Room temperature molecular and lattice structures of a homologous series of anhydrous zinc(II) n-alkanoate

Article abstract of DOI:10.1016/j.saa.2006.11.007

The room temperature structures and lattice arrangements of a homologous series of zinc(II) n-alkanoates from chain length, n_C = 4-20, inclusive, have been studied using infrared spectroscopy, X-ray diffraction and polarizing light microscopy. Lattice parameters from single crystal and powder diffraction data, for zinc(II) hexanoate, are compared to validate the use of the powder method. Since they are in excellent agreement, the powder data are analyzed by a software programme to determine lattice parameters for all the homologues. These are used, in conjunction with infrared, X-ray, density and molecular model calculations to determine molecular and lattice structures. The compounds are isostructural, in that, each zinc atom is tetrahedrally coordinated to oxygen atoms from four different carboxylate groups and each ligand forms a Z,E-type bidentate bridge with two tetrahedral zinc atoms resulting in a syn-anti arrangement. The hydrocarbon chains are in the fully extended all-trans configuration and are tilted at an average angle of 60° to the zinc basal plane. For the short chain length compounds with n_C ≤ 8, a double bilayer in-plane-perpendicular-perpendicular-in-plane arrangement of hydrocarbon chains, with two molecules per unit cell, is indicated. For the others, an interdigitating in-plane-in-plane bilayer with head-to-tail interactions, with one molecule per unit cell, is proposed. A geometric model is presented to account for odd-even chain effects and to explain the differences in melting points and densities between these adducts. All the compounds crystallize in the monoclinic space group with P symmetry and are arranged in a two-dimensional network along the ac plane within the unit cell.

Full text of DOI:10.1016/j.saa.2006.11.007

Spectrochimica Acta Part A 68 (2007) 99–107
Room temperature molecular and lattice structures of a homologous
series of anhydrous zinc(II) n-alkanoate
Richard A. Taylor, Henry A. Ellis∗
Department of Chemistry, University of the West Indies, Mona, Kingston 7, Jamaica
Received 4 September 2006; accepted 9 November 2006
Abstract
The room temperature structures and lattice arrangements of a homologous series of zinc(II) n-alkanoates from chain length, nC = 4–20, inclusive,
have been studied using infrared spectroscopy, X-ray diffraction and polarizing light microscopy. Lattice parameters from single crystal and
powder diffraction data, for zinc(II) hexanoate, are compared to validate the use of the powder method. Since they are in excellent agreement,
the powder data are analyzed by a software programme to determine lattice parameters for all the homologues. These are used, in conjunction
with infrared, X-ray, density and molecular model calculations to determine molecular and lattice structures. The compounds are isostructural,
in that, each zinc atom is tetrahedrally coordinated to oxygen atoms from four different carboxylate groups and each ligand forms a Z,E-type
bidentate bridge with two tetrahedral zinc atoms resulting in a syn–anti arrangement. The hydrocarbon chains are in the fully extended all-trans
configuration and are tilted at an average angle of 60◦ to the zinc basal plane. For the short chain length compounds with nC ≤ 8, a double
bilayer in-plane-perpendicular–perpendicular-in-plane arrangement of hydrocarbon chains, with two molecules per unit cell, is indicated. For the
others, an interdigitating in-plane–in-plane bilayer with head-to-tail interactions, with one molecule per unit cell, is proposed. A geometric model
is presented to account for odd–even chain effects and to explain the differences in melting points and densities between these adducts. All the
compounds crystallize in the monoclinic space group with P symmetry and are arranged in a two-dimensional network along the ac plane within the
unit cell.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Zinc(II) n-alkanoates; Infrared spectroscopy; X-ray diffraction; Polarizing light microscopy; Lattice parameters; Densities
1. Introduction
P1c1 and P symmetry, respectively. One complicating factor
is that the long chain compounds are of such low symmetry
that it is difficult to grow single crystals from them for X-ray
studies. Another is the presence of polymorphic forms of these
structures arising, in part, from their methods of synthesis and
other unknown factors. These are characterized by hydrocarbon
chains interacting or orientated differently in the various lat-
tices. For example, Segedin et al. [5], on the basis of infrared,
X-ray diffraction and solid state 13C NMR studies, demonstrated
the presence of polymorphic structures for zinc(II) hexanoate
and heptanoate though they did not report structures for them.
Indeed, a definitive picture of the molecular units within the lat-
tice has not been reported, except for a few short chain length
compounds [4,7–10].
In a previous publication [1], room temperature molecu-
lar and lattice structures were proposed for anhydrous zinc(II)
hexanoate and pentadecanoate. Infrared and X-ray diffraction
studies pointed to syn–anti bridged structures, with the zinc
atom coordinated to oxygen atoms from four different carboxy-
late groups; a result in excellent agreement with other published
molecular structuresoftheselongchain compounds [2–6]. How-
ever, there have been few systematic studies on the effect of
hydrocarbon chain length on the lattice structures and chain
packing within a unit cell. At least for the hexanoate and pen-
tadecanoate, hydrocarbon chains are packed within lamellae and
tilted at an average angle of 60◦ to the zinc basal plane [1].
Additionally, they crystallize within a monoclinic unit cell with
In this paper, the evolution of the structural properties of
anhydrous zinc(II) n-alkanoate, Zn (CnH2n−1O2)2, containing
4–20 carbon atoms (n = 4–20) inclusive, is studied in relation
to the number of carbon atoms in the hydrocarbon chain and
whether the chains contain an odd or even number of carbon
∗
Corresponding author. Tel.: +876 9271910; fax: +876 9771835.
E-mail address: henry.ellis@uwimona.edu.jm (H.A. Ellis).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.11.007

100
R.A. Taylor, H.A. Ellis / Spectrochimica Acta Part A 68 (2007) 99–107
Table 1
atoms. This should give some insight in to the relationship
between hydrocarbon chain length, molecular and lattice struc-
tures.
Elemental analyses for anhydrous zinc(II) n-alkanoates
Compound
Carbon (%)
Hydrogen (%)
Experimental
Experimental
Calculated
Calculated
2. Experimental
ZnC4
ZnC5
ZnC6
ZnC7
ZnC8
ZnC9
ZnC10
ZnC11
ZnC12
ZnC13
ZnC14
ZnC15
ZnC16
ZnC17
ZnC18
ZnC19
ZnC20
44.94
44.45
49.09
51.92
54.30
57.18
58.04
59.84
61.88
63.42
64.31
65.73
66.33
66.49
65.90
67.63
68.84
40.11
44.89
48.75
51.94
54.63
56.92
58.89
60.61
62.13
63.47
64.66
65.73
66.70
67.58
68.38
69.11
69.79
7.06
6.96
7.82
8.43
8.93
5.89
6.78
7.50
8.09
8.60
9.02
9.39
9.71
9.99
10.24
10.46
10.66
10.84
11.07
11.16
11.29
11.42
The compounds were prepared by refluxing zinc oxide with
an excess of the appropriate carboxylic acid in ethanol for
approximately 2 h. The plate-like crystals, which precipitated,
on cooling, were filtered off, washed with ethanol, repeatedly,
collected and kept over silica gel in a vacuum desiccator (yield
∼85%). The compounds were white microcrystalline solids.
They were very thin ∼150–200 ␮m and lath-like in shape with
vertical cleavage. When viewed under polarized light, they
exhibited pleochroism (displayed faint colour changes as the
microscope stage was rotated), were anisotropic (birefringent
values in the region of 0.003–0.004) and first order grey. Zinc(II)
hexanoate single crystals were prepared as described previously
[1].
9.51
9.73
10.09
10.43
10.71
10.82
10.66
11.32
11.08
11.35
11.65
11.80
The melting points of the compounds were determined by the
capillary melting technique and differential scanning calorime-
try (DSC). DSC melting points were determined using a Mettler
TC 10A Processor attached to a DSC 20 standard cell. Between
3 and 10 mg of powder samples were sealed in standard alu-
minium pans with pierced lids and heated at a rate of 2 K/min
over a temperature range of 323–353 K. Each run was performed
in triplicate.
agreement between experimental and calculated values, it is
reasonable to conclude that the compounds are of high purity.
Capillary and DSC melting points, in addition to density
data, are given in Table 2. The sharp, final endothermic tran-
sition observed in the DSC trace is taken as the true melting
point. Clearly, the capillary and DSC data are in good agreement
though their values are consistently higher than the correspond-
ing literature values. This is not surprising since, in the early
literature, the melting points of these long chain soaps were col-
lected, for the most part, from impure compounds. For example,
basic (M(OH)x(RO)2) and or acid soaps (RCO2)x(M(RO2H))
are well-known contaminants. In some cases, using the capil-
lary method, a high temperature phase transition was sometimes
mistaken for the melting point [15].
Both melting point and density data, as a function of chain
length, nC areshowninFig. 1. Bothvaluesdecreasewithincreas-
ing nC. An odd–even effect, with increasing nC, is observed for
nC ≥ 9 with the values being somewhat higher for the even-chain
length compounds. Odd–even melting behaviour in n-alkanes
and other n-alkane derivatives is not unusual and has been stud-
ied quite extensively by Boese et al. [16,17]. They accounted
for the effect by suggesting different packing arrangements of
hydrocarbon chains in a crystal lattice. A geometric model,
adapted from those workers, is presented here to explain the dif-
ferent packingarrangements for nC ≥ 9. Theeven andoddchains
are fitted to a parallelogram (Fig. 2B) and trapezoid (Fig. 2F),
respectively. The stacking of the methylene, methyl and car-
boxylate groups are the important factors here. For both odd and
even chains, methylene groups are inter-grooved into adjacent
chains above and below them and the terminal methyl groups are
arranged in a staggered conformation but anti-parallel to neigh-
bouringchainsalongthesameplane. Ifthemodelsarestackedon
top of each other, as shown in Fig. 2C and G, then the same vol-
umeofvoidspacesforbothevenandoddchainareobtained. This
would mean that they are of similar densities and hence melting
points. This is clearly not in accord with the experimental results.
Infrared (IR) spectra were recorded in KBr pellets, at room
temperature, on a Perkin-Elmer FTIR 1000 spectrometer over
the range of 4000–400 cm−1
.
Single crystal diffraction measurements were carried out
on a Bruker AXS P4 Diffractometer with Mo K␣ radiation
(λ = 0.71073). Cell parameters were collected in a 2θ range
of 9.7–25◦ and refined from 2494 reflections. The data were
reduced by SHELXTL [11] and the structure refined and solved
bySHELXS97[12]. ORTEP3[13]wasthegraphicsprogramme
used.
Powder diffraction data were collected, at room temperature,
on a Bruker D5005 diffractometer with nickel filtered Cu K␣
˚
radiation (λ = 1.54056 A) from ungrounded samples mounted in
standard plastic holders. The X-ray tube was operated at 45 kV
and 35 mA at a time/step of 2 s for 2 h between 2◦ and 60◦.
Longer runs did not improve the diffraction patterns.
Microscopic studies were performed on samples mounted
between glass slides and coverslips, using a Swift polarizing
light microscope. For conoscopic studies, both slide and cover-
slips were pre-treated with an aqueous solution of 0.2% PVA,
heated to approximately 373 K, for a few minutes, and cooled
to ambient temperatures to produce good homeotropic textures
for examination.
The densities of the compounds were determined by flotation
in heptane/carbon tetrachloride mixtures.
3. Results and discussion
Elemental analyses of carbon and hydrogen of the synthe-
sized compounds are given in Table 1. Since there is excellent

R.A. Taylor, H.A. Ellis / Spectrochimica Acta Part A 68 (2007) 99–107
101
Table 2
Density and melting point data for zinc(II) n-alkanoates
Compound
Visual melting point (◦C)
This study
DSC melting point ( 0.2 ◦C)
Density (g cm−3
)
Literature [14]
This study
Literature [14]
ZnC4
ZnC5
ZnC6
ZnC7
ZnC8
ZnC9
ZnC10
ZnC11
ZnC12
ZnC13
ZnC14
ZnC15
ZnC16
ZnC17
ZnC18
ZnC19
ZnC20
156–157
154–156
145–147
140–142
140–141
134–137
137–138
135–137
133–135
135–136
132–134
132–133
133–135
132–133
135–136
134–135
136–136
–
–
158.5
157.9
149.9
142.9
141.4
138.4
138.8
135.2
134
134.3
133.2
130.3
133.8
131.2
134.2
131.8
134.8
–
–
144
–
132
–
131
–
127
–
125
–
123
–
121
–
1.36
1.35
1.30
1.24
1.19
1.12
1.14
1.10
1.09
1.07
1.08
1.05
1.06
1.05
1.06
1.04
1.05
141–143
–
135–137
–
128–130
–
127–130
–
126–128
–
126–128
–
124–126
–
–
–
However, as these units stack in a geometrically allowed manner
(with an off-set to the right) aided by the conformation of the ter-
minal methyl groups, the even-chain length lattice has few void
spaces (Fig. 2D). This results in a high packing efficiency and
high density. The odd chains are stacked in a manner that result
in increased void spaces between the molecular units result-
ing in a lower packing efficiency and lower density (Fig. 2H).
Consequently, the more densely packed even-chain length com-
pounds will have a higher melting point than the less densely
packed odd chain length ones. Also, the overall decrease in melt-
ing point with increasing nC may be ascribed to the energetics
resulting from competition between zinc–carboxylate binding,
which is favoured in the lower chain length compounds, and
van der Waals interaction between hydrocarbon chains, which
is a weaker force, and predominates in the longer chain length
compounds.
4. Infrared spectroscopy
Infrared spectroscopy of long chain metal carboxylates pro-
vides information on metal carboxylate coordination, chain
conformation and packing [18–20]. Thus, representative spec-
tra, collected between 4000 and 400 cm−1, at room temperature,
are shown in Fig. 3 .The spectra are characterized by strong
absorptions in the region of 1700–1200 cm−1 and weak to
medium bands between 1400 and 500 cm−1. The regular pro-
gression of narrow bands, especially pronounced for short
chain length compounds, between 1400 and 700 cm−1, are
assigned to the methylene progressive bands, typical for these
compounds [21–23]. The absence of a carbonyl absorption
band in the region of 1730 cm−1 and its replacement by car-
bonyl absorptions in the region of 1500, 1400, 950, 580 and
540 cm−1 indicates that there is complete resonance in the
C–O bonds of the carbonyl group as a result of coordination
between zinc and carboxylate ion; as well as the absence of
free acid. Further, the absence of hydroxyl absorption bands
in the region of 3500–3300 cm−1 confirms that the com-
pounds are indeed anhydrous. The carbonyl bands are assigned
as: the antisymmetric stretch, νa(COO) ∼1538–1531 cm−1
;
symmetric stretch, νs(COO) ∼1409–1394 cm−1; deforma-
tion, νd(C–C)COO ∼949–957 cm−1
; bending, ν␦(COO)
∼745–743 cm−1; out of plane twisting, ν (COO) ∼580 cm−1
␶
and rocking, ν (COO) ∼550–547 cm−1. The νa(COO) and
Fig. 1. Density and melting point vs. nC.
␳

102
R.A. Taylor, H.A. Ellis / Spectrochimica Acta Part A 68 (2007) 99–107
Fig. 2. Proposed geometrical models for nC ≥ 9 for even, A and B and odd, E and F chain length compounds. B and F are the parallelogram and trapezoid, respectively.
C–H are the various packing arrangements. C and G represent intermediate packing when the terminal groups are not properly orientated. The dark lines are the void
spaces. D and H represent the off-set packing arrangement.

R.A. Taylor, H.A. Ellis / Spectrochimica Acta Part A 68 (2007) 99–107
103
pounds. Each zinc atom is tetrahedrally coordinated to oxygen
atoms from four different carboxylate groups and each ligand
forms a Z,E-type bidentate bridge with two tetrahedral zinc
atoms resulting in a syn–anti arrangement. Since the νa(COO)
and νs(COO) absorptions are of unequal intensities through-
out, because of the difference in zero-point energies between
the asymmetric and symmetric modes, it implies that the C–O
bonds coordinated to zinc are non-equivalent; that is, Zn–O
bond lengths are not equal and, therefore, bonding around zinc
is asymmetrical. Moreover, the near invariance of the ρ(COO)
absorption in the region of 550–547 cm−1 supports a similar
molecular structure for all the homologues. Any significant dif-
ferences in frequency in this absorption would point to a change
in molecular structures [3]. This is supported by the ꢀν data
collected in Table 3, where the values are in the right range for
bridged structures [22–25]. Additionally, for triclinic or hexago-
nal chain packing, a single band for ρ(CH2) is expected; splitting
of this band is indicative of orthorhombic or monoclinic pack-
ing [3,26]. In the present case, the band is a doublet, except for
butanoate and pentadecanoate, and indicates that the compounds
crystallize within a monoclinic or orthorhombic crystal system.
These appear to be the preferred crystal systems. For example,
Peultier et al. [9,10] reported an orthorhombic system for anhy-
drous zinc(II) heptanoate and a monoclinic one for anhydrous
zinc(II)octanoate;hintingatapossiblechainlengthdependence.
Moreover, in these long chain compounds, hydrocarbon chains
are usually packed in small periodic structures (subcells) within
the unit cell. There are two possible types of subcells, depending
on the lateral packing of the chains. The nature of the packing
can be inferred from ρ(CH2) and δ(CH2) absorption bands. If
these bands are split, horizontal packing is indicated; if not, par-
allel packing is assumed [3]. In this case, there is no correlation
split in the bands and so lateral packing is of the parallel-type.
Further, the progression of regularly spaced absorptions, in the
region of 1347–1232 cm−1, arising from the wagging vibra-
tions of methylene groups, are typical for aliphatic chains and
Fig. 3. Room temperature infrared spectra for ZnC5, ZnC9, ZnC14 and ZnC20
.
νs(COO) absorptions are very strong bands in the regions of
1542–1530 and 1401–1397, respectively, for the entire homol-
ogous series. Interestingly, both are doublets for nC = 4–9 and
singlets for nC > 10 (Table 3). The splitting of these degenerate
modes correlates with a lowering of symmetry and the rela-
tive strengths of the zinc–carboxylate bond and van der Waals
interactions between hydrocarbon chains. It is expected that
zinc–carboxylate binding will be favoured for short chain homo-
logues, resulting in a stronger interaction between metal and
ligandleadingtosplittingoftheseabsorptions. Forthelongchain
length homologues, there is no corresponding splitting as the
metal–ligand bond becomes progressively weaker with increas-
ing nC, while van der Waals interchain interactions become
progressively stronger. Ellis et al. [1–6] have provided conclu-
siveevidence, frominfrared, X-rayandotherstudiesforbridging
bidentate coordination as the mode of bonding for these com-
Table 3
Frequencies of main bands and COO separation (cm−1) for Zn(II) n-alkanoates
Compound
Frequency of main bands and COO− separation (cm−1
)
νas(COO)
νs(COO)
ꢀν
ρ(CH2)
ρ(COO)
ZnC4
ZnC5
ZnC6
ZnC7
ZnC8
ZnC9
ZnC10
ZnC11
ZnC12
ZnC13
ZnC14
ZnC15
ZnC16
ZnC17
ZnC18
ZnC19
ZnC20
1549/1532
1549/1535
1547/1531
1549/1533
1550/1530
1550/1530
1537
1534
1542
1537
1539
1538
1539
1539
1538
1539
1539
1411/1401
1410/1401
1409/1399
1409/1400
1408/1398
1410/1400
1397
1399
1397
1398
1398
1397
1398
1397
1398
1397
1397
131
134
132
133
132
130
140
135
143
139
141
141
141
142
140
142
142
744/736
744/736
743/727
747/723
747/723
746/722
745/722
747/721
743/723
745/722
744/723
745/723
743/723
743/723
744/723
744/723
745/723
547
547
547
545
551
552
548
553
549
554
551
550
550
550
550
550
550

104
R.A. Taylor, H.A. Ellis / Spectrochimica Acta Part A 68 (2007) 99–107
indicate all-trans conformation of hydrocarbon chains in these
structures.
planes. Clearly, there are four hexanoate ligands within a unit
cell. For the longer chain length compounds, no suitable crystal
could be found for single crystal X-ray diffraction or conoscopic
studies. For these, the compounds are analyzed by X-ray powder
diffraction.
5. X-ray diffraction
Typical powder patterns, collected at room temperature, are
shown in Fig. 5 as intensity of the diffractive ray versus angle
of diffraction, 2θ for ZnC5, ZnC12 and ZnC20. No visible
reflections are observed beyond a 2θ of 60◦. The patterns are
characterized by evenly spaced reflections throughout for those
compounds with nC = 4–8, as shown by the pattern for ZnC5.
However, the patterns for nC ≥ 9 are somewhat different, as
shown by that for ZnC20, where evenly spaced reflections are
observed only at low angles; with those at intermediate and
higher angles being irregularly spaced and of low intensity. The
patterns in the low angle region are typical for reflections from
a lamellar structure [23,25–27]. Since there are different pat-
terns for the short and long chain compounds, in the region of
20◦ ≤ 2θ ≤ 25◦, associated with side chain packing, suggest that
the packing of hydrocarbon chains within the lamellae are not
the same for these adducts.
Profile analyses are performed on powder patterns where
peaks are chosen based on intensity and shape using Diffracplus
EVA V.9.0 software (a part of the machine operating system)
and are subsequently converted using the same software to a
line profile appropriate for auto-indexing. The lines are auto-
indexed using commercially available Win-Metric LS V. 2.1 for
windows at a tolerance of <0.1 and subsequently refined at a tol-
erance in the region of 0.06 with figures of merit, FN ∼ 13 and
M20 ∼ 19.0. Most lines are completely single indexed except the
very low intensity ones which are few. To validate the procedure,
the powder lattice parameters for zinc(II) hexanoate are com-
pared with the single crystal data. Indeed, the values are almost
identical as given in Table 4. Additionally, the single crystal
lattice parameters are used to generate powder patterns, which
are in excellent agreement with the experimentally derived one.
Typical results of the refinements and comparison with exper-
imental data are shown in Fig. 6 for ZnC5, ZnC12 and ZnC20.
The observed and experimental lines match to a high degree
of accuracy. All the compounds crystallize in the monoclinic
space group, as predicted by the infrared data, with P symme-
Details of the single crystal data and structure refinements, for
anhydrous zinc(II) hexanoate, are given in a previous paper [1].
There are two crystallographically different zinc atoms, Zn(1)
˚
and Zn(2) as shown in Fig. 3. The Zn(1)–O bond (1.972(3) A)
˚
is slightly longer than Zn(2)–O (1.947(3) A), which points to
unsymmetrical bonding around zinc, as indicated by the infrared
data. Also, the results confirm that the chains are in the fully
extended all-trans conformation. Although Segedin et al. [5]
pointed to the presence of a polymorphic structure for this com-
pound, they did not report a possible structure for it. Since their
compound crystallized in the monoclinic space group, P21/c and
this in P1c1, it is not unreasonable to propose our structure as a
polymorph. This proposal is confirmed, at least for nC ≤ 9 by the
small difference in structure, when compared with nC ≥ 9, of the
strong band centered at ∼1460 cm−1 corresponding to antisym-
metric methyl absorptions and scissoring modes for methylene
group absorptions. A close examination of Fig. 4 shows that
the hydrocarbon chains, in the plane of the paper, are parallel
to each other along a zinc plane and perpendicular to the plane
on the other side of the zinc atom. Conoscopic studies support
this arrangement of chains within the lattice. For example, under
polarized light two cleavage planes are observed, perpendicular
to each other, most probably resulting from this type of lat-
tice arrangement. Moreover, on rotating the microscope stage
a biaxial interference figure is obtained. This is clear evidence
for tilted hydrocarbon chains. In fact, molecular models indi-
cate that the chains are tilted at an average angle of 60◦, in good
agreement with previous studies [9,10]. Indeed, the evidence for
tilting of hydrocarbon chains in metal carboxylate compounds is
overwhelming [24,27,28]. Additionally, two cleavage planes are
observed, perpendicular to each other, most probably as a result
of the syn–anti arrangement of chains within the lattice. Thus,
thechainsarepacked:in-plane-perpendicular–perpendicular-in-
plane (Fig. 4) throughout the whole lattice structure. The unit
cell, for this short chain homologue, contains two molecules
(Z = 2) and is defined by the distance between two Zn(1) or Zn(2)
Fig. 4. Molecular projection of structure of zinc(II) hexanoate. Displacement ellipsoids drawn at 30% probability level.

R.A. Taylor, H.A. Ellis / Spectrochimica Acta Part A 68 (2007) 99–107
105
Fig. 6. Observed (a) and calculated (b) powder diffraction line patterns for ZnC5,
ZnC12 and ZnC20
.
try (unfortunately, the powder method is not accurate enough
to determine the precise space group). Lattice parameters are
collected in Table 4 and a plot of the experimentally derived
lamellae distances between two Zn(1) or Zn(2) planes, dexp ver-
sus nC shows a discontinuity at nC = 8 (Fig. 7), which is also
replicated in the plots of unit cell side a (the principal axis) and
Fig. 5. X-ray powder diffraction patterns for ZnC5, ZnC12 and ZnC20
.
Table 4
Powder X-ray data for zinc(II) n-alkanoates
Compound
a (A)
b (A)
β (◦)
dL (A)
dexp (A)
˚
˚
˚
c (A)
˚
˚
ZnC4
ZnC5
ZnC6
ZnC7
ZnC8
ZnC9
ZnC10
ZnC11
ZnC12
ZnC13
ZnC14
ZnC15
ZnC16
ZnC17
ZnC18
ZnC19
ZnC20
25.52
28.26
32.27 (32.31)
36.54
42.36
24.40
25.79
28.05
29.53
31.45
33.91
36.69
38.36
40.95
42.67
45.18
47.27
4.96
4.99
5.02 (4.71)
5.31
5.22
6.02
5.57
5.50
5.41
5.62
5.65
5.42
5.50
5.49
5.50
5.64
5.54
9.69
9.04
9.08 (9.33)
10.31
10.21
9.12
9.25
9.47
9.70
9.48
9.61
9.58
9.39
9.50
9.09
8.99
9.09
94.70
90.13
90.78 (93.72)
97.53
99.93
92.85
95.88
95.23
94.96
93.54
95.17
95.63
94.93
94.80
93.66
91.93
93.09
6.97
8.24
9.50
25.43
28.26
32.27
36.22
41.72
24.38
25.65
27.93
29.41
31.39
33.78
36.52
38.21
40.81
42.58
45.16
47.20
10.76
12.02
13.28
14.54
15.80
17.07
16.33
19.59
20.85
22.11
23.37
24.64
25.90
27.16
The values in parentheses denotes single crystal data.

106
R.A. Taylor, H.A. Ellis / Spectrochimica Acta Part A 68 (2007) 99–107
Table 5
Cell volume, Vcell, tilt angles, r (◦) and number of molecules in a unit cell, Z for
zinc(II) n-alkanoates
3
r (◦)
Z
˚
Compound
Vcell (A )
ZnC4
ZnC5
ZnC6
ZnC7
ZnC8
ZnC9
ZnC10
ZnC11
ZnC12
ZnC13
ZnC14
ZnC15
ZnC16
ZnC17
ZnC18
ZnC19
ZnC20
1222.79
1275.09
1472.42
1982.09
2225.19
1339.03
1322.10
1468.46
1541.98
1674.22
1833.71
1895.51
1977.51
2127.90
2128.63
2291.00
2375.94
55.80
58.86
58.13
57.39
60.19
66.62
61.89
62.04
59.48
58.9
59.51
61.14
59.78
60.82
59.77
60.67
60.33
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
perpendicular cleavage planes observed under polarized light.
The longer chain length structures, for which an interdigitated
bilayer arrangement is proposed, are packed differently, with a
head-to-tail arrangement of hydrocarbon chains within the lat-
tice. Such an arrangement should lead to side chain interactions
in the long chain compounds, which are indeed seem in the pow-
der traces. Hence, the structural models previously proposed [1]
must be modified, somewhat, in light of these results (Fig. 2).
˚
Fig. 7. Unit cell side, a (A), molecular length (dL) and experimental lamellar
spacing, dexp vs. nC.
cell volume, Vcell versus nC. However, sides b, c and angle, β
remain fairly constant with increasing nC. It is clear then that
the packing arrangements of hydrocarbon chains in the lattice
for nC ≤ 8 and nC ≥ 9 are different. To confirm this, dexp values
determined from 1 0 0 (h k l) reflections and calculated values,
dL determined from the fully extended all-trans configuration
from the expression:
References
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2+
dL = dC–H + (n − 1)dC–C sin 55◦ + dC–O + rZn
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,
˚
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˚
˚
˚
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