Q. Is the word “to” capitalized in a title or heading when used as part of an infinitive verb?
A. Whoever decided that “to” should be considered part of the infinitive verb form in English has caused more trouble than such a small word is worth (see “infinitives, split”). It certainly doesn’t merit capitalization in titles. The first eleven editions of CMOS said to capitalize all “important” words in a title: “nouns, pronouns, adjectives, adverbs, verbs, first words, and last words.” Starting with the twelfth edition, that advice was expanded to clarify that “verbs” did not extend to the “to” in infinitives, which should remain lowercase in titles (see CMOS 8.169 in the current, or seventeenth, edition). So,
Born to Run (where “to” marks the infinitive)
“Midnight Train to Georgia” (where “to” is a preposition)
But, when “to” is the first or last word,
To Kill a Mockingbird (infinitive marker)
To the Lighthouse (preposition)
“If You Asked Me To” (infinitive marker [verb implied])
Exceptions in the middle of a title would be rare. Here’s one, in the title of an article from volume 2012 of the journal Supreme Court Review:
“ ‘To Regulate,’ Not ‘To Prohibit’: Limiting the Commerce Power”
CMOS doesn’t cover that scenario. (If we did, we might say to capitalize the first and last words in a quoted phrase within a title.) Rarer still would be a title that featured “to” as an adverb—as in the phrase “come to” in the sense of “regain consciousness.” You’ll have to take our word for it that we’d capitalize the T in that case.