According to RDF 1.1 IRI definition, IRIs should conform to the syntax defined in RFC 3987, which defines following grammar:
IRI = scheme ":" ihier-part [ "?" iquery ] [ "#" ifragment ]
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
RFC 3986 defines similar grammar for URIs as well.
So, scheme part of at least one ALPHA followed by colon is mandatory, thus IRI <node16s114au1x1> is rejected by org.openrdf.rio.helpers.RDFParserBase.createURI()
Although, note that rejection does not come from Sesame code itself, it uses BigdataValueFactoryImpl, which calls
com.bigdata.rdf.model.BigdataURIImpl.BigdataURIImpl(BigdataValueFactory, String) and the last one throws IllegalArgumentException since commit 4a2155e91ca7d6159c612011520ccbc6aea73792 (thompsonbry, 2008-04-16 22:59:01)