Molecular ions, losses from

Amine Characteristic Fragments Rearrangement Ions Losses from the Molecular Ion... 

There are three main reasons for this choice. Firstly, it becomes more and more difficult to obtain recordable, molecular-ion signals from un-derivatized carbohydrates as their M, increases significantly above 3000. Secondly, the mass spectrometers that have been used in all high-mass-carbofiydrate studies published at the time of writing this article are not capable of very sensitive analysis above —3800 mass units (see later). Thirdly, at masses >4000, it is usually not practicable to work at the resolution necessary for adjacent peaks to appear as separate signals in the spectrum. To do so would require that the source and collector slits be narrowed to such a degree that there would be an unacceptable loss in sensitivity. Thus, spectra acquired at mass >4000 are usually composed of unresolved clusters. 

Molecular ions obtained from thianthrenes are normally the base peak in their mass spectra. The principal fragmentation involves loss of sulfur (87PS377), and this is interpreted as formation of a dibenzothiophen radical cation (16). Further loss of sulfur then occurs. CSH is lost from both the dibenzothiophen fragment ion and from the molecular ion species such as 17, from the parent ion, are proposed (74JHC287). The mass spectroscopic fragmentation pattern of fluorothianthrenes is comparable (720MS373). 

All of the group 5 heterobenzenes show mass spectra typical of aromatic compounds (Table 4). The compounds show intense molecular ions which are the base peaks for all the heterobenzenes except bismabenzene. Important fragmentation involves loss of either C2H2 or HCE from the molecular ion. Loss of HCE becomes relatively less important for the heavier heterobenzenes. This observation may be explicable in... 

Mass spectra of arsilidene complexes, [Cp Mn(CO)2]2AsR, 17-24 (Table 3), contain molecular ion peaks. The most common fragment ions correspond to elimination of 2CO, R and Cp Mn(CO)2. The abundances of these ions depend strongly on the R substituent at the As atom (Table 4). Substituents R such as F, I, OCS, NCS and N3 are easily eliminated directly from the molecular ion. Loss of Ph, however, occurs only after elimination of all carbonyl ligands, and R = H, R = c-Hex are not lost at all. Only compounds with R = Ph, H and c-Hex have mass spectra which display peaks corresponding to (M—2CO) and (M-4CO) ". The formation of (M — R — nCO) (n = 2,4) ions is characteristic of complexes containing R = I, OCS, NCS and N3. The complex having R = F is intermedi-... 

The authors also suggested a possible pathway to account for the involvement of a phenyl ring carbon atom in the loss of HCN from the molecular ion produced from benzonitrile (equation 91). 

The authors also proposed a mechanism to account for the loss of CH3 from the molecular ion formed from the j -allenic alcohol 35. This favourable fragmentation reaction occurs via several steps (Scheme 11). 

As another example, aliphatic hydrocarbons contain a large number of bonds, all with nearly equal energies. The likelihood that at least one of the bonds will break is high, so molecular ion peaks from aliphatic hydrocarbons will be of low intensity if observed at all. Cyclic hydrocarbons, on the other hand, must cleave two carbon-carbon bonds before any loss of mass occurs (neglecting the relatively unimportant loss of a hydrogen atom), with the result of greatly increased molecular ion intensities. 

One important type of fragmentation is the simple cleavage of a single bond. With a radical cation this cleavage can take place in at least two ways each way produces a cation and a radical. Only the cations are detected in a positive ion mass spectrometer. (The radicals, because (hey are not charged, are not detected.) With (he molecular ion obtained from propane by loss of one carbon-carbon tr bonding electron, for example, two possible modes of cleavage are... 

During the course of biochemical studies (138). the mass spectrum of 2-acetamidothiazole was recorded its main peaks are the molecular ion (m/e= 142, relative intensity = 26%) and fragments 100 (100), 58 (2. 5), and 43 (39). For 2-acetamido-5-bromothiazole the main peak results again from the loss of C2H2O by the molecular ion. 2-AcetyIacet-amido-4-methylthiazole (2S) exhibits significant loss of from the... 

Unlike the case of benzene in which ionization involves loss of a tt electron from the ring electron impact induced ionization of chlorobenzene involves loss of an elec tron from an unshared pair of chlorine The molecular ion then fragments by carbon-chlorine bond cleavage... 

Mass Spectrometry The molecular ion peak is usually quite small m the mass spec trum of an alcohol A peak corresponding to loss of water is often evident Alcohols also fragment readily by a pathway m which the molecular ion loses an alkyl group from the... 

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular ion peak m their mass spectra Aldehydes also exhibit an M— 1 peak A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acylium ions) by cleavage of an alkyl group from the carbonyl The most intense peak m the mass spectrum of diethyl ketone for example is m z 57 corresponding to loss of ethyl radi cal from the molecular ion... 

Mass Spectrometry Aside from a peak for the molecular ion which is normally easy to pick out aliphatic carboxylic acids undergo a variety of fragmentation processes The dominant fragmentation m aromatic acids corresponds to loss of OH then loss of CO... 

The peak at rn/z 70 corresponds to loss of water from the molecular ion The peaks at m/z 59 and 73 correspond to the cleavages indicated... 

Molecular ion (Section 13 22) In mass spectrometry the species formed by loss of an electron from a molecule Molecular orbital theory (Section 2 4) Theory of chemical bonding in which electrons are assumed to occupy orbitals in molecules much as they occupy orbitals in atoms The molecular orbitals are descnbed as combinations of the or bitals of all of the atoms that make up the molecule Molecularity (Section 4 8) The number of species that react to gether in the same elementary step of a reaction mechanism... 

The ion (M +) derived from the parent molecule by loss of an electron is called a molecular ion. Depending on the structure of substance M and the energy of the incident electron, the resulting ion (M"+) may break up (fragment) to give ions of smaller mass (A+, B, etc.). 

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

The mass spectra of pyridopyrimidines in general show many features in common with those of other related N-heterocycles, in particular quinazolines and pteridines. The pyridopyrimidines show strong molecular ions, and when breakdown of the hetero ring occurs, fragments arising from loss of CO, CN, HCN and HCNO are observed. 

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