Spectrochimica Acta Part A 79 (2011) 443–450 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Ab initio HF, DFT and experimental (FT-IR) investigation of vibrational spectroscopy of P-N,N-dimethylaminobenzylidenemalononitrile (DBM) M.M. El-Nahass, M.A. Kamel, A.F. El-deeb, A.A. Atta, S.Y. Huthaily ∗ Physics Department, Faculty of Education, Ain Shams University, Roxy, 11757 Cairo, Egypt article info Article history: Received 15 December 2010 Received in revised form 22 February 2011 Accepted 28 February 2011 Keywords: FT-IR spectrum Ab initio and DFT DBM Vibrational analysis abstract The optimized geometry and vibrational frequencies of P-N,N-dimethylaminobenzylidenemalononitrile (DBM) were obtained by ab initio HF and DFT/B3LYP levels with complete relaxation in the potential energy surface using 6-31++G(d,p) and 6-311++G(d,p) basis sets. The Fourier-transform infrared (FT-IR) spectrum of DBM has been recorded in the region 4000–400 cm -1 . The harmonic vibrational frequen- cies were calculated and the scaled values have been compared with experimental FT-IR spectrum. The calculated frequencies are in good agreement with the experimental frequencies. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Disubstituted benzenes of the type D-ph-A, where (D) is an elec- tron donor and (A) an electron acceptor, constitute a broad class of organic molecules with both technological and biological interest. A charge separation may result from the transfer of an electron upon excitation from a donor to an acceptor site within a suit- able molecule. This new charge distribution generally leads to an increase of the molecular dipole moment as well as to structural and electronic rearrangements within the molecule. These com- pounds often undergo relaxation towards a highly polar state and exhibit anomalous emission properties like dual fluorescence [1]. P-N,N-dimethylaminobenzylidenemalononitrile (DBM) dye belongs to a class of organic compounds known as molecular rotors [2], which have great importance by using it as probes to study microscopic structural changes in surrounding media occurring on the picoseconds time scale [2,3]. It has received some interest as a cytotoxic agent against tumours, and as stabilizer in plastics against ultraviolet radiation [3]. Literature survey reveals that to the best of our knowledge no ab initio Hartree–Fock (HF) and density functional theory (DFT) fre- quency calculations of DBM have been reported so far. Therefore, the present investigation was undertaken to study the vibrational spectra, for the first time, of the DBM molecule completely and to identify the various normal modes with greater wavenumber accu- racy. Ab initio HF and DFT calculations have been performed to support our wavenumber assignments. In spite of recent study on ∗ Corresponding author. Tel.: +20 2109707062. E-mail address: s huthaily@yahoo.com (S.Y. Huthaily). vibrational spectra of DBM, we represent a combined theoretical and experimental study for FT-IR spectroscopy using ab initio HF and DFT levels. 2. Experimental details DBM dye was synthesized and used without further purification. FT-IR spectrum of DBM in a powder form was performed using Jasco FT-IR plus 460 spectrophotometer (Japan) in the infrared spec- tral range 4000–400 cm -1 . Good signal-to-noise ratio was obtained from the accumulation of 128 scans with spectral resolution of 4.0 cm -1 . For this investigation 1 mg of DBM powder was mixed with 50 mg of vacuum dried IR-grad KBr. 3. Computational details The entire calculations were performed at HF and DFT/B3LYP levels on a Pentium IV/2.8 GHz personal computer using Gaussian 03W [4] program package, invoking gradient geometry opti- mization [5]. The geometry of the DBM molecule in the ground state is fully optimized. The input geometry of DBM molecule has been first optimized without any constraint in the poten- tial energy surfaces at HF and DFT/B3LYP levels adopting the 6-31++G(d,p) and 6-311++G(d,p) basis sets for C, N and H atoms. The resultant-optimized geometry has been used as input for vibrational frequency calculations at HF and DFT/B3LYP levels to characterize all stationary points as minima. The polarization func- tions have been added for the better treatment of amino group. DFT method, including local or non-local functionals, yields molecular force fields and vibrational wavenumbers in excellent agreement with experiment. Among the numerous available DFT 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.02.055