N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.6 Heteroaromatics
387
N-Oxides of Pyridines and Quinolines
Fragmentation: The [M - O] +· radical ion, of variable intensity, is probably due
to thermal decomposition. The fragments [M - CO] +· and, if an alkyl group is
present on the neighboring C atom, [M - OH] + are relevant for quinoline N-oxides.
Rearrangements with ring formation including the N–O moiety if alkyl or aryl
groups are present in the neighboring positions.
Ion series: As for the corresponding heteroaromatics, aromatic hydrocarbon
fragments, C n H n , C n H n±1 and C n H n±1 N (m/z 39–41, 51–54, 63–67, 75–80, …), are
observed.
Intensities: Dominant peaks for M +· and products of benzylic-type cleavage.
Molecular ion: Strong, except when [M - O] +· dominates due to experimental
conditions or when benzylic-type cleavage is possible. Odd mass for odd number of
N atoms in the molecule. No tendency to protonate.
Pyridazines and Pyrimidines
Fragmentation: Loss of N 2 or CH 2 N · (Δm 28) from pyridazines. Also, loss of
N 2 H · (especially important in methylpyridazines) to give [M-29] + . In pyridazine
N-oxides, consecutive loss of NO · and HCN. Consecutive losses of two HCN
(2 × Δm 27) molecules from pyrimidines. From 2-, 4-, and 6-methylpyrimidines,
CH 3 CN (Δm 41) is eliminated and the ion CH 3 CNH + (m/z 42) occurs.
Ion series: Aromatic hydrocarbon fragments (C n H n , C n H n±1 ) and, for pyrimidines,
C n H n±1 N, at low masses (m/z 39, 51–53).
ESI-MS/MS: Pyrimidines also eliminate NH=C=NH (Δm 42).
Intensities: Dominant peak for M +· .
Molecular ion: Strong. No tendency to protonate. For pyrimidines, [M-1] + is
usually observable.
Pyrazines
Fragmentation: Consecutive losses of two HCN (2 × Δm 27) molecules. For
methylpyrazines, elimination of CH 3 CN (Δm 41) and formation of CH 3 CNH + (m/z
42).
Ion series: No aromatic character of the spectra.
Intensities: Dominant peak for M +· .
Molecular ion: Strong. No tendency to protonate. Usually, [M-1] + is observable; it
can be stronger than M +· in alkyl-substituted (C n > 1 ) pyrazines.
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.6 Heteroaromatics
387
N-Oxides of Pyridines and Quinolines
Fragmentation: The [M - O] +· radical ion, of variable intensity, is probably due
to thermal decomposition. The fragments [M - CO] +· and, if an alkyl group is
present on the neighboring C atom, [M - OH] + are relevant for quinoline N-oxides.
Rearrangements with ring formation including the N–O moiety if alkyl or aryl
groups are present in the neighboring positions.
Ion series: As for the corresponding heteroaromatics, aromatic hydrocarbon
fragments, C n H n , C n H n±1 and C n H n±1 N (m/z 39–41, 51–54, 63–67, 75–80, …), are
observed.
Intensities: Dominant peaks for M +· and products of benzylic-type cleavage.
Molecular ion: Strong, except when [M - O] +· dominates due to experimental
conditions or when benzylic-type cleavage is possible. Odd mass for odd number of
N atoms in the molecule. No tendency to protonate.
Pyridazines and Pyrimidines
Fragmentation: Loss of N 2 or CH 2 N · (Δm 28) from pyridazines. Also, loss of
N 2 H · (especially important in methylpyridazines) to give [M-29] + . In pyridazine
N-oxides, consecutive loss of NO · and HCN. Consecutive losses of two HCN
(2 × Δm 27) molecules from pyrimidines. From 2-, 4-, and 6-methylpyrimidines,
CH 3 CN (Δm 41) is eliminated and the ion CH 3 CNH + (m/z 42) occurs.
Ion series: Aromatic hydrocarbon fragments (C n H n , C n H n±1 ) and, for pyrimidines,
C n H n±1 N, at low masses (m/z 39, 51–53).
ESI-MS/MS: Pyrimidines also eliminate NH=C=NH (Δm 42).
Intensities: Dominant peak for M +· .
Molecular ion: Strong. No tendency to protonate. For pyrimidines, [M-1] + is
usually observable.
Pyrazines
Fragmentation: Consecutive losses of two HCN (2 × Δm 27) molecules. For
methylpyrazines, elimination of CH 3 CN (Δm 41) and formation of CH 3 CNH + (m/z
42).
Ion series: No aromatic character of the spectra.
Intensities: Dominant peak for M +· .
Molecular ion: Strong. No tendency to protonate. Usually, [M-1] + is observable; it
can be stronger than M +· in alkyl-substituted (C n > 1 ) pyrazines.