How did "djēm" avoid palatalization?

Question

While posting an answer to this question, I looked back at an answer I'd written on another site, which mentions the strange case of Latin diēs < PIt *diēm.

In particular:

The accusative form of [the Proto-Indo-European root behind Zeús and Juppiter], *dyēm (cf AGrk Zēn), also survived in Latin, in the form *diēm > diem. Uniquely, the dy here didn't become *dj > j; some scholars suggest that it evolved in a different Italic language/dialect, which didn't have the palatalization rules, then got borrowed back into Latin.

But, to my shame, I can't seem to figure out who these "some scholars" I cited actually are: I recently lost my web history, and with it all records of which particular articles I'd been reading.

And so I'm asking here: how did *diēm avoid the palatalization that created Jovis? I was under the impression that this palatalization happened back in Proto-Italic times, before Latin had split off; was there actually some Italic language that avoided the process entirely, from which Latin could have borrowed an unpalatalized *diēm? And if not, where could this notorious exception have come from?

Answer 1

Diem seems to be due not to borrowing, but to Lindeman's Law. This is a reconstructed rule of PIE whereby monosyllabic words beginning with a stop+sonorant cluster (CR-) could optionally insert the vowel equivalent of the sonorant between C and R.

So in this case, a monosyllable *di̯ēm would have two alternative forms: *di̯ēm and *dii̯ēm, where the second is the "Lindeman variant".

Now in Latin, *di̯- became j-, but *di- stayed intact. So diem can be explained as the reflex of the Lindeman variant, while Jovis etc. come from the *di̯- forms.