Modulated symmetric and anti-symmetric stretching vibrations SRS of –CH ₃ in aqueous EGME solution by hydrogen bonding formed with –O– functional groups

The immiscibility between ether and water hinders the hydrogen bonding (HB) formed between -OH and -O- functional groups. In this study, regulating the dominant vibrational mode of -CH₃ by HB formed with -O- functional groups was demonstrated by hydrophilic ethylene glycol methyl ether (EGME) mixed with H₂O, which realized switching stimulated Raman scattering (SRS) peak between symmetric vibrations (2829 cm^-1) and anti-symmetric vibrations (2939 cm^-1) of -CH₃ at critical volume fractions V_EGME = 0.7 and 0.4. The self‑association of EGME transforms to the hydration structures of EGME-H₂O, because the intermolecular HB network restructures. Density functional theory calculation results reveal that HB formed with -O- group alters electron cloud density in the O-C-H bond, resulting in an asymmetric shift of the polarization close to the O-C-H bond. In the anti-symmetric vibrations of -CH₃, one C-H bond elongates while the others shorten, and this asymmetric motion is highly coupled to an asymmetric electronic environment that is already polarized, leading to an increase in the derivative of polarizability (∂<i>α</i>/∂Q), which preferentially enhances the SRS of -CH₃ anti-symmetric vibration. The work show that HB formed with -O- functional groups can selectively regulate SRS of intramolecular vibrational modes, providing a framework for the HB modulating coherent radiation technique.