From 1,3-dicyanobenzene to 3-cyanobenzamide - Its molecular structure in the gas-phase and in the crystal

Article abstract of DOI:10.1016/S0022-2860(02)00410-6

The crystals of 3-cyanobenzamide have been obtained in the reaction of MgPcH₂O with 1,3-dicyanobenzene. 3-Cyanobenzamide crystallises in the P2₁/c space group of the monoclinic system with four molecules per unit cell. In the crystal, the 3-cyanobenzamide molecule is non-planar, the amide group is rotated by 21.8(1)° around the C_ar-C_sp² bond in relation to the phenyl ring. Full optimised molecular orbital calculations performed on the isolated 3-cyanobenzamide molecule have shown a similar conformation (the rotation angle equals to 22.46°). Two pairs of symmetric equivalent N-H?O and N-H?N hydrogen bonds with two neighbouring form pseudo one-dimensional chains interconnect each 3-cyanobenzamide molecule in the crystal. The chains of 3-cyanobenzamide molecules form a stacking structure with π?π interactions between the aromatic phenyl rings within the stacks. The molecular orbital calculations and conformational analysis have shown two equivalent pairs minimum of energy on the potential energy surface (PES) - one global, other local. During rotation of the amide group from 0 to 360° around the C_ar-C_sp² bond four rotation barriers are observed. The highest rotation barrier energy of 48.84 kJ mol^-1 is compared to the hydrogen bond energy.

Full text of DOI:10.1016/S0022-2860(02)00410-6

Journal of Molecular Structure 644 (2003) 13–21
www.elsevier.com/locate/molstruc
From 1,3-dicyanobenzene to 3-cyanobenzamide—its molecular
structure in the gas-phase and in the crystal
*
Jan Janczak , Ryszard Kubiak
W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrocław, P.O. Box 1410, Poland
Received 9 July 2002; accepted 30 July 2002
Abstract
The crystals of 3-cyanobenzamide have been obtained in the reaction of MgPcH2O with 1,3-dicyanobenzene. 3-
Cyanobenzamide crystallises in the P21=c space group of the monoclinic system with four molecules per unit cell. In the crystal,
the 3-cyanobenzamide molecule is non-planar, the amide group is rotated by 21.8(1)8 around the Car–C2sp bond in relation to the
phenyl ring. Full optimised molecular orbital calculations performed on the isolated 3-cyanobenzamide molecule have shown a
similar conformation (the rotation angle equals to 22.468). Two pairs of symmetric equivalent N–H· · ·O and N–H· · ·N
hydrogen bonds with two neighbouring form pseudo one-dimensional chains interconnect each 3-cyanobenzamide molecule in
the crystal. The chains of 3-cyanobenzamide molecules form a stacking structure with p· · ·p interactions between the aromatic
phenyl rings within the stacks. The molecular orbital calculations and conformational analysis have shown two equivalent pairs
minimum of energy on the potential energy surface (PES)—one global, other local. During rotation of the amide group from 0
to 3608 around the Car–C2sp bond four rotation barriers are observed. The highest rotation barrier energy of 48.84 kJ mol21 is
compared to the hydrogen bond energy.
q 2002 Elsevier Science B.V. All rights reserved.
Keywords: 3-Cyanobenzamide; Crystal structure; Gas-phase structure; Conformation; Rotation barriers
1. Introduction
where Pc ¼ C32H16N822 and M ¼ divalent metal,
possesses usually the excellent stability to chemical
and thermal treatments. However, the magnesium
phthalocyanine is exceptional and unusual [7].
Although it is thermally stable, chemically is active
even in the ambient air [7,8]. In particular, it has
been shown by us that magnesium phthalocyanine
during annealing process with 1,2-dicyanobenzene,
coordinates the 1,2-dicyanobenzene molecule and
forms the MgPc(C6H4(CN)2) complex [9]. How-
ever, with increasing temperature the ligated
1,2-dicyanobenzene molecule releases from
the complex and forms the tris(2-cyanophenyl)-
1,3,5-triazine, a product of the catalytic and
The work discussed in this paper has its origin
in our interest in the investigations of the
cyanocompounds such as dicyanobenzene isomers
[1–5] and dicyanonaphthalene [6] and their fea-
tures as N-donating ligands, especially in the field
of complexation and coordination chemistry of
metallophthalocyanines. Metallophthalocyaninato
complexes with the general formula of MPc,
*
Corresponding author. Tel.: þ48-71-343-5021; fax: þ48-71-
441-029.
E-mail address: janczak@int.pan.wroc.pl (J. Janczak).
0022-2860/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(02)00410-6

14
J. Janczak, R. Kubiak / Journal of Molecular Structure 644 (2003) 13–21
selective cyclisation of 1,2-dicyanobenzene [10]. It
has been also shown that magnesium phthalocya-
nine forms with pyridine the MgPc(py)2 complex
that in the ambient atmosphere transforms into
hydrated complex with the composition of
MgPcH2O·2py. In this complex (MgPcH2O·2py)
the water molecule is directly coordinated to the
central magnesium cation of MgPc and the pyridine
molecules as acceptors form hydrogen bonds with
the coordinated water molecule [11,12]. For the
above reasons, we decided to examine the products
obtained during thermal process of magnesium
phthalocyanine or hydrated magnesium phthalocya-
nine with 1,3-dicyanobenzene.
2.2. X-ray crystallography
A colourless single crystal of 3-cyanobenzamide
with the approximate dimensions of 0.32 £ 0.25 £
0.20 mm3 was use for data collection on a four-circle
KUMA KM-4 diffractometer equipped with a two-
dimensional area CCD detector. The graphite mono-
ꢀ
chromatised Mo Ka radiation ðl ¼ 0:71073 AÞ and
v-scan technique with Dv ¼ 0:758 for one image
were used for data collection. The 960 images for six
different runs covering over 95% of the Ewald sphere
were performed. Initially the lattice parameters were
refined in 126 reflections obtained from 40 images for
eight runs with different orientation in the reciprocal
space. Finally the lattice parameters were refined by
least-squares methods on the basis of all collected
reflections with F2 . 2sðF2Þ: One image was mon-
itored as a standard for every 40 images. Integration of
the intensities, correction for Lorenz and polarisation
effects was performed using a KUMA KM-4 CCD
software [13]. The face-indexed analytical absorption
was calculated using the SHELXTL program [14].
The structure was solved by direct methods and
refined with anisotropic thermal parameters for all
non-hydrogen atoms using the SHELXL-97 program
[15]. The hydrogen atoms were located from differ-
ence Fourier synthesis and their positional parameters
were refined. The temperature factors of hydrogen
atoms were constrained as Uisso ¼ 1:2 Uisso; i.e. 20%
higher than the carbon or nitrogen atoms directly
joined the H atom. The final difference Fourier maps
showed no peaks of chemical significance. The final
agreement factors and details of data collections were
collected in Table 1. The final atomic coordinates and
equivalent thermal parameters are listed in Table 2.
Herein, we report the synthesis and structural
characterisation of 3-cyanobenzamide, the product of
the thermal treatment of 1,3-dicyanobenzene in
presence of hydrated magnesium phthalocyanine
(MgPcH2O). The experimental X-ray geometry of 3-
cyanobenzamide is compared with the geometry in
the gas-phase obtained by the molecular orbital
calculations at the HF/6-31G(d,p) level.
2. Experimental
2.1. Synthesis
A powder sample of magnesium phthalocyanine
was purchased from Sigma Aldrich Co. and was
purified by zone refining. The b-MgPc crystalline form
of the purified sample was identified from X-ray
diffraction pattern (sample a). The part of b-MgPc
sample was converted to hydrated form by grinding
under water in the mortar. Next the excess of the water
was decanted and the sample drained (sample b). Each
of the prepared magnesium phthalocyanine samples
was mixed with 1,3-dicyanobenzene in the molar
proportion of about 1:2, pressed into pallet (each of
0.8 g), degassed, sealed of in the glass ampoule and
annealed at 170 8C (the hot zone of the furnace) by 1
day. After such processing no new phase was formed in
the sample (a) and only the 1,3-dicyanobenzene was
resublimed at the end of the ampoule (150 8C), as was
evidenced by the X-ray diffraction technique. How-
ever, in the sample (b) the colourless parallelepipedal
crystals of the new phase were present in the hot zone.
One of them was selected for X-ray data collection
2.3. Theoretical calculations
Ab initio molecular orbital calculations were
performed with the GAUSSIAN94 program package
[16] at the Hartree–Fock level of theory. Full
geometry optimisations were carried out with
the 6-31G(d,p) basis set functions starting from the
X-ray geometry. As a convergence criterion the
threshold limits of 0.00025 and 0.0012 a.u. were
applied for the maximum force and the displacement,
respectively. The rotation barriers of the amide group
(CONH2) were calculated at the HF/6-31G(d,p)

J. Janczak, R. Kubiak / Journal of Molecular Structure 644 (2003) 13–21
15
Table 1
Table 2
Crystallographic data for 3-cyanobenzamide, details for data
collection and refinement parameters
Final atomic coordinates and equivalent isotropic displacement
ꢀ2
P P
parameter for 3-cyanobenzamide, Ueq ¼ ð1=3Þ
jUijaiajaiaj ðA Þ
i
Formula
C4H4(CN)(CONH2)
146.15
Atom
x
y
z
Ueq
Mol. wt.
Crystal system, space group
˚
Unit cell dimensions a, b, c, (A)
Monoclinic, P21=c
3.8120(8), 15.503(3),
11.824(2)
O1
N1
N2
C1
C2
C3
C4
C5
C6
C7
C8
H1
H2
H3
H4
H5
H6
0.8238(2)
0.8673(3)
0.0739(3)
0.6413(3)
0.4870(3)
0.3615(3)
0.3919(3)
0.5457(3)
0.6668(3)
0.7840(3)
0.1996(3)
0.865(4)
0.972(3)
0.458(3)
0.296(3)
0.568(3)
0.773(3)
0.60664(5)
0.49433(6)
0.58812(6)
0.63135(6)
0.59790(7)
0.65302(7)
0.74199(7)
0.77509(8)
0.72073(7)
0.57621(7)
0.61678(7)
0.4767(8)
0.4589(8)
0.5389(7)
0.7788(7)
0.8364(8)
0.7409(7)
0.46432(6)
0.34556(9)
0.0616(3)
0.0508(3)
0.0588(3)
0.0344(3)
0.0365(3)
0.0372(3)
0.0441(3)
0.0488(3)
0.0430(3)
0.0411(3)
0.0427(3)
0.071(4)
0.072(4)
0.042(3)
0.056(4)
0.061(4)
0.048(3)
20.10192(8)
0.27090(8)
0.17133(8)
0.08426(8)
0.09573(9)
0.19590(9)
0.28252(9)
0.36822(9)
20.01962(9)
0.2710(12)
0.4088(11)
0.1587(9)
b (8)
Volume, V (A )
94.05(3)
3
˚
697.0(2)
Z
4
F(000)
152
D
calc (g/cm3)
1.393
Dobs measured, flotation (g/cm3)
1.39
˚
Radiation, Mo Ka, (A)
2u range
l ¼ 0:71073
3–56
Reflections collected
Independent reflections
Observed reflections
Absorption coefficient, m (mm21
Correction
6691
1616, ðRint ¼ 0:0218Þ
1051
)
0.096
0.0326(10)
0.2039(10)
0.3518(10)
Lorenz, polarisation,
face-indexed analytical
absorption,
Tmax ¼ 0:9810;
Tmin ¼ 0:9699
Refinement on F 2
R(F 2 . 2s(F 2))a
0.0364
amide group takes place only in the sample (b), i.e. in
the presence of the hydrated magnesium phthalocya-
nine, MgPcH2O. At the first the CxN group with the
lone pair of electron at the N atom interacting with the
hydrogen atom of the coordinated by central mag-
nesium atom of MgPc water molecule and finally
accepts the hydrogen atom form intermediate
moieties, i.e. the protonated at the cyano group cation
and MgPcOH2 counter ion. Both intermediate units
interacting according to Scheme 1 yielding 3-
iminohydroxy cyanobenzene (I). As has been shown
by single crystal X-ray analysis in the magnesium
phthalocyanine the magnesium cation is displaced by
wR (F 2 all reflections)b
Reflections/L.S. parameters
Goodness-of-fit, S
0.0482
13
1.002
23
˚
Residual electron density (e A
)
þ0.121, 20.102
ꢀ
P
P
a
R ¼ llFol 2 lFcll F :
oꢀ
P
P
1=2
b
2
wR ¼
½wðFo2 2 Fc2Þ ꢀ wF4
;
{
}
o
w21 ¼ s2ðFo2Þ þ ð0:0126PÞ where P ¼ ðFo2 þ 2Fc2Þ=3:
2
level using GAUSSIAN94. In this case, the all-full
optimised parameters were kept fixed while the amide
group was rotated in steps of 58 along the Cring–Camide
bond (C1–C7). The high of the rotation barrier was
than derived from the variation of the total energy as a
function of the rotation angle (torsion angle of C2–
C1–C7–N1).
˚
about 0.5 A from the weighted least-squares plane
defined by the four isoindole atoms [7], and the Mg
atom in the MgPc has about 0.5 positive charge,
which was confirmed by the molecular orbital
calculation [17]. Therefore, MgPc easy forms 4 þ 1
coordinated complexes. Additionally, it has been
stated that magnesium phthalocyanine prefers the
4 þ 1 coordination in the complexes with the O-
ligands compare to the N-ligands [11]. For example,
the MgPc(py)2 complex under moist atmosphere or in
the water solution is unstable and transforms into
hydrated complex according to the reaction:
3. Results and discussion
3.1. Synthesis
The selective catalytic transformation of only one
of two cyano groups in the 1,3-dicyanobenzene to the

16
J. Janczak, R. Kubiak / Journal of Molecular Structure 644 (2003) 13–21
Scheme 1.
MgPc(py)2 þ H2O ! [MgPcOH2]·2py. In the com-
plex of [MgPcOH2]]·2py (4 þ 1 coordination)
the water molecule is coordinated by O atom to the
central magnesium cation of MgPc, while the pyridine
molecules interact with H atoms of coordinated water
molecule form O–H· · ·N hydrogen bonds. Taking this
into account we suggest that the compound (I)
(Scheme 1) transforms into 3-cyanobenzamide that
interacts with positively charged magnesium atom of
MgPc forms complex (II). However, during prolonged
thermal process (at 170 8C, 1 day) the Mg–O bond
breaks and the 3-cyanobenzamide migrates to the
cooler zone forms good quality single crystal.
In the sample (a), i.e. in presence of pure MgPc at
the used condition (,170 8C) the 1,3-dicyanobenzene
does not undergo transformation. This is consistent
with the observation obtained during the experiment
of MgPc with 1,2-dicyanobenzene [9,10]. The pure

J. Janczak, R. Kubiak / Journal of Molecular Structure 644 (2003) 13–21
Table 3
17
MgPc catalyses the selective transformation of one
Comparison of the X-ray and gas-phase geometry of 3-cyanoben-
˚
zamide (A, 8)
of two cyano groups of 1,2-dicyanobenzene into
tris(2-cyanophenyl)-1,3,5-triazine, but only at tem-
perature about 408 and higher (,210 8C) [10], at
lower temperature the 1,2-dicyanobenzene is coordi-
nated by N atom of one cyano group to MgPc form
4 þ 1 coordinated complex [9], and similarly to the
1,3-dicyanobenzene during prolonged thermal pro-
cess in the similar conditions the Mg–N bond breaks
yielding MgPc and crystals of 1,2-dicyanobenzene.
Bond lengths
X-ray
1.3797(13)
HF/6-31G(d,p)
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C1
C1–C7
C7–N1
C7–O1
C3–C8
C8–N2
N1–H1
N1–H2
C2–H3
C4–H4
C5–H5
C6–H6
1.386
1.390
1.389
1.391
1.383
1.396
1.503
1.358
1.198
1.448
1.144
0.992
0.995
1.074
1.074
1.075
1.073
1.3962(13)
1.3901(14)
1.3827(16)
1.3804(15)
1.3952(14)
1.5040(14)
1.3404(13)
1.2297(12)
1.4484(15)
1.1439(12)
0.927(11)
1.001(12)
0.935(9)
3.2. X-ray and gas-phase geometry
The molecular geometry of the 3-cyanobenzamide
obtained by the single crystal X-ray analysis with the
atomic labelling is shown in Fig. 1. The X-ray
experimental geometrical parameters are given in
Table 3, which also comprises the ab initio full-
optimised parameters calculated at the HF/
6-31G(d,p) level. The absolute energy calculated
using the 6-31G(d,p) basis functions was found to be
2490.238 Hartrees and the calculated dipole
moment of the 3-cyanobenzamide molecule is
2.565 D.
0.984(10)
0.966(11)
0.945(10)
Angles
C3–C8–N2
C1–C7–O1
C1–C7–N1
N1–C7–O1
C1–C2–C3
C2–C3–C4
C3–C4–C5
C4–C5–C6
C5–C6–C1
C6–C1–C2
C2–C3–C8
C4–C3–C8
C2–C1–C7
C6–C1–C7
C7–N1–H1
C7–N1–H2
C1–C2–H3
C3–C2–H3
C3–C4–H4
C5–C4–H4
C4–C5–H5
C6–C5–H5
C5–C5–H6
C1–C6–H6
179.56(13)
120.21(10)
117.61(10)
122.18(10)
120.16(10)
120.90(10)
118.63(11)
120.56(12)
121.06(11)
118.68(10)
119.42(10)
119.68(10)
123.27(10)
118.05(9)
117.8(6)
179.79
121.11
116.59
122.29
119.85
120.45
119.58
120.06
120.57
119.49
119.57
119.98
122.85
117.64
115.59
120.90
121.08
119.05
119.76
120.66
119.86
120.08
120.87
118.56
The aromatic six-membered ring of 3-cyanoben-
zamide in the crystal is planar, but shows noticeable
distortion from the ideal hexagonal shape. The C–C
˚
bonds in the ring with the average value of 1.387 A
119.7(7)
123.3(6)
116.6(6)
118.4(6)
123.4(6)
119.1(6)
120.3(6)
122.9(6)
116.0(6)
Torsion angles
C2–C1–C7–N1
21.8(1)
22.46
are comparable to those observed in the other
1,3-disubstituted benzene derivatives [18]. The
differences between the internal C–C–C angles
within the ring (118.6(1)–121.1(1)8) results from
Fig. 1. View of the X-ray experimental geometry of 3-cyanobenza-
mide molecule with atom labelling. Thermal ellipsoids are shown at
a probability level of 50%. H atoms are shown as spheres with
arbitrary radii.

18
J. Janczak, R. Kubiak / Journal of Molecular Structure 644 (2003) 13–21
the substitution effects of the both cyano (CxN) and
amide (CONH2) groups. The single C–CN bond
C6–C1–C7 angles observed in the crystal are similar
as found in the optimised molecule (Table 3).
˚
The CyO as well as the C–NH2 bonds in the
crystal are slightly longer in those in the gas-phase
structure. The slightly elongated CyO and C–NH2
bonds in the crystal are undoubtedly due to the
intermolecular interaction, since the oxygen as well as
both hydrogen atoms of the NH2 group are involved in
the hydrogen bonds. A slightly longer CxN bond
with a distance of 1.448(1) A well correlate with the
˚
distance of 1.436–1448 A observed in several
compounds, which contain the Car–Csp bond [19].
This bond (C–CN) is shorter than the single C–C
˚
bond that links the amide group (1.504(2) A). This
value corresponds to the Car–C2sp bond with the
˚
length of 1.489–1.510 A; shorter if the substituted
˚
(1.144(1) A) in the crystal than in the optimised
group is coplanar with the aromatic ring, otherwise
longer, i.e. longer if the substituted group is not
coplanar. The shortening of the Car–Csubstituent bond
distance is a result of partial delocalisation of the p-
electron and consequently the slight p-character of
the bond, as has been shown in the dicyanobenzene
isomers [5], 2,3-dicyanonaphthalene [6] or in
cyanoaniline [20]. In all cases the p-character of
the s single C–CN bond has been calculated on the
basis of the theoretical charge density distribution.
The most interesting feature of the molecular
structure of 3-cyanobenzamide is that the amide group
is not coplanar with the aromatic six-membered ring.
The orientation of the almost planar amide group in
relation to the benzene ring can be described by the
torsion angle of C2–C1–C7–N1, that describe the
rotation of the amide group (CONH2) around
the single Car–C2sp bond. In the crystal, this angle is
equal to 21.8(1)8. Ab initio molecular orbital calcu-
lations performed on the isolated molecule show a
similar value of the C2–C1–C7–N1 torsion angle
(22.468). Thus the non-planarity of the 3-cyanoben-
zamide molecule mainly results from the intramole-
cular interactions, and to a lesser degree from the
intermolecular interaction and the crystal packing.
The rotation of the amide group around the Car–Cs2p
bond is likely due to the intramolecular interaction
(repulsion force) between the H1 atom of the NH2
group and the H3 atom in the ring. The intramolecular
˚
molecule (1.136 A) is also due to the interactions with
the hydrogen atom of the NH2 group of the
neighbouring molecule with the formation of the
weak hydrogen bond (N–H· · ·N). A similar corre-
lation between the X-ray experimental and ab initio
optimised CxN bond is observed in the dicynoben-
zene isomers [5], 2,3-dicyanonaphthalene [6] and in
4-cyanoaniline [20].
The interactions of the dipole CxN group with
both ortho-H atoms (H3 and H4, see also Fig. 1) leads
to the decrease of the both C3–C4–H4 and C3–C2–
H3 angles by about 1.58 from 1208, thus the repulsion
interaction between the H3 ant H1 atoms leads
additionally to the decreasing of the C3–C2–H3
angle by about 28. All discussed details are on the
basis of the relatively high accurate X-ray crystal
structure determination including the refinement of
the all hydrogen atoms (relatively low final agreement
˚
H1· · ·H3 distance is equal to 2.207(2) A in the crystal
˚
and 2.246 A in the optimised molecule related to the
˚
gas-phase. This value is about 0.25 A longer than the
distance if amide group is coplanar with the ring.
The intramolecular H1· · ·H3 interaction besides the
rotation of the amide group around the Car–C2sp bond
leads to the increasing of the C2–C1–C7 angle and
simultaneously decreasing of the opposite C6–C1–
C7 angle. The relation between the C2–C1–C7 and
Fig. 2. View of the crystal packing showing the stacking structure.
Dashed lines represents the N–H· · ·O and N–H· · ·N hydrogen
bonds in the chains.

J. Janczak, R. Kubiak / Journal of Molecular Structure 644 (2003) 13–21
19
Fig. 3. Pseudo one-dimensional chain of the hydrogen bonded 3-cyanobenzamide molecules: (a) view perpendicular to the benzene ring and (b)
view parallel to the benzene ring.
˚
factors and the residual electron density in the final
difference Fourier maps).
molecules are separated by ,3.52 A. The value of the
distance indicates on the p–p interactions between
the aromatic six-membered rings along the stacking
axis, since this distance is comparable to the van der
Waals distance between aromatic ring system [21].
The two symmetric equivalent hydrogen bonds N–
H· · ·O are almost linear (178(1)8) and stronger than
the other two N–H· · ·N hydrogen bonds, that invol-
ving the nitrogen atom of the cyano groups (Table 4).
Besides above mentioned intermolecular interactions,
the interactions between the cyano groups of two
neighbouring 3-cyanobenzamide molecules with
the local non-zero dipole moment that are oriented
antiparallel are present. In this orientation, the cyano
3.3. Intermolecular interactions and crystal packing
The arrangement of the 3-cyanobenzamide mol-
ecules in the crystal is shown in Fig. 2. Each molecule
of 3-cyanobenzamide in the crystal is involved in four
hydrogen bonds, in two of them as H donor and in
other two as H acceptor, and with two neighbouring
molecules forms chains as illustrated in Fig. 3. In the
chains, the aromatic six-membered rings are parallel
to each other, and the eight atoms fragment of the
hydrogen bonding system
Table 4
˚
Hydrogen bonding geometry (A, 8)
is almost planar and inclined to the benzene ring by an
angle of ,21.68. The chain of the hydrogen-bonded
3-cyanobenzamide molecules are almost parallel to
the b axis and is inclined by ,63.4 and ,26.58 to the
a and c axis, respectively. The chains in the crystal
form a stacking structure (Fig. 2) with a stacking axis
almost perpendicular to the OY. In the stacks, the
D–H· · ·A
D–H
H· · ·A
D· · ·A
/(D–H· · ·A)
N1–H1· · ·N2a 0.927(11) 2.257(12) 3.174(2) 170(1)
N1–H2· · ·O1b 1.001(12) 1.918(12) 2.918(1) 178(1)
a
Symmetry code: 1 2 x, 1 2 y, 2z.
Symmetry code: 2 2 x, 1 2 y, 2z.
b

20
J. Janczak, R. Kubiak / Journal of Molecular Structure 644 (2003) 13–21
groups of the neighbouring molecules in the chains
interact electrostatically, the more negatively charged
nitrogen atom of one CxN group lies opposite to the
second CxN group. This kind of the intermolecular
interactions and charge compensation gives rise to the
relatively great crystal density (,1.393 g/cm3).
360 2 22.46 ¼ 337.548) and local (at ,145 and
,2158) are observed, thus the total energy of
molecule is equal for the four symmetrically equiv-
alent conformations (the same value for the rotation
from 0 to ^1808). During rotation of the amide group
from 0 to 3608 for different rotation barriers are
observed. The lowest barrier is observed at the angle 0
(or 3608), and the highest at the rotation angle of 908
(and equivalent angle of 2708). The energy barrier at
3.4. Molecular conformation and rotational barriers
the rotation angle of )8 equals to ,1.26 kJ mol21
.
The rotation of the amide group in the crystal of
3-cyanobenzamide is hindered due to the intermole-
cular interactions that leads to the formation N–
H· · ·O and N–H· · ·N hydrogen bonding system as
well as by the crystal packing force. In the gas-phase,
the rotation of amide group around the single Car–Cs2p
bond is possible. Full optimised molecular orbital
calculations performed on the isolated 3-cyanobenza-
mide molecule show similar conformation as present
in the crystal. The total energy of the most stable
conformation was found to be 2490.238 Hartree. The
calculations exposed two equivalent global minima
and two equivalent local minima on the potential
energy surface (PES). The values of the rotation
barriers were derived from the variation of the total
energy as the function of the rotation angle (the
torsion angle of C2–C1–C7–N1, Fig. 1) and results
are illustrated in Fig. 4. As can be seen from Fig. 4
during the rotation of the amide group from 0 to 3608
two pairs minimum of energy—global (at 22.468 and
This relatively lower barrier energy is due to the
repulsion interaction between the H1 and the H3
atoms, therefore the rotation of the amide group by the
angle of ,22.468 leads to the increasing of this
˚
H1· · ·H3 distance by ,0.25 A. The highest barrier
energy of ,48.84 kJ mol21 is found for the rotation
angle of amide group by 908 in relation to the ring.
The relatively high energy barrier (as well as the total
energy of molecule) is undoubtedly due to the whole
breaking the delocalisation of the p-electron between
the aromatic ring and the amide group along the
Car–C2sp (amide) bond, since the p orbital of the C2sp
(amide) is perpendicular to the p–p orbitals of
the carbon atoms of the aromatic ring. Next, if the
amide group rotates from ,145 to ,2158 the barrier
energy of ,6.35 kJ mol21 is observed. Although at
the rotation angle of 1808, the amide group is coplanar
with the rings (Fig. 5) and the mentioned above
delocalisation of the p-electron over the Car 2 Cs2p
(amide) bond is possible (the delocalisation of the p-
electron usually leads to the decreasing of the total
energy of molecule). However, the increasing of the
energy by the ,6.35 kJ mol21is observed. This is
likely due to the intramolecular interaction (repulsion
force) of the negatively charged oxygen atom (with
two lone pairs) and the CxN group. The back of
Fig. 4. Rotational barriers of the amide group in relation to the
benzene ring computed by a molecular orbital calculation at the
HF/6-31G(d,p) level.
Fig. 5. Schematic representation of the rotational conformation of
the 3-cyanobenzamide, the rotation angle of the amide group 0, 90
and 1808 in relation the ring.

J. Janczak, R. Kubiak / Journal of Molecular Structure 644 (2003) 13–21
21
the molecule to the most stable conformation from
the local minimum (145 or 2158) requires to add the
References
energy of DE3 ¼ 17:48 kJ mol21
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Acknowledgments
This work was supported by a grant no. 3 T09A
180 19 from the Polish State Committee for Scientific
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