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Combretastatin-A2 (CA2), a potential anticancer drug in advanced preclinical development, is extracted from the medicinal plant Combretum caffrum. The NIR-FT Raman and FT-IR spectral studies of the molecule were carried out and ab initio calculations performed at the B3LYP/6-31G(d) level to derive the equilibrium geometry as well as the vibrational wavenumbers and intensities of the spectral bands. The vibrational analysis showed that the molecule has a similar geometry as that of cis-stilbene, and has undergone steric repulsion resulting in twisting of the phenyl ring with respect to the ethylenic plane. Vibrational analysis was used to investigate the lowering of the stretching modes, and enhancement of infrared band intensities of the C-H stretching modes of Me2 may be attributed to the electronic effects caused by back-donation and induction from the oxygen atom. Analysis of phenyl ring modes shows that the CA2 stretching mode 8 and the aromatic C-H in-planebending mode are equally active as strong bands in both IR and Raman spectra, which can be interpreted as the evidence of intramolecular charge transfer (ICT) between the OH and OCH 3 groups via conjugated ring path and is responsible for bioactivity of the molecule.

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Single crystals of methyl 3-(4-methoxyphenyl)prop-2-enoate were grown by the slow evaporation technique and vibrational spectral analysis was carried out using near-IR Fourier transform Raman and Fourier transform IR spectroscopy. Ab initio quantum computations were also performed at the HF/6-311G (d,p) level to derive the equilibrium geometry, vibrational wavenumbers and intensities and first hyperpolarizability. The large NLO efficiency predicted for the first time in this new class of compounds was confirmed by powder efficiency experiments. Hartree-Fock calculations reveal that the endocyclic angle at the junction of the propeonate group and the phenyl ring is decreased from 120 • by 2.5 • , whereas the two neighbouring angles around the ring are increased by 2.1 • and 1.2 • , associated with chargetransfer interaction. Vibrational analysis indicates the lowering of asymmetric stretching modes of Me1 and Me2 due to the electronic effects simultaneously caused by back-donation and induction due to the presence of the oxygen atom. The occurrence of Fermi resonance is also identified. The carbonyl stretching vibrations were lowered owing to conjugation and the hydrogen bonding network inside the crystal. The vibrational spectra confirm that the charge-transfer interaction between the -COOCH 3 group and phenyl ring through the ethylenic bridge must be responsible for simultaneous IR and Raman activation of C 7 C 18 stretching and ring modes 8 and 19. The large intensity differences observed between the 8a and 8b modes in both the IR and Raman spectra are due to the algebraic difference of the electronic effects of the substitutents. The charge transfer interaction between the -COOCH 3 group and phenyl ring through the ethylenic bridge resulting in p-electron cloud movement from donor to acceptor can make the molecule highly polarized. Intramolecular charge transfer must be responsible for the NLO activity of MMP.