J. Phys. Chem. 1982, 86, 298-303
298
Calorimetric and Potentiometric Investigation of Diethylenetriamine and Its N-Methyl-Substituted Derivatives in Aqueous Solution J. Yperman, J. Mullens, J.-P. Frangols, and L. C. Van Poucke' Limburgs Universitair Centrum (Opt. SBM), 83610 Diepenbeek, Belgium (Received: June 1. 198 I: In Final Form: September 18, 198 1)
A potentiometric and calorimetric study in aqueous solution has been carried out on eight different Nmethyl-substituted diethylenetriamines. The protonation constants have been determined at 25.00 "C and at an ionic strength of 1.3. AHc and ASc have also been calculated for the different deprotonating processes. A possible reaction scheme has been given to explain the experimental values.
Introduction The stepwise ionization in aqueous solution of the protonated form of diethylenetriamine and its N-methylsubstituted derivatives can be represented by the equations
H3L3++ H+
+ H2L2+
H2L2++ H+ + HL+ HL+ + H+
+L
(14
these stability constants in the presence of a large amount of an indifferent electrolyte. The reaction heats of eq la, lb, and ICare determined by a direct calorimetric method. From AG; and AH; values, AS; values are calculated.
Met hod
Ob)
Determination of the Acidity Constants by Potentionmetric Titration. The protonation function iiH,which is defined as the average number of H+ ions bounded to L,
(IC)
is given by eq 2.
AG;, AH?,and AS; are the changes of the thermodynamic functions in the molarity scale of reactions la, lb, and IC, describing the ionization of HiLi+. As diethylenetriamine is an important complexing agent, AG; values have been determined quite often; AH; and AS: values are also reHowever, there is a considerable lack of similar data concerning the N-methyl-substituted derivatives. To our knowledge, only two investigation^^^^ reported AG; values for some of these products, but no AH; or AS; values are given. As the thermodynamic functions of diethylenetriamine exhibit some interesting features, e.g., the greater value found for AH2c than for the Nmethyl derivatives are, in this respect, worthwhile to be investigated. Furthermore, the knowledge of these thermodynamic functions is also useful for the study of the steric influence of methyl substituents on the protonation and the complexation ability of the N-donor atom. In this paper AG;, AH;,and AS; values are reported for the following products: diethylenetriamine (PSP), 1-methyldiethylenetriamine (SSP), 1,l-dimethyldiethylenetriamine (TSP),1,1,4-trimethyldiethylenetriamine (TTP), 174-dimethyldiethylenetriamine (STP), 1,4,7-trimethyldiethylenetriamine (STS), 1,1,7,7-tetramethyldiethylenetriamine (TST), and 1,1,4,7,7-pentamethyldiethylenetriamine (TTT). The shorthand notation, given in parentheses, indicates whether the amino groups, in the sequence end, central and end, are primary (P), secondary (S), or tertiary (T). Acidity constants, and consequently AG: values, can be obtained from potentiometric pH measurements. As it is one of the purposes of this study to obtain conditional acidity constants, necessary for the investigations of metal ion complex equilibria, it is most appropriate to determine (1)Ciampolini, M.; Paoletti, P. J. Phys. Chem. 1961, 65, 1224. (2)Jonassen, H.B.;Le Blanc, R. B.; Meibohm, A. W.; Rogan, R. M. J. Am. Chem. SOC.1950, 72, 2430. (3) McIntyre, C. H.; Block,R. P.; Femelius, W. C. J. Am. Chem. SOC. 1959, 81, 529. (4)Allison, J. W.;Angelici, R. J. Znorg. Chem. 1971, 10,2233. ( 5 ) Fbmetach, R.; Marxer, A.; Miescher, K. Helu. Chim. Acta 1951,34, 1611.
AH
The
=
Pii(H+) + ~ P ~ Z ( H + +3&3(H+)3 )~ 1 + Pll(H+) + P12(H+)2+ Pl3(H+I3
(2)
Pli are defined by eq 3. (3)
Charges are partially omitted for the sake of simplicity. When the tribasic acid H3L3+,with total concentration CL,is titrated with a strong base with total concentration C,, it can be easily shown from the mass-balance equations that i i H can be calculated from eq 4 if (H+) and (OH-) are known.
From potentiometric measurements, pH and pK, values, both expressed in concentration units, can be obtained according to a method described by Rossotti and Rossotti6 using a Gran plots7 Thus the protonation function i i H can be obtained in terms of pH. From this function approximate values of Pll, P12,and &3 can be calculated using eq 5, 6, and 7 . = [iiH(H+)1'(H+)2+ ( i i H - 2)(H+)2((H+)2 (H+)I3}- 2(H+)3(H+)12(fi~ - 3)1/[(H+)(H+)i2(H+),((1 iiH)(H+)2* - 2(Hf)1(H+)(2- i i ~+) (H+I2(3- fiH)11 (5)
Pi2
Pl2
=
(H+)23- 4(H+)13- 2Pl1(H+)2(H+)l3 (H+)12(H+)23
(6)
(H+)l and (H+)zare the values of (H+) at iiH= 1 and iiH = 2, respectively. These approximate values of Pll, P12,and (6) Rossotti, F. J. C.; Rossotti, H. J. Chem. Educ. 1965, 42,375. (7) Gran, G.Analyst (London) 1952, 77, 661.
0022-3654/02/2086-0298$0l.25/00 1982 American Chemical Society
The Journal of Physical Chemlstry, Vol. 86, No. 2, 1982 299
Calorimetric Study of Diethylamlnetriamine
Ol3 are then refined using the computer program MINIQUADa8 Determination of the Reaction Enthalpy by Thermometric Titration Calorimetry. To a solution of the triamine in the ammonium trinitrate form, placed in the reaction vessel of the calorimeter, a solution of KOH is added at a constant speed. QR(t),the heat developed in the reaction vessel due to chemical reactions only, can be related with the reaction enthalpy values by means of eq 8. -QR(t) = (m' - m)AHc ( m [ - ml)(AHlc AHc) ( m i - mz)(AHlc AHz, 2AH9 ( m i - m3)(AHc AH$ + AH3' + 3AHc) (8)
+
+
+
+
+
+
+
ml, m2,m3,and m are the number of moles of H2L2+,HL+, L, and H+, respectively, at t = 0, Le. a t the start of the addition of the base. The values of these quantities at a time t are indicated by a prime. AHc is defined as the reaction enthalpy of eq 9a. H+ + OH- + HzO
(94
AH; are the reaction enthalpies of eq la, lb, and IC.AHc can be obtained from a thermometric titration of HN03 with KOH. The number of moles in eq 8 can be calculated from the acidity constants and the mass-balance equations using the computer program EQUILeg The calorimetric experiment yields linear equations of the form Q(tj)- Q(ti),which can be derived from eq 8, and where t j and ti are two successive experimental points. If the number of these points is n + 1,the number of linear equations is n. The unknowns in these equations are AH?, AH,',and AH3'. In an isoperibolic experiment the heat Q(t),developed in the calorimeter reaction vessel due to reaction and dilution heats, can be calculated from
Eguation 9b can be derived from eq 2.20 given by BarthePo using the transformations proposed by this author. The symbols used in eq 9b have the following meanings: E, the difference between the voltages of the thermistors placed in the reaction vessel and the thermostated bath, %, the cooling constant of Newton's law, C,, the heat capacity of the calorimeter at t = 0, i.e., before addition of the titrant, C, the heat capacity of the calorimeter at time t , CB, a specific heat related with C and Co by the equation C = Co + CBUt, where u represents the addition speed of the titrant, and W,,the thermal power of the stirrer'and thermistor. Equation 9b has been derived on the assumption that E and T are related by eq 10. E = k ( T - To) (10) T represents the temperature of the reaction vessel, Tois the temperature of the surroundings (the thermostated bath), and k is a proportionality constant. C, C,, CB, H ,W,,and k are determined from calibration experiments using the following method according to Christensen et al." In the E w. t curve of each calibration (8) Sabatini, A.; Vacca, A.; Gans,P. Talanta 1974, 21, 53. (9) Ting-Po,I.; Nancollas, G . H. Anal. Chem. 1972,44, 1940. (10) Barthel, J., ThermometricTitrations";Wiley New York, 1975. (11) Christensen, J. J.; Izat, R. M.; Hansen L. 0. Rev. Sci. Instrum. 1965, 36, 779.
experiment three parts can be distinguished a preperiod, a heating period, and an afterperiod. For each period a specific equation can be written, for the preperiod eq l l a , for the heating period eq l l b , and for the afterperiod eq llc. Cok(Eb - E,) - Wo(tb - t,) - Co%k(t, - t,)E, = 0 (1h)
Cok(E, - Eb) - Wo(t, - tb) - Co%k(t, - tb)&
8,
(llb)
C,k(E,- E,) - W&t, - t,) - C,%k(t, - t,)Ec = 0
up)
Index b indicates the beginning and index c the end af heating; x and y are points of time of the preperiod and the afterperiod, respectively; Q, is the calibration heat. It is supposed that the thermal power of the calibration is constant. Each calibration yields a set of three eq lla, 114 and llc. The calibration before the titration run givea C&, W,,and C&. The second calibration gives C,k, Wo, and C,k%. C, is the heat capacity of the calorimeter when all the titrant has been added. CBk is then obtained from eq 12. CBk = (Cnk - c&)/ v (12)
V is the total volume of the titrant. By subtracting Qv(t),the heat developed during addition of the titrant due to dilution, from the corresponding Q(t) values, the QR(t)values are obtained. These Qv(t)values are obtained by repeating the same experiments as described before, but with no tribasic acid in the solution of the reaction vessel of the calorimeter. Experimental Section Apparatus. pH measurements are performed using 1 Radiometer pHM-64, equipped with an Ingold HA-2Ql glass electrode and a saturated calomel electrode (Radiometer K 1301),provided with a 2 M KN03 electrolyte bridge. The reaction vessel is surrounded by a thermostated jacket (25.00 f 0.05 OC) and placed in a thermostated cage. Calorimetric measurements are carried out using a Tronac calorimeter Model 550. The calculations are performed using an IBM 370/125 computer. All programs are written in Fortran IV. Reagents. The KOH solutions are prepared using Titrisol (Merck p.a.). A Gran plot of a titration of the KOH solution with HN03 solution revealed the absence of carbonate in the KOH solution. KN03(Merck p.a) is 4 as indifferent electrolyte without further purification. The N-methyl-substituted triamines are purchased from the Ames Laboratories, Melford, CT. The triamines are converted into the ammonium form by adding three equivalents of HN03 to a dilute solution at 0 OC. The solution is concentrated by vacuum distillation. The salt is washed with ethanol and ether, and is dried in a vacuum desiccator (P4Olo). All solutions are prepared using bidistilled deionized water. Potentiometric Titrations. All potentiometric wsurements are done in a nitrogen atmosphere. Before we, the nitrogen is successively washed with sodalime, 3 M H2S04,saturated Ca(OH)2solution, distilled water, and 1.3 M KN03 solution. For each product at least two titrations, differing in the initial concentrations (0.008M and 0.040 M), are performed. The most concentrated solutions are titrated with 0.4 M KOH, the others with 0.1 M KOH. The KOH solution is added with a very precise Strohlein piston buret. For all measurements the concentration of KNOBis 1.3 M. Gran plots are obtained for titrations of a 0.01 M HN03 solutions with KOH solutions, ranging from 0.1 to 0.4 M,
The Journal of Physical Chemistry, Vol. 86, No. 2, 1982
300
Yperman et ai.
TABLE I : Log 4,i Values for the Listed Triamines
Q mJ m m l ”
_._ .
ligand __
-
10.101 (I) 10.241 ( 2 ) 9.910 ( 2 ) 10.237 (1) 9.865 ( 3 ) 10.355 ( 2 ) 9.592 ( 2 ) 9.551 13)
PSP SSP TSP STP TTP
ST s TST ‘YT 7’
19.487 (1) 19.815 ( 2 ) 19.031 ( 2 ) 19.920 (1) 19.093 ( 2 ) 20.192 ( 2 ) 18.551 ( 2 ) 18.422 ( 2 )
24.376 24.453 23.389 23.301 22.278 23.323 22.390 21.164
(2) (3) (4) (2) (3) (3) (4) (5)
TABLE 11. p K k ZValues for the Listed Triamines -__- -- ligand pRA, P K PKA, - __ - _ _ _ _ A -, _________ 9 386 (1) 10.101 (1) PSP 4 889 ( 2 ) SSP 4 638 (1) 9 574 ( 3 ) 10.241 ( 2 ) TSP 4 358 ( 5 ) 9.121 ( 3 ) 9.910 ( 2 ) STP 3 381 ( 2 ) 9 683 (1) 10 237 (1) 9226(4) 9.876 ( 3 ) TTP 3185 ( 5 ) s’rs 3 131 ( 4 ) 9.837 ( 3 ) 10.355 ( 2 ) TST 3 839 ( 5 ) 8.959 ( 3 ) 9.592 ( 2 ) TTr 2 ’742 ( 5 ) 8 871 ( 4 ) 9 551 ( 4 )
90
80 -
.
701I
w-l
1 7 I
TABLE 111. Thermodynamic Functions for Reaction 1 3 ~A G , k~J L J H ,k~J AS,^ J ligand mol ’ mol mol-’ K-I .. . - __ - _________________ PSP 139.2 136.9 -7.8 SSP 139 6 130 0 -32.2 TSP 133 6 116.7 -56.7 STP 133 1 116 3 - 56.3 127 2 TTP 103 4 - 79.8 sm i33 2 109 5 -74 7 TST 127 9 96.6 -105.0 TTT 1’0 7 82 6 - 127 8
and in such conditions that the total concentration of KNOBis always 1.3 M. Calorimetric Titrations. A 0.2 M KOH solution is added to a solution of 0.008 M tribasic acid at a speed of 0.005556 cm3 s-l during 450 s. The voltage of the thermistor is measured every 10 s. This means that for each product 44 equations of the type of eq 8 are obtained. The concentration of KNOBfor each point is here again 1.3 M. In Figure 1the thermometric titration curves are shown of some of the investigated triamines. The heat production Q, expressed in mJ mmol-’, is given as a function of the added quantity of KOH (in mmol).
1
40
i
301 1
x aTTT .TTP aTSP 0 .PSP A
& O
o 0
I
I
I
I
I
Results and Discussion For each substance the 7tH vs. pH curves coincide and are consequently independent of the total concentration uf tribasic acid. This means that no association products are present. All iiHvs. pH curves have an horizontal inflection point at tiH = 2; however, such an inflection point is not found at AH = 1. Thus, reaction l a can clearly be distinguished from the others, but reactions l b and IC occur mainly simultaneously. The values for log pl, obtained with MINIQUADs are shown in Table I. The more familiar pKA,values are given in Table 11. TABLE IV: Thermodynamic Quantities for the Stepwise Reactions l a , l b , and IC
-
ligand .
- .
PSP SSP TSP STP TTP QTS
TST ITT
-
AG,,‘ kJmol
27.9 26.4 21.9 19.3 18.2 17.9 21.9 15.6
AGa,C
’
k J mol-’ 53.6 54.7 52.1 55.3 52.6 56.1 31.2 50.6
57.7 58.5 56.6 58.5 56.4 59.2 54.8 54.5
37.3 35.8 34.0 19.2 18.9 17.2 29.3 16.6
52.0 49.9 40.9 53.0 43.8 49.5 38.0 38.6
47.6 44.3 41.8 44.1 40.7 42.8 29.3 27.4
31.5 31.5 30.5 -0.3 2.4 2.4 24.8 3.4
AS,: J mol-’ K-’
AS3,=J mol-’ K-’
-5.4 -16.1 -37.6 -7.7 -29.5 -22.1 -44.3 -40.3
-33.9 -47.6 -49.6 -48.3 -52.7 -55.0 -85.5 -90.9
*
The Journal of Physical Chemistry, Vol. 86, No. 2, 1982 301
Caiorlmetric Study of Diethylaminetriamine
-150
I 0
* 1
2
3
5
1
nM
Figure 3. Plot of C,AS,cE ASc vs. nM,the number of methyl groups.
0
1
Flgurs 2. Plot of
2
3
-
9
C,AGIo AGO vs. nT,the number of tertiary am-
moniun groups.
shown in Figure 2. This linear relationship is given by eq 14. AGc = - 6 . 1 2 ~+ 139.5 (r = -1.00)
(14)
( r is the correlation coefficient). The linear relationship demonstrates the fact that the acid-base behavior of a tertiary amino group is different from that of a primary or a secondary. In fact, the tertiary amino group is a less stronger base than the corresponding primary and secondary, while the latter two show almost identical acidbase behavior. Inductive effects, caused by the methyl substituents, would not only give a more regular sequence in function of the number of methyl groups but probably a reversed sequence as has been demonstrated by recent calculations performed by Marchington, Moore, and Richards.12 According to these authors, methyl substitution increases the electron density on N. This is in agreement with the findings of Munson13concerning the basicities of some hydrocarbon derivatives in the gas phase. Methyl substitution also increases the polarizability of the molecule. This effect is more important than the inductive effect, but is in the aqueous phase considerably attenuated by dispersion of the charge to the H-bonded solvent molecules as has been demonstrated by Taft and others.14 The lower basicity of the tertiary amino group is usually explained by hydration effects. According to TrotmanDickins~n,'~ the primary ammonium ions will be, in a hydrogen bounded solvent, more solvated than the sec(12) Marchington,A. F.; Moore, S. C.; Richards,W. G. J. Am. Chem. SOC.1979,101,5529. (13) Muneon, M. 5.R. J.Am. Chem. SOC.1965,87, 2332. (14) Taft, R. W.; Taagepera, M.; Abbond, J. L. M.; Wolf,J. F.; De Frees,D. J.; Hehre, W. J.; Badmesa,J. E.; McIver R. T., Jr. J. Am. Chem. SOC.1978, 100, 7765. (15) notman-Dickenson, A. F. J. Chem. SOC.1949,1293.
ondary ammonium ions, which in turn will be more solvated than the tertiary ammonium ions. Ha1116proposed a somewhat modified model: only one water molecule can hydrate a tertiary ammonium ion, but much larger numbers may hydrate secondary and primary ammonium ions. Indeed, acid-base behavior is not proportional to the number of protons in the ammonium ion; in the Nmethyl-substituted ethylenediamines the secondary amino group seems to be the stronger base.17 Probably, as has been mentioned by Gurney,18steric effects are responsible for the anomalous behavior of the tertiary amino group. Hancock19 also emphasizes the importance of the steric effects in the acid-base sequence in aqueous solution for the methyl-substituted amines. In this study a better linear relationship is found between AHc and nM,the number of methyl groups, than between AH"and nT. This happens also for AS", where the linear relationship is very good. This relationship is shown in Figure 3 and is represented by eq 15. ASc = -23.89nM - 7.8 (r = -1.00)
(15)
Because of the relationship between AG" and nT, a twoparameter equation was tried to fit the AH" values. The relationship obtained in this way is given by eq 16. AHc = -6.97nM - 6.41nT
+ 136.9
(r = 1.00) (16)
Equations 15 and 16 can be combined to give eq 17. AGc = AH"- TASC = -6.97nM - 6.41nT 136.9 7.13nM+ 2.3 (17)
+
+
The linear relationship between AG" and nT can be explained, according to eq 17, by the fact that the first term of AH" is completely compensated by the first term of TAP. This allows the following interpretation of the terms in eq 16: the first term can be related to the influence of the methyl groups upon the structure of the solvent: the second term is related with the decrease of bounding strength between N and the hydronium ion, when a primary or secondary amino group is replaced by a tertiary one. (16) Hall, H. K. J. Am. Chem. SOC.1957, 79,5441. (17) Creyf, H. S.; Van Poucke, L. C.; Eeckhaut, Z. Talanto 1973,20, 973. (18) Gurney, R. W. "Ionic Processes in Solution"; New York, 1953. (19) Hancock, R. D. J. Chem. SOC.,Dalton Trans. 1980, 416.
JG?
Yperman et al.
The Journal of Physical Chemistry, Vol. 86, No. 2, 1982
902
TABLE V : Thermodynamic Quantities for Reaction 20
4
k J 1-
kJ mol-'
kJ mol-'
AS2-3rc J mol-' K - l
-4.1 -3.8 -4.5 -3.2 -3.8 -3.1 -3.6 -3.9
4.4 5.6 -0.9 8.9 3.1 6.7 8.7 11.2
28.5 31.5 12.0 40.6 23.2 32.9 41.2 50.6
AGP31C
-
ligand
PSP SSP C
1
4
Flgwe 4. Plots of AG,O vs. n M for triamine wlth a
and with a central tertiary amino group.
"51
central secondary
__-_
_______
TSP STP TTP STS TST TTT
For diethylenetriamine (PSP) Prue and Schwarzenbach2" suggest that, due to electrostatic repulsion, the ionization of the central ammonium group is the first ionization step. The inflection point in the protonation c w e at tiH= 2 seems to confirm this suggestion. Further evidence can be found from a plot of AGlc against nM. Two different linear relationships are found depending on the central amino group. These relationships can be represented by eq 18 and 19 and are shown in Figure 4.
AGlc = 27.9 - 1.50nM(S central) (r = 1.00) (18) I
AGIC= 19.6 - 1.23nM(T central) (r = 1.00) (19)
tWIcdecreases with enhanced methyl substitution. The slope of each line is small and the intercept determines mainly the AGlc values. However, the intercept obtained from eq 18 is markedly greater than that obtained from eq 19. This sustains the hypothesis20 that reaction l a corresponds mainly with the deprotonation of the central group. Furthermore, the tertiary ammonium group is a stronger acid than the secondary, although no deviations are found for TSP and TST. Therefore it can safely be accepted that the suggestion of Prue an$.Schwarzenbachm holds for all the triamines considered in this investigation. As to the second (lb) and third reaction step (IC), Ciampolini and Paolettil suggest that for PSP HL+ consists of two tautomeric forms: one with the proton on the primary and one with the proton on the secondary amino group. This suggestion is based on the somewhat greater value found for AH2c than for AH3'. This fact can be explained by assuming that in the second step, the deprotonation of one of the end groups is partly followed by a jump of the remaining proton from the other end group toward the central group. This proton jump consists of @ more endothermic ionization of the primary ammonium and a less exothermic proton association of the m d a r y amino group, giving consequently a net endoh r m i c effect. Using 'H NMR measurements, Sudmeier and Reilley21found, in neutralizing PSP with HCI, that a t the first equivalence point the end nitrogens are somewhat more protonated than the central nitrogen, partly due to the presence of diprotonated species. At the second equivalence point they found almost complete protonation of the end nitrogens and very slight protonation of the central nitrogen. These findings are not in disagreement with the proposed reaction scheme of Ciampolini and Paoletti.' Using '3c NMFt measurements, Delfini et al.22found, in neutralizing PSP with HC1, that in the first two steps the protonation sites are the external primary amino groups with no protonation of the secondary inner one. These findings are in contradiction with those of Ciampalini and Paoletti.' From Table IV it can (20) Prue, J. E.; Schwarzenbech, G. Helu. Chim. Acta 1960,33, 985. (21) Sudmeier, J. L.; Reilley, C. N. Anal. Chem. 1964, 36, 1698. (22) Delfiii, M.;Segre, A.L.; Conti, F.; Barbucci, R.; Barone, V.; Ferruti, P. J. Chem. Soc., Perkin Trans. 2 1980, 900.
Figure 5. Plot of
?
.
:
E
E
3
13
1'
?H, > k I P~O
against
be seen that for all triamines, except one (TSP), AH2' > AH3'. This feature holds even for STP, where the reaction scheme of Ciampolini and Paolettil is very unlikely as the tertiary ammonium group is a stronger acid than the corresponding secondary and primary ammonium groups. Consequently, the suggestion of Ciampolini and Paoletti' does not explain all the experimental facts. A closer look at these problems can be obtained by considering reaction 20. H2L2+(aq)+ L(aq) + 2HL+(aq)
(20)
The thermodynamic quantities of this reaction are given in Table V. From this table it can be seen that the AGc2-3 values have a small spreading, but that strong differences occur between the AHc2-3,respectively, the AS"-3 values. As can be seen from Table V, the compounds with S central have, for the same end groups, a lower value for AHc2-3than for the T central analogues. However, the existence of two tautomeric forms for the latter is less probable than for the former. So on the basis of the theory of Ciampolini and Paoletti,' and considering that the deprotonation of the end group (T)is less endothermic than the deprotonation of the central group (S), one would expect a negative value of AH'2-3 for TST. However, this value is positive and the third greatest of the series. There is a good linear relationship between the AS'2-3 and the corresponding AHc,-, values, which can be represented by eq 21. As"-3 = 3.11Mc2-3
+ 14.05
( r = 1.00)
(21)
Here AScz-3 and AHC2+ are given in there usual units. Figure 5 is a plot of AS"-, against AH'2-3. Using relationship 21 AGC2-3can be calculated as in eq 22. AG'2-3 = AHc2-3 - TAS'2-3 = AH'2-3 - 0.92AHc2-3- 4.2 (22) Equation 22 shows that the value for AG'2-3 is almost completely determined by the constant value of -4.2 kJ, which comes from the entropy term. It should be remarked that this value does not differ very much from -RT In 3, the value of the statistical effect if all amino groups were equivalent. Thus the cause that makes the AH'2-3 values of reaction 20,positive, gives also increased AS'Z-3
J. Phys. Chem. 1082, 86. 303-306
values, so that both effects are almost completely compensated by eq 22. Such compensations have been attributed to hydration effects and a possible explanation of the positive M C 2 - 3 values can be given by assuming that
303
two HL+ molecules are more structure breaking than the combination of an H2L2+and an L molecule, giving an endothermic effect, but, by increasing the number of free water molecules, enhances also AScz-3.
Intramolecular Hydrogen Bonding and Fluorescence of Salicylaldehyde, Salicylamide, and o -Hydroxyacetophenone In Gas and Condensed Phases J. Catalk, F. Torlblo, and A. U. Acufia” lnstltuto de Qdmlca &Ice “Rocasolano“, C.S.I.C., Serano 119, MadrM 6, Spaln, end Departamento de Qdmlca A i c a y Qihlca CuEnHca, Unlversklad Aut6noma de MadM, Cantoblanco, Madrhl34, Spain (Received: April 29, 198 1; I n Flnal Form: October 6, 198 1)
The fluorescence emission and excitation spectra of salicylaldehyde, salicylamide, and o-hydroxyacetophenone in gas and solution phases were recorded, and quantum yield measurements were made in cyclohexane and N,”-dimethylformamide solutions. The single broad fluorescence band observed in the visible region is assigned to zwitterionic structures. It is proposed that, unlike methyl salicyclate, these compounds can have, under the particular experimental conditions, only one ground-state conformation and therefore a single emission band. Moreover, the abrupt falloff seen in the excitation spectrum of collision-free methyl salicylate is absent here. Evidence is presented indicating that proton phototransfer is not necessarily followed by ultrafast radiationless processes. Thus, the ground-state rotameric model discussed here, though capable of accounting for multiple fluorescence in salicilate derivatives, needs to be supplementedwith a yet unknown electronic factor to explain the large range of the observed lifetimes. Hidden n,n* states are suggested for that role.
Introduction Following electronic excitation the proton affinity of some aromatic molecules undergoes highly localized changes markedly dependent on the kind of substitution on the aromatic rings.172 If a proton-donating hydroxyl group is hydrogen bonded already in the ground state, as happens in some rotameric forms of salicyclic compounds, intramolecular transfer of a proton during the lifetime of the excited moleculew becomes very probable. The influence of other ground-state geometries in the photophysics of these compounds remains unclear; a particular spectroscopic parameter cannot be associated to any one of the plausible rotamers, unlike the situation in, e.g., diarylethylenes.’ In an effort to establish unambigous assignments, we carried out some work8s9on the emission of methyl salicylate (MS) and related compounds in the gas phase and in nonpolar solvents and concluded that only “closed” structures IC and IIC (Figure 1) are relevant to the dual fluorescence observed in these particular conditions. One corollary is that molecules which cannot attain structure IIC must show only one emission band, which contradicts apparently the recent findinglo of two fluorescence bands in salicylamide dissolved in cyclohexane. Furthermore, we foundg that the gas-phase blue (450 nm) fluorescence from MS, assigned to the excited species (1)Ireland, J. F.; Wyatt, P. A. H. Adu. Phys. Org. Chem. 1976,12,131. (2)Freiser, B. S.;Beauchamp, J. L. J. Am. Chem. SOC.1978,99,3214. (3)Weller, A. Naturwissemchajten 1955,42,175. (4)Klapffer, W.; Naudorf, G. J. Lumin. 1974,8,457. (5) Kosower, E.M.; Dodiuk, H. J. Lumin.1975/76,11,249. (6)Sandros, K. Acta Chem. Scand., Ser A 1976,30,761. (7)Fischer, E.J.Phys. Chem. 1980,84,403. (8)Acuiia, A. U.; Amat-Guem, F.; C a t a h , J.; Gonzalez-Tablas,F. J. Phys. Chem. 1980,84,629. (9)AcuKa, A. U.;C a t a h , J.; Toribio, F. J. Phys. Chem. 1981,85,241. (10)Schulman, G. S.; Underberg, W. J. M. Photochem. Photobiol. 1979,29,937. 0022-3654/82/2086-0303$01.25/0
in which the proton transfer has occurred, was quenched by rather low excess excitation energies (700 f 200 cm-’). This fast deactivation route, if due only to the protolytic reaction, should be observed also in other salicylic derivatives. In the present work we investigate the wavelength dependence of the fluorescence quantum yield and the solution emission of salicylaldehyde (SCHO), o-hydroxyacetophenone (HA), and salicylamide (SAM). These particular compounds were selected because of the gasphase emission can be recorded at near room temperature and, according to the proposed a single band should appear. Experimental Section Materials. Methyl salicylate (Merck) was purified by fractional vacuum distillation. Salicylaldehyde (Fluka), salicylamide (Merck),and o-hydroxyacetophenone (Merck) were purified by column chromatography on Kieselgel60 (Merck) until a single spot on TLC plates of Kieselgel60 f254 (Merck) and various solvents was obtained. Cyclohexane (for fluorescence spectroscopy, Merck) and N,N’-dimethylformamide (Merck “Uvasol”)were dried with sodium mirror and molecular sieves. Methods. The vapors from the salicylic derivatives were handled in a vacuum line using 10-mm rectangular cells as beforeg and also a 70-mm spherical cell. The spectra from S A M had to be run at &50 OC, to increase the vapor pressure in the cell, while the other compounds were at room temperature. Samples of SAM kept at 50 “C for 4 h did not show any measurable spectral changes. Emission and excitation spectra were recorded in a SLM photon-counting instrument and corrected as described earlier;8i9because of the low red sensitivity of the photomultiplier, reliable correction factors could not be found for wavelengths longer than 600 nm. Fluorescence quantum yields in air-equilibrated liquid solution were determined by the usual procedures, using the methyl salicylate 0 1982 American Chemical Society
