Molecular structure and O-H?O hydrogen bond in 1-aryl-1,3-diketone malonates

Article abstract of DOI:10.1016/j.molstruc.2012.09.010

The molecular structure of four 1,3-diketone malonates 1-4 were studied by analysis of geometric parameters obtained from single crystal X-ray diffraction experiments at 293 K, displaying three sections: aromatic ring (4-substituted phenyl or naphthyl rin

Full text of DOI:10.1016/j.molstruc.2012.09.010

Journal of Molecular Structure 1034 (2013) 43–50
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Molecular structure and OAHÁ Á ÁO hydrogen bond in 1-aryl-1,3-diketone malonates
⇑
Federico Jiménez-Cruz , Lubanski Fragoza Mar, Jose Luis García-Gutierrez
Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, Colonia San Bartolo, Atepehuacan, CP 07730 México, DF, Mexico
h i g h l i g h t s
"
"
"
"
X-Ray and DFT shows three moieties for 1,3-diketone malonates: aromatic, enolone and a linear chain.
Aromatic and enolone system are conjugated resulting in an strong OAHÁ Á ÁO bonding.
p
-Delocalization parameters (Q and kQ) showed more enol–keto delocalization.
Hydrogen bonding (RAHB) may be affected by the additive effect of CH2CH2CH(COOCH3)2.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2012
Received in revised form 30 August 2012
Accepted 4 September 2012
Available online 12 September 2012
The molecular structure of four 1,3-diketone malonates 1–4 were studied by analysis of geometric
parameters obtained from single crystal X-ray diffraction experiments at 293 K, displaying three sec-
tions: aromatic ring (4-substituted phenyl or naphthyl rings), H1O1C1C2C3O3 enol–keto (EK) system
and a linear chain with dimethyl malonate. In the malonate moiety which is involved in C6, enolizable
fragments are not observed. The two firsts moieties (aromatic ring and enolone system) are almost in
a planar conformation. These results are well-supported by DFT B3LYP/6-31++G(d,p) calculations and
Keywords:
MO analysis. These compounds are in the enol–keto (EK) tautomer. In comparison with a full
ized enolone (kQ = 0.5), diketones 1–4 have a EK
kQ values are between 0.283 and 0.321 Å and showing values of OÁ Á ÁO length between 2.488 and 2.567 Å,
which describe a resonance assisted hydrogen bonding (RAHB) and may be influenced by the additive
effect of the CH2CH2CH(COOCH3)2 chain.
p delocal-
1,3-Diketone malonates
Tautomerization
Resonance assisted hydrogen bonding
DFT
X-ray diffraction
Benzoylacetone
p
-delocalization in the OAHÁ Á ÁO moiety because of the
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Usually the strong H-bonds are characterized by a short bond
length and a hydrogen bond energy exceeded by 12 kcal/mol [7].
Molecular structure of 1,3-diketones have attracted a great
attention for several years. Indeed, the fact of the intramolecular
H-bond (HB) described in these systems, in which two oxygen
atoms are interconnected through the hydrogen atom by a conju-
gated single and double bonds, has received numerous studies [1–
3]. The two main techniques used to determine the molecular
structure are X-ray and neutron diffraction. According to these
techniques, crystals of 1,3-diketones consist of packings of the enol
tautomers stabilized by a strong intramolecular H-bond; that is,
the OAHÁ Á ÁO tautomerization and their interconversion.
(Scheme 1) These investigations have been resulted in many pub-
lications in order to elucidate that the strengthening of the homo-
These HB include configurations in which the hydrogen atoms
are close at the midpoint of the formal proton donor and proton
acceptor atoms; (XÁ Á ÁY) length is usually less than 2.5 Å and have
wells potential with a low-barrier in which the position of the pro-
ton is variable and appears to be sensitive to the molecular envi-
ronment beyond its immediate vicinity [7]. Gilli and co-workers
described this bond type by the following interrelated features:
(1) very short OÁ Á ÁO lengths 2.432–2.554 Å; (2) strong delocaliza-
tion in the heteroconjugated fragment; (3) lengthening of the
OAH bond (1.20 Å); (4) lowering of the v(OH) frequencies (2566–
2675 cmÀ1) and downfield shifting of the enolic proton resonance
(15.3–17.0 ppm) [1,2]. RAHBs are characterized by synergistic
interplay between the strengthening of the hydrogen bonding
nuclear OAHÁ Á ÁO bond is paralleled by
a transition from
dissymmetric to symmetric of the XAHÁ Á ÁY systems properties in
which RAHB are often classified as a strong hydrogen bonding
[1,4–6].
and increasing delocalization of the p conjugated fragment con-
necting the H-bond donor and acceptor atoms. This confers to
RAHBs specific geometric features which must be quantified by
suitable geometrical descriptors such as the antisymmetric delo-
calization parameter Q and the delocalization parameter kQ, which
⇑
Corresponding author. Tel.: +52 55 9175 6611; fax: +52 55 9175 7178
E-mail address: jimenezf@imp.mx (F. Jiménez-Cruz).
describe the
p electron delocalization, it says describing the RAHB
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.09.010

44
F. Jiménez-Cruz et al. / Journal of Molecular Structure 1034 (2013) 43–50
H
H
H
O
O
O
O
O
O
O
O
1
1
1
1
R
R
R
R
R
R
R
R
keto-enol
full delocalized
enol-keto
EK
keto-keto
KK
KE
Scheme 1.
0
intermediates and final products 1–4 were purified by either
recrystallization or chromatographic column and full characterized
by spectroscopic methods. Crystal samples of diketone malonates
1–4 were obtained from slow evaporation of the compounds dis-
solved in ethanol. Scheme 4
+Q
0.320
-Q
-0.320
H
O
H
H
O
O
O
O
O
DELOC
0.5
KE
EK
λ
2.2. Single crystal X-ray diffraction studies
0
1.0
Scheme 2.
Single crystal X-ray diffraction data of compounds 1–4 were
collected at 293 K using graphite-monochromated MoKa radiation
(k = 0.71073 Å) on a Siemens P4/PC diffractometer. The data collec-
tion [15], cell refinement and data reduction were carried out using
XSCANS. The structure was solved with direct methods and refined
by full matrix least square methods based on F2 using SHELXS97
and SHELXL97 packages (Table 1) [16,17].
in the localized enol–keto (EK) or keto–enol (KE) tautomers, or the
full delocalized fragment (DELOC), see Scheme 2 [1,8].
The cooperative nature of hydrogen bonding is of great interest
to us because it has occurred in the molecular structure of the 1,3-
diketone malonates 1–4. (Scheme 3) The synthesis of these com-
pounds has been described and characterized previously by our
group [9,10]. These compounds have a very important chemical
reactivity because they can be used in the synthesis of heterocyclic
organic [11,12] or coordination compounds such as organotin(IV)
complexes [13,14].
1,3-Diketone malonates present features such as a benzoylace-
tone and a malonate structure moieties, in which the enolizable
hydrogen in benzoylacetone moiety allows the equilibrium to the
enol–keto, keto–enol or keto–keto tautomers because is an unsym-
metrical structure [11].
2.3. Computational details
The molecular structures of the diketone malonates 1–4 were
built and visualized with Gauss View 03 tool; all calculations were
carried out using Gaussian 03 program [18]. For modeling, the ini-
tial guess of the molecule was obtained by optimization of the
probes at the semiempirical level of theory with AM1 Hamiltonian
and then submitted to geometry optimization at DFT level of the-
ory (the B3LYP exchange correlation corrected functional was
used) [19,20] with the 6-31++G(d,p) basis set. Frequency calcula-
tions were performed in order to find none imaginary frequency
assuring the minimal energy structures. HOMO and LUMO maps
were generated from the cube files performed in the same Gauss-
ian suite.
In this paper we describe the molecular structure of the enolone
tautomer of the 1-aryl-1,3-diketone malonates 1–4 (Scheme 3) and
the hydrogen bonding analysis in the OAHÁ Á ÁO system of the ben-
zoylacetone moiety in solid state by single crystal X-ray diffraction
and the
p
-delocalization properties in the OAHÁ Á ÁO system. In
addition, in order to show the electronic contribution in the eno-
lone (HOAC@CAC@O) system, DFT calculations of these probe
molecules were performed.
3. Results and discussion
3.1. Molecular structure
2. Experimental and computational section
According to Table 1, the yellow crystals of the compound 2 and
the colorless crystals of compounds 1, 3 and 4 have the appropriate
features for the measurement of some selected geometric parame-
ters outlined in Table 2. The ORTEP plot of compounds 1–4 are dis-
played in Figs. 1 and 2, and the numbering of selected atoms is
showed in Scheme 5.
Molecular structures display three sections: aromatic ring (4-
substituted phenyl or naphthyl rings), H1O1C1C2C3O3 enolone
system and a linear chain with dimethyl malonate. In the malonate
moiety which is involved in C6, enolizable fragments are not ob-
served. According to the Fig. 3, a top perspective view for diketone
1–4 is depicted; in these features, for the 1-naphthyl-substituted
diketone malonate 4, the corresponding torsion angle defined by
2.1. Preparation of 1,3-diketone malonates 1–4
1-Aryl-1,3-diketone malonates 1–4 were prepared according to
our method described elsewhere [9,10]. It consists in the C-aroyla-
tion of the dimethyl malonate anion (NaH in THF) with the corre-
sponding aroyl chlorides to produce an aroylmalonate. Further
Michael addition of the aroylmalonates to methyvinyl ketone pro-
ceeded in the presence of Triton B or Et3N in THF, yielding the 1,5-
diketones. Finally the 1,5-diketones were submitted to a 1,5 ? 1,3-
diketone rearrangement with piperidinium acetate in refluxing
benzene to produce the titled compounds 1–4 (Scheme 3). Both
OH
O
O
*
*
*
*
R
OCH3
OCH3
in wich R is
O2N
H
H3C
1
2
O
4
3
Scheme 3.

F. Jiménez-Cruz et al. / Journal of Molecular Structure 1034 (2013) 43–50
45
O
HC- OCH3
O
O
O
O
O
O
O
O
O
O
OCH3
THF
O
Piperidine, Acetic acid
benzene reflux
R
R
OCH3
OCH3
R
OCH3
OCH3
O
O
R
Cl
Triton B, THF
O
H3CO
OCH3
1,3-diketone malonates
Scheme 4.
Table 1
Crystal data, collection data and structure refinement for 1,3-diketone malonates 1–4.
1
C
2
3
4
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Crystal shape (color)
Crystal size (mm)
Space group
16H18O6
C16H17O8N
351.31
293(2)
MoK
Triclinic
Prism (yellow)
0.24 Â 0.18 Â 0.08
P1
C17H20O6
320.33
293(2)
MoK
Monoclinic
Prism (colorless)
0.40 Â 0.20 Â 0.10
P21/c
C20H20O6
356.36
293(2)
MoKa radiation (0.71073)
Monoclinic
Prism (colorless)
0.38 Â 0.28 Â 0.24
P21/n
306.30
293(2)
MoK
Orthorhombic
Plates (colorless)
0.60 Â 0.48 Â 0.20
P bca
a
radiation (0.71073)
a
radiation (0.71073)
a
radiation (0.71073)
ꢀ
Unit cell dimensions
a = 10.245(1) Å,
a
= 90.00°
a = 7.258(4) Å,
a
= 90.59°
a = 19.7001(17) Å,
a
= 90.00°
a = 14.4410(10) Å, a = 90.00°
b = 8.309(1) Å, b = 90.00°
b = 8.867(3) Å, b = 91.87°
b = 10.4448(15) Å, b = 92.215°
b = 5.6720(10) Å, b = 105.98°
c = 36.614(3) Å,
3116.8(5)
8
c
= 90.00°
c = 12.947(6) Å,
818.8(7)
2
c
= 100.49°
c = 8.084(3) Å,
1662.2(6)
4
c
= 90.00°
c = 22.586(2) Å, c = 90.00°
1778.5(4)
4
Volume (Å3)
Z
Density (calculated) (g/
1.306
1.425
1.280
1.331
cm3)
Absorption coefficient
0.100
0.116
0.097
0.098
(mmÀ1
)
F(000)
1296
368
680
752
h range for data collection
1.50–25.5
2.34–30.00
2.07–25.0
1.88–30.0
(°)
Diffractometer used/scan
mode
Siemens P4/PC diffractometer/ Siemens P4/PC diffractometer/
h–2h
x
Siemens P4/PC diffractometer/
x
Siemens P4/PC diffractometer/
x
Index ranges
0 6 h 6 12, 0 6 k 6 10,
À44 6 l 6 0
0 6 h 6 10, À12 6 k 6 12,
À18 6 l 6 18
À23 6 h 6 23, À12 6 k 6 0,
0 6 h 6 17, 0 6 k 6 6,
À26 6 l 6 25
0 6 l 6 9
Reflections collected
2890
5117
3170
4732
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
2890 [R(int) = 0.0000]
2890/0/254
1.022
4765 [R(int) = 0.0487]
4765/0/229
0.792
2936 [R(int) = 0.0306]
2936/0/212
0.840
4586 [R(int) = 0.0447]
4586/0/239
0.874
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
r
(I)]
R1 = 0.0733, wR2 = 0.1298
R1 = 0.2311, wR2 = 0.1989
0.216 and À0.218 e ÅÀ3
R1 = 0.0587, wR2 = 0.1446
R1 = 0.1426, wR2 = 0.1806
0.245 and À0.190 e ÅÀ3
R1 = 0.0491, wR2 = 0.1018
R1 = 0.1225, wR2 = 0.1231
0.150 and À0.148 e ÅÀ3
R1 = 0.0507, wR2 = 0.0965
R1 = 0.1255, wR2 = 0.1149
0.229 and À0.171 e ÅÀ3
Table 2
Selected geometric parameters by X-ray for 1,3-diketone malonates 1–4.
the compound 4. The torsion angles described in the enol–keto
moiety, H1O1C1C2 and O3C3C2C1 showed an almost planar geom-
etry, 1,3-diketone 3 showed the less planarity (13.6° and À3.9°),
these features are related to the presence of a resonance between
1
2
3
4
H1Á Á ÁO3a
O1Á Á ÁO3
CaC1
C1C2
C2C3
C3O3
C1O1
O1H1
1.654(7)
2.488(6)
1.482(8)
1.362(8)
1.420(8)
1.249(7)
1.306(7)
0.963(7)
111.2(5)
121.9(6)
120.0(6)
142.4(7)
1.758(3)
2.567(3)
1.488(4)
1.354(4)
1.440(4)
1.261(3)
1.346(3)
0.865(3)
103.0(2)
122.3(2)
121.6(2)
155.0(3)
1.731(3)
2.511(3)
1.475(4)
1.364(4)
1.420(4)
1.259(3)
1.323(4)
0.904(3)
109.0(2)
120.5(3)
121.5(3)
143.0(3)
1.750(3)
2.537(2)
1.474(3)
1.363(3)
1.421(3)
1.244(3)
1.325(3)
0.870(3)
107.5(2)
121.0(19)
122.3(2)
149.0(3)
the
p electrons and the hydrogen bonding, which is object of a
more detailed analysis in the next section.
3.2. Electron delocalization parameters and hydrogen bonding analysis
H1O1C1b
O1C1C2
C1C2C3
O1H1Á Á ÁO3
The cooperative nature of hydrogen bonding involves two as-
pects: (a) the hydrogen bonding between molecules with conju-
gated multiple
p bonds which is described as a resonance
assisted hydrogen bonding (RAHB) [1], and (b) a second type of
cooperativity occurs when hydrogen bonding in functional groups
such as hydroxyl groups with both donor and acceptor properties
forms continuous chains or cycles. It was first recognized in the
crystal structures of small carbohydrate molecules by Jeffrey, Gress
and Takagi [21]. According to Gilli any OAHÁ Á ÁO bond can be con-
sidered as a stationary point of a bimolecular reaction pathway,
OAHÁ Á ÁO ? OÁ Á ÁHAO, by the OÁ Á ÁHÁ Á ÁO transition state [1,2].
According to Table 2 and Scheme 5, hydrogen bonding lengths
(H1Á Á ÁO3) are rounded between 1.65 and 1.76 Å, the highest value
a
Length (Å).
Bond angle (°).
b
O1C1CaCb and C2C1CaCf for the aromatic ring and the enolone sys-
tem moieties shows a value of 2.94° and 1.52° respectively, which
indicate that these moieties are almost in a planar conformation; in
the case of the 1-nitrophenyl-substitued diketone malonate 2 the
values of O1C1CaCb and C2C1CaCf are À17.7° and À18.9° respec-
tively, that is, the compound 2 is less planar in comparison with

46
F. Jiménez-Cruz et al. / Journal of Molecular Structure 1034 (2013) 43–50
Fig. 1. Ortep plots for 1,3-diketone malonates 1 and 2.
Fig. 2. Ortep plots for 1,3-diketone malonates 3 and 4.
1
antisymmetric delocalization parameter Q and the delocalization
parameter kQ [1,8]. The former is calculated as Q = d1–d4 + d3–d2
and is zero for the total -delocalized fragment, and it has been
H
3
O
O
O
O
1
d4
q1=d1-d4 C---O
5
d1
b
d2
d3
p
6
1
3
q2=d3-d2 C---C
Q = q1 + q2
O
a
f
shown to amount Q = Q0 = 0.320 Å for the extreme enol–keto
(EK) and keto enol forms (KE) as an effect of the formation of
the intramolecular H-bond. The latter is calculated as kQ = (1 À Q/
4
2
O
(1 - Q/0.320)
=
λ
Q0)/2 in which kQ = 0, 1 and 0.5 for EK, KE and complete
p delocal-
2
ization, respectively.
Scheme 5.
By considering the appropriate geometric parameters, Q and kQ
can be calculated for diketone malonates 1–4 as is shown in Table 3.
In order to compare with other diketonic systems from the X-ray
parameters reportedintheliterature,thecompounds1–4wereeval-
uatedwithacetylacetone5 [22]; dibenzoylmethanecompounds 6, 8,
9, 10 [2] and 11 [23]; benzoylacetones 7 [24] and 12 [25]; curcumi-
noid compounds 13 [26] and 14 [27], a trifluoromethyl b-diketone
15 [28] and a very short O1Á Á ÁO3 length and symmetrical hydrogen
bond observed in 4-cyano-2,2,6,6-tetramethyl-3,5-heptanedione
16 [29], which are described in Fig. 4. Q, kQ and dO1Á Á ÁO3 values for
compounds 5–16 are also shown in Table 3.
corresponds to the NO2-substituted compound 2. In the case of the
O1Á Á ÁO3 bond lengths, the values are 2.488, 2.567, 2.511 and
2.537 Å for H-, NO2-, CH3-, and naphthyl-substituted compound
2, respectively. As we describe above, planarity in the enolone moi-
ety also shows the possible resonance in the system by
delocalization.
p
In order to describe the
p electron delocalization in the system
O1C1C2C3O3 two parameters have been described (Scheme 5), the
Fig. 3. Top perspective view of the molecules 1–4 showing the planarity by measuring torsion angles: 1. O1C1CaCb, 2. C2C1CaCf, 3. H1O1C1C2, 4. O3C3C2C1 in (°).

F. Jiménez-Cruz et al. / Journal of Molecular Structure 1034 (2013) 43–50
47
Table 3
Values of Q and kQ with appropriate geometric parameters for 1,3-diketone malonates 1–4 and related compounds 5–16.a
Entry
d1
d2
d3
d4
q1
0.0566
q2
0.0577
Q
kQ
dO1Á Á ÁO3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1.3058
1.3456
1.3234
1.3252
1.3210
1.3130
1.2930
1.3621
1.3542
1.3639
1.3628
1.3590
1.3910
1.4050
1.4198
1.4396
1.4205
1.4210
1.4430
1.3970
1.4140
1.2492
1.2605
1.2594
1.2444
1.2620
1.2880
1.2860
0.1143
0.1705
0.1206
0.1390
0.1430
0.0310
0.0160
0.0690
0.0540
0.1410
0.0450
0.0270
0.3214
0.2336
0.3116
0.2828
0.2766
0.4516
0.4750
0.3922
0.4156
0.2797
0.4297
0.4578
0.5900
0.6453
0.3297
0.5052
2.488
2.567
2.511
2.537
2.592
2.465
2.502
2.463
2.470
2.554
2.461
2.481
2.459
2.498
2.553
2.388
0.0851
0.0640
0.0808
0.0590
0.0250
0.0070
0.0380
0.0260
0.0740
0.0150
0.0350
À0.0313
À0.0540
0.0740
À0.0005
0.0854
0.0566
0.0582
0.0840
0.0060
0.0090
0.0310
0.0280
0.0670
0.0300
À0.0080
À0.0263
À0.0390
0.0350
À0.0028
1.3040
1.3210
1.3070
1.3380
1.2797
1.2590
1.3170
1.2731
1.3800
1.3570
1.3880
1.3980
1.4203
1.4250
1.3780
1.4324
1.4080
1.4240
1.4180
1.3900
1.3940
1.3860
1.4130
1.4296
1.2780
1.2470
1.2920
1.3030
1.3110
1.3130
1.2430
1.2736
À0.0576
À0.0930
0.1090
À0.0033
a
The values are in Å.
OH
O
OH
O
OH
O
OH
O
OH
O
H3C
CH3
OCH3
NO2
8
7
6
5
9
O
OH
OH
O
OH
O
OH
O
NO2
O
Br
H3CO
O2N
11
13
12
10
OH
O
OH
O
O
OH
CF3
O2N
N
16
15
14
Fig. 4. Structure of the 1,3-diketones 5–16 cited in Table 3.
2,60
2,55
2,50
2,45
2,40
2,35
2,60
5
5
2
2
15
15
10
4
10
4
2,55
2,50
2,45
2,40
2,35
3
3
7
7
14
14
1
1
12
12
9
9
6
6
8
13
8
13
11
11
16
16
KE
KE
EK
EK
-0,2
0,0
0,2
0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,65 0,70
π-delocalization index, λQ
Q (Å)
Fig. 6. Plot depicting the relation between O1Á Á ÁO3 length (Å) and
index kQ.
p-delocalization
Fig. 5. Plot depicting the relation between O1Á Á ÁO3 length (Å) and Q parameter (Å).
features, 1,3-diketone malonates 1–4 are
enol–keto (EK) tautomer in comparison to the OÁ Á ÁHÁ Á ÁO full
delocalized diketone 16 [29] which has the least dO1Á Á ÁO3 value
p delocalized towards
A graphical analysis between the O1Á Á ÁO3 lengths (a criteria for
a RAHB), the antisymmetric delocalization parameter Q and the
delocalization parameter kQ is described in Figs. 5 and 6. In these
p

48
F. Jiménez-Cruz et al. / Journal of Molecular Structure 1034 (2013) 43–50
Table 4
2,58
Selected geometric parameters and delocalization parameters Q and
k from the
optimized 1,3-diketone malonates 1–4 at B3LYP/6-31++G(d,p) level of theory.
2
1
2
3
4
2,56
2,54
2,52
2,50
2,48
Total energy
au
À1071.98067 À1276.48785 À1111.3022 À1225.63226
H1Á Á ÁO3a
O1Á Á ÁO3
CaC1
1.547
2.493
1.478
1.385
1.434
1.261
1.327
1.020
105.8
120.2
120.3
151.8
À0.4
1.567
2.504
1.481
1.379
1.443
1.256
1.328
1.017
105.9
120.8
120.2
150.8
À0.3
1.573
2.509
1.476
1.383
1.436
1.259
1.330
1.014
106.0
120.3
120.6
151.1
0.5
1.580
2.513
1.477
1.382
1.438
1.257
1.331
1.012
106.1
120.3
120.7
150.9
0.2
4
C1C2 (d2)
C2C3 (d3)
C3O3 (d4)
C1O1 (d1)
O1H1
4*
3
3*
2*
H1O1C1b
O1C1C2
C1C2C3
O1H1Á Á ÁO3
H1O1C1C2c
O3C3C2C1
O1C1CaCb
C2C1CaCf
Qd
1*
EK
1
0,22
0,24
0,26
0,28
0,30
0,32
À0.3
À0.4
À0.3
À0.3
11.2
12.1
10.7
11.7
À8.0
À0.8
π-delocalization index, λQ
À8.7
À0.9
0.1157
0.3192
0.1358
0.2878
0.1242
0.3059
0.1296
0.2975
Fig. 7. Plot depicting the relation between O1Á Á ÁO3 length (Å) and
p-delocalization
kQ
index kQ of both X-ray and B3LYP (marked with asterisk) calculated geometries for
a
b
c
diketone malonates 1–4.
Length (Å).
Bond angle (°).
Torsion angle (°).
Delocalization parameters (Å).
geometries have the best approach to full EK
p-delocalization in
d
the 2 (NO2) and 4 (naphthyl) diketone malonates. Likewise, the
corresponding dO1Á Á ÁO3 values are lesser than those estimated by
X-ray, however, in general the tendency to EK delocalization is
similar in both X-ray and DFT results. In contrast the determina-
tions in diketone malonates 1 and 3 are similar in both X-ray
and DFT. Bond and torsion angle values are comparable for both so-
lid and gas phase molecular structures.
In these features, the top view projection of the HOMO and
LUMO contribution for the title diketones was outlined (Fig. 8). A
planarity in the aromatic-enolone system is observed in the gas
phase of the calculated molecules as well as in X-ray described
in Fig. 3, in which the CH2CH2CH(COOCH3)2 chain shows the best
conformation of the optimized geometry. The main contribution
to the HOMO is in the aromatic-enolone system and in the first
CH2 chain moiety. Contributions to the frontier MO from the rest
of the chain, CH2CH(COOCH3)2, are not observed. However, in the
naphthyl-substituted diketone malonate 4 the CH2 contribution
is not observable and a very little contribution to HOMO in O1 is
remarkable. In gas phase, HB may be also affected by the additive
effect of the CH2CH2CH(COOCH3)2 chain by their conformational
motions on 1,3-diketone malonates.
(2.388 Å), Q = 0 and kQ = 0.5. H-substituted diketone malonate 1
has more tendency towards the full delocalized diketone, thus
the order of this behavior is H (kQ = 0.3214) > CH3 (kQ = 0.3116) > -
Naphthyl (kQ = 0.2828) > NO2 (kQ = 0.2336). By inspection of Figs. 5
and 6, related molecular structures such as benzoylacetones 7 and
12 are closer to the full
nates 1–4, which have the attached CH2CH2CH(COOCH3)2 chain
and its possible conformational effect on the complete delocal-
ization in the enolone system. In contrast, EK -delocalized dike-
tones 1–4 are far from the behavior of curcuminoid diketones 13
and 14 which are more KE -delocalized. Dibenzoylmethane 6
and substituted compounds 8, 9, 10 [2] have closer behavior to a
complete delocalization but the tendency is to EK -delocalized
p
p delocalization than 1,3-diketone malo-
p
p
p
p
p
tautomer; whereas, the crowded and more substituted dib-
enzoylmethane 10 is closer to the behavior of diketone malonates
1–4 and the electron deficient aromatic 1,3-diketone 15.
In addition, benzoylacetone 7, with kQ = 0.4750 and
d
O1Á Á ÁO3 = 2.502, showed a near p-delocalization in contrast with
the relative benzoylacetone moiety in the diketone malonate 1.
This behavior can be produced by the effect of the aliphatic chain
CH2CH2CH(COOCH3)2 attached to the benzoylacetone moiety and
packing effects in the solid state. The symmetrical acetylacetone
5 can be either KE or EK localized system through of the dynamic
hydrogen motion in OAHÁ Á ÁO which not only involve the O1Á Á ÁO3
coordinate but also changes in skeletal geometry [22]. Besides, ste-
ric O1Á Á ÁO3 stretching can be produced by the chain moiety [23]. In
the case of the NO2-substituted diketone malonate 2, both the elec-
tron withdrawing effect and the additive effect of the CH2CH2-
CH(COOCH3)2 chain in the O1Á Á ÁO3 length and delocalization
parameters is well observable.
4. Conclusions
The molecular structure of four 1,3-diketone malonates 1–4
were studied by analysis of geometric parameters obtained from
single crystal X-ray diffraction experiments at 293 K, displaying
three sections: aromatic ring (4-substituted phenyl or naphthyl
rings), H1O1C1C2C3O3 enol–keto system and a linear chain with
dimethyl malonate. In the malonate moiety which is involved in
C6, enolizable fragments are not observed. The two firsts moieties
(aromatic ring and enol–keto system) are almost in a planar con-
formation. These compounds are in the enol–keto (EK) tautomer.
It is noteworthy that the calculated geometric parameters of
1,3-diketone malonates 1–4 at the B3LYP/6-31++G(d,p) level of
theory obtained in the gas phase (Table 4) are slightly different
from those estimated by X-ray. In the case of the dH1Á Á ÁO3 lengths,
X-ray values are greater than the DFT values. By the other hand,
the lengths described in the calculation of Q and kQ delocalization
parameters (d1, d2, d3, d4) are also slightly different and this is re-
flected in the calculated values of Q and kQ (1Ã, 2Ã, 3Ã, 4Ã), see Ta-
ble 4, and in the plot of calculated X-ray (Fig. 7). DFT gas phase
In comparison with a full
p delocalized enolone (kQ = 0.5), dike-
tones 1–4 have a EK
p-delocalization in the OAHÁ Á ÁO moiety be-
cause of the kQ values are between 0.283 and 0.321 Å and
showing values of OÁ Á ÁO length between 2.488 and 2.567 Å, which
describe a resonance assisted hydrogen bonding (RAHB) and may
be influenced by the additive effect of the CH2CH2CH(COOCH3)2
chain. The main contribution to the HOMO is in the aromatic-eno-
lone system and in the first CH2 chain moiety. However, in the
naphthyl-substituted diketone malonate 4 the CH2 contribution

F. Jiménez-Cruz et al. / Journal of Molecular Structure 1034 (2013) 43–50
49
O
O
O
O
O
O
H
1
1
2
2
3
3
4
4
HOMO
LUMO
Fig. 8. Top view projection of the HOMO and LUMO contributions for the l,3-diketone malonates 1–4.
is not observable and a very little contribution to HOMO in O1 is
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