Radicals Fourier transform spectroscopy

Fourier transform methods have come into their own as a means of studying the optical spectra of gas-phase radicals. Both infrared (FTIR) and ultraviolet/visible spectroscopy (FTUV/VIS) are now used to scrutinize these reactive molecules. We discuss the underlying principles of Fourier transform spectroscopy (FTS) with particular emphasis on the advantages and drawbacks of FTIR and FTUV/VIS measurements. Extensive tables are presented of metastable molecules that have been studied by Fourier transform methods. 

Laboratory spectroscopic studies of atmospherically important radicals using Fourier transform spectroscopy,... 

NH radicals in higher electronic states can be detected by emission as mentioned above for the A state at 336 nm [54, 58, 62, 97] this A Tl-X emission was also observed with Fourier transform spectroscopy [47, 53]. Those electronically excited NH states, which have allowed transitions to lower electronic states, can easily detected by emission, but also a spin-forbidden, dipole-allowed radiative transition b S - X was observed... 

Since the mid sixties there has been a continual evolution in Fourier transform spectrometers which has recently culminated in a very radical expansion in the applications of Fourier transform spectroscopy. This evolution has arisen particularly from the truly remarkable evolution in microelectronic circuitry and computer technology. 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

The primary 1-methylthymine cation radical (44) and three successor radicals, the N3-deprotonated cation radical (45), the C6-OH 5-yl radical (46) and a C6-0P032 5-yl addition radical (47), were characterized using Fourier Transform EPR spectroscopy (FT EPR) in aqueous solution at room temperature.36... 

FTIR Spectroscopy and Mechanisms on Electrode. The basis of Fourier transform infrared spectroscopy was described in Section 6.2.6. One of the more difficult aspects of detecting the mechanism of electrode reactions is that of knowing the nature of the intermediate radicals on the electrode surface. Infrared spectroscopy measures chemical bonds, so it is an ideal method for detecting which bonds are present and hence which intermediate radicals are taking part in a surface reaction at a given potential, etc. 

The pronounced relaxation acceleration observed for slowly relaxing nuclei in the presence of paramagnetic compounds is exploited in Fourier transform, 3C NMR spectroscopy. On use of the fast pulse sequences that are frequently necessary, the spin-lattice relaxation of slow 13C nuclei can no longer follow excitation, and the corresponding 13C signals have low intensities. In such cases, addition of small amounts of relaxation accelerators, such as radicals or transition metal salts, to the sample amplifies these signals [153]. 

The radical anions may be formed by reacting the nucleobases with eaq which may be either generated radiolytically or in a two-step reaction, e.g in the laser flash photolysis of anthraquinonedisulfonate in the presence of pyrimidines (yielding the pyrimidine radical cation and an anthraquinonedisulfonate radical anion) and subsequent photoionization of the anthraquinonedisulfonate radical anion (Lii et al. 2001). The latter approach, combined with Fourier transform EPR spectroscopy, yielded detailed information as to the conformation of the radical anions of Ura and Thy in aqueous solution (for a discussion see Close 2002 Naumov and Beckert 2002). Similarly valuable EPR information has been obtained from y-irradiated single crystals (cf. Box and Budzinski 1975 Boxet al. 1975 Sagstuen et al. 1998). 

Dimethyl sulfoxide (DMSO) has recently been detected in marine air masses. To date nothing is known about the atmospheric fate of DMSO in the gas phase. Reported here are product and kinetic studies on the reactions of OH, NO3 and Cl radicals with DMSO. The investigations were performed in a 420 1 reaction chamber at atmospheric pressure using long path in situ Fourier transform (FTIR) absorption spectroscopy for detection of reactants and products. 

X. Fourier Transform Infrared Spectroscopy of Jet-Cooled Radicals.180... 

Photolysis is a selective way to make radicals from a suitable precursor. It is very clean and particularly useful in emission experiments. The energy required to photolyze a precursor is typically about 6 electron volts. This means that photolysis sources will be pulsed lasers or resonance lamps. Lasers such as XeCl (A. = 308 nm) or ArF (A = 193 nm) with pulse widths of about 10 ns and repetition rates of around 100 Hz will thus make transients species in short bursts. It is a difficult task to couple such a system to a Fourier transform spectrometer but it has been done in a dynamics study [23]. Resonance lamps, while providing the appropriate energy, usually do not provide the flux necessary to generate the high concentrations of unstable species needed for a successful spectroscopy experiment [24],... 

IX. FOURIER TRANSFORM ABSORPTION SPECTROSCOPY FOR THE STUDY OF PHOTOREACTIVE RADICALS... 

X. FOURIER TRANSFORM INFRARED SPECTROSCOPY OF JET-COOLED RADICALS... 

Once deposition is complete and the initial reaction product is trapped in an inert gas matrix, characterization is carried out spectroscopically. Several spectroscopic techniques have been used the most common is infrared spectroscopy, either dispersive or Fourier transform. Raman spectroscopic studies have been carried out as well, but low signal levels have made this approach difficult. When the trapped intermediate is a free radical, electron spin resonance techniques are valuable as well. Finally, a number of researchers are employing electronic spectroscopy, when the species of interest has an absorption in the visible or ultraviolet tegion. 

It has been suggested that the first step of reaction (6) may be the formation of a carboxylic species COOHads. Carboxyl radicals have indeed been observed by Zhu et al." for potentials lower than 0.65 V using Fourier Transform infrared Reflectance Absorption Spectroscopy with the Attenuated Total Reflection mode (ATR-FTtR). Moreover Anderson et al." made numerical simulation which indicated that the formation of an adsorbed carboxylic species was energetically more favorable. Here, it has to be noted that the electro-oxidation of CO being a stracture sensitive reaction (sensitive to the superficial stracture symmehy" and to the presence of surface defects) this species can be used to study the activity of a catalyst but also as a molecular probe to characterize the catalytic surface. ... 

The photooxidation rates of nylon polymers have been monitored using Fourier Transform Infra-red spectroscopy. Norrish type-II reactions of carbonyl groups were considered to be the most important process. Another group of workers have studied the photooxidations rates of a nylon 6- polypropylene glycol copolymerIn this case polyether sequences are the major source of free radical attack resulting in high levels of hydroperoxides. Irradiation of nylon and polyester fibres with an excimer laser resulted in... 

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