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Electrostatic proximity effects in gas-phase acidities and basicities: a comparison of theoretical methods (INDO, MNDO, AM1 and ab initio)
Journal
of Molecular
Structure
(Theochem),
205 (1990) 367-372
Elsevier Science Publishers B.V., Amsterdam -
367
Printed in The Netherlands
Short communication
ELECTROSTATIC PROXIMITY EFFECTS IN GAS-PHASE ACIDITIES AND BASICITIES: A COMPARISON OF THEORETICAL METHODS (INDO, MNDO, AM1 AND AB INITIO)
J. CATALAN and J.L.G. DE PAZ Departamento Cantoblanco,
de Quimica, Facultad 28049.Madrid
de Ciencias,
Universidad
Autdnoma
de Madrid,
(Spain)
J. ELGUERO* and A. MARTINEZ Instituto
de Quimica Mkdica,
CSZC, Juan
de la Cierva 3,28006
Madrid
(Spain)
R.W. TAFT and F. ANVIA Department
of Chemistry,
University
of California
at Irvine,
Irvine,
CA 92717 (U.S.A.)
(Received 31 October 1988; in final form 23 June 1989)
For a chemist, it is of the utmost importance to have a theoretical tool that allows the study of large series of compounds including the complete optimization of geometries. To avoid the limitations of ab initio methods, it is necessary to use reliable semi-empirical methods. By using the latter it is possible to rationalize the behaviour of a family of compounds and to make predictions within it of compounds that have not yet been studied. In 1967, Pople et al. [ 1 ] proposed the INDO method and in 1985 Dewar and co-workers, starting with MINDO/l [2], MINDO/B [3], MINDO/B’ [4], MIND0/3 [ 51 and MNDO [ 61, proposed the AM1 method [ 71. According to Dewar, the latter method is particularly well suited for basicity (60 compounds) and acidity studies (80 compounds) [8]. In this communication, we would like to call attention to the theoretical calculations of the acid-base equilibria of azoles and azines which involve what have been termed “electrostatic proximity effects” [ 91. With this purpose, we present in Table 1 the experimental (in gas phase) and theoretical results of a series of acid-base equilibria. Within each method, the geometries were fully optimized. MNDO and AM1 methods give very comparable values which differ considerably from the experimental results; for instance, contrary to experience (eqn. 4) these methods predict that pyrazole is more acidic than imidazole which invalidates all previous theoretical analyses on this equilibrium [ 171. The difference, about 9 kcal mol-’ for the first eight examples, clearly indicates that
0166-1280/90/$03.50
0 1990 Elsevier Science Publishers B.V.
368
”
9
2
10
9 I.0
I c
9
9 co
2
- -? c c r-i /
-2
@
‘I
/-\ P
-2.
ox +
+
+
-r - r _7-; ,-y Jr ,-y
- -? c c CP 11
r-i /
-2
+
11 / \ 11
11
--I
-2
+
20 /
-2
r-d /
369
c?
‘4
9 t-
@3
q
a!
c?
‘Eb ti
‘a:
m
co
0
I
rl
I
E:
/\ -e + +
z--I
-l
z-
(0
” fI
+
c7 oz=/ I’ ‘*
/
h
OJ
a,
370
Fig. 1. 1,2 - Interactions
in neutral, protonated
and ionic forms.
both methods do not deal correctly with electrostatic proximity 1,2-interactions in neutral, protonated or anionic forms such as those shown in Fig. 1. In order to study this point further, we have measured, for the first time, the gas-phase basicity of l,&naphthyridine. It is now possible to determine the energy balance of eqn (9) in which the “proximity” effect is peri and not 1,2. Surprisingly enough, both methods provide correct results for this equilibrium. As can be seen, the INDO method correctly describes the equilibria involving azoles [eqns. (l)-(6)]. Concerning azines [ eqns. (7)- (9) ] there is an almost constant shift of about 5 kcal mol-l. This seems to indicate that this method does not treat azine nitrogen hybridization as reliably as that for azoles. 6-31G calculations (6-31G//6-31G) describe qualitatively all equilibria (cations or anions, azoles or azines) in a satisfactory manner. It is even possible to find a linear relationship, dJ&, = 0.12+0.72dE, n=6 [eqns. (l)-(4), (7) and (S)], P=0.996, that ho Ids f or all compounds studied, but for the moment the calculation of benzo condensed rings [ eqns. (5 ) , (6 ) and (9 ) ] must be excluded. TABLE 2 Calculated deprotonation
values (kcal mol-‘) Position”
Compound
INDO
AEd,
dA&,
1-Methylimidazole
2 5 4
619.3 622.1 638.8
0 2.8 19.5
1-Methylpyrazole
5 4 3
618.4 625.1 627.4
0 6.7 9.0
Pyridine
3,5 4 2,6
616.2 618.7 627.9
0 2.5 11.7
“In order of decreasing
acidity.
371
Confident in the reliability of INDO calculations, we present in Table 2 our results concerning the acidity of the C-H protons of three heterocycles, two azoles and one azine. There are no results available for the gas phase, but solution studies [ 19-221 are consistent with these predictions, although the mechanisms involved in the hydrogen-deuterium exchange in solution are quite complex. In the case of pyridine, 6-31G//INDO calculations confirm that position 2 is less acid than position 4 by 9.8 kcal mol-‘, whereas AM1 calculations lead to an opposite conclusion by 4.9 kcal mol-‘, in contradiction, at least, with solution results [ 201. In conclusion, the AM1 method fails in the description of electrostatic proximity effects and we propose instead the INDO method which has given good results for acidity-basicity problems of azoles [ 231 and other kinds of compounds [ 24 1. ACKNOWLEDGMENT
The financial support from CICYT (Project no. 87045) and from CCHNCCT (no. 86/04/009) is gratefully acknowledged. All calculations were performed at the UAM/IBM, CC/UAM and Instituto Rocasolano (CSIC) centres, Madrid.
REFERENCES
6 7 8 9 10 11 12 13 14 15 16 17 18 19
J.A. Pople, D.L. Beveridge and P.A. Dobosh, J. Chem. Phys., 47 (1967) 2026. N.C. Baird and M.J.S. Dewar, J. Chem. Phys., 50 (1969) 1262. M.J.S. Dewar and E. Haselbach. J. Am. Chem. Sot., 92 (1970) 590. N.C. Baird, M.J.S. DewarandD.H. Lo, J. Am. Chem. Sot., 94 (1972) 5303. R.C. Bingham, M.J.S. Dewar and D.H. Lo, J. Am. Chem. Sot., 97 (1975) 1285,1294,1302, 1307. M.J.S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4899,4907. M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Sot., 108 (1985) 8075. M.J.S. Dewar and K.M. Dieter, J. Am. Chem. Sot., 108 (1986) 8075. R.W. Taft, F. Anvia, M. Taagepera, J. Catalan and J. Elguero, J. Am. Chem. Sot., 108 (1986) 3237. J. Catalan, R.M. Claramunt, J. Elguero, J. Laynez, M. Menendez, F. Anvia, J.H. Quian, M. Taagepera and R.W. Taft, J. Am. Chem. Sot., 110 (1988) 4105. M. Meot-Ner, J.F. Liebman and J.E. Del Bene, J. Org. Chem., 51 (1986) 1105. S.G. Lias, J.F. Liebman and R.D. Levin, J. Phys. Chem. Ref. Data, 13 (1984) 695. 0. M6, J.L.G. de Paz and M. Yafiez, J. Phys. Chem., 90 (1986) 5597. 0. M6, J.L.G. de Paz and M. Yafiez, Theor. Chim. Acta, 73 (1988) 307. R.W. Taft, F. Anvia, J. Catalan, J.L.G. de Paz and J. Elguero, unpublished results. 0. M6, J.L.G. de Paz and M. Yariez, J. Mol. Struct. (Theochem), 150 (1987) 135. S. Olivella and J. Vilarrasa, J. Heterocycl. Chem., 18 (1981) 1189. G.P. Ford and J.D. Scribner, J. Comput. Chem., 4 (1983) 594. (a) KS. Schofield, M.R. Grimmett and B.R.T. Keene, The Azoles, Cambridge University Press, Cambridge, 1976.
372
20 21 22 23
24
(b) A.R. Katritzky and C.W. Rees (Eds.), Comprehensive Heterocyclic Chemistry, Pergamon Press, Oxford, 1984. J.A. Zoltewicz, G. Grahe and CL. Smith, J. Am. Chem. Sot., 91 (1969) 5501. Y. Takeuchi, H.J.C. Yeh, K.L. Kirk and L.A. Cohen, J. Org. Chem., 43 (1978) 3565. Y. Takeuchi, K.L. Kirk and L.A. Cohen, J. Org. Chem., 43 (1978) 3570. J. Catalan, J. Elguero, R. Flammang and A. Maquestiau, Angw. Chem. Int. Ed. Engl., 22 (1983) 323; J. Catalan and J. Elguero, J. Chem. Sot., Perkin Trans. 2, (1983) 1869; J. Catalan, P. Perez and J. Elguero, Heterocycles, 20 (1983) 1717; J. Catalan, M. Menendez, J. Laynez, R.M. Claramunt, M. Bruix, J. de Mendoza and J. Elguero, J. Heterocycl. Chem., 22 (1985) 997. J. Catalan and A. Macias, J. Chem. Sot., Perkin Trans. 2, (1979) 1632; J. Catalan, J.L.G. de Paz and M. Yaiiez, J. Mol. Struct. (Theochem), 107 (1984) 257.
of Molecular
Structure
(Theochem),
205 (1990) 367-372
Elsevier Science Publishers B.V., Amsterdam -
367
Printed in The Netherlands
Short communication
ELECTROSTATIC PROXIMITY EFFECTS IN GAS-PHASE ACIDITIES AND BASICITIES: A COMPARISON OF THEORETICAL METHODS (INDO, MNDO, AM1 AND AB INITIO)
J. CATALAN and J.L.G. DE PAZ Departamento Cantoblanco,
de Quimica, Facultad 28049.Madrid
de Ciencias,
Universidad
Autdnoma
de Madrid,
(Spain)
J. ELGUERO* and A. MARTINEZ Instituto
de Quimica Mkdica,
CSZC, Juan
de la Cierva 3,28006
Madrid
(Spain)
R.W. TAFT and F. ANVIA Department
of Chemistry,
University
of California
at Irvine,
Irvine,
CA 92717 (U.S.A.)
(Received 31 October 1988; in final form 23 June 1989)
For a chemist, it is of the utmost importance to have a theoretical tool that allows the study of large series of compounds including the complete optimization of geometries. To avoid the limitations of ab initio methods, it is necessary to use reliable semi-empirical methods. By using the latter it is possible to rationalize the behaviour of a family of compounds and to make predictions within it of compounds that have not yet been studied. In 1967, Pople et al. [ 1 ] proposed the INDO method and in 1985 Dewar and co-workers, starting with MINDO/l [2], MINDO/B [3], MINDO/B’ [4], MIND0/3 [ 51 and MNDO [ 61, proposed the AM1 method [ 71. According to Dewar, the latter method is particularly well suited for basicity (60 compounds) and acidity studies (80 compounds) [8]. In this communication, we would like to call attention to the theoretical calculations of the acid-base equilibria of azoles and azines which involve what have been termed “electrostatic proximity effects” [ 91. With this purpose, we present in Table 1 the experimental (in gas phase) and theoretical results of a series of acid-base equilibria. Within each method, the geometries were fully optimized. MNDO and AM1 methods give very comparable values which differ considerably from the experimental results; for instance, contrary to experience (eqn. 4) these methods predict that pyrazole is more acidic than imidazole which invalidates all previous theoretical analyses on this equilibrium [ 171. The difference, about 9 kcal mol-’ for the first eight examples, clearly indicates that
0166-1280/90/$03.50
0 1990 Elsevier Science Publishers B.V.
368
”
9
2
10
9 I.0
I c
9
9 co
2
- -? c c r-i /
-2
@
‘I
/-\ P
-2.
ox +
+
+
-r - r _7-; ,-y Jr ,-y
- -? c c CP 11
r-i /
-2
+
11 / \ 11
11
--I
-2
+
20 /
-2
r-d /
369
c?
‘4
9 t-
@3
q
a!
c?
‘Eb ti
‘a:
m
co
0
I
rl
I
E:
/\ -e + +
z--I
-l
z-
(0
” fI
+
c7 oz=/ I’ ‘*
/
h
OJ
a,
370
Fig. 1. 1,2 - Interactions
in neutral, protonated
and ionic forms.
both methods do not deal correctly with electrostatic proximity 1,2-interactions in neutral, protonated or anionic forms such as those shown in Fig. 1. In order to study this point further, we have measured, for the first time, the gas-phase basicity of l,&naphthyridine. It is now possible to determine the energy balance of eqn (9) in which the “proximity” effect is peri and not 1,2. Surprisingly enough, both methods provide correct results for this equilibrium. As can be seen, the INDO method correctly describes the equilibria involving azoles [eqns. (l)-(6)]. Concerning azines [ eqns. (7)- (9) ] there is an almost constant shift of about 5 kcal mol-l. This seems to indicate that this method does not treat azine nitrogen hybridization as reliably as that for azoles. 6-31G calculations (6-31G//6-31G) describe qualitatively all equilibria (cations or anions, azoles or azines) in a satisfactory manner. It is even possible to find a linear relationship, dJ&, = 0.12+0.72dE, n=6 [eqns. (l)-(4), (7) and (S)], P=0.996, that ho Ids f or all compounds studied, but for the moment the calculation of benzo condensed rings [ eqns. (5 ) , (6 ) and (9 ) ] must be excluded. TABLE 2 Calculated deprotonation
values (kcal mol-‘) Position”
Compound
INDO
AEd,
dA&,
1-Methylimidazole
2 5 4
619.3 622.1 638.8
0 2.8 19.5
1-Methylpyrazole
5 4 3
618.4 625.1 627.4
0 6.7 9.0
Pyridine
3,5 4 2,6
616.2 618.7 627.9
0 2.5 11.7
“In order of decreasing
acidity.
371
Confident in the reliability of INDO calculations, we present in Table 2 our results concerning the acidity of the C-H protons of three heterocycles, two azoles and one azine. There are no results available for the gas phase, but solution studies [ 19-221 are consistent with these predictions, although the mechanisms involved in the hydrogen-deuterium exchange in solution are quite complex. In the case of pyridine, 6-31G//INDO calculations confirm that position 2 is less acid than position 4 by 9.8 kcal mol-‘, whereas AM1 calculations lead to an opposite conclusion by 4.9 kcal mol-‘, in contradiction, at least, with solution results [ 201. In conclusion, the AM1 method fails in the description of electrostatic proximity effects and we propose instead the INDO method which has given good results for acidity-basicity problems of azoles [ 231 and other kinds of compounds [ 24 1. ACKNOWLEDGMENT
The financial support from CICYT (Project no. 87045) and from CCHNCCT (no. 86/04/009) is gratefully acknowledged. All calculations were performed at the UAM/IBM, CC/UAM and Instituto Rocasolano (CSIC) centres, Madrid.
REFERENCES
6 7 8 9 10 11 12 13 14 15 16 17 18 19
J.A. Pople, D.L. Beveridge and P.A. Dobosh, J. Chem. Phys., 47 (1967) 2026. N.C. Baird and M.J.S. Dewar, J. Chem. Phys., 50 (1969) 1262. M.J.S. Dewar and E. Haselbach. J. Am. Chem. Sot., 92 (1970) 590. N.C. Baird, M.J.S. DewarandD.H. Lo, J. Am. Chem. Sot., 94 (1972) 5303. R.C. Bingham, M.J.S. Dewar and D.H. Lo, J. Am. Chem. Sot., 97 (1975) 1285,1294,1302, 1307. M.J.S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4899,4907. M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Sot., 108 (1985) 8075. M.J.S. Dewar and K.M. Dieter, J. Am. Chem. Sot., 108 (1986) 8075. R.W. Taft, F. Anvia, M. Taagepera, J. Catalan and J. Elguero, J. Am. Chem. Sot., 108 (1986) 3237. J. Catalan, R.M. Claramunt, J. Elguero, J. Laynez, M. Menendez, F. Anvia, J.H. Quian, M. Taagepera and R.W. Taft, J. Am. Chem. Sot., 110 (1988) 4105. M. Meot-Ner, J.F. Liebman and J.E. Del Bene, J. Org. Chem., 51 (1986) 1105. S.G. Lias, J.F. Liebman and R.D. Levin, J. Phys. Chem. Ref. Data, 13 (1984) 695. 0. M6, J.L.G. de Paz and M. Yafiez, J. Phys. Chem., 90 (1986) 5597. 0. M6, J.L.G. de Paz and M. Yafiez, Theor. Chim. Acta, 73 (1988) 307. R.W. Taft, F. Anvia, J. Catalan, J.L.G. de Paz and J. Elguero, unpublished results. 0. M6, J.L.G. de Paz and M. Yariez, J. Mol. Struct. (Theochem), 150 (1987) 135. S. Olivella and J. Vilarrasa, J. Heterocycl. Chem., 18 (1981) 1189. G.P. Ford and J.D. Scribner, J. Comput. Chem., 4 (1983) 594. (a) KS. Schofield, M.R. Grimmett and B.R.T. Keene, The Azoles, Cambridge University Press, Cambridge, 1976.
372
20 21 22 23
24
(b) A.R. Katritzky and C.W. Rees (Eds.), Comprehensive Heterocyclic Chemistry, Pergamon Press, Oxford, 1984. J.A. Zoltewicz, G. Grahe and CL. Smith, J. Am. Chem. Sot., 91 (1969) 5501. Y. Takeuchi, H.J.C. Yeh, K.L. Kirk and L.A. Cohen, J. Org. Chem., 43 (1978) 3565. Y. Takeuchi, K.L. Kirk and L.A. Cohen, J. Org. Chem., 43 (1978) 3570. J. Catalan, J. Elguero, R. Flammang and A. Maquestiau, Angw. Chem. Int. Ed. Engl., 22 (1983) 323; J. Catalan and J. Elguero, J. Chem. Sot., Perkin Trans. 2, (1983) 1869; J. Catalan, P. Perez and J. Elguero, Heterocycles, 20 (1983) 1717; J. Catalan, M. Menendez, J. Laynez, R.M. Claramunt, M. Bruix, J. de Mendoza and J. Elguero, J. Heterocycl. Chem., 22 (1985) 997. J. Catalan and A. Macias, J. Chem. Sot., Perkin Trans. 2, (1979) 1632; J. Catalan, J.L.G. de Paz and M. Yaiiez, J. Mol. Struct. (Theochem), 107 (1984) 257.