- Email: [email protected]
The role of solvent in inorganic reaction mechanisms, as elucidated by high pressure studies
w.
#--,-.,-L-A
---r-r-
&de-
,Ar
--A.
----
-c-
--1--s
--..
-a-
.R
3+-n*
l
LkvB B
AV,’
4.5 3.6
-7.5 -6.1
2.9 2.4
9
-5. i
1.9
11.6 5.7 3.9
-18.3
20.6
-20.7
-9.1 -6.2
10.5 7.0
-13.8 -7 3
11.1
-17.8
16.1
-16.3
-14.7
200
5.8 4.0
-9.3 -6.4
9.3 6.5
-9.7 -6.9
-7.9 -5.5
0 100
12.6 6.1
-18.4 -9.3
49.7 19.5
47.6
-18.3
200
4.2
-6.4
12.1
-11.3
-12.5 -6.4 -4.4
P
IMPa
0 100
9M e-d methanol
0 100 200
acetonitrile
acetone ‘I-0.1
0 100
AV-’
/1o”o Pa-’ /cm3mar’ /cm3mol”
molkg”,r=WOpm,a=
Al+k)
/cm3mar’ /cm3moT’ -3.4 -2.8 -2.2
-6.3
-5.1 4.1 -14.9 -7.6
-5.2
1.ONII, a = 800 pm, T= 25”C, AVi* = 0.6 cm3mol-‘.
averaged,sincetheii press dependence is nonlinear’)or its value at mid-range(100 MPa). In practice,exper&U valuesof In A often appearto be linear fhctions of P witbin the error limits, altbougbnon-lineuity does seemto exist in some caseswhere 1Ati 1is~~~l~~~~~~~ with theoryis then possible. . expaimentalresultsonpresawee&ctsonthe kineticsof ?MM&qww e&troll tmnsfer reacti-. Table 1 gives bexmticai vahe3 of AVi* and AVOOUL’ and, , crlthoughfew 3+/Z+ reactionshave been studied in no-w
media to date (largely becauseof sohbi&ty aud redox stabi&y problems),the point emergeJthrd~opposinscontn’butionscsn~sdverylargethatthecalculationof Av’ bewmesnumericallyunstable.Even for O/- or +/O couples,for which the DebyeNacltelandd~~wo~tams~iapriaciplcbe~~~~onewwpectsthat second-orderionic Srength aud Coulombic&%cts could contribute siguific8ntIyand possiblyunequallyto Ap. Furthermore,ion pairing of the reauants(at least one of wbicb has to be ionic) with the wunteri0nstbatareinevitablypresentwillbew~;e increh@yevidentasDdecrews. Ifthepairedion(s)aremarereactivethanthefiee remant ions,the conuibutiooAVt$ of ion pair formationto Ap will be positive,since,as Fuosstheoryshows,’increasingpressurearts throughincrea@ D to disfavorion pairing. Conversely,if pairedreactantions are fecesrwtive in electrontransfh than the parent species,AVLP:will be negative- Wberlad’ has shownthat this is more generaNythe case, although the assumptim made Er convenier#;e06 &a&ion in some of our publications,‘ht ” ion pairs are inac+iv~iF electronBan&r, is excessiveClf co\~cse, the valueof A&p1will approximateto a constantonly if ion’pairsconstitutea very smallor a very large fraction, re&y, of the totai reactantir thesetwo cases.
242
Tabk2.Volwmesofactivation
at 100 MFa for sekxchsnge in ML,(z’lw couples in
Fe(pimh’~+ co([9jane&~3+n* w=P~3~+ CO(v)3*Rf Co(~~)‘+“+ Co(Ul)33~+ Co@harb3’R’
3.0 25.0 25.0 25.0 25.0 65.0 25.0 25.0 85.0 45.0 45.0 45.0 25.0
A>“” MnoI “TT
NaeMnO~~w gy$&g4&
0.3 0.1 0.2 0.1 0.63 0.5 O.l(NO3-)
0. I(Cl-) 0.5 1.1 1.1 ;;
-2.2 f 0. I -4.8 f 0.2 -6.4 f 0.2 -10.5 f 0.6 -9.4 f 0.9 -15.5 f 0.8’ -16.0 f 0.7 -17.6 f 0.7 -3.2 f 0.3 -17k2b +3 f 1 +22*2 -1 f 1
-2.5’ -5.3 -6.4 -7.3 -9.2 -5.3 -1.7 -2.2 -9’ -6* -ad -5”
14 15 I5 ;6 16 10 17 17 18 9 9 19 9
11 -11 2.0 +0.8 f 0.9 Fe(H&Oli*/ FQ~+ 015 ’ AVmztakea as 0.6 an3 md’ except as indicated. ” Ak4 shows possii~e pressure depembce. ’ AV$ taken ns 0. ‘Includes an es&ate of the e&t of ion p&1%18.
Rdcreaca 1. 2. 3. 4. 5.
D. R §tra+, Pure Appl. C$cm., 38,303 (1974). T. W. Swa&ile, Adv. kxg Bioinorg. Me&. 2,95 (1983). W. E. Jones,L. R Carey and T. W. Swddle, Can. J. Ghan. SO,2739 (1972). T. W. Swnddle, Can J. Phys,, in press. R A lb+hus, J. Chm. Phys., 24, 966, 979 (1956); Discus Fmby Sot, 29, 2 1 (MO); Faraday Disc. Chem. Sot., 74.7 (1982). 6. N. S. Hid, Trans. Famiay Sot., 57,557 (1961). 7. T. W. Swaddle, Inotg Cbem., 29,5017 (1990).
cobatt(m) ammineexample,the volumeof auivation AY’(= -RT(d In k/a&, wherek is the rate cqstant and P is the pressure)for reaction 1
Cow3h*3=p +
Hz0 -+ CO(NH~),O&~+ + x”
(1)
varieswidely Tom +l to -18 an3 mar’ at the atmospheric-pressure knit as z is increased fkom 0 (X = Ha) to -2 (X = S0,2), and is markedlypressuredependemfor anionicX Theseresults may be interpretedto dlean that bond making/breakingcontributesonly about+1cm3mof“toAVIintheseries,ihebalanse(andthep~dependenceofAVt) being due to aolvationof the emerging3+ and z- ions with concomitantloss of both vdumeandcomp~~~ofpartofthGsolvent-~anestimatedeight~atermoleculeJ per co, in the caseofthe sulfato-comp1ex.’ Apamcular~eintheuseofp~toprobesohgtioaal~~b~ pressmecan~“theGharacteristcbuDproperti~ofthesolvetItwithoutchangmgit chemically.The use of mixed s&ems to vaty the diektric constant,for example,wig likelybeconfoundedby~alsdvstionbyoneofthesdvents,whilethe~will bechemicallydiffierentifsoiutionsindiffeaentneatsohrentsareconsidered.~ studies,however,have their own limitstio~. Technicalcomplications4 limit the presswe rangenonnaliyuaedinchemicalkineticshtdiestoabout0.1-200MPa,andthisproduces only smallchangesin suchsohmt parsmeters.Furthermore,one needsto haveavailablea ~cthearyttratrelates~~in~l~parameterstotheiranticipated~on reactionrcltesandequilibria,ifp~istobeusedasareliablepmbeofsolvation phenom~oUr~efibrtsincal~hevebeendireaedtowardsestablishingand testing such a &eoreti& Barrreworkfor suitablemodel reactions.The simplestof such subatimtionproceases)andthe reactionaaresoiventexchaugeonmetaiions(represeMing outer-sphereehxmm tmnsfbr reactions of tranahion metal complexes(reprewnting redox).For the iatter, the theoriesof Marcus’and Hush6haveprovideda startingpoint for a theoryof pressureefFwtson reactionratea.‘*’ IL Solvent exchange kinetics
Pressureeffectson the kineticsof solventex&auge on metalcationshavebeendeactibed atlength~“aradwillaotbeconsidesedin~here.Sufficeittosaythatitis possible,usingan empiricalrelationthat predictsthe molar voh.tmesof aqueouscations,to predictthe volumechangesthat would acwmpaqthegainorlossofonesokeutligand and so to set limits upon Ati for associative(A) and disso&ive (D) water exchange (about -!3 and +13 cm3mar’, respe&eiy).” GI fact, Ap vaiuesfor all water exchange reactionsof metal 2+ and 3+ ‘ions studied to date have fallen between these limits (Ti(H~)63’is closeto the associativelimit’), implying that the water exchangeprocessis appropriatelydewibed as inr4tcAange(I,, 4). Iu particular,the D limit is not ‘approached ataUclosely,asmightbeexpectedinviewofthehigh abundme of water molecules ~themetalion,withthe~~~oftheirdipolespointinginwardsona timea~.At~~thereareinsutlici~moiarvohrmedatafornretalioruin nonaque0ussolventsto permit cotwuction of empiricalreMon&@ like that estabiiahed for aqueoussohrtiona,and so this approachhasso far beenlimited to solutionsin water.
2.39
UL Theory of premwc efkb
on 0ateMpbere sdf-crcltrn~
reatiioa rate4
ln the hence of ion pairing and rate limit&ion by solvent dynamics,the volume of activation Av( for adiabatic outer-sphereelectron tram&r in cwples of the type A4QL”~~ can,in principle,be c&a&ted as in equation2 fiom an adapt&n of MarcusHush theory.le’In equation2, the suhsuipts refix rcspectivdy to vohme ““‘fibufiy~ fromint~@rimarityM-Lbondlength;aMi~~feorgarritatiant~an~ for electrontransfer,medium@ebye+Huckel)e@!ec&, the C@ombic work of bringingthe m&ants together,andthe formationof the precursofcmpkx. Ati = At@ + A&t’+ AVmt+ AVcaRt+ AVpaecs
(2)
Of thesecontributions, Avis? is calculated to be between0 and 1 cm’mar’ for rigid complexes’ (a valueof +0.6 cm3tnor’ is adoptedfor the complexesdiscussedin this paper), Ah& is typicaliy between+I and i-2 cm3 nd,’ mi, for practicalvaluesof the ionic stmngthI, AV’& and AV~twillbeofoppositesignbutsimikumagnitudcunless the relativepermihity R of the solventis very low:
AV&t = (Rzz*&@/(1
+ BuP)*)[(a ln DIdpM3 + 2&P)
- g))
AVcorn! = (&?,&e2/4xe,,u)(~D~~~~
(3) (4)
lfwe assumethe reactantsto be hard spheresofroughly equalradii r and electrontransfer to occur w .zhhighestprobabilityat an M-M sepamtionu that varies with compressionof the solvent(&thermal compress&ii b), we havefor solventreorganization AVsR’= (N*2/167cE’l)[(f-’- d){d(n -2- R-‘)EP}r - (n” - D’)gnu]
(5)
ln equations3-5, (I is the anion-cationcloseappfoeehd&ant, B and C are the DebyeHiiclrelpanunetenatO.lMPa,nistire~iadexofthesolvan,andtheother symbolshavetheusualSImeaniags.Thus,theorypredictsthstA~~beequated tqqmbately with AVsat. Sii the’presswe dep&mesofbothnaadRare&isdy linked to that of the densityp of the solvent,i.e., to 8, it follows that dY’ is bferwtined in kqe part by the coIlDpplesFibiii~of tk sohmt. Sample datlations for modeI 3+i2+
couplesare givenin Table 1. As Table 1 shows,however,the compres&Ues of liquids, pahdady organic solvents,decreasemarkedlywith bcre&ng pressure, and so the dilated AV,t ad indeedaMthepammetersinTabie1deuease in ~&solutemagnihde ovex the typiad exfhened premtlre range,especiaUyat low pressums.TheresultisthatAVtitselfcan beexpectedtodependmarkedlyupo~pnssure,butthee~isleJ;sJisnificlntforwater than for nonaqueoussolvents.The implication is that, given the usual expaima#ai unce&mies,onecanmext&@ydiscusspmssureeffeasondectron~kinetiu,in aqrreoussystansoverthe0-200NParaneeintermsofeithofImavrrogevatueofA~ overthat~e(butthisraisesthequestionofbowthetheoreticalvaluesofAY”antobe
244
8. A E. Ibleb&, Pure Appf. Ckn. S9, 161 (1987), and in H&h hsure Ckm&by adBiochernisby,,~RRvan~andJ.Jo~,p.311~idelDofdrecht, 1987). 9. L. Spiccia and T. W. Swaddie,inorg Chem., 26,226s (1987). 10. W. H. Jolley, D. R Stranksaad T. W. Swaddle, Iaorg. Cbem., 29,385 (MO). II. W. H. Jo&y, D. R Strauh and T. W. Sm hrg. Ghan., 29,1948 (1990). 12. S. whaland. Cod. them. Rev. 123, i69 (1993) 13. H. Doine and T. W. Swaddh, Inorg. Chwn., 27,665 (1988). 14. Ii. Doii ami T. W. Swddk, Cat. J. Chem. 66.2763 (1988). 15. Ii. Doiw ad T. W. Swaddle,hiq. Chari., 30,1858 (1991). I4 RD. ShakhandT. W. Swaddk,~pubiication. 17. M. R Grace and T. W. IFwaddle,Iaarg Chem., 32,5597 (1993). 18. W. H. Jo&y, D. R Stranks end T. W. Swaddlq Inorg. Chem., 31,507 (1992). 19. H. TakagiaadT. W. Swaddle,Inorg Ckn. 31,4669(1992). 20. R A Biaaendand J. K. Beak, Inerg. Chn. zs, 1481(1!X6). tl.RD.Sbeldas,HT~endT.W.Swsddle,unplbfishedworlr. 22. S. P. Doiin, R R Do~onadzeand E. D. Gemtan, J. Cheat. Sot. Faraday Trans. I, 73, 23. D. V. Matyuhov, CRem.Phys. 174, 199 (1993); D. V. Matyushov and R S&mid, J.
Phys. cbl.
98,5152 (1994).
#--,-.,-L-A
---r-r-
&de-
,Ar
--A.
----
-c-
--1--s
--..
-a-
.R
3+-n*
l
LkvB B
AV,’
4.5 3.6
-7.5 -6.1
2.9 2.4
9
-5. i
1.9
11.6 5.7 3.9
-18.3
20.6
-20.7
-9.1 -6.2
10.5 7.0
-13.8 -7 3
11.1
-17.8
16.1
-16.3
-14.7
200
5.8 4.0
-9.3 -6.4
9.3 6.5
-9.7 -6.9
-7.9 -5.5
0 100
12.6 6.1
-18.4 -9.3
49.7 19.5
47.6
-18.3
200
4.2
-6.4
12.1
-11.3
-12.5 -6.4 -4.4
P
IMPa
0 100
9M e-d methanol
0 100 200
acetonitrile
acetone ‘I-0.1
0 100
AV-’
/1o”o Pa-’ /cm3mar’ /cm3mol”
molkg”,r=WOpm,a=
Al+k)
/cm3mar’ /cm3moT’ -3.4 -2.8 -2.2
-6.3
-5.1 4.1 -14.9 -7.6
-5.2
1.ONII, a = 800 pm, T= 25”C, AVi* = 0.6 cm3mol-‘.
averaged,sincetheii press dependence is nonlinear’)or its value at mid-range(100 MPa). In practice,exper&U valuesof In A often appearto be linear fhctions of P witbin the error limits, altbougbnon-lineuity does seemto exist in some caseswhere 1Ati 1is~~~l~~~~~~~ with theoryis then possible. . expaimentalresultsonpresawee&ctsonthe kineticsof ?MM&qww e&troll tmnsfer reacti-. Table 1 gives bexmticai vahe3 of AVi* and AVOOUL’ and, , crlthoughfew 3+/Z+ reactionshave been studied in no-w
media to date (largely becauseof sohbi&ty aud redox stabi&y problems),the point emergeJthrd~opposinscontn’butionscsn~sdverylargethatthecalculationof Av’ bewmesnumericallyunstable.Even for O/- or +/O couples,for which the DebyeNacltelandd~~wo~tams~iapriaciplcbe~~~~onewwpectsthat second-orderionic Srength aud Coulombic&%cts could contribute siguific8ntIyand possiblyunequallyto Ap. Furthermore,ion pairing of the reauants(at least one of wbicb has to be ionic) with the wunteri0nstbatareinevitablypresentwillbew~;e increh@yevidentasDdecrews. Ifthepairedion(s)aremarereactivethanthefiee remant ions,the conuibutiooAVt$ of ion pair formationto Ap will be positive,since,as Fuosstheoryshows,’increasingpressurearts throughincrea@ D to disfavorion pairing. Conversely,if pairedreactantions are fecesrwtive in electrontransfh than the parent species,AVLP:will be negative- Wberlad’ has shownthat this is more generaNythe case, although the assumptim made Er convenier#;e06 &a&ion in some of our publications,‘ht ” ion pairs are inac+iv~iF electronBan&r, is excessiveClf co\~cse, the valueof A&p1will approximateto a constantonly if ion’pairsconstitutea very smallor a very large fraction, re&y, of the totai reactantir thesetwo cases.
242
Tabk2.Volwmesofactivation
at 100 MFa for sekxchsnge in ML,(z’lw couples in
Fe(pimh’~+ co([9jane&~3+n* w=P~3~+ CO(v)3*Rf Co(~~)‘+“+ Co(Ul)33~+ Co@harb3’R’
3.0 25.0 25.0 25.0 25.0 65.0 25.0 25.0 85.0 45.0 45.0 45.0 25.0
A>“” MnoI “TT
NaeMnO~~w gy$&g4&
0.3 0.1 0.2 0.1 0.63 0.5 O.l(NO3-)
0. I(Cl-) 0.5 1.1 1.1 ;;
-2.2 f 0. I -4.8 f 0.2 -6.4 f 0.2 -10.5 f 0.6 -9.4 f 0.9 -15.5 f 0.8’ -16.0 f 0.7 -17.6 f 0.7 -3.2 f 0.3 -17k2b +3 f 1 +22*2 -1 f 1
-2.5’ -5.3 -6.4 -7.3 -9.2 -5.3 -1.7 -2.2 -9’ -6* -ad -5”
14 15 I5 ;6 16 10 17 17 18 9 9 19 9
11 -11 2.0 +0.8 f 0.9 Fe(H&Oli*/ FQ~+ 015 ’ AVmztakea as 0.6 an3 md’ except as indicated. ” Ak4 shows possii~e pressure depembce. ’ AV$ taken ns 0. ‘Includes an es&ate of the e&t of ion p&1%18.
Rdcreaca 1. 2. 3. 4. 5.
D. R §tra+, Pure Appl. C$cm., 38,303 (1974). T. W. Swa&ile, Adv. kxg Bioinorg. Me&. 2,95 (1983). W. E. Jones,L. R Carey and T. W. Swddle, Can. J. Ghan. SO,2739 (1972). T. W. Swnddle, Can J. Phys,, in press. R A lb+hus, J. Chm. Phys., 24, 966, 979 (1956); Discus Fmby Sot, 29, 2 1 (MO); Faraday Disc. Chem. Sot., 74.7 (1982). 6. N. S. Hid, Trans. Famiay Sot., 57,557 (1961). 7. T. W. Swaddle, Inotg Cbem., 29,5017 (1990).
cobatt(m) ammineexample,the volumeof auivation AY’(= -RT(d In k/a&, wherek is the rate cqstant and P is the pressure)for reaction 1
Cow3h*3=p +
Hz0 -+ CO(NH~),O&~+ + x”
(1)
varieswidely Tom +l to -18 an3 mar’ at the atmospheric-pressure knit as z is increased fkom 0 (X = Ha) to -2 (X = S0,2), and is markedlypressuredependemfor anionicX Theseresults may be interpretedto dlean that bond making/breakingcontributesonly about+1cm3mof“toAVIintheseries,ihebalanse(andthep~dependenceofAVt) being due to aolvationof the emerging3+ and z- ions with concomitantloss of both vdumeandcomp~~~ofpartofthGsolvent-~anestimatedeight~atermoleculeJ per co, in the caseofthe sulfato-comp1ex.’ Apamcular~eintheuseofp~toprobesohgtioaal~~b~ pressmecan~“theGharacteristcbuDproperti~ofthesolvetItwithoutchangmgit chemically.The use of mixed s&ems to vaty the diektric constant,for example,wig likelybeconfoundedby~alsdvstionbyoneofthesdvents,whilethe~will bechemicallydiffierentifsoiutionsindiffeaentneatsohrentsareconsidered.~ studies,however,have their own limitstio~. Technicalcomplications4 limit the presswe rangenonnaliyuaedinchemicalkineticshtdiestoabout0.1-200MPa,andthisproduces only smallchangesin suchsohmt parsmeters.Furthermore,one needsto haveavailablea ~cthearyttratrelates~~in~l~parameterstotheiranticipated~on reactionrcltesandequilibria,ifp~istobeusedasareliablepmbeofsolvation phenom~oUr~efibrtsincal~hevebeendireaedtowardsestablishingand testing such a &eoreti& Barrreworkfor suitablemodel reactions.The simplestof such subatimtionproceases)andthe reactionaaresoiventexchaugeonmetaiions(represeMing outer-sphereehxmm tmnsfbr reactions of tranahion metal complexes(reprewnting redox).For the iatter, the theoriesof Marcus’and Hush6haveprovideda startingpoint for a theoryof pressureefFwtson reactionratea.‘*’ IL Solvent exchange kinetics
Pressureeffectson the kineticsof solventex&auge on metalcationshavebeendeactibed atlength~“aradwillaotbeconsidesedin~here.Sufficeittosaythatitis possible,usingan empiricalrelationthat predictsthe molar voh.tmesof aqueouscations,to predictthe volumechangesthat would acwmpaqthegainorlossofonesokeutligand and so to set limits upon Ati for associative(A) and disso&ive (D) water exchange (about -!3 and +13 cm3mar’, respe&eiy).” GI fact, Ap vaiuesfor all water exchange reactionsof metal 2+ and 3+ ‘ions studied to date have fallen between these limits (Ti(H~)63’is closeto the associativelimit’), implying that the water exchangeprocessis appropriatelydewibed as inr4tcAange(I,, 4). Iu particular,the D limit is not ‘approached ataUclosely,asmightbeexpectedinviewofthehigh abundme of water molecules ~themetalion,withthe~~~oftheirdipolespointinginwardsona timea~.At~~thereareinsutlici~moiarvohrmedatafornretalioruin nonaque0ussolventsto permit cotwuction of empiricalreMon&@ like that estabiiahed for aqueoussohrtiona,and so this approachhasso far beenlimited to solutionsin water.
2.39
UL Theory of premwc efkb
on 0ateMpbere sdf-crcltrn~
reatiioa rate4
ln the hence of ion pairing and rate limit&ion by solvent dynamics,the volume of activation Av( for adiabatic outer-sphereelectron tram&r in cwples of the type A4QL”~~ can,in principle,be c&a&ted as in equation2 fiom an adapt&n of MarcusHush theory.le’In equation2, the suhsuipts refix rcspectivdy to vohme ““‘fibufiy~ fromint~@rimarityM-Lbondlength;aMi~~feorgarritatiant~an~ for electrontransfer,medium@ebye+Huckel)e@!ec&, the C@ombic work of bringingthe m&ants together,andthe formationof the precursofcmpkx. Ati = At@ + A&t’+ AVmt+ AVcaRt+ AVpaecs
(2)
Of thesecontributions, Avis? is calculated to be between0 and 1 cm’mar’ for rigid complexes’ (a valueof +0.6 cm3tnor’ is adoptedfor the complexesdiscussedin this paper), Ah& is typicaliy between+I and i-2 cm3 nd,’ mi, for practicalvaluesof the ionic stmngthI, AV’& and AV~twillbeofoppositesignbutsimikumagnitudcunless the relativepermihity R of the solventis very low:
AV&t = (Rzz*&@/(1
+ BuP)*)[(a ln DIdpM3 + 2&P)
- g))
AVcorn! = (&?,&e2/4xe,,u)(~D~~~~
(3) (4)
lfwe assumethe reactantsto be hard spheresofroughly equalradii r and electrontransfer to occur w .zhhighestprobabilityat an M-M sepamtionu that varies with compressionof the solvent(&thermal compress&ii b), we havefor solventreorganization AVsR’= (N*2/167cE’l)[(f-’- d){d(n -2- R-‘)EP}r - (n” - D’)gnu]
(5)
ln equations3-5, (I is the anion-cationcloseappfoeehd&ant, B and C are the DebyeHiiclrelpanunetenatO.lMPa,nistire~iadexofthesolvan,andtheother symbolshavetheusualSImeaniags.Thus,theorypredictsthstA~~beequated tqqmbately with AVsat. Sii the’presswe dep&mesofbothnaadRare&isdy linked to that of the densityp of the solvent,i.e., to 8, it follows that dY’ is bferwtined in kqe part by the coIlDpplesFibiii~of tk sohmt. Sample datlations for modeI 3+i2+
couplesare givenin Table 1. As Table 1 shows,however,the compres&Ues of liquids, pahdady organic solvents,decreasemarkedlywith bcre&ng pressure, and so the dilated AV,t ad indeedaMthepammetersinTabie1deuease in ~&solutemagnihde ovex the typiad exfhened premtlre range,especiaUyat low pressums.TheresultisthatAVtitselfcan beexpectedtodependmarkedlyupo~pnssure,butthee~isleJ;sJisnificlntforwater than for nonaqueoussolvents.The implication is that, given the usual expaima#ai unce&mies,onecanmext&@ydiscusspmssureeffeasondectron~kinetiu,in aqrreoussystansoverthe0-200NParaneeintermsofeithofImavrrogevatueofA~ overthat~e(butthisraisesthequestionofbowthetheoreticalvaluesofAY”antobe
244
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