N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.9 Nitrogen Compounds
407
Aromatic Nitriles (R–CN)
Fragmentation: Consecutive elimination of HCN and acetylene.
ESI-MS/MS: HCN-Elimination (Δm 27) from [M+H] + .
Ion series: Aromatic fragments corresponding to C n H n and C n H n±1 (m/z 39, 51–53,
63–65, 75–77, …).
Intensities: Intensive peaks in the M +· region.
Molecular ion: Dominant intensity, often base peak. In contrast to aliphatic and
benzylic nitriles, [M-1] + is usually not important.
Aliphatic Isonitriles (R–NC)
Fragmentation: In general, the spectra are similar to those of the corresponding
nitriles. The most important difference lies in the loss of CN · (Δm 26) and the
higher probability of losing HCN (Δm 27). Further important fragmentations
are the elimination of alkyl radicals to give (CH 2 ) n CN + ions and the McLafferty
rearrangement to yield CR 2 =N=CH +· (m/z 41 for R: H).
Ion series: Saturated and unsaturated alkyl ions mainly in the lower mass range
(C n H 2n+1 , m/z 29, 43, 57, … and C n H 2n-1 , m/z 27, 41, 55, …). Rearrangement
products corresponding to C n H 2n-1 N contribute, to a significant extent, to the ion
series of m/z 41, 55, 69, ….
Intensities: Intensive peaks in the lower mass range.
Molecular ion: Weak, decreasing with increasing chain length and degree of
branching. Both [M+H] + and [M-H] + can be stronger than M +· .
Aromatic Isonitriles (R–NC) [4]
Fragmentation: Dominant loss of HCN ([M-27] +· ). In methylphenyl and benzyl
isocyanides also formation of isocyanotropylium ion, [M-1] + , followed by loss of
HCN to [M-28] + .
Ion series: Aromatic (C n H n and C n H n±1 ; m/z 39, 51–53, 63–65, 75–77, …).
Intensities: Intensive peaks in the higher mass range.
Molecular ion: Dominant; base peak for phenyl isocyanide.
8.9.6 Cyanates, Isocyanates, Thiocyanates, and Isothiocyanates
Aliphatic Cyanates (R–OCN) [8]
Fragmentation: Spectra often very similar to those of the corresponding isocyanates
(see below). Cleavage of the C–C bond next to O, with the charge remaining on
CH 2 OCN (m/z 56) for short-chain cyanates and preferably on the alkyl substituent
if it has a C n > 2 chain (m/z 29, 43, 57, …). Cleavage of the C–O bond with H rearrangement to give HCNO +· (m/z 43) or alkene +· (m/z 42, 56, 70, …). For cyanates
with C n > 5 substituents, alkene elimination yields m/z 99.
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
8.9 Nitrogen Compounds
407
Aromatic Nitriles (R–CN)
Fragmentation: Consecutive elimination of HCN and acetylene.
ESI-MS/MS: HCN-Elimination (Δm 27) from [M+H] + .
Ion series: Aromatic fragments corresponding to C n H n and C n H n±1 (m/z 39, 51–53,
63–65, 75–77, …).
Intensities: Intensive peaks in the M +· region.
Molecular ion: Dominant intensity, often base peak. In contrast to aliphatic and
benzylic nitriles, [M-1] + is usually not important.
Aliphatic Isonitriles (R–NC)
Fragmentation: In general, the spectra are similar to those of the corresponding
nitriles. The most important difference lies in the loss of CN · (Δm 26) and the
higher probability of losing HCN (Δm 27). Further important fragmentations
are the elimination of alkyl radicals to give (CH 2 ) n CN + ions and the McLafferty
rearrangement to yield CR 2 =N=CH +· (m/z 41 for R: H).
Ion series: Saturated and unsaturated alkyl ions mainly in the lower mass range
(C n H 2n+1 , m/z 29, 43, 57, … and C n H 2n-1 , m/z 27, 41, 55, …). Rearrangement
products corresponding to C n H 2n-1 N contribute, to a significant extent, to the ion
series of m/z 41, 55, 69, ….
Intensities: Intensive peaks in the lower mass range.
Molecular ion: Weak, decreasing with increasing chain length and degree of
branching. Both [M+H] + and [M-H] + can be stronger than M +· .
Aromatic Isonitriles (R–NC) [4]
Fragmentation: Dominant loss of HCN ([M-27] +· ). In methylphenyl and benzyl
isocyanides also formation of isocyanotropylium ion, [M-1] + , followed by loss of
HCN to [M-28] + .
Ion series: Aromatic (C n H n and C n H n±1 ; m/z 39, 51–53, 63–65, 75–77, …).
Intensities: Intensive peaks in the higher mass range.
Molecular ion: Dominant; base peak for phenyl isocyanide.
8.9.6 Cyanates, Isocyanates, Thiocyanates, and Isothiocyanates
Aliphatic Cyanates (R–OCN) [8]
Fragmentation: Spectra often very similar to those of the corresponding isocyanates
(see below). Cleavage of the C–C bond next to O, with the charge remaining on
CH 2 OCN (m/z 56) for short-chain cyanates and preferably on the alkyl substituent
if it has a C n > 2 chain (m/z 29, 43, 57, …). Cleavage of the C–O bond with H rearrangement to give HCNO +· (m/z 43) or alkene +· (m/z 42, 56, 70, …). For cyanates
with C n > 5 substituents, alkene elimination yields m/z 99.