The structure of 3-iodobenzaldehyde (3IB) was characterized by FT-IR, Raman and single-crystal X-ray diffraction techniques. The conformational isomers, optimized geometric parameters, normal mode frequencies and corresponding vibrational assignments of 3IB were examined using density functional theory (DFT) method, with the Becke-3-Lee-Yang-Parr (B3LYP) functional and the 6-311+G(3df,p) basis set for all atoms except for iodine. The LANL2DZ effective core basis set was used for iodine. Potential energy distribution (PED) analysis of normal modes was performed to identify characteristic frequencies. 3IB crystallizes in monoclinic space group P21/c with the O-trans form. There is a good agreement between the theoretically predicted structural parameters, and vibrational frequencies and those obtained experimentally. In order to understand halogen effect, 3-halogenobenzaldehyde [XC6H4CHO; X=F, Cl and Br] was also studied theoretically. The free energy difference between the isomers is small but the rotational barrier is about 8kcal/mol. An atypical behavior of fluorine affecting conformational preference is observed.

10 ray Crystallography Unit School of Physics Universiti Sains Malaysia 11800 USM Penang Malaysia

10 ray Crystallography Unit School of Physics Universiti Sains Malaysia 11800 USM Penang Malaysia; Department of Engineering Chemistry Alva's Institute of Engineering and Technology Mijar Modbidri 574225 Karnataka India

10 ray Crystallography Unit School of Physics Universiti Sains Malaysia 11800 USM Penang Malaysia; Department of Pharmaceutical Chemistry College of Pharmacy King Saud University Riyadh 11451 Saudi Arabia

Computational Chemistry Group Department of Chemistry Faculty of Science University of Mauritius Réduit 80837 Mauritius

Department of Pharmaceutical Chemistry College of Pharmacy King Saud University Riyadh 11451 Saudi Arabia

Department of Pharmaceutical Chemistry College of Pharmacy King Saud University Riyadh 11451 Saudi Arabia; Computational Chemistry Group Department of Chemistry Faculty of Science University of Mauritius Réduit 80837 Mauritius

Department of Physics Dumlupinar University Kütahya 43100 Turkey

Department of Sugar Technology and Chemistry Sir M Visvesvaraya PG Center University of Mysore Tubinakere Mandya 571402 India

Institute of Physics and Biophysics Faculty of Science University of South Bohemia Branišovská 31 České Budějovice 370 05 Czech Republic

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