Electrochemistry of Ir-Bisesqui- and Tris(phen) Complexes
The Journal of Physical Chemistty, Vol. 83, No. 20, 7979
(19) H. Caldararu, K. DeArmond, K. Hanck, and V. Sahini, J. Am. Chem. Soc., 98, 4455 (1976). (20) R. Ballardini, G. Varani, L. Moggi, V. Balzani, K. Olsen, F. Scandola, and M. Hoffman, J . Am. Chem. Soc., 97, 728 (1975).
201 1
(21) W. L. Huang, C. M. Carlin, and M. K. DeArmond, Inorg. Chem., in press. (22) S. Roffia and M. Ciano, J . Electroanal. Chem., 87, 267 (1978). (23) S. Roffia and M. Ciano, J. Electroanal. Chem., 77, 349 (1977).
Electrochemistry of Iridium Tris- and Bisesquis(phenanthro1ine) Complexes J. L. Kahl, Kenneth W. Hanck,“ and Keith DeArmond” Department of Chemistty, North Carolina State University, Raleigh, North Carolina 27650 (Received August 14, 1978; Revised Manuscript Received April 9, 1979) Publication costs assisted by North Carolina State University
Cyclic voltammetry data have been obtained for the tris- and bisesquis(1,lO-phenanthroline)(phen) complexes of iridium(II1)in acetonitrile. A reaction mechanism is proposed on the basis of the analysis of electrochemical and luminescence data. For the bisesqui- and tris(phen) complexes the mechanisms are similar to the corresponding 2,2’-bipyridine (bpy) Ir(II1) complexes except that an insoluble deposit forms after three electrons ] ~ + , comparison have been transferred. Similar behavior has been observed for the reduction of [ R ~ ( b p y ) ~however, of the reverse (oxidation) scan for the first reduction wave of the tris Ir(II1) and Ru(I1) complexes indicates distinctly different behavior. These data are interpreted as resulting from the multiring delocalized redox orbital of the Ru(I1) complex and the single ring delocalized redox orbital of the Ir(II1) complex.
Introduction In the preceding paper,’ the electron transfer sequence was determined for the bis(1,lO-phenanthroline)(phen) and bis (5,6-dimethyl-1,lO-phenanthroline) (5,6-Mephen) complexes of Ir(II1). A Zeiss type adsorption2i3interaction of the phen ring system with the platinum electrode was postulated and verified by comparison with the sterically hindered 5,6-Mephen complex. These results are basically consistent with predictions from the luminescence behavior of the bis ~ o m p l e x .The ~~~ luminescence spectra at 77 K for the difficult to synthesize [ I r ( ~ h e n ) ~and ]~+ bisesquis (meaning two and one-half) [Ir(phen)2(OH)(phen)]2+ complexes have been published elsewhere7and are similar to their bpy analogues.8-10 Room temperature emission spectra are consistent with the low temperature spectra although weak and less well resolved. The emission spectra for the two analogous bpy c o m p l e ~ ehave s ~ ~ been ~ assigned as a mr*phosphorescence,consequently the two tris(phen) complexes can be similarly assigned, therefore “delocalized” redox orbitals are predicted. The further description4p5of the delocalized orbitals as “single ring” indicating that the redox orbital utilizes only one of the three chelate rings or “multiring” to indicate that the redox orbital spans two or more of the three chelate rings has further ramifications for the electron transfer sequence. For example, the rapid splitting out of a ligand for the [ R h ( b p ~ ) ~complex1 ]~+ where luminescence indicates a delocalized orbital emission can be rationalized as resulting from the instability of the species produced which contains a full electron charge on a single ring. Cyclic voltammetry and coulometry data have been acquired to permit description of the electron transfer sequence and the adsorption interactions for these tris(phen) complexes. From these data and the heterogeneous rate constants, k,, the redox orbital ~ h a r a c t e r ~can , ~ Jbe ~ determined and compared with the luminescence behavior. Experimental Section The reagents, electrodes instrumentation, and experimental conditions have been described previ0us1y.l~~ All potentials are in reference to the aqueous 0022-365417912083-2611$0 1.OO/O
Synthesis. The preparation of [Ir(phen)2(OH)(phen)](NO3), was identical with the bisesqui(bpy) complex previously de~cribed.~JO ‘H NMR in Me2sO-d~ showed a singlet at 8.76 ppm, doublets at 10.16, 9.49,9.33, 9.19,8.97,8.42,8.30,8.20,8.08, and 7.30 ppm, and a triplet at 8.66 ppm vs. Me4Si. Anal. Calcd for [Ir(phen)z(OH)(phen)](N03)z.5H20: C, 44.86; H, 3.66; N, 11.62. Found: C, 44.92; H, 3.00; N, 11.56. [Ir(~hen),](NO~)~ was prepared by the method of Flynn and Demas7 and purified by the method used for the tris(bpy) c ~ m p l e x . ~lH J ~ NMR in Me2sO-d~showed a singlet at 8.78 ppm and doublets at 9.34, 8.40, 8.22, and 8.16 ppm vs. Me4Si. 13C NMR in Me2SO-d6 showed chemical shifts of 153.1,146.4,141.8,131.9,128.9,and 127.7 ppm vs. Me4Si. Anal. Calcd for [Ir(phen)3](N03)3.3H20: C, 44.44; H, 3.11; N, 12.96. Found: C, 44.04; H, 3.02; N, 12.90. [Ru(phen),] (C104)2was prepared by a method similar to Tokel-Takvoryan et a1.12 The emission spectra of the bisequis (acid and base media) and tris complexes at 77 K in ethanol-methanol glass and in 0.1 M TEAP-AN were used to check the purity of the materials and are published in ref 10. Results Table I contains peak potentials as a function of scan rate for the bisesqui- and tris(phen) complexes. Reduction peaks are designated by increasing positive Roman numerals, I through VI, proceeding toward more negative potentials. No peaks were observed between +1.90 and -0.90 V. The bisesqui- and tris(phen) complexes both show an irreversible oxidation (designated -I) at a potential more positive than +1.90 V (Table I). [Ir(phen),(OH)@hen)lz+.Cyclic voltammograms of the first two reduction peaks are shown in Figure 1. A stepwise controlled potential coulometry (CPC) study done first at -1.17 V (n = 0.96) and then at -1.40 V ( n = 0.87) yields an electrode coated with an insoluble black deposit. Figure 2 shows the voltammetric behavior of the bisesquis complex over all reduction peaks as a function of scan rate. A stepwise CPC study was done first at -1.17 V (n = 0.96), 0 1979 American Chemical Society
2612
J. L. Kahl, K. W. Hanck, and K. DeArmond
The Journal of Physical Chemistry, Vol. 83, No. 20, 1979 I
I
I
I
,
b.
I
I
-0.6
I
I
I
-1 4
-0 6
-1 0
I
-1.0
I
V vs SCE Flgure 1. Cyclic voltammograms of [Ir(phen)z(OH)(phen)]2+,4.0 X M: (a) 0.1 V/s, (b) 100 VIS, (c) 0.1 V/s, (d) 1.0 VIS, (e) 20 VIS, (f) 100 V/s. I
I
I
I
I
I
I
I
1
I
-1.'
-0.6
I
-1.0
I
-1.q
I
-0.6
I
-1.0
I
-1.'
V vs SCE Figure 3. Cyclic voltammograms of [Ir(phen),13+, 6.2 X M: (a) 0.1 V/s, (b) 1.0 V/s, (c) 5.0 V/s, (d) 10 V/s, (e) 20 V/s, (f) 100 V/s.
l
0
F
z c x LT w
3
"
0
0
.-L --0.6
-1.0
-1.4
-1.8
I
-2.2
-0.6
-1.0
-1.4
-1.8
-2.2 -2.6
I
-2.1
V vs SCE Flgure 2. Cyclic voltammograms of [Ir(phen),(OH)(phen)]2+, 4.0 X M: (a) 0.1 V/s, (b) 10 V/s, (c) 100 V l s .
then at -1.40 V (n = 0.87), and finally at -1.80 V ( n = 0.11). A CPC experiment on the starting material at -1.80 V ( n = 1.89) yields an electrode coated with an insoluble black deposit. Emission spectra at 77 K for all CPC reduction products produced no evidence for free phen in s ~ l u t i o n . ~ [ I r ( ~ ~ h e n ) Cyclic ~ ] ~ + voltammograms . of the first three reduction peaks are shown in Figure 3. A stepwise CPC experiment done first at -0.98 V ( n = 1.06), then at -1.10
V vs SCE Flgure 4. Cyclic voltammograms of [ I r ( ~ h e n ) ~ ] 2.9 ~ + ,X lo-' M: (a) 10 V/s, (b) 20 V/s, (c) 100 Vls.
V ( n = 1.02), and finally at -1.30 V ( n = 0.95) yields an electrode coated with an insoluble black deposit. Coulometric reduction of the starting material at -1.30 V yields n = 3.08 and a black insoluble deposit on the electrode. Further reductions of the solution were prevented by the coating on the electrode. Emission spectra at 77 K for all CPC reduction steps produced no evidence for free phen in s ~ l u t i o n .Figure ~ 4 shows the voltammetric behavior of the tris complex as a function of scan rate.
Electrochemistry of Ir-Bisesqui- and Tris(phen) Complexes
The Journal of Physical Chemistry, Vol. 83, No. 20, 1979 2613
TABLE I: Variation of Anodic (Epa)and Cathodic ( E c ) Peak Potentials as a Function of v for Cyclic Voltammograms of Bisesqui- and Tris(phen) Complexes in 0.1 M TEAP-A8 at 32 mm2 Pt Wire, E, in V vs. SCE, v in V/s, EpAverage of at Least Three Scans, All Values Are k 5 mV A. [Ir(phen),(0H)(phen)l2+, 4.0 x
I
-I
v
Epa
100 20 10 5 1
+2.118 +2.116 +2.030 +2.016 +1.986 t1.952
Epc b b b b b 0.1 b
I1
Epc
Epa
-1.153 -1.014 c -1.111 -1.108 c -1.099 a -1.083 d -1.070 d
I11
Epc
Epa
Epc
Epa
Epc
-1.370 -1.314 -1.305 -1.294 -1.272 -1.259
-1.190 -1.194 c a
-1.720 -1.630 -1.622
-1.480 -1.565 -1.560
a a a
C
C
C
u
C C
c
-1.820 -1.786
a
v 100 20 10 5 1 0.1 e
Epc b b b b b b
I
a
b b b b b b
V
EPC
Epa
EPC
Epa
-2.286 -2.144
-1.998
-2.610 -2.462
-2.258
C
a a a
C C
C C
a a
a a
c
C
a a
a
c
M
I11
I1
IV
II1ads
B. [Ir(phen),13+,4.1 x -I
M
IV
V
VI
Epa
EPC
Epa
Epc
Epa
Epc
Epa
EPC
Epa
Epc
Epa
EPC
C2.808 +2.726 +2.665 +2.600 +2.651 t2.595
-1.003 -0.973 -0.965 -0.957 -0.947 -0.928
-0.882
-1.143 -1.091 -1.083 -1.072 -1.058 -1.041
-1.023
-1.310 -1.250 -1.242 -1.230 -1.216 -1.208
-1.209
-2.070 -2.018 -2.005
-1.941 -1.944 -1.957
-2.366
-2.210
-2.642
-2.438
c c
C
C C
C C
C
a
C
a
a
a a
a a
C C a
c c a
e e
I
:J
20uA
a
a a
c
c u
a a
Not present at any scan rate.
Not present at this scan rate. Stripping spike at -0.918. I
c c
Not a well-defined peak.
C
a a a
a
a a
Stripping spike at -0.970.
I
+2+J
C Ir Lz (OH) M I " -I
r-e-
r-
CIrLz (OH) MI''
C I r LZ(OH 1MI'-
c I r LZ(OH 1M I "
C I r L z(OH) MI"' ne-
+e-J
r
I1
1
+
+e-J
IV
[I rL2 ( OH 1M I 2 -
.
1 OmA
+e-J
+
Figure 5.
:
v
[I r LZ(OH 1MI3-
1
I
I
I
I
I
-1.0
-1.4
-1.8
-1.0
-1.4
-1.8
/
V vs SCE Cyclic voltammograms of [Ru(phen),]*',
6.0 X M: (a) 0.1 V/s, (b) 5.0 V/s, (c) 0.1 V/s, (d) 1.0 V/s, (e) 10 VIS, (f) 100 V l s .
The heterogeneous standard rate constant, k,, was calculated for the free phen ligand, free 5,6-Mephen ligand, bis(phen) complex, bis(5,6-Mephen) complex, bisesqui(phen) complex, and tris(phen) complex. The method used has been outlined by Nich01son.l~ The kinetic data for couples I, 11, and V of the bis complexes are based on fast scan rates to avoid errors due to the elimination of chloride from the bis complexes at slow scan rates.l The kinetic data for couples I and I1 of the bisesquis complex and couples I, 11, and I11 of the tris complex are based on fast scan rates since irreversible behavior occurs at slow scan rates (Table I, Figure la). [ R ~ ( p h e n ) ~ Cyclic ] ~ + , voltammograms of the first two reduction peaks are shown in Figure 5.
Figure 6. Proposed reaction mechanism for [Ir(phen),(OHXphen)] '+, L = phen, M = monodentate phen. Roman numerals indicate peak number.
Discussion [Ir(phen)z(Olcn(phen)]2+. An electron transfer mechanism is proposed in Figure 6 which is consistent with the Coulometric, voltammetric, and spectroscopic (no free phen) results obtained. At fast scan rates the bisesquis complex undergoes five consecutive one electron transfers (Figure 2 4 . At slow scan rates an adsorption spike at -1.79 V (Figure 2a) is present and is likely associated with the deposit of [Ir(phe&(OH)(phen)]" on the electrode. This wave (HIadB) would then be a post wave for the third reduction step.14 This zero charged species [Ir(phen),(OH)(phen)]" is also responsible for the oxidation stripping spike at -0.97 V (Figure IC).This black coating on the electrode during CPC experiments prevents further reductions from occurring. Wave I does not show an anodic peak at slow scan rates but such a peak is clearly seen at fast scan rates
2614
The Journal of Physical Chemistry, Vol. 83, No. 20, 1979
J. L. Kahl, K. W. Hanck, and K. DeArmond
TABLE 11: Standard Rate Constants for Iridium-Phenanthroline Complexes and Free Ligands, k , in cm/s [Ir(phen), -
[Ir(phen),Cl,]’ [Ir(5,6-Mephen),Cl2]+ (OH)(phen)12+ -I I I1 I11
0.23
?:
0.18
?:
V
0.17
a
0.16 0.18 0.15
.t.
i
0.05
i: i
0.02 0.02
a b
a b
IV VI
0.05
0.02 0.19 i 0.03
0.01
Peaks poorly defined.
0.09
f
0.01
b 0.19
0.11
t i
0.03 0.03
a a a
[Wphen), 13+
phen
b
b
0.30 i: 0.03 0.29 i 0.03 0.39 i: 0.04 a a a
0.19 5 0.05
5,6-Mephen b 0.13
i:
0.02
Irreversible process.
contrast to the behavior4 of [Rh(~hen)~]O.
C IrL31n+ r-”C-
r
C IrL31’+ +e-j
III
Figure 7. Proposed reaction mechanism for [Ir(phen),13’, Roman numerals indicate peak number.
L = phen.
(Figure l a and lb). This behavior does not result from a loss of a phen ligand as free phen cannot be detected spectroscopically following Coulometric reductions of the solution. [ I r ( p h e r ~ ) ~ ] ~An + . electron transfer mechanism is proposed in Figures7 which is consistent with the voltammetric (Figures 3 and 4),Coulometric, and spectroscopic (no free phen) results obtained. At fast scan rates the tris complex undergoes six consecutive one electron transfers. At slow scan rates a black deposit (CPC n = 3.08) of [Ir(phen)3]0forms after wave I11 which undergoes an oxidative stripping reaction at -0.92 V (Figure 3a). At slow scan rates, reversing the scan immediately after wave I1 produces no corresponding oxidation peak while at fast scan rates a peak is clearly seen (Table I). CPC products show no evidence (free phen) of ligand loss. [ R u ( p h e r ~ ) ~ ] ~The + . overall electrochemistry of the Ru(I1) tris(phen) complex has been described.1° We have reexamined the voltammetry of the first two reduction steps to better compare the electrochemistry of [Ru(phen),lZc with its Rh and Ir analogues (Figure 5). [ R ~ ( p h e n )like ~ ] ~the tris(phen) and bisesqui(phen) Ir complexes is of limited solubility and precipitates onto the electrode at slow scan rates. The oxidative stripping peak disappears at fast scan rates because insufficient time is allowed for nucleation and precipitation of [Ru(phen),lo. The Ir and Ru zero charged species block the electrode thus inhibiting further electron transfer; this is in marked
Redox Orbitals The redox orbitals can be classified as delocalized on the basis of the Vlcek criteria previously discussed.l The luminescence of these Ir(II1) phen complexes9JoJ5indicates that the first redox orbital should be delocalized. Moreover, the character of the successive reductions of the tris (at least two additional steps) and the bisesquis (at least one additional step) implied6that these processes also involve delocalized redox orbitals. The magnitudes of the rate constants for the Ir(II1) phen complex couples (Table 11) are all comparable and in agreement with those reported for the analogous couples of the diimine complexes of Rh(III),2Ir(III),l Rh(II),17and Os(II).17 Thus the kinetic data corroborate the characterization of these redox orbitals as delocalized. The presence of a reverse peak for the first reduction step of [ R u ( ~ h e n ) ~at] ~ slow + scan rates contrasts sharply with the absence of a comparable peak for the Ir bisesquiand tris(phen) complexes (Figure 5a, Figure la). We believe this difference could be evidence for a single ring delocalized orbital picture for the Ir(II1) phen complexes rather than a multiring delocalized orbit all^^*^ as for the Ru(I1) phen complex. The [R~(phen)~]’ with an electron delocalized throughout three phenanthroline ligands can approach the electrode from any direction and undergo an oxidation. On the other hand, the [ I r ( p h e ~ ~ )with ~ ] ~an + electron localized in only one of three phens must approach the electrode in a specific orientation (phen-‘ toward the electrode) to permit oxidation. At fast scan rates the Ir complexes show a reverse peak because the [ I r ( ~ h e n ) ~ ] ~ + has not had sufficient time to diffuse away from the electrode surface and hence still has the “reduced ligand” oriented toward the electrode surface. Such reverse scan behavior is not observed for the tris Ir(II1) and tris Rh(II1) complexes, systems for which single ring delocalized orbitals have been postulated. The origin of this apparent anomaly is not clear. Conclusions The similarity between the electrochemistry of the Ru(I1) tris(phen) complex13and the Ir(II1) bisesqui(phen) and tris(phen) complexes indicates that the Ir(II1) mechanisms are similar to the analogous Fe(II), Ru(II), and Os(II1) mechanisms rather than those found for the C O ( I I I )and ~ ~ Rh(III)2J9complexes. Such a mechanism is consistent with the delocalized “redox orbital” character postulated for these Ir(II1) bisesqui- and tris(phen) starting materials. The difference in the oxidative stripping behavior of the tris Ir(II1) and tris Ru(I1) complexes can be interpreted as resulting, respectively, from “single ring” and “multiring” redox orbital character. Ultimately, the emission polarization dataz0 of the starting material and ESR21i22 of the stable reduced species will be useful in determining the relative amount of metal
N-H Hydrogen Bond in Nucleic Acid Bases
and ligand character and the interaction between ligands within these complexes. Acknowledgment. This research was supported by the National Science Foundation (CF 40894 and CHE 7605716). The authors express their thanks to Clifford Carlin for his assistance with the preparation of Figures 1-5.
References and Notes (1) J. Kahl, K. Hanck, and K. DeArmond, J . Phys. Chem., preceding paper in this Issue. (2) G. Kew, K. Hanck, and K. DeArmond, J. fhys. Chem., 79, 1828 (1975). (3) R. Lane and A. Hubbard, J . Phys. Chem., 77, 1401 (1973). (4) J. Kahl, K. Hanck, and K. DeArmond, J. phys. Chem., 82,540 (1978). ( 5 ) K. DeArmond and K. Hanck, to be published. (6) J. Kahl, Ph.D. Thesis, North Carolina State Unlversity, 1978. (7) C. Flynn, Jr., and J. Demas, J. Am. Chem. SOC.,96, 1959 (1974). (8) C. Flynn, Jr., and J. Demas, J. Am. Chem. Soc.,97, 1988 (1975).
The Journal of Physical Chemistty, Vol. 83, No. 20, 1979 2615
(9) R. Watts, J. Harrington, and J. Van Houten, J. Am. Chem. Soc., 99, 2179 (1977). (10) J. Kahl, K. DeArmond, and K. Hanck, J. Inorg. Nucl. Chem., 41, 495 (1979). (11) A. Vlcek, Rev. Chim. Mlner., 5, 299 (1968). (12) N. TokeCTakvciyan, R. Hemingway,and A. Bard, J. Am. Chem. Soc., 95, 6582 (1973). (13) R. Nlcholson, Anal. Chem., 37, 1351 (1965). (14) R. Wopschall and I. Shain, Anal. Chem., 39, 1514 (1967). (15) J. Demas, E. Harris, and R. McBrkle, J. Am. Chem. Soc., 99, 3547 (1977). (16) I. Hanazaki and S. Nagakua, 5uU. Chem. Soc.Jpn., 44, 2312 (1971). (17) T. Saji and S. Aoyagui, J. Elecfroanal. Chem., 63, 31 (1975). (18) S. Margel, W. Smtth, and F. Anson, J. Electrochem. Soc., 125, 241 (1978). (19) G. Kew, K. DeArmond, and K. Hanck, J. phys. Chem., 78, 727 (1974). (20) M. K. DeArmond, C. M. Carlin, and W. L. Huang, Inorg. Chem., In Dress. (21) N. Tanaka, T. Ogata, and S.Niizuma, Bull. Chem. SOC.Jpn., 46, 3299 (1973). (22) H. Caklararu, K. DeArmond, K. Hanck. and V. Sahlnl, J. Am. Chem. Soc., 98, 4455 (1976).
The N-H Hydrogen Bond. 2. Models for Nucleic Acid Bases J. N. Spencer,* Jeffrey E. Glelm, Charles H. Blevins, Robert C. Garrett, Fred J. Mayer, Johanna E. Merkle, Susan L. Smlth, and M. Louise Hackman Department of Chemistty, Lebanon Valley College, Annville, Pennsylvania 17003 (Received December 28, 1978; Revlsed Manuscript Received June 4, 1979) Publication costs asslsted by the Petroleum Research Fund
The pure-base calorimetric method has been used to determine enthalpies of formation for hydrogen-bonded complexes of pyrrole, indole, and imidazole with various bases. These enthalpies are compared to those obtained by other methods. Frequency shifts determined in CCld solvent have been used to find AH-Av relationships for N-H-eO and N-H-N adducts. From these relationships, hydrogen-bond enthalpies have been calculated for bases not determined by the pure-base method. The systems investigated have been used as models to calculate enthalpies of formation for the more complex nucleic acid base pairs. Comparison of enthalpies calculated from the model compounds is made to enthalpies determined by theoretical and other experimental methods.
Introduction This is the second article of a series reporting on calorimetric and spectroscopic investigations of N-H.-N and N-H-00 hydrogen bonds. These hydrogen bonds have particular significance for studies of molecules of biological interest, but, because of the difficulty of obtaining thermodynamic parameters for such weak hydrogen-bonded systems, few reliable data are available. The pure-base calorimetric procedure used for this study has several advantages over other calorimetric or spectroscopic means of investigation for the study of weakly hydrogen-bonded systems.
Experimental Section The calorimetric and spectroscopic procedures have been previously described, as have the purifications of most reagent~.l-~Aldrich imidazole was recrystallized from benzene, vacuum dried, and further dried over Pz05. Aldrich 99+ 5% N-methylimidazole was used without further purification. Dioxane was dried over NaOH and distilled from NaOH in an N2 atmosphere. Acetonitrile was allowed to stand over Pz06until no orange coloration of Pz05was noted and then distilled under N2 Indole was purified by vacuum sublimation. N-Methylindole was refluxed over CaO and distilled under N2. Aldrich pyrimidine was used without further purification. 0022-365417912083-26 15$01.OO/O
For the spectroscopic studies the N-H stretch at about 3500 cm-l was monitored. Indole concentrations were about 0.006 M, pyridine and DMF concentrations were about 0.1 M, and N-methylimidazole concentration was about 0.03 M. A slight correction for overlap of indole and indole-DMF complex bands was made. Thermodynamic parameters were calculated from the temperature dependence of the equilibrium constant as previously reported.2 Frequency shifts were determined in CC14solvent for all systems. Base concentrations were varied to detect any possible dependence of the complex frequency on base concentration.6 Enthalpies of solution were determined by injecting about 0.5-5 mmol of acid into 200 mL of solvent. With the exception of imidazole in CHCl,, no concentration dependencies of the enthalpies of solution were noted. The imidazole-CHC1, data were extrapolated to infinite dilution. The calorimetric pure-base approach to the determination of enthalpy changes is independent of the equilibrium constant, provided that the equilibrium constant is large enough to ensure complete complexation. According to the pure-base method, if a small quantity of hydrogen-bonding acid is injected into the base as a solvent, two contributions to the heat observed are involved: the heat due to hydrogen bonding and that heat 0 1979 American Chemical Society
