A readily assembled triple spectrometer for pulsed laser‐excited Raman scattering with multichannel detection: Resonance Raman spectra in solution of a bridged species, [CuI(4,4′ ‐bipyridyl)(PPh3)2 ClO4]m, and of photogenerated transients in metal carbene complexes, (CO)5WC(OR)R′

Abstract

AbstractA simple two‐stage spectrometer for multichannel Raman spectroscopy is described which permits detection by an intensified diode array of Q‐switched laser‐excited Raman scattering at frequencies ≥200 cm−1 from the laser line. It is assembled from a double grating spectrometer operated in subtractive dispersion and coupled to a 0.5 m spectrograph with a 1200 grooves mm−1 grating. Excitation is by means of Q‐switched Nd/YAG laser in tandem with an H2 or CH4 Raman wavelenth‐shifting cell.Raman spectra are reported for two types of metal‐centred system. Solutions in CH2Cl2 or MeOH of a yellow solid (A) formed by reacting the copper (I) salt [Cu(PPh3)4]+[ClO4−] with the ligand 4,4′‐bipyridine (L) exhibit Raman spectra which are dominated by bands in the range 750‐1620 cm−1. The bands are assignable to modes of L enhanced in resonance with a CuI → L charge‐transfer transition. When an excess of PPh3 is added the UV absorption of A shifts to shorter wavelengths and the enhanced modes of L virtually disappear, leaving non‐resonant PPh3 features, consistent with the probe laser lines being off‐resonance with the CuI → L charge‐transfer transition. The findings are consistent with a tentative proposal that A has a chain structure with L as a bridging ligand.Resonance Raman spectra are also reported for solutions in CCl4 of the metal carbene complex (CO)5 WC(OCH3)CH3 at excitation wavelengths of 354.7, 396 and 416 nm. Changes with λexc in the relative intensities of vibrational bands show that the spectra are due to a laser‐generated transient, possibly a solventcoordinated fragment, (CO)4W[S]C(OCH3)CH3, formed following CO photodissociation. Studies using increasing beam focus at 416 nm suggest that solvent substitution at the metal centre causes only minor changes in the vibrational spectrum. Preliminary low‐frequency spectra for a laser‐generated transient in the analogous complex (CO)5WC(OCH3)Ph are described.