1585 Research Article Received: 15 December 2008 Accepted: 15 March 2009 Published online in Wiley Interscience: 5 June 2009 (www.interscience.wiley.com) DOI 10.1002/jrs.2303 FT-Raman, FTIR and density functional theory studies of a hydrogen-bonded formamide : pyridine complex Filipe S. F. Jacinto, a Leonardo J. A. Siqueira b and Wagner A. Alves a* Raman and IR experiments have been carried out on formamide (FA) and pyridine (Py) mixtures at different compositions. The appearance of a new Raman band at 996 cm -1 (ν 1 region of Py), whose intensity depends on the FA concentration, is assigned to an FA : Py adduct and this result is in excellent agreement with those of other authors who employed noisy light-based coherent Raman scattering spectroscopy (I (2) CARS). Another band at 1587 cm -1 (ν 8 region of Py) has been observed for the first time by using Raman and IR spectroscopies. Its intensity shows the same dependence on the FA concentration and this fact allows us to also attribute it to an FA : Py adduct. The good relationship between the Raman and IR data demonstrates the potential of the vibrational spectroscopy for this kind of study. Owing to higher absolute Raman scattering cross section, the ν 1 region of Py has been chosen for the quantitative analysis and a stoichiometry of 1 : 1 FA : Py is reported. The experimental data are very well supported by the density functional theory (DFT) calculation, which was employed for the first time to the present system. Furthermore, the actual investigation shows an excellent agreement with those reported from computational calculations for similar systems. A comparison with our previous studies confirms that the solvent dielectric constant determines the stoichiometry of a given Lewis acid – base adduct in the infinite dilution limit. Copyright c  2009 John Wiley & Sons, Ltd. Keywords: Raman spectra; infrared spectra; binary mixtures; formamide; pyridine Introduction Hydrogen-bonded complexes or Lewis acid–base adducts have been and continue to be intensely studied because of their im- portance in many branches of science. The formation of these compounds in some reaction steps can be one of the reasons for many preferential interactions that take place in the living matter. [1] Several hydrogen-bonded formamide (FA) complexes have been investigated in our laboratory and the results are im- portant for a better understanding on the nature of the hydrogen bond in systems exhibiting the peptide type of bonding and DNA structures. [2] In those works, FA essentially acts as a Lewis acid where its N-H site causes wavenumber shifts of the character- istics bands of acetonitrile (ACN), [3] dimethylsulfoxide (DMSO), [4] tetrahydrofuran (THF), [5] and dioxane (DX). [6] In addition, interac- tions between FA molecules lead to wavenumber shifts of the C O stretching mode (basic site), [7,8] and both these shifts are in good agreement with its amphoteric character. Other compounds often studied in biochemistry are the nitro- gen heterocycles. Such species take part in several vital activities and are omnipresent in biological systems. [9] A large number of researchers has then chosen pyridine (Py) as the simplest nitrogen heterocycle for studying biologically important interactions. [9 – 11] Based on this information, we believe that it would be interesting to investigate mixtures of FA and Py with the purpose to evaluate the effects of hydrogen bonding. A recent study performed by Berg et al. [9] has investigated FA/Py, water (W)/Py and acetic acid (AA)/Py mixtures. Such systems have been studied by means of noisy light-based coherent Raman scattering spectroscopy (I (2) CARS) and the results show that FA forms a weaker hydrogen bond with Py than with W and AA. So far, all spectroscopic studies on hydrogen-bonded Py complexes have used the Raman technique as a tool and the bands between 990 and 1030 cm −1 (ring-stretching modes of Py) as markers for monitoring changes in the local environment of this molecule. [9 – 15] In this paper, we report FT-Raman and FTIR studies of FA and Py mixtures at different compositions and the experiments are designed to (1) show for the first time another spectral region of Py which is strongly perturbed by coordination with hydrogen and (2) determine the stoichiometry of the FA : Py adduct as being an initial step for a future study on the FA adducts’ stability. Owing to its good performance, density functional theory (DFT) was also employed in this investigation to support the experimental evidences, and a good agreement was achieved. Comparisons with similar systems are also presented aiming at a better understanding on the nature of the hydrogen bonding. Experimental Raman spectra were recorded on a Bruker FT-Raman RFS 100/S using the 1064-nm line of a neodymium-doped yttrium aluminium garnet (Nd : YAG) laser and a Ge detector operating at liquid nitrogen temperature. The samples were inserted in nuclear ∗ Correspondence to: Wagner A. Alves, Departamento de Química, Instituto de Ciˆ encias Exatas, Universidade Federal Rural do Rio de Janeiro, Serop´ edica, RJ 23890-000, Brazil. E-mail: waa@ufrrj.br a Departamento de Química, Instituto de Ciˆ encias Exatas, Universidade Federal Rural do Rio de Janeiro, Serop´ edica, RJ 23890-000, Brazil b Instituto de Química, Universidade de S˜ ao Paulo, C. P. 26077, S˜ ao Paulo, SP, Brazil J. Raman Spectrosc. 2009, 40, 1585–1590 Copyright c  2009 John Wiley & Sons, Ltd.