According to the NC Department of revenue (extracted below), if your separation has been legally sanctioned by a court (court order for separation) then you can be considered unmarried and file single. Otherwise you can file as married filing jointly or married filing separately until the the divorce is final. In some cases, if the spouses lived apart for the last 6 months of the tax year and provided a home for a qualifying dependent, that person could file head of household.
See Publication 501 for more details on filing status.
"Therefore, a separation agreement must be sanctioned by a court in the form of a decree or judgment for parties to be considered unmarried for purposes of section 2(b) of the Code."