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Intramolecular hydrogen bonding of malondialdehyde and its monothio and dithio analogues studied by the PM3 method
Journal of Molecular Structure (Theochem), 208 (1990) 253-260 Elsevier Science Publishers B.V., Amsterdam
253
INTRAMOLECULAR HYDROGEN BONDING OF MALONDIALDEHYDE AND ITS MONOTHIO AND DITHIO ANALOGUES STUDIED BY THE PM3 METHOD
GIUSEPPE BUEMI Dipartimento di Scienze Chimiche, Universitti di Catania Viale A.Doria nr.6, 95125 Catania (Italy) (Received 30 November 1989)
ABSTRACT The molecular geometries of all the possible conformations of malondialdehyde (MDA ) , thiomalondialdehyde (TMDA) and dithiomalondialdehyde (DTMDA ) were optimized by means of the most recent semiempirical method MNDO-PM3 to test the performance of the new Hamiltonian in evaluating the most probable structures as well as the hydrogen-bond strengths. PM3 results are better than AM1 ones in predicting the C-S and C=S bond lengths and the energy of theS-H* . *S bridge, but unreliable geometries for the hydrogen-bonded and open tautomer of DTMDA were obtained. Ab initio 4-3lG calculations carried out for these two conformations gave geometrical parameters which agree better with the AM1 than with the PM3 ones.
INTRODUCTION
Intermolecular and intramolecular hydrogen bonds are of fundamental interest to chemists and biochemists because they play a very important role in chemical reactions and biochemical processes. The knowledge of hydrogenbond energies is therefore extremely interesting both for theoretical and practical purposes, but their evaluation is still not simple and up to a few years ago, only ab initio methods were able to give satisfactory values. However, ab initio calculations, especially if geometry optimization is carried out and an extended basis set is adopted, are too expensive because they need too long computation time. Semiempirical methods are now available (MNDO/H [ 11, AM1 [ 21)) which, in a reasonably low computation time, allow good hydrogen-bonding energies to be obtained; however, the related “absolute” values are still not perfectly in line with the available experimental data and/or ab initio predictions. In order to test the performance of such methods, in previous studies [ 3-91 we adopted AM1 for calculating intramolecular and intermolecular hydrogen bonds [3] and it was applied successively to the study of malondialdehyde
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(MDA) and acetylacetone (ACAC) [41 together with their mono [5] and dithio analogues [6] as well as salicylic acid [7], 3-(4’-biphenyl)pentane-2,4dione [ 81 and usnic acid [91. Although AM1 utilizes the MNDO parameters for sulphur, rather satisfactory results were obtained for the molecular geometries, whilst failures occurred in predicting correctly the stability order of the keto and enol tautomers of thiomalondialdehyde [5] and of some chelated structures of usnic acid [9]. Notwithstanding this, the stability order of OH. - *O, O-H. l S, S-H* *-0 and S-H* - *S bridges is predicted correctly; some energies appear to be underestimated and others overestimated (e.g. S-H- - -0 and S-H* - *S are predicted to be too strong in comparison with 0-H. *-0 and 0-H. **S hydrogen bridges). The difference between MNDO and AM1 is mainly in the presence in the latter method of radial gaussian orbitals which correct the excessively longrange core-core repulsions. Recent improvements of the MNDO approach, consisting essentially in a new procedure for parameter optimization and also in taking into account compounds containing ipervalent bonds, led to a new Hamiltonian, labelled MNDO-PM3 [lo]. This Hamiltonian is available in the MOPAC-Version 5.00 computation package [ 111, distributed by QCPE. As far as hydrogen bonding is concerned, the original paper reports the heats of association for various pairs of molecules calculated to test the method, but only the intramolecular hydrogen-bonding distances of salicylaldoxime (poorly reproduced by MNDO and AM1 ) are tabulated. In the present work the MNDO-PM3 Hamiltonian was adopted to reinvestigate malondialdehyde (MDA), thiomalondialdehyde (TMDA) and dithiomalondialdehyde (DTMDA) in order to ascertain whether, and to what extent, improvements can be obtained both in the molecular geometry and the intramolecular hydrogen-bond energy evaluation for different hydrogen bridges involving 0 and S atoms. l
CALCULATIONS
All the possible keto and enol tautomers of MDA, TMDA and DTMDA (shown in Fig. 1) already studied at the AM1 level were taken into account. The geometrical parameters selected to define the molecular geometry were always optimized simultaneously. Attempts to include the option “CI = 4” (four molecular orbitals enclosed in the CI treatment to take into account partially the correlation energy, thereby obtaining results directly comparable with the AM1 results in Refs. 4-6) were made and later abandoned, because, differently from AMl, error messages were obtained [ 121. Since no ab initio study on DTMDA was found, we also carried out a full geometry optimization at the ab initio 4-31G level, limited to the III-ES tautomer, in order to evaluate its hydrogen-bond energy, to be compared with the analogous findings for MDA and TMDA reported in the literature.
255
Fig. 1. Possible conformations and adopted numbering systems of malondialdehyde (I or MDA, X = Y = 0 ), thiomalondialdehyde (II or TMDA, X = 0 and Y = S ) and dithiomalondialdehyde (III or DTMDA, X = Y = S ) . K, keto; EO, enol; ES, enethiol. R6 = R, = H. Obviously, some of the above structures are redundant when X = Y.
The AMPAC [ 2b] and MOPAC [ 111 computation packages furnished by QCPE and the GAUSSIAN 80 program [13] were used. Calculations were carried out on a VAX-STATION 2000 and on VAX 11-750 computers. RESULTS
AND DISCUSSION
Although the molecular geometries of all the tautomers shown in Fig. 1 were optimized, only the 4-3lG geometries of the hydrogen-bonded and open structures of the III-ES tautomer are reported herein (see Fig. 2). However, the PM3 geometries are available from the author on request. The enol trans conformations were found to be always less stable than the hydrogen-bonded ones, except for TMDA. In particular, the enethiol II-ETS3 tautomer was calculated to be about 10 kJ mol-’ more stable than the hydrogen-bonded one and nearly isoenergetic with the II-ETSl one. The stability order predicted by PM3 for the enol and enethiol tautomers of the title compounds is shown in Table 1. Generally, for the trans conformers it is different from that predicted by AM1 owing to the fact that some energy differences between the various tautomers are rather low. AM1 predicts the II-K3 keto tautomer of TMDA to be about 7 kJ mol-’ more stable than the enol one [ 51 in contrast to the experimental evidence [ 171. The PM3 results
256
Fig. 2. Geometries, energies and dipole moments of the hydrogen-bonded and open conformations of DTMDA (III-ES) as obtained at the ab initio 4-31G level.
TABLE I Energy differences (kJ mol-i) predicted by MNDO-PM3
MDA TMDA (enol ) TMDA (enethiol) DTMDA
among the various tautomers of MDA, TMDA and DTMDA, as
EO (ES)
E-TO1 (E-TSl)
E-TO2 (E-TS2)
E-TO3 (E-TSB)”
0.0 0.0 10.1 0.0
14.8 9.1 1.3 40.5
10.2 7.9 10.0 33.7
11.0 6.0 0.0 37.3
“Lower labels refer to DTMDA and to the enethiol tautomers of TMDA.
not only confirm this result but also the keto tautomer of MDA is calculated to be more stable than I-E, so giving rise to erroneous enolization energies. Comparison of bond lengths and bond angles of MDA and TMDA with the literature ab initio data shows good agreement, not appreciably different from that observed previously for AM1 results [ 4,5], but PM3 predicts longer C=S and C-S bond lengths making them closer to the experimental data available in the literature [ 181. However, rather unreliable results are predicted for the III-ES tautomer of DTMDA, where the hydrogen-bonded structure shows CzV symmetry with Hg equidistant from the two S atoms. Moreover, in the corresponding open conformation the S*-* S distance is estimated as 1.926 A, an extremely low value, being nearly equal to the van der Waals radius of the sulphur atom. A comparison of the molecular geometries of the I-E0 and III-ES open conformations points out a dramatic decrease in S1_2_3,$_3_4 and S3-4_5,which indicates an abnormally strong S **-S attraction in the latter tautomer. The analysis of the most significant energy-partitioning terms reported in Table 2
+ 33.427 + 204.311 +4%.611 +71.520 + 191.948
-0.156 - 0.093 -5.124 - 0.056 - 6.326
- 6.307 - 2.051 -0.530 - 2.050 -0.076 - 6.307
Dithiomalondialdehyde s1-G - 14.202 G-H, - 4.980 s,-ss +0.876 SS-& - 4.980 C*-Cz +0.201 G-S, - 14.203 -312.443 - 73.323 - 126.483 - 73.324 - 167.19% - 312.44%
- 407.127 - 76.17% - 389.163 - 112.849 - 154.154 - 392.336
+ 162.772 + 42.41% -t 147.423 +42.417 +82.623 + 162.772
+ 222.988 +42.373 + 185.350 l-69.781 i-83.245 + 209.088
E
- 14.016 - 0.472 - 5.086 - 7.476 f0.401 - 12.189
- 28.122 - 0.002 - 0.004 - 14.363 +0.110 - 18.558
-6.340 - 0.403 - 1.396 -3.779 -0.079 -5.190
- 10.134 0.000 -0.019 -5.617 - 0.037 - 6.035
B
+ 157.836 + 28.348 + 194.612 +47.51% + 94.009 + 147.241
+ 196.044 + 18.864 + 189.257 f 52.658 +71.975 + 192.771
C
- 316.463 - 53.476 - 403.040 - 94.952 - 179.330 -302.541
- 408.427 - 41.223 - 362.084 - 117.331 - 153.003 - 391.545
D
“A, resonance terms; B, exchange terms; C, electron-electron repulsion; D, electron-nuclear attraction; E, nuclear-nuclear repulsion.
+ 154.075 -l-31.%86 I- 149.298 +31.8%6 -t&1.791 + 154,077
+ 195.783
- 9.650
~alond~a~ehyde - 26.969 0,-G -0.927 01-H, +0.061 01-G - 13.625 0,-H, +0.109 G-G? - 19.375 C&l
D
A
C
A
B
*en
Hydrogen-bonded
Most significant PM3 energy partitioning terms” of hydrogen-bonded and open tautomers of MDA and DTMDA
TABLE 2
+ 162.834 +25.173 +210.%%0 + 49.569 + 85.818 + 158.536
+ 224.848 +22.135 + 173.216 f 70.769 +%1.419 + 207.246
E
258
confirms that the most anomalous contribution is the S* **S electron-nuclear attraction term: unlike the value for I-EO, it jumps from - 126.5 eV to -403.0 eV on passing from the hydrogen-bonded to the open structure of III-ES and is not balanced by an appropriate increase in the electron-electron and/or nuclear-nuclear repulsion terms, probably because of the opposite net charges predicted on the two S atoms (Table 3). Incidentally, the negative charge found on H, also appears to be unjustified. Now, apart from the correctness of the above-discussed terms, an H-centred conformation implies a very strong hydrogen-bonding energy, which is only 8.19 kJ mol-l, a value in good agreement with the prediction of the Lippincott potential function [ 191 and with the well-known weakness of the S-H* **S bridge. Indeed, no experimental information on DTMDA is known since this compound has still not been synthesized and only some theoretical results have been reported in the literature [ 5,201, and to the best of our knowledge, no ab initio study has been performed. For this reason we carried out a geometry optimization at the 4-31G level in order to obtain alternative geometrical data and hydrogen-bond energy values. These geometries (Fig. 2) confirm the asymmetrical hydrogen-bonded structure of DTMDA found by the AM1 approach. Table 4 shows the hydrogen-bonding values predicted by MNDO-PM3 and AM1 methods and evaluated as the difference between the total energies of the hydrogen-bonded tautomers and those of the corresponding open structures. Ab initio values are also reported for comparison. For MDA‘and for the enethiol tautomers of TMDA, both AM1 and PM3 give nearly equal values: however, the value for MDA is underestimated and that of TMDA overestimated with respect to the ab initio predictions. Also, the hydrogen-bond energies of TABLE 3 PM3 net charges of the hydrogen-bonded and open tautomers of MDA and DTMDA Q
DTMDA (III-E)
MDA (I-E) Hydrogen bonded
Open
Hydrogen bonded
Open
-0.2578 +0.3538 -0.4561 +0.1942 - 0.3854 + 0.0549 + 0.1079 +0.1338 +0.2547
-0.1945 + 0.3309 - 0.3526 +0.1126 -0.3178 + 0.0350 + 0.0807 +0.1150 +0.1908
-0.0870 -0.0734 -0.2746 - 0.0734 -0.0871 +O.llBl +O.llBl +0.1260 +0.2333
+0.3217 -0.2084 -0.1241 -0.2180 - 0.0285 +0.1254 +0.1264 +0.1225 -0.1170
TABLE 4 Calculated hydrogen-bonding dithio analogues
energies (kJ mol-‘)
for malondialdehyde
and its monothio and
Hydrogen-bond energy
Malondialdehyde Thiomalondialdehyde” Thiomalondialdehyde’ DithiomaIondiaIdehyde
AM1
PM3
Ab initio
39.2” 33.ld 25.3d 18.18
36.9 41.6 22.0 a.2
56.gb 52.2’ 16.3 10.3h
“Ref. 3. bHF/6-31G [21]. “Enol form. dRef. 5. eHF/6-31G work: 4-31G basis set.
[ 161. ‘Enethiol form. %ef. 6. hThis
the enol tautomer of TMDA predicted by AM1 and PM3 are lower than the ab initio ones, but AM1 and ab initio results indicate that the hydrogen bridge in TMDA is about 5 kJ mol-’ weaker than in MDA, in contrast with PM3 calculations which give the reverse order of stability. Finally, PM3 predicts the H-S** S bridge to be much weaker than does AMl, although the DTMDA geometries giving rise to this conclusion are highly improbable. The weakness of such a bond is also confirmed by the 4-31G ab initio results which give 10.31 kJ mol-‘. l
CONCLUSION
On the grounds of the results obtained, the performance of the MNDO-PM3 Hamiltonian is less good than that of the AM1 one. Even if the former method calculates better values for the C-S bond lengths, failure occurs in predicting some molecular geometries and some stability ordering of keto and enethiol tautomers, so that this method is to be used with caution. Notwithstanding this, the PM3 intramolecular hydrogen-bond strengths are comparable with, or better than, those obtained with the AM1 approach. However, a previous PM3 study [22 J on thiophene-l-oxide and dithienothiophene sulphoxides pointed out its good performance in calculating inversion barriers which appear to be more reliable than the MNDO ones. ACKNOWLEDGEMENT
Financial support from the Italian Minister0 della Pubblica Istruzione (Roma) is gratefully acknowledged.
260 REFERENCES 1 2
3 4 5 6 7 8 9 10 11 12
13
14 15 16 17 18 19 20 21 22
K.Ya. Burst&n and A.N. Isaev, Theor. Chim. Acta, 64 (1984) 397. (a)M.J.S. Dewar, E.G. Zoebisch, E.F. Healy and J.J.P. Stewart, J. Am. Chem. Sot., 107 (1985) 3902. (b)Q.C.P.E. Program No. 506. G. Buemi, F. Zuccarello and A. Raudino, J. Mol. Struct. (Theochem), 164 (1988) 379. G. Buemi and C. Gandolfo, J. Chem. Sot. Faraday Trans. 2,85 (1989) 215. G. Buemi, unpublished results. G. Buemi, J. Chem. Sot., Faraday Trans. 2,85 (1989) 1771. F. Zuccarello and G. Buemi, Gazz. Chim. Ital., 118 (1988) 359. G. Buemi, J. Mol. Struct. (Theochem), 201 (1989) 193. G. Buemi and F. Zuccarello, J. Mol. Struct. (Theochem), in press. J.J.P. Stewart, J. Comput. Chem., 10 (1989) 209,221. J.J.P. Stewart, QCPE Bull., 9 (1989) 10; QCPE Program No. 455, version 5.00. The error originates from degenerate energy levels detected in MECI (multi-electron configuration interaction calculation). All components of degenerate irreducible representation are used. If not, then results will be nonsense. Such an error message was never given by AMPAC when used for the same molecules, whilst it was obtained by MOPAC for some conformers also when “CI = 6” was invoked. Since it was not possible to obtain homogeneous results for all structures, “CI” was excluded in all. J.S. Binkley, R.A. Whiteside, R. Krishnan, R. Seeger, D.J. DeFrees, H.B. Schlegel, S. Topiol, L.R. Kahn and J.A. Pople, Gaussian 80, QCPE Program No. 406; QCPE New& 71 (1980) 36. J. Emsley, N.J. Freeman, R.J. Parker and R.E. Overill, J. Chem. Sot., Perkin Trans. 2, (1986) 1479. J. Bicerano, H.F. Schaefer, III and W.H. Miller, J. Am. Chem. Sot., 105 (1983) 2550. S. Millefiori and A. Millefiori, J. Chem. Sot., Faraday Trans. 2,85 (1989) 1465. F. Duus, J. Am. Chem. Sot., 108 (1986) 630. Tables of Interatomic Distances and Configurations in Molecules and Ions, Special Publication No. 18, The Chemical Society, London, 1965. R. Schroeder and E.R. Lippincott, J. Phys. Chem., 61 (1957) 921. L. Carlsen and F. Duus, J. Chem. Sot. Perkin Trans. 2, (1980) 1080. S. Millefiori, A. Millefiori and G. Granozzi, J. Mol. Struct. (Theochem), 105 (1983) 135. G. Buemi, submitted for publication.
253
INTRAMOLECULAR HYDROGEN BONDING OF MALONDIALDEHYDE AND ITS MONOTHIO AND DITHIO ANALOGUES STUDIED BY THE PM3 METHOD
GIUSEPPE BUEMI Dipartimento di Scienze Chimiche, Universitti di Catania Viale A.Doria nr.6, 95125 Catania (Italy) (Received 30 November 1989)
ABSTRACT The molecular geometries of all the possible conformations of malondialdehyde (MDA ) , thiomalondialdehyde (TMDA) and dithiomalondialdehyde (DTMDA ) were optimized by means of the most recent semiempirical method MNDO-PM3 to test the performance of the new Hamiltonian in evaluating the most probable structures as well as the hydrogen-bond strengths. PM3 results are better than AM1 ones in predicting the C-S and C=S bond lengths and the energy of theS-H* . *S bridge, but unreliable geometries for the hydrogen-bonded and open tautomer of DTMDA were obtained. Ab initio 4-3lG calculations carried out for these two conformations gave geometrical parameters which agree better with the AM1 than with the PM3 ones.
INTRODUCTION
Intermolecular and intramolecular hydrogen bonds are of fundamental interest to chemists and biochemists because they play a very important role in chemical reactions and biochemical processes. The knowledge of hydrogenbond energies is therefore extremely interesting both for theoretical and practical purposes, but their evaluation is still not simple and up to a few years ago, only ab initio methods were able to give satisfactory values. However, ab initio calculations, especially if geometry optimization is carried out and an extended basis set is adopted, are too expensive because they need too long computation time. Semiempirical methods are now available (MNDO/H [ 11, AM1 [ 21)) which, in a reasonably low computation time, allow good hydrogen-bonding energies to be obtained; however, the related “absolute” values are still not perfectly in line with the available experimental data and/or ab initio predictions. In order to test the performance of such methods, in previous studies [ 3-91 we adopted AM1 for calculating intramolecular and intermolecular hydrogen bonds [3] and it was applied successively to the study of malondialdehyde
0166-1280/90/$03.50
0 1990 -
Elsevier Science Publishers B.V.
254
(MDA) and acetylacetone (ACAC) [41 together with their mono [5] and dithio analogues [6] as well as salicylic acid [7], 3-(4’-biphenyl)pentane-2,4dione [ 81 and usnic acid [91. Although AM1 utilizes the MNDO parameters for sulphur, rather satisfactory results were obtained for the molecular geometries, whilst failures occurred in predicting correctly the stability order of the keto and enol tautomers of thiomalondialdehyde [5] and of some chelated structures of usnic acid [9]. Notwithstanding this, the stability order of OH. - *O, O-H. l S, S-H* *-0 and S-H* - *S bridges is predicted correctly; some energies appear to be underestimated and others overestimated (e.g. S-H- - -0 and S-H* - *S are predicted to be too strong in comparison with 0-H. *-0 and 0-H. **S hydrogen bridges). The difference between MNDO and AM1 is mainly in the presence in the latter method of radial gaussian orbitals which correct the excessively longrange core-core repulsions. Recent improvements of the MNDO approach, consisting essentially in a new procedure for parameter optimization and also in taking into account compounds containing ipervalent bonds, led to a new Hamiltonian, labelled MNDO-PM3 [lo]. This Hamiltonian is available in the MOPAC-Version 5.00 computation package [ 111, distributed by QCPE. As far as hydrogen bonding is concerned, the original paper reports the heats of association for various pairs of molecules calculated to test the method, but only the intramolecular hydrogen-bonding distances of salicylaldoxime (poorly reproduced by MNDO and AM1 ) are tabulated. In the present work the MNDO-PM3 Hamiltonian was adopted to reinvestigate malondialdehyde (MDA), thiomalondialdehyde (TMDA) and dithiomalondialdehyde (DTMDA) in order to ascertain whether, and to what extent, improvements can be obtained both in the molecular geometry and the intramolecular hydrogen-bond energy evaluation for different hydrogen bridges involving 0 and S atoms. l
CALCULATIONS
All the possible keto and enol tautomers of MDA, TMDA and DTMDA (shown in Fig. 1) already studied at the AM1 level were taken into account. The geometrical parameters selected to define the molecular geometry were always optimized simultaneously. Attempts to include the option “CI = 4” (four molecular orbitals enclosed in the CI treatment to take into account partially the correlation energy, thereby obtaining results directly comparable with the AM1 results in Refs. 4-6) were made and later abandoned, because, differently from AMl, error messages were obtained [ 121. Since no ab initio study on DTMDA was found, we also carried out a full geometry optimization at the ab initio 4-31G level, limited to the III-ES tautomer, in order to evaluate its hydrogen-bond energy, to be compared with the analogous findings for MDA and TMDA reported in the literature.
255
Fig. 1. Possible conformations and adopted numbering systems of malondialdehyde (I or MDA, X = Y = 0 ), thiomalondialdehyde (II or TMDA, X = 0 and Y = S ) and dithiomalondialdehyde (III or DTMDA, X = Y = S ) . K, keto; EO, enol; ES, enethiol. R6 = R, = H. Obviously, some of the above structures are redundant when X = Y.
The AMPAC [ 2b] and MOPAC [ 111 computation packages furnished by QCPE and the GAUSSIAN 80 program [13] were used. Calculations were carried out on a VAX-STATION 2000 and on VAX 11-750 computers. RESULTS
AND DISCUSSION
Although the molecular geometries of all the tautomers shown in Fig. 1 were optimized, only the 4-3lG geometries of the hydrogen-bonded and open structures of the III-ES tautomer are reported herein (see Fig. 2). However, the PM3 geometries are available from the author on request. The enol trans conformations were found to be always less stable than the hydrogen-bonded ones, except for TMDA. In particular, the enethiol II-ETS3 tautomer was calculated to be about 10 kJ mol-’ more stable than the hydrogen-bonded one and nearly isoenergetic with the II-ETSl one. The stability order predicted by PM3 for the enol and enethiol tautomers of the title compounds is shown in Table 1. Generally, for the trans conformers it is different from that predicted by AM1 owing to the fact that some energy differences between the various tautomers are rather low. AM1 predicts the II-K3 keto tautomer of TMDA to be about 7 kJ mol-’ more stable than the enol one [ 51 in contrast to the experimental evidence [ 171. The PM3 results
256
Fig. 2. Geometries, energies and dipole moments of the hydrogen-bonded and open conformations of DTMDA (III-ES) as obtained at the ab initio 4-31G level.
TABLE I Energy differences (kJ mol-i) predicted by MNDO-PM3
MDA TMDA (enol ) TMDA (enethiol) DTMDA
among the various tautomers of MDA, TMDA and DTMDA, as
EO (ES)
E-TO1 (E-TSl)
E-TO2 (E-TS2)
E-TO3 (E-TSB)”
0.0 0.0 10.1 0.0
14.8 9.1 1.3 40.5
10.2 7.9 10.0 33.7
11.0 6.0 0.0 37.3
“Lower labels refer to DTMDA and to the enethiol tautomers of TMDA.
not only confirm this result but also the keto tautomer of MDA is calculated to be more stable than I-E, so giving rise to erroneous enolization energies. Comparison of bond lengths and bond angles of MDA and TMDA with the literature ab initio data shows good agreement, not appreciably different from that observed previously for AM1 results [ 4,5], but PM3 predicts longer C=S and C-S bond lengths making them closer to the experimental data available in the literature [ 181. However, rather unreliable results are predicted for the III-ES tautomer of DTMDA, where the hydrogen-bonded structure shows CzV symmetry with Hg equidistant from the two S atoms. Moreover, in the corresponding open conformation the S*-* S distance is estimated as 1.926 A, an extremely low value, being nearly equal to the van der Waals radius of the sulphur atom. A comparison of the molecular geometries of the I-E0 and III-ES open conformations points out a dramatic decrease in S1_2_3,$_3_4 and S3-4_5,which indicates an abnormally strong S **-S attraction in the latter tautomer. The analysis of the most significant energy-partitioning terms reported in Table 2
+ 33.427 + 204.311 +4%.611 +71.520 + 191.948
-0.156 - 0.093 -5.124 - 0.056 - 6.326
- 6.307 - 2.051 -0.530 - 2.050 -0.076 - 6.307
Dithiomalondialdehyde s1-G - 14.202 G-H, - 4.980 s,-ss +0.876 SS-& - 4.980 C*-Cz +0.201 G-S, - 14.203 -312.443 - 73.323 - 126.483 - 73.324 - 167.19% - 312.44%
- 407.127 - 76.17% - 389.163 - 112.849 - 154.154 - 392.336
+ 162.772 + 42.41% -t 147.423 +42.417 +82.623 + 162.772
+ 222.988 +42.373 + 185.350 l-69.781 i-83.245 + 209.088
E
- 14.016 - 0.472 - 5.086 - 7.476 f0.401 - 12.189
- 28.122 - 0.002 - 0.004 - 14.363 +0.110 - 18.558
-6.340 - 0.403 - 1.396 -3.779 -0.079 -5.190
- 10.134 0.000 -0.019 -5.617 - 0.037 - 6.035
B
+ 157.836 + 28.348 + 194.612 +47.51% + 94.009 + 147.241
+ 196.044 + 18.864 + 189.257 f 52.658 +71.975 + 192.771
C
- 316.463 - 53.476 - 403.040 - 94.952 - 179.330 -302.541
- 408.427 - 41.223 - 362.084 - 117.331 - 153.003 - 391.545
D
“A, resonance terms; B, exchange terms; C, electron-electron repulsion; D, electron-nuclear attraction; E, nuclear-nuclear repulsion.
+ 154.075 -l-31.%86 I- 149.298 +31.8%6 -t&1.791 + 154,077
+ 195.783
- 9.650
~alond~a~ehyde - 26.969 0,-G -0.927 01-H, +0.061 01-G - 13.625 0,-H, +0.109 G-G? - 19.375 C&l
D
A
C
A
B
*en
Hydrogen-bonded
Most significant PM3 energy partitioning terms” of hydrogen-bonded and open tautomers of MDA and DTMDA
TABLE 2
+ 162.834 +25.173 +210.%%0 + 49.569 + 85.818 + 158.536
+ 224.848 +22.135 + 173.216 f 70.769 +%1.419 + 207.246
E
258
confirms that the most anomalous contribution is the S* **S electron-nuclear attraction term: unlike the value for I-EO, it jumps from - 126.5 eV to -403.0 eV on passing from the hydrogen-bonded to the open structure of III-ES and is not balanced by an appropriate increase in the electron-electron and/or nuclear-nuclear repulsion terms, probably because of the opposite net charges predicted on the two S atoms (Table 3). Incidentally, the negative charge found on H, also appears to be unjustified. Now, apart from the correctness of the above-discussed terms, an H-centred conformation implies a very strong hydrogen-bonding energy, which is only 8.19 kJ mol-l, a value in good agreement with the prediction of the Lippincott potential function [ 191 and with the well-known weakness of the S-H* **S bridge. Indeed, no experimental information on DTMDA is known since this compound has still not been synthesized and only some theoretical results have been reported in the literature [ 5,201, and to the best of our knowledge, no ab initio study has been performed. For this reason we carried out a geometry optimization at the 4-31G level in order to obtain alternative geometrical data and hydrogen-bond energy values. These geometries (Fig. 2) confirm the asymmetrical hydrogen-bonded structure of DTMDA found by the AM1 approach. Table 4 shows the hydrogen-bonding values predicted by MNDO-PM3 and AM1 methods and evaluated as the difference between the total energies of the hydrogen-bonded tautomers and those of the corresponding open structures. Ab initio values are also reported for comparison. For MDA‘and for the enethiol tautomers of TMDA, both AM1 and PM3 give nearly equal values: however, the value for MDA is underestimated and that of TMDA overestimated with respect to the ab initio predictions. Also, the hydrogen-bond energies of TABLE 3 PM3 net charges of the hydrogen-bonded and open tautomers of MDA and DTMDA Q
DTMDA (III-E)
MDA (I-E) Hydrogen bonded
Open
Hydrogen bonded
Open
-0.2578 +0.3538 -0.4561 +0.1942 - 0.3854 + 0.0549 + 0.1079 +0.1338 +0.2547
-0.1945 + 0.3309 - 0.3526 +0.1126 -0.3178 + 0.0350 + 0.0807 +0.1150 +0.1908
-0.0870 -0.0734 -0.2746 - 0.0734 -0.0871 +O.llBl +O.llBl +0.1260 +0.2333
+0.3217 -0.2084 -0.1241 -0.2180 - 0.0285 +0.1254 +0.1264 +0.1225 -0.1170
TABLE 4 Calculated hydrogen-bonding dithio analogues
energies (kJ mol-‘)
for malondialdehyde
and its monothio and
Hydrogen-bond energy
Malondialdehyde Thiomalondialdehyde” Thiomalondialdehyde’ DithiomaIondiaIdehyde
AM1
PM3
Ab initio
39.2” 33.ld 25.3d 18.18
36.9 41.6 22.0 a.2
56.gb 52.2’ 16.3 10.3h
“Ref. 3. bHF/6-31G [21]. “Enol form. dRef. 5. eHF/6-31G work: 4-31G basis set.
[ 161. ‘Enethiol form. %ef. 6. hThis
the enol tautomer of TMDA predicted by AM1 and PM3 are lower than the ab initio ones, but AM1 and ab initio results indicate that the hydrogen bridge in TMDA is about 5 kJ mol-’ weaker than in MDA, in contrast with PM3 calculations which give the reverse order of stability. Finally, PM3 predicts the H-S** S bridge to be much weaker than does AMl, although the DTMDA geometries giving rise to this conclusion are highly improbable. The weakness of such a bond is also confirmed by the 4-31G ab initio results which give 10.31 kJ mol-‘. l
CONCLUSION
On the grounds of the results obtained, the performance of the MNDO-PM3 Hamiltonian is less good than that of the AM1 one. Even if the former method calculates better values for the C-S bond lengths, failure occurs in predicting some molecular geometries and some stability ordering of keto and enethiol tautomers, so that this method is to be used with caution. Notwithstanding this, the PM3 intramolecular hydrogen-bond strengths are comparable with, or better than, those obtained with the AM1 approach. However, a previous PM3 study [22 J on thiophene-l-oxide and dithienothiophene sulphoxides pointed out its good performance in calculating inversion barriers which appear to be more reliable than the MNDO ones. ACKNOWLEDGEMENT
Financial support from the Italian Minister0 della Pubblica Istruzione (Roma) is gratefully acknowledged.
260 REFERENCES 1 2
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