Tetraaryl-methane analogues in group 14 - V. Distortion of tetrahedral geometryin terms of through-space π-π and π-σ interactions andNMR sagging in terms of π-σcharge transfer

Article abstract of DOI:10.1016/S0277-5387(98)00256-3

44 members of thecompound series Ph_4-nMR_n (M=Si, Ge, Sn, Pb; R=o-, m-, p-Tol; n=0-4) were synthesized (15 newcompounds). The crystal structures of Ph₃Sn (o-Tol) and PhSn (o-Tol)₃ were determined and compared to 16 known structures. Subject to the distanced (M-C), an interplay between through-space π-π repulsion and π-σ attraction leads to either elongated or compressed tetrahedral geometry. ²⁹ Si-, ¹¹⁹ Sn- and ²⁰⁷ Pb-NMR chemical shifts were determined in solution and in the solid state. ⁷³ Ge chemical shifts were measured only in solution. Anupfield or downfield sagging of the chemical shifts along each series is rationalized in terms of a π-σcharge transfer which is constrained by torsion of the aromatic groups.

Full text of DOI:10.1016/S0277-5387(98)00256-3

Polyhedron Vol[ 06\ Nos[ 14\ 15\ pp[ 3386Ð3495\ 0887
Þ 0887 Elsevier Science Ltd
Pergamon
All rights reserved[ Printed in Great Britain
9166Ð4276:87 ,08[99¦9[99
\
PII] SS9166!4276"87#99145!2
Tetraaryl!methane analogues in group 03*V[
Distortion of tetrahedral geometry in terms of
through!space pÐp and pÐs interactions and
NMR sagging in terms of pÐsꢀ charge transfer
Michael Charissea\ Andrea Zickgrafa\ Heike Stengera\ Elmar Braua\
ꢀ
Ã
Cristelle Desmarqueta\ Martin Dragera\ ꢁ\ Silke Gerstmannb\
Ã
Dainis Dakternieksb\ꢁ and James Hookc
aInstitut fur Anorganische Chemie und Analytische Chemie der Johannes Gutenberg!Universitat\
Ã
Ã
D!44988 Mainz\ Germany
bSchool of Biological and Chemical Sciences\ Faculty of Science and Technology\ Deakin University\
Geelong\ Vic[ 2106\ Australia
cNMR Facility\ The University of New South Wales\ Sydney\ NSW 1941\ Australia
"Received 13 February 0887^ accepted 12 June 0887#
Abstract*33 members of the compound series Ph3−nMRn "MꢂSi\ Ge\ Sn\ Pb^ Rꢂo!\ m!\ p!Tol^ nꢂ9Ð3# were
synthesized "04 new compounds#[ The crystal structures of Ph2Sn"o!Tol# and PhSn"o!Tol#2 were determined
and compared to 05 known structures[ Subject to the distance d"MÐC#\ an interplay between through!space
pÐp repulsion and pÐs attraction leads to either elongated or compressed tetrahedral geometry[ 18 Si!\ 00 8 Sn!
and 19 6 Pb!NMR chemical shifts were determined in solution and in the solid state[ 62 Ge chemical shifts were
measured only in solution[ An up_eld or down_eld sagging of the chemical shifts along each series is rationalized
in terms of a pÐsꢁ charge transfer which is constrained by torsion of the aromatic groups[ Þ 0887 Elsevier
Science Ltd[ All rights reserved
Keywords] silicon^ germanium^ tin^ lead^ through!space interaction^ heteronuclear NMR in solution and solid
state^ pÐsꢁ charge transfer[
———————————————————————————————————————————————
INTRODUCTION
the solution state18 Si!\ 00 8 Sn! and19 6 Pb!NMR chemi!
cal shifts was rationalized by means of a pÐsꢁ charge
transfer from the aromatic groups into the LUMO
associated mostly with the hetero atom M ð0Ł[
As part of ongoing studies about tetraarylmethane
analogues ð0Ð3Ł\ the series of compounds Ph3−nM"p!
Tol#n "MꢂSi\ Ge\ Sn\ Pb^ nꢂ9Ð3# was synthesized
and investigated using X!ray di}raction and NMR
spectroscopy ð0Ł[ The structural _nding of elongated
tetrahedra for MꢂSi and Ge and compressed
tetrahedra for MꢂSn and Pb "see Ref[ ð4Ł# was ration!
alized by means of an interplay between pÐs attrac!
tions and pÐp repulsions of the aromatic groups ð0\ 5\
6Ł[ The sagging e}ect "see Refs[ ð7Ð09Ł# observed in
In this paper the subject is extended synthetically
to the groups o! and m!tolyl ðfour series of compounds
Ph3−nMRn\ "0Ð3#Ł[ In addition to the 05 crystal struc!
tures known previously ð0Ð3Ł\ the structures of
Ph2Sn"o!Tol# and PhSn"o!Tol#2 were determined[ Het!
eronuclear NMR chemical shifts in solution and also
in the solid state were determined for all compounds
of series eq[ "0#\ eq[ "2# and eq[ "3#[ In addition\62 Ge!
NMR chemical shifts for the _ve compounds of series
eq[ "1# in solution are given ð00Ð05Ł[ Semi!empirical
MO calculations have been done to support the con!
cept of a pÐsꢁ charge transfer[ As will be seen from
ꢁ Author to whom correspondence should be addressed[
Tel[] ¦38!5020!284646^ Fax] ¦38!5020!284279[
3386

3387
M[ Drager et al[
Ã
the results\ these exhaustive calculations concluded to angles are listed in Table 0[ Both molecules possess
be altogether unrewarding and fruitless[
crystallographically a S3 symmetry\ i[e[ the ortho
methyl groups exhibit a statistical disorder[ One of
the four possible ordered arrangements is given in Fig[
0\ respectively[
Ph3−nSiRn Rꢀo−Tol\ m−Tol\ p−Tol nꢀ9Ð3[
"0#
"1#
Ph3−nSnRn Rꢀo−Tol\ m−Tol\ p−Tol nꢀ9Ð3[
"2#
Ph3−nPbRn Rꢀo−Tol\ m−Tol\ p−Tol nꢀ9Ð3[
"3#
As outlined for tetraaryls with phenyl and p!tolyl
substitution ð0Ł\ the tetrahedra with MꢂC\ Si\ Ge are
elongated "two bond angles less and four bond angles
greater than 098[4># whereas the tetrahedra with
MꢂSn\ Pb are compressed "two bond angles greater
and four bond angles less than 098[4>#[ This result is
valid also for the cases of o!tolyl and m!tolyl substi!
tution[ Figure 1 gives a plot of d"MÐC# vs the twofold
angle F"CÐMÐC#Ðð1xŁ for all 07 cases Ph3−nMRn\
which have been investigated by X!ray crys!
Ph3−nGeRn Rꢀp−Tol nꢀ9Ð3[
Ä
RESULTS AND DISCUSSION
Structural results
tallography[ At d"MÐC#ꢂ1[95 A "cubic _t curve# an
overall repulsion between the aromatic groups trans!
forms into an overall attraction[ For short MÐC dis!
tances the through!space pÐp repulsion predominates^
Figure 0 shows the molecular structures of Ph2Sn"o! for long MÐC distances the through!space pÐs attrac!
Tol# and of PhSn"o!Tol#2[ Relevant bond lengths and tion prevails[
Fig[ 0[ Ortep drawings of Ph2Sn"o!Tol# "top# and of PhSn"o!Tol#2 "continued#[ Only one of the four possible ordered
arrangements is given\ respectively[ Thermal ellipsoids are at the 19) probability level^ hydrogen atoms are omitted for
better clarity[ The molecules are compressed along the outlined pseudo S3 axes[ The group identi_cations include the
¹
symmetry operators of the space group P310c[

Tetraaryl!methane analogues
3388
Fig[ 0[ Continued[
Table 0[ Bond lengths "A# and bond angles "># of Ph2Sn"o!Tol# and PhSn"o!Tol#2 and of Ph3Sn and Sn"o!Tol#3a for comparison
Ä
¹
"all compounds crystallize in the space group P310c#
Compound
Distance SnÐC"0#
Two!fold angle
C"0#ÐSnÐC"0a#
F"CÐSnÐC#1×
Four!fold angle
C"0#ÐSnÐC"0b#
F"CÐSnÐC#3×
Ph3Sn
1[028"3#
1[98"3#
1[046"7#
1[041"4#
000[1"1#
000"1#
001[7"3#
001[5"2#
097[5"0#
098"0#
096[7"3#
096[8"0#
Ph2Sn"o!Tol#
PhSn"o!Tol#2
Sn"o!Tol#3
a See Ref[ ð06Ł[
NMR chemical shifts
Table 2 gives the solid state18 Si\00 8 Sn and19 6 Pb chemi!
cal shifts of the compounds of the series eqs[ "0\ 2Ð
Table 1 gives the heteronuclear NMR chemical 3#[ Figure 3 shows exemplarily for MꢂSn the six
shifts for the homoleptic and heteroleptic tetraaryls sequences of d"00 8 Sn# in the solution "top# and in the
Ph3−nMRn in CDCl2 solution[ The series eq[ "1# is the solid "bottom# state[ The values are connected by
_rst systematic investigation of 62 Ge!NMR chemical cubic _t curves[
shifts\ which has been described "Figure 2# ð00Ð05Ł[
With regard to the aliphatic compounds Me3M\

3499
M[ Drager et al[
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Ä
Fig[ 1[ Plot of the bond distances d"MÐC# "A# vs the two!fold bond angle F"CÐMÐC#!1× "># for 07 compounds Ph3−nMRn
"MꢂC\ Si\ Ge\ Sn\ Pb^ Rꢂo!\ m!\ p!Tol^ nꢂ9Ð3# in relation to the distortion of the tetrahedra around M[ Data from Ref[
Ä
ð0Ð3Ł and references cited therein[ Cubic _t curve "change of distortion at 1[95 A#[
Table 1[ Solution!state 18 Si!\ 62 Ge!\ 00 8 Sn! and 19 6 Pb! NMR chemical shifts
"ppma# for the series Ph3−nMRn eqs[ "0Ð3#
M
Si
R
Ph3M
Ph2MR
Ph1MR1
PhMR2
MR3
o!Tol
m!Tol
p!Tol
p!Tol
o!Tol
m!Tol
p!Tol
o!Tol
m!Tol
p!Tol
−02[87
−03[28
−03[14
−03[18
−21[38
−015[6
−029[1
−018[0
−069[3
−070[4
−068[4
−02[89
−03[13
−03[25
−20[60
−013[6
−029[0
−016[6
−050[1
−079[8
−065[2
−02[31
−03[12
−03[31
−29[66
−010[6
−029[9
−015[0
−041[8
−079[2
−063[9
−01[41
−03[23
−03[44
−20[13
−013[4
−018[7
−013[5
−055[6
−068[4
−060[2
Ge
Sn
−21[89
−017[0
Pb
−068[9
a Solvent CDCl2^ relative to the external standards Me3Si:Ge:Sn:Pb\
respectively\ ambient temperature[
the chemical shifts d"18 Si:62 Ge:00 8 Sn:19 6 Pb# of the homoleptic end members Ph3Sn and SnR3\ a sagging
homoleptic members Ph3M and MTol3 are all up_eld pattern ð7Ð09Ł holds[ The sagging e}ect is di}erent
and this shielding had been explained by a p"aro! for the solution and for the solid state[ Furthermore\
matic#:sꢁ"M# charge transfer0 ð0Ł[ Between the the e}ect changes from the "o!Tol# to the "m!Tol#
to the "p!Tol# series[ A possible explanation may be
0 A seeming exception is Ph3C with d"02 C#ꢂ¦54[9 ppm
"Me3C¦17[9 ppm#[ The central carbon atom has four short
distances "cf[ Figure 1# to the deshielding side!on position of
the aromatic ring current ð0Ł[
that pÐsꢁ charge transfer depends on the torsion of
the aromatic groups about the MÐCipso axis[ In
solution the torsion angles easily arrange themselves\
such as\ to optimize the energetic interactions among

Tetraaryl!methane analogues
3490
In accordance with the similar molecular forms of
the compounds with MꢂSn and Pb "see Section struc!
tural results#\ the three solution state d"19 6 Pb# sagging
curves are nearly identical to those of d"00 8 Sn# "Table
1 and Fig[ 3#[ Also the three solid state d"19 6 Pb# sag!
ging curves show a close correspondence to those of
d"00 8 Sn#[ Similar torsions of the aromatic groups and
similar crystal structures can be assumed[
In solution the d"62 Ge# and d"18 Si# sagging curves
show similar trends to those described for d"00 8 Sn# and
d"19 6 Pb#[ The shortage of charge "down_eld shift# is
enhanced for the case of the sterically very demanding
o!tolyl group at Si[ However the known solid state
structures of the Si series are di}erent and this fact
manifests itself in sagging curves de_nitely di}erent
from those of Fig[ 3[
Semi!empirical MO calculations
To support the concept of a pÐsꢁ charge transfer\
semi!empirical MO calculations at the PM2 level
"MOPAC 5[9 program package\ parameter set] ð07\
08Ł "all 33 compounds Ph3−nMRn\ molecular geo!
metries optimized starting from the known crystal
Fig[ 2[62 Ge!NMR spectra for the series of compounds Ph3−n!
Ge"p!Tol#n in CDCl2 solution[ Chemical shifts are given with
reference to Me3Ge[
structures# and at the Extended Huckel level ð19Ł
Ã
"PhSn"o!Tol#2\ _xed geometry of the structure deter!
mination# were performed[ The results are similar for
all compounds[ On the basis of the main coe.cients\
the energetic sequences of the MO|s are in the order
eq[ "4#[ With regard to this order\ a de_nite higher
transition probability is needed for the assumed pÐsꢁ
charge transfer "high energy gap# than for a pÐpꢁ
charge transfer "low energy gap# ð10Ł "A quantitative
correspondence between the PM2 energy di}erences
and the energies of the experimental UV bands
between 139 and 169 nm does not exist[#
the four aromatic groups whereas in the solid state
the torsion angles are constrained by the crystal
packing ð6Ł[ In solution\ the sterically undemanding
groups m!tolyl and p!tolyl lead to an up_eld sagging
"excess of charge\ with regard to a linear course#[
On the other hand\ the sterically crowded o!tolyl
group leads to a distinct down_eld sagging "shortage
of charge#[
"several p−HOMO?s#:"several p(−LUMO?s#
:"four s(−LUMO?s#[ "4#
Table 2[ Solid!state 18 Si!\ 00 8 Sn!\ and 19 6 Pb!NMR chemical shifts "ppma# for
the series Ph3!nMRn eqs[ "0\ 2Ð3#
M
Si
R
Ph3M
Ph2MR
Ph1MR1
PhMR2
MR3
o!Tol
m!Tol
p!Tol
o!Tol
m!Tol
p!Tol
o!Tol
m!Tol
p!Tol
−03[96
−03[97
−01[70
−03[59
−012[0
−019[1
−008[8
−035[8
−036[7
−034[8
−06[90b
−02[15
−02[64
−011[0
−004[1
−006[6
−040[0
−035[3
−031[9
−03[68
−01[54
−02[37
−012[5
−000[3
−007[7
−049[1
−017[9
−049[9
−02[26
−01[92
−02[20
−016[3
−096[4
−007[7
−048[4
−008[2
−037[7
Sn
Pb
−010[0
−035[6
a Conditions as given in Section 2[
b Additional trace signal at −03[6 ppm[

3491
M[ Drager et al[
Ã
Fig[ 3[ Course of 00 8 Sn!NMR chemical shifts vs n for the series of compounds Ph3−nSnRn "Rꢂo!\ m!\ p!Tol^ nꢂ9Ð3# in
CDCl2 solution "top# and in the solid state "bottom#[ Cubic _t curves[
In detail\ all PM2 calculations result in the energetic
sꢁÐLUMO sequences eq[ "5#[ With regard to the
NMR behavior of eqs[ "0Ð3#\ no de_nite order or
E"Si#×E"Pb#×E"Sn#×E"Ge#[
"5#
correlation exist with E"sꢁÐLUMO# or with its di}er! In fact\ it can be concluded that semi!empirical cal!
ence to the pÐHOMO\ apart from the fact that the culations have to lower a sophistication\ to be of any
MO sequences are most disturbed if the group "o!Tol# purpose in discussing the postulated pÐsꢁ charge
is involved[
transfer[

Tetraaryl!methane analogues
3492
EXPERIMENTAL
of the small number of re~ections of signi_cant inten!
sity\ only the Sn atom was given anisotropic thermal
parameters\ the C atoms were handled isotropically[
The hydrogen atoms were calculated as riding on their
Synthesis
The 03 homoleptic compounds and the 01 het! carbon atoms[ For calculations and drawings local
eroleptic p!tolyl compounds were prepared by litera! versions of SHELX!65 and ORTEP were used[ Tables
ture methods ð0Ð3Ł[ For previous work on the o!tolyl listing details of crystal data and structure deter!
substituted silicon compounds\ see Ref[ ð11Ð14Ł[ The minations\ full sets of parameters "Sn\ C and H#\ all
mixed ortho! and meta!substituted tin and lead com! bond lengths and angles and torsion angles and lab!
pounds Ph3−nM"o!\ m!Tol#n "nꢂ0Ð2^ MꢂSn\ Pb# elled plots have been deposited at the Cambridge
were synthesized via a Grignard reaction with the Crystallographic Data Centre as supplementary
corresponding halides Ph3−nMXn "XꢂCl\ Br\ I#[ material[
Compounds of the series Ph3−nSiM"m!Tol#n "nꢂ0Ð2#
were obtained by reaction of Ph3−nSiCln with Li"m!
Tol#[ A representative detailed procedure follows for
Solution state NMR
the compound PhGe"p!Tol#2\ which had been
18 Si!\ 00 8 Sn! and 19 6 Pb!NMR spectra were recorded
described in Ref[ ð0Ł as contaminated with Ge1"p!
on a Bruker WP 79:DS instrument "digital resolution
Tol#5[ To 04[5 mmol of "p!Tol#Li in 49 ml ether is
9[4 Hz# at 04[81 MHz "18 Si#\ 18[77 MHz "00 8 Sn# and
slowly added\ at ambient temperature 0[91 g "3 mmol#
05[63 MHz "19 6 Pb#[ Solution state 62 Ge!NMR spectra
of freshly distilled PhGeCl2\ dissolved in 49 ml ether[
were recorded on a Jeol JNM!GX 169 spectrometer
After 0 h under re~ux\ the ether is distilled o} and
" frequency 8[2 MHz\ sweep width 1999 Hz\ acqui!
substituted continuously by toluene[ After further stir!
sition time 1 s\ scans 0499#[ The chemical shifts are
ring for 3 h under re~ux and for 19 h at ambient tem!
relative to the external standards Me3Si\ Me3Ge\
perature\ the solution is hydrolyzed by slow addition
Me3Sn and Me3Pb[ Solutions of 099Ð399 mg of com!
of 0 M HCl[ The organic layer is dried with Na1SO3\
pound:2 mL of CDCl2 were used[
the solvent distilled o} and methanol added[ The
crude product is separated by _ltration\ washed with
cold methanol and recrystallized from methanol: Solid state NMR
toluene "2]0#[ Table 3 summarizes the results obtained
in this study[
18 Si!\00 8 Sn! and19 6 Pb!NMR spectra were measured
Melting points were determined in glass capillaries on a Bruker MSL!299S spectrometer operating at
in a Ko~er melting block[ Elemental analyses "C and 48[530\ 000[811 and 51[44 MHz\ respectively[ Samples
H# were obtained from the Institut fur Organische of 299 mg of the organosilicon and lead compounds
Ã
Chemie\ Universitat Mainz\ with a Perkin!Elmer were packed into 6 mm zirconia rotors and spun at
Ã
CHN!Analyser 139[
the magic!angle\ at speeds of 1[4 kHz[ The organotin
compounds "ca[ 099 mg# were measured in 3 mm zir!
conia rotors at 4 kHz[ Spectra were obtained with
broad!band double air!bearing cross!polarization
Bruker WBÐBL probes\ at ambient temperature
Crystal structure determinations
Colorless single crystals of Ph2Sn"o!Tol# and "184 K# from single contact cross!polarization "CP#
PhSn"o!Tol#2 were obtained by slow evaporation of a experiments[
CDCl2 solution[ As previously ð0Ð3Ł\ the crystals of
Solid state18 Si!NMR] spectral width\ 19 kHz^ pulse
the heteroleptic compounds were all of a poor quality[ width\ 4[4 ms "89>#0H pulse^ contact time\ 0 ms^ recycle
An inspection by means of Weissenberg exposures time\ 19 s^ chemical shifts with respect to kaolinite
was essential[ Both eventually chosen crystals had a "d"18 Si#ꢂ−80[4 ppm "Q2 connected Si# ð15\ 16Ł^ set up
low di}raction power[ A summary of crystal data\ of the CP conditions and use as an external standard[
intensity data collections and re_nements\ is given in Solid state 00 8 Sn!NMR] spectral width\ 22 kHz^ pulse
Table 4[ The crystals were _xed with glue and sealed width\ 3[9 ms "89># 0H pulse^ contact time\ 09 ms^
in thin!walled glass capillaries[ The experimental den! recycle time\ 09 s^ chemical shifts with respect to Sn"c!
sities were determined by ~otation in an aqueous poly! Hex#3 "d"00 8 Sn#ꢂ−86[3 ppm ð17Ł^ set up of the CP
tungstate solution[ Integrated intensities were conditions and use as an external standard[ Acqui!
collected on an Enraf!Nonius CAD3 di}ractometer[
sition of 017 scans was su.cient to obtain spectra
Both compounds crystallize in the same tetragonal with a satisfactory signal to noise ratio[ Solid state
space group P310c as their homoleptic end members 19 6 Pb!NMR] spectral width\ 51[4 kHz^ pulse width\
¹
"see the _rst footnote of Table 4#[ Trials of re_nement 2[4Ð3[9 ms "89># 0H pulse^ contact time\ 4Ð19 ms^ recy!
in a monoclinic setting resulted in distinctly worse cle time\ 29 s^ chemical shifts with respect to Pb"p!
geometries and esd|s[ The space group symmetry Tol#3 "d"19 6 Pb#ꢂ−037[7 ppm ð18\ 29Ł^ set up of the
requires a S3 symmetry for both molecules\ i[e[ the CP conditions and use as an external standard#[
ortho!methyl groups are statistically disordered "site Acquisition of 53Ð017 scans was su.cient to obtain
occupations 9[14 and 9[64\ respectively#[ On account spectra with a satisfactory signal to noise ratio[

3493
M[ Drager et al[
Ã

Tetraaryl!methane analogues
3494
Table 4[ Summary of crystal data\ intensity data collection "Mo radiation\ ambient
temperature# and re_nement for Ph2Sn"o!Tol# and PhSn"o!Tol#2
Ph2Sn"o!Tol#
PhSn"o!Tol#2
¹
¹
Cryst[ syst[\ space group
tetragonal\ P310c
tetragonal\ P310c
a
Ä
Unit cell dimen[] a\ c "A#
01[991"2#\ 6[193"3#
0927"1#\ 1\ 333
0[300\ 0[396
9[484
01[921"0#\ 6[742"1#
0025"0#\ 1\ 365
0[261\ 0[261
9[693
2
Ä
V "A #\ Z\ F"999#
Dcalcd\ Dexptl "g cm−2
sin umax:9[60958 "A
#
−0
Ä
#
Re~[] meas[\ indep[ "int[R#
Re~[] used\ limit
Variab[\ ratio re~[:var[
Final R
2585\ 656"9[091#
162\ I×0 s"I#
25\ 6[5
2553\ 0239"9[915#
571\ I×0 s"I#
24\ 08[4
9[9804
9[9431
a
2
Ä
Ä
Ph3Sn] aꢂ01[947"0#\ cꢂ5[470"0# A\ Vꢂ846 A ^ Sn"o!Tol#3] aꢂ01[910"0#\ cꢂ7[943"0#
2
Ä
Ä
A\ Vꢂ0053 A "see Ref[ ð06Ł#[
Acknowled`ements*We gratefully acknowledge _nancial 00[ Harris\ R[ K[\ Kennedy\ J[ D[ and McFarlane\
support from the Deutsche Forschungsgemeinschaft\ Bonn\
Germany and the Fonds der Chemischen Industrie\ Frank!
furt:Main\ Germany[ C[ D[ thanks the European Com!
munity for a grant under the Erasmus scheme[
W[\ First tabulation of 62Ge NMR data\ In NMR
and the Periodic Table\ ed[ R[ K[ Harris and B[ E[
Mann[ Academic Press\ London\ 0867\ p[ 239[
01[ Kennedy\ J[ D[ and McFarlane\ W[\ tabulation of
62Ge NMR data\ In Multinuclear NMR\ p[ 209\
Ed[ Mason\ J[\ Plenum Press\ New York\ 0876[
02[ Kennedy\ J[ D[ and McFarlane\ W[\ In Multi!
nuclear NMR\ ed[ J[ Mason\ Plenum Press\ New
York\ 0876\ p[ 209[
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