsits closer to one ironion

than the other in the mixed‑valence cation. On the other、thanhand，the X =工 or X = Br じriiodidestransfer electrons faster‑6‑τhe Mδssbauer techniョue can sense even at 斗 .
2K. 工n the solid‑sta:a3Zアu
ctures of these two compounds the environme
ロt，including the了1 t
heposition o~ the 工3anion，is equivalきnt ~or both iron ions imixed‑valence cation. Rapid 告lectron 乞unnelingoccurs.
，o/Ith ali<yl groups as X exhibit aThe mixed‑valence biferrocenes 'heir、Mδssbauerspectra. At temperaturestempera七ure dependence in t
II and an Fe~~~II工 dbelow ‑200K，each compound shows both an Fe~~oublet.
門An increase in temperature eventually leads to the two‑doublet spec‑
trum changing to a spectrum with a single average‑valence doublet. Asample‑history dependence in the temperature dependence of thesespectra has been noted.
Changing the anion from I3 to Br2I or to other anions can leadto a dramatic change in the electron‑transfer char、acteristics of themixed‑valence biferrocenes. For example，if the I3 anion of theX =H compound is changed to the very similar Br
・
2工 anion，the tempera‑ture a七 which the electron‑transfer rate is faster than can be sensed
by the Mossbauer technique is reduced by ‑150 degrees.
工乞 is likely t
hat the electron‑transfer processes in the mixed‑

valence biferrocenes are coupled to phase transitions in the solidstate. Evidence for this comes from the sample‑history dependence
and anion dependence of the electron‑transfer rates，as well as
七‑capacitymeasurements on the X =H triiodide compound.
hea

Mixed‑valence trinuclear iron aceta乞ecomplexes (see drawing
工工 nd two Fe~~~工I iabove) formally contain one 門
Fe~~ aons. The ligand L ispyridine or a substituted pyridine. Usually these uncharged com
目‑7‑plさxes crysta工lizewi:h a solvate羽 olecule in the lattice. Mdssbauer
spectroscopyhas played a significant role in the interesti~g obser‑
vations whichhave been made for these iron acetate comolョxes:1) Simply changing the solvate molecule dramatically affects
therate ofi
nじramolecular electron transfer.
2) For several of these complexes there is a change in thevibrational nature of the solidwhichapparentlyreflects
the onset of dynamics associated with the ligandL and/or
the solvatemolecules. 2~H N
MR data support this.

3) For several of the complexes the molecular.dimensions change
appreciablywith changes in temperature.
持) Phase transitions which affecじ七herate of electron transfer

have been seen for several complexes.
A detailed theoretical model has been formulaむedto delineate
the type of phase transitions that can be seen in these trinuclear
mixed‑valence iron acetate complexes. The intrinsic electronic
structure of the complex，together with the nature of intermolecularinteractions present in the solid state，dictate the types of phasetransitions which are seen.
Finally，an overview of results bearing on the factor、s which。
ontrol therate of conversion of an ironspin‑crossover c lplex
佃

between its low‑spin ground state and high‑spin excited statewill begiven. 57FeMdssbauer spectroscopy has again proved to be a powerful
technique in studying the spin‑sta包einte
rconversion in 乞hese com‑
plexes.

‑8‑