4
1 Nitrile Imines and Their Properties
1.3 Spectroscopy and Isolation
A number of spectroscopic techniques have been employed in an effort to suitably
characterise NIs. As would perhaps be expected in a nitrillium betaine 1,3-dipole,
the inherent reactivity of the species is a major stumbling block in this process.
Following on from their initial application, [1] any preliminary evidence of the
existence of an NI intermediate within a reaction profile was indirect, [18] with the
first direct detection of an NI reported in 1973, using UV spectrometry at cryogenic
temperatures [50].
Multiple approaches towards the spectroscopic characterisation of NIs were
reported throughout the 1980s. The UV spectra of diphenyl NI and derivatives were
reported independently by both Holm and Heimgartner, employing 12 K matrix
isolation and immobilisation using EPA or PVC glass [51–53]. The IR spectra were
also disclosed [53]. The palette of diaryl NIs characterised using this technique
increased towards the middle of the decade, with over 20 UV and IR spectra reported
[54–57]. The utility of this approach was significantly bolstered with the application
of “flash” photolysis, enabling the direct capture of the UV spectrum of an NI at
room temperature [58, 59].
Other spectroscopic approaches were also developed, principally using high
temperature gas phase techniques [60]. Generation of the NI in this way
enabled analysis through mass spectrometry, photoelectron spectroscopy, or IR via
immobilisation of the NI on a KBr disc [24, 61]. Modern-day advances in IR
technology have recently seen the re-emergence of this approach as the most valuable
analytical technique in the characterisation of reactive NIs [26, 49, 62].
14 N NMR has also been shown to be a feasible approach towards NI identification,
although a limitation of this technique is that the NI is required to have
enhanced stability relative to other spectroscopic approaches [63]. More recently,
photocrystallography was also employed as a method of generating crystal structures
of diaryl NIs [64]. Formyl NI has also been shown to be detectable, via either
gas-phase neutralisation-reionisation mass spectroscopy, or low-temperature IR
spectroscopy [36, 37].
1.3.1 Ultraviolet
NI UV spectra are dominated by a large signal around 240–275 nm (heteroatomsubstituted NIs) or 370–465 nm (diaryl NIs) [50–54, 56, 58, 59, 61, 65]. This is
postulated to originate from a π-π* transition, with, in the case of diaryl NIs, an
extremely large extinction coefficient leading to a very broad signal [58]. Electronrich species on the N-terminus of the NI and electron-deficient species on C-terminus
will further increase absorption wavelength [58]. The UV spectrum of formyl NI has
never been recorded, however it is predicted to have a maximum absorbance of
around 230 nm [25].
1 Nitrile Imines and Their Properties
1.3 Spectroscopy and Isolation
A number of spectroscopic techniques have been employed in an effort to suitably
characterise NIs. As would perhaps be expected in a nitrillium betaine 1,3-dipole,
the inherent reactivity of the species is a major stumbling block in this process.
Following on from their initial application, [1] any preliminary evidence of the
existence of an NI intermediate within a reaction profile was indirect, [18] with the
first direct detection of an NI reported in 1973, using UV spectrometry at cryogenic
temperatures [50].
Multiple approaches towards the spectroscopic characterisation of NIs were
reported throughout the 1980s. The UV spectra of diphenyl NI and derivatives were
reported independently by both Holm and Heimgartner, employing 12 K matrix
isolation and immobilisation using EPA or PVC glass [51–53]. The IR spectra were
also disclosed [53]. The palette of diaryl NIs characterised using this technique
increased towards the middle of the decade, with over 20 UV and IR spectra reported
[54–57]. The utility of this approach was significantly bolstered with the application
of “flash” photolysis, enabling the direct capture of the UV spectrum of an NI at
room temperature [58, 59].
Other spectroscopic approaches were also developed, principally using high
temperature gas phase techniques [60]. Generation of the NI in this way
enabled analysis through mass spectrometry, photoelectron spectroscopy, or IR via
immobilisation of the NI on a KBr disc [24, 61]. Modern-day advances in IR
technology have recently seen the re-emergence of this approach as the most valuable
analytical technique in the characterisation of reactive NIs [26, 49, 62].
14 N NMR has also been shown to be a feasible approach towards NI identification,
although a limitation of this technique is that the NI is required to have
enhanced stability relative to other spectroscopic approaches [63]. More recently,
photocrystallography was also employed as a method of generating crystal structures
of diaryl NIs [64]. Formyl NI has also been shown to be detectable, via either
gas-phase neutralisation-reionisation mass spectroscopy, or low-temperature IR
spectroscopy [36, 37].
1.3.1 Ultraviolet
NI UV spectra are dominated by a large signal around 240–275 nm (heteroatomsubstituted NIs) or 370–465 nm (diaryl NIs) [50–54, 56, 58, 59, 61, 65]. This is
postulated to originate from a π-π* transition, with, in the case of diaryl NIs, an
extremely large extinction coefficient leading to a very broad signal [58]. Electronrich species on the N-terminus of the NI and electron-deficient species on C-terminus
will further increase absorption wavelength [58]. The UV spectrum of formyl NI has
never been recorded, however it is predicted to have a maximum absorbance of
around 230 nm [25].