word order - "Not to" or "to not"

What is the difference between:

1. "I promise not to misbehave."


2. "I promise to not misbehave."

as in something a kid would say to convince its parents that it will behave well?

Answer

Some people think it's important to avoid "splitting infinitives." In these sentences, "to misbehave" is an infinitive verb and "splitting" it means inserting a word between the "to" and the "misbehave." In your example 2, "promise to not misbehave" is a split infinitive.

I happen to think this is an entirely pointless rule. Sometimes it makes more sense, either because of clarity, or as Andrew says, because of the sound of the sentence to split the infinitive.

One commonly cited example of a split infinitive that sounds much better split is that Star Trek slogan, "to boldly go where no one has gone before." This breaks the split infinitive rule - which would tell you that it must be "to go boldly" or "boldly to go" - but sounds much nicer.

Additionally, in some circumstances repositioning the "not" can subtly change the meaning of the statement:

Be aware that putting "not" or another adverb between "to" and its verb adds some emphasis to that adverb. For example, in the sentence "They decided not to stay another night" the phrase "they decided" is the most important information, but the sentence "They decided to not stay another night" tells us that maybe they decided to stay another night before, but now it is important that they will not stay.
http://learnersdictionary.com/qa/Split-Infinitives