Jabeen, Saima (2007) Experimental (spectroscopic) and theoretical studies of rhodanine (2-Thio-4-Oxothiazolidine) and its derivatives. PhD thesis, University of Greenwich.

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Abstract

There is paucity of data in the scientific literature pertaining to detailed assignments of the vibrational spectra of nitrogen containing thiocarbonyl compounds. Herein, three five membered heterocyclic compounds –rhodanine, 3-aminorhodanine, and 3-methylrodanine-containing both amide and thioamide moieties have been investigated from both an experimental and theoretical perspective. Experimental Raman (?o = 632.8 nm) and infrared spectra of the three compounds have been recorded in the solid state (protonated and deuteriated). Deuteriation studies revealed that not only are the protons attached to the nitrogen atom labile, but the protons of the active methylene group also undergo hydrogen-deuterium exchange. Comparisons of protonated and deuteriated spectra enable discrimination of the bands associated with N-H, NH2, CH2 and CH3 vibrations. Ab initio calculations, using density functional theory (DFT; B3-LYP) employing the cc-pVTZ basis set, have been conducted in order to obtain the geometry optimized, energy minimized structures of the isolated molecules in the gas phase. The data generated was then used to (a) compare, where possible, the calculated structure with the experimentally determined X-ray crystallographic structure, and (b) carry out normal coordinate analysis in order to obtain the potential energy distributions (PEDs) of each normal vibrational mode thus allowing detailed vibrational band assignments for the three molecules. The assignment of the bands associated with the amide and thioamide groups have been made possible by comparing the spectra of the three molecules and DFT calculations. In the case of rhodanine, the cis amide I mode is attributed to the bands at ~1713 and 1779 cm-1, whereas a band at ~1457 cm-1 is assigned to the amide II mode (Raman and IR). The vibrational bands for the thioamide II and III modes of rhodanine, 3-aminorhodanine, and 3-methylrhodanine are observed at 1176 and 1066/1078; 1126 and 1044; 1107 and 984 cm-1 in the Raman and 1187 and 1083; 1230 and 1074; 1116 and 983 cm-1 in the IR spectra, respectively.

Surface enhanced Raman spectroscopy (SERS) studies of the three molecules have been conducted as a function of pH, concentration, and time in order to understand their interaction/absorption behaviour with citrate-reduced Ag and Au colloidal (substrate) nanoparticles, as well as for semi-quantitative trace analysis. The pH studies revealed that the optimum conditions for SERS analysis is the presence of HC1 and poly(L-lysine) at pH 2.3 (?o = 632.8 nm). The SERS studies also revealed the presence of a strong band at 1566/1575/1572 cm-1 (for rhodanine, 3-aminorhodanine and 3-methylrhodanine, respectively), irrespective of experimental conditions used. The 1566 cm-1 band is only present in the solid state dispersive Raman spectrum of rhodanine ( as a very weak band) and absent in the solution (methanol) state Raman spectra of all three molecules. It is hypothesized that upon interaction with Ag and Au SERS substrates all three molecules undergo photo-initiated condensation reactions, which result in the formation of dimers. Ab initio calculations have also been performed on the dimers of rhodanine (B3-LYP/cc-pVTZ basis set), and for its derivatives (B3-L YP/D95 basis sets), followed by normal coordinate analysis. Calculated PEDs of the dimers suggest that the bands at 1566/1575/1572 cm-1 can be assigned to C=C stretching modes. Data from concentration-dependent SER spectra have been used to hypothesize that changes in the SERS profiles are related to the adsorbate concentration which could be due to changes in either the orientation of the analyte molecule on the Ag surface, adsorption via different coordinating sites or different surface coverage effects. The bands associated with thioamide groups in solid state Raman spectra show a large shift upon adsorption to the substrates, which suggest that the analyte molecules are coordinating through the exo-sulfur atom. For semi-quantitative analysis by SERS, low limits of detection (LODs) and good linear (signal vs concentration) correlations have been achieved for all three molecules. SERS studies of rhodanine have been conducted using two laser exciting wavelengths (514.5 and 632.8 nm); the low LODs (i.e., femtomolar, 10- mol/dm-3) are achieved using 632.8 nm excitation. For the rhodanine derivatives, trace analysis has been carried out using 632.8 nm excitation only. The LOD values for the three molecules (632.8 nm excitation) are as follows: rhodanine (10- mol/dm-3) > 3-methylrhodanine (10-13 mol/dm-3) > 3-aminorhodanine (10-12 mol/dm-3).

In order to investigate the aggregation behaviour of Ag and Au colloidal nanoparticles, photon correlation spectroscopic studies have been conducted using a
number of colloidal aggregating agents [poly(L-lysine), KCl, Na2SO4, and Na2S2O3] as a function of concentration and time. The concentration of the three inorganic electrolytes required to achieve optimum SERS signals range from 17-26 mmol dm-3. Size (hydrodynamic diameter) measurements of the SERS substrate nanoparticles show that the aggregation process is dynamic, and depends on time and concentration of added aggregating agent. Zeta ( ½)-potential measurements highlighted the fact that the presence of both poly(L-lysine) and HCl in the colloidal system increases the stability of the system, and slows down the aggregation process. The hydrodynamic diameter ands-potential measurements for rhodanine and its derivatives adsorbed on Ag and Au surf aces, under different experimental conditions, have also been investigated. The results show that rhodanine, in contrast to its derivatives, causes aggregation of Ag and Au colloidal nanoparticles in the absence of aggregating agents. The aforementioned observations are supported by SERS studies of the three molecules (using both Ag and Au substrates) without any aggregating agents; SER spectra are only obtained for rhodanine.

Item Type:	Thesis (PhD)
Additional Information:	uk.bl.ethos.571400
Uncontrolled Keywords:	chemical compounds, heterocyclic compounds, Raman spectroscopy, SERS, quantum chemistry,
Subjects:	Q Science > QC Physics Q Science > QD Chemistry
Pre-2014 Departments:	School of Science School of Science > Department of Chemistry
Last Modified:	30 Oct 2019 15:51
URI:	http://gala.gre.ac.uk/id/eprint/8397

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