Matrix isolation electron spin resonance

The direct measurement of CH,- radicals, using a matrix isolation electron spin resonance system (MIESR), has previously demonstrated that surface-generated CH,-radicals are produced during the oxidative coupling of CH4 and emanate into the gas phase... 

Matrix isolation-electron spin resonance. A third method used to measure HOz and ROz is matrix isolation with ESR (see earlier description of matrix isolation). Because HOz and ROz have distinct ESR signals, they can be differentiated (Mihelcic et al., 1985, 1990, 1993). For example, Fig. 11.53, part A, shows the ESR spectrum obtained when approximately... 

L.B. Knight et al., Laser vaporization generation of the SiB and SiAl radicals for matrix isolation electron spin resonance studies comparison with theoretical calculations and assignment of their electronic ground states as X 4Z. J. Chem. Phys. 98, 6749-6757... 

Fig. 5. Experimental equipments for matrix isolation electron spin resonance (MIESR) spectroscopy (1) catalyst (2) gas inlet (3) thermocouple well (4) pressure probe (5) metal valve (6) O-ring joints (7) gate valve (8) butterfly valve (9) two vacuum pump (10) vacuum shroud (11) sapphire rod (12) microwave cavity and (13) quadrupole mass spectrometer inlet. Reprinted from Reference 45).

The basic methods of the identification and study of matrix-isolated intermediates are infrared (IR), ultraviolet-visible (UV-vis), Raman and electron spin resonance (esr) spectroscopy. The most widely used is IR spectroscopy, which has some significant advantages. One of them is its high information content, and the other lies in the absence of overlapping bands in matrix IR spectra because the peaks are very narrow (about 1 cm ), due to the low temperature and the absence of rotation and interaction between molecules in the matrix. This fact allows the identification of practically all the compounds present, even in multicomponent reaetion mixtures, and the determination of vibrational frequencies of molecules with high accuracy (up to 0.01 cm when Fourier transform infrared spectrometers are used). 

Matrix isolation photolytic studies on tetrazolo[l,5- ]pyridazine 25 have been reported by Hill and Platz <2003PCP1051> (Scheme 5) and formation of the l-cyano-3-diazopropene 27, triazacycloheptatetraene 28, and cyano-cyclopropene 29 was detected. Upon the absence of electron spin resonance absorptions at 7 K, the authors concluded that triplet nitrene was not formed but, instead, the resulting singlet nitrene rapidly underwent further ring openings. 

This approach has also been used in the analysis of air by infrared spectroscopy (e.g., see Griffith, 1994). As discussed later, matrix isolation has also been used in conjunction with electron spin resonance (ESR) to measure free radical species, including N02, NO, H02, and ROz. 

Finally, matrix isolation combined with electron spin resonance has been used for N02 as well as for other free radicals such as HOz, ROz, and N03 (Mihelcic et al., 1985, 1990, 1993 Zenker et al., 1998). Trace gases in a sample of air (typically about 8 L) are trapped in a D20 matrix at 77 K and the ESR spectrum obtained. Any paramagnetic species present has a characteristic ESR spectrum that can be used to identify it and, using reference spectra, obtain its concentration. Since NOz is the paramagnetic species present in the largest concentration, it is easily detected and measured. 

Mihelcic, D P. Miisgen, and D. H. Ehhalt, An Improved Method of Measuring Tropospheric N02 and R02 by Matrix Isolation and Electron Spin Resonance, J. Atmos. Chem., 3, 341-361 (1985). 

It is interesting to note that the catalysts that show good selectivities at the higher temperatures generally do not contain easily reducible metal ions, such as V, Mo, or Sb. Many of the catalysts for the lower-temperatures operation, on the other hand, contain these reducible cations. In a study using a Li-Mg oxide, it was established that gas-phase ethyl radicals could be generated by reaction of ethane with the surface at about 600°C (17). These radicals could be trapped by matrix isolation and identified by electron spin resonance spectroscopy. 

The problem of bringing a large magnet into the field for ambient measurements has been overcome in electron paramagnetic resonance (EPR, also called electron spin resonance, ESR) by Mihelcic, Helten, and coworkers (93-99). They combined EPR with a matrix isolation technique to allow the sampling and radical quantification to occur in separate steps. The matrix isolation is also required in this case because EPR is not sensitive enough to measure peroxy radicals directly in the atmosphere. EPR spectroscopy has also been used in laboratory studies of peroxy radical reactions (100, 101). 

Richardson NA, Gu J, Wang S, Xie Y, Schaefer HF III (2004) DNA nucleosides and their radical anions molecular structures and electron affinities. J Am Chem Soc 126 4404-4411 Riederer FI, Fluttermann J (1982) Matrix isolation of free radicals from 5-halouracils. 3. Electron spin resonance of base oxidation in aqueous acidic glasses. J Phys Chem 86 3454-3463 Riederer FI, Fluttermann J, Symons MCR (1978) cr -Electron addition to 5-halogenouracils in neutral glasses. J Chem Soc Chem Commun 313-314... 

Despite of this inherent limitation, several spectacular results have been obtained. It should be noted that the initiation mechanism of the cationic polymerization of styrene described above was also deduced from the results of pulse radiolysis experiments. The pulse radiolysis combined with other techniques, such as the matrix isolation technique, the electron spin resonance technique and usual polymerization techniques, definitely provides a powerful means for investigating fundamentals of polymerization. 

In previous electron spin resonance (ESR) studies of matrix isolated Naj (X, 1) and K3 3, 4), alkali trimers have been shown to be chemically bound and well described both by simple bonding ideas (1, 3) and by the more sophisticated calculations recently employed for Li3 (5), Na3 6) and Kj 7). For the potassium trimer in argon, two distinct ESR spectra are observed 3). An obtuse angled isomer corresponds to one of three static Jahn-Teller distortions from 03 symmetry, and is surprisingly similar to the... 

The vibrational frequencies for FOO(g), adopted here, were assumed to be the same as those obtained from matrix isolation by Noble and Pimentel (4). These authors have observed two more frequencies for isotopically substituted FOO and added to those observed by Spratley et al. ( ), and performed a normal coordinate analysis. The results confirm the fact that the bond distances and angles in FOO are essentially the same as those reported for F 02(g) by Jackson ( ). The same molecular structure for FOO was also deduced from electron spin resonance and electron paramagnetic resonance data reported by Fessenden and Schuler (7) and Kasai and Kirschenbaum (8), respectively. Hence this molecular structure was adopted. Two vibrational... 

Vibrational frequencies for example result from the appUcation of infrared, laser-induced fluorescence, Raman, and Raman resonance spectroscopy. Spectroscopy in the visible and near-UV regions yields information on electronic transitions. Electron spin resonance spectroscopy is used in determining the geometric and electronic structure. These methods were applied to study the gaseous species trapped at low temperatures in a solid inert rare gas matrix (matrix isolation technique) as well as in the free state. 

Experimental techniques that are frequently used in physicochemical studies should be applied to reactive plasmas. These techniques are, for example, the use of deuterated compounds, the analysis of stable products in the gas phase, and the use of matrix isolation or trapping of reactive species at low temperatures combined with electron-spin resonance (ESR) or optical spectroscopy. 

Mihelcic, D., M. Helten, H. Park, P. Musgen, H. W. Patz, M. Trainer, D. Kempa, and D. H. Ehhalt. 1982. Tropospheric airborne measurements of NOj and ROj using the technique of matrix isolation and electron spin resonance. 2nd Sympos. on Composition of the Nonurban Troposphere, Williamsburg, VA. Abstracts, p. 327-329. 

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