Fig. 6. Raman shift of the H₂ stretching vibration in different isostructural X₂H₂ compounds.

Upper panel. Table reporting the Raman frequency (cm⁻¹) of the H–H stretching vibration of H₂ molecules in different isostructural X₂H₂ crystals (${\nu }_{H_2}$(X₂H₂) with X = CH₄¹⁹, PH₃, H₂S²⁶, H₂Se²⁷, and HI⁵⁷) and their frequency shift (Δν) with respect to bulk H₂ (${\nu }_{H_2}$(H₂)) in comparable pressure conditions. Ref. ¹⁹ directly provides the frequency shift value for CH₄. Lower panel. The molecular structures of CH₄, PH₃, H₂S, H₂Se, and HI, drawn according to the VSEPR theory and fulfilling the octet rule for the electron outer shell, are shown to highlight the absence of lone pairs on C in CH₄, comparing to the other hydride forming elements.