Time-Resolved Infrared Studies, TRIR

Because the carboxylates have relatively long-lived photoexcited states, femtoseconds TRIR studies allow one to probe the charge distribution in the SI and T1 states as a function of time. Particularly helpful in this regard are ligands that have reporter IR groups that are located in a region of the infrared spectrum that is devoid of other vibrations. Thus, C=X bonds, where X = 0, N, or C, that are IR active in the region 2000-2200 cm are ideal. 

DFT calculations for the anion [M2(02CH)2(02CR)2] . In the case of molybdenum complexes, this signal is lost within a few picoseconds, typictJly 2-20 ps, as the T1 state is formed. This is MoMo55, and this state has little effect on the ligand C=X vibration. 

The homoleptic compounds Mo2(02CR)4, where R = CgH -ZJ-CsCH and C(,H -p-CN, have also been examined by TRIR in the region of v(C=X) [81]. With symmetry, the OjCR % orbitals 

Compound Experimental Ground state v(C=X) MLCT state v(C=X) Shift Av(CsX) Calculated Anion shift Av(C=X) References 

Listed in Table 6.6 are the observed values of v(C=X) for these two compounds, where a comparison is also made with related frans-substituted compounds. 

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Time-resolved Infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR spectroscopy (ns), has been outstandingly successful in identifying reactive intermediates [5] and excited states [6] of metal carbonyl complexes in solution at room temperature. We have used infrared spectroscopy to probe the mechanism of photo-17] and electrochemical [8] catalytic reduction of COj. We have used TRIR to study organometallic reactions in supercritical fluids on a nanosecond time-scale [9-10]. 

Time-Resolved Infrared (TRIR) Studies of Organic Reactive Intermediates... 

TIME-RESOLVED INFRARED (TRIR) STUDIES OF ORGANIC REACTIVE INTERMEDIATES... 

Nanosecond time resolved infrared (TRIR) spectroscopy has recently become available to physical organic chemists. This spectroscopy is an attractive tool for studying carbonyl nitrenes. Such work is in progress in several laboratories ... 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

Perutz reported the time resolved infrared (TRIR) study of CpRh(CO)2 in cyclohexane solution. A species was observed with a lifetime of 15 ms, assigned as CpRh(CO)(c-hexyl)H. If CO is present (1.5 atm), the intermediate decays with a half-life of 1.7 ms. Similar observations were made if CpRh(CO)(C2H4) was used to prepare the reactive intermediate. Laser flash photolysis experiments show the formation of the hydrocarbon activation adduct within 400 ns of the flash, but did not provide evidence for an intermediate prior to its formation [34]. 

X-Z Sun, MW George, SG Kazarian, SM Nikiforov, M Poliakoff. Can organometal-lic noble gas compounds be observed in solution at room temperature A time-resolved infrared (TRIR) and UV spectroscopic study of the photochemistry of M(C0)6 (M = Cr, Mo, and W) in supercritical noble gas and CO2 solution. J Am Chem Soc 118 10525-10532, 1996. 

Spectroscopic methodologies have provided a wealth of information concerning the amination of ketenes. Scaiano and co-workers have measured rate constants for ketene reactions with various classes of amines in acetonitrile. The reaction rate is influenced by the basicity of the amine as well as by steric factors in both the ketene and the amine. Ketene amination has also been studied by time-resolved infrared (TRIR) spectroscopy. The strong ketene IR band near 2100 cm" makes it an excellent candidate for study by this spectroscopic technique. Scaiano, Wagner, Lusztyk, and co-workers provided evidence for the first nucleophilic attack being rate determining and that the transition state involves an enol amide. They further found that the asymmetric stretching IR band of substituted ketenes... 

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