N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.9 Nitrogen Compounds
401
8.9 Nitrogen Compounds [1,2]
General rule: Odd molecular mass and many even-mass fragments for molecules
with an odd number of nitrogen atoms.
8.9.1 Amines
Saturated Aliphatic Amines [3]
Fragmentation: Dominating loss of alkyl residues by cleavage of the C–C bond
next to the N atom ("N-cleavage"). Larger substituents are eliminated preferably.
When a γ-H is available, subsequent elimination of alkenes by McLafferty-type
reactions:
N
R
1
R
2
R
3
N
R 1
R
2
+
– R
1
-CH=CH 2
H
N R
2
+
– R 3
+ .
.
Otherwise, unspecific H transfer onto the N atom:
R
1
N R
2
R
3
– R
3
R
1
N R
2
+
– R
1
-CH=CH 2
H
N R
2
+
H
+ .
.
NH 3 , RNH 2 , and RR'NH eliminations from primary, secondary, and tertiary amines,
respectively, are negligible except from some multifunctional compounds (e.g.,
diamines and phenyl-phenoxy-substituted amines).
ESI-MS/MS: Contrary to EI-MS, the cleavage of the C–N bond is a common
process in ESI-MS/MS. Primary amines R–NH 2 eliminate NH 3 from [M+H] + to an
alkyl cation R + . Depending on the proton affinity of the products, the cleavage of an
R–N bond in secondary and tertiary amines leads either by alkene elimination to the
protonated amine or by amine elimination to a carbocation:
[R 1 R 2 NH + H] +
[R 2 NH 3 ] + + [R 1 - H]
[R 1 R 2 NR 3 + H] +
[R 1 R 2 NH 2 ] + + [R 3 - H]
[R 1 R 2 NH + H] +
[R 1 ] + + R 2 NH 2
[R 1 R 2 NR 3 + H] +
[R 3 ] + + R 1 R 2 NH
The cleavage of the C–C bond next to the N is also in ESI-MS/MS a common
process. In protonated primary amines, alkene elimination leads to the NH 4
+ ion.
Ion series: Even-mass fragments of the type C n H 2n+2 N (m/z 30, 44, 58, 72, 86, …).
Intensities: Mainly peaks in the low mass range. Dominating base peak from
“N-cleavage” at [28 + m(R 1 ) + m(R 2 ) + m(R 4 ) + m(R 5 )] + for R 1 R 2 R 3 CNR 4 R 5
(e.g., m/z 30 for RCH 2 NH 2 , m/z 44 for RCH 2 NHCH 3 , m/z 58 for RCH 2 N(CH 3 ) 2 ,
and m/z 86 for RCH 2 N(CH 2 CH 3 ) 2 ). Local maximum at m/z 86 (C 5 H 12 N + ) for
n-alk–NH 2 (protonated piperidine, 6-membered ring).
Molecular ion: Usually weak or absent, especially if the α-C atom is substituted.
Decreasing intensity with increasing molecular weight. Tendency to protonate to
[M+H] + .
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.9 Nitrogen Compounds
401
8.9 Nitrogen Compounds [1,2]
General rule: Odd molecular mass and many even-mass fragments for molecules
with an odd number of nitrogen atoms.
8.9.1 Amines
Saturated Aliphatic Amines [3]
Fragmentation: Dominating loss of alkyl residues by cleavage of the C–C bond
next to the N atom ("N-cleavage"). Larger substituents are eliminated preferably.
When a γ-H is available, subsequent elimination of alkenes by McLafferty-type
reactions:
N
R
1
R
2
R
3
N
R 1
R
2
+
– R
1
-CH=CH 2
H
N R
2
+
– R 3
+ .
.
Otherwise, unspecific H transfer onto the N atom:
R
1
N R
2
R
3
– R
3
R
1
N R
2
+
– R
1
-CH=CH 2
H
N R
2
+
H
+ .
.
NH 3 , RNH 2 , and RR'NH eliminations from primary, secondary, and tertiary amines,
respectively, are negligible except from some multifunctional compounds (e.g.,
diamines and phenyl-phenoxy-substituted amines).
ESI-MS/MS: Contrary to EI-MS, the cleavage of the C–N bond is a common
process in ESI-MS/MS. Primary amines R–NH 2 eliminate NH 3 from [M+H] + to an
alkyl cation R + . Depending on the proton affinity of the products, the cleavage of an
R–N bond in secondary and tertiary amines leads either by alkene elimination to the
protonated amine or by amine elimination to a carbocation:
[R 1 R 2 NH + H] +
[R 2 NH 3 ] + + [R 1 - H]
[R 1 R 2 NR 3 + H] +
[R 1 R 2 NH 2 ] + + [R 3 - H]
[R 1 R 2 NH + H] +
[R 1 ] + + R 2 NH 2
[R 1 R 2 NR 3 + H] +
[R 3 ] + + R 1 R 2 NH
The cleavage of the C–C bond next to the N is also in ESI-MS/MS a common
process. In protonated primary amines, alkene elimination leads to the NH 4
+ ion.
Ion series: Even-mass fragments of the type C n H 2n+2 N (m/z 30, 44, 58, 72, 86, …).
Intensities: Mainly peaks in the low mass range. Dominating base peak from
“N-cleavage” at [28 + m(R 1 ) + m(R 2 ) + m(R 4 ) + m(R 5 )] + for R 1 R 2 R 3 CNR 4 R 5
(e.g., m/z 30 for RCH 2 NH 2 , m/z 44 for RCH 2 NHCH 3 , m/z 58 for RCH 2 N(CH 3 ) 2 ,
and m/z 86 for RCH 2 N(CH 2 CH 3 ) 2 ). Local maximum at m/z 86 (C 5 H 12 N + ) for
n-alk–NH 2 (protonated piperidine, 6-membered ring).
Molecular ion: Usually weak or absent, especially if the α-C atom is substituted.
Decreasing intensity with increasing molecular weight. Tendency to protonate to
[M+H] + .