Due to the subtlety of this effect, the exact reasons for this rule have been vigorously debated in the chemical literature and is yet unsettled.
dˈuː tə ðə sˈʌɾəlɾi ʌv ðɪs ɪfˈɛkt, ðɪ ɛɡzˈækt ɹˈiːzənz fɔːɹ ðɪs ɹˈuːl hɐvbɪn vˈɪɡɚɹəsli dᵻbˈeɪɾᵻd ɪnðə kˈɛmɪkəl lˈɪɾɚɹᵻtʃɚ ænd ɪz jˈɛt ʌnsˈɛɾəld.