"NOT TO" or "TO NOT"?

reason not to...or reason to not?

I'd say that it supposed to be 'reason not to'

e.g. There must be a reason for them not to speak English.

Other similar example:
There must a reason for not doing it.

However, there is a slight change in meaning between 'not to' and 'to not'

1) They forced me not to do it

(I was going to do it but they forced me to give up the idea)

2) They forced me to not do it

(I was doing it but they stopped me forcibly)

P/S: But I have never come across any book or statement that explains about this. DO COMMENT IF YOU FIND ANY. THANKS.