13C NMR calculations on azepines and diazepines

Abstract

Ab initio and DFT calculations of ¹³ C NMR chemical shifts of 1H-, 2H- and 3H-azepines as well as recently synthesised 1H- and 5H-1,3-diazepines are reported. The reliabilities of the computational methods used for this purpose are evaluated by examining a large number of combinations of basis sets and geometry optimisations. Generally, the Becke3LYP/6-31+G* and HF/6-31G* or HF/6-31+G* single-point calculations based on MP2/6-31G* geometries give the best agreement with experiment (3–4 ppm deviation), better than the corresponding BLYP calculations. Localised orbital methods such as IGLO or LORG do not improve the accuracy. The ¹ H NMR chemical shifts are also calculated, but the smaller chemical shift range for protons makes the calculated data inherently less precise. Again, Becke3LYP/6-31G* or BLYP/6-31G* with MP2/6-31G* geometries and the HF/6-311+G(3df,2p)//HF/6-31G* combination give the best results. Overall, the ¹³ C NMR calculations in particular are sufficiently precise to be a valuable tool in the identification of novel compounds of this type.