The vibration of the formic acid molecule is composed of 9 normal modes. Four modes are stretching the molecule along the OH, CH, C=O, and C-O bonds respectively. All are are symmetric with respect to the σh reflection plane. The other five modes describe angular deformations. Two of the five deformation modes are antisymmetric with respect to the σh reflection plane. The remaining deformation modes are symmetric with respect to the σh reflection plane.
Formic acid is in the Cs point group
with the symmetry element: σh reflection plane (cyan).
normal modes of the formic acid molecule
ν1(A') | νOH
ν2(A') | νCH
ν3(A') | νC=O
ν4(A') | δsCH
ν5(A') | δsOH
ν6(A') | νCO
ν8(A'') | δasCH
ν9(A'') | δasOH
ν7(A') | δsOH