Orientation change of adsorbed pyrazine on roughened rhodium electrodes as probed by surface-enhanced Raman spectroscopy

A surface-enhanced Raman spectroscopic (SERS) study of pyrazine adsorbed on roughened Rh electrodes was performed. Potential and concentration effects on the adsorption behavior of pyrazine were investigated. The SER spectra display four pairs of overlapping bands with the relative intensity of each pair being highly potential dependent, which has not been observed on other metals. The orientation change of the adsorbed pyrazine from the end-on to N/pi bonded edge-on configuration is proposed to account for this potential-dependent relative intensity change. This hypothesis is further supported by the SERS results obtained at different pyrazine concentrations. In conjunction with the orientation effect, the interaction of Rh with hydrogen and oxygen generated at different potentials has a great influence on the adsorption configuration of pyrazine.     

The effects of protonation on the surface-enhanced Raman scattering from ATP molecules adsorbed on the Ag electrode

The voltage dependence of the SER spectra of adsorbed ATP molecules has been found to vary with the pH values of the solutions. The differences in the spectra are explained by the protonation of the adsorbed ATP molecules.     

Raman spectra of pyridine adsorbed on a copper electrode

The surface enhanced Raman scattering (SERS) of pyridine adsorbed on a copper electrode was observed with 647.1 nm excitation and the dependence of the Raman intensity as a function of electrode potential was obtained for the most intense bands. The results are compared with reported intensity and frequency values for the silver/pyridine system.     

Simultaneous Raman spectroscopic and differential double-layer capacitance measurements of pyridine adsorbed on roughened silver electrodes

It is shown that differential double-layer capacitance data can be obtained simultaneously with the recording of the Raman spectra of adsorbates at a roughened silver microelectrode. The use of such data to provide information about the structure of the silver electrode/aqueous KCl (0.1 M)/pyridine (0.1 M) interface is demonstrated.     

Time development of surface enhanced Raman intensities of the adsorbed pyrazine molecules on the silver electrode as the applied voltage varies

In this work,the time development of surface enhanced Raman intensities of the vibrationalmodes involving the ring skeleton and the C—H motions of the pyrazine molecule as the applied voltageon the silver electrode is shifted between -0.2 V and -0.5 V (vs.saturated calomel electrode)isreported.The observation shows that there are two processes of enhancement involved during thisshift of applied voltage.One is vibrational mode dependent while the other is not.The detailedbehavior of these two processes of enhancement is discussed along with the retarded response of theHelmholtz double layer structure to the change of the applied voltage on the electrode.     

Voltage-dependent excitation profiles of pyridine adsorbed on a silver electrode

Excitation profiles of Raman modes of pyridine adsorbed on an Ag electrode were measured at three applied voltages. A resonant type of contribution was observed, the resonance energy was voltage dependent. Our observations were attributed to the voltage-dependent effects of charge transfer transitions.     

Variation of bond polarizabilities of adsorbed pyrazine molecules on the silver electrode as a function of applied voltage from surface enhanced Raman scattering

An algorithm using the Wolkenstein approximation to calculate the bond polarizabilities of adsorbed pyrazine molecules on the Ag electrode from the surface Raman intensities has been employed. The result shows that the pyrazine molecule is end-on adsorbed on the electrode surface instead of the face-on configuration. The large variation of the CN bond polarizability with applied voltage shows that there is strong perturbation on the ring charge from the electrode and electron sharing between them is expected. The variation of the CC and CH bond polarizabilities is however relatively small. As the applied voltage is shifted toward −0.8 V (with respect to a saturated calomel electrode) the bond polarizabilities decrease, in general, while in the piperidine case the situation is opposite. This may be due to the different behaviour of the aromatic and saturated ring in surface enhanced Raman scattering (SERS).     

Vibronic coupling by out-of-plane modes in pyridine,pyrazine and quinoxaline

Detailed spectroscopic observations on the first ¹n, π^* states of three different N-heterocycles are successfully modelled through vibronic coupling calculations incorporating a small number of adjusted but non-arbitrary parameters: in pyridine, the “pseudo-planar” potential of the out-of-plane vibration 5b₁(=ν_16b); in pyrazine, the very anharmonic vibration of surprisingly low frequency, 1b_1g(=ν_10a); in quinoxaline, vibronic coupling through more than one frequency.     

Investigation of condensed pyrimidine,pyrazine, and pyridine systems

The reaction of 5-hydroxy-6-aminopyrimidines with chloromalonic ester in alcohol or with bromomalonic ester in dimethylformamide leads to 6-carbethoxypyrimido-7-oxazinones, whereas a mixture of 6-carbethoxypyrimido-7-oxazinones and 6,6-spirodipyrimido-7-oxazinones is formed in the reaction with bromomalonic ester in alcohol. The latter oxazinones were obtained by reaction of 5-hydroxy-6-aminopyrimidines with dibromomalonic ester. Some of the properties of the synthesized compounds were studied.See [1] for communication XXV.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 391–396, March, 1978.     

Derivatives of condensed pyrimidine,pyrazine, and pyridine systems

Reaction of 2-mercapto-3-amino-5,6-dimethyl- and 2-mercapto-3-amino-5,6-diphenylpyrazines withα-halo acid esters gave 2-carbethoxy-3-aminopyrazines, which are converted by the action of sodium ethoxide to 5,6-dimethyl- and 5,6-diphenylpyrazino[2,3-b]-[1,4]thiazin-6-ones. The latter are more conveniently obtained from 2-chloro-3-amino-5,6-dimethyl- and 2-chloro-3-amino-5,6-diphenylpyrazines and thioglycolic acid. 5,6-Dihydropyrazinothiazine is formed by reduction of 5,6-dimethyl pyrazino[2,3-b][1,4]thiazin-6-one, whereas the 2,3-dimethyl-5-amino-6-sulfonic acid and its N-oxide are formed by oxidation.     

Derivatives of condensed pyrimidine,pyrazine, and pyridine systems

4-Amino-, 4-methylamino-, and 4-dimethylamino-6-aminopyrimido[4,5-b]-1,4-thiazines were obtained. Some of the properties and transformations of 6-aminopyrimido[4,5-b]-1,4-thiazines — their behavior with respect to acids, reductive desulfuration under the influence of Raney nickel, and reaction with hydrazine — were studied.     

Derivatives of condensed pyrimidine,pyrazine, and pyridine systems

Pyrazinothiazines and the corresponding 3-hydroxy-3,4-dihydroquinoxalinothiazines, which are dehydrated to quinoxalinothiazines under the influence of acidic agents, are formed in the reaction of 2-mercapto-3-amino-5,6-dimethylpyrazine and 2-mercapto-3-aminoquinoxalines with - or -chloroacetoacetic esters in the presence of an equimolar amount of alkali. The IR, UV, and PMR spectra are presented.See [1] for communication XXXIV.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 695–697, May, 1977.     

Study of condensed pyrimidine,pyrazine, and pyridine systems

A new method was developed for the synthesis of pyrimido [5, 4-b][l, 4] oxazin-7-ones by O-alkylation of 5-hydroxy-6-aminopyrimidines with -halo carboxylic acids and subsequent cyclization of the resulting pyrimdyloxyacetic acids in acetic anhydride. The reaction of chloropyrimidooxazinones with hydrazine gives the corresponding hydrazinopyrimidooxazinones, from which the azides were obtained. Unsubstituted pyrimido [5, 4-b][l, 4] oxazin-7-one was synthesized.See [1] for communication XXXIII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 681–685, May, 1976.     

Derivatives of condensed systems based on pyrimidine,pyrazine, and pyridine

Pyrimido[4,5-b]-1,4-thiazin-6-one and pyrimido[4,5-b]-1,5-thiazepin-6-one derivatives were obtained by reaction of 5-amino-6-chloropyrimidines with thioglycolic acid and 5-amino-6-mercaptopyrimidines with -bromopropionyl chloride. The IR spectra of the compounds are presented.See [1] for communication XXXV.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1332–1335, October, 1977.     

Derivatives of condensed pyrimidine,pyrazine, and pyridine systems

It is shown by means of IR, UV, and PMR spectroscopy that 5-amino-6-thio-1,6-dihydropyrimidines exist primarily in the thione form both in the crystalline state and in solution.See [1] for communication XXXIII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 398–401, March, 1977.     

Direct visual evidence for chemical mechanism of SERRS of pyrazine adsorbed on Ag nanoparticle via charge transfer

In this paper, the chemical enhancement of surface-enhanced resonance Raman scattering (SERRS) of pyrazine adsorbed on Ag nanoparticles through charge transfer was experimentally and theoretically investigated. Based on the calculations by density functional theory (DFT) and time-dependent DFT (TD-DFT), we theoretically analyzed the absorption spectra and SERS spectrum of the S-complex of pyrazine–Ag₂₀. The charge transfer in the process of resonant electronic transitions between adsorbed molecule and metal cluster can be visualized by the method of charge difference density. It is a direct evidence for the chemical enhancement mechanism of SERRS of pyrazine molecule adsorbed on Ag nanoparticle via charge transfer between molecule and metal. Additionally, the intracluster charge redistribution was also considered as an evidence for the electromagnetic enhancement. By comparing the experimental and theoretical results, it was demonstrated that the SERRS of the pyrazine molecule absorbed on silver clusters in different incident wavelength regions is dominated by different enhancement mechanisms via the chemical and electromagnetic enhancements.     

Free radical chlorination of methyl derivatives of pyridine,pyrazine, and thiazole by N-chlorosuccinimide

When methylazines (2-, 3-, and 4-methylpyridines, methylpyrazine) are treated with N-chlorosuccinimide they undergo successive chlorination of the methyl group to give 2-chloromethylpyridine, 2-dichloromethylpyridine, and dichloromethylpyrazine in preparative yields. 3-Dichloromethylpyridine was synthesized from pyridine-3-aldehyde and PC1₅. The primary chlorination products of 4-methylthiazole are 4-methyl-5-chlorothiazole and 5-chloro-4-chloromethylthiazole.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 543–546, April, 1989.     

ESR Spectra of radicals produced by reduction of pyridine and pyrazine

Radicals produced by reaction of e^?_aq with pyridine, pyrazine, and pyrazinedicarboxylic acid have been studied by electron spin resonance using the in situ radiolysis steady-state ESR technique. The radical anions initially produced have been found to undergo rapid protonation on nitrogen to form pyridinyl and pyrazinyl radicals. The NH proton of pyridinyl radical does not dissociate even at pH 13.7. The radical from pyrazine has been observed only in the doubly protonated positively charged form in acid and neutral solutions, but no spectrum was observed in alkaline media. With 2,3-pyrazinedicarboxylic acid the doubly protonated radical has been observed at pH 4–8 and the singly protonated one at pH 11–12. The pK for this dissociation is 9.2. The hyperfine constants of the pyridinyl radical are compared with those obtained from INDO molecular orbital calculations.     

Redox transformations of adsorbed NO molecules on a Pt(100) electrode

The electrochemical behavior of adsorbed NO molecules on a Pt(100) electrode has been studied in perchloric acid solutions by means of cyclic voltammetry. According to the literature data, a saturated NO adlayer with a coverage of ～0.5 monolayers (MLs) is formed under open circuit conditions in an acidic nitrite solution as a result of a disproportionation reaction. The saturated adlayer is stable in the potential range of 0.4–0.9 V vs. a reversible hydrogen electrode in 0.1 M HClO₄. NO molecules are oxidized at 0.9–1.1 V with the formation of adsorbed nitrite anions, and they can be reduced to ammonia at potentials less than 0.4 V. In this paper it has been shown that the adlayer stability depends on the surface coverage and extent of ordering. An unsaturated NO adlayer demonstrates NO ? NH₃ redox transformations at 0.5–0.8 V.