1.3 Spectroscopy and Isolation
5
1.3.2 Infra-red
IR spectroscopy is one of the most valuable techniques in determining whether a
specific NI will adopt an allenic or propargylic structure, as the most prominent
peak of the spectrum corresponds to the C–N antisymmetric stretch, with the
stretching frequency increasing with increasing bond order. All NIs with a C–N
stretch frequency below 2100 cm
−1 are almost entirely allenic (C–N bond order of
2), while those with a stretch frequency above 2200 cm
−1 are likely to primarily exist
as the propargylic resonance form. Those with a value between 2100 and 2200 cm
−1
exhibit both allenic and propargylic character [66].
Most diaryl, or even monoaryl NI species have been shown to possess a
predominantly propargylic structure through this technique (C–N stretch typically
ranges from 2215 to 2250 cm
−1 ) [53, 55, 57, 61]. The addition of any conjugative
functional group onto either of the aromatic rings, electron donating or withdrawing,
will lower the bond order of the C–N bond and will lower the stretching frequency
accordingly [57]. NIs substituted by heteroatoms typically adopt allenic or mixed
resonance forms, with C–N stretch values between 1990 and 2170 cm
−1 [26, 43,
45, 48, 49, 66–68]. An IR spectrum of formyl NI has been recorded, with a C–N
stretching frequency of 2027 cm
−1 [36]. This is in relatively good agreement with
the relevant calculated spectral values for this dipole species [25, 36].
1.3.3 Nuclear Magnetic Resonance
There has been considerably less focus on the characterisation of NIs by NMR
in comparison to IR and UV spectroscopy, likely due to the relatively specialised
conditions required to capture an NMR spectrum making the stability of the
compound a necessity.
In the specific cases where NI NMR spectra have been examined,
13 C resonances
have been observed between 45 and 70 ppm [25, 63]. This is complicated by large
T 1 values and small T 2 values, which can lead to extremely broad and undetectable
peaks [63].
14 N NMR is a viable alternative, which generates a signal for the α-N at
around −215 to −170 ppm [63] (Fig. 1.3).
1.3.4 Isolation of NI Compounds
While spectroscopic analysis of a multitude of different NI species rapidly became
commonplace during the 1980s, the high reactivity of the dipole still prevented
isolation. Alternative syntheses of the NI would prove necessary for any further
progress to be made in the area.
5
1.3.2 Infra-red
IR spectroscopy is one of the most valuable techniques in determining whether a
specific NI will adopt an allenic or propargylic structure, as the most prominent
peak of the spectrum corresponds to the C–N antisymmetric stretch, with the
stretching frequency increasing with increasing bond order. All NIs with a C–N
stretch frequency below 2100 cm
−1 are almost entirely allenic (C–N bond order of
2), while those with a stretch frequency above 2200 cm
−1 are likely to primarily exist
as the propargylic resonance form. Those with a value between 2100 and 2200 cm
−1
exhibit both allenic and propargylic character [66].
Most diaryl, or even monoaryl NI species have been shown to possess a
predominantly propargylic structure through this technique (C–N stretch typically
ranges from 2215 to 2250 cm
−1 ) [53, 55, 57, 61]. The addition of any conjugative
functional group onto either of the aromatic rings, electron donating or withdrawing,
will lower the bond order of the C–N bond and will lower the stretching frequency
accordingly [57]. NIs substituted by heteroatoms typically adopt allenic or mixed
resonance forms, with C–N stretch values between 1990 and 2170 cm
−1 [26, 43,
45, 48, 49, 66–68]. An IR spectrum of formyl NI has been recorded, with a C–N
stretching frequency of 2027 cm
−1 [36]. This is in relatively good agreement with
the relevant calculated spectral values for this dipole species [25, 36].
1.3.3 Nuclear Magnetic Resonance
There has been considerably less focus on the characterisation of NIs by NMR
in comparison to IR and UV spectroscopy, likely due to the relatively specialised
conditions required to capture an NMR spectrum making the stability of the
compound a necessity.
In the specific cases where NI NMR spectra have been examined,
13 C resonances
have been observed between 45 and 70 ppm [25, 63]. This is complicated by large
T 1 values and small T 2 values, which can lead to extremely broad and undetectable
peaks [63].
14 N NMR is a viable alternative, which generates a signal for the α-N at
around −215 to −170 ppm [63] (Fig. 1.3).
1.3.4 Isolation of NI Compounds
While spectroscopic analysis of a multitude of different NI species rapidly became
commonplace during the 1980s, the high reactivity of the dipole still prevented
isolation. Alternative syntheses of the NI would prove necessary for any further
progress to be made in the area.