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I
RelativeSignsof the 3rP-rHand "P-C-'H
Coupling
NuclearMagneticResonance
Constants
By GeorgeM. Whitesides,JesseL. Beauchamp,and John D. Roberts
[Reprinted from the Journal of the American Chemical Society, E6,2065 (1963).]
1963 by the Americau Chemical Society aud reprinted
by permission of the copyright
Copyright
Relative Signs of the 31P-'H and 3tP-C-rH Nuclear
Magnetic Resonance Coupling ConstantsI
Sir:
The possibility that terms other than Fermi contact
interaction make significant contributions to nuclear
spin-spin coupling rvith heavy nuclei has stimulated
considerableinterest in the n.m.r. spectraof compouncls
containing heteroatoms. In particular, relative signs
and magnitudes of -/xcs and /xccn have beenreported
for ethyl derivatives of several heavy isotopesof spin
',1r.' In order to understand better the factors responsiblefor spin coupling rvith a representativeheavy
nucleus, we have ana"Tvzedthe spectra of (CH3)3P,
(C Hr ) r pH, and (C H r)PH : a n c l h a v e o b ta i n e clrel ati ve
signs and magnitudes of -/ps and /pcH.
The Jrroton spectrum of methylphosphine (Fig. l)
rnav be treated as the superpositionoi an ArB.:and an
ArX., spectrum, using the methocl of effectivechemical
shifts.r Inspection oi the spectrum suggeststhat the
i
I
I
I
i'
\^
\--/
I
t1
A
fii lt /l i1i i
n
tl
vl
L
i.
/l
r_;L
-,.----i"a-,l,\
I
,\,1
-_^_t)
I
lti
I
/\
"i,J
I
.--l
w--n-!
(upper; and calculated (lorver) proton
Fio. 1.-Obsen'ed
n.m.r. spectra of methylphosphine at 60 Mc.p.s. The spectral
parameters used in the calculated spectrum are given in Table I.
(1)
Sut:ported
in part
by
Science Foundatioo.
(2)
For
(1962);
(1961).
a surnmary
P. T.
and
Narasimhan
the
Office of Naval
Research
and
"{
Klose, Ann.
references, see G.
'f.
and NI.
Rogers.
J.
Chem.
the National
Physih,
Plys,
methvl region can be separatedinto a triplet at lorver
fielcl ancl a more complicated group of lines at higher
field. Associationof the triplet with the simpler half of
the P-H spectrum, also occurring as the lower fielcl
part, immediately suggeststhat -/pn and Jpcn have
the sarne sign. The spectrunr observeclfor CH,TPH:
ancl the spectrum calculated using the parameters irr
the table are shorvnin Fig. 1. In both methyl- and climethvlphosphine -/pH anci -/pcn have the same sign;
double-resonanceexperiments performed by Drs. G.
Juvi nal l and S . L. l l anatt of the Jet P ropul sionLabor atorv confirm this conclusion.
T.{sLe I"
Cireurc.r.r, Snrnrs ar*o CoupLrxc Coxsr,rNTS roR Prrospnrxn
DeRtvertves
C()rnp()und
Jptt
Jyr:6
/rtcl'tt
yg
y(tlfr
PHl"
i8l.l
-r-186.-l -+-+.1 3.1
CH,PH.1
I57.7
58.5
-!191.6
(cI-Ii),pH
*;1.6
7.;
I87.7
63 S
(cr-r3),P
) 7
56.3
o C h e n r i c a ls h i f t s a n d c o u p l i n gc o n s t s n t sr v e r e m o d e r a t e i l '
t e n r l ) r r l t l l r ct l e p e r r d e n tt:h e v a l u e ir e p o r t c da r e f o r 3 3 o . C h e r n i
c e l s h i i t sa r e g i v e ni n c . p . s a
. t t j O] I c . p . s .f r o m t e t r a m e t h l ' l s i l a n e
as internrl standard. ' R. IL Ll'nden-Bell,Trans.FaradaltSoc.,
5 7 ,E 3 8( 1 9 6 1) .
Anet'J has recentlv demonstrated by double-resonance
experinrents that /"cH and -/ncs have opposite sign in
HLDITCOH; Lvnden-Bell and Sheppard{ have reached
the same conclusion for /"cH and /ttccn
in Hrr3Cr3CH1 by analy'sis of the high resolution spectrum.r
In contrast, -/cn and -Ipcn have the Same Sigx. Theoretical treatments have indicated that contact interactiorr
rvill dominate the coupling of directly bonded atomsi;
more approKimate treatments of HCH6 and I3CCH7 ieacl
to the same conclusion.
The observ'ed difference in relative sign of /xn and
-Ixcn for X : rH, r3C and for X - 3rP suggests that
some mechanism other than the usual o-bond interactions contribute to spin-pin
coupling with phosphorus.
Spin-orbital
and electron-dipole interaction
would be expected to be relativelv more important in
I
/\
owncr.
g,:62
34,
10-19
( 3 ) F . A . L . A n e t , J . A n . C h e m . S o c . , 8 43, 7 6 7( 1 9 6 2 ) .
( { ) R . } [ . L y n d e n - B e l l a n d N . S h e p p a r d ,P r o c . R o y . S o c . ( L o u d o n ) . L 2 6 9 ,
3 8 , ' r( 1 s 6 2 ) .
(.t) lt. Karplus and D. \[. Grant, Proc. S*all.Acad. Sci. Lr.S.,46, l2tj0
( 1 9 5 9 ); N . \ [ u l l e r a n d D . E . P r i t c h a r d . J . C h e m .P h y s . , 3 1 , 7 6 8 , 1 4 7 1 ( 1 9 . - r s;)
J. \. Shoolery, ibid.,31, l{?1 (1959).
(6) H. S. Gutorvsky, \I. Karplus, and D.
1278 (19;tf), aod references therein.
(7) G. T. Kerabatsos,
1i;5; (l961).
J. D.
Grahem,
and
\I. Grant, J. Chcm. Phys., 31,
F. Vane, J. Phys. Chen., 65,
ColruuNrcATroNs To rHE Enrron
coupling to 3rP than to rH or r3C due both to the
greater anisotropy of the screeningaround phosphorusE
and to the relatively small degree of s-character of its
bonding orbitals. e However, Klose has been able to
justifl' the spectrum of (CHrCHr)3P semiquantitatively
bl- a valence-bond treatment using oniy contact interactions.2 Alternately, the d orbitals and nonbonding
electrous of phosphorus may be involved in coupling
through structures of the "ylene" type (I).to Spinorbital or magnetic-dipole interactions might give
H
\,/
H_C=P
,/\
(8) J. A.Pople, Mol. Phys.,1,216(1958).
(9) The bond aogles in methylphosphine suggest that tbe phosphorus
bonding orbitals iu this compound bave predomiuaotly p-character: ZCpH
: 97o30', ZIJPIJ = 93e23'; seeE. L. Breigand C. C. Liu, J. Chem. phys.,
36,2r39 (1961).
(10) This structure is analogous to the hyperconjugative structures used
to erplain long range coupling in h;rdrocarbons. Although uniml)ortant in
determining the total energy of tbe molecule, such structures may be very
important in spin-spin coupling. See R. A. Hofiman and S. Gronon,itz.
Arhia. Kcmi., L6, 471 (1960).
Vol. S5
JpcH the observed sign; important contributions from I
certainll' would. It is not possible to clecicle*'hich
factor is most important on the basisof availabie experimental evidence.
It is unfortunately not possibleto assignan abs.iute
sign to -Ipr.r,although such an assignmentrvould be of
considerable interest. Recent calculations for several
paramagnetic transition metal ions indicate that the
contributions of a spin-polarized 3s orbital to the contact term rnay be opposite in sign to that of a Is or es
orbitallr; this result suggestslhat JpH may be either
positive or negative, depending upon the relative contributions of the ls, 2s, and 3s electronsto the coupling.
Acknowledgment.-We are deeplv indebted to Oi.
F. C. Caserio, Jr., for the phosphinesamplesused in
this research.
( 1 1 ) V . H e i n e , P h y s . R e t . , 1 0 ? , 1 0 0 2 ( 1 9 5 7 ) ; R . E . \ \ , ' a t s o na n d A .
J.
Freeman, ibid., LzO,1125,1134(1960).
(12) National Science Foundation Undergraduate Researcb participant,
I 962.
Coxrnrsurlox 2967
GroRoB M. WHrrBsroes
GerBs aNn CRer-Lrx LasoneroRrEs or Cue:rssrRv
Cer-rronxra IxsrrrurB on TBcnNoI-ocy Jessn L. Bneucu.aMpr3
Pasaoewa, CalmoRNra
Jous D. Roeenrs
RBcnrvBo Jr,'lr' 1, 1963