Correlation of methane Raman ν₁ band position with fluid density and interactions at the molecular level

The CH 4 Raman ν 1 symmetric stretching band position was measured at various temperatures and pressures over the range 1–600 bars and 0.3–22 °C. The ν 1 symmetric stretching band shifts to lower wavenumbers with increasing pressure and/or decreasing temperature, and shows a systematic correlation with density over this same range. Ab initio calculations (MP2/aug‐cc‐pVTZ) show that the changes in peak shape and position reflect fundamental distortion of the methane structure as the density increases.