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

The CH₄ Raman ν₁ symmetric stretching band position was measured at various temperatures and pressures over the range 1–600 bars and 0.3–22 °C. The ν₁ 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. Copyright © 2007 John Wiley & Sons, Ltd.