McLafferty determination

Characteristic fragment ions Aliphatic ketones also give abundant McLafferty rearrangement ions at m/z 58, 72, 86, and so forth. Methyl ketones produce an abundant ion at m/z 43. Low-intensity ions at m/z 31, 45, 59, 73, and so on reveal oxygen in the unknown ketone and are especially useful in distinguishing ketone spectra from isomeric paraffin spectra. Subtract 43 from the mass of the rearrangement ion to determine R. 

Computer Techniques McLafferty (Ref 63) has pointed out that the usefulness of elemental composition information increases exponentially with increasing mass, since the number of elemental combinations with the same integral mass becomes larger. There are compilations of exact masses and elemental compositions available (Refs 12a, 13 18a). Spectral interpretation will be simplified in important ways if elemental compositions of all but, the smallest peaks are determined. Deriving the elemental compositions of several peaks in a spectrum is extremely laborious and time-consuming. However, with the availability of digital computers such tasks are readily performed. A modern data acquisition and reduction system with a dedicated online computer can determine peak centroids and areas for all peaks, locate reference peaks, interpolate between them to determine the exact masses of the unknown peaks, and find within minutes elemental compositions of all ions in a spectrum (Refs 28b 28c)... 

Senko, M. W. Beu, S. C. McLafferty, F. W. Determination of monoisotopic masses and ion populations for large biomolecules from resolved isotopic distributions. J. Am. Soc. Mass Spectrom. 1995, 6,229-233. 

Example Isotopic labeling does not only reveal the original position of a rearranging atom, but can also reveal the rate-determining step of multi-step reactions by its marked influence on reaction rates. Thus, the examination of H/D and isotope effects led to the conclusion that the McLafferty rearrangement of aliphatic ketones (Chap. 6.7) rather proceeds stepwise than concerted. [68]... 

McLafferty, F. W., "Mass Spectrometry," in "Determination of Organic Structuresby Physical Methods," F. C. Nachod andW. D. Phillips, eds., Academic Press, New York, 1962. 

Focus identification on the odor-active regions as determined by GC-sniffing and RI values as published in the Flavomet (http //www.nysaes.cornell.edu/flavornet). The fragmentation pattern obtained for the compounds can be compared with those in data banks like the Wiley/NBS Registry of Mass Spectral data (McLafferty and Stauffer, 2000) or the NIST 98 library (National Institute of Science and Technology). 

The structure of diphosphallenic radical cations, generated from the allene ArP=C=PAr by electrochemical oxidation, has been examined using EPR spectroscopy. Ab initio calculations including correlation effects at the MP2 and MCSCF levels have determined that two rotamers exist compatible with Jahn-Teller distortion of the allene.146 Anodically generated radical cations of alkyl phosphites [(RO P] and silylphosphites [(RO)2POSiMe3] reacted with alkenes by initial attack at the C=C bond followed by electron transfer, deprotonation, and elimination of an alkyl or trimethylsilyl cation to form identical alkyl phosphate adducts.147 The electron ionization-induced McLafferty rearrangement of n-hexylphosphine afford the a-distonic radical cation CTEPH, the distinct reactivity of which suggests there is no... 

In the 1950s, Benyon, Biemann and McLafferty clearly demonstrated the chemistry of functional groups in directing fragmentation, and the power of mass spectrometry for organic structure determination began to develop. 

The McLafferty rearrangement in certain carboxylic acids initially forms the ion (CH2 = C(OH)2)t, which then undergoes metastable ion decomposition to lose a hydroxyl radical. With the labelled ions [CD2 = C(OH)(OD)]t and [CH2=C(OH)(OD)]t, the isotope effect 7oh/ od has been measured as 0.38 [526, 753], 0.56 [342] and 0.42 [407], i.e. OD- loss was more probable than OH - loss. This observation has been interpreted [407, 526] in terms of the rate-determining step... 

Isotope effects on the McLafferty rearrangement have been determined from the peak heights in the El mass spectra for a variety of ketones and other compounds. Intramolecular [554, 555, 556, 913] and inter-molecular [194, 907] isotope effects for this rearrangement via a 6-membered cyclic transition state have fallen in the region 1—2. The literature on this rearrangement, including isotope effects has been thoroughly reviewed [205],... 

Intramolecular carbon isotope effects have been determined for the McLafferty rearrangement in metastable a-ethylbutyrophenone ions using molecules specifically labelled with 13C at either one of the [3 or at one of the 7 positions. The isotope effects /c2H4/fi3CCH obtained were 0.095 0.005 for the 7-13C compound and 1.017 0.d07 for the 3-13C compound [849]. 

McLafferty, F.W. Tandem Mass Spectrometry (MS/MS) A Promising New Analytical Technique for Specific Component Determination in Complex Mixtures, Acc. Chem. Res. 13, 33-39 (1980). 

McLafferty and co-workers [150,151] were the first to apply these reactions to the determination of ion structures in organic chemistry. 

McLafferty [173b] showed the possibility of detecting the presence of thiophene, THF and n-propylbenzene at 25,50 and 500 ppm, respectively, in gasoline, which is a particularly complex mixture. Hass and co-workers [190,192] showed how GC/MS can be complemented by MS/MS, and thus lead to an approach for determining complex structures, such as that of humic acid. In addition, they demonstrated the presence of very small quantities of tetrachlorodibenzo-p-dimethoxane (TCDD) (>5 pg) in methyl stearate samples. 

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