The guanidine and benzoic acid (1:1) complex. the polarized vibrational studies and theoretical investigations

Article abstract of DOI:10.1016/j.saa.2014.10.064

The structure of guanidinium benzoate was discovered by Silva et al. On the basis of these X-ray crystallographic studies the detailed DFT investigation are performed. According to this result the infrared spectrum for one theoretical molecule was calculated. On the basis of potential energy distribution (PED) analysis the clear-cut assignments of observed bands were performed. For the calculated molecule with energy minimum the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were obtained. The energy difference between HOMO and LUMO was analyzed. According to theoretical calculations the direction of dipole moments (TDM) for bands observed in infrared spectra are analyzed. Verification of theoretical TDM behaviors is performed on the basis of experimental polarized specular reflection infrared spectra. The detailed assignments of observed bands is presented. Both theoretical and experimental spectra are compared. Crucial role of three different hydrogen bonds is studied in detail. Additionally, on the basis of differential scanning calorimetric study no phase transition was found in investigated crystal in the range 100-400 K.

Full text of DOI:10.1016/j.saa.2014.10.064

Accepted Manuscript
The Guanidine and Benzoic Acid (1:1) Complex. The polarized vibrational
studies and theoretical investigations
M. Drozd, D. Dudzic
PII:
S1386-1425(14)01561-3
http://dx.doi.org/10.1016/j.saa.2014.10.064
SAA 12879
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
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Revised Date:
Accepted Date:
17 June 2014
8 October 2014
15 October 2014
Please cite this article as: M. Drozd, D. Dudzic, The Guanidine and Benzoic Acid (1:1) Complex. The polarized
vibrational studies and theoretical investigations, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.10.064
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The Guanidine and Benzoic Acid (1:1) Complex. The polarized
vibrational studies and theoretical investigations.
M. Drozd, D. Dudzic*
Institute of Low Temperature and Structure Research of Polish Academy of Sciences
Okólna 2 str., 50-422 Wrocław, email: d.dudzic@int.pan.wroc.pl
*corresponding author
Abstract:
The structure of guanidinium benzoate was discovered by Silva at all. On the basis of these X-
ray crystallographic studies the detailed DFT investigation are performed. According to this result the
infrared spectrum for one theoretical molecule was calculated. On the basis of potential energy
distribution (PED) analysis the clear-cut assignments of observed bands were performed. For the
calculated molecule with energy minimum the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) were obtained. The energy difference between HOMO
and LUMO was analyzed. According to theoretical calculations the direction of dipole moments
(TDM) for bands observed in infrared spectra are analyzed.
Verification of theoretical TDM behaviors is performed on the basis of experimental polarized
specular reflection infrared spectra. The detailed assignments of observed bands is presented. Both
theoretical and experimental spectra are compared. Crucial role of three different hydrogen bonds is
studied in detail.
Additionally, on the basis of differential scanning calorimetric study no phase transition was
found in investigated crystal in the range 100 – 400 K.
Keywords: Guanidine, DFT, hydrogen bonds, specular reflection, monocrystal
1

Introduction
The general conviction is that the hydrogen bonds play crucial role in determination of many
physical and chemical properties in biological and inorganic materials. It is known that this
interaction with different strength determine behaviors of molecular crystals where the molecules of
substrates with regular chemical bonds are joint with different molecules forming the network
constructed by relatively weak hydrogen bonds. It is clear that such physical phenomena as
ferroelectricity or nonlinear optical properties can be better understood, when the nature of
hydrogen bond is clearly elucidated.
For determination of hydrogen bonds properties the vibrational spectroscopy seems to be
very attractive. The characteristic bands assigned to stretching vibrations of hydrogen bonds are
observed on powder IR and Raman spectra very well1, but only spectroscopy on monocrystals2, 3 with
polarized light gives full information about behavior of hydrogen bond network in investigated
compounds.
The polarized vibrational spectra were successfully used for detailed analysis of hydrogen
bonds network in relation to the Betaine*H2SeO3 crystal structure4, hydrochloride diglycine
hydrochloride5, monoglycine nitrate6, alfa-glycine7, pentacaesium trihydrogentetraselenate
monohydrate8, gamma-glycine9, monoglycine dihydrogenphosphate10 and ferroelectric glycinium
hydrogrenphospite (GPI)11.
Unfortunately, a vibrational studies with using polarized light for guanidinium compounds
are scarced. The detailed IR measurement was made for diguanidinium hydrogenarsenate
monohydrate12 and bisguanidinium hydrogenphospahe monohydrate13 monocrystals. Both
compounds belong to noncentrosymmetric crystallographic system and demonstrate NLO properties.
These detailed studies shown that hydrogen bond network determines physical and chemical
properties of these materials.
On the other hand the two small monocrystals without NLO properties were studied with
using polarized light and IR microscope. In this investigation the detailed IR spectra and its
correlation with crystallographic structure were performed for guanidinium hydrogenselenate14 and
its sulphur analogue15. In this case for better interpretation of polarised infrared spectra oriented gas
model16 was used.
In present studies a polarized specular reflection spectra of guanidinium benzoate
monocrystals are presented. The detailed vibrational analysis of bands observed in vibrational
spectra is performed. These results are discussed in relation to crystallographic structure.
Additionally the comparison between experimental and theoretical data is made. The directional
properties of transition dipole moments for observed bands are presented for both experimental and
2

theoretical data. Our results allow the better understanding the role of hydrogen bond network in
molecular crystals.
Experimental
The guanidinium benzoate (abbreviation as GUAB) crystals were grown from the aqueous
solution containing guanidinium cations and benzoate anions in a stoichiometric ratio 1:1. The slow
evaporation at constant room temperature method was used. The big colorless and transparent
monocrystals were obtained (size of approximately ca. 1 cm x 1 cm x 1 cm).
The powder IR spectra were measured as Nujol and hexachlorobutadiene suspensions using
a Bruker IFS-88 spectrometer with a resolution 2 cm-1. The samples were put between KBR windows.
The powder Raman spectra were taken with the FT-Raman FRA-106 accessory attached to
the Bruker IFS-88 spectrometer and using Nd:YAG laser pumped by the diode laser; power ca. 180
mW, resolution 2 cm-1.
For bigger GUAB single crystal the specular reflection spectra were measured using Perkin-
Elmer reflection unit. The reflectance spectra were measured parallel to three crystallographic axis
(a, b and c). The single crystals were oriented applying a polarizing microscope. The radiation was
polarised (wire grid polariser on AgBr or polyethylene substrate) perpendicular to the incidence
plane.
The reflection spectra were calculated with the Kramers-Kronig transformation by OPUS
Bruker software. In the discussion term “refraction spectra” is reserved for the spectra of the
imaginary part of the refractive indices, .
Differential scanning calorimetric studies were performed on Perkin-Elmer DSC-7
calorimeter equipped with CCA-7 low temperature attachment with a heating/cooling rate 20°/min.
The samples of the mass ca. 20 mg were sealed in the aluminum pans. The measurements were
performed for the temperature region 100 – 400 K.
The calculation of theoretical geometry, IR and Raman spectra were performed with the
Gaussian program (09 revision D.01)17 configured on the Wroclaw High Performance Computer
Cluster. For calculation the structural data from X-ray investigations of crystal were used. The
coordinates for particular atoms were established from CIF crystallographic data. The optimized
structure has been calculated by the DFT/B3LYP method. The 6-311++G (d,p) basis set have been
employed.
For optimized structure the harmonic frequencies and infrared intensities were calculated by
the density functional triply-parameter hybrid model (DFT/B3LYP). The 6-311++G (d,p) basis set was
used. The obtained frequencies of stretching vibrations were scaled by 0.96. The calculated pseudo
potential energy distribution (PPED) for the investigated molecule enabled us to make detailed band
3

assignment in infrared spectra. For established of direction for transition dipole moment for bands in
theoretical spectra the IOP (7/33=1) command.
The NBO charges, HOMO and LUMO orbitals energies were analyzed directly from Gaussian
program log files. The graphic interpretation of mentioned properties was made by Gaussview
program. In the cases of HOMO, LUMO and electrostatic potentials graphic illustrations of the
isosurface with value equal to 0.01 was used.
X-ray structure versus theoretical calculation
The X-ray crystallographic structure for guanidinium benzoate was discovered by Silva18 at all.
According to these data crystal belongs to Orthorhombic Pnma crystallographic group with
macroscopic point of inversion. The a parameter is ca. two times bigger (15.7347(8) Å) than b and c
(8.1216(4) and 7.8885(4) Å, respectively). In the title compound the cations and ions lie on
crystallographic mirror plane. It should be mentioned that in skeletal flat guanidinium cation two C-N
bonds are identical and equal to 1.3162 Å, whereas the third one is a little bit longer (1.328 Å).
Additionally, in crystallographic studies the delocalization of the over both oxygen atoms of the COO-
group was noticed. Both C-O bonds in carboxylic group are identical and equal to 1.2519 Å. All C-H
bonds in benzoate anion are identical and equal to 0.9300 Å, whereas for guanidinium cation lengths
of N-H bods are different and included in the range 0.859 – 0.931 Å.
According to crystallographic data three different hydrogen bonds of type N-H…O were found. The
lengths of HB are typical for this weak bonds and equal ca.: 2.83, 2.88 and 2.97 Å. Hydrogen bonds
forms the ions into layers in the bc plane. The typical chains created by hydrogen bond in
investigated crystal are shown in Figure 1.
For investigated compound the theoretical calculation of equilibrium structure was
performed. The geometry of calculated molecule is presented in Figure 2. The calculated geometrical
parameters with selected experimental results are shown in Table 1.
For guanidinium cation the general relationship from X-ray experiment (two C-N bonds
identical and the third one longer) is confirmed by theoretical calculation. Of course in this
approximation the C-N bonds lengths are longer and equal ca. 1.32 Å for C15-N16 and C15-N18
bonds and 1.35 Å for third C15-N17 chemical bond. Contrary to crystallographic data the all
calculated N-H bonds are longer that 1 Å and included in the range 1.006 – 1.026 Å, but similar as in
experimental data the all N-H bonds are different. Additionally, both theoretical and x-ray results
shown that skeletal N-C-N of guanidinum cation is flat. Its confirm previous study performed for this
molecule19.
4

Theoretical results in the case of benzoate anion are different than experimental one. Firstly,
the C2-O1 and C2-O3 bonds lengths are not identical and equal to ca. 1.24 and 1.28 Å, respectively.
Nonequivalent character of these bonds was confirmed by NBO analysis (see next chapter).
In ring of benzoate anion only three types of C-C bonds are noticed in X-ray experiment. Two
C-C bonds are equal to ca. 1.37 Å, other two: 1.383 Å and last two 1.385 Å. Generally, this
relationship is confirmed by theoretical approach. All calculated values of C-C bonds are a little bit
longer than experimental and included in the very narrow range (1.399 – 1.405 Å).
Good agreement between experimental and theoretical data is found for longer C2-C4 bond.
According to X-ray data this bond is equal to 1.502 Å, whereas that it’s theoretical counterpart is
equal to ca. 1.53 Å. This relationship (the experimental bonds lengths are shorter than theoretical) is
observed in the case of all bonds in investigated crystal.
According to X-ray data all C-H bonds in benzoate anion are identical and equal to 0.93 Å. It
should be mentioned that this values are a little bit bigger than for N-H bonds in guanidinium cation.
This observation is confirmed by theoretical data where C-H bonds lengths are longer (ca. 1.08 Å)
than N-H bonds.
It should be mentioned that very good agreement between experimental and theoretical
data (in according to use in theoretical approach model of isolated molecule) is observed for O3-H20
and O3-H24. Both H20 and H24 protons are involved in hydrogen bonds network. In experimental
data the O3-H20 length is equal to 2.080 Å, whereas this theoretical value is 1.915 Å. Similar situation
is noticed for second O3-H24 interaction. In this case the experimental and theoretical values are
equal to 2.174 and 2.039 Å, respectively. It relationship (experiment lengths are bigger than
theoretical one) is in opposite to other bond values investigated for this molecule.
NBO charges
The results of Natural Bond Order analysis for investigated molecule are shown in Figure 3.
For guanidine molecule a calculated charge for carbon atom is positive and equal to 0.828 e, whereas
the negative charges noticed for all nitrogen atoms in this molecule are identical, practically and
equal to -0.820 e. Of course for all protons in guanidine molecule the charges are positive and
included in the range 0.399 – 0.488 e. These results may be considered as typical for this molecule,
but two interesting facts can be closely described despite of this. Firstly, the value of positive charge
of carbon noticed for guanidine molecule is different in many investigated guanidinium based
compounds. The calculated values of this charge are ca. 0.2 e for diguanidinium arsenate
monohydrate20 and guanidinium maleate21 (but the different calculation method was used) and
equal ca. 0.62 e for guanidinium complexes with acetic and its derivatives acid22. The using of
Mulliken and Lewdin methods for calculation of charges in isolated guanidine molecule19 have given
5

similar results ca. 0.59 and 0.21 e, respectively. It should be emphasized that the calculation
performed for guanidine with acrylic acid complex have given completely different result. In this case
this charge is equal to 0, practically, but in this complex the anomaly character of guanidine molecule
was postulated23.
Of course in all calculations performed for guanidinium based compounds the nitrogen
atoms have negative charges. On the other hand the calculated charge for nitrogen atom involved in
hydrogen bond, in previously investigated compounds of guanidine, had different value than two
other. In guanidinium benzoate all charges obtained for nitrogen atoms in guanidinium cation are
identical. Similar insensitive to hydrogen bonds creation charge is noticed for all protons presents in
guanidinium cation. According to performed calculations it should be expected that hydrogen bonds
between benzoic acid and guanidine will be very weak.
The NBO analysis for benzoate anion has shown that in this case the charges for carbon
atoms in the ring are negative and include in the range -0.151 - -0.216 e. It should be noticed that
smaller value of charge (-0.151 e) was calculated for carbon atom C4 which is connected with two
ring carbon atoms and C2 atom from hydroxycarboxylic group. On the contrary for atom C2 the
positive value of charge is noticed. In this case the charge is equal to 0.902 e. In hydroxycarboxylic
group the charge calculated for O3 atom (involved in hydrogen bond) is equal to -0.969 e, whereas
for other oxygen atom O1 from this group this value is -0.767 e. These results confirmed that the O3-
H20-N16 hydrogen bond is created in investigated molecule.
All charges calculated for protons (except H7) are positive and included in the range 0.185 –
0.192 e. A little bit bigger value is noticed for H7 proton. In this case the calculated NBO charge is
equal to 0.228 e. It should be noticed that these charges values are smaller than noticed for protons
in guanidinium cation.
HOMO and LUMO orbitals
The shapes of HOMO and LUMO isopotential surfaces are shown in Figure 4. The calculated
HOMO energy is equal to -0.30829 [a. u.], whereas the value of LUMO energy is positive (0.07544 [a.
u.]). The GAP energy is equal to -0.38373. Surprisingly, in the case of investigated compound this
value is negative. In other guanidinium based compounds with organic acids the GAP has positive
value24, in contrary to compounds with inorganic acids where the positive GAP was not noticed. On
the basis of calculated GAP values for many guanidinium compounds seems to be clear that the
relationship between GAP value and used acid is noticed. For strongest acids (as inorganic and
benzoic acid) the GAP has negative value, whereas for weak organic acids this value is positive.
According to theoretical calculation the HOMO orbital is localized on benzoate ion, only. It
shape corresponds to p valence of carbon atoms of benzoate ion ring, but a symmetry of this orbital
6

is disturbed. For two C4 and C12 atoms, character of HOMO orbital is strongly localized. The typical
dumbbells shape are observed in this case. Different situation is observed for C5 and C8 atoms. In
this case the HOMO orbital has delocalized feature. The center of this shape is asymmetrically moved
from atoms C6 and C10 mentioned above carbon atoms nr 5 and 8. It is characteristic that calculated
HOMO orbital is not localized on the hydroxycarboxylic group. It suggest that a hydrogen bonds of O-
H...N type observed in this complex are weak. The fully localization of HOMO orbital on the benzoate
ring shown that the chemical behavior of this structure can be interpreted as typical “Lewis base”,
whereas opposite properties (“Lewis acid”) should be assigned to COOH group.
Other behavior of molecular orbitals is noticed in the case of calculated LUMO orbital. This
orbital is localized on guanidinium cation, entirely. For carbon atom its character is strongly
delocalized and pronounced distortion from sphere is observed. It confirms a fact that on carbon
atom the hybridization of sp2 is realized and skeleton of guanidinium cation is almost flat. Other
character has LUMO orbital for all nitrogen atoms. In this case the shape of this orbital corresponds
to p valence electrons of nitrogen atoms and has strongly localized character. The shapes of orbitals
localized on each nitrogen atoms are similar to dumbbells. As in case of HOMO orbital the moving of
electron cloud to hydrogen bonds is not observed. It should be mentioned that in other guanidinium
based compounds the moving of electron cloud from ions to hydrogen bonds were observed24.
According to theoretical approach it seems to be clear that the strongly basic character of
guanidinium ion is preserved, whereas the acid center of this compound is localized on the benzoate
ion.
Electrostatic potential
The shape of electrostatic potential calculated for investigated molecule is shown in Figure 5.
On the basis of this study it is clear that huge differences in electrostatic potential surface are noticed
for benzoate ion. The positive charge isosurface observed for benzoate part of investigated molecule
is practically parallel, for ring, whereas the negative charges are almost symmetrically and are divided
into two oxygen atoms from hydroxycarboxylic group. It confirms previous NBO detailed study that
the differences between O1 and O3 atoms were too small for typical COOH group, which was
postulated by crystallographic investigations. On the other hand the positive isosurface is observed in
minimal part on the guanidinium cation, especially created by two nitrogen atoms a hydrogen bonds.
Nevertheless the guanidinium cation preserve the neutral character at all.
Vibrational spectra
The FTIR and FT-Raman spectra of the title crystal measured for powder sample at ambient
temperature are shown in Figure 6. The imaginary part of refractive index calculated on the basis of
7

reflectance spectra measured for investigated monocrystalline sample is shown in Figure 7. The
bands observed in the measured spectra arise from hydrogen bonds (O-H...N type), guanidinium
cation (C-N, N-H) and benzoate anion (C-C, C-H, COOH group) vibrations. All observed bands and its
assignment are collected in Table 2. The detailed assignments of bands observed for guanidinium
based compounds was presented in our previous paper19, 21-24
.
In Table 3 the correlation diagram for guanidinium cation and benzoate ion is shown. Free
guanidinium ion can be described by D3h symmetry. In this ideal assumption the 24 vibrations with
symmetry A’1(3), A’2(2), E’ (6), A’’1(1) A’’2(2) and E’’ (2) should be observed. For free benzoate anion
the 36 A type of vibrations should be considered. According to performed analysis in real crystal (D2h
factor group with Z=4, site symmetry C2v) the 44 vibrations for guanidinium cation with symmetry Au
(12) and B3u (32) should be active in IR spectra, whereas the 52 vibrations with Ag (36) and B3g (16)
are active in Raman spectra.
For benzoate ion in real crystal the 72 vibrations with symmetry B1u (36) and B3u (36) should
be active in infrared spectrum and 72 (with symmetry Ag (36) and B2g (36)) should be observed in
Raman spectroscopy.
Additionally, the 48 translations and rotations (included 3 acoustic vibrations) are calculated
for this crystal.
Specular reflection spectra
The analysis of bands observed in specular reflection spectra was made on the basis of
calculation of cosines direction for proper bands. The results of these calculations for selected group
of atoms are collected in Table 4.
4000 – 2000 cm-1
In this spectral region the bands derive from stretching vibrations of NH and CH groups
should be considered. According to literature data all bands visible on IR spectra above 2800 cm-1 can
be assignment to stretching NH vibrations of guanidinium cation and stretching CH vibrations of
benzoate anion.21,26, 28 The band at 3427 cm-1 observed in spectrum measured in b direction should
be assigned to asymmetric stretching vibrations of H24N18H23, whereas the band at 3347 cm-1 and
3337 cm-1 noticed in spectra measured parallel to a and c directions derives from symmetric
stretching vibrations of all NH group in guanidinium cation. A clear conclusion followed from Table 4
for stretching vibrations of H24N18H23. The band assigned to symmetric stretching vibrations will
not be active in polarized spectrum in b direction (the component of cosines in Y direction is equal to
0), whereas the band derive from asymmetric stretching should be visible in polarized spectra
measured in b direction. According to Table 4 in spectra measured at all directions the band derive
8

from symmetric stretching of H22N17H21 and H20N16H19 should be observed. On the basis of this
assumption the band at 3294 cm-1 in a spectra, 3303 in b spectra and 3290 in c spectra should be
assigned to this type of vibrations.
On the basis of simple cosines directions calculation is not possible to perform clear-cut
analysis for bands observed in polarized infrared spectra for stretching vibrations of CH groups of
benzoate ion. According to performed calculation it seems to be clear that band noticed at 3087 and
3027 cm-1 in spectrum measured in b direction should be assigned to asymmetric stretching
vibrations of CH groups, whereas the band noticed at 3068 cm-1 in spectra measured parallel to a and
c direction derive from symmetric stretching vibrations of these atoms (if symmetry axis of
considered ion is created by C2C4 and C12H14 bonds).
2000 – 1350 cm-1
In the region 1800 – 1350 cm-1 the bands derive from stretching vibrations of carboxylic
group, aromatic ring and asymmetric stretching vibrations of CN bonds should be observed.26, 28
Additionally, the bands which could be assigned to bending vibrations (scissoring) of NH2 groups can
be noticed.23,24 As the most characteristic in this spectral region the band derive due to asymmetric
stretching vibrations of deprotonated carboxylic group should be considered. As follow in Table 4
(C2O1 and C2O3) the band derive from asymmetric stretching vibrations of carboxylic group should
be active in spectrum measured in b direction, only. The band due to symmetric stretching vibrations
can be observed in both other spectra. On the basis of this assumption and previous literature data
the band visible at 1683 cm-1 in b spectrum is assigned to asymmetric stretching vibrations of
O1C2O3 group, whereas the bands noticed at 1402 and 1393 cm-1 in spectra c and a, respectively
should derive from symmetric stretching vibrations of this group.27,28
In this region the characteristic band due to scissoring vibrations of NH2 groups is observed.
According to literature data this band is noticed on the spectra at ca. 1650 cm-1.23,24,28 A calculated
direction cosines values clearly shown, that this type of vibrations should be manifested in polarized
spectra measured in a and c directions and is not active in b spectrum. On the basis of this approach
the bands noticed at 1653 and 1654 cm-1 was assigned to above mentioned type of vibrations.
As follow from calculations of directional cosines most bands observed in b direction
spectrum are due to asymmetric vibration, whereas in spectra parallel to a and c directions the
bands derived from symmetric vibrations are noticed. This observation is confirmed in the case of
bands assigned to vibrations of CN bonds. The band noticed at 1592 cm-1, which is active only in b
spectrum, was assigned to asymmetric stretching vibrations of CN, whereas its counterpart derive
from symmetric CN stretching vibrations is noticed in spectra parallel to a and c directions only (band
visible at ca. 1000 cm-1).23,24,28
9

Other bands observed in this region of spectra should be qualified as deriving from stretching
vibrations of aromatic ring. The band at 1600 cm-1 visible in spectrum measured parallel to a
direction was assigned to symmetric stretching vibrations of C-C bonds. The counterpart of this band
observed at 1545 cm-1 in spectra of b direction derives from asymmetric stretching of this type of
vibrations. Similar assignment can be made for bands noticed at 1441 in b spectra and 1412 cm-1 in a
and c spectra.
1350 – 800 cm-1
This region is dominated by very characteristic bands observed in all vibrational spectra of
guanidinium based compounds19. The band due to symmetric stretching of C-N bonds was observed
in previous infrared spectra of guanidinium compounds in the range 1020 - 990 cm-1. In infrared
spectrum this band has very small intensity, but in Raman spectra it is very often the most intensive
peak. According to performed calculation this band should be not active in spectrum measured
parallel to b direction (see Table 4). Because the differences in bond lengths between CN bonds are
observed (C15N16, C15N17 and C15N18) it is clear that in spectra parallel to a and c directions the
two bands can be observed. Therefore two bands at 1012 and 1003 cm-1 were assigned to this type
of vibrations.
The bands which are visible above 1250 cm-1 in vibrational spectrum can originate from
bending in-plane vibrations of aromatic ring. Our calculations shown that these bands should be
active in spectrum measured parallel to b direction (in opposite to out-of-plane bending vibrations).
On the basis of our approach the bands noticed at 1302 and 1311 cm-1 were assigned to this type of
vibrations.
In this spectral region the bands originated from rocking vibrations of NH2 groups should be
observed. In opposite to other bands derived from bending vibrations of NH2 groups this band can be
active in polarized spectrum measured in b direction and non-active in both other spectra. This
assumption was made on the basis of transition dipole moment calculation for rocking vibrations for
H24N18H23 group. According to this calculation the band noticed at 1071 cm-1 was assigned to
above mentioned type of vibrations.
A deformation out-of-plane vibration of NH2 groups (according to our calculation bands
derived from this type of vibrations are not active in spectrum parallel to b direction) wagging
vibrations manifest as band in spectrum close to 1050 cm-1 28. On the basis of these facts the band
noticed at 1024 cm-1 can be assigned to wagging type of vibrations of NH2 group.
There is only one specific stretching vibration of C-C type in benzoate ring due to C2 and C4
atoms. The band assigned to this type of vibrations can be visible in wide range of spectrum (1200 –
800 cm-1). For the sake of fact that the C2-C4 bond is perpendicular to b direction the band of
10

stretching vibration of C-C type should be active in spectra parallel to a and c directions. In discussed
range of spectra two bands at 1180 and 839 (in a direction, 841 in c) cm-1 can be assigned to this type
of vibrations. On the basis of literature data second band should be assigned to out-of-plane
deformation vibration of aromatic ring, which are observed in many spectra in the range 850 – 800
cm-1.26,27,28 According to our analysis of transition dipole moment for this type of vibrations this band
will be active in spectra in a and c directions. Because only one band at 839 cm-1 in a spectra and 841
in c spectra fulfils these conditions, therefore the band at 1180 cm-1 should be assigned to stretching
vibrations of C2-C4 bond.
On the basis of our directional cosines analysis and literate data the unambiguous
assignment for only one band (at 935 cm-1 observed in spectrum parallel to c direction) in this
spectral range is not possible.
800 – 400 cm-1
According to our transition dipole moments calculation the clear-cut assignment of bands
observed in this range bands is almost impossible. On the basis of literature data the bands noticed
in the range 740 – 660 cm-1 should be assigned to deformation vibrations of CH groups in aromatic
ring, but the detailed analysis of these bands seems to very difficult28. This also makes difficult the
presence of different bands which were assigned in previously paper19 as due to wagging vibrations
of C-N groups. According to literature data23, 24 the bands noticed in the range 560 – 520 cm-1 (556,
548 and 533 cm-1) should be assigned to twisting vibrations of C-N groups. Last band observed at 407
cm-1 is due to twisting vibrations of NH2 groups, because is active in spectra measured parallel to a
and c directions.
PED, transition dipole moments calculations
The assignment of bands observed in spectra was made on the basis of our previous studies
of guanidine based compounds and pseudo PED analysis, which is presented in Supplementary
Material. The theoretical vibrational spectra are presented in Figure 8. It should be noticed that
pseudo PED calculation were performed for isolated molecule (without weak intramolecular
interaction). This fact can disturb obtained results (especially in the case of weak hydrogen bonds).
According to our calculations the bands noticed in the range 3584 – 3177 cm-1 in theoretical
spectrum are assigned to the asymmetric or symmetric stretching vibrations of NH groups.
Calculated frequencies are a little bit higher than observed in experimental spectrum. On the basis of
dipole moments calculations for these groups the bands assigned to asymmetric stretching vibrations
should be visible in spectra parallel to a and c direction whereas in b direction spectrum the bands
11

derived from symmetric stretching vibrations of NH can be noticed. It is clear that this conclusion is in
opposite to behavior of TDM calculated directly for our experimental specular reflection spectra.
The PED calculations shown that in the range 3108 – 3051 cm-1 the bands derived from
stretching vibrations of CH group are expected. This results is in good agreement with experimental
results. The theoretical bands noticed at 3108 and 3084 cm-1 should be visible in spectrum parallel to
a direction, mainly. The other three bands derived from this type of vibrations can be observed in
spectrum measured in b direction. In case of these bands the both calculation methods are in good
agreement.
Pretty good agreement between two calculation methods used is noticed in the case of
bands due to asymmetric stretching vibrations of carboxylic group. Firstly the Gaussian calculations
predict perfectly the frequency of this band. Additionally for this band the theoretical calculation
gave unambiguous information about direction of TDM. On the basis of this calculations the band
derived from asymmetric stretching vibrations of CO group should be visible in polarized spectrum
measured in b direction, only. Our study for symmetric stretching vibrations shows similar
relationship. The frequency calculated in Gaussian program is in very good agreement with
experimental data. The nice agreement is noticed for TDM calculation. In this case the band should
be observed in spectra parallel to a and c direction.
According to PED calculation the bands due to scissoring vibrations of NH2 groups can be
visible in spectrum in the range 1840 – 1670 cm-1. These frequencies are much higher than
experimental one. On the basis of TDM calculations the bands derived from symmetric stretching
type of vibrations should be visible in spectrum measured in b direction. The Gaussian results
obtained for symmetric in-plane bending vibrations suggest that these bands should be visible in the
spectrum measured in b direction. This is not consistent with experimental data.
The characteristic band assigned to asymmetric stretching vibrations of CN group is noticed
in theoretical calculation a little bit higher than its experimental counterpart, but very good
agreement for TDM calculation in his case must be emphasized. In opposite to this result in the
theoretical approach for symmetric stretching vibration the prediction of CN group is very good, but
according to TDM calculation of Gaussian program this band can be observed in spectra parallel to b
and c direction. According to our experimental data this band is noticed in spectra measured in a and
c (not in b) direction.
On the basis of pseudo PED analysis the bands noticed at 1674 and 1635 cm-1 are assigned to
stretching vibrations of C-C bonds in aromatic ring. These results are in good agreement with
experimental data. Both bands mentioned above should be present in spectrum b, only. This
assumption is not in good agreement with our experimental data where band due to asymmetric
stretching vibrations of C-C bonds is visible in spectrum parallel to a direction. Additionally, the
12

counterpart of this band derived from asymmetric stretching vibration was noticed in spectrum
measured in b direction.
The PED analysis for other bands due to deformation vibration of NH2 and CH groups shown
good agreement between theoretical and experimental results. A worse agreement is noticed for
TDM calculation, but in the case of this type of vibration the prediction of TDM direction on the basis
of experimental data and simply calculation model is very difficult. Therefore the clear-cut opinion
about reliability of results obtained from Gaussian calculation seems to be impossible.
Conclusions
On the basis of performed theoretical and experimental studies following conclusions are
formulated:
1. Very good agreement between X-ray crystallographic studies and theoretical calculations of
equilibrium geometry is noticed.
2. No phase transitions were found in investigated compounds in the range 100 – 400 K.
3. Non typical value of GAP for guanidinium with organic acids is noticed in the case of
guanidinium benzoate compound.
4. The clear-cut assignment of observed bands was made on the basis of specular reflection
spectra measured on monocrystalline sample with polarized light.
5. Good agreement between experimental and theoretical vibrational spectra is noticed.
6. Calculations of directions properties for transition dipole moments in Gaussian program
cannot be used in good interpretation of polarized vibrational spectra measured on
monocrystalline sample.
Acknowledgment
The calculation was performed, partially on the computers of Wrocław Supercomputer and
Networking Center.
This work was supported by the Ministry of Science and Higher Education (grant nr: N N507
221840).
13

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15

Figures captions
Figure 1: Hydrogen bonds chains in investigated crystal (Picture based on data from
reference paper).
Figure 2: Atoms numbering of calculated molecule.
Figure 3: Natural Bond Order.
Figure 4: HOMO and LUMO orbitals.
Figure 5: Electrostatic potential.
Figure 6: Experimental powder IR and FT-Raman spectra.
Figure 7: The imaginary part of refractive index on the basis of reflectance spectra
measured for investigated monocrystalline sample.
Figure 8: Theoretical vibrational spectra (IR – above and Raman – below).
Tables captions
Table 1: Calculated geometrical parameters. The bond lengths are compared with
experimental data.
Table 2: Experimental frequencies for IR and Raman powder and specular reflection spectra.
Table nr 3: Correlation diagram for guanidinium cation and benzoate anion for k=0.
Table nr 4:Calculated cosines direction for selected vibrations.

Figure 1: Hydrogen bonds chains in investigated crystal (Picture based on data from
reference [15]).
2.83 Å
2.83 Å
2.88 Å
2.97 Å
2.88 Å
Oxygen
Carbon
Hydrogen
nitrogen

Figure 2: Atoms numbering of calculated molecule.

Figure 3: Natural Bond Order.

Figure 4: HOMO and LUMO orbitals.
EHOMO=-0.30829
ELUMO=0.07544

Figure 5: Electrostatic potential.

Figure 6: Experimental powder IR and FT-Raman spectra.

Figure 7: The imaginary part of refractive index on the basis of reflectance spectra
measured for investigated monocrystalline sample.
 - kappa

Figure 8: Theoretical vibrational spectra (IR – above and Raman – below).
2000
1800
1600
1400
1200
1000
800
600
400
200
0
4000
3500
3000
2500
2000
1500
1000
500
0
Wavenumbers [cm-1]
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
4000
3500
3000
2500
2000
1500
1000
500
0
Wavenumbers [cm-1]

Table 1: Calculated geometrical parameters. The bond lengths are compared with
experimental data.
O1-C2
C2-O3
C2-C4
O3-H20
O3-H24
C4-C5
C4-C6
C5-H7
C5-C8
C6-H9
C6-C10
C8-H11
C8-C12
C10-C12
C10-H13
C12-H14
C15-N16
C15-N17
C15-N18
N16-H19
N16-H20
N17-H21
N17-H22
N18-H23
N18-H24
1.2418 1.2519
1.283 1.2519
O1-C2-O3
O1-C2-C4
O3-C2-C4
C2-O3-H20
C2-03-H24
H20-O3-H24
C2-C4-C5
C2-C4-C6
C5-C4-C6
C4-C5-H7
C4-C5-C8
H7-C5-C8
C4-C6-H9
126.7528
117.1078
116.1306
115.668
148.0546
66.5491
119.749
121.7793
118.4709
O1-C2-O3-H20
O1-C2-O3-H24
C4-C2-O3-H20
C4-C2-O3-H24
O1-C2-C4-C5
O1-C2-C4-C6
O3-C2-C4-C5
O3-C2-C4-C6
C2-O3-H20-N16
-104.975
168.6809
73.9104
-12.4338
-1.3697
178.9582
179.6336
-0.0384
-150.622
-5.7619
110.1261
8.7841
1.5326 1.502
1.9153 2.080
2.0388 2.174
1.4027 1.3846
1.4048 1.3846
1.0849 0.93
1.4003 1.383
1.0846 0.93
1.4013 1.383
1.0861 0.93
1.4011 1.371
1.3998 1.371
1.0867 0.93
1.0865 0.93
1.3238 1.3162
1.3508 1.328
1.3294 1.3162
1.0071 0.913
1.0243 0.913
1.0064 0.928
1.0123 0.859
1.0084 0.928
1.0203 0.859
118.1595 H24-O3-H20-N16
120.7373
121.1031
118.8393
121.0193
120.1414
119.827
120.3724
119.8004
120.0216 C5-C4-C6-H9
120.1094 C5-C4-C6-C10
119.8683 C4-C5-C8-H11
119.3764
120.3467
120.2767
120.981
118.7591
120.2593
121.6812
116.765
C2-O3-H24-N18
H20-O3-H24-N18
C2-C4-C5-H7
C2-C4-C5-C8
C6-C4-C5-H7
C6-C4-C5-C8
-0.009
C4-C6-C10
179.8698
179.6738
-0.4474
0.136
-179.828
-179.54
0.4957
-179.849
0.2825
0.0261
-179.842
-0.3768
179.9307
179.6594
-0.0331
-0.1524
179.6828
179.9792
-0.1856
0.1982
-179.637
179.8915
0.0562
-3.0206
179.5847
176.7117
-0.6829
2.7013
H9-C6-C10
C5-C8-H11
C5-C8-C12
C2-C4-C6-H9
C2-C4-C6-C10
H11-C8-C12
C6-C10-C12
C6-C10-H13
C12-C10-H13
C8-C12-C10
C8-C12-H14
C10-C12-H14
N16-C15-N17
N16-C15-N18
N17-C15-N18
C15-N16-H19
C15-N16-H20
H19-N16-H20
C15-N17-H21
C15-N17-H22
H21-N17-H22
C15-N18-H23
C15-N18-H24
H23-N18-H24
O3-H20-N16
O3-H24-N18
C4-C5-C8-C12
H7-C5-C8-H11
H7-C5-C8-C12
C4-C6-C10-C12
C4-C6-C10-H13
H9-C6-C10-C12
H9-C6-C10-H13
C5-C8-C12-C10
C5-C8-C12-H14
121.501
120.5677
120.3636
118.9928
121.1161
118.1693
120.3356
153.1772
146.0831
H11-C8-C12-C10
H11-H8-H12-H14
C6-C10-C12-C8
C6-C10-C12-H14
H13-C10-C12-C8
H13-C10-C12-H14
N17-C15-N16-H19
N17-C15-N16-H20
N18-C15-N16-H19
N18-C15-N16-H20
N16-C15-N17-H21
N16-C15-N17-H22
N18-C15-N17-H21
N18-C15-N17-H22
N16-C15-N18-H23
N16-C15-N18-H24
N17-C15-N18-H23
N17-C15-N18-H24
C15-N16-H20-03
H19-N16-H20-O3
C15-N18-H24-O3
H23-N18-H24-O3
179.5017
-177.027
-0.2266
176.1168
3.1621
-4.1489
-177.104
4.2959
-173.104
-9.7367
177.2515

Table 2: Experimental frequencies for IR and Raman powder and specular reflection spectra.
IR Frequency (polarized light)
IR Frequency
[cm-1]
IR
int.
Raman Frequency
[cm-1]
[cm-1]
Assignments
X
Y
Z
3420
3348
3289
3084
3068
3056
3026
2954
2922
2869
2852
1674
1649
1599
1590
1537
1464
1441
1408
1397
1385
1311
1301
1179
s
s
s
s
s
s
s
s
s
s
s
s
s
3427
νasNH
3347
3337
νsNH
3294
3303
3087
3290
νsNH
νasCH
3072
3063
3028
2969
3068
3060
3068
3060
νsCH
3058
3027
2953
2919
2869
2841
1683
νsCH
νasCH
2953
2919
2953
2919
NUJOL, νsCH
NUJOL, νsCH
NUJOL, νasCH
NUJOL, νsCH
νasCOO
δNH2
2829
2831
1654
1678
1652
1600
1653
1600
sh
m
m
m
w
νsCC(ar)
1592
1545
νasCN
1551
1409
νasCC(ar)
NUJOL
1441
νasCC(ar)
νsCC(ar)
sh
sh
s
1412
1393
1412
1402
νsCOO
1384
1311
νsCOO
vw
w
1311
1302
def. i.p. CCC(ar)
def. i.p. CCC(ar)
νCC
w
1179
1180
1180

1070
1024
1010
1003
839
w
1071
ρNH2
m
w
1025
1011
1003
837
1024
1011
1003
839
1024
1009
1003
841
ωNH2
νsCN
w
νsCN
m
sh
m
w
def. o.o.p. CCC(ar)
def. CH
def. CH
def. CH
def. CH
ωCN
741
720
718
686
666
556
728
694
725
682
680
677
661
vw
w
554
548
sh
m
m
547
524
405
548
407
ωCN
530
533
ωCN
405
407
τNH2
Abreviations: -stretching; as-antisymmetric; s-symmetric; -out of plane bending; τ- twisitng; def.
– deformation; def o.o.p. – out of plane deformation; def.i.p. – in plane deformation

Table nr 3: Correlation diagram for guanidinium cation and benzoate anion for k=0.
Guanidinium cation
D3h
Molecular
group
C2v
Site
D2h
Factor
Group
(Z=4)
36 Ag
3 A’1
2 A’2
6 E’
A’’1
2 A’’2
2 E’’
9 A1
3 A2
4 B1
8 B2
12 Au
16 B3g
32 B3u
Benzoate anion
C1
C2v
Site
D2h
Molecular
group
Factor
Group
(Z=4)
36 Ag
9 A1
9 A2
9 B1
9 B2
36 B2g
36 B1u
36 B3u
36 A
Number of all free degree: (240+48)=288
Number of internal vibration: 96 (guanidine cation) + 144 (benzoate anion)=240
Number of translations and rotations (included 3 aqustic vibrations) = (6 (one molecule translations
and rotations) x2 (number of molecules)) x4 (Z)=48

Table nr 4:Calculated cosines direction for selected vibrations.
Calculated cosines directions
Group of atoms
Type of vibration
X
Y
Z
N18H24
-0,3525
0,8716 -0,3405 ν
N18H23
C2O1
-0,3525 -0,8716 -0,3405 ν
0,3770 -0,8800 -0,2891 ν
C2O3
0,3770
0,8798 -0,2891 ν
C15N16
C15N17
0,3188 -0,8666
0,3838 ν
0,3838 ν
0,3188
0,8666
C15N18
-0,6388
0,3300 -0,8795
0,0000 -0,7696 ν
0,3431 δ
0,3432 δ
0,0000 -0,6949 δ
1,0000 0,0000 ρ
0,0148 -0,6730 def. o.o.p.
H20N16H19
H22N17H21
H24N18H23
H24N18H23
H20N16H19
H24N18H23
N16C15N17
N16C15N18
H20N16C15
H19N16C15
H24N18C15
O1C2O3
O1C2C4
C2C4C5
C4C5C8
C5C8C12
C8C12C10
H7C5C4
0,3301
-0,7193
0,0000
0,7395
-0,6948
-0,7694
0,7695
-0,7394
0,8796
0,0000
0,0000
0,7192 def. o.o.p.
0,6388 def. o.o.p.
0,0000 -0,6387 def. o.o.p.
0,0261 0,6727 def. o.o.p.
0,7616 -0,0070 -0,6480 def. o.o.p.
0,7680 0,0616 -0,6374 def. o.o.p.
0,6082 -0,0001 0,7937 def. o.o.p.
-0,6163 -0,0055 -0,7876 def. o.o.p.
-0,6163 0,0045 -0,7876 def. o.o.p.
0,6158 -0,0043
0,6158
0,6103
0,6157 -0,0042
0,7878 def. o.o.p.
0,7878 def. o.o.p.
0,7921 def. o.o.p.
0,7879 def. o.o.p.
0,0039
0,0000
H7C5C8
-0,6159
0,6158
0,0041 -0,7879 def. o.o.p.
0,0037 0,7878 def. o.o.p.
H11C8C5
H11C8C12
H14C12C8
-0,6146 -0,0038 -0,7889 def. o.o.p.
0,6104 0,0001 0,7920 def. o.o.p.
Abreviations: -stretching; δ – in plane bending; ρ - rocking; def
o.o.p. – out of plane deformation