HOckel rule

In accord with the HOckel rule of 4/i + 2 electrons, both cyclobutadiene and cyclooctatetraene (cot) are nonaromatic. Cyclooctatetraene contains alternating bond lengths and has a tub-shaped conformation ... 

A thiepine is formally isoelectronic with the 871-electron cyclooctatetraene and cycloheptatrienide anion as well as other heteropines, and if planar, should be antiaromatic as the HOckel rule predicts. However, as discussed under x-ray diffraction (Section 9.03.2.1), the thiepine ring is boat shaped. H NMR spectral analysis indicates that thiepines are not paramagnetic but atropic (Section 9.03.2.2) (see also Section 9.01.1). 

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