Discrimination between hydrogen bonding and protonation in the spectra of a surface-enhanced Raman sensor

Adsorbed mercaptopyridine can sense hydrogen-bonding because the ring breathing mode has a different frequency from bare and protonated species.

Surface enhanced Raman scattering of single 1,4-Benzenedithiol molecular junction

Here, we present simultaneous electronic and optical measurements of a single 1,4-benzenedithiol (BDT) molecular junctions to investigate the electronic and structural details in the molecular junction and to understand the charge transport property at the single molecular scale. The electronic property was investigated by DC conductance measurement while structural property was characterized using surface enhanced Raman scattering (SERS) measurement. The single BDT junctions sandwiched between Au nanogap-electrodes were prepared by the mechanically controllable break junction method at ambient conditions. The simultaneous conductance and SERS measurements demonstrate that ring deformation mode coupled to C–S stretching mode, ring breathing mode, and C=C stretching mode are detectable for the single BDT molecular junctions with electronic conductance of [Formula: see text] [Formula: see text]. The single molecule origin is supported by the characteristic variability of SERS within samples. Time evolution of the conductance and SERS signals indicated that the molecular conductance and the vibrational energy of the ring breathing mode exhibits anti-correlated relationship. This relationship can be mediated by time evolution of structural change in the single molecular junction and corresponding change in strength of metal–molecular coupling. The larger metal–molecular coupling causes higher electronic conductance of the molecular junction while charge transfer effect leads to weakening of molecular bonds and thus a resulting decrease in the vibration energy of the ring breathing mode.

Photoionisation of the tropyl radical

We present a study on the photoionisation of the cycloheptatrienyl (tropyl) radical, C7H7, using tunable vacuum ultraviolet synchrotron radiation. Tropyl is generated by flash pyrolysis from bitropyl. Ions and electrons are detected in coincidence, permitting us to record mass-selected photoelectron spectra. The threshold photoelectron spectrum of tropyl, corresponding to the X + 1A1’ ← X 2E2” transition, reveals an ionisation energy of 6.23 ± 0.02 eV, in good agreement with Rydberg extrapolations, but slightly lower than the value derived from earlier photoelectron spectra. Several vibrations can be resolved and are reassigned to the C–C stretch mode ν16 + and to a combination of ν16 + with the ring breathing mode ν2 +. Above 10.55 eV dissociative photoionisation of tropyl is observed, leading to the formation of C5H5 + and C2H2.

ANALYSIS OF THE VIBRATIONAL SPECTRA OF 3,4-DIHYDROXY- AND 4-HYDROXY-3-METHOXYPHENYLACETIC ACIDS AND THE RING BREATHING MODE IN THEIR RAMAN SPECTRA

The structural stability of 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylacetic acid were investigated using the DFT-B3LYP and the ab initio MP2 methods together with the 6-311G** basis set. The two molecules were predicted at the DFT and MP2 levels of calculation to have the non-planar (np) forms as the lowest energy conformer of the acids. Another very low lying conformer of the former acid in the gas phase seems to play no role in the condensed phase. The observed spectral intensities of the acids were consistent with the np conformation being the predominant form at room temperature. The vibrational wavenumbers were computed at the B3LYP level of theory and vibrational assignments were provided on the basis of combined theroretical and experimental infrared and Raman data of the molecules. As reported earlier in the case of chlorine substituted anilines, the ring breathing mode, which is expected to have a very high intensity in the Raman spectra of substituted benzenes, diminishes in intensity with increasing electron density in the ring (activation for electrophilic attacks).

Detection of Chlorinated Hydrocarbons in Aqueous Surfactant Solutions by Near-IR Raman Spectroscopy

Near-IR Raman spectroscopy is used to detect chlorinated hydrocarbons under surfactant-enhanced solubilization conditions. The Raman bands of tetrachloroethylene (PCE) and 1,2-dichlorobenzene (DCB) in micelle solutions could be observed in the presence of humic acid when a 784-nm diode laser was used. With 532- or 632.8-nm excitation, humic acid fluorescence obscured the Raman signals. For quantification, the integrated area of the carbon-chlorine stretch mode (PCE) or the phenyl ring-breathing mode (DCB) was used. Test results for samples with unknown concentrations based on linear calibration curves were in agreement with results from an accepted gas chromatography method. Detection limits were calculated to be 240 ppm for tetrachloroethylene and 500 ppm for 1,2-dichlorobenzene. Our study has shown the feasibility of this technique for field applications.