2
12
I. N eˇ mec et al. / Journal of Molecular Structure 476 (1999) 203–213
addition, the total number of bands is much lower than
the maximum number expected. This fact could be
explained by small inter-ion interactions in the unit
cell and also in terms of the fact that all measurements
were carried out on polycrystalline samples.
The low-intensity bands found particularly in the
low-temperature IR spectrum from 2400 to
1800 cm can be assigned to the overtones and
Ϫ1
combination bands of the fundamental vibrations.
The band of the CyO stretching vibration, which
confirms the existence of the b-alaninium cation in
Ϫ1
the crystal structure, is located at 1700 cm . When
3.3. Vibrational spectra of MbAN
the sample temperature is decreased, it is split into
two symmetric branches (probably due to interactions
in b-alaninium dimers). In the deuterated compound,
splitting into two bands with different intensities
The IR spectra of MbAN recorded at laboratory
and low temperatures (90 K) are depicted in Fig. 4
together with the Raman spectrum; the peak positions
are listed in Table 8. The IR spectra of deuterated
MBAN (recorded at 298 K and 90 K), together with
the Raman spectrum, are depicted in Fig. 5 and the
maxima are listed in Table 9. The overall character of
the vibrational spectra is in accordance with the
results of the MbAN crystal structure determination;
i.e., a compound consisting of b-alaninium and nitrate
ions, interconnected by a network of hydrogen bonds.
Assignment of the bands in the vibrational spectra
of MbAN and its deuterated analogue was based on
the results of a previous study on interpretation of the
vibrational spectra of b-alanine [7, 8], nitrate [9] and
the addition compounds of glycine [10].
Ϫ1
(1686 and 1644 cm ) was observed even at labora-
tory temperature. The presence of the weak satellite
Ϫ1
peaks in the 1780–1760 cm region can be explained
by interactions between the CyO stretching vibration
and overtones or combination vibrations.
ϩ
The deformation vibrations of the NH group were
3
Ϫ1
recorded in the range 1630 to 1500 cm . A great
multiplication of the number of bands appears espe-
cially for antisymmetric vibrations in the Raman and
low-temperature spectra. The corresponding bands of
ϩ
the dND3 vibrations were observed in the 1175–
Ϫ1
1145 cm region in the spectra of the deuterated
compound.
The stretching vibrations of the N–H and O–H
groups interconnected by the hydrogen bond system
in the crystal appear in the IR spectrum as broaϪd1,
medium-intensity bands in the 3250–2500 cm
region (2400–1800 cm region in the deuterated
compound). It is apparent from the crystal structure
Manifestations of the deformation vibrations of the
CH groups can be observed from ca. 1470 to ca.
2
Ϫ1
1430 cm . The number of bands of these vibrations
also increases in number on a decrease in temperature.
The presence of b-alaninium ions in the crystal
structure is also reflected in the highly mixed bands
Ϫ1
Ϫ1
(
H
Table 3) that these are hydrogen bonds of the N–
of the C–O stretching vibration (ca. 1415 cm ) and
…
˚
O type with a length of 2.9–3.2 A and of the
the C–O–H in-plane bending vibration (ca.
Ϫ1
…
˚
O–H O type with a length of ca. 2.6 A. On the
basis of correlation curves [11] between the wave-
number of the nO–H vibration and the length of the
1230 cm ), together with the C–O–H out-of-plane
Ϫ1
bending vibration (815 cm ).
The expected manifold multiplication of all
bands of the internal vibrations of the nitrate
groups (see Table 7) was not observed even in
low-temperature IR spectra. Splitting is apparent
only for the originally doubly degenerate vibrations
hydrogen bond, the band corresponding to the O–
Ϫ1
…
H
O bonds can be expected at 2600 cm . The
temperature-sensitive band corresponding to out-of-
…
plane O–H O bending vibrations, characteristic for
this type of intermediate hydrogen bond, is located at
ca. 950 cm .
The bands of the stretching vibrations of CH2,
n (doublet) and n (doublet to triplet). In order to
3
4
Ϫ1
Ϫ
confirm the interpretation of the NO3 internal
vibrations, the spectra of the isotope-substituted
ϩ
15
Ϫ
similar to those for b-alanine [8], were observed in
compound NH CH CH COOH NO3 has been
3 2 2
Ϫ1
the 3030–2940 cm region. These vibrations, which
studied. Shifts were observed in the bands of both
Ϫ1
are quite intense in the Raman spectra, are almost
masked in the IR spectra of natural MbAN by the
broad bands of the stretching N–H and O–H
vibrations.
branches of the n
vibration by about 30 cm towards
3
lower wavenumbers (IR and Raman) and of the band
Ϫ1
of the n
(IR). The positions and character of the bands of the n
vibration by 22 cm in the same direction
2
1
Nemec, Ivan
