Chandler is worth reading on this.
EDIT: See below for links to some of the sources.
932.
In conformity with the best Greek manuscripts, though contrary to the express precepts of the ancient grammarians, the following monosyllables are unaccented when they precede the words to which they belong; ὁ ἡ, οἱ αἱ of the prepositive article (ὅ=ὅς, ἥ, οἵ, αἵ, ὅ of the postpositive article or relative pronoun are accented), the relative adverb ὡς, the negative οὐ οὐκ or οὐχ, the conjunction εἰ or αἰ, and the prepositions ἐκ ἐξ, ἐν εἰν, ἐς εἰς ὡς, as ἐκ κεφαλῆς ἐς πόδας: ὁ μάντις ἦν ἐν τῇ τέχνῃ: εἰν Αἵδου: σάφα οὐκ οἶδ’ εἰ θεός ἐστιν: ὡς ἐκ κακῶν ἐχάρη: εἰς Φωκέας ὡς πρὸς συμμάχους ἐπορεύετο: ἡ οὐ διάλυσις: οἱ ἄνδρες καὶ αἱ γυναῖκες.
Ὡς, > as> , is oxytone when it follows the word to which it belongs, as θεὸς δ’ ὣς τίετο δήμῳ. When it stands for οὕτως some make it oxytone, but there seems more authority for writing ὧς, as ὣς or ὧς εἰπών.
Οὐ (or οὐκ), when it means > No> , or stands at the end of a sentence, is oxytone, as
P. Ζεὺς δ’ ἔστ’ ἐκεῖ τις, ὃς νέους τίκτει θεούς;
D. οὔκ, ἀλλ’ ὁ Σεμέλην ἐνθάδε ζεύξας γάμοις.
Eurip. Bacch. 467.
B. νὴ τὸν Δί’, αὕτη πού ’στί σοί γ’ ἡ Δαρδανίς.
P. οὔκ, ἀλλ’ ἐν ἀγορᾷ τοῖς θεοῖς δᾲς κάεται.
Aristoph. Vesp. 1371.
S. 1. αἰβοῖ· φέρ’ ἄλλην, χἀτέραν μοι χἀτέραν,
καὶ τρῖβ’ ἔθ’ ἑτέρας. S. 2. μὰ τὸν Ἀπόλλω ’γω μὲν οὔ.
Aristoph. Pax 15.
δίδωμ’ ἕκουσα τοῖσδ’ ἀναγκασθεῖσα δ’ οὔ.
Eurip. Heraclid. 551.
The prepositions are also oxytone under the conditions mentioned above, #913.
933.
Note 1.–‘There are in Greek, as in other languages, words so unimportant of themselves that they have no accent of their own, but are associated by the speaker with the really accented word to which they belong, in the same way as if the two formed one word. In Greek, however, a distinction is observed in such words: (1) those which stand > before> , and (2) those which stand > after> , the word that they refer to. The former of these unaccented words are called > proclitics > [by Hermann, not by any ancient authority], and are not furnished by the Greeks with a sign of accent: the others are called > enclitics> . They differ from each other merely by position; for e.g. τοι belongs to both in τοιγάρτοι, the first τοι being proclitic, the second enclitic;’ Göttling, Greek Accent. p. 99. This passage expresses the common doctrine concerning the nature of proclitics and enclitics. The Greek grammarians know nothing whatever about proclitics. Ὁ, ἡ, οἱ, and αἱ are oxytone, Joh. Alex. 22. 26; Apoll. de Pron. 62; Arc. 178. 12; Charax, ap. A. G. 1153: so too is εἰ, Arc. 185. 6; Joh. Alex. 40. 17; and οὐ, Arc. 183. 26; Joh. Alex. 25. 31: on the whole subject, see Göttling, Accent. pp. 388-9; Reiz, de Inclin, Accent. p. 43. Dindorf, in his edition of Sophocles, Lips. 1863, reads in (Ed. Tyr. 182, ἓν δ’ ἄλοχοι, where others leave ἑν unaccented; one cannot help wishing that editors would once for all make up their minds as to the principles on which they mean to accent their Greek.
And of course:
977.
…Editors do as well as they can in such awkward cases. Sometimes perhaps they reproduce the accents of a manuscript, and when they do, they print what may be the faint echo of a tradition going back to the best ages of classical antiquity, but which probably represents no more than the practice of the scribe’s own times. The oldest manuscript of any classical author continuously accented is comparatively modern. When manuscripts are not followed, theories of what the Greek accents must have been are generally acted on, and the result is an amount of variety in the accentuation of printed books which could hardly have been reached in any other manner. The curious reader should by all means peruse Lobeck’s unfinished essay, ‘De interpunctione cum enclisi et synalœphe conjuncta,’ in the Pathologiæ Græci Sermonis Elementa. Pars posterior, pp. 321-337.
EDIT:
The first reference to pg. 99 of Göttling is here, and from an English translation: https://books.google.com/books?id=lXESAAAAIAAJ&pg=RA1-PA99
The references to pages 388 and 389 later on are not from that translation, however, but from the German. Chandler is clearly getting much of his argument from it.
https://archive.org/stream/allgemeinelehrev00goet#page/388/mode/2up
The Lobeck essay is here, but it’s Latin so I can’t read a word of it:
http://babel.hathitrust.org/cgi/pt?id=coo.31924021604313;view=1up;seq=337