Depicting the Molecular Ion

The ions in mass spectrometry may be formed in a variety of ways. One method for converting molecules to ions (ionization) in a mass spectrometer is to place a sample under high vacuum and bombard it with a beam of high-energy electrons (—70 eV, or —6.7 X 10 kJ mol ). 

This method is called electron impact (El) ionization mass spectrometry. The impact of the electron beam dislodges a valence electron from the gas-phase molecules, leaving them with a +1 charge and an unshared electron. This species is called the molecular ion (M ). We can represent this process as follows  

The molecular ion is a radical cation because it contains both an unshared electron and a positive charge. Using propane as an example, we can write the following equation to represent formation of its molecular ion by electron impact ionization  

In many cases, the choice of just where to localize the odd electron and charge is arbitrary, however. This is especially true if there are only carbon-carbon and carbon-hydrogen single bonds, as in propane. When possible, though, we write the structure showing the molecular ion that would result from the removal of one of the most loosely held valence electrons of the original molecule. Just which valence electrons are most loosely held can usually be estimated from ionization potentials (Table 9.3). [The ionization potential of a molecule is the amount of energy (in electron volts) required to remove a valence electron from the molecule.] 

Radical cations from ionization of nonbonding or n electrons. 

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Notice that we have written the above formula for the propane radical cation in brackets. This is because we do not know precisely from where the electron was lost in propane. We only know that one valence electron in propane was dislodged by the electron impact process. However, depicting the molecular ion with a localized charge and odd electron... 

Some classes of compounds are so prone to fragmentation that the molecular ion peak IS very weak The base peak m most unbranched alkanes for example is m/z 43 which IS followed by peaks of decreasing intensity at m/z values of 57 71 85 and so on These peaks correspond to cleavage of each possible carbon-carbon bond m the mol ecule This pattern is evident m the mass spectrum of decane depicted m Figure 13 42 The points of cleavage are indicated m the following diagram... 

In the case of weak chemisorption of a Cl atom depicted in Fig. 2a, the bonding electron is that drawn from the Cl ion of the lattice, the latter ion serving as the adsorption center. In other words, the bonding is due to a hole being drawn from the Cl atom into the lattice. We have here a bond of the same type as in the molecular ion Ch". The dipole moment which arises in this case is opposite in direction to that of the preceding case. 

The fragmentation of 2-methyl-2//-chromene follows an analogous pathway to that described for (4). a-Cleavage results in loss of a methyl radical giving the benzopyrylium ion (4a) as the base peak and subsequent fragmentations are identical to those depicted in Scheme 3. The alternative loss of a hydrogen radical from the molecular ion is not observed... 

Figure 1.13 The ground (lower solid line) and excited (dashed line) potential energy curves of the molecular ion H2+.The upper potential curve represents the ground electronic potential curve shifted by the energy ha> of one photon of the electromagnetic radiation. The ground vibrational wavefunction in the ground electronic state is coupled to the continuum of scattering states of the excited electronic potential depicted here by dense set of energy levels.

Gross and coworkers129 also studied the unimolecular dissociation of protonated acy-lanilines, viz. A-[2-(benzoyloxy)phenyl]benzamides formed via both fast-atom bombardment (FAB) and electrospray ionization (ESI). They found that cyclization occurs upon the loss of a molecule of benzoic acid, and that a similar process occurs for the molecular ion under El. This gas-phase reaction is analogous to a solution reaction leading to phenyl-benzoxazoles. The proposed cyclization process, for which concurrent mechanisms were proposed (Scheme 38 depicts only the displacement reaction route), was corroborated by accurate mass measurements, tandem mass spectrometric experiments with comparison with reference ions, isotopic labeling and theoretical calculations. 

Aldononitrile acetates are suitable for analysis of sugars by g-l.c., and give characteristic mass spectra that are easy to interpret. The molecular ion is not seen, but ions are found at (M — 73) and (M — 98) , corresponding to loss of CH2OAC and CH(OAc)CN moieties, respectively. Primary fragments are formed by a-cleavage of the alditol chain, as depicted in 40. The ions m/e 98 and 170, expected from fission... 

The reaction scheme shown in Equation 2.1 depicts how electron (El) ionization employs a high-energy electron beam to excite molecules M in the vapor state to high electronic and vibrational energy levels M. When enough energy is supplied to a molecule, an electron is ejected to produce a positively charged species M+. This is termed the molecular ion. 

The mass spectrum of Ge(C3H7-i)4 (at 70 eV, depicted as a line diagram) shows the molecular ion in low intensity (1 89) compared with [Ge(C3H7-i)2H]+ as the most abundant species. Most of the major fragments in the fragmentation scheme below are produced by molecular elimination reactions, since the appropriate metastable ions were clearly detected [1] ... 

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