Vibrational and Theoretical Studies of the Structure and Adsorption Mode of m‑Nitrophenyl α‑Guanidinomethylphosphonic Acid Analogues on Silver Surfaces Edyta Proniewicz,* ,† Ewa Pięta, † Andrzej Kudelski, ‡ Natalia Piergies, † Dominika Skoluba, † Younkyoo Kim, § and Leonard M. Proniewicz † † Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland ‡ Faculty of Chemistry, University of Warsaw, ul. L. Pasteura 1, 02-093 Warsaw, Poland § Department of Chemistry, Hankuk University of Foreign Studies, Yongin, Kyunggi-Do, 449-791, Korea * S Supporting Information ABSTRACT: This work presents Fourier transform Raman (FT-Raman), Fourier transform absorption infrared (FT-IR), and surface-enhanced Raman scattering (SERS) spectroscopic investigations of three m-nitrophenyl α-guanidinomethylphonic acids, including m-NO 2 PhG(cHex)P, m-NO 2 PhG(Morf)P, and m-NO 2 PhG(An)P, adsorbed onto colloidal and roughened silver surfaces. The SERS spectra were deconvoluted to determine the overlapped bands from which the specific molecular orientation can be deducted. The vibrational wavenumbers are calculated through density functional theory (DFT) at the B3LYP/6-31+ +G** level with the Gaussian 03, Raint, GaussSum 0.8, and GAR2PED software packages. The experimental and calculated vibrational bands are compared to those from SERS for the investigated compounds adsorbed on colloidal and roughened silver surfaces. The geometry of these molecules on the SERS-active silver surfaces is deduced from the observed changes in both the intensity and width of the Raman bands in the spectra of the bound species relative to the free species. ■ INTRODUCTION Over the last few decades, there has been increasing interest in the applications of phosphorus analogues of amino acids composed of at least one CP(O)(OH) 2 group because phosphonates are chemically and enzymatically stable, 1 are highly water-soluble, and are sparingly soluble in organic solvents. 1,2 In addition, the functional groups of these compounds characterize a wide range of applications from medicine to agriculture. 3,4 For example, analogues of these compounds have been used as enzyme inhibitors (i.e., cholesterol, angiotensin, HIV protease, phenylalanine ammo- nia-lyase, and the parasite that causes malaria (Plasmodium falciparum)), antibacterial agents, antibiotics, neuroactive compounds, andmost promisinglyanticancer agents. 1,5-8 The analogues of amino acids can penetrate cancer cells much more easily than normal cells. 2 Phosphonates are also known to be substrates for the treatment of osteoporosis. 1 They are commonly employed to control pests as insecticides, herbicides (i.e., against cress (Lepidium sativum) and cucumber (Cucumis sativus)), and growth regulators for plants. 2,8,9 α-Guanidino- phosphonic acids additionally contain the guanidine group,  NHC(NH)NH 2 . This structural modification is thought to be responsible for neuroactive properties and for fungicidal and herbicidal activities. 5,9 Another modification with nitrobenzene (a compound highly soluble in organic solvents, such as alcohol or benzene, and in lipids, but slightly soluble in water) demonstrates interesting properties. 10,11 Although many nitro-substituted aromatics are classified as carcinogenic, mutagenic, and toxic (generally, these effects are more severe with the ortho- and para-isomers than with the meta-isomer), 12 these compounds are also widely used as pharmaceuticals, food additives, antimicrobial agents, and Received: March 27, 2013 Revised: May 17, 2013 Published: May 17, 2013 Article pubs.acs.org/JPCA © 2013 American Chemical Society 4963 dx.doi.org/10.1021/jp403045g | J. Phys. Chem. A 2013, 117, 4963-4972