Right
塞 supplies the sound - sāi exactly, no shift. Pure phonetic role; the 'block / stuff' meaning of
塞 contributes nothing here. Same phonetic family:
寨 stockade,
赛 to compete,
鳃 gill.
噻 is one of many
口-radical morphemes coined to transliterate foreign chemistry terms - see also
吡 /
吖 /
嘧 for similar IUPAC syllables.