.

$$H_{1-4} = \left ( \beta^h sin \theta_{1-2} sin \theta_{3-4} + \beta cos \theta_{1-2} cos \theta_{3-4} \right )$$ (9)

where we are defining a ``horizontal'' $\beta^h$ in analogy with the ``parallel'' one normally used in these kinds of calculations. Since there is no $\sigma_{1-4}$ bond (from pre-hybridized spⁿ orbitals), the normal Hückel rules tell us that the second term in Equation 9 vanishes. Thus

$$H_{1-4} = \beta^h sin \theta_{1-2} sin \theta_{3-4}$$ (10)