Vibrational Raman optical activity in forward scattering: Trans-pinane and β-pinene

The first measurements of vibrational Raman optical activity (ROA) have been performed employing a forward scattering geometry. Forward-scattered ROA spectra of (+)-trans-pinane and (−)-β-pinene are presented as first examples. Comparison with the corresponding backward-and right-angle-scattered spectra confirmed that the bond polarizability theory of ROA is a good approximation for trans-pinane, and has identified unequivocally a large isotropic contribution to the ROA in β-pinene bands at 716 and 765 cm−1 ascribed to interactions between the olefinic methylene twist and a skeletal mode of the pinane structure. Unless large isotropic contributions are present, which is unusual, forward scattering is by far the least favourable of the ROA measurement strategies, which might explain the failure of recent attempts to observe ROA in coherent anti-Stokes Raman scattering