Step-scan FTIR

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

Step-growth reactions, 24 16 Step-scan ftir photoacoustic analysis, 19 564... 

Figure 4 Step-scan FTIR spectra of 1-naphthyl acetate photodissociation and carbonyl species). (Adapted from Ref. 56.)...

These conclusions were supported by transient absorption spectroscopy, which revealed signals corresponding to the formation of the diimine radical anion, with lifetimes in close agreement with the luminescence lifetimes. Time-resolved infrared spectroscopy of the acetylide C = C bonds provides further conclusive evidence for the MLCT assignment. Thus, in the ground state IR spectrum of 4, there are two v(C=C) bands at 2115 and 2124 cm-1, whilst the step-scan FTIR difference spectrum obtained 50 ns after irradiation at 355 nm reveals bleaching of the parent bands, and the formation... 

The fact that the final product 3-Tp Rh(CO)(H)(R) does not appear on the ultrafast time scale (<1 ns,) (Fig. 4) indicates a free energy barrier greater than 5.2 kcal/mol for the alkane C-H bond activation. Nanosecond step-scan FTIR experiments on the 3-Tp Rh(CO)2/cyclohexane system show that the remnant of the 2-Tp Rh(CO)(S) peak persists for 280 ns after photoexcitation, while the product CO stretch at 2032 cm-1 rises with a... 

Step-scan FTIR systems are available commercially from several vendors. Unfortunately, they are (despite manufacturer s claims) unsuitable for time-resolved measurements without extensive system modifications. Typically, the illumination optics must be modified to permit a tighter focus of the... 

IR and Raman spectra of copper(II) complexes of histamine gave evidence for the formation of [Cu2(L H)2]2+, CuL2 and CuL2+ at high pH, Cu(LH)2, CuL2- and CuL2+ at lower pH all with coordination through the imidazole moiety.251 Time-resolved step-scan FTIR spectroscopy was used to probe the... 

Rammelsberg, R., Boulas, S., Chorongiewski, H. and Gerwert, K. (1999) Set-up for time-resolved step-scan FTIR spectroscopy of noncyclic reactions. Vih. Spectrosc., 19, 143-149. 

To obtain IR spectra on a time scale of nanoseconds, the sample cell in conventional spectrometers is usually excited by an Nd YAG laser. Flow cells with a pathlength of at least 0.1 mm must be used for photoreactive samples and the pulse repetition frequency is then limited to 1 Hz. In step scan FTIR spectroscopy,211 the time evolution is collected at single points of the interferogram, which is then reconstructed point-by-point and subsequently transformed to time-resolved IR spectra. Alternatively, dispersive instruments equipped with a strong IR source can be used.212 The time resolution of both methods is about 50 ns. FTIR instruments provide a triggerable fast-scan mode to collect a complete spectrum within a few milliseconds.213... 

The overall diagram of evolution of the excited states and reactive intermediates of a photoinitiating system working through its triplet state can be depicted in Scheme 10.2 [249]. Various time resolved laser techniques (absorption spectroscopy in the nanosecond and picosecond timescales), photothermal methods (thermal lens spectrometry and laser-induced photocalorimetry), photoconductivity, laser-induced step scan FTIR vibrational spectroscopy, CIDEP-ESR and CIDNP-NMR) as well as quantum mechanical calculations (performed at high level of theory) provide unique kinetic and thermodynamical data on the processes that govern the overall efficiency of PIS. 

Time-resolved infrared spectroscopy (TRIR) has been outstandingly successful in identifying reactive intermediates and excited states of both metal carbonyl [68,69] and organic complexes in solution [70-72]. Some time ago, the potential of TRIR for the elucidation of photochemical reactions in SCFs was demonstrated [73]. TRIR is particularly suited to probe metal carbonyl reactions in SCFs because v(CO) IR bands are relatively narrow so that several different species can be easily detected. Until now, TRIR measurements have largely been performed using tunable IR lasers as the IR source and this has restricted the application of TRIR to the specialist laboratory [68]. However, recent developments in step-scan FTIR spectroscopy promise to open up TRIR to the wider scientific community [74]. 

Like laser-based TRIR [68], step-scan FTIR uses repetitive UV photolysis to build up the data. The difference is that the photolysis and data collection in step-scan FTIR is fully automated. Therefore a wider spectral range can be covered much more rapidly than with laser-based TRIR. The difference between laser based and step-scan TRIR measurements is shown in Figure... 

Figure 3.1-9 TRIR data obtained following irradiation of W(CO)e in seXe in the presence of CO. (a) A series of TRIR spectra (in 1 ps increments) obtained using the step-scan FTIR set-up displayed in Figure 3.1-8. (b) A TRIR spectrum obtained I ps after photolysis using the IR laser based TRIR. In (b) represent the data points and the solid line is a curve fit through these data. In both (a) and (b) negative peaks indicate loss of W(CO)6 and positive peaks indicate formation of W(CO)5(Xe). The step-scan data show that W(CO)5(Xe) decays to reform W(CO)e and to form a new species that can be assigned to W(CO)s(H20). This demonstrates that trace impurities in supercritical fluids may have a dramatic effect on the course of a reaction.

Figure 3.1-10 Step-scan FTIR spectra obtained (a) 1 ps after irradiation (532 nm) of trans-[CpMo(CO)3l2 in SCCO2. The TRIR spectrum shows formation of the [CpMo-(COlsl radical formed by cleavage of the Mo-Mo bond, (b) The [CpMo(CO)3l radical decays by second-order kinetics at a diffusion-controlled rate to form rra/tJ-[CpMo(CO)3]2 and the unstable gauche-[CpMo(CO)3 2 (see spectrum (c) obtained 20 ps after irradiation), (d) The unstable isomer decays via first-order kinetics to form the more stable trans-[CpMo(CO)3l2 isomer.

Figure 3.1-10 illustrates how step-scan FTIR can be used to monitor more complicated photochemical reactions in SCFs. Visible photolysis of trans-[CpMo(CO)3l2 in scCOa generates [CpMo(CO)3] radicals which recombine at a diffusion controlled rate to form the stable trans and unstable gauche forms of [CpMo(CO)3]2. Gauche-[CvMo(CO)-i 2 slowly isomerizes to trans-[CpMo(CO)3l2 (Scheme 3.1-2). 

Using step-scan FTIR, we can obtain spectra of both transient species, the [CpMo(CO)3] radical and gauche-[CpMo(CO)-i 2- In addition, we can measure the reaction kinetics of both the radical recombination and the gauche to trans isomerization. In the near future, this type of monitoring is likely to make a major contribution towards understanding the mechanistic aspects of synthetic chemistry in SCFs. 

To obtain TRIR spectra with sufficient sensitivity, we typically signal average several thousand laser shots at each IR frequency of interest. A flow cell, therefore, is necessary to prevent excessive sample decomposition, especially when photo-irreversible processes are monitored. A reservoir of solution (typically 10-20 mL) is continually circulated between two calcium or barium fluoride salt plates. To maintain sample integrity for non cyclic systems, one is usually forced in the dispersive TRIR experiment to acquire data in a series of short (e.g., 100-200 cm ) scans rather than in one complete scan. Thus, a substantial amount of sample may be required. Sample integrity is also of significant concern in the step-scan FTIR experiment because data must be collected at each mirror position. To address this concern, very large reservoirs of solution are required alternatively, a sample changing wheel [33] or very focused pump-probe beams in combination with sample translation [34] have been used with thin film samples. 

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