C{double bond, long}N{single bond}N{double bond, long}C conformational isomers of 2′-hydroxyacetophenone azine: FTIR matrix isolation and DFT study

Article abstract of DOI:10.1016/j.molstruc.2010.04.017

2′-hydroxyacetophenone azine (APA) has been studied by matrix isolation infrared spectroscopy and quantum chemical calculations. The DFT/B3LYP/6-311++G(2d,2p) calculations demonstrated the existence of two conformers for the lowest energy E/E configuratio

Full text of DOI:10.1016/j.molstruc.2010.04.017

Journal of Molecular Structure 976 (2010) 371–376
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
C@NAN@C conformational isomers of 20-hydroxyacetophenone azine: FTIR
matrix isolation and DFT study
*
Joanna Grzegorzek, Zofia Mielke , Aleksander Filarowski
Faculty of Chemistry, University of Wrocław, Joliot Curie 14, 50-383 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
20-hydroxyacetophenone azine (APA) has been studied by matrix isolation infrared spectroscopy and
quantum chemical calculations. The DFT/B3LYP/6-311++G(2d,2p) calculations demonstrated the exis-
tence of two conformers for the lowest energy E/E configuration of APA, a s-trans and a gauche ones.
The conformers are characterized by similar energies and differ in the value of a C@NAN@C angle, that
was calculated to be 180° for a planar s-trans conformer and 155° for a non-planar gauche one. The cal-
culated barrier for conformational interconversion is also very low, ca. 1 kJ molꢀ1 for the conversion from
a gauche conformer to a trans one. The FTIR spectra of an argon matrix doped with APA from a vapour
above solid sample evidence the presence of both conformers that exhibit reversible interconversion at
matrix temperatures. The comparison of the theoretical spectra with the experimental ones and revers-
ible temperature dependence of the experimental spectra allowed for unambiguous spectroscopic char-
acterization of the trans and gauche conformers. The experiment also demonstrated that a gauche
conformer is more stable than a trans one. The spectra analysis indicates that transformation from a trans
conformer to a gauche one weakens the intramolecular OAHꢁ ꢁ ꢁN bonds in the molecule.
Article history:
Received 9 February 2010
Received in revised form 9 March 2010
Accepted 13 April 2010
Available online 20 April 2010
Keywords:
Azine
Infrared spectra
Matrix isolation
Conformational analysis
Theoretical calculation
Hydrogen bonding
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
respect to the NAN single bond is determined by the s1 angle,
for
a s-trans conformation s1 = 180° and for a gauche one
Azines, and in particular aromatic azines, are receiving increas-
ing attention for their biological, chemical and physical properties
[1–4]. They play an important role in synthetic chemistry [5,6] and
are potential nonlinear optical materials [7,8]. A series of salicylal-
dehyde azine derivatives were found to exhibit interesting aggre-
gation-induced emission enhancement characteristics that is a
promise of their potential applications in such devices like optical
sensors or light emitting diodes [9].
In light of the great utility of aromatic azines, there has been
significant interest in studies of their stereochemistry and isomer-
ism [8,10–17]. Most of the experimental studies were performed
for the crystalline state [12]; recently a number of quantum chem-
ical calculations that refer to the gaseous state has been also re-
ported [14,15]. A survey of the conformations of a number of
derivatives of the parent compound benzaldazine was given re-
cently by van Schalkwyk and Stephen [12].
s1 – 180°; in the s-trans conformation conjugation of two halves
of azine is at a maximum. Most of the crystal structures of azines
show either NAN s-trans or gauche conformation, only in a few
cases both conformations occur in polymorphous crystals. The
most marked conformational changes result from substitution of
various bulky groups for H on the azine carbon atoms. The set of
compounds containing the group [ArCMe@NA]2 (Ar – differently
substituted aromatic rings, Me – methyl group) is markedly differ-
ent structurally from the set involving the [ArCH@NA]2 group [12].
The steric hindrance of Me group in the first set of compounds
causes distortion about the NAN bond which results in s1 value
strongly deviating from 180° and the general form of the molecule
in some cases approximates rather to an orthogonal than to a pla-
nar structure. There are, however, exceptions from this general
trend. The crystal structure of 20-hydroxyacetophenone azine
(which involves [(o-OHAr)CMe@NA]2 group) consists of discrete
almost planar molecules characterized by the s1 value equal to
177°. This no doubt follows from the strong OAHꢁ ꢁ ꢁN bonding of
the OAH group to N [18].
The conformation of aromatic azine is controlled by the chain of
four atoms: C@NAN@C as depicted in Scheme 1.
Almost all studied aromatic azines exist in the preferred (E,E)
configuration in which the large groups attached to the C@N bonds
are trans to the NAN one. The stereochemistry of the azines with
The 20-hydroxyacetophenone azine (APA) is a derivative of sal-
icylaldehyde azine (SAA) in which the two hydrogen atoms at
azine carbons [(o-OHAr)CH@NA]2 are replaced by methyl groups.
The salicylaldehyde azine is the smallest possible symmetric
aromatic Schiff base with two hydrogen bonding centres and the
* Corresponding author. Tel.: +48 71 3757475; fax: +48 71 3282348.
E-mail address: zm@wchuwr.chem.uni.wroc.pl (Z. Mielke).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.04.017

372
J. Grzegorzek et al. / Journal of Molecular Structure 976 (2010) 371–376
3. Results and discussion
OH
3.1. Calculated properties of EE-trans and EE-gauche conformers
N
N
The calculations resulted in eleven stationary points on the po-
tential energy surface of APA at the DFT/B3LYP/6-311++G(2d,2p)
level of theory (only the configurations in which the methyl
group is in a trans position to the (CAC(O))ring bond were taken
into account). The two most stable structures (I, II) correspond
to E/E configuration, they are presented in Fig. 1. The next con-
former (taking as a criterion the stability) has Z/Z configuration
and is by ca. 31 kJ molꢀ1 less stable than conformers I and II;
the structures of all optimized conformers are presented in
Fig. 1, Supporting material. The calculations also demonstrated
the stability of a keto tautomer of conformer I in which one
hydrogen atom in intramolecular OAHꢁ ꢁ ꢁN hydrogen bond is
transferred from an oxygen atom to a nitrogen atom (Fig. 2).
The keto tautomer of I is by 19.94 kJ molꢀ1 less stable than the
enol form. The calculations did not result in minimum corre-
sponding to a keto structure in which two protons are transferred
to the N atoms. This is in accord with earlier reported experimen-
tal and theoretical data for symmetric Schiff bases that demon-
strated single proton transfer process during excitation of
symmetric Schiff bases [27,28].
HO
Scheme 1.
molecule was a subject of many studies [19]. In contrast with SAA
there are only few literature data on APA properties; the structural
characteristics [17,18,20] and the infrared and Raman spectra of
the molecule in the crystalline state have been recently reported
[16,17,21]. We performed the FTIR matrix isolation study and
DFT/B3LYP calculations of APA with the aim to obtain characteris-
tics on its low energy conformers. The calculations performed for
the methyl(para-methylphenyl) ketone azine [11] demonstrated
that the activation energy required for
s variations between the
NAN s-trans and gauche structures is very small, below
0.5 kcal molꢀ1 for this compound. If the potential energy barrier
between the two lowest conformers of APA lies also below
1 kJ molꢀ1 the two conformers should be in conformational equi-
librium in the matrix [22]. Matrix isolation has been proved to
be extremely powerful technique in the conformational study.
In Table 1, the calculated geometrical parameters for I and II are
presented and compared with the experimental values reported for
crystalline APA [18,20]. The calculated value of s1 angle (u
2. Experimental section
C10N11N28C26) equals to ꢀ180.0° for I and to ꢀ155.4° for II which
is characteristic for trans and gauche conformers, respectively. In
Table 2, the energies of the two conformers are compared; the en-
ergy minimum corresponding to a I-trans conformer lies 0.66 kJ
molꢀ1 above that corresponding to a II-gauche one when ZPVE is
not taken into account but after including ZPVE correction the min-
imum corresponding to a I-trans conformer is 0.09 kJ molꢀ1 below
that corresponding to a II-gauche one. ZPVE correction leads to the
reverse of the stability order of the two conformers.
2.1. Infrared matrix isolation studies
The APA/Ar matrix was obtained by simultaneous deposition of
vapour above solid APA, kept at 333 K, and argon on a gold-plated
copper mirror kept at 17 K by a closed cycle helium refrigerator
(Air Products, Displex 202A). The matrix concentration was con-
trolled by the matrix gas flow rate that was adjusted to minimize
the concentration of APA dimers and higher aggregates. The spec-
tra were recorded for the matrix kept at 9 K, 17 K and 32 K; after
deposition at 17 K the matrix was cooled down to 9 K, then heated
to 32 K and again cooled down to 9 K.
24
36
20
17
34
35
The infrared spectra (resolution 0.5 cmꢀ1) were recorded in a
reflection mode with Bruker 113v FTIR spectrometer using a MCT
detector cooled by liquid N2.
19
13
1
23
14
13
9
15
11
6
The synthesis of 20-hydroxyacetophenone azine was performed
from 2:1 stoichiometric mixtures of 2-hydroxyacetophenone and
saturated ammonia water solution in methanol according to Fur-
nuss et al. [23].
26
16
22
21
25
5
10
18
28
8
27
30
2
4
29
32
3
31
2.2. Computational methods
7
12
I, trans ΔE=0.66; ΔEZPVE = 0.00
The calculations were performed with the Gaussian03 suite of
programs [24]. The DFT/B3LYP/6-311++G(2d,2p) method was ap-
plied to optimize all the structures and to calculate the harmonic
frequencies. Binding energies were corrected by the Boys-Bernardi
full counterpoise correction [25]. Potential-energy torsional pro-
files were obtained by optimizing all geometrical parameters ex-
cept for the CNNC torsional angle that was fixed at a given value
(using increments of 5°).
All the stationary points were unambiguously characterized as
minima or transition states by their vibrational spectra. The com-
puted harmonic frequencies were scaled down by a single factor
0.982 to correct them for the basis set limitations and anharmonic-
ity effects. A potential energy distribution (PED) of the normal
modes for each isomer of 20-hydroxyacetophenone azine was com-
puted in terms of internal coordinates with the Gar2ped program
[26].
II, gauche ΔE= 0.0; ΔEZPVE = 0.09
Fig. 1. DFT/B3LYP/6-311++G(2d,2p) optimized s-trans and gauche conformers of E/E
configuration of 20-hydroxyacetophenone azine.

J. Grzegorzek et al. / Journal of Molecular Structure 976 (2010) 371–376
373
Table 2
Relative energies, kJ molꢀ1
,
(not corrected,
D
E, and corrected for zero-point
vibrational contributions,
DEZPVE), and estimated populations for a trans and a gauche
conformers of 20-hydroxyacetophenone azine.
Conformer
20-hydroxyacetophenone azine
EZPVE
Population (%)a
D
E
D
E
EZPVE
I Trans
II Gauche
0.66
0 (ꢀ879.261192)b
28
72
34
65
0 (ꢀ879.550782)b
0.09
a
According to Boltzmann distribution at 60 °C (temperature of deposition).
In parantheses the values of electronic energy, E, or electronic energy with zero-
b
point contribution, EZPVE, in hartree are given.
Fig. 2. The DFT/B3LYP/6-311++G(2d,2p) optimized structure of a keto tautomer of a
s-trans conformer 20-hydroxyacetophenone azine.
Fig. 3 presents the potential-energy profile for internal rotation
around the NAN bond (C@NAN@C dihedral angle) in APA calcu-
lated at the DFT/B3LYP/6-311++G(2d,2p) level of approximation;
the profile is based on energy values of the potential minima
(not corrected for ZPVE). As one can see the minima corresponding
to gauche configuration are separated from the minimum corre-
As one can see in Table 1, the experimental values reported for
crystalline APA [20] show good agreement with the calculated
parameters for I indicating that the molecule in solid state shows
s-trans conformation. The largest discrepancies concern the OH
and Hꢁ ꢁ ꢁN distances which is due to uncertainty of H position
determination by X-ray study. The comparison of the geometrical
parameters for I and II demonstrates that the rotation of the [(o-
OHAr)CMe@NA]2 grouping around NAN bond by ca. 25° (s1 angle
changes from ꢀ180° to ꢀ155.4°) when going from a trans to a
gauche conformation is accompanied by rotation of two CH3 groups
around CAC bond by ca. 180°. The dihedral u (C2C10C29H32) and u
(C23C26C33H36) angles change from 0.96°, ꢀ0.94° to ꢀ176.9° (for
both angles) when going from a trans to a gauche form. This is
accompanied by the change of the dihedral u (N11N28C26C33), u
(N28N11C10C29) angles from ca. 0° to 6.3°. There are also other small
structural differences in geometrical parameters of the
H3CAC@NAN@CACH3 grouping between trans and gauche con-
formers as can be seen in Table 1.
sponding to a s-trans conformer by very low barrier, 1.06 kJ molꢀ1
;
the energy difference between the two minima corresponding to
trans and gauche forms is also very low (0.66 kJ molꢀ1). The zero-
point corrected electronic energies will slightly modify the energy
profile, however the general picture, i.e. small energy difference
and very low energy barriers between the two conformers, will
remain.
In Table 2, the electronic energies, zero-point corrected elec-
tronic energies and relative abundances for the optimized I and II
conformers are collected. The relative abundances of the two con-
formers were estimated from Boltzmann distribution at the depo-
sition temperature 333 K, on the basis of their calculated relative
energies (both
DE and DE(ZPVE) were taken into account) and tak-
ing into account the weighting factors (degeneracy) for the trans
and gauche structures as equal to 1 and 2. The obtained results
indicate that both trans and gauche conformers should be present
Table 1
The comparison of the DFT/B3LYP/6-311++G(2d,2p) calculated geometrical parameters for a trans and gauche conformers of 20-hydroxyacetophenone azine with the reported
crystallographic data.
Trans
1.374
Gauche
1.378
Ref. [18]
1.401(5)
Ref. [20]
1.394(4)
r N11N28
r N11(28)C10(26)
r C10(26)C2(23)
r C10(26)C29(33)
r C1(21)O13(25)
r O13(25)H14(27)
r O13(25)H14(27)ꢁ ꢁ ꢁN11(28)
1.305
1.467
1.507
1.340
0.999
1.640
1.300
1.469
1.506
1.343
0.993
1.678
1.302(6), 1.285(6)
1.478(6), 1.477(7)
1.477(7), 1.493(8)
1.347(7), 1.355(6)
1.300(5), 1.295(5)
1.464(5), 1.460(6)
1.497(5), 1.495(5)
1.346(5), 1.352(5)
0.8200
1.579, 1.862
1.80
h N11(28)N28(11)C26(10)
h N11(28)C10(26)C2(23)
h N11(28)C10(26)C29(33)
h C2(23)C10(26)C29(33)
h C2(23)C1(21)O13(25)
h C1(21)C2(23)C10(26)
116.49
117.14
120.90
121.95
122.47
121.36
106.04
148.78
118.12
117.40
123.37
119.23
122.43
122.02
106.28
148.08
115.1(4), 115.4(4)
115.8(4), 116.3 (5)
124.8(5), 123.6(5)
122.6(5), 122.8(5)
122.6(5), 122.8(5)
121.9(5), 121.8(5)
115.8(3), 115.7(3)
116.5(4)
124.0(4), 123.2(4)
119.6(4), 120.3(4)
122.1(4), 122.0(4)
122.1(4), 122.3(4)
109.5
h C21(1)O25(13)H27(14)
. . .
h O13(25)H14(27)
N
11(28)
141.5, 155.8
177.0
146, 147
u C10N11N28C26
u N11N28C26C33
u N28N11C10C29
u N11N28C26C23
u N28N11C10C2
u N11C10C2C1
ꢀ180.00
ꢀ0.063
ꢀ0.072
179.93
ꢀ179.92
ꢀ0.35
ꢀ155.41
6.33
ꢀ177.9(4)
ꢀ0.6(6)
6.34
ꢀ0.2(5)
ꢀ174.15
ꢀ174.14
3.39
179.9(3)
ꢀ179.2(3)
2.7(5)
u N28C26C23C19
u C29C10C2C1
0.38
3.40
2.2(5)
ꢀ179.63
ꢀ177.055
0.96
ꢀ177.064
ꢀ179.63
ꢀ176.93
ꢀ176.96
0.15
ꢀ176.4(4)
u C33C26C23C21
u C2C10C29H32
u C23C26C33H36
u C2C1O13H14
u C23C21O25H27
ꢀ177.4(4)
ꢀ0.94
ꢀ0.22
ꢀ0.23
ꢀ0.18

374
J. Grzegorzek et al. / Journal of Molecular Structure 976 (2010) 371–376
to a gauche conformation all the symmetry elements are lost and
5
4
3
2
1
0
120
100
80
60
40
20
0
all APA vibrations become active in infrared spectrum. However
the intensities of all normal modes that became active in a gauche
conformer are very weak (see Table 1, Supporting material), so, the
spectra of the two forms are, in fact, quite similar.
In Table 3 the experimental frequencies are compared with the
calculated ones for the s-trans and gauche conformers; in Fig. 4 the
experimental and theoretical spectra are presented. The simulated,
theoretical IR spectrum was built by adding the calculated individ-
ual spectra of the two conformers scaled by the corresponding esti-
mated populations at 333 K. One has to remember that due to the
very low barrier between the two conformers and small difference
between their electronic energies one can expect conformational
equilibration in the matrix at 20 K [22]. Low energy barriers pro-
mote also the relaxation of the higher energy local minima into
more stable structures, this effect is known as the conformational
cooling effect [29,30]. The two conformers are characterized by
very similar set of frequencies and the simulated spectrum repro-
duces relatively well the observed one in the 1800–400 cmꢀ1 re-
gion. In Fig. 4 are presented the 1675–1400 cmꢀ1 and 1425–
1150 cmꢀ1 spectral regions of the APA/Ar matrix at 9 K (a), after
its annealing to 32 K (b) and then after cooling down again to 9 K
(c). One can see clearly the reversible temperature dependence of
a number of bands pairs which indicates a reversible interconver-
sion of the two conformers at matrix temperatures. The compari-
son of the experimental spectra with the calculated ones for the
s-trans and gauche conformers allowed us to assign unambigously
all the component bands whose intensities increase at higher
matrix temperature to s-trans conformer and those whose intensi-
ties decrease to a gauche one. For example, the 1511, 1516 cmꢀ1
1.06
0.66
0
+
60
+
120
180
C=N-N=C / O
-
120
-
60
0
Fig. 3. DFT/B3LYP/6-311++G(2d,2p) calculated potential-energy profile for internal
rotation around the NAN bond.
in an argon matrix after deposition of the vapour above solid APA
and, moreover, they are expected to exhibit reversible interconver-
sion at matrix temperature due to the very low barrier between
them [22].
4. Infrared spectra
APA in a trans form has C2h symmetry, and its vibrations can
thus be distributed as Cvib = 35Ag + 34Bu + 17Au + 16Bg. When the
[(o-OHAr)CMe@NA] grouping is rotated around a C@NAN@C angle
Table 3
Comparison of the experimental and calculated frequencies (cmꢀ1) and intensities (km molꢀ1) for a trans and gauche conformers of 20-hydroxyacetophenone azine; the
results of potential energy distribution are also given.
[16]
Trans
Gauche
Ar matrix
mobs
Ar matrix
mobs
Calculated
Calculated
mobs
mharm.
I
PED %
mharm.
I
PEDa
%
3165
3143
3136
29
28
11
83
95
89
m
m
m
CH
CH
CH
3153
3143
3135
3122
3119
3104
2999
1627
1605
1574
1511
19
28
14
17
14
1338
19
235
280
286
45
95
89
100
m
m
CH
CH
mCH
2816–2651
2816–2651
66m
86m
97m
91m
38m
20m
38m
OH + 24
CH
OH
CH3
mCH
3120
3004
2995
1627
1605
1560
1516
1485
1459
1446
1424
1388
1308
1252
1242
1169
1131
21
71m
71m
76m
36m
10m
46m
22m
CH3 + 14
CH3 + 24
m
m
CH
OH
2651–2555
316
1005
259
202
367
62
OH + 22
CCring+18dCOH
CN + 16 CCring + 12dCOH + 7dCCC(N)
CN + 9dCOH
CCring + 22dCOH + 16dCHring
m
CH3
1624.1 w
1611.7 s
1563.3 m
1501.7 w
1627.6 w
1608.2 s
1563.3 m
1492.4 w
CCring + 14dCOH
CN + 12 CCring + 8dCOH + 8dCCC(N)
CN + 10dCOH
CCring + 18dCHring
1605 vs
1569 vs
1497 s
m
m
22dCOH + 19
m
56
17
50dCH3
90dCH3
33dCHring + 12
20dCOH + 18dCH3 + 14dCHring
90dCH3
1454.7 w
1444.9 vw
1396.8 w
1368.6 m
1305.6 vs
1249.9 w
1239.3 w
1165.2 w
1129.8 vw
1450.1 w
1447.5 w
1400.4 vw
1368.6 m
1305.6 vs
1253.8 m
1231.8 w
1161.4 w
1129.8 vw
1038.5 vw
830.2 vw
820.2 vw
807.3 vw
753.2 s
1460
1446
1432
1383
1307
1254
1240
1166
1130
1040
869
56
92
40
30dCH3 + 15dCHring
38dCH3 + 12 CN
28dCOH + 28dCHring
1439 s
101
128
28
111
171
69
mCN
m
1362 s
1300 s
1247 s
38
136
103
106
56
15
11
47
45
17
31
95
46
82dCH3
24
mCO + 18
m
CC(N)+14dCCring
CO + 10dCHring
26m
30m
36m
CO + 14
CCring + 24
CCring + 18dCHring + 17dCOH
m
CC(N)+14dCCring
30m
CCring + 20
m
mCO + 16dCHring
28dCHring + 22
50dCHring + 14
38dCHring + 12
m
m
m
CCring
CCring
CCring
1158 m
1135 w
1031 w
839 s
62
13
52dCHring + 12
38dCHring + 12
46dCH3 + 17
66
33
40dCCCring + 16m
36
26
m
m
CCring
CCring
c
CC(C)N
CHring
CHring + 14 CCO + 14
CO
CCCCring + 24 CCO + 18
CCC(N)+24 CCCCring + 18
CCNN
840.8 w
820.2 vw
805.1 vw
896
858
840
750
746
663
76
16
16
32
103
45
97
31
c
c
COH
CCHring + 17
c
c
COH + 14c
s
CCCCring + 15
CO
CCN + 16
CCO
c
CCO
859
840
751
749
c
s
CCCCring + 12
c
COH
40dCCring + 16
46
54
m
758 s
sCCCCring + 18
c
c
c
CCO
s
c
c
cCCC(N)+16
c
CHring
749.7 m
662.4 w
cCCHring + 20
751.9 vs
664.0 w
s
c
CCH
662 w
44dCCN + 21dCCring
669
34dCCN + 23dCCCring + 14
s
a
Only PED values greater than 7% are presented. In the Gar2ped program that was used to calculate PED the internal coordinates were determined as follows: all stretches,
methyl group deformations, C6 ring deformations, CrCr (H)Cr, CrCr(O)Cr and CrCr(C)Cr deformations, Cr–C(C)@N deformations, COH in plane deformation and torsions around
CAC(H3), CrAC, C@N and NAN bonds (r indicates ring C atom). Definition of coordinates: , bond stretching, d, bending, , out of (ring) plane bending, , torsion. All the
m
c
s
contributions to PED present antisymmetric vibrations of two corresponding coordinates in the two halves of azine. Experimental intensities are presented in qualitative
terms: vs, very strong; s, strong; m, medium; w, weak; vw, very weak.

J. Grzegorzek et al. / Journal of Molecular Structure 976 (2010) 371–376
375
A 1.0
B
1.2
1.0
0.8
0.6
0.4
0.2
0.0
+
0.8
0.6
0.4
0.2
0.0
+
+
+
+
+
c
b
a
3000 2700
1600
1400
1200
1000
800
600
1650
1600
1550
1500
1450
Wavenumber/cm-1
+
1.2
1.0
0.8
0.6
0.4
0.2
0.0
+
+
+
+
+
c
b
a
1400
1350
1300
1250
1200
1150
3200 3000 1600
1400
1200
1000
800
600
Wavenumber/cm-1
Wavenumber/cm-1
Fig. 4. (A) Infrared spectra of 20-hydroxyacetophenone azine trapped in an argon matrix (upper curve) and simulated spectrum obtained by summing the calculated spectra
for a trans and a gauche conformers at the DFT/B3LYP/6-311++G(2d,2p) level of theory, scaled by their estimated Boltzmann populations at 333 K. (B) The 1670–1425, 1400–
1150 cmꢀ1 regions in the spectra of APA/Ar matrix. Spectra were recorded directly after matrix deposition at temperature 9 K (a), after heating up the matrix to 32 K (b) and
after cooling the matrix to 9 K (c). The bands due to a gauche and s-trans conformers are marked by ‘‘+” and ‘‘ꢂ”, respectively.
calculated frequencies of the gauche and s-trans conformers, are as-
signed in the experimental spectrum to the bands at 1492.4 and
1501.7 cmꢀ1, respectively. The intensity of the 1492.4 cmꢀ1 com-
ponent due to a gauche form decreases and that at 1501.7 cmꢀ1
due to a trans one increases after matrix annealing. The observed
spectral characteristics indicates that a trans conformer is less
stable than a gauche one as indicated by the DFT/B3LYP/6-
311++G(2d,2p) calculated electronic energy difference between
the two conformers however without zero-point correction. Taking
into account ZPVE correction leads to the reverse of the energy or-
der, so, the calculations do not reproduce correctly the small en-
ergy difference between the two conformers.
stretching of a gauche one. The shape and the position of these
bands are similar to the characteristics of the corresponding bands
observed for the ortho-hydroxy acylaromatic Schiff bases contain-
ing chelate ring with intramolecular OAHꢁ ꢁ ꢁN bond [31]. One may
note that the two bands appear in the lower frequency region than
that predicted by calculations. It is a known fact that the matrix
material, even as inert as solid argon, strongly effects the structure
and, in consequence, the vibrations of strongly hydrogen bonded
systems. The most spectacular example is the ammonia-hydrogen
chloride complex for which the hydrogen stretching vibration was
identified at 2084 cmꢀ1 in solid Ne and at 1370.9 cmꢀ1 in solid ar-
gon [32–34].
The d(COH) vibrations in both conformers are strongly coupled
4.1. The COH group vibrations
with the mCN, mCCring, and dCHring vibrations; the normal modes
involving the C@N, CCring stretching and COH, CHring bending vibra-
tions occur in the 1630–1400 cmꢀ1 region (Table 3); however the
effect of conformational change on these vibrations is less clear
The COH groups serve as proton donors to N atoms forming two
internal OAHꢁ ꢁ ꢁN hydrogen bonds. The calculations show that the
frequencies of the COH groups vibrations are very sensitive to the
than on
The
at 869 cmꢀ1 in a gauche conformer (in a gauche conformer
vibration gives a small contribution to COH), and the correspond-
ing bands are identified at 840.8 cmꢀ1 and at 830.2 cmꢀ1, respec-
tively. The higher COH frequency for a trans than for a gauche
conformer is corresponding with the lower OH value of the first
conformer. The relative values of the OH frequencies as well as
OH ones of the two conformers indicate that the intramolecular
OAHꢁ ꢁ ꢁN bond is stronger in a trans form than in a gauche one.
m
OH and d(OH) ones.
OH mode is calculated to occur at 896 cmꢀ1 in a trans and
CHring
rotation around C@NAN@C angle. The two
m
OH vibrations are cal-
c
culated to appear at ca 3000 cmꢀ1 for a trans isomer and at ca.
c
3100 cmꢀ1 for a gauche one; in trans conformer the vibration is
c
strongly coupled with
pling between one of the two
m
CH3 whereas in a gauche one a weak cou-
OH vibrations and CH vibration oc-
m
m
c
curs. The analysis of the experimental spectra allowed us to assign
a broad, diffuse band with some structure in the 2650–2555 cmꢀ1
region to the OH stretching vibration of s-trans conformer and a
similar band extending in the 2815–2650 cmꢀ1 region to the OH
m
m
c

376
J. Grzegorzek et al. / Journal of Molecular Structure 976 (2010) 371–376
The weakening of the OAHꢁ ꢁ ꢁN hydrogen bond in a trans form is
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2010.04.017.