Raman laser temperature-jump technique

Raman earth halide clusters, 46 2-3 Raman laser temperature-jump technique, 32 17-18... 

A major advance in the investigation of the intramolecular dynamics of spin equilibria was the development of the Raman laser temperature-jump technique (43). This uses the power of a laser to heat a solution within the time of the laser pulse width. If the relaxation time of the spin equilibrium is longer than this pulse width the dynamics of the equilibrium can be observed spectroscopically. At the time of its development only two lasers had sufficient power to cause an adequate temperature rise, the ruby laser at 694 nm and the neodymium laser at 1060 nm. Neither of these wavelengths is absorbed by solvents. Various methods were used in attempts to absorb the laser power, with partial success for microsecond relaxation times. 

The Raman laser temperature-jump technique has been used in studies of a variety of spin-equilibrium processes. It was used in the first experiment to measure the relaxation time of an octahedral spin-equilibrium complex in solution (14). Its applications include investigations of cobalt(II), iron(II), iron(III), and nickel(II) equilibria. 

The dynamics of an octahedral spin equilibrium in solution was first reported in 1973 for an iron(II) complex with the Raman laser temperature-jump technique (14). A relaxation time of 32 10 nsec was observed. Subsequently, further studies have been reported with the use of this technique, with ultrasonic relaxation, and with photoperturbation. Selected results are presented in Table III. 

Since all relaxations for these reactions are faster than 1 ysec., the Raman laser temperature jump technique was used (see Fig.l)... 

This report has been written in order to demonstrate the nature of spin-state transitions and to review the studies of dynamical properties of spin transition compounds, both in solution and in the solid state. Spin-state transitions are usually rapid and thus relaxation methods for the microsecond and nanosecond range have been applied. The first application of relaxation techniques to the spin equilibrium of an iron(II) complex involved Raman laser temperature-jump measurements in 1973 [28]. The more accurate ultrasonic relaxation method was first applied in 1978 [29]. These studies dealt exclusively with the spin-state dynamics in solution and were recently reviewed by Beattie [30]. A recent addition to the study of spin-state transitions both in solution and the... 

Consideration of the thermodynamics of a representative reaction coordinate reveals a number of interesting aspects of the equilibrium (Fig. 5). Because the complex is in spin equilibrium, AG° x 0. Only complexes which fulfill this condition can be studied by the Raman laser temperature-jump or ultrasonic relaxation methods, because these methods require perturbation of an equilibrium with appreciable concentrations of both species present. The photoperturbation technique does not suffer from this limitation and can be used to examine complexes with a larger driving force, i.e., AG° 0. In such cases, however, AG° is difficult to measure and will generally be unknown. 

The spin-equilibrium dynamics of iron(III) complexes in solution have been examined with the techniques of Raman laser temperature-jump, ultrasonic relaxation, and photoperturbation. The complexes investigated, the relaxation times observed, and one of the derived rate constants are presented in Table IV. Many of the relaxation times are quite short, and some of the original temperature-jump results (45) were found to be inconsistent with more accurate ultrasonic experiments (20) and later photoperturbation experiments (102). It has not been possible to repeat some of these laser temperature-jump observations. Instead, the expected absorbance changes and isosbestic points were found to occur within the heating rise time of the laser pulse, consistent with the ultrasonic and photoperturbation experiments (20). Consequently, none of the original Raman laser temperature-jump results is included in Table IV. 

T2(Oh) 1A1(Oh) spin equilibrium in iron(II) complexes based on the hy-drotris(l-pyrazolyl)borate ligand, was established for [Fe(HB(me-pz)3)2] (for abbreviations see Sect. 8.1) in the solid state 172>173 and for [Fe(HB(pz)3)2] in solution174 . Sutin et al.17S studied the dynamics of the spin interconversion in CH2C12/CH30H solutions with the laser Raman temperature-jump technique between 0 and 25 °C. The relaxation was observed to be first order with a lifetime of 32 10 ns, independent of temperature and concentration over the range studied. The ki and k j values for the process... 

Both ultrasonic and laser Raman temperature jump techniques have been applied to the measurement of the spin state interconversions (equation 21) in solution.361-369 On the basis of measurements on a range of complexes it was concluded that intersystem crossing rates are faster in solution than in the solid state, as judged from Mossbauer spectra, and faster for the bis complexes of the tridentate ligands than for those of the hexadentate ligands. Data pertaining to some differently substituted [Fe(X-salmeen)2]+ complexes are in Table 5.367... 

Dose et al. have directly measured the kinetics of the interconversion between the two spin states of ferric myoglobin hydroxide using the laser-stimulated Raman temperature-jump technique. For an idealized Fe haem centre of Oh symmetry, the spin-equilibrium is between a low-spin r(A =i) state and a high-spin A(S= ) state ... 

The most significant results obtained for complexes of iron(II) are collected in Table 3. The data derive from laser Raman temperature-jump measurements, ultrasonic relaxation, and the application of the photoperturbation technique. Where the results of two or three methods are available, a gratifying agreement is found. The rate constants span the narrow range between 4 x 10 and 2 X 10 s which shows that the spin-state interconversion process for iron(II) complexes is less rapid than for complexes of iron(III) and cobalt(II). 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

---