EXERCISE 12.3: C2H2, acetylene, is shown here. HCCH (a) Is acetylene (C2H2) centrosymmetric? (b) Acetylene has several vibrational modes. One vibrational mode is shown here: Is this vibrational mode Raman active, IR active, or both? EXERCISE 12.3: C2H2, acetylene, is shown here. HCCH (a) Is acetylene (C2H2) centrosymmetric? (b) Acetylene has several vibrational modes. One vibrational mode is shown here: Is this vibrational mode Raman active, IR active, or both