Raman, Surface-Enhanced Raman, and Density Functional Theory Characterization of (Diphenylphosphoryl)(pyridin-2‑, -3‑, and -4- yl)methanol Edyta Proniewicz,* ,† Ewa Pięta, ‡ Krzysztof Zborowski, ‡ Andrzej Kudelski, § Bogdan Boduszek, ∥ Tomasz K. Olszewski, ∥ Younkyoo Kim, ⊥ and Leonard M. Proniewicz ‡ † Faculty of Foundry Engineering, AGH University of Science and Technology, ul. Reymonta 23, 30-059 Krakow, Poland ‡ Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland § Department of Chemistry, University of Warsaw, ul. L. Pasteura 1, 02-093, Warsaw, Poland ∥ Department of Organic Chemistry, Faculty of Chemistry, Wroclaw University of Technology, Wybrzeż e Wyspiańskiego 27, 50-370 Wroclaw, Poland ⊥ Department of Chemistry, Hankuk University of Foreign Studies, Yongin, Kyunggi-Do 449-791, Korea * S Supporting Information ABSTRACT: This work presents near-infrared Raman spectroscopy (FT-RS) and surface- enhanced Raman scattering (SERS) studies of three pyridine-α-hydroxymethyl biphenyl phosphine oxide isomers: (diphenylphosphoryl)(pyridin-2-yl)methanol (α-Py), (diphenylphosphoryl)(pyridin-3-yl)methanol (β-Py), and (diphenylphosphoryl)(pyridin- 4-yl)methanol (γ-Py) adsorbed onto colloidal and roughened in oxidation−reduction cycles silver surfaces. The molecular geometries in the equilibrium state and vibrational frequencies were calculated by density functional theory (DFT) at the B3LYP 6- 311G(df,p) level of theory. The results imply that the most stable structure of the investigated molecules is a dimer created by two intermolecular hydrogen bonds between the H atom of the α-hydroxyl group (in up (HO U ) or down (HO D ) stereo bonds position) and the O atom of tertiary phosphine oxide (O) of the two monomers. Comparison the FT-RS spectra with the respective SERS spectra allowed us to predict the orientation of the hydroxyphosphonate derivatives of pyridine that depends upon both the position of the substituent relative to the ring N atom (in α-, β-, and γ-position, respectively) and the type of silver substrate. ■ INTRODUCTION Organophosphorus compounds, utilized in the area of chemistry connected with medicine, agriculture, and industry, possess very interesting biological, chemical, and physical properties. 1,2 Among them, α-hydroxy functionalized phospho- nates and phosphine oxides are the most important. These molecules play a crucial role in inhibition of enzymes 3,4 and are potent agents in the treatment for bacterias, 5,6 viruses, 7,8 and tumors. 9,10 This is because these molecules are able to bind a metal. 11 For example, the hydroxyl group is a potent binding site at high pH. It was also found that the phosphine oxides containing pyridine (Py) are effective and highly stable bifunctional homogeneous metal complexes for oxidative catalysis and phase transfer reactions, which show hepatotoxic activity. 12−14 Metal complexes of α-hydroxyphosphine oxides owe their stability to the basic lone pair of electrons on the N atom of pyridine. This lone pair of electrons does not belong to the pyridine π-electrons system. Thus, pyridine serves as a base that has chemical properties like those of tertiary amines. Additionally, at lower pH, the phosphinic oxygen may be also used as an another electron donor. Although the biological significance of tertiary phosphine oxides containing both the pyridine and α-hydroxy fragments is high, the data on this class of compounds are limited. 15−17 Thus, particular interest we placed on α-, β-, and γ- isomers (α, β, and γ are positions of the substituent in respect to the N atom of Py) of pyridine-α-hydroxymethyl biphenyl phosphine oxide (α-Py, (diphenylphosphoryl)(pyridin-2-yl)methanol; β- Py, (diphenylphosphoryl)(pyridin-3-yl)methanol; γ -Py, (diphenylphosphoryl)(pyridin-4-yl)methanol) (Table 1 presents molecular structures of the investigated compounds), performing vibrational spectroscopy studies on these molecules. We applied near-infrared Raman (FT-RS) and absorption infrared (FT-IR) spectroscopies that give the characteristic frequencies of the compounds and provide information on the structure of molecules over a broad range of physical states and temperatures. 18−23 To exactly and certainly explain the measured spectra, we extended our experiments by theoretical analysis using density functional theory (DFT) calculations at Received: April 7, 2014 Revised: July 9, 2014 Published: July 10, 2014 Article pubs.acs.org/JPCA © 2014 American Chemical Society 5614 dx.doi.org/10.1021/jp503392e | J. Phys. Chem. A 2014, 118, 5614−5625