198
S.F. Tayyari et al. / Journal of Molecular Structure 782 (2006) 191–199
this band greatly increases upon deuteration. This behavior
is caused by decoupling from OD bending and increasing the
CaC and CaO character of this mode.
6. Conclusion
We have tried to analyze the vibrational spectra of
nitromalonaldehyde and its deuterated analogues by
establishing one to one correlations between theoretically
calculated frequencies at fairly high level with the
experimental results. From the OD stretching and OH/OD
out-of-plane bending modes a stronger intramolecular
hydrogen bond in NO2MA than that in MA was concluded.
By applying the Natural Bond Orbital calculations, the
electron inductive, steric, and conjugation effects of NO2
group were analyzed. According to these calculations,
steric, and conjugation effects increases the hydrogen bond
strength, while the electron withdrawing effect decreases the
strength of the bond.
The strong bands at 1525 and 1358 cmK1 are assigned to
the asymmetric and symmetric NO2 stretching vibrations,
respectively. In aliphatic compounds nasNO2 and nsNO2
appear at 1556–1545 cmK1 and 1390–1355 cmK1, respect-
ively, and by conjugation with aromatic groups both of them
show red shift [37]. With increasing the electron with-
drawing ability of the substituted groups the frequency of
the asymmetric NO2 stretching increases [38,39]. Therefore,
the relatively low frequencies of these two bands in NO2MA
could be attributed to the conjugation between the NO2
group and enol ring. Therefore, it could be concluded that
the enolated ring has a character near to that of aromatic
rings and acts as electron donor to the NO2 group.
The band at 1250 cmK1, which upon deuteration
disappears, assigned to the OH in-plane bending mode.
This band is coupled with C–O stretching and C–H in-plane
bending modes and upon deuteration disappears and a new
band appears at 1061 cmK1, which is mainly dOD.
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Another very important band in this region is due to the
O/O stretching, which appears at 287 cmK1
.
The strong band at 200 cmK1, according to our
calculation, is due to the NO2 out-of-plane rocking mode.