The ACS recently announced that its new pre-print server ChemRxiv (chem archive), is available in beta at https://chemrxiv.org/. ChemRxiv is one of several new pre-print repositories modeled after ArXiv, the long running pre-print archive for physics.
A pre-print is a pre-publication version of a paper, posted online so that research can be widely shared and made freely available as soon as possible. Pre-prints are not peer-reviewed, and are not edited, they are usually published as-is. ACS does only a minimal check of submissions, stating that “ChemRxiv conducts a basic screen for plagiarism, offensive language, and non-scientific content.”
Before posting a paper on ChemRxiv or elsewhere, you should check the publication policies of any journal you intend to publish in. The ACS policies on prior publication can be found at http://pubs.acs.org/page/policy/prior/index.html
Please feel free to contact me at email@example.com about this, or with any other questions you may have about open access. The Libraries have specialists who can help!