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A method for polarizing protons by proton-chlorine rotating frame cross-relaxation in high magnetic fields
Volume 32,.~umber 3 :_
.’
: .. ‘(.’
:
._, “. .:.’
.,
1 Kay 1975
PHYSICS LEI%RS :
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:
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,.‘.’ ‘. ”‘,.
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& STOOD FOR ?O~Z~~ ~ROT~~~ &’ ~~~~~ __gfjJ&& C CROSS-REUTION IN HIGH MAGNETIC FIELDS * ‘.
RDTAT~G
FI&&iE
:
WS: VEE&
** and C.S.’ YAKTONI
IB~~~~~~~~~
’ ,.
.’
_:
,.
.’
San Jose, California 95193, USA
Ladamiary,
‘Re+ivcd 10 &emb&‘1974 ‘.. Revisedmanuscrijt received 13 January 1975 : -.
:.
: f
:
.’ ._
.’
.I ‘_, ., . . A Ned tecJ&ique is demonstrated f& poiaking protons in high dc magnetic fields, ‘YE&methorf has been used sucessfully &I polycrystalline samples at an arbitrary dc mame,tic Geld strcngtb w&out recourse to rotation . . .‘(‘, of the Sam@? or cycling to low.or zero BeId,
..
: 1. ~~oduc~on
Ti (for purposes of this ~s~ussio~, it is assumed that. ._ 1~=I~=!/2). Until now, tiowever, this technique reGenerally, the d&e&on of rare spin species in sol; quired slow rotation of the sample in a’magnetic Reid. ids by nut&adouble xesonance requires the contic& of @ecified strength f5] , or removal of the sample to of abu~d~~ spins (A> in an ordered state in the’rotazero or very low field: [rj,?]. fin this note we report a ting f&e v&h rare spins (B) El]_ If, prior t& this conmethod for crc)s-reltig A spins~with Q spins at any ‘- malefic field strength, withaut the necessity far metact, the B spins are heated, tben~during the contact, char&ally moviug the sample, A~pin magnekation will be ‘destroyedland B-spin ‘. ~~etization built up [Z] . The B-spin’s_~c~rum can :_ then be mapped out either by measuring the.A-spiri ,[1,2] or:B-spin ma~eti~tion 131. The ordeied state “. Z!..Wethod .’ of the A spins usually is a spin-locked [l] or adiabati-” .. : k&y demagnetized state in the rotating frame ..JTp idea for our method stems in part from a pre-, (ADRF) [4J khich 1asts.a time T1, or Y“,,; respecvious experiment carried out at zero magnetic field tively. To achieve
a
satisfactory
E-spin sisal,
it often’
[7]Jn
'is micewirytorepeat thicontact many~ties; tberea fore, except in ~o~e.favor~ble oases where the TIP or 5”‘@ very long, one’has to ie@$arize the A sp’insvia spi&attice* relaxation after a sm&.i number of dontacts. This can be very time-consuming .drh&nthe spin- -.’ lattice relaxation time ?jk. is very long., .: ‘,, & varietl of techniques can be used to ~ir~~rn~ent ” ‘this diffrc;iIty,‘onc of the most successful being rep& c ‘, ‘, lariialian.o[the d spins by cross-relaxation with: ;i : ” : &jrd .I: i.midI~~ qu&upolar, nucleus (Q) wifh a she+ : ;..
t&
ca513,the Q spin-resonance
I?+ is narrow
and the entire Qspiri system ‘can Mocked along a' strong rnonoc~orna~~ rf l%fdH1q. ~ra~-rel~ti~~ then occurs bets&n Qspins in the interaction repre&n&&ion and’A s&s in the dipolar state in zero magneticfieId The s,ample is then returned to high field .for detection of the B spins via contact with the now-
.polatized A s&ns. ,. The &tigti of noetic
fIeId stretch
te‘chniq~e somewhstcumbersome
makes this
and one would !ike
to bb,able to do,#is experiment at high magnetic Geld being fdrced to go to zero field. l=Iowe%rr;-in ‘2 . . ,:, .higb field; the. $uad&pofe resonance of the Q s@ns is . .‘. .,; ” ,, * $,uuppozted by the U.S. Armi k&$rch O&k. ; ** P&sent address: Labortltbriu~‘vdot‘Fysiscfie Chemie,‘, ,“. ‘, :.b;oade’ned c~~~~er~bi~ by Zeeman interactions, :’ ToBmooiveld, Nijmegen,Thc Net.hBr&nds,., : : ,: only a neg.”_’ ., es@iaIiy in’ golycrystaIline samples, and .,I . . : . _..I .,. .,., ‘. I., .‘.‘. .. _,. ; : . . : _’ ‘.I .: ~. .’ ‘. ‘.‘, ” 499 .. ,: ..’ ;_ ..’ . . . . ‘., _ ‘. ;. : _. ‘I ,. . .:.:. ,I“ .). :; 1, .,; ,, ,, ‘. ‘., ,‘I. ;; .:-.: ‘.” _.._.,,,, . . .:. _.‘_:_,,’ ” .’ ., ‘,. ,“,. .;
‘:
.(’
(., “.
:
,: ,’
:
withti~t
Volume‘32, number 3 :
,‘.,
,_ i.
CHEM~CALPHYSICS LkTERS
1 hl,ay~1975,
Proton pblarization was achieved in ihe following polycryst&ine samples at 77K: paradichlorobenz&e, 2,5- nnd 2,6dichlorqbenzoquinone, lJ,5-trichloro&ikne and ! .2,4,51tetiachlorobenzene: TheTI of, HIA
:
., ,.
,-
Receiver(A) Time Fig 1. P&e ,$~ough
se&mce
used to
LI lb’
-
polarizeprotonsby-sweeping
the %I raonance. The dashed cu:Jes show the ex-
periment when the sweep direction is reversed. legible fraction of the Q spins can be brought -tinto thermal contact with be A spins by locking with a monocl?Jqmatid rf field. A method which:u:es level ‘crossing in the laboratory frame overcom& this problem, but dictates the field strkngth at which the tiR experiment cari be performed [S]. In the.technique reported here, the entire Q spin reservoir can be coupled to the A spins at tiy field s’uength merely by sweeping HIQ through the Q spin resonance line IvKle dkngnotting to the A spins. During rile sweep,the &spin dip013reservoirbecornes.ordered, and this order is subsequently transferre’d io.the A-spin Zeeman reservoir in the rotating frame by adiabaticdy applying an rf field HlA on resonance [4]. I+ the experiments reported here, the magnetization along HIA is measured by turning off HlA suddenly and measuriag
the initial amplitude
of the-free in-
the protons in these compounds at 77K is very long (for paradichlorobe&ene we measured 4 -C1 hour), but using the above technique, we were able to polarize tl~ protons to an appreciable amount in only fivk seconds! The initial amplitude of the FID after turning &f the magnetizing field HlA is about 50% of the initial amplitude of the FID following a rr/2 pulse after the protons have come into equtibrium with the lattiti.
The direction of the sweep affects the sign of the magnztization: after the “Cl spkctrum is swept from low to high frequency (fig. 1) then a proton polarization gows alongHI,, cotiesponding to a positive spin temperature of ‘Lheproton bipolar reservoir. However, when the sweep direction is reversed (dashed curve in fig. 1) the magnetization builds up antiparallel to HIA, which means that the proton dipolk reservoir kmperature
must havi been negative
[4].
Preliminary results also indicate that (i) the degree of polarization HIQ, provided
is relatively insensitive to the value of t.hatHlq is beater than ==7 G (rotating
component) - the largestvalueof HI, unedwas kl5 G; (ii) repetition of the sweep results in increased polarization -about 90% of the full proton Boltz,nann polarization was achieved with six con.’ secutive 7-second sweeps; (iii) no significant increase in polarization resulted from long (B 15 S) sweeps. A comp’letely equitient proton magnetization has been
duction decay(FID) of the A spins*, ,Thepulse se-
achie\fedwhen, duringthe sweepthrough the Cl sgecl
quence used is shown in fig. 1. In our experiments the Qs ins are 35Cl nuclei and the A spins’are protpns. The R Cl nuclei are also intioived, but give a smaller effect because of their lower abundance; in ordei to keep the discussion simple,’ f$e effect of 37Cl nuclei will be neglected.
trum, the 4 spins are either locked along HlA or have been subjected to an ADRF;
..
: l An equivalent signal is obtained when Lhe magnetization is .Y xun@edin ‘he laboktory frame by going adiabatically off resonance kfok turning HI k off, iher, returning io reso-mmCea@tibsetig the FID.follcwi$ a 742 pu&e. _. ‘. .‘-, ‘s& :.., .~’ : _, .-_ : ~‘. ,. ..:- : . .. . . .’ : ,I,“. :_ .,
4. Experimelital details Paradichlorobenzene (Matheson, Col&man and Bell) was iefluxed over Al+, (using n-pentane as a solvent), then sublimed at 5OOCtid zone refined three times., The 6 ther mat,erials were cotiercially available (E-X&m Kod?k) and ,were used,wfthbut further pm-i; fixation.
: Coil syitei.operating
A&&
at 23 MHz (5400 G) ‘.
.’ :
‘,.’ ,: .- :.,
.,
.‘,‘,, ., ;, :
::
.,.
. . .‘,
..,’
: . .‘,
Volume 32,,tiumber 3
CI3E?t%ICiiL PHYSICS
:
LEFRS
1 May 1975
.’ was used to magrktize &td detect the polarized pro-
tons. The 35& nuclei were excited by an rf field pro- : c@ed in a saddle-shaped coil with its axis p&allel to the e’xtemal dc field. The Coil was driven by a pushpull oscillator; the amplitude of Nlrr was kept conslant over the sweep _range (2731 MHz) by continGously tuning the +_a& citctit WitA a rn~tor.d~ve~ capacitor. Ail me~ureme~ts were made at 77K. ’
5. Discussion Several experiments have been reported in which (1) polarizdtion is transferred from.one spin system to anothei under the effect of radiation, and (2) sweep reversal could affect $e sign of the induced polar&ation. Roughly speaking, these experiments fail into. two categories, depending on the state of the,spin systern (in our case, the Q spins) from which poIarization is transferred. If these spins are in equilibrium with the combined effect of rf irradiation and spin-lattice relaxation, then transfer of polarization to another sfin system can occur through the solid effect iri the rotating frame [ 81, and thermal mitig by con~uous off-resonance irradiation 161. lf, however, the Q spins : are in a quasi~q~briu~ (e.g., sp~.locked oi AI)RF) state, then thermal mixing between spin systems can atso occur [‘?I. We believe that our experiment resembles the latter.
For para~c~orobenzene at 7X, the spin-lattice relaxation times in the laboratory frame and spinlocked state are 390 and 45 ms respectively, while the time cans”%.&for %LIH cross relaxation is ~2 rns. [7]. From a measurement of our sweep rate, we esti; mate that we spend less than a, cross-relaxation
time
~~sferred into the proton Zeeman reservoir in the rotating frame&y adiebaticaliy sp&ing an rf field at the proton resonance frequency [4f. When the sweep is started far below the resonance frequency of the chlorine spin packets or of some of the spin packets, then near resonance,
these
spins are
cooled to a low but positivespin temperature i.n their interadirjg the proton
representation. The spin temperature of dipolar reservoir also becomes positive. Thus, a @roton magnetiwtion wiU grow in a direction parallel to the adiabaticay app!ied rr field: However, w-hen l&e sweep through the CI spectrum is started far above the chlorine resonance frequencies, then the temperature of the spin packets becomes negative and therefore
&o
the spin temperature
of the d&alar
re-
servo&. By applying the proton rf field, a magnetization then gows in an a&par&M direction. This explains tl@ observed depndence of the sign of the FrCt on the sweep direction. The insensitivity of the proton poIa~zation to the magnitude of Hla is probabIy due to the wide range of effective fields (in the interaction representation) expc~enced by the %l nuclei in a polycrystine sample. The increase in pofarization resuIting from sweep repetition is due to additional cooling of the proton dipolar reservoir by the 35CI reservoir (ii the interaction representation) with each contact. In the absence of relaxation in the d&&r state, the proton qin temperature would &entutiy reach that of the 35c1 reservoir. .Thti polarization of the protons (in paradichlorobenzene at 77K) produced in a singe sweep through the Cl spectrum may be c+pared with a prediction usGig spin thermodykmics. The spin temperature of the protons after the sweep is obtained by equating the initial and ftial energies of the Cl and ‘E-ispin re$ervoirs, YiJlthan itiizid inFmite proton spin temptature, the fi~al proton temperature is given by:
within the (homogeneous)linewidth of a.Cl spin packet @MOOHz 191). ‘Therefore, the poI~~zation is transferred to the protons while the CI spins are isoIated from the lattice, but lo&e{ along tlie effective r, = Pa Cl + &&_J’ (1) field in the interaction represenfation pr’oduced by where $!a is the i&&al Cl temperature in the spin HIQ. When individual spins become locked, ?hen near iesonance; their splitting in the interaction representi; locked state,,and,+_r-r is +&eratio of the heat capacities at a common spin temperature of the proton tion is of *ti same order of magriitude as the splittings dipolar reservoir and the (T!nuclei in the interaction . in the diipolar energy reservoir ctimptised of dipolar ,, representation [7 J. This &tio is given by: interactions among all the spins. Consequen~y, order .can be transferred from-the quad~pole system (in the : cPH)+& ~tera~tio~ ~epre~~~atio~) do ‘tie $pokr’u%ffoir .. ., e&-H= [cy@ 7.3 cpcr)] tXlj3 UZY ’ at hi’ ~~.~~etic fiela and this order-cr+n *tin be ,. ,- ; : ..
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501 1
Volume 32, number 3.‘.
:-
_CH@ICAL pHYSI@ LETTERS:_
., : ._-: W-I&G we haYe.usedone-halfof the 37a heat capacity since @I sweep’covered only th@upper liqf of the .’ 37Cl spectrum centered, at.27 e.
II, is the ‘magnitide of the lo&l dipolar field at a proion and 2Hl/?UT.is the effect&field experienced by g spin5!2 qua’drupolar nucleus with an tiy-etry parameter .0.08 ina polycrystzlline sample [7,9]. The .nuclear .Cu& constant
:
is given by:
C(i)'=ij;;z b2I&t I)/3 k,
’
[7], whiie HI was I.0 G. Using the appropriate values for N, 7, and I in paradichlorobenzene, +I_ H = 22.6.
The initial (51spin temperature in the interaction representation is zO.OlK, so that from (11, TfmO.236K By elquating entroyp iti fie local dipolar field at this ‘t&m@tuTe to entropy. in the final applied field (Ho), the proton spin temperature after adiabztic remagneti-. g&n
T, 7 Tf.I&/H,
by: -.
for double resonance &x_xriments. ,.
the lattice temperature;in
:
..
sequence .,
.’
.. y
._...
Home .desiE;nedthe sweep oscillattir an! J. Duran pro-’ vidtd &hehighly purified paradichlorbbenzene. We thank E.L. Hahnfor communicating the deMs.of some of his cross-relaxation studies prior to pub&ation.
References. [i] S.R. H&gann
[3] [4] [5]
reasonable agreement-with
”
1975
The caprlble technical assistbnce of R.D.- Hendrick in dl phase:; of the experiment is acknowlerlged. D.
is slightly more than twice the lattice temperatuie, resulttig,$ an equivalent protdn polarization slightiy less than one&if the full Boltzmann polarization at
J.@Y
nical apparntus iixthe field re&dn. ” Work is in progr&s to use,this poltization
..
using Ho = 5400 G. Thus, the firial proton spin ternperature after a’ single. sweep through the Cl resclnance
.‘experim&t..
:
useful.&II%it ban I&a&&d at.anny field stren&.to. a sample in situ,.and without the nec&ity for mecfia-
[2]
;; j6>K, .’
::
LwcIlowledgi~~ri~~-.~ .- ,~
whe& 4, ri, and Ij are the number, gyromagnetic ratio and spinquantum number. of the i th nuclear species, and .I!and k the Planck and Boltzmann constants. H, is’takeri to be 7..?5 G from proton TZ measurements
‘titionis
:
;’
[6]
and EL Hahn,Phys. Rev. 128 (1962) 2042. F.M. Lurie and C-.P.Slichter, Phys. Rev. 133A (1964) 1108. A. Pines,‘M.G. gibbyand J.S. Waugh, J. Chem. Phys. 59 (1973) 569. A.G. Anderson and S.R. H~ITMM, Phys. Rev. 128 (1962) 2023. D.E Woe;sner and H.S; Gutowsky, J. Chem. Phys. 29 (1058) 804; Y. Hsieh,‘J.C. Koo and E.L Hahn, Chem. Fhys. Letters 13 (1972; 563. M. Goldman ard A. Landesmqn, Phys. Rev. 132 (1963)
610, ‘6. Conclusions ,. A new method
; for, pol&tig nuclei v$ cross-relaxanuclei in hi.& maztetic field
[7] M. Sch~-~ and EL. Hahn, J; Chem. Phys. 52 (1970) ‘3152. [g] N. BloemL&gen and P.P. Sorokin, Phys. Rev. 110 (1958) 865. . [9] M. Schwab, Thesis,~U~iver$ty of California at Berkeley .(1968), unpublished.
.’
: .. ‘(.’
:
._, “. .:.’
.,
1 Kay 1975
PHYSICS LEI%RS :
:fkEhlfCAi
.~
:
.
,.‘.’ ‘. ”‘,.
‘.
‘. .,“, ,, ~
‘)
_.
;
‘_
& STOOD FOR ?O~Z~~ ~ROT~~~ &’ ~~~~~ __gfjJ&& C CROSS-REUTION IN HIGH MAGNETIC FIELDS * ‘.
RDTAT~G
FI&&iE
:
WS: VEE&
** and C.S.’ YAKTONI
IB~~~~~~~~~
’ ,.
.’
_:
,.
.’
San Jose, California 95193, USA
Ladamiary,
‘Re+ivcd 10 &emb&‘1974 ‘.. Revisedmanuscrijt received 13 January 1975 : -.
:.
: f
:
.’ ._
.’
.I ‘_, ., . . A Ned tecJ&ique is demonstrated f& poiaking protons in high dc magnetic fields, ‘YE&methorf has been used sucessfully &I polycrystalline samples at an arbitrary dc mame,tic Geld strcngtb w&out recourse to rotation . . .‘(‘, of the Sam@? or cycling to low.or zero BeId,
..
: 1. ~~oduc~on
Ti (for purposes of this ~s~ussio~, it is assumed that. ._ 1~=I~=!/2). Until now, tiowever, this technique reGenerally, the d&e&on of rare spin species in sol; quired slow rotation of the sample in a’magnetic Reid. ids by nut&adouble xesonance requires the contic& of @ecified strength f5] , or removal of the sample to of abu~d~~ spins (A> in an ordered state in the’rotazero or very low field: [rj,?]. fin this note we report a ting f&e v&h rare spins (B) El]_ If, prior t& this conmethod for crc)s-reltig A spins~with Q spins at any ‘- malefic field strength, withaut the necessity far metact, the B spins are heated, tben~during the contact, char&ally moviug the sample, A~pin magnekation will be ‘destroyedland B-spin ‘. ~~etization built up [Z] . The B-spin’s_~c~rum can :_ then be mapped out either by measuring the.A-spiri ,[1,2] or:B-spin ma~eti~tion 131. The ordeied state “. Z!..Wethod .’ of the A spins usually is a spin-locked [l] or adiabati-” .. : k&y demagnetized state in the rotating frame ..JTp idea for our method stems in part from a pre-, (ADRF) [4J khich 1asts.a time T1, or Y“,,; respecvious experiment carried out at zero magnetic field tively. To achieve
a
satisfactory
E-spin sisal,
it often’
[7]Jn
'is micewirytorepeat thicontact many~ties; tberea fore, except in ~o~e.favor~ble oases where the TIP or 5”‘@ very long, one’has to ie@$arize the A sp’insvia spi&attice* relaxation after a sm&.i number of dontacts. This can be very time-consuming .drh&nthe spin- -.’ lattice relaxation time ?jk. is very long., .: ‘,, & varietl of techniques can be used to ~ir~~rn~ent ” ‘this diffrc;iIty,‘onc of the most successful being rep& c ‘, ‘, lariialian.o[the d spins by cross-relaxation with: ;i : ” : &jrd .I: i.midI~~ qu&upolar, nucleus (Q) wifh a she+ : ;..
t&
ca513,the Q spin-resonance
I?+ is narrow
and the entire Qspiri system ‘can Mocked along a' strong rnonoc~orna~~ rf l%fdH1q. ~ra~-rel~ti~~ then occurs bets&n Qspins in the interaction repre&n&&ion and’A s&s in the dipolar state in zero magneticfieId The s,ample is then returned to high field .for detection of the B spins via contact with the now-
.polatized A s&ns. ,. The &tigti of noetic
fIeId stretch
te‘chniq~e somewhstcumbersome
makes this
and one would !ike
to bb,able to do,#is experiment at high magnetic Geld being fdrced to go to zero field. l=Iowe%rr;-in ‘2 . . ,:, .higb field; the. $uad&pofe resonance of the Q s@ns is . .‘. .,; ” ,, * $,uuppozted by the U.S. Armi k&$rch O&k. ; ** P&sent address: Labortltbriu~‘vdot‘Fysiscfie Chemie,‘, ,“. ‘, :.b;oade’ned c~~~~er~bi~ by Zeeman interactions, :’ ToBmooiveld, Nijmegen,Thc Net.hBr&nds,., : : ,: only a neg.”_’ ., es@iaIiy in’ golycrystaIline samples, and .,I . . : . _..I .,. .,., ‘. I., .‘.‘. .. _,. ; : . . : _’ ‘.I .: ~. .’ ‘. ‘.‘, ” 499 .. ,: ..’ ;_ ..’ . . . . ‘., _ ‘. ;. : _. ‘I ,. . .:.:. ,I“ .). :; 1, .,; ,, ,, ‘. ‘., ,‘I. ;; .:-.: ‘.” _.._.,,,, . . .:. _.‘_:_,,’ ” .’ ., ‘,. ,“,. .;
‘:
.(’
(., “.
:
,: ,’
:
withti~t
Volume‘32, number 3 :
,‘.,
,_ i.
CHEM~CALPHYSICS LkTERS
1 hl,ay~1975,
Proton pblarization was achieved in ihe following polycryst&ine samples at 77K: paradichlorobenz&e, 2,5- nnd 2,6dichlorqbenzoquinone, lJ,5-trichloro&ikne and ! .2,4,51tetiachlorobenzene: TheTI of, HIA
:
., ,.
,-
Receiver(A) Time Fig 1. P&e ,$~ough
se&mce
used to
LI lb’
-
polarizeprotonsby-sweeping
the %I raonance. The dashed cu:Jes show the ex-
periment when the sweep direction is reversed. legible fraction of the Q spins can be brought -tinto thermal contact with be A spins by locking with a monocl?Jqmatid rf field. A method which:u:es level ‘crossing in the laboratory frame overcom& this problem, but dictates the field strkngth at which the tiR experiment cari be performed [S]. In the.technique reported here, the entire Q spin reservoir can be coupled to the A spins at tiy field s’uength merely by sweeping HIQ through the Q spin resonance line IvKle dkngnotting to the A spins. During rile sweep,the &spin dip013reservoirbecornes.ordered, and this order is subsequently transferre’d io.the A-spin Zeeman reservoir in the rotating frame by adiabaticdy applying an rf field HlA on resonance [4]. I+ the experiments reported here, the magnetization along HIA is measured by turning off HlA suddenly and measuriag
the initial amplitude
of the-free in-
the protons in these compounds at 77K is very long (for paradichlorobe&ene we measured 4 -C1 hour), but using the above technique, we were able to polarize tl~ protons to an appreciable amount in only fivk seconds! The initial amplitude of the FID after turning &f the magnetizing field HlA is about 50% of the initial amplitude of the FID following a rr/2 pulse after the protons have come into equtibrium with the lattiti.
The direction of the sweep affects the sign of the magnztization: after the “Cl spkctrum is swept from low to high frequency (fig. 1) then a proton polarization gows alongHI,, cotiesponding to a positive spin temperature of ‘Lheproton bipolar reservoir. However, when the sweep direction is reversed (dashed curve in fig. 1) the magnetization builds up antiparallel to HIA, which means that the proton dipolk reservoir kmperature
must havi been negative
[4].
Preliminary results also indicate that (i) the degree of polarization HIQ, provided
is relatively insensitive to the value of t.hatHlq is beater than ==7 G (rotating
component) - the largestvalueof HI, unedwas kl5 G; (ii) repetition of the sweep results in increased polarization -about 90% of the full proton Boltz,nann polarization was achieved with six con.’ secutive 7-second sweeps; (iii) no significant increase in polarization resulted from long (B 15 S) sweeps. A comp’letely equitient proton magnetization has been
duction decay(FID) of the A spins*, ,Thepulse se-
achie\fedwhen, duringthe sweepthrough the Cl sgecl
quence used is shown in fig. 1. In our experiments the Qs ins are 35Cl nuclei and the A spins’are protpns. The R Cl nuclei are also intioived, but give a smaller effect because of their lower abundance; in ordei to keep the discussion simple,’ f$e effect of 37Cl nuclei will be neglected.
trum, the 4 spins are either locked along HlA or have been subjected to an ADRF;
..
: l An equivalent signal is obtained when Lhe magnetization is .Y xun@edin ‘he laboktory frame by going adiabatically off resonance kfok turning HI k off, iher, returning io reso-mmCea@tibsetig the FID.follcwi$ a 742 pu&e. _. ‘. .‘-, ‘s& :.., .~’ : _, .-_ : ~‘. ,. ..:- : . .. . . .’ : ,I,“. :_ .,
4. Experimelital details Paradichlorobenzene (Matheson, Col&man and Bell) was iefluxed over Al+, (using n-pentane as a solvent), then sublimed at 5OOCtid zone refined three times., The 6 ther mat,erials were cotiercially available (E-X&m Kod?k) and ,were used,wfthbut further pm-i; fixation.
: Coil syitei.operating
A&&
at 23 MHz (5400 G) ‘.
.’ :
‘,.’ ,: .- :.,
.,
.‘,‘,, ., ;, :
::
.,.
. . .‘,
..,’
: . .‘,
Volume 32,,tiumber 3
CI3E?t%ICiiL PHYSICS
:
LEFRS
1 May 1975
.’ was used to magrktize &td detect the polarized pro-
tons. The 35& nuclei were excited by an rf field pro- : c@ed in a saddle-shaped coil with its axis p&allel to the e’xtemal dc field. The Coil was driven by a pushpull oscillator; the amplitude of Nlrr was kept conslant over the sweep _range (2731 MHz) by continGously tuning the +_a& citctit WitA a rn~tor.d~ve~ capacitor. Ail me~ureme~ts were made at 77K. ’
5. Discussion Several experiments have been reported in which (1) polarizdtion is transferred from.one spin system to anothei under the effect of radiation, and (2) sweep reversal could affect $e sign of the induced polar&ation. Roughly speaking, these experiments fail into. two categories, depending on the state of the,spin systern (in our case, the Q spins) from which poIarization is transferred. If these spins are in equilibrium with the combined effect of rf irradiation and spin-lattice relaxation, then transfer of polarization to another sfin system can occur through the solid effect iri the rotating frame [ 81, and thermal mitig by con~uous off-resonance irradiation 161. lf, however, the Q spins : are in a quasi~q~briu~ (e.g., sp~.locked oi AI)RF) state, then thermal mixing between spin systems can atso occur [‘?I. We believe that our experiment resembles the latter.
For para~c~orobenzene at 7X, the spin-lattice relaxation times in the laboratory frame and spinlocked state are 390 and 45 ms respectively, while the time cans”%.&for %LIH cross relaxation is ~2 rns. [7]. From a measurement of our sweep rate, we esti; mate that we spend less than a, cross-relaxation
time
~~sferred into the proton Zeeman reservoir in the rotating frame&y adiebaticaliy sp&ing an rf field at the proton resonance frequency [4f. When the sweep is started far below the resonance frequency of the chlorine spin packets or of some of the spin packets, then near resonance,
these
spins are
cooled to a low but positivespin temperature i.n their interadirjg the proton
representation. The spin temperature of dipolar reservoir also becomes positive. Thus, a @roton magnetiwtion wiU grow in a direction parallel to the adiabaticay app!ied rr field: However, w-hen l&e sweep through the CI spectrum is started far above the chlorine resonance frequencies, then the temperature of the spin packets becomes negative and therefore
&o
the spin temperature
of the d&alar
re-
servo&. By applying the proton rf field, a magnetization then gows in an a&par&M direction. This explains tl@ observed depndence of the sign of the FrCt on the sweep direction. The insensitivity of the proton poIa~zation to the magnitude of Hla is probabIy due to the wide range of effective fields (in the interaction representation) expc~enced by the %l nuclei in a polycrystine sample. The increase in pofarization resuIting from sweep repetition is due to additional cooling of the proton dipolar reservoir by the 35CI reservoir (ii the interaction representation) with each contact. In the absence of relaxation in the d&&r state, the proton qin temperature would &entutiy reach that of the 35c1 reservoir. .Thti polarization of the protons (in paradichlorobenzene at 77K) produced in a singe sweep through the Cl spectrum may be c+pared with a prediction usGig spin thermodykmics. The spin temperature of the protons after the sweep is obtained by equating the initial and ftial energies of the Cl and ‘E-ispin re$ervoirs, YiJlthan itiizid inFmite proton spin temptature, the fi~al proton temperature is given by:
within the (homogeneous)linewidth of a.Cl spin packet @MOOHz 191). ‘Therefore, the poI~~zation is transferred to the protons while the CI spins are isoIated from the lattice, but lo&e{ along tlie effective r, = Pa Cl + &&_J’ (1) field in the interaction represenfation pr’oduced by where $!a is the i&&al Cl temperature in the spin HIQ. When individual spins become locked, ?hen near iesonance; their splitting in the interaction representi; locked state,,and,+_r-r is +&eratio of the heat capacities at a common spin temperature of the proton tion is of *ti same order of magriitude as the splittings dipolar reservoir and the (T!nuclei in the interaction . in the diipolar energy reservoir ctimptised of dipolar ,, representation [7 J. This &tio is given by: interactions among all the spins. Consequen~y, order .can be transferred from-the quad~pole system (in the : cPH)+& ~tera~tio~ ~epre~~~atio~) do ‘tie $pokr’u%ffoir .. ., e&-H= [cy@ 7.3 cpcr)] tXlj3 UZY ’ at hi’ ~~.~~etic fiela and this order-cr+n *tin be ,. ,- ; : ..
..‘. .
,_p ,.
: : .-.. ;
.: :. ..
:.
._
‘.
., : ‘. ;,:
,)
:
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II.. : 1 -. .’ ., ,,’
‘, .,
.. ‘. :
,:
1 :.
(’
‘.,
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.’
: :
501 1
Volume 32, number 3.‘.
:-
_CH@ICAL pHYSI@ LETTERS:_
., : ._-: W-I&G we haYe.usedone-halfof the 37a heat capacity since @I sweep’covered only th@upper liqf of the .’ 37Cl spectrum centered, at.27 e.
II, is the ‘magnitide of the lo&l dipolar field at a proion and 2Hl/?UT.is the effect&field experienced by g spin5!2 qua’drupolar nucleus with an tiy-etry parameter .0.08 ina polycrystzlline sample [7,9]. The .nuclear .Cu& constant
:
is given by:
C(i)'=ij;;z b2I&t I)/3 k,
’
[7], whiie HI was I.0 G. Using the appropriate values for N, 7, and I in paradichlorobenzene, +I_ H = 22.6.
The initial (51spin temperature in the interaction representation is zO.OlK, so that from (11, TfmO.236K By elquating entroyp iti fie local dipolar field at this ‘t&m@tuTe to entropy. in the final applied field (Ho), the proton spin temperature after adiabztic remagneti-. g&n
T, 7 Tf.I&/H,
by: -.
for double resonance &x_xriments. ,.
the lattice temperature;in
:
..
sequence .,
.’
.. y
._...
Home .desiE;nedthe sweep oscillattir an! J. Duran pro-’ vidtd &hehighly purified paradichlorbbenzene. We thank E.L. Hahnfor communicating the deMs.of some of his cross-relaxation studies prior to pub&ation.
References. [i] S.R. H&gann
[3] [4] [5]
reasonable agreement-with
”
1975
The caprlble technical assistbnce of R.D.- Hendrick in dl phase:; of the experiment is acknowlerlged. D.
is slightly more than twice the lattice temperatuie, resulttig,$ an equivalent protdn polarization slightiy less than one&if the full Boltzmann polarization at
J.@Y
nical apparntus iixthe field re&dn. ” Work is in progr&s to use,this poltization
..
using Ho = 5400 G. Thus, the firial proton spin ternperature after a’ single. sweep through the Cl resclnance
.‘experim&t..
:
useful.&II%it ban I&a&&d at.anny field stren&.to. a sample in situ,.and without the nec&ity for mecfia-
[2]
;; j6>K, .’
::
LwcIlowledgi~~ri~~-.~ .- ,~
whe& 4, ri, and Ij are the number, gyromagnetic ratio and spinquantum number. of the i th nuclear species, and .I!and k the Planck and Boltzmann constants. H, is’takeri to be 7..?5 G from proton TZ measurements
‘titionis
:
;’
[6]
and EL Hahn,Phys. Rev. 128 (1962) 2042. F.M. Lurie and C-.P.Slichter, Phys. Rev. 133A (1964) 1108. A. Pines,‘M.G. gibbyand J.S. Waugh, J. Chem. Phys. 59 (1973) 569. A.G. Anderson and S.R. H~ITMM, Phys. Rev. 128 (1962) 2023. D.E Woe;sner and H.S; Gutowsky, J. Chem. Phys. 29 (1058) 804; Y. Hsieh,‘J.C. Koo and E.L Hahn, Chem. Fhys. Letters 13 (1972; 563. M. Goldman ard A. Landesmqn, Phys. Rev. 132 (1963)
610, ‘6. Conclusions ,. A new method
; for, pol&tig nuclei v$ cross-relaxanuclei in hi.& maztetic field
[7] M. Sch~-~ and EL. Hahn, J; Chem. Phys. 52 (1970) ‘3152. [g] N. BloemL&gen and P.P. Sorokin, Phys. Rev. 110 (1958) 865. . [9] M. Schwab, Thesis,~U~iver$ty of California at Berkeley .(1968), unpublished.