Overview of Raman Spectroscopy: Fundamental to Applications
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academia and industrial communities [79, 80]. ILs attracted many researchers owing
to their structural diversity, distinctive, intriguing characteristics such as low vapor
pressure, low melting points, non-volatility, non-flammability, and excellent heat
and chemical stability as well as the unique physicochemical properties [81–86].
The structure of ILs consist of bulky cations and various anions, reveals them as
task-specific configurable, customizable, and designer solvents. Due to the cationanion complex in ILs, various forces such as weak forces (dispersion forces, van
der Walls), specific (charges, dipole and hydrogen bonding (HB), etc.) and strong
(Coulomb) forces are responsible for the novel physical properties. Coulomb force
is only 70% of total energy, suggesting that the weaker interactions cannot be overlooked [87]. In particular, HB is deemed important for ILs structure and interaction
modes [88–94]. Raman spectroscopy is useful not only in HB study in ILs but also in
the identification of the various conformers in crystals and liquid forms of ILs [95–
100]. Alkylammonium nitrate ILs are perhaps the most investigated ILs because they
are the first synthesized ILs in the laboratory [101]. Bodo et al. reported methylammonium nitrate (MAN) crystal structure and revealed the presence of HB interaction
using Raman spectroscopy, where nitrate anion asymmetrically coordinated with
MA cation [95]. Raman spectra revealed in longer alkyl chain ILs that propylammonium cations exhibit trans-conformations in a crystalline low-temperature state
and undergo a crystal polymorphism transition with increasing temperature, where
propylammonium cations exhibited in gauche conformation [96, 102]. Further, in the
case of Imidazolium cation based ILs, Raman spectroscopy revealed the existence
of trans-trans and gauche-gauche conformations in monoclinic and orthorhombic
crystal structures of 1-butyl-3-methylimidazolium (C 4 mim) cation in combination
with Cl, Br, I, BF 4 , and PF 6 anions [97, 99, 100]. Recently, our group has extensively
studied the HB interactions in C n mim X ILs where n = 2, 4 and X = Cl, Br, I, and
BF 4 and revealed the presence of gauche-trans conformations [103–105]. In C 4 mim
X ion-pairs, two characteristic Raman bands were identified at 600 and 624 cm
−1
for two isomers (gauche and trans) of the butyl chain in imidazolium cation. Also,
significant changes in intensities of these bands were observed with different anions
(Cl, Br, I, and BF 4 ) [103]. In the Raman spectra of C 2 mim X ILs, the vibrational
bands at 3061, 3074, 3090, and 3125 cm
−1 for the C 2 mim X (X = Cl, Br, I, and
BF 4 ) ion-pairs, respectively, are attributed to the C2 − H9 stretch vibrational band
which is a marker band for structural changes of ion-pair interactions [105]. Here,
the noticeable red shift for C 2 mim Cl indicates a stronger HB between cation and
anion compared to ILs composed of Br, I, and BF 4 anions [105]. Additionally, 598
and 620 cm
−1 Raman vibrational bands represent the gauche (non-planar) and trans
(planar) conformation, respectively [105]. Increasing the complexity of anions, from
single atom halide anions and highly symmetric multiatom (BF 4 ) to less symmetric
CF 3 SO 3 (trifluoromethanesulfonate) and NTf 2 (bis(trifluoromethylsulfonyl)imide)
anions, greatly affect the strength of HBs in ion-pairs. In C 2 mim TfO (1-ethyl3-methylimidazolium trifluoromethanesulfonate) IL, the C2-H9 vibrational bands
occurs at 3116 cm
−1 which is at higher wavenumber side in comparison to C 2 mim
X ILs indicates multiple HBs are forming in this ion-pair and exhibits the bifurcated
and chelated structure [104].
161
academia and industrial communities [79, 80]. ILs attracted many researchers owing
to their structural diversity, distinctive, intriguing characteristics such as low vapor
pressure, low melting points, non-volatility, non-flammability, and excellent heat
and chemical stability as well as the unique physicochemical properties [81–86].
The structure of ILs consist of bulky cations and various anions, reveals them as
task-specific configurable, customizable, and designer solvents. Due to the cationanion complex in ILs, various forces such as weak forces (dispersion forces, van
der Walls), specific (charges, dipole and hydrogen bonding (HB), etc.) and strong
(Coulomb) forces are responsible for the novel physical properties. Coulomb force
is only 70% of total energy, suggesting that the weaker interactions cannot be overlooked [87]. In particular, HB is deemed important for ILs structure and interaction
modes [88–94]. Raman spectroscopy is useful not only in HB study in ILs but also in
the identification of the various conformers in crystals and liquid forms of ILs [95–
100]. Alkylammonium nitrate ILs are perhaps the most investigated ILs because they
are the first synthesized ILs in the laboratory [101]. Bodo et al. reported methylammonium nitrate (MAN) crystal structure and revealed the presence of HB interaction
using Raman spectroscopy, where nitrate anion asymmetrically coordinated with
MA cation [95]. Raman spectra revealed in longer alkyl chain ILs that propylammonium cations exhibit trans-conformations in a crystalline low-temperature state
and undergo a crystal polymorphism transition with increasing temperature, where
propylammonium cations exhibited in gauche conformation [96, 102]. Further, in the
case of Imidazolium cation based ILs, Raman spectroscopy revealed the existence
of trans-trans and gauche-gauche conformations in monoclinic and orthorhombic
crystal structures of 1-butyl-3-methylimidazolium (C 4 mim) cation in combination
with Cl, Br, I, BF 4 , and PF 6 anions [97, 99, 100]. Recently, our group has extensively
studied the HB interactions in C n mim X ILs where n = 2, 4 and X = Cl, Br, I, and
BF 4 and revealed the presence of gauche-trans conformations [103–105]. In C 4 mim
X ion-pairs, two characteristic Raman bands were identified at 600 and 624 cm
−1
for two isomers (gauche and trans) of the butyl chain in imidazolium cation. Also,
significant changes in intensities of these bands were observed with different anions
(Cl, Br, I, and BF 4 ) [103]. In the Raman spectra of C 2 mim X ILs, the vibrational
bands at 3061, 3074, 3090, and 3125 cm
−1 for the C 2 mim X (X = Cl, Br, I, and
BF 4 ) ion-pairs, respectively, are attributed to the C2 − H9 stretch vibrational band
which is a marker band for structural changes of ion-pair interactions [105]. Here,
the noticeable red shift for C 2 mim Cl indicates a stronger HB between cation and
anion compared to ILs composed of Br, I, and BF 4 anions [105]. Additionally, 598
and 620 cm
−1 Raman vibrational bands represent the gauche (non-planar) and trans
(planar) conformation, respectively [105]. Increasing the complexity of anions, from
single atom halide anions and highly symmetric multiatom (BF 4 ) to less symmetric
CF 3 SO 3 (trifluoromethanesulfonate) and NTf 2 (bis(trifluoromethylsulfonyl)imide)
anions, greatly affect the strength of HBs in ion-pairs. In C 2 mim TfO (1-ethyl3-methylimidazolium trifluoromethanesulfonate) IL, the C2-H9 vibrational bands
occurs at 3116 cm
−1 which is at higher wavenumber side in comparison to C 2 mim
X ILs indicates multiple HBs are forming in this ion-pair and exhibits the bifurcated
and chelated structure [104].