J. Michalski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 112 (2013) 263–275
271
compounds are 4.6557 debye for PH5N; 6.1986 debye for
PH5N3M; 4.5604 debye for PH5N4M and 4.1841 debye for
PH5N6M.
presented here and their comparison with our results published
earlier [14,15,38]. They appear in the following ranges:
mas(NO2) 1618 (1614)–1604 (1601),
ms(NO2) 1340 (1340)–1305 (1300),
m(/-NO2) 1165 (1161)–1153 (1150) and
Results and discussion
1072 (1079)–1071 (1069),
d(NO2) 849 (843)–818 (821),
Vibrational and DFT calculations
x
(NO2) 767 (769)–743 (739),
The optimized structures of the all studied compounds in their
monomer form are shown in Figs. 1 and 2 illustrates molecular
structure of PH5N6M in which the inter- and intra-molecular
interactions are taken into account. The IR and Raman spectra of
the studied compounds are presented in Figs. 3 and 4. Experimen-
tal wavenumbers of the all studied compounds are collected in Ta-
ble 1, where the assignment of the vibrational normal modes to the
respective bands is proposed. Table S1, enclosed as supplementary
material, collects calculated wavenumbers with PEDs of the all
studied compounds.
q(NO2) 538 (526)–513 (509),
q(/-NO2) 312 (314)–281 (281),
c
s
(/-NO2) 219 (216)–205 (207),
(NO2) 171 (176)–169 (124) cmꢁ1
.
Fig. 5a and b illustrates the relationships of some of these wave-
numbers to the substitution position of methyl group in the ring.
The methyl group as a substituent in the pyridine ring acts as
electron-donating unit making the ring more negative. It signifi-
cantly influences the melting points of the studied compounds
that clearly differ in their stereochemistry. The substitution place
of this group also influences the position of some bands of the
Pyridine and phenyl ring vibrations
The assignment of phenyl and pyridine ring vibrations of the
studied compounds was proposed in our previous work in which
6-methyl-3-nitro-2-phenylhydrazopyridine was studied. Its crystal
structure and quantum chemical calculations were discussed [15].
This compound is a structural isomer of the derivatives studied
here. The same nomenclature was used as in the papers on free
pyridine [27,28] and its different derivatives [29–33]. The phenyl
ring vibrations were assigned on the basis of quantum chemical
calculations performed in the present paper. The calculated wave-
numbers agree well with those reported for benzene and its deriv-
atives [34–37]. The following description of the bands observed in
pyridine ring and nitro-group. For the modes
NO2), as(CH3), (/) and (CH)/ the increasing trends are observed
for the series 4 M < 3 M < 6 M (see Fig. 5a), whereas for the
modes s(NO2), (/-NO2), (NN), s(CH3) and (CH3) such trend
appears in the series 3 M < 4 M < 6 M (see Fig. 5b). These trends
agree with the tendency observed for the melting points of the
studied isomers.
mas(NO2), m(/-
m
m
c
m
m
m
m
q
Hydrazo bond vibrations
Nitrogen atoms in hydrazo group appear in the sp3 hybridisa-
tion. Three nitrogen electrons of this unit are involved in the for-
mation of NAC, NAN and NAH bonds and remaining two
electrons form a lone pair distributed inside the whole system. This
effect is not steady for the bonds of this system and the length of
the three bonds differs from one another. For the monomeric forms
of the studied compounds the average bond lengths are as follows:
C/AN 1.375 Å, NAN 1.394 Å and ChAN 1.411 Å. The bond lengths
calculated for the particular molecules in their monomeric and di-
meric forms are listed in Table 2.
the spectra for pyridine ring is proposed:
1598–1542, 1384–1370 and 1357–1135; d(/): 1079–509;
998–770; and
(/): 769–419 cmꢁ1. The respective vibrations of
the phenyl ring appear in the ranges: (CH): 3105–3006; (h):
1618–1601; (h) + d(h): 1272–1240, 1112–1082; d(h): 1009–544;
(CH): 918–813, 769–764;
(h): 698–679, 412–408 cmꢁ1
m
(CH): 3150–3062;
m
(/):
c(CH):
c
m
m
m
c
c
.
Methyl and nitro group vibrations
The methyl and nitro group vibrations of pyridine derivatives
were well recognized and described in details in our previous pa-
pers [14,15,38]. The experimental wavenumbers recorded for the
studied here compounds fit well those previously reported and
these data agree with commonly accepted assignment [34–37].
The vibrations of the bridge C/ANHANHACh moiety are de-
scribed by 12 degrees of freedom. Five of them refer to the stretch-
ing modes:
seven modes to the bending vibrations: das(HNANH), ds(HNANH),
d(NNH)h, d(NNH)/, (HNANH), (NHANH) and (C/NNCh). The
m(NH)h, m(NH)/, m(C/AN), m(ChAN) and m(NAN); and
The calculated wavenumbers of methyl group:
2955 cmꢁ1 (100% contribution) and s(CH3) 3038–2909 cmꢁ1
(100% contribution) agree well with the experimental values found
at 2989–2919 cmꢁ1 for as(CH3) and 2845–2838 cmꢁ1 for
s(CH3).
mas(CH3) 3070–
c
s
s
m
bands corresponding to these vibrations are observed in the IR
and (RS) spectra at the following wavenumbers (cmꢁ1):
m
m
The other weak bands in this range arise from a Fermi resonance
with the bending vibrations of these groups.
m
m
(NH)inter HB 3436 (3340)–3328 (3329),
(NH)intra HB 3265 (3264)–3168 (3198),
das(CH3) vibrations give rise to the bands observed in the range
1488–1408 cmꢁ1 giving a 52–71% contribution to the normal
mode. The symmetric ds(CH3) vibrations also appear in a typical
range and are observed at 1384–1370 cmꢁ1 giving a 57–91% con-
tribution to the normal modes. The other bands involving the
das(HNANH) 1598 (1595)–1590 (1585),
ds(HNANH) 1541 (1537)–1520 (1514),
d(NNH)h 1509 (1507)–1500 (1506),
d(NNH)/ 1466 (1470)–1434 (1448),
m(C/AN) 1293 (1299)–1274 (1278),
m(ChAN) 1270 (1272)–1243 (1240),
m(NAN) 1165 (1161)–1135 (1140),
c(HNANH) 666 (669)–633 (636),
methyl groups are observed in the following ranges:
q
.
(CH3):
1038–967, (/-CH3): 603–376 and
x
s
(CH3): 257–176 cmꢁ1
The nitro group bonded to the pyridine ring (C/–NO2) gives rise
to the nine vibrational normal modes. Three of them correspond to
603 (601)–589 (581),
the stretching
scribe in-plane vibrations (bending d(NO2), rocking
rocking of the whole group (/-NO2)), and finally, three corre-
spond to out-of-plane modes (wagging (NO2), twisting (NO2)
and torsional (/–NO2)). Observed main IR and (RS) modes of the
C/–NO2 group were identified on the basis of DFT calculations
m
as(NO2),
m
s(NO2) and
m
(/–NO2) vibrations, three de-
538 (526)–419 (421),
q(NO2) and
s(C/NNCh) 258 (257)–251 (254),
q
s(HNANH) 171 (176)–169 (124).
x
s
c
Fig. 6 illustrates selected normal modes of hydrazo ANHANHA
bond.
Michalski
