of Phosphinesand Carbonylnickel
SuspensionNMR Spectroscopy
ComplexesImmobilizedon Silica
K. D. Behringer, J. Bliimel*
Institut der TU Miinchen,
Anorganisch-chemisches
4, D - 85747 Garching,Germany
Lichtenbergstrasse
Dedicatedto Prof. H. P. Fritz on the occasionof his retirement
Z. Naturforsch.50 b, 1723-1128(1995);receivedJune6, 1995
3rP SuspensionNMR Spectra,Immobilized Bifunctional Phosphines,Immobilized Carbonylnickel PhosphineComplexes,Modifled Silica, "P CPA,{ASNMR Spectra
The 31PsuspensionNMR spectraof silica-immobilizedbifunctional phosphinesand their
di- and tricarbonylnickelcomplexeswere recordedusing conventionalhigh resolutionNMR
31P
equipment.The practical advantagesof suspensionNMR spectroscopy,as comparedto
The
suspentendencies
are
demonstrated:
General
spectroscopy,
are
discussed.
NMR
CP/MAS
sion NMR signalsbecomebroaderwith increasingsolvent viscosity and decreasingsolvent
than alkyl chains.The linewidths
polarity. Phenylgroupsas spacerslead to broaderresonances
increasein the caseof chelation.
Introduction
The immobilization of homogeneouscatalysts
on inert supportsis of growing interest lI, 21,becausethe advantagesof homogeneousand heterogeneouscatalysis,like high selectivityand easyrecycling,can in principlebe combined.Sincemost
metals form stable phosphinecomplexes,bifunctional phosphineslike 1,2, 3, or 4 f3,41 are often
usedas linkers.
(1),
PPh2(CH2)4OH
(2),
PPh2(CH2)3Si(OEt)3
(3),
PPh2(C6H4)SiMerOEt
(4).
PPh2(C6H4)Si(OE03
Silica serves as a popular support, since it is
cheap,inert againstelevatedtemperaturesand most
brganic solvents[5], and alreadywell explored [5,
61.The complexescanbe preformedwith the shown
ligandsandattachedto the supportsubsequently[3].
Alternatively, the silica is modified with the phosphines1 - 4 to form li to 4i [3, 4) via condensation
reactionswith surfacesilanol groupsor additionreactionsto surfacesiloxane groups [7] prior to the
attachmentof metal complexes.The latter procedure hasthe advantagethat largerbatchesof modified silica with uniform quality canbe preparedand
reactedwith different metal complexes.
* Reprint requeststo Dr. J. Bliimel.
The most powerful and versatile technique to
ed silica and surfacecharacteizephosphine-modifi
bound complexesis multinuclear solid-stateNMR
spectroscopyt8 - 101.Usually,crosspolarization
(CP) and magic angle spinning (MAS) alone are
appliedfor sensitivityand resolutionenhancement,
but therehavealso beendescribedfruitful methods
for line narrowingvla adsorption[11] or sonication [12].
Alternatively, modified silicas have been investigated by NMR measurementsof suspensionsin
orderto elucidatechromatographymechanismsunder realisticconditionstl3 - 241.Hereby,the silicas
are most often coatedby long alkyl chainsof eight
to eighteencarbon atoms.The mobility and ordering on the su-rface
are probedby t'C [13 - 18, 20,
'H
in the
22 - 241and
ll9, 2Il NMR spectroscopy
presence
ofvarioussolvents.Reportson "P suspensionNMR spectroscopyof immobilizedphosphines
and metal complexesthereof are rare 125- 291 and
mostlydealwith solventswollenpolymersf25,26,
28, 291or phosphinemoietiesat the end of long
alkyl chains[27].
In this contributionwe demonstratethat31PNMR
spectroscopycan successfullybe applied to suspensionsof li to 4i in differentsolvents.It is important to establisha corelation between solvent
and mobility of surfacespecies,because,as in the
caseof chromatography,the successof a catalytic
0939-5015/95/1100-1723$ 06.00 O 1995 Verlagder Zeitschrift fiir Naturforschung.All rights reserved.
t]24
K. D. Behringer- J. Bltimel . Suspension
NMR Spectroscopy
of Phosphines
TableI. Chemicalshiftsd(31P)andhalfwidths2172of the t'P NMR signalsof suspensions
of Li to 6i in the indicated
(298 K), if not statedotherwise.The 6i31P)andu112valuesof the 3rPCP/MAS NMR
solventsat room temperature
signalsare takenfrom refs. [3, 4]. MeasurementconditionsseeExperimental.
Compoundu
No.
PPh2(CH2)4O-{SiOr}
1i
Solvent
toluene
hexane
cyclohexane
nitromethane
PPh2(CH2)3
Si(O-i SiO' ) )3
2i
toluene
hexane
cyclohexane
nitromethane
-16.8
-16.1
-16.3
-16.7
-16.2
-16.3
-16.5
-t].1
-11.0
-17.1
vt rt lHz]'
suspension
toluene
toluene(60'C)
PPh2(C6H4)Si(O-{ SiO' })3
4i
(co)3NiPPh2
(cH2)4o-{si02}
5I
-
1t
J.+
A<
420
r760
520
550
290
710
910
tl0
730
1550
1200
750
toluene
- 4.7
23.5
1300
toluene
z3- |
1540
,olu"n"
22.9
22.5
3030
6i
u, r, lHzl
solid-state
860
980
/<
PPh2(C6H4)SiMe2O-{ SiO2}
(co)2Ni[pph2
(cH2)3
s(o-{ sio2})3] 2
6(ttP)o
400
830
" {SiO2} denotesthe silica support,Ph a phenylgroup (C6H5);compounds2i,4i, and6i are not necessarilybound
to silicaviathreesiloxanebridges(seetext);b513rP)t 0.2 ppm and al ppm (5i, 6i);" u112t 10 Hz (li - 4i) and
t 50Hz (li in cyclohexane,
3i, 5i, 6i).
reaction dependson the proper choice of solvent.
We also present here the first suspensionNMR
spectraof surface-bounddi- and tricarbonylnickel
complexes.The advantagesand limitations of this
method are discussedand comparedto solid-state
CP/MAS NMR spectroscopy.
2. Results
The dry phosphine modified silica li to 4i
f3, 4), as well as the immobilized carbonylnickel
complexes(CO)3NiPPh2(CH2)4O-{SiO,
} (5i) and
(CO)2Ni[PPh2
(CH2)3Si(O-{SiO2})3]z (6i) [4] can
be characterizedbytheir rtP CPA4AS NMR spectra. When thesedry materialsare subjectedto 31P
NMR spectroscopyunder conditions of conventional high resolution(see4.), no signalat all can
be obtained.The sameis true for CClo suspensions.
However,a plethoraof other solventsgivesreasonably narrow signalsunder the conditionsdescribed
in 4. TableI sivesthe chemicalshiftsandlinewidths
of the 31Psuspensionsignalsof silica li to 6i in
toluene.Additionally, the correspondingvalues of
hexane,cyclohexane,andnitromethanesuspensions
of li and 2i are given. A detailedstudy of a variery
of other solvent/silicasystemsincluding T, values
is underway [33].
The suspension31PNMR spectraof 1i in cyclohexane,hexane.and nitromethaneare shown in
Fig. 1, thoseof 5i and 6i in toluenein Fig. 2. It
is known from silica esterificationstudieswith butanol, that the Si-O-C group is easily hydrolyzed
l3I,321. Although siloxanesare more stable,the
possibility of hydrolysisalso existsfor the Si-OSi group. Therefore,in the casesof relatively narrow signals(liltoluene; li/nitromethane;2iltoluene;
2ilnitromethane)it was carefully checkedin analogy to [13] that the resonanceswere not due to
purely adsorbedspecies:The supernatantsolution
of the measuredsuspensions
was subjectedto 31P
31P
NMR separately;no
signalsweredetectedinany
case.Furthermore,a traceof the correspondingfree
phosphinewas addedto the suspensions
and these
were again measuredafter stirring for t h. A narrow resonancewith about30 to 50 Hz halfwidth was
K. D. Behringer* J. Bliimel . Suspension
NMR Spectroscopy
of Phosphines
always found in the suspensionrlP NMR spectra,
superimposedon the broad signalsof the immobilized phosphines.The intensity of the latter did not
change,as was checkedby subtractingthe spectra
acquiredbefore and after phosphineadditionunder
identical NMR conditions with equal numbers of
scans.As demonstratedin the pioneeringworks of
Albert et al. lI3l and Gilpin [17], the linewidths
of suspensionNMR signalsincreasewith the surface coverage.Therefore,samplesof Ii,2i and 3i,
4i with similar surfacecoveragesof about 5 Voand
7 7o carboncontent,respectively,as determinedby
elementalanalysis,were applied.For the measurementsof 1i and 2i in different solvents,portions of
the samebatchof li and2i, respectively,were used.
The chemicalshifts are in good agreementwith the
ones determinedby 3rP CP/MAS NMR (Table I)
andby solutionNMR spectroscopyof the dissolved
compounds[4].
r725
3. Discussion
The residuallinewidthsof the 31PCPAzIASNMR
signalsdeterminedearlier 13,4l ne given in Table
I. With the exceptionof the systems2iltolueneand
2ilnitromethane,the suspensionNMR resonances
are broaderthan the solid-state31pCpA,tAS NUR
lines of the dry samples.Even though there is the
possibility to apply more favorable solvents (see
below and [33]), evenat this preliminarystatesuspensionNMR spectroscopyoffers someadvantages
ascomparedto'P CP/MAS NMR:
1. The sample preparation is simple. The silica materialsdo not have to be dried before measurement.After the modificationprocedure[3] and
washingwith the appropriatesolvent,the sluny can
be filled into the NMR tube directly. Impurities
(mostlyfrom oxidation [3]) showup in the spectrum
quitewell (Fig. 1A), andif the silica doesnot meet
the desiredpurity requirements,it can be discarded
before a tediouswork-up and drying procedure.
2. Quantitative data may be obtained conveniently. For simultaneous quantitative measurements of all the possible speciespresent on the
surface,the CP parametershave to be optimized
carefully[3]. Ifone setofparametersdoesnot give
optimal signal intensity for all surface species,a
multitude of spectrahave to be recordedin order
- 100
0
-100
.
100
0
PPM
PPM
31P
Fig. 1. 121.5MHz suspension NMR spectraof phos31PNMR spectraof surface
phine modified silica li in nitromethane(A), hexane(B), Fig.2.121.5MHz suspension
immobilized tri- and dicarbonylnickelcomplexes5i (A)
andcyclohexane(C). Measurementconditionsand NMR
dataseeExperimentalandTableI, respectively.S denotes and 6i (B) in toluene.Measurmentconditionsand NMR
dataseeExperimentaland TableI, respectively.
someoxidationproducts.
100
1126
K. D. Behringer- J. Bltimel . Suspension
NMR Spectroscopy
of Phosphines
to get quantitativeinformation. SuspensionNMR
spectroscopy,however, gives quantitatively reliable signal intensitieswithin one spectrumfor oxidic speciesand phosphinesunder the (uncritical)
measurementconditionsapplied (see 4.), as was
checkedby "P CP/MAS NMR t3l. Furthermore,
suspensionNMR spectroscopyallows quantitative
rneasurements
of physicallyadsorbedspecies[13].
This is not easily done with CPA4AS NMR [7].
However, suspensionNMR and CP/MAS NMR
signals of adsorbedspecieshave the samenalrow
linewidths. The latter distinguishesthem from signalsof chemicallyboundspecies[7].
3. There are no rotational sidebandsthat could
lead to overlapping.
4. The measurement
timesareshort.31PCP/MAS
NMR spectraof good S/N ratio of li - 6i require
measurementtimes of aboutthreeto four hours [3,
4]. SuspensionNMR spectra,on the otherhand,are
donein about15min. This is mostly dueto thefavorable T, times of thesesurface-attached,
in the presence of solventsmoderatelymobile species([34]
and seebelow).Additionally, an approximatelytenfold larger volume of silica can be usedin a 10 mm
glasstube than in a conventional7 mm rotor.
5. No spinninggasis consumed.In an inorganic
NMR laboratory, where air-sensitivecompounds,
diluted in a solid are mostly measured,nitrogen is
best used as the drive- and bearinggas, because
the otherwiseuseful aitight rotor inserts [35] need
too much volume within the rotor at the expense
of samplevolume. With a large quantity of routine
samples,this canbecomevery expensive.However,
suspensionNMR spectroscopydoes not need any
nitrogen for spinningat all [30].
6. The catalystsare probedunderrealistic condiNMR spectroscopy
providesintions. Suspension
formation about the mobility of surface attached
speciesin the presenceof solvents,which is indispensablefor a corelation of catalyst activity and
appliedsolvent.This information can of coursenot
be obtainedfrom 3lP CP/MAS NMR measurements
of thedry samples.
From the presentresultsthe following trendsfor
3lP
speciesemerge:The suspension
surface-bound
NMR resonancesof the immobilized phosphines
3i and 4i are substantiallybroader than those of
1i and 2i with the samesolvent (TableD. Broader
lines areindicativeof restrictedmobility, sincethen
the chemical shift anisotropyand dipolar coupling
are reducedless effectively [8]. Therefore,we assume, that the phenyl spacerbetween phosphine
moiety and silica surfaceleads to a more rigid arrangementof the phosphineson the surfacethanthe
alkyl chainswith threeandfour segments.This is in
line with refs. [25, 26]. Additionally,it was shown
for alkyl modified sllica via suspension13CNMR
measurements,
that the signalsare broaderfor less
mobile and more rigid segmentsof the alkyl chains
t13- 2r1,.
The "P NMR signalsof phosphines2i and 4i,
which can be bound to the surfacevia tp to three
ethoxy groups,are narrowerthan the onesof li and
3i, which allow just one bond to the surface.At first
sight this is in contrastto intuition, sincea more stable "anchoring" shouldreducethe mobility. But the
phosphorusnucleusis in every casepresentedhere
far away from the silica surface.Furthermore,as it
was nicely demonstratedby the study of Albert et
al. [I4], small alkyl groups,adjacentto longer alkyl
chains,reducethe mobility of the latter.Keepingin
mind that ethoxysilanereagentspreferentially add
to surfacesiloxanegroupsofrigorously dried silica
[7], the triethoxysilanes2i and4i would be expected
to have more neighboring surface bound ethoxy
groups and thereforedisplay broader31Psuspension NMR lines thanli and 3i. This is not the case.
Therefore,we concludethat the effectof line broadeningcausedby adjacentsmallalkyl groups[14] is
not dominant in the case of our phosphineswith
shorl alkyl and rigid phenyl spacers.We tentatively
interpretthe narrowerlinewidths of 2i and 4i asbeing the result of cross-linkingand the formation of
mobile, surfacedetachedpolysiloxane chains [36,
371.This would alsoexplainthebroaderresonances
of Li as comparedto 2i in all solvents.But one has
to be carefulin the discussionof this case,because
of the different alkyl spacerlengthsof li and 2i. T,
studiesof terminal methyl resonancesof long surface bound alkyl groups show that their rotational
motion doesnot changewith the chainlength [17].
However,othersystemswith shortern-alkylligands
display a nonlinear changeof mobility of terminal
methyl groups with chain lengths and a maximal
mobility for alkyl groups with six carbon atoms
[38] in the solid state.
Regarding Table I, it is obvious that the solvent employed for the suspensionNMR measurementsplays a crucial role regardinglinewidths.Nitromethaneleads by far to the narrowestsignals,
NMR Spectroscopy
of Phosphines
K. D. Behringer- J. Bliimel . Suspension
followed by toluene,hexane,and cyclohexane.As
abeadydescribedearlier [15, 16], solventviscosity is one of the factors dominating the halfwidths
NMR resonances.
A comparisonbeof suspension
tween the spectraobtainedfrom cyclohexane(r7=
(ry=0.33 cP at20
l.02cP atlT "C [39])andhexane
of li and2i showsthedramatic
"C [39]) suspensions
effect of viscosity.The polarity of thesesolventsis
similar. The linewidth increaseswith the viscosity
of the solvent.If the solventplays a crucial role for
u 11,of the 31PNMR signalsof immobilizedphosphines,thenu 11,shoulddecreasewith q at elevated
temperaturesfor a given solvent. Indeed, when a
toluene suspension(ri = 0.59 cP at 20 "C [39])
of 3i is measuredat 60 'C (n = 0.38 cP at 60 "C
[39]), the linewidthis reducedby 350 Hz (TableI).
Another important factor governingthe halfwidths
of suspensionNMR sigrialsis the solventpolarity
fzl, 22, 241. This can be demonstratedusing the
solventstoluene and nitromethane(4 = 0.62 cP at
25 'C). Although their viscosities are similar, nitromethaneleadsto the narrowestlines. This might
be due to the different polarities of toluene(e = 2.4
at 25 'C [39]) and nitromethane(e = 38.6 at 2O"C
[39]). Sincetolueneand nitromethanedissolvethe
free phosphines1 and 2 equallywell, we suppose,
in analogyto refs. I21,22,241, that nitromethane
is better suitedto adsorbat the polar silica surface
and detachthe phosphinemoietiesfrom there.This
would enhancethe mobility of the surfacebound
phosphinesand decreasethe linewidths of the suspension"P NMR resonances.
The immobilized nickel complexes 5i and 6i
give the largest linewidths. The lineshapes are
somewhatunsymmetrical and resemblethe overall signalshapesof the conesponding:tP CP/MAS
NMR spectra[4], meaningthat the chemical shift
anisotropyof the signalsin not removedby mobility of the bound complexes.Reducedmobility of
immobilized phosphinesafter coordinationof metals has alreadybeen reportedfor Rh 126- 281 and
Ptl25,29l complexes.In the caseof 6i, chelation
probably causesadditional reduction of mobility.
This chelationeffect has for examplebeendemonstratedfor platinum complexesimmobilized with
amine ligands [22].
chelating^
With "P CP/MAS NMR it is easily possible
to distinguish the di- from the tricarbonylnickel
complexes[4]. This is diffrcult using suspension
dueto the similar chemicalshift
NMR spectroscopy,
1121
and large linewidths (Fig. 2). Nevertheless,impurities andespeciallyuncomplexedphosphineswould
showup in the suspensionNMR spectra.Therefore,
evenhere a "preselection"of samplesfor CP/MAS
measurements
can be made.
In conclusion,this work has shownthat suspenis a usefulmethodas
sion 31PNMR spectroscopy
a supplementto conventional31PCP/MAS NMR
spectroscopy.Its advantageslie mostly in the ease
andconvenienceof applicationandin the savedtime
for routinemeasurements.
It may help to understand
why andhow catalytic activity changes,if the metal
complexesareimmobilized.The aboveresultsshow
that the solvent plays a crucial role. For analytical purposes,thereis somepotential in suspension
NMR spectroscopyin caseswhereparamagneticor
metallic impuritieqon the silica surfacedo not allow
the recording of "P CP/MAS NMR spectra.With
theproperpolar solventit might be possibleto move
the immobilized complexesfurther away from the
surface and to allow at least a suspensionNMR
spectrumto be recorded.A more detailedstudy of
the mobility effectsdiscussedabove,including determinationof T, values,is underway [33].
4. Experimental
For all the 3lP suspension
NMR measurements,
a
BRUKERMSL 300NMR spectrometer,
operating
in the
low powermodeat 121.5MHz andequipped
with a conprobehead,
ventionall0 mmbroadband
wasapplied.The
drymodifiedsilicaof anapproximate
volumeof 2 ml was
filled underN2 into 10 mm glassNMR tubes,equipped
with groundglassjoints, and stirredwith an excessof
therespective
driedandoxygenfreesolvent(3 to 6 ml).
After reversiblesealingof the NMR tubewith a glass
stopper,the silicawasallowedto settledownfor about
t h prior to themeasurement.
Thefollowingparameters
arenot optimized:Theappliedpulsewidth of 8 ps correspondsapproximately
to a 50opulse.The deadtime
was10irs,thepulsedelaycouldbe asshortas200ms.
Usually,for a spectrum
of a modifiedsilica(5 Vocarbon
content,asdetermined
by elemental
analysis)with good
SA{ratio,3000scans
wasenough.
Thiscorresponds
to a
timerequirement
for themeasurement
of abouta quarter
of anhour.Chemicalshiftsarereferenced
withresoectto
external
859oH jPOaidlrrPl= 9.1.
Acknowledgment
We thank the DeutscheForschungsgemeinschaft,
the
LeonhardLorenz foundation,and the Fondsder ChemischenIndustriefor financial support.
NMR Spectroscopy
K. D. Behringer- J. Bliimel . Suspension
of Phosphines
1728
U. Deschler, P. Kleinschmit, P. Panster,Angew.
Chem.,Int. Ed. 8ng1.,25, 236 (1986).
[2] F. R. Hartley, Supported Metal Complexes, D.
Reidel Publishing Company, Dordrecht, Holland
(1985),and literaturecited therein.
[3] J. Bliimel, Inorg.Chem.33, 5050(1994).
[4] K. D. Behringer,J. Bliimel, Inorg.Chem.,in press.
t5l R. K. Iler, The Chemistryof Silica;JohnWiley, New
York (1979).
[6] R. P.W Scott,SilicaGel andBondedPhases;John
Wiley & Sons,New York (1993).
[7] J. Bliimel,J.Am. Chem.Soc.ll7,2ll2 (1995),and
literaturecited therein.
tSl C. A. Fyfe, Solid-StateNMR for Chemists,C. F. C.
P-ress,
Guelph,Canada,(1983),and literaturecited
tnereln.
[9] G. Engelhardt and D. Michel, High-Resolution
Solid-StateNMR of Silicates and Zeolites, John
Wley, Chichester,U. K. 1987, and literaturecited
tnereln.
in
tl0l A. T. Bell and A. Pines,Eds.,NMR Techniques
Catalysis,Marcel Dekker,New York (1994).
[11] M. Ebener,G. von Fricks,H. Giinther,Helv. Chim.
Acta74, 1296(1991).
tl2l J. Homer, P. McKeown, W. R. McWhinnie, S. U.
Patel,G. J. Tilstone,J. Chem. Soc.,FaradayTrans.
8 7 , 2 2 5 3( 1 9 9 1 ) .
[13] K. Albert, B. Evers,E. Bayer,J. Magn. Reson.62,
428 fl985).
t14l E. Bayer,A. Paulus,B. Peters,G. Laupp,J. Reiners,
K. Albert,J. Chromatogr.364,25(1986).
[15] R. K. Gilpin, M. E. Gangoda,J. Magn. Reson.64,
408 (1985).
t16l R. K. Gilpin,Anal.Chem.57,1465(1985).
llTl R. K. Gilpin, M. E. Gangoda,Anal. Chem. 56,
1410 0984\.
tl8l R. K. Gilpin, M. E. Gangoda,J. Chromatogr.Sci.
2 r , 3 5 2( 1 9 8 3 ) .
[19] R. C.Zeigle1G. E. Maciel,J. Am. Chem.Soc.113,
[]
6349(r99r).
C. Zeigler, G. E. Maciel, J. Phys. Chem. 95,
t20l R.
'734s
(r99r).
C. A. Fyfe, J. Am. Chem.Soc.1.08,
t2ll E. C. Kelusky,
1146,1986\.
[22] S. Shinoda, Y. Saito, Inorg. Chim. Acta 63, 23
(1982).
l23l K. Tanaka,S. Shinoda,N. Takai, H. Takahashi,Y.
Saito,Bull. Chem.Soc.Jpn.53, 1242(1980).
l24l K. Tanaka,S. Shinoda,Y. Saito,Chem.Lett. 1979,
119.
l25l H. C. Clark, J. A. Davies,C. A. Fyfe, P. J. Hayes,
R. E. Wasylishen,Organometallics2, 177 (1983).
[26] S. J. Fritschel,J. J. H. Ackerman,T. Keyser,J. K.
Stille,J. Org. Chem.44,3152(1979).
l27l S. Shinoda,K. Nakamura,Y. Saito,J. Mol. Catal.
17,17 (1982).
R. J. M. Nolte, W Drenth,J.
t28l A. J. Naaktgeboren,
Am. Chem.Soc.102.3350(1980).
t29l N. Kawata,T. Mizoroki,A. Ozaki,Bull. Chem.Soc.
Ipn.47,1807(1914).
[30] Rotation (20 Hz) of the tubes did not enhancethe
resolutionof the signalsand thereforewas omitted,
as well as a teflonplug.
t3ll C. C. Ballard,E. C. Broge,R. K. Iler, D. S. StJohn,
J. R. McWhorter,J. Phys.Chem.65, 20 (1961).
[32] W. K. Lowen, E. C. Broge,J. Phys.Chem.65, 16
(1961).
t33l K. D. Behringer,J. Bliimel, in preparation.
t34l C. P. Slichter, Principles of Magnetic Resonance,
chapter5, Harper& Row,New York (1963).
L.
H. Merwin,A. Sebald,J. E. Espidel,R. K. Harris,
t35l
(1989).
J. Magn.Reson.84,367
t36l D. W. Sindorf,G. E. Maciel,J.Am. Chem.Soc.105,
3767(1983).
[37] R. K. Gilpin, M. E. Gangoda,J. Chromatogr.Sci.
28,277(1990).
K. Albert,K. D. Lork, K. K. Unger,H.
i38l B. Pfleiderer,
Briickner, E. Bayer, Angew. Chem., Int. Ed. Engl.
28,32;7(1989).
[39] "Handbookof Chemistryand Physics",eds.R. C.
Weast,S. M. Selby,The ChemicalRubber Co.,
Cleveland,Ohio (1967).