Download metal-water interactions and hydrogen bond strength

M. Georgiev,
D. Stoilova
Journal of the University of Chemical
Technology
and Metallurgy, 42, 2, 2007, 211-216
METAL-WATER INTERACTIONS AND HYDROGEN BOND STRENGTH
M. Georgiev1, D. Stoilova2
1
University of Chemical Technology and Metallurgy
8 Kl. Ohridski, 1756 Sofia, Bulgaria
E-mail: [email protected]
Received 05 February2007
Accepted 20 May 2007
2
Institute of General and Inorganic Chemistry
Bulgarian Academy of Sciences
“Akad. G. Bonchev” str., bl.11, 1113, Sofia, Bulgaria
E-mail:[email protected]
ABSTRACT
The strength of the hydrogen bonds formed in some acetates, Ba(CH3COO)2·H2O, Zn(CH3COO)2·2H2O and
BaZn(CH3COO)4·2H2O, as deduced from the infrared wavenumbers of the respective uncoupled OD stretching modes
(matrix-isolated HDO molecules) is discussed in terms of hydrogen bond lengths Ow···O, metal-water interactions (synergetic effect) and proton acceptor capabilities of the acetate oxygen atoms (competitive effect). The spectroscopic experiments reveal that water molecules bonded to Zn2+ ions form stronger hydrogen bonds due to both the shorter Zn-OH2
bond distances and the increasing covalence of the respective bonds as compared to those coordinated to Ba2+ ions. Four
OD oscillators are deduced from the infrared bands in the spectrum of BaZn(CH3COO)4·2H2O, thus indicating the
existence of at least two water molecules in the double salt which are bonded to both metal ions. The intramolecular O-H
distances are derived from the novel νOD vs. rOH correlation curve [J. Mol. Struct. 404 (1997) 63].
Keywords: Metal acetate hydrates (Me = Ba, Zn); IR matrix - spectroscopy, hydrogen bond strength, synergetic effect.
INTRODUCTION
The strength of the hydrogen bonds in crystal
hydrates is governed by both the hydrogen bond donor
strength of the hydrogen bond donors and the hydrogen
bond acceptor capability of the hydrogen bond acceptors. Bonding interactions of the donors and acceptors
with other entities in the structure additionally modify
this strength: (i) metal-water interactions (synergetic
effect) [1-4 and Refs. therein]; (ii) cooperative effect
(water molecules as hydrogen bond donors and acceptors) [1,2,5 and Refs. therein]; (iii) anti-cooperative or
competitive effect (different proton acceptor strength
of the atoms building up one acceptor group) [1-3 and
Refs. therein]; (iv) repulsive potential forces at the respective lattice sites (i.e. unit-cell volumes) [1-4], etc.
In the case of synergetic effect, i.e. the bonding
of water molecules to metal ions, the O-H bonds of the
water molecules are both weakened and polarized with
the increasing strength of the respective Me−OH2 bonds
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Journal of the University of Chemical Technology and Metallurgy, 42, 2, 2007
and, hence, the acidity of the respective hydrogen atoms is increased. The synergetic effect increases with
increasing charge and decreasing size of the metal ions
as well as with increasing covalence of the Me-OH2
bonds. Thus, the alkali and alkali-earth metal ions exhibit a weaker synergetic effect as compared to those of
the first transition metal series [1,2]. As a result of the
different metal-water interactions the hydrogen bonds
formed in crystal hydrates of the d-metals are stronger
than those formed in crystal hydrates of alkali and alkali-earth metals, i.e. the respective νOH(D) are shifted to
lower frequencies.
The present paper aims at studying the influence
of the metal-water interactions on the strength of the
hydrogen bonds present in the metal acetates,
Ba(CH 3COO) 2 ·H 2 O and Zn(CH 3 COO) 2 ·2H 2 O. An
attempt is made to analyze the hydrogen bond strength
in BaZn(CH3COO)4·2H2O (a compound with unknown
structure) and to deduce the coordination of the water
molecules to the metal ions. The method of infrared
matrix-spectroscopy (matrix-isolated HDO molecules)
is used to investigate the hydrogen bonding systems in
the above acetates.
observed (infrared spectra using Nujol mulls were also
measured).
RESULTS AND DISCUSSION
Crystal structures of Ba(CH 3COO) 2·H 2O and
Zn(CH3COO)2·2H2O
According to the structural data
Ba(CH3COO)2·H2O crystallizes in the triclinic centric
space group [8]. A part of the crystal structure is shown
in Fig. 1. Due to the low symmetry of the unit - cell all
species (two crystallographically different barium ions,
Ba1 and Ba2 – nine and eight coordinated, respectively;
four crystallographically different acetate ions, and two
crystallographically different water molecules, Ow1 and
Ow2) are in C1 site symmetry. The water molecules are
EXPERIMENTAL
Ba(CH3COO)2·H2O and Zn(CH3COO)2· 2H2O
were prepared by re-crystallization of commercial products in aqueous solutions at 30oC according to the solubility polytherm (barium acetate monohydrate) [6] and
at 25oC (zinc acetate dihydrate). A slight excess of acetic acid was added to the solutions in order to prevent
the hydrolysis processes. The crystals were filtered,
washed with alcohol and dried in air. The double salt,
BaZn(CH3COO)4·2H2O, was obtained according to the
solubility diagram of the Zn(CH3COO)2 - Ba(CH3COO)2
- H2O system at 30oC [7]. Isotopically dilute samples
containing matrix isolated HDO molecules (ca 10 %
D2O) were prepared using the same crystallization procedure in the presence of heavy water. The reagents used
were “p.a.” quality (Merck).
The infrared spectra were recorded on the Bruker
model IFS 25 and IFS 113 Fourier transform interferometers (resolution < 2 cm-1) at ambient and liquid
nitrogen temperatures using KBr discs as matrices. Ion
exchange or other reactions with KBr have not been
212
Fig. 1. Crystal structure of Ba(CH3COO) 2·H 2O (dash lines –
hydrogen bonds)
Fig. 2. Local environments of the acetate oxygen atoms in
Ba(CH 3COO) 2·H2O
M. Georgiev, D. Stoilova
Table 1. Assignments of the hydrogen bonds in metal acetates.
Hydrogen
νOH
νOH
bonds
RT
LNT RT
νOD
νOD
Ow···O Ow···O H···O
exp
O−H
Me−OH2 ΣO
cal
exp
cal
cal
Ow1···O21 3566 3566 2620 2620 3.109
3.062
2.125 0.952 2.791
1.65
Ow1···O41 3340 3320 2474 2452 2.750
2.769
1.816 0.971
1.60
Ow2···O21 3240 3240 2474 2440 2.738
2.756
1.802 0.973 2.744
1.65
Ow2···O41 3490 3485 2570 2570 2.823
2.943
1.999 0.957
1.60
LNT
(v.u.)
Ba(CH3COO)2·H2O
Zn(CH3COO)2·H2O
Ow1···O2
3137 3218 2352
2325
2.675
2.652
1.692
0.984
Ow1···O3
3073 2352
2325
2.711
2.652
1.692
0.984
1.987
1.69
1.71
BaZn(CH3COO)4·2H2O
3446 3405 2568
2546
2.899
1.953
0.958
3413 3390 2520
2509
2.841
1.892
0.962
3170 3014 2334
2282
2.620
1.659
0.989
2976 2334
2212
2.575
1.611
0.998
Fig. 3. Crystal structure of Zn(CH3COO)2·2H2O (dash lines – hydrogen bonds).
coordinated to Ba1 and the respective bond lengths have
values of 2.791 and 2.744 Å (Ba-Ow1 and Ba-Ow2, respectively). The different local environments of the acetate ions are shown in Fig. 2. It is seen that the oxygen
atoms O21 and O41 are monodentately bonded to Ba2+
ions and as a result they exhibit the lowest Brown’s bond
valence sums among all oxygen atoms – 1.6508 and 1.6034
v. u., respectively (BVS are calculated according to Brese
and O’Keeffe [9]). Due to the small values of the bond
valence sums the oxygen O21 and O41 act as acceptors of
two protons (Table 1). Each water molecule forms two
hydrogen bonds with both the oxygen atoms O21 (located
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Journal of the University of Chemical Technology and Metallurgy, 42, 2, 2007
between the layers) and the oxygen atoms O41 (located
within the layers) (hydrogen bonds are calculated from
the structural data as Ow···O bonds, see Table 1).
Zn(CH3COO)2·2H2O belongs to the monoclinic
space group C2/c [10]. Four acetate oxygen atoms are
bidentately (chelate) bonded to the Zn2+ ions. Two water
molecules complete the coordination of the Zn2+ ions
(Zn-Ow bond length is 1.987 Å) (Fig. 3). Due to the C1
site symmetry the water molecule (one crystallographical type) forms two hydrogen bonds (2.675 and 2.711 Å
bond lengths, see Table 1). The bond valence sums of the
oxygen atoms have close values – 1.69 and 1.71 v. u.
BaA c2 .H 2 O
2620
3240
2570 2474
3340
3566
3490
2620
2440
2570
2452
3240
3566
Z nA c2 .2H 2 O
3485
3320
2352
3137
2325
3218
BaZ nA c4 .2H 2 O
3073
2334
3170
2568
2520
3446
3413
2282
3014
3405
3600
2976
3390
3200
2800
2212
2509
2546
2400
Wavenumbers, cm
-1
Fig. 4. Infrared spectra of the metal acetates in the regions of
OH and OD vibrations (…, ambient temperature, - liquid nitrogen
temperature)
214
Infrared spectroscopy study
Infrared spectra of the metal acetates under study
in the region of the vibrations O-H and O-D (matrix
isolated HDO molecules) are shown in Fig. 4 (see also
Table 1). Three bands at 2620, 2570 and 2474 cm -1
corresponding to three OD oscillators are observed in
the spectrum of the barium acetate monohydrate at ambient temperature. The band at 2474 cm-1 shifts to lower
frequencies and transforms into two bands (2452 and
2440 cm-1) at liquid nitrogen temperature. The bands at
the higher wavenumbers (2620 and 2570 cm-1) are attributed to hydrogen bonds Ow1···O21 and Ow2···O41,
respectively, (bond distances 3.109 and 2.823 Å, Table 1).
The band at the lowest frequency (2440 cm-1) is assigned to hydrogen bonds of the type Ow2···O21 due to
both the shorter bond length (2.738 Å) and the shorter
Ba-Ow2 bond distance (2.744 Å, i.e. stronger synergetic
effect) as compared to hydrogen bonds formed by Ow1
(band at 2452 cm-1 and Ba-Ow1 bond length – 2.791 Å).
The spectroscopic findings evidence that the water molecules Ow1 are more asymmetrically hydrogen bonded
than Ow2 (∆ν have values of 168 and 130 cm-1, respectively).
The water molecule in Zn(CH3COO)2·2H2O is
expected to exhibit two bands corresponding to two different hydrogen bonds. However, as Fig. 4 shows, the
water molecule in Zn(CH3COO)2·2H2O forms hydrogen bonds of equal strength, irrespective of the different hydrogen bond lengths (band at 2352 cm-1 at ambient temperature and 2325 cm-1 at liquid nitrogen temperature) owing to both the very strong synergetic effect of the Zn2+ ions and the close proton acceptor capabilities of the oxygen atoms (see Table 1).
M. Georgiev, D. Stoilova
BaZn(CH3COO)4·2H2O exhibits three bands in
the region of the OD vibrations of the matrix-isolated
HDO molecules (2568, 2520 and 2334 cm-1, ambient
temperature) which shift to lower frequencies on cooling. Furthermore, the band at the lowest wavenumber
transforms into two bands at 2282 and 2212 cm-1 (liquid nitrogen temperature, see Fig. 4). The spectroscopic
experiments allow us to deduce the existence of at least
four different OD oscillators, i.e. the existence of at least
two crystallographically different water molecules which
form hydrogen bonds of quite different bond strengths.
Taking into account the hydrogen bond strengths (i.e.
the infrared band positions) in the simple salts,
Ba(CH3COO)2·H2O and Zn(CH3COO)2·2H2O, the bands
at lower frequencies in the spectrum of
BaZn(CH3COO)4·2H2O are assumed to be due to hydrogen bonds formed by water molecules coordinated
to Zn2+ ions and those at higher frequencies to hydrogen bonds formed by water molecules coordinated to
Ba2+ ions.
Almost all infrared bands display a positive temperature dependence (∆ν/∆T > 0) (see Table 1), thus
indicating the formation of nearly linear hydrogen bonds
(i.e. OH···O > 140oC) [1,3,5,11]. The intermolecular
H···O and Ow···O bond distances calculated according
to the traditional correlation curve of Mikenda [12] (i.e.
νOD vs. H···O and Ow···O bond lengths) are presented in
Table 1. Recently linear correlations between the intramolecular bond valences sOH(D) and s’OH(D) of water
molecules in condensed materials and the wavenumbers
of the respective uncoupled OD stretching modes of
matrix-isolated HDO molecules have been established
[13,14]. The correlation curves allow calculations of the
intramolecular O-H bond lengths using infrared and
Raman wavenumbers of the OD vibrations in isotopically dilute samples [14] (see Table 1).
The formation of hydrogen bonds of different
strengths in the metal acetates reflects on the shape of the
spectra in the high frequency region (3200-3600 cm-1)
where the O-H stretching modes occur. The high frequency
bands in the spectrum of Ba(CH3COO)2·H2O display small
half-widths owing to the formation of comparatively weak
hydrogen bonds. The bands at 3566 and 3320 cm-1 are
attributed to νas and νs of Ow1, and those at 3485 and 3240
cm-1 – to νas and νs of Ow2, respectively. The isotopic
ratios νOH/νOD have values of 1.36 for the bands at 2620,
2570 and 2452 cm-1, and 1.33 for the lowest wavenumbered
band at 2440 cm-1. The spectrum of Zn(CH3COO)2·2H2O
exhibits a broad band with a maximum at 3137 cm-1 ( ambient temperature) which shifts to lower frequencies at liquid nitrogen temperature (bands at 3218 and 3073 cm-1).
The large half-width of the bands is owing to the strong
interactions of the identical oscillators, i.e. to the formation of strong hydrogen bonds in the zinc compound.
Two doublets are distinguished in the spectrum of the
double salts – two bands at 3405 and 3390 cm-1 which
could be assigned to water molecules bonded to Ba2+
ions and 3014 and 2976 cm-1 assigned to water molecules bonded to Zn2+ ions, respectively. The isotopic
ratios νOH/νOD have values of 1.34 for the high frequency
bands (3405 and 3390 cm-1) and 1.31 and 1.35 for the
lower wavenumbered bands (3014 and 2976 cm-1).
CONCLUSIONS
The analysis of the infrared spectra of
Ba(CH 3 COO) 2 ·H 2 O, Zn(CH 3 COO) 2 ·2H 2 O and
BaZn(CH3COO)4·2H2O reveals that: (i) The Zn2+ ions
exhibit strong Me-OH2 interactions (i.e. strong synergetic effect) due to the covalent character of the respective bonds, thus facilitating the formation of strong hydrogen bonds; (ii) The water molecules coordinated to
Ba2+ ions are slightly polarized and as a result comparatively weak hydrogen bonds are formed; (iii) on
the basis of the spectroscopic experiments the water
molecules in BaZn(CH3COO)4·2H2O are assumed to be
bonded to both metal ions.
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