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LCAO-MO-calculations on the enol form of acetylacetone and its metal complexes
Volume
3. number
6
CHEhIICAL
LCAO-MO-CALCULATIONS
ON AND
ITS
PHYSICS
THE
ENOL
METAL
(LCAO-MO-Studies
on
LETTERS
June 1969
FORM
OF
ACETYLACETONE
COMPLEXES
molecular
structure
III
*)
Peter SCHUSTER ** Max-PZanck-Znstitutfiir
Physrkalische Received
Ckemre,
28 April
GSftingen,
Gemzany
1969
Energy surfaces descrlbmg the motion of the hydrogen bonded proton or the metal atom m the en01 form of acetylacetone and its Li complex were calculated by a semtemptrical LCAO-MO-SCF procedure for all valence electrons (CNDO/B). CNDO/Z calculattons on the structure of the Be complex of aceeylacetone are presented as well. The conclusions drawn from the different energy nurfaces and the calculated bond lengths are m good agreement unth the available expertmental data.
In the previous papers of this series * the semiempirical all-valence-electron LCAO-MOSCF method of Popie, Segal and Santry [l-3] (CNDO/Z) was shown to be suitable for a successful treatment of hydrogen bonded structures. Now we present CNDO model calculations for the enol form of acetylacetone and its Li and Be complexes One of the most interesting problems of these structures is the position of the proton or the metal atom between the two oxygens. We can dmtmguish two possibilities: the central atom (X) is moving between two eqmvalent positions and we are dealing with an equilibrium of two identical tautomers (1)or the favoured position of the X-atom lies on the C2-axis of the symmetric molecule (2).
H,c/
2
1
literature [4] concerns the aromatic character of acetylacetone metal chelates. The starting point of our calculabons is the symmetrical enolate amon (3a) The proton or the Li atom was placed in different positions, whereas the coordmates of all the other atoms were kept constant m all calculations. In order to save computation time the methyl groups *m the acetylacetone molecule were replaced by hydrogens (3 a.nd 4).
3cr
Anran
The bond lengths used are summarized m table 1. All bond angles on the_sp2-hybridized C-atoms are set equal (W = 120”) The original parametrization of the CNDO/2-method (31 IS used without any modification. In both structures the arrangeluent of all
X = “.LI
Ancther question discussed very much in the * Part I. Chem. Phys. Letters 2 (1968) 587; Part II: Intern J. Quantum Chem. 3 (1969). in press. ** Permanent address: Institut fdr Theorettsche Chemie. Untversitat Wien, Austria.
Table 1 Bond lengths used tn the CNBO/Z Bond
Length (A)
c-o c-c C-H
1.36 1.40 1.08
calculations.
433
Volume 3, number 6
CHEMICAL PHYSICS LETTERS
y, = IT-+=
_60:$:. .,I_ooo -t_ 169
4
-60 !Eo
June 1969
I Fig. 1. Energy curve for an out-of-plane movement of the central atom (X) in the enol form of malondmldehyde (3) and its Li complex (4).
atoms in one plane (x, y) shows the lowest energies (fig. 1). Therefore only a two-dimensional variation vnth the proton or the Li atom m the plane of the whole molecule was carried out. The total energies of the calculated structures 3 and 4 are presented as functions of the coor&nates (x, y) of the central atom (H, Li) (figs. 2 and 3). In spite of the completely symmetrical arrangement of the other atoms m the molecule, the hydrogen bonded proton prefers a position outside the C2-axis of the enolate anion (fig. 2). Ar the energy mmtmum the two O-H distaqes differ considerably in length - rolH = 1.13 A and The energy barrier ‘y’&$ = 1.33 d respectively. between the two mmima of the potential surface is extremely low - AE = 0.5 kc&/mole without considering zeropoint energies. From these data we would expect the proton moving to and fro very rapidly. We did not calculate the frequency from our potential energy surface, because in the molecule the motion of the proton will be coupled with changes of the other bond lengths. Therefore one would have to mimmize the energy with respect to all bond lengths m order to get correct numerical values. In the energy diagram of the Li complex (fig. 3) we find the other possibility &scussed
0.1
G
0 0.1 -at Fig. 3. Energy surface for an m-plane movement of the central hydrogen in the en01 form of malondiaIdeWe (3). 434
Fig. 3. Energy surface for an in-plane movement of the Li atom in the Li complex of malonciialdehy4e (4).
Volume
Charge Atom
3, number
&strlbutlon
6
CHEMICAL
in the enol form
PHYSICS
LETTERS
Table 2 of malondlaldehyde (3), the
En01 (3), X = H o-density Net charge z-den&y
Net charge
June 1969
corresponding amon (aa) and Its Li complex (4).
Anion (38) n-density
Lr complex u-density
Net charge
(4).
i&den&y
X = Li u-density
01
-0
313
1.750
4 563
- 0.445
1.524
4.921
- 0.370
1.555
4.815
C2
+O 298
0.720
2.982
~0.246
0.788
2 966
+o 294
0.741
2.965
C3
- 0.217
1.266
2.951
- 0.277
1 376
2.901
- 0.236
1.304
2.932
C4
+a.303
0.703
2.994
+ 0.246
0.788
2 -966
to.294
0.741
2.965
05
- 0.371
1.561
4.810
- 0.445
1.524
4.921
- 0.370
1.555
4.815
X
+ 0.311
0.689
+0.493
0.105
0.403
H2
- 0.010
1 .OlO
-0
128
1.128
- 0.052
2.052
H3
+0.023
0.977
- 0.068
1.068
- 0.002
I.002
H4
- 0.026
1.026
-0 128
1.128
- 0.052
1 .O52
above (2). At the energy mmimum the central atom is placed on the C2-ax& and the whole molecule possesses true C&-symmetry. As expected the Li-0 distance 1s much greater than the H-O distiOce in the previous example rti_O = 2.21 A. Charge distributions of both structures are presented in table 2 In the complex the LI atom carries a net charge of about +0.5. In order to CLzscussthe aromatic character of the complex we look at the charge distribution on the Ll atom in detil. An appreciable ring current m the NMR-spectrum can only be expected if the LI atom shows considerable a-electron density in the complex. In fact the calculated n-electron density is extremely low on the Li atom (P&I) = 0.1) and we would not expect to fmd a detectable ring current in the NMR-spectra of the acetylacetone complex. In the last year experimental data concerning the structure of the enol of acetylacetone and its metal complexes were presented by Muss0 and Junge [5]. From their IR-spectroscopical investigations of partially 13C-substituted compounds they concluded that the enol consists of a very rapid equilibrium of two identical tautomers (1, X = H). Since no splitting of NMR-signals is observed the frequency of the tautomerlzation is much higher than the frequency difference m the NMR-spectrum [S]. On the contrary the splitting ul the IR-speCtrum of partially l%-substituted metal complexes (X = Li, Na etc.) demonstrates the true C&-symmetry of these structures. In a recent pa_oer Kuhr and Muss0 [4] verified that no magnetic anisotropy due to an aromatic ring current can be detected in appropriate substituted metal chelates of acetylacetone .
E zeu
-122.1
Fig. 4. Energy dependence on the Be-O distance in the Be complex of maionchaldehyde for two extreme orientations, 58 (%h) and 5 b
435
Volume 3. number 6
CHEMICAL
Because of the size of the Be chelate (5) no extensive
5b variation of the structural parameters as m the Lr complex was performed. The metal atcm was placed in a symmetric position between the two ligands and the total energy was calculated as a function of the Be-O &stance for two extreme orientations of the llgands showing D2h- or D2dsymmetry (5a or 5 b). The “tetrahedral” arrangement of the oxygen3 around the central Beatom (5b) show3 the iower energy (fig. 4) The
Charge distrlbutlon
Table 3 m the Be complex hyde (sb).
Atom
436
h’et charge
Be
to
392
01
-0
297
C2
+o 320
C3
-0216
H2
-0
H3
+ 0.018
022
of malondmlde-
PHYSICS
LETTERS
June 1969
energy minimum 1s fou.iid at a metal oxygen bond length of ~~-0 = 1.74 A. The charge distribution in the Be complex is presented in table 3. The structure of the Be complex of acetylacetone was investigated by X-ray Mraction in the crystal [?I. A “tetrahedral” arrangement of the ligands is found. Tb,e experimental Be-O distance &Be_0 = 1.70 A) i3 very close to the calculated value. The results of the CNDO/2 calculations agree surprisingly well with the experunental data Despite Its approxlmatlons and simplifications thrs method seem3 to be an appropriate tool to an3wer structural questions. The author is indebted to Professor Dr. M. Elgen and Dr Th Funck for valuable discussions on this subject.
REFERENCES [l] J.A Pople, D. P Santry and G.A Seea!. I , J. Chem. Phys. 43 (1965) 129. J A Pople md G.A Segal, J Chem. Phys. 43 (1965) 136 J.A. Pople md G.A.Segal. J. Chem. Phys 44 (1966) 3289. M.Kuhr and H.Musso, Angew. Chem 81 (1969) 150. H Muss0 aria1H <.cge, Chem. Ber. 101 (1968) 801. J.L Burdett and M T Rogers, J. Am. Chem Sot 86 (1964) 2105. V.Amwthalmgam, V.M.Padmanabhan and J.Shankar. Acta Cryst. 13 (1960) 201.
3. number
6
CHEhIICAL
LCAO-MO-CALCULATIONS
ON AND
ITS
PHYSICS
THE
ENOL
METAL
(LCAO-MO-Studies
on
LETTERS
June 1969
FORM
OF
ACETYLACETONE
COMPLEXES
molecular
structure
III
*)
Peter SCHUSTER ** Max-PZanck-Znstitutfiir
Physrkalische Received
Ckemre,
28 April
GSftingen,
Gemzany
1969
Energy surfaces descrlbmg the motion of the hydrogen bonded proton or the metal atom m the en01 form of acetylacetone and its Li complex were calculated by a semtemptrical LCAO-MO-SCF procedure for all valence electrons (CNDO/B). CNDO/Z calculattons on the structure of the Be complex of aceeylacetone are presented as well. The conclusions drawn from the different energy nurfaces and the calculated bond lengths are m good agreement unth the available expertmental data.
In the previous papers of this series * the semiempirical all-valence-electron LCAO-MOSCF method of Popie, Segal and Santry [l-3] (CNDO/Z) was shown to be suitable for a successful treatment of hydrogen bonded structures. Now we present CNDO model calculations for the enol form of acetylacetone and its Li and Be complexes One of the most interesting problems of these structures is the position of the proton or the metal atom between the two oxygens. We can dmtmguish two possibilities: the central atom (X) is moving between two eqmvalent positions and we are dealing with an equilibrium of two identical tautomers (1)or the favoured position of the X-atom lies on the C2-axis of the symmetric molecule (2).
H,c/
2
1
literature [4] concerns the aromatic character of acetylacetone metal chelates. The starting point of our calculabons is the symmetrical enolate amon (3a) The proton or the Li atom was placed in different positions, whereas the coordmates of all the other atoms were kept constant m all calculations. In order to save computation time the methyl groups *m the acetylacetone molecule were replaced by hydrogens (3 a.nd 4).
3cr
Anran
The bond lengths used are summarized m table 1. All bond angles on the_sp2-hybridized C-atoms are set equal (W = 120”) The original parametrization of the CNDO/2-method (31 IS used without any modification. In both structures the arrangeluent of all
X = “.LI
Ancther question discussed very much in the * Part I. Chem. Phys. Letters 2 (1968) 587; Part II: Intern J. Quantum Chem. 3 (1969). in press. ** Permanent address: Institut fdr Theorettsche Chemie. Untversitat Wien, Austria.
Table 1 Bond lengths used tn the CNBO/Z Bond
Length (A)
c-o c-c C-H
1.36 1.40 1.08
calculations.
433
Volume 3, number 6
CHEMICAL PHYSICS LETTERS
y, = IT-+=
_60:$:. .,I_ooo -t_ 169
4
-60 !Eo
June 1969
I Fig. 1. Energy curve for an out-of-plane movement of the central atom (X) in the enol form of malondmldehyde (3) and its Li complex (4).
atoms in one plane (x, y) shows the lowest energies (fig. 1). Therefore only a two-dimensional variation vnth the proton or the Li atom m the plane of the whole molecule was carried out. The total energies of the calculated structures 3 and 4 are presented as functions of the coor&nates (x, y) of the central atom (H, Li) (figs. 2 and 3). In spite of the completely symmetrical arrangement of the other atoms m the molecule, the hydrogen bonded proton prefers a position outside the C2-axis of the enolate anion (fig. 2). Ar the energy mmtmum the two O-H distaqes differ considerably in length - rolH = 1.13 A and The energy barrier ‘y’&$ = 1.33 d respectively. between the two mmima of the potential surface is extremely low - AE = 0.5 kc&/mole without considering zeropoint energies. From these data we would expect the proton moving to and fro very rapidly. We did not calculate the frequency from our potential energy surface, because in the molecule the motion of the proton will be coupled with changes of the other bond lengths. Therefore one would have to mimmize the energy with respect to all bond lengths m order to get correct numerical values. In the energy diagram of the Li complex (fig. 3) we find the other possibility &scussed
0.1
G
0 0.1 -at Fig. 3. Energy surface for an m-plane movement of the central hydrogen in the en01 form of malondiaIdeWe (3). 434
Fig. 3. Energy surface for an in-plane movement of the Li atom in the Li complex of malonciialdehy4e (4).
Volume
Charge Atom
3, number
&strlbutlon
6
CHEMICAL
in the enol form
PHYSICS
LETTERS
Table 2 of malondlaldehyde (3), the
En01 (3), X = H o-density Net charge z-den&y
Net charge
June 1969
corresponding amon (aa) and Its Li complex (4).
Anion (38) n-density
Lr complex u-density
Net charge
(4).
i&den&y
X = Li u-density
01
-0
313
1.750
4 563
- 0.445
1.524
4.921
- 0.370
1.555
4.815
C2
+O 298
0.720
2.982
~0.246
0.788
2 966
+o 294
0.741
2.965
C3
- 0.217
1.266
2.951
- 0.277
1 376
2.901
- 0.236
1.304
2.932
C4
+a.303
0.703
2.994
+ 0.246
0.788
2 -966
to.294
0.741
2.965
05
- 0.371
1.561
4.810
- 0.445
1.524
4.921
- 0.370
1.555
4.815
X
+ 0.311
0.689
+0.493
0.105
0.403
H2
- 0.010
1 .OlO
-0
128
1.128
- 0.052
2.052
H3
+0.023
0.977
- 0.068
1.068
- 0.002
I.002
H4
- 0.026
1.026
-0 128
1.128
- 0.052
1 .O52
above (2). At the energy mmimum the central atom is placed on the C2-ax& and the whole molecule possesses true C&-symmetry. As expected the Li-0 distance 1s much greater than the H-O distiOce in the previous example rti_O = 2.21 A. Charge distributions of both structures are presented in table 2 In the complex the LI atom carries a net charge of about +0.5. In order to CLzscussthe aromatic character of the complex we look at the charge distribution on the Ll atom in detil. An appreciable ring current m the NMR-spectrum can only be expected if the LI atom shows considerable a-electron density in the complex. In fact the calculated n-electron density is extremely low on the Li atom (P&I) = 0.1) and we would not expect to fmd a detectable ring current in the NMR-spectra of the acetylacetone complex. In the last year experimental data concerning the structure of the enol of acetylacetone and its metal complexes were presented by Muss0 and Junge [5]. From their IR-spectroscopical investigations of partially 13C-substituted compounds they concluded that the enol consists of a very rapid equilibrium of two identical tautomers (1, X = H). Since no splitting of NMR-signals is observed the frequency of the tautomerlzation is much higher than the frequency difference m the NMR-spectrum [S]. On the contrary the splitting ul the IR-speCtrum of partially l%-substituted metal complexes (X = Li, Na etc.) demonstrates the true C&-symmetry of these structures. In a recent pa_oer Kuhr and Muss0 [4] verified that no magnetic anisotropy due to an aromatic ring current can be detected in appropriate substituted metal chelates of acetylacetone .
E zeu
-122.1
Fig. 4. Energy dependence on the Be-O distance in the Be complex of maionchaldehyde for two extreme orientations, 58 (%h) and 5 b
435
Volume 3. number 6
CHEMICAL
Because of the size of the Be chelate (5) no extensive
5b variation of the structural parameters as m the Lr complex was performed. The metal atcm was placed in a symmetric position between the two ligands and the total energy was calculated as a function of the Be-O &stance for two extreme orientations of the llgands showing D2h- or D2dsymmetry (5a or 5 b). The “tetrahedral” arrangement of the oxygen3 around the central Beatom (5b) show3 the iower energy (fig. 4) The
Charge distrlbutlon
Table 3 m the Be complex hyde (sb).
Atom
436
h’et charge
Be
to
392
01
-0
297
C2
+o 320
C3
-0216
H2
-0
H3
+ 0.018
022
of malondmlde-
PHYSICS
LETTERS
June 1969
energy minimum 1s fou.iid at a metal oxygen bond length of ~~-0 = 1.74 A. The charge distribution in the Be complex is presented in table 3. The structure of the Be complex of acetylacetone was investigated by X-ray Mraction in the crystal [?I. A “tetrahedral” arrangement of the ligands is found. Tb,e experimental Be-O distance &Be_0 = 1.70 A) i3 very close to the calculated value. The results of the CNDO/2 calculations agree surprisingly well with the experunental data Despite Its approxlmatlons and simplifications thrs method seem3 to be an appropriate tool to an3wer structural questions. The author is indebted to Professor Dr. M. Elgen and Dr Th Funck for valuable discussions on this subject.
REFERENCES [l] J.A Pople, D. P Santry and G.A Seea!. I , J. Chem. Phys. 43 (1965) 129. J A Pople md G.A Segal, J Chem. Phys. 43 (1965) 136 J.A. Pople md G.A.Segal. J. Chem. Phys 44 (1966) 3289. M.Kuhr and H.Musso, Angew. Chem 81 (1969) 150. H Muss0 aria1H <.cge, Chem. Ber. 101 (1968) 801. J.L Burdett and M T Rogers, J. Am. Chem Sot 86 (1964) 2105. V.Amwthalmgam, V.M.Padmanabhan and J.Shankar. Acta Cryst. 13 (1960) 201.