Vibrational spectral studies of methyl 3-(4-methoxyphenyl)prop-2-enoate, a new organic non-linear optic crystal

Abstract

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 charge-transfer 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 COOCH3 group and phenyl ring through the ethylenic bridge must be responsible for simultaneous IR and Raman activation of C7C18 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 COOCH3 group and phenyl ring through the ethylenic bridge resulting in π-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. Copyright © 2004 John Wiley & Sons, Ltd.