21.5 ± 0.3 kJ mol
−1 . The crystals of polymorph I are monoclinic, P2 1 /c, with
Z = 4, a = 5.601(10) Å, b = 12.178(3) Å, c = 8.090(2) Å, a = c = 90°, and
b = 105.62(2)° [28]. The most relevant pattern of hydrogen bonds noticeable in the
crystal of polymorph I is the centrosymmetric ring formed by N–H⋯O hydrogen
bond interactions, which group 1MH molecules in dimeric units. In turn, polymorph II is orthorhombic, Pna2 1 space group, with Z = 4, a = 19.0258(4) Å,
b = 3.91210(10) Å, and c = 6.82880(10) Å [29]. The striking difference between
the H-bond network in the two polymorphs is that while in polymorph I, the N–
H⋯O interactions group the molecules in dimers (which are then associated by C–
H⋯O weak interactions); in polymorph II, the N–H⋯O interactions are used to
form zigzag chains, which are interspersed with other chains through the interactions involving oxygen atoms and form an angle of ca. 36° between them [29].
In the case of 5MH, four polymorphs were identified. Polymorph I corresponds
to the commercial sample, which could be transformed into the remaining forms
(II–IV) by thermal treatment. The melting points of the polymorphs (following
their numbering) were found to be 147.2 ± 0.6, 143.0 ± 0.4, 120.1 ± 0.8, and ca.
94–96 °C [30]. Only the structure of polymorph III could be solved by XRD,
which resulted to be triclinic, P-1 space group, with Z = 2, a = 4.3618(2) Å,
b = 6.1535(2) Å, c = 10.3145(5) Å, a = 76.196(2)°, b = 80.860(3)°, and
c = 84.904(3)° [30]. In this crystal, the molecules are packed in layers, where the
molecules are joined in infinite chains through a head-to-tail pattern of hydrogen
bonding involving the C=O and NH groups.
The different polymorphs of the two compounds were investigated by infrared
and Raman spectroscopies.
In the case of 1MH, analysis of the H-bond sensitive regions of the infrared
spectra of the two polymorphs allowed to estimate the average energy per H-bond
in each polymorph. This was achieved by use of Rozenberg and coworkers’
empirical correlations [38, 39], which relate the frequency redshifts of the stretching
vibrations and/or the frequency blueshifts of the out-of-plane bending modes of the
H-bond donors (compared to the corresponding frequencies for the free groups;
matrix isolation data was used as reference) with H-bond properties, including
energies. The results indicate that slightly stronger N–H bonds exist in polymorph
I than in polymorph II (DH (H-bond) = −28 and −26 kJ mol
−1 , respectively). These
results are in agreement with the structural crystallographic data, which indicate that
the N–H bond lengths are longer, and both the N⋯O and H⋯O distances associated
with the N–H⋯O dominant intermolecular interactions are shorter in polymorph
I than in polymorph II.
A very interesting result was obtained by following the transformation of a
single crystal of polymorph II of 1MH into polymorph I, by temperature variation
Raman microspectroscopy. As shown in Fig. 7.11, upon heating, the crystal of
polymorph II converts into polymorph I in a transformation that kept crystal
integrity [29].
For 5MH, the Raman spectra of polymorphs I and II were found to be almost
identical (Fig. 7.12), except in relation to relative band intensities, indicating that
the orientation of the 5MH molecules in both crystals shall be very similar. On the
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R. Fausto et al.
−1 . The crystals of polymorph I are monoclinic, P2 1 /c, with
Z = 4, a = 5.601(10) Å, b = 12.178(3) Å, c = 8.090(2) Å, a = c = 90°, and
b = 105.62(2)° [28]. The most relevant pattern of hydrogen bonds noticeable in the
crystal of polymorph I is the centrosymmetric ring formed by N–H⋯O hydrogen
bond interactions, which group 1MH molecules in dimeric units. In turn, polymorph II is orthorhombic, Pna2 1 space group, with Z = 4, a = 19.0258(4) Å,
b = 3.91210(10) Å, and c = 6.82880(10) Å [29]. The striking difference between
the H-bond network in the two polymorphs is that while in polymorph I, the N–
H⋯O interactions group the molecules in dimers (which are then associated by C–
H⋯O weak interactions); in polymorph II, the N–H⋯O interactions are used to
form zigzag chains, which are interspersed with other chains through the interactions involving oxygen atoms and form an angle of ca. 36° between them [29].
In the case of 5MH, four polymorphs were identified. Polymorph I corresponds
to the commercial sample, which could be transformed into the remaining forms
(II–IV) by thermal treatment. The melting points of the polymorphs (following
their numbering) were found to be 147.2 ± 0.6, 143.0 ± 0.4, 120.1 ± 0.8, and ca.
94–96 °C [30]. Only the structure of polymorph III could be solved by XRD,
which resulted to be triclinic, P-1 space group, with Z = 2, a = 4.3618(2) Å,
b = 6.1535(2) Å, c = 10.3145(5) Å, a = 76.196(2)°, b = 80.860(3)°, and
c = 84.904(3)° [30]. In this crystal, the molecules are packed in layers, where the
molecules are joined in infinite chains through a head-to-tail pattern of hydrogen
bonding involving the C=O and NH groups.
The different polymorphs of the two compounds were investigated by infrared
and Raman spectroscopies.
In the case of 1MH, analysis of the H-bond sensitive regions of the infrared
spectra of the two polymorphs allowed to estimate the average energy per H-bond
in each polymorph. This was achieved by use of Rozenberg and coworkers’
empirical correlations [38, 39], which relate the frequency redshifts of the stretching
vibrations and/or the frequency blueshifts of the out-of-plane bending modes of the
H-bond donors (compared to the corresponding frequencies for the free groups;
matrix isolation data was used as reference) with H-bond properties, including
energies. The results indicate that slightly stronger N–H bonds exist in polymorph
I than in polymorph II (DH (H-bond) = −28 and −26 kJ mol
−1 , respectively). These
results are in agreement with the structural crystallographic data, which indicate that
the N–H bond lengths are longer, and both the N⋯O and H⋯O distances associated
with the N–H⋯O dominant intermolecular interactions are shorter in polymorph
I than in polymorph II.
A very interesting result was obtained by following the transformation of a
single crystal of polymorph II of 1MH into polymorph I, by temperature variation
Raman microspectroscopy. As shown in Fig. 7.11, upon heating, the crystal of
polymorph II converts into polymorph I in a transformation that kept crystal
integrity [29].
For 5MH, the Raman spectra of polymorphs I and II were found to be almost
identical (Fig. 7.12), except in relation to relative band intensities, indicating that
the orientation of the 5MH molecules in both crystals shall be very similar. On the
216
R. Fausto et al.