Base peak 3 cleavage

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... 

Molecular ions are usually intense for low-molecular weight compounds. Alkyl cleavage with the charge remaining on the unsaturated portion is very often the base peak. A series of fragment ions with m/z 41, 55. 69, 83, and so forth are characteristic. Methods are available to locate the position of the double bond in aliphatic compounds.1... 

The potassium salts of the pyrophosphonic acids gave FAB spectra free from matrix interferance.238 Chemical ionisation induced competing and consecutive heterolytic ring cleavage in the mass spectra of the oxides (88).239 The base peaks of imphos and cyclophane were guite different and base peaks appeared at m/z 131 and 21 respectively.240 The mass spectra of phenylphosphonates and phosphates such as (89) have been reviewed.24 ... 

In the case of secondary amine 56 the base peak is due to cleavage of p bond with respect to the side chain nitrogen, while in both amide 57 and urethane 58 the bond is broken along with elimination of H2. 

In the mass spectra of all these substances (4,5,14,15,87,92,94), the molecular ions peaks are present. The base peak at m/z 58, typical for all the seco alkaloids with the N,iV-dimethylaminoethyl chain, corresponding to the CH2=N(CH3)2+ ion is observed. The main mass fragmentation under electron impact seems to involve bond cleavage between C-l and C-9. This can be evidenced by the presence of ions of the 122 and 123 type found in spectra of all compounds except for N-methylhydrastines (98, 99), where the 123 — C02 ion is formed instead. Finally, the carbonyl group absorption... 

The main mass fragmentation of secobenzylisoquinoline alkaloids involves bond cleavage between the two benzylic carbonyls. This process is evidenced by the presence of peaks representing fragment ions at m/z 151, found in spectra of all these bases and attributed to the lower portion of the molecules, and ions at m/z 220,236, and 222, found in spectra of 159,160, and 161, respectively, formed from the upper part of the compounds. Similarly, as in the mass spectra of other secoisoquinoline alkaloids incorporating the amino-ethyl substituent, the [H2C=N(CH3)2]+ ion at mjz 58 is the base peak. 

The high-resolution mass spectrum of compound 7 (R, R1 = H) shows that the molecular ion constitutes the base peak at m/z = 170 (100%) and suffers the cleavage of C-C and C-N with hydrogen transfer giving rise to different heterobicyclic ions <2003PS2055>. However, for compound 4, in addition to the expected molecular ion, the fragment peaks appeared at m/z = [M-28]+, [PhCN]+, [R]+, [ArN]+, and [Ar]+ <2000JPR342>. 

The molecular peak of the spiro[adamantine-2,2 -(l,3,4)-thiadiazolidine]-3, 4 -dicarboxylate 29 appears with 99% intensity and shows four distinct fragmentations <1999T12783>. The molecular formulas of the fragments were confirmed by the intensities of 13C and (34S+13C2) isotope peaks. The two major fragmentations involve cleavage of the weak C-S bonds and afford the base peak at mjz 208 (100%) corresponding to and a peak at m/z 239... 

The spectrum of benzyl acetate is shown in Figure 9.56. The parent peak at m/z 150 is prominent (Rule 1) as is the tropylium ion peak at m/z 91 (Rule 2). The base peak at m/z 108 is due to a rearrangement (Rule 4) after cleavage of the acetyl group which itself gives a prominent peak at m/z 43... 

Lastly, Figure 9.57 shows a spectrum of 2-pentanol. The base peak at m/z 45 is due to cleavage at the C-C bond adjacent to the OH group (Rule 3) and several peaks appear corresponding to the loss of H20 and C2H2 (Rule 4). 

The mass spectrum of dobutamine hydrochloride given in figure 5 shows the molecular ion of the free base at m/e 301. The major fragmentation consists of a cleavage beta to the nitrogen to yield peaks at m/e 123 and 178 with relative intensities, to the base peak, of 15.2 and 97.6%, respectively. The base peak is at m/e 107. 

Two new pseudotabersonine-related alkaloids were isolated by Husson et al. from the stem bark of T. albiflora from French Guiana (29) and characterized as (20/ )-18,19-dihydroxypseudovincadifformine (166, C2iH26N204, MP 190°C, [a]D +264°) and 19-hydroxy-20-epipandoline (167, C21H26N204, MP 204°C, [a]D +511°. Both compounds exhibited similar UV maxima attributed to a (3-anilinoacrylic chromophore, and their mass spectra showed, in addition to a molecular ion at m/z 370, a base peak at m/z 156 formed by retro-Diels-Alder fragmentation in ring C, followed by C-6—C-5 bond cleavage. 

In this context, the striking difference of the standard 70 eV El mass spectra of the isomeric cyclooctadienes may be mentioned here (see below)87. Whereas the radical cations of the stereoisomeric 1,5-cyclooctadienes, containing two bis-allylic C—C bonds, give the products of the two-fold allyl cleavage as the base peak ([C4H6]+ /[M]+ s=ss 10 1), the cis,cis-l,4 and cis.cis-].3-cyclooctadicnc ions are reluctant to do so (IC4II511 / [M]+ 1 3). Clearly again, 1,2- and 1,3-H shifts cannot efficiently compete with dissociation of the bis-allylic C—C bond. 

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