Molecular ion peak

The amino add analysis of all peptide chains on the resins indicated a ratio of Pro Val 6.6 6.0 (calcd. 6 6). The peptides were then cleaved from the resin with 30% HBr in acetic acid and chromatogra phed on sephadex LH-20 in 0.001 M HCl. 335 mg dodecapeptide was isolated. Hydrolysis followed by quantitative amino acid analysis gave a ratio of Pro Val - 6.0 5.6 (calcd. 6 6). Cycll2ation in DMF with Woodward s reagent K (see scheme below) yielded after purification 138 mg of needles of the desired cyc-lododecapeptide with one equiv of acetic add. The compound yielded a yellow adduct with potassium picrate, and here an analytically more acceptable ratio Pro Val of 1.03 1.00 (calcd. 1 1) was found. The mass spectrum contained a molecular ion peak. No other spectral measurements (lack of ORD, NMR) have been reported. For a thirty-six step synthesis in which each step may cause side-reaaions the characterization of the final product should, of course, be more elaborate. 

The mass spectra of phenylthiazoles are characterized by the presence of intense molecular ion peaks, due to the aromatic nature of the molecules, which represent 35, 41, and 44% of the total ionization for 2-, 4-, and 5-phenylthiazoles, respectively. 

Not only the molecular ion peak but all the peaks m the mass spectrum of benzene are accompanied by a smaller peak one mass unit higher Indeed because all organic com pounds contain carbon and most contain hydrogen similar isotopic clusters will appear m the mass spectra of all organic compounds... 

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... 

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... 

This example can be used in reverse to show the usefulness of looking for such isotopes. Suppose there were an unknown sample that had two molecular ion peaks in the ratio of 3 1 that were two mass units apart then it could reasonably be deduced that it was highly likely the unknown contained chlorine. In this case, the isotope ratio has been used to identify a chlorine-containing compound. This use of mass spectrometry is widespread in general analysis of materials, and it... 

A diagrammatic illustration of the effect of an isotope pattern on a mass spectrum. The two naturally occurring isotopes of chlorine combine with a methyl group to give methyl chloride. Statistically, because their abundance ratio is 3 1, three Cl isotope atoms combine for each Cl atom. Thus, the ratio of the molecular ion peaks at m/z 50, 52 found for methyl chloride in its mass spectrum will also be in the ratio of 3 1. If nothing had been known about the structure of this compound, the appearance in its mass spectrum of two peaks at m/z 50, 52 (two mass units apart) in a ratio of 3 1 would immediately identify the compound as containing chlorine. 

For other elements that occur with major relative abundances of more than one isotope in the natural state, the isotope pattern becomes much more complex. For example, with chlorine and bromine, the presence of these elements is clearly apparent from the isotopes Cl and for chlorine and Br and Br for bromine. Figure 47.2a shows the molecular ion region for the compound chlorodecane. Now, there are new situations in that C, C, C1, and Cl isotopes all have probabilities of occurring together. Thus, there are molecular ion peaks for + Cl, C + Cl, + Cl, and so on. Even so, the isotopic ratio of 3 1 for Cl to Cl is very clear... 

The ion having greatest abundance is said to form the base peak in the spectrum. The base peak may or may not be the same as the molecular ion peak. 

These are discussed in (B-71MS4). Oxirane itself shows a strong molecular ion peak and a slightly stronger base peak at mje 29 (CHO ) due to isomerization to ethanal and loss of a methyl radical. Substituted oxiranes tend to show only weak molecular ion peaks, because of rearrangement and fragmentation. 

A molecular ion peak at 196.1447 (calcd. for Ci2H2o02 196.1463) is displayed in its mass spectrum. 

The mass spectra of free carbohydrates and their glycosides, obtained by ionization upon electron impact, are limited in their usefulness for structural studies. Peaks corresponding to molecular ions are generally not observed due to extensive fragmentation to ions of low m/e (4,9,11, 24, 26). In contrast, positive ions produced by field ionization do not give fragment spectra as characteristic as do those produced by electron impact, but the molecular ion peaks are intense, often the most intense in the spectra (3). 

O-Isopropylidene derivatives of carbohydrates form structural isomers from carbohydrates which themselves are epimers. Since structural isomers often fragment differently whereas epimers do not, mass spectra of these derivatives may permit interpretation in terms of stereochemistry. Although molecular-ion peaks are not observed, the molecular weight can be determined readily from a relatively intense M-CH/ peak, resulting from loss of a methyl radical from a 1, 3-dioxolane ring (12). 

Diethyl dithioacetal derivatives of carbohydrates are generally crystalline compounds which are easily prepared from sugars in the combined or free form. In contrast to the mass spectra of the carbohydrate derivatives discussed above, the mass spectra of diethyl dithioacetals allow direct determination of molecular weight from molecular-ion peaks of... 

No molecular-ion peak is observed in the mass spectrum of isomer 5, although the fragment at m/e 117 [119] is present from loss of the C-l... 

The 5-deoxy isomer, methyl 5-deoxy-/ -D-xt/Zo-furanoside (6), gives a mass spectrum (Figure 3) which contains no molecular-ion peak, but does contain peaks at m/e 117 [119] and m/e 116 [117] for the losses of the C-l methoxyl group and a molecule of methanol, respectively. 

No molecular-ion peak is observed from either isomer. The loss of a methoxyl group to give m/e 131 is shown in Equation 16 to be from the C—1 position, on the basis of analogy to reported studies using per-O-methylated sugars with a methoxyl-d3 substituent on C-l (24). 

Molecular-ion peaks are not generally observed, but molecular weight can be readily determined from a relatively intense M-15 (M-CH3)... 

Once again no molecular-ion peak is seen, but the M-CH3 peak is prominent at m/e 189 in Figure 7. An important peak at m/e 159 from C-4-C-5 cleavage with charge retention on C-4 establishes the presence of a furanose ring and of a 6-deoxy function. Charge retention on C-5 leads to the ion at m/e 45 (see Equations 19 and 20). 

Both straight-chain and branched aliphatic aldehydes show molecular ion peaks up to a minimum of Cl4 aldehydes. 

Molecular ion As is expected, the molecular ion peak is intense for aryl ethers. 

The molecular ion peaks in the mass spectra of aromatic halogenated compounds are fairly intense. The molecular ion abundances... 

Figure 18.2 is the mass spectrum of a branched hydrocarbon. Note the intensity of the molecular ion peak and the presence of an M - 15 peak. The M - 15 peak is typical, particularly if the side chain is a methyl group. The position of the side chain is indicated by the m/z 112 and 113 ions (CH3(CH2)5CHCH3). Usually the site of branching is more difficult to establish. 

The presence of chlorine and/or bromine is easily detected by their characteristic isotopic patterns (see Appendix 11). As in many aliphatic compounds, the abundance of the molecular ion decreases as the size of the R group increases. For example, in the El mass spectra of methyl chloride and ethyl chloride, the molecular ion intensities are high, whereas in compounds with larger R groups such as butyl chloride, the molecular ion peak is relatively small or nonexistent. 

The molecular ion intensity decreases with increased branching, therefore the molecular ion peak may be nonexistent. The loss of 15 Daltons from the molecular ion indicates a methyl side chain. The mass spectra of branched alkanes are dominated by the tendency for fragmentation at the branch points, and hence are difficult to interpret. 

In general, the relative height of the molecular ion peak decreases in the following order. 

Verification of the molecular weight of thiirene dioxides by mass spectrometry, employing the conventional electron-impact (El) ionization method, has been unsuccessful due to the absence or insignificant intensity of molecular ion peaks in their mass spectra. The base peak is rather characteristic, however, and corresponds to the formation of the disubstituted acetylene ion by loss of sulfur dioxide91 (equation 3). 

The acid-base interaction in group 13-stibine and -bismuthine adducts seems to be very weak as is indicated by mass spectroscopic studies, which never showed the molecular ion peak but only the respective Lewis acid and Lewis base fragments. The extreme lability in the gas phase may also account for the fact that there are only very few reports on thermodynamic data of group 13-stibine or bismuthine adducts in the literature. Therefore, multinuclear NMR spectroscopy and single crystal X-ray diffraction are the most important analytical tools for the characterization of such adducts. 

Considering these situations, the observation of molecular weights, particularly by matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MASS), is essential [33]. The operation is simple and enables us to observe the molecular ion peaks of CPOs with molecular weights exceeding 10,000. The quahty of the measurement is strongly dependent on the choice of the matrix. Therefore, the search for the best matrix for each CPO should be pursued. 

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