160
D. K. Pandey et al.
[52, 53]. In addition, the influence of temperature and pressure contributes to major
changes in the structure and physical properties, which will reflect in the Raman
spectra. Hu et al. recorded in situ Raman spectra of water at a constant pressure of
30 MPa and over a large temperature range from 253 to 753 K [54]. The measurement revealed two significant findings, one is the central frequency blue-shifted
by 200 cm
−1 indicates HB network collapsed above 673 K where water enters
into the supercritical state due to which widespread HB tetrahedron disappeared.
Another is that the Gaussian deconvolution of the Raman contour gives five components below 533 K and four components above 533 K shows that OH groups in
water are involved in stretching vibrations with various energetic states showing
high-temperature dependence. Intermolecular vibrational couplings (IVC) and Femi
resonance (FR) may be the crucial factors in influencing the Raman water spectra
other than HB interactions [55]. The key to spectra interpretation is to consider the
relationship between the FR, inhomogeneous HB structure, couplings, and spectral
characteristics, but it is a difficult task. Many studies suggested that the OH/OD
stretch band represents not just the involvement of HB interactions [44, 55–59], but
it is the participation of IVC/FR whereas others have recognized the isolated water
molecule’s symmetric and antisymmetric modes [59]. In this regard, most recently,
the Hu group discussed the water structure using Raman spectroscopy at a temperature range from 303 to 573 K based on the isotopic substitution (IS) effect [60]. Five
dominant HB configurations are reported in water: two types of tetrahedral, single
donor (SD) HB configuration, single hydrogen-bonded water (SHW), and free water
(FW) without any HBs, represented by five sub-bands. The rise in temperature splits
the HB arrangement and IS further favors the transfer of structure from tetrahedral
to SD, SHW, and FW. Then, temperature and IS considerably reduce the number of
HBs in water. To the current status, this fundamental field still attracts researchers
around the globe, and with the aid of Raman spectroscopy, they are studying the
structure of water not only in pristine form but in various confinement states [45, 61,
62], on polymer surfaces [38, 43, 63, 64], different interfaces [56, 65, 66] and in their
binary mixtures with other solvents or molecular systems [67–74]. Several reviews
articles summarized the different studies that address basic questions on the theory
that governs the interaction between different surfaces and water. Most recently,
the Tian group has studied the influence of 11 hydrated ions on the OH stretching
vibration of water using Raman spectroscopy and revealed that ions primarily break
the tetrahedral HB and promote the formation of partly hydrogen-bonded and free
water molecules [75]. Understanding the structure of HB networks in binary solutions such as ethanol–water is essential for elucidating the role of water molecules
in many biological and chemical processes that occur in aqueous solutions, and the
anomalous properties of water itself [76, 77]. Most recently, the Men group studied
formic acid–water binary mixtures with the aid of Raman spectroscopy and revealed
the effect of formic acid (FA) on the OH stretching band indicating that the structure
of FA-water undergoes two phase transitions [78].
Similar to water, a new class of solvent is ionic liquids (ILs), which have emerged
since the last two decades as a revolutionary material and have many applications
in energy, medical, and advanced material design which makes it attractive between
D. K. Pandey et al.
[52, 53]. In addition, the influence of temperature and pressure contributes to major
changes in the structure and physical properties, which will reflect in the Raman
spectra. Hu et al. recorded in situ Raman spectra of water at a constant pressure of
30 MPa and over a large temperature range from 253 to 753 K [54]. The measurement revealed two significant findings, one is the central frequency blue-shifted
by 200 cm
−1 indicates HB network collapsed above 673 K where water enters
into the supercritical state due to which widespread HB tetrahedron disappeared.
Another is that the Gaussian deconvolution of the Raman contour gives five components below 533 K and four components above 533 K shows that OH groups in
water are involved in stretching vibrations with various energetic states showing
high-temperature dependence. Intermolecular vibrational couplings (IVC) and Femi
resonance (FR) may be the crucial factors in influencing the Raman water spectra
other than HB interactions [55]. The key to spectra interpretation is to consider the
relationship between the FR, inhomogeneous HB structure, couplings, and spectral
characteristics, but it is a difficult task. Many studies suggested that the OH/OD
stretch band represents not just the involvement of HB interactions [44, 55–59], but
it is the participation of IVC/FR whereas others have recognized the isolated water
molecule’s symmetric and antisymmetric modes [59]. In this regard, most recently,
the Hu group discussed the water structure using Raman spectroscopy at a temperature range from 303 to 573 K based on the isotopic substitution (IS) effect [60]. Five
dominant HB configurations are reported in water: two types of tetrahedral, single
donor (SD) HB configuration, single hydrogen-bonded water (SHW), and free water
(FW) without any HBs, represented by five sub-bands. The rise in temperature splits
the HB arrangement and IS further favors the transfer of structure from tetrahedral
to SD, SHW, and FW. Then, temperature and IS considerably reduce the number of
HBs in water. To the current status, this fundamental field still attracts researchers
around the globe, and with the aid of Raman spectroscopy, they are studying the
structure of water not only in pristine form but in various confinement states [45, 61,
62], on polymer surfaces [38, 43, 63, 64], different interfaces [56, 65, 66] and in their
binary mixtures with other solvents or molecular systems [67–74]. Several reviews
articles summarized the different studies that address basic questions on the theory
that governs the interaction between different surfaces and water. Most recently,
the Tian group has studied the influence of 11 hydrated ions on the OH stretching
vibration of water using Raman spectroscopy and revealed that ions primarily break
the tetrahedral HB and promote the formation of partly hydrogen-bonded and free
water molecules [75]. Understanding the structure of HB networks in binary solutions such as ethanol–water is essential for elucidating the role of water molecules
in many biological and chemical processes that occur in aqueous solutions, and the
anomalous properties of water itself [76, 77]. Most recently, the Men group studied
formic acid–water binary mixtures with the aid of Raman spectroscopy and revealed
the effect of formic acid (FA) on the OH stretching band indicating that the structure
of FA-water undergoes two phase transitions [78].
Similar to water, a new class of solvent is ionic liquids (ILs), which have emerged
since the last two decades as a revolutionary material and have many applications
in energy, medical, and advanced material design which makes it attractive between