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Dynamics of proton tautomerism in 2,5-dihydroxy-p-benzoquinone: A 13C NMR and CNDO study
Volume 62. number Z
CHEhIlCAL
DYNAMICS OF PROTON TAUTOMERISM
PHYSICS
I AprrllP79
LE-l-l-ERS
IN 3,,5-DIHYDROXY-p-BENZOQUINONE:
A 13CNMR AND CNDO STUDY F. GRAF* IBM Research Laborarory.
Smz Jose. CaIifomia 95193. US.4
Received 9 June 1978; in final form 8 September
1978
hare been imestiThe temperature snd sobent dependence of the 13C N.\IR spectm of 2.5~dtydro~y-p-benzoquinone gated_The variations of the line shapes are xxdyzed in terms of chcmicel exchange of the two equrv&nt mtramolecular hy drogen bonds. The interpretation of the rate parameters su,,-wsts an interconrersion mechanism in n hich the two hydroxylic protons move in a two consecuthe step mechnnism. This is supported by the shape of the potential surface for the motion of the hydroxylic protons, calculated by CNDO.
Two classes of compounds
have recently
been given
attention by NMR spectroscopists with the aim of elucidating the structure and the kinetics of intramolecular hydrogen bonds_ On *&e one hand. there are porphyrins for which detailed data about the mechanism and the activation parameters associated with the motion of the two internal protons have been reported [ 1 ?_I _On the other hand. there are the enol forms of &diketones [3,4] and tropolone [S] in which only an averaged structure can be detected by magnetic resonance owing to the very rapid interconversion between the tautomeric forms_ It is true, though, that for the acetyl and trirmethyl siIyl derivatives of, e.g., tropolone [6,7], the activation parameters couid be derived from the temperature-dependent NMR line shape, but the very problem of the proton transfer across a five or six membered -0-H.._O=hydrogen chelate was not tackIed thereby_ In this letter, we wish to report the successful attempt to study the mechanism and the kinetics of the tautomerism in the 2,5-dihydroxy-p-benzoquinone (2,5-DHBQ). The reievant intramolecular reaction for this molecule is shown in fig. la which illustrates the two proton transfer process associated with this degenerate rearrangement. Obviously, the two olefinic special
* Permanent address: Physical Chemistry Laboratory. CH-8006 Zurich, Swi:zerland.
ETH,
“\ \\c-c /”
0
H-c/ L” \\c-c’
d
\
z
k-
$0
H2
H2
Gl
1. Qn2
c2 (a)
(b)
l‘ig_ 1. IntramolecuIrrr dynamics in the 2,5dIhydro\y-p-benzoquinone molecule- (a) t\xo proton transfer process;(b) molecular system and displacement coordinates of the hydrogen bridge protons.
atoms ( ZC-H) are related by symmetry. and wdl lead to a single (proton decoupled) r3C resonance because of their identical chemical shifts. Conversely, the carbonyl (I= C=O) and the enol (ZC-OH) carbon atoms interchange their role upon the protomeric process, so that this part of the r3C spectrum will be affected by chemical exchange. As pointed out elsewhere, r3C NMR spectra offer some advantages for dynamic studies including notably the larger chemical shifts than in proton NMR spectroscopy [8]. On the other hand, because of the lower sensitivity achieved with the former method, more concentrated solutions have to be employed in which intermoIecular processes may conceivably incarbon
291
Volume 63. number 2
1000
Hz 4
I
r-i_ 7__r3C h\lR spec:m of 2.5-dthydroxy-p-bcnroquinonerecordrd on s Vzrkm CI-T-20 spectrometer_ The sohzrionsare xbour 03 M (3) in THF& at --19=C: tbk in DMSO-& af 3-C. ppm scale dowG5Ad solvent peak_ terfere wirh the intr3mofccular process 3imed 3f in this study_ Therefore, the concentration of the soiutions of 25DHBQ was kept as fox as possible comprttibly with signal-tcFnoise ntio and/o: chemic;lI sfabiliry constnints- TypicAiy, the solutions (~0.3 M) were tmnsferred to the prcdried s3mpIe tubes under inert tltmosphcre- The deuter3ted solvents were dried over fr,-silly heated ahuninium oxide immediateIy prior to use in order to minimize possibte catalytic efticts of h_vdroxyIic impurities_ The proton decoupled I3C spectrum of 2.5DHBQ under different conditions is shown in fig_ 2_ At low temperature
in THFG&
(fig_ 21). the spectrum
three isolated peaks of obvious assignment 3s indicated in the figure_ the two downfield peaks being sepamted by about 24 ppm_ Fig_ 26 shows the spectrum of ?,5-DHBQi16 solution at room tempemture- QnIy a downfield
line is apparent,
Q,tBl c
.a -4
--4 -2
whereas
the line of the olefinic carbon remains sharp_ CIesrIy
for this last spectrum. the fst exchange limit has been re.&ed for the process fig_ !a.*111generzd, analysis of 3 comples exchange problem would require 3 computerized iine shrtpe 3nalysis, but due to the simple form of the spectrum, the rate of exchange can be calculated from zmalytical stow exchange or fst exchzmge limiting formuizle [9,10]. 292
The Arrhenius plot for the exchange rate in THFrig leads to the activation parameters: AE = 5.4 f OJ kul/moIe, In A = 11 .O i I and in DMSO-d6 to the v;lIues: AE = 4.7 i OS kcaI/moIe. In A = 17.6 * I _ It is worthwhile to renll that the determination ofactjvation parameters may suffer from v3rious systematic errors which may originate in this p3rticular c3se from, e.g., second order kinetic contributions to the Iine shape [S]_ Such contributions tend to 3cceIerate the exchange, so th;lt the AE v3lues found in this work would become Iower bounds to the true activation energies for the process fig_ I _ These solvent effects may provide some clue about the nature of the mechanism invoIved, but, obviousIy, such a discussion is most n3turAy conducted within the framework of 3 Born-Oppenheimer potentizd function for the electronic ground st3te [l I] _ For this purpose, 3 CNDO calculation W.LSperformed in which the hydroxylic protons HI and H2 3re considered independent and moving along 3 displacement coordinate @tl and Q!J~, respectiveiy, cf. fig. lb. Standard Pople geometries [ 121 were used for the C6H_,04 system, which was allowed to relax sccording to the positions of the protons HI and H2 in the
con-
sists of
singIe, broadened
1 April 1979
CIIEMICAL PtlYSICS LETTERS
~
-Al
-_k
:0
2
_‘Ei
Fig. 3. CNDO potential energy surface for the motion of the two protons Ii1 and H? within their hydrogen bridges. The reaction coordinates Q and Q,, are defined according to tis. lb. Points C. Tan“CIS denote?he ground state. the transition state and the saddle point, respectively. The energy increment (or decrement) between consecutive energy Ievel lines corresponds to 10 kcalfmole.
Volume
6’7. number 2
CHEWCAL
&I &I2 plane. In this way, a smooth structur31 transition between the structures G I and G3 of fig. la was achieved. The CNDO potential surface thus obtained, visualized in Iig. 3, is characterized by a welldefined mlnhnum G, a mxximum T and a saddle point S_ With respect to the value of such a calculation. it should be pointed out that 3 more reIi3ble approach would require the nctu3I geometry optimization of ali the ground state and transition st3te structures involved. 3s has been done on seveml occasions for the much simpler c3se of 3 single proton transfer [ 13_14]_ Nevertheless. the features of the potential surface of fig. 3 dre so clex thtlt it is believed that they cm form the basis for .t qwlitative discussion_ In fact, it seems worthwhile to recall that with respect to the two proton transfer process. the main issue is to establish whether this occurs via 3 two cxrsecutive jump mechanism or, rather, via a simultaneous double jump mechrtnism. 3s is currently debated in the case of the porphyrins [ I,?] _For our molecule. the simultaneous double jump corresponds to the dashed line 3long the Qttt = QtIl diagonal joining Gl and G2 and pxsing through T, cf. lig. 4. The tlctivation profile encounrered 3long this path is ch3rscterizcd by 3 high and shsrp b.rrrier of height E2_ The two consecutive single jumps correspond to the full line G I -S2-G2 (or G I -SI -G2)_ where first H3 (HI) moves along QI12 (or, I) for 3 distance d of .tbout 1-G A passing d potentidl barrier of height E, and tften proceeds from S2 (Sl) for another distmce of about d passing a very low barrier gt on its way down to G2. Even though the absolute CNDO values cannot be used 3s such for the rezrson mentioned above. E1 turns out to be more thm twice 3s large 3s Et _which is a strong xgument in favor of the two consecutive steps mechanism. Along the Qttt = QH-, diagon31, the molecule ret3ins its center of symmetry and has therefore zero dipole moment afl the way along the path from G I 2nd G2 md therefore no dependence of the exch3nge r3te upon the polxity of the solvent would be expected_ Along QHt (or QHz) alone. the molecuie loses its center of symmetry, and Sl (or S2) hss indeed a very huge calculated dipole moment of about 5 debye. For the two step mechanism 3 dependence upon the polarity of the solvent is espetted, since a high dielectric constant of the solvent will selectively stabilize the transition state, thus accelerating the interconversion reaction. Now, it h3s to be pointed out that the choice of suita-
PHkSICS
LfTI-ERS
1 Aprd 1979
4
-8
-4
.
d
00
4
tiu > -8
%,
Fig_ 4. Pztthwz>s for the mtrmnolrculx proton trzmsfa in 2.5 DHBQ. Cl tnd G2 are the tv.o itable equthbrium structures \\ith the protons on opposite sides of the? = pl.mc. In the structures Sl nnd S2. the t\\o h) dro\> hc protons .z:e on the same side of rhe_;s plane. The dashed pathxts)- Cl -T-G? is associated 1%ith an encr~? pro!ile chxxterized by Ez (double proton jump), \\herezs the contimtous pathxxa) Cl -S1G3 IS nuocuted rrith tile txxo bxriers Et, 5; (consec~tlre single proton Jumps-L The quantity d denotes rhc dlaance cowred b) a ample proton betnern t\to taurorneric forms.
ble solvents for 2,5DHBQ is severely hmited by its poor solubility m nonpolar solvents: however, DMSO (E = -%6.-l) IS 3 much more polar sohent than TliF (E = 7.-i>. and the lower activatron energy found in the former 31~0 points toward a consecutive step mechanism. it is interesting th3t in their latest paper [Z!] Eaton md E&on claim that their kinetic d3ta for porp!ryrins 3re consisrent with a simultaneous movement of two hydrogens through 3 symmetrtcaf transition state. This conviction is based upon the f3cr that a large kinetic (isotope) effect kli/kD of about 20 is found st room temperature wheress 3 single N-H vibration could account for 3 factor of about 10 at most [ 15]_The latter statement beingundoubtedly correct, it is also true that more than one vibration can be involved in *the intramolecular transfer of a single proton, as has been postulated for a molecule with one intramolecular live-membered hydrogen chelate for which a kinetic isotope effect of about lo@ 113s been found at room temperature [ 14]_ In other words. 293
Volume 62. number 2
CHEMICAL
even though the double jump mechanism does require a Iarge kinetic isotope effect, the latter by itself is not sufficient proof urdess substantiated by more experimental evidence or quantum chemical arguments. Finally. a Iast comment applies to the preexponential factors of the Arrhenius equation, albeit with the reserva:ions mentioned above. In both THF and DMSQ these turn out to be lower than for porphyrins, especiaIIy in THF. thus pointing toward large negative activation entropies_ Besides, in intermolecular hydrogen bonding, \+hose rote and importance have to be investigated more thoroughly, the reason for it could be found in the structural difference of the hydrogen bonds in the two types of moiecutes. In porphyrins, the protons are well localized in a planar cage and ahvays in favorable positions for the jump, whereas the hydroxylic protons in X,5-DHBQ may find themseives out of the II-plane or even on the other side of the hydrogen bond owing to the internal rotation of the hydroxylic groupsAs 3 conclusion, it should be emphasized that the t= o proton transfer process in 2,5_dihydrox~-p-benzoquinone appears to be rather different from that operating in porphyrins_ the on& other case to our knowledge which could be studied by NMR spectroscopy so farAdditional experimental work, e.g._ on isotope and solvent effects as well as better quantum chemicai calcuIations, are bound to reveal more information about the intramolecular dynamics of this interesting moiecute_
PHYSICS LETTERS
1 Aprit 1979
References [ 11 C.S. Irving and A. lapidot.
J_ Chem_ Sot_ Chem. Com-
mun. (1977) 184. [21 S.S. Eaton and G-R. &ton, 1604.
J_ Am. Chem. Sot. 99 (1977)
[31 iv_ J&W. G. Gunnzss on_ T-E BuU and S_ Forsen. J_ Am. Chem. Sot. 99 (1977) 4568_ [4] AU-T- Robinson, KM. Rosen ar.d J.D.B. Workman, Tetmhedron 33 (1977) 1655. [S] L. WciIer, Can. J. Chem- 50 (1972) 1975. 161 S. bfasamune, A-V_ Kemp-Jones, J. Green. D-L. bbenstein. &I_Yasunzuni. K_ T&ase and T_ Nozoe. J. Chem. Sot_ Chem_ Commun. (1973) 283. 171 NJ_ Reich and D.A_ Murk. J. Am. Chem. Sot. 95 (1973) 3418. [S] F_A.L- Anrt and R_ Anet. in: Dynamic nuclear magnetic resonance spectroscopy, eds. L-M. Jackman and F-A. Cotton (Academic Press, New York. 1975) ch. 14. 191 _A_Cz.urin=tonand A-D. McLachIan, Introduction to ma=netic resonance (Harper and Row* London, ? 969) ch. 12. [ 10; A. Bferbach and J-C_ Biinzli. Helv. Chim. Acta 55 (1972) 1903. [ I1 1 P. Schuster, in: The hydrogen bond, Vol. 1, eds_ P_ Schuster, C. Zundel and C. Sandorfy (North-tIolkmd_ Amsterdam. 1976) ch_ 2_ 1121 J.A. Popie, D.L. Bevetidge and P-A_ Dobosh. J_ Am_ ChcmSot_ 90 (1968) 42OI_ [ I3 1 G. Karstriim. H. WennerstrGm. B_ J&won. S_ Worsen. J. AlmlGf and B. Roos, J. Am. Chem. Sot. 97 (1975) 4188[ 141 Ii_ Loth, F. Graf and Hs. H. Giinthard, Chem. Phys. 13 (1976) 95 [ 1.5 1 R-P_ Bell. The proton in chemistry, 2nd Ed_ (Cornell Univ. Press. Ithaca, 1973) ch. 12.
CHEhIlCAL
DYNAMICS OF PROTON TAUTOMERISM
PHYSICS
I AprrllP79
LE-l-l-ERS
IN 3,,5-DIHYDROXY-p-BENZOQUINONE:
A 13CNMR AND CNDO STUDY F. GRAF* IBM Research Laborarory.
Smz Jose. CaIifomia 95193. US.4
Received 9 June 1978; in final form 8 September
1978
hare been imestiThe temperature snd sobent dependence of the 13C N.\IR spectm of 2.5~dtydro~y-p-benzoquinone gated_The variations of the line shapes are xxdyzed in terms of chcmicel exchange of the two equrv&nt mtramolecular hy drogen bonds. The interpretation of the rate parameters su,,-wsts an interconrersion mechanism in n hich the two hydroxylic protons move in a two consecuthe step mechnnism. This is supported by the shape of the potential surface for the motion of the hydroxylic protons, calculated by CNDO.
Two classes of compounds
have recently
been given
attention by NMR spectroscopists with the aim of elucidating the structure and the kinetics of intramolecular hydrogen bonds_ On *&e one hand. there are porphyrins for which detailed data about the mechanism and the activation parameters associated with the motion of the two internal protons have been reported [ 1 ?_I _On the other hand. there are the enol forms of &diketones [3,4] and tropolone [S] in which only an averaged structure can be detected by magnetic resonance owing to the very rapid interconversion between the tautomeric forms_ It is true, though, that for the acetyl and trirmethyl siIyl derivatives of, e.g., tropolone [6,7], the activation parameters couid be derived from the temperature-dependent NMR line shape, but the very problem of the proton transfer across a five or six membered -0-H.._O=hydrogen chelate was not tackIed thereby_ In this letter, we wish to report the successful attempt to study the mechanism and the kinetics of the tautomerism in the 2,5-dihydroxy-p-benzoquinone (2,5-DHBQ). The reievant intramolecular reaction for this molecule is shown in fig. la which illustrates the two proton transfer process associated with this degenerate rearrangement. Obviously, the two olefinic special
* Permanent address: Physical Chemistry Laboratory. CH-8006 Zurich, Swi:zerland.
ETH,
“\ \\c-c /”
0
H-c/ L” \\c-c’
d
\
z
k-
$0
H2
H2
Gl
1. Qn2
c2 (a)
(b)
l‘ig_ 1. IntramolecuIrrr dynamics in the 2,5dIhydro\y-p-benzoquinone molecule- (a) t\xo proton transfer process;(b) molecular system and displacement coordinates of the hydrogen bridge protons.
atoms ( ZC-H) are related by symmetry. and wdl lead to a single (proton decoupled) r3C resonance because of their identical chemical shifts. Conversely, the carbonyl (I= C=O) and the enol (ZC-OH) carbon atoms interchange their role upon the protomeric process, so that this part of the r3C spectrum will be affected by chemical exchange. As pointed out elsewhere, r3C NMR spectra offer some advantages for dynamic studies including notably the larger chemical shifts than in proton NMR spectroscopy [8]. On the other hand, because of the lower sensitivity achieved with the former method, more concentrated solutions have to be employed in which intermoIecular processes may conceivably incarbon
291
Volume 63. number 2
1000
Hz 4
I
r-i_ 7__r3C h\lR spec:m of 2.5-dthydroxy-p-bcnroquinonerecordrd on s Vzrkm CI-T-20 spectrometer_ The sohzrionsare xbour 03 M (3) in THF& at --19=C: tbk in DMSO-& af 3-C. ppm scale dowG5Ad solvent peak_ terfere wirh the intr3mofccular process 3imed 3f in this study_ Therefore, the concentration of the soiutions of 25DHBQ was kept as fox as possible comprttibly with signal-tcFnoise ntio and/o: chemic;lI sfabiliry constnints- TypicAiy, the solutions (~0.3 M) were tmnsferred to the prcdried s3mpIe tubes under inert tltmosphcre- The deuter3ted solvents were dried over fr,-silly heated ahuninium oxide immediateIy prior to use in order to minimize possibte catalytic efticts of h_vdroxyIic impurities_ The proton decoupled I3C spectrum of 2.5DHBQ under different conditions is shown in fig_ 2_ At low temperature
in THFG&
(fig_ 21). the spectrum
three isolated peaks of obvious assignment 3s indicated in the figure_ the two downfield peaks being sepamted by about 24 ppm_ Fig_ 26 shows the spectrum of ?,5-DHBQi16 solution at room tempemture- QnIy a downfield
line is apparent,
Q,tBl c
.a -4
--4 -2
whereas
the line of the olefinic carbon remains sharp_ CIesrIy
for this last spectrum. the fst exchange limit has been re.&ed for the process fig_ !a.*111generzd, analysis of 3 comples exchange problem would require 3 computerized iine shrtpe 3nalysis, but due to the simple form of the spectrum, the rate of exchange can be calculated from zmalytical stow exchange or fst exchzmge limiting formuizle [9,10]. 292
The Arrhenius plot for the exchange rate in THFrig leads to the activation parameters: AE = 5.4 f OJ kul/moIe, In A = 11 .O i I and in DMSO-d6 to the v;lIues: AE = 4.7 i OS kcaI/moIe. In A = 17.6 * I _ It is worthwhile to renll that the determination ofactjvation parameters may suffer from v3rious systematic errors which may originate in this p3rticular c3se from, e.g., second order kinetic contributions to the Iine shape [S]_ Such contributions tend to 3cceIerate the exchange, so th;lt the AE v3lues found in this work would become Iower bounds to the true activation energies for the process fig_ I _ These solvent effects may provide some clue about the nature of the mechanism invoIved, but, obviousIy, such a discussion is most n3turAy conducted within the framework of 3 Born-Oppenheimer potentizd function for the electronic ground st3te [l I] _ For this purpose, 3 CNDO calculation W.LSperformed in which the hydroxylic protons HI and H2 3re considered independent and moving along 3 displacement coordinate @tl and Q!J~, respectiveiy, cf. fig. lb. Standard Pople geometries [ 121 were used for the C6H_,04 system, which was allowed to relax sccording to the positions of the protons HI and H2 in the
con-
sists of
singIe, broadened
1 April 1979
CIIEMICAL PtlYSICS LETTERS
~
-Al
-_k
:0
2
_‘Ei
Fig. 3. CNDO potential energy surface for the motion of the two protons Ii1 and H? within their hydrogen bridges. The reaction coordinates Q and Q,, are defined according to tis. lb. Points C. Tan“CIS denote?he ground state. the transition state and the saddle point, respectively. The energy increment (or decrement) between consecutive energy Ievel lines corresponds to 10 kcalfmole.
Volume
6’7. number 2
CHEWCAL
&I &I2 plane. In this way, a smooth structur31 transition between the structures G I and G3 of fig. la was achieved. The CNDO potential surface thus obtained, visualized in Iig. 3, is characterized by a welldefined mlnhnum G, a mxximum T and a saddle point S_ With respect to the value of such a calculation. it should be pointed out that 3 more reIi3ble approach would require the nctu3I geometry optimization of ali the ground state and transition st3te structures involved. 3s has been done on seveml occasions for the much simpler c3se of 3 single proton transfer [ 13_14]_ Nevertheless. the features of the potential surface of fig. 3 dre so clex thtlt it is believed that they cm form the basis for .t qwlitative discussion_ In fact, it seems worthwhile to recall that with respect to the two proton transfer process. the main issue is to establish whether this occurs via 3 two cxrsecutive jump mechanism or, rather, via a simultaneous double jump mechrtnism. 3s is currently debated in the case of the porphyrins [ I,?] _For our molecule. the simultaneous double jump corresponds to the dashed line 3long the Qttt = QtIl diagonal joining Gl and G2 and pxsing through T, cf. lig. 4. The tlctivation profile encounrered 3long this path is ch3rscterizcd by 3 high and shsrp b.rrrier of height E2_ The two consecutive single jumps correspond to the full line G I -S2-G2 (or G I -SI -G2)_ where first H3 (HI) moves along QI12 (or, I) for 3 distance d of .tbout 1-G A passing d potentidl barrier of height E, and tften proceeds from S2 (Sl) for another distmce of about d passing a very low barrier gt on its way down to G2. Even though the absolute CNDO values cannot be used 3s such for the rezrson mentioned above. E1 turns out to be more thm twice 3s large 3s Et _which is a strong xgument in favor of the two consecutive steps mechanism. Along the Qttt = QH-, diagon31, the molecule ret3ins its center of symmetry and has therefore zero dipole moment afl the way along the path from G I 2nd G2 md therefore no dependence of the exch3nge r3te upon the polxity of the solvent would be expected_ Along QHt (or QHz) alone. the molecuie loses its center of symmetry, and Sl (or S2) hss indeed a very huge calculated dipole moment of about 5 debye. For the two step mechanism 3 dependence upon the polarity of the solvent is espetted, since a high dielectric constant of the solvent will selectively stabilize the transition state, thus accelerating the interconversion reaction. Now, it h3s to be pointed out that the choice of suita-
PHkSICS
LfTI-ERS
1 Aprd 1979
4
-8
-4
.
d
00
4
tiu > -8
%,
Fig_ 4. Pztthwz>s for the mtrmnolrculx proton trzmsfa in 2.5 DHBQ. Cl tnd G2 are the tv.o itable equthbrium structures \\ith the protons on opposite sides of the? = pl.mc. In the structures Sl nnd S2. the t\\o h) dro\> hc protons .z:e on the same side of rhe_;s plane. The dashed pathxts)- Cl -T-G? is associated 1%ith an encr~? pro!ile chxxterized by Ez (double proton jump), \\herezs the contimtous pathxxa) Cl -S1G3 IS nuocuted rrith tile txxo bxriers Et, 5; (consec~tlre single proton Jumps-L The quantity d denotes rhc dlaance cowred b) a ample proton betnern t\to taurorneric forms.
ble solvents for 2,5DHBQ is severely hmited by its poor solubility m nonpolar solvents: however, DMSO (E = -%6.-l) IS 3 much more polar sohent than TliF (E = 7.-i>. and the lower activatron energy found in the former 31~0 points toward a consecutive step mechanism. it is interesting th3t in their latest paper [Z!] Eaton md E&on claim that their kinetic d3ta for porp!ryrins 3re consisrent with a simultaneous movement of two hydrogens through 3 symmetrtcaf transition state. This conviction is based upon the f3cr that a large kinetic (isotope) effect kli/kD of about 20 is found st room temperature wheress 3 single N-H vibration could account for 3 factor of about 10 at most [ 15]_The latter statement beingundoubtedly correct, it is also true that more than one vibration can be involved in *the intramolecular transfer of a single proton, as has been postulated for a molecule with one intramolecular live-membered hydrogen chelate for which a kinetic isotope effect of about lo@ 113s been found at room temperature [ 14]_ In other words. 293
Volume 62. number 2
CHEMICAL
even though the double jump mechanism does require a Iarge kinetic isotope effect, the latter by itself is not sufficient proof urdess substantiated by more experimental evidence or quantum chemical arguments. Finally. a Iast comment applies to the preexponential factors of the Arrhenius equation, albeit with the reserva:ions mentioned above. In both THF and DMSQ these turn out to be lower than for porphyrins, especiaIIy in THF. thus pointing toward large negative activation entropies_ Besides, in intermolecular hydrogen bonding, \+hose rote and importance have to be investigated more thoroughly, the reason for it could be found in the structural difference of the hydrogen bonds in the two types of moiecutes. In porphyrins, the protons are well localized in a planar cage and ahvays in favorable positions for the jump, whereas the hydroxylic protons in X,5-DHBQ may find themseives out of the II-plane or even on the other side of the hydrogen bond owing to the internal rotation of the hydroxylic groupsAs 3 conclusion, it should be emphasized that the t= o proton transfer process in 2,5_dihydrox~-p-benzoquinone appears to be rather different from that operating in porphyrins_ the on& other case to our knowledge which could be studied by NMR spectroscopy so farAdditional experimental work, e.g._ on isotope and solvent effects as well as better quantum chemicai calcuIations, are bound to reveal more information about the intramolecular dynamics of this interesting moiecute_
PHYSICS LETTERS
1 Aprit 1979
References [ 11 C.S. Irving and A. lapidot.
J_ Chem_ Sot_ Chem. Com-
mun. (1977) 184. [21 S.S. Eaton and G-R. &ton, 1604.
J_ Am. Chem. Sot. 99 (1977)
[31 iv_ J&W. G. Gunnzss on_ T-E BuU and S_ Forsen. J_ Am. Chem. Sot. 99 (1977) 4568_ [4] AU-T- Robinson, KM. Rosen ar.d J.D.B. Workman, Tetmhedron 33 (1977) 1655. [S] L. WciIer, Can. J. Chem- 50 (1972) 1975. 161 S. bfasamune, A-V_ Kemp-Jones, J. Green. D-L. bbenstein. &I_Yasunzuni. K_ T&ase and T_ Nozoe. J. Chem. Sot_ Chem_ Commun. (1973) 283. 171 NJ_ Reich and D.A_ Murk. J. Am. Chem. Sot. 95 (1973) 3418. [S] F_A.L- Anrt and R_ Anet. in: Dynamic nuclear magnetic resonance spectroscopy, eds. L-M. Jackman and F-A. Cotton (Academic Press, New York. 1975) ch. 14. 191 _A_Cz.urin=tonand A-D. McLachIan, Introduction to ma=netic resonance (Harper and Row* London, ? 969) ch. 12. [ 10; A. Bferbach and J-C_ Biinzli. Helv. Chim. Acta 55 (1972) 1903. [ I1 1 P. Schuster, in: The hydrogen bond, Vol. 1, eds_ P_ Schuster, C. Zundel and C. Sandorfy (North-tIolkmd_ Amsterdam. 1976) ch_ 2_ 1121 J.A. Popie, D.L. Bevetidge and P-A_ Dobosh. J_ Am_ ChcmSot_ 90 (1968) 42OI_ [ I3 1 G. Karstriim. H. WennerstrGm. B_ J&won. S_ Worsen. J. AlmlGf and B. Roos, J. Am. Chem. Sot. 97 (1975) 4188[ 141 Ii_ Loth, F. Graf and Hs. H. Giinthard, Chem. Phys. 13 (1976) 95 [ 1.5 1 R-P_ Bell. The proton in chemistry, 2nd Ed_ (Cornell Univ. Press. Ithaca, 1973) ch. 12.