Bohlmann bands, in infrared spectra

The presence or the lack of the Bohlmann bands in the infrared (IR) spectra of perhydropyrido[l,2-c][l,3]oxazines was used to identify /ram-fused <1997TA109, 2003TL8609> orm-fused <2005EJ01378> conformations of the bicycle. 

A review has appeared regarding the infrared and nuclear magnetic resonance spectroscopic investigations of quinolizidine alkaloids (303). The connection between the C/D ring junction and the existence of Bohlmann bands in the IR spectra of indolo[2,3-a]quinolizidines has been reinvestigated and interpreted (304). 

The Bohlmann bands at 2805 and 2761 cm-1 in the infrared spectra indicated trans-fusion of the heterorings in 3,3-diethyl-l,3,4,6,7,116-hexahydro-2//-pyrimido[2,l-a]isoquinoline (69YZ649). The presence of Bohlmann bands in the infrared spectra of l,3,4,6,ll,llu-hexahydro-2//-pyrimido[l,2-6]-isoquinoline and its 2-oxo derivative justified the trans-type ring junction of the heterorings [68JCS(CC)1423]. 

The IEs on amine basicity are also due to changes in vibrational frequencies, not only computationally but also experimentally. Gas-phase IEs of O.lOkcalmol-1 per CD3 group on basicities of methylamine, dimethy-lamine, and trimethylamine can be reproduced by ab initio force constants for C-H stretching, which increase on TV-protonation.100 Infrared spectra of amines show characteristic bands (called Bohlmann bands ) in the 2700-2800 cm-1 region, lower than the 2900 cm-1 of a typical C-H stretch.169,170 Upon /V-protonation these bands revert to a typical, higher frequency. Therefore the zero-point energy of the C-H increases on... 

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