Have you found C:\Program Files\Microsoft SQL Server\MSSQL11.SQL01\MSSQL\Binn\msxmlsql.dll? I think msxmlsql.dll is in the C:\Program Files\Microsoft SQL Server\110\Shared. Please check it. You could try to Copying msxmlsql.dll to the Binn folder to see
if it would be OK.
Or you could try to repair the SQL Server through SQL Server Installation Center.