Thermodynamic stability of the molecules YC2, YC3, YC4, YC5, YC6, YC7, and YC8 by high temperature Knudsen effusion mass spectrometry

The molecules YC2, YC3, YC4, YC5, YC6, YC7, and YC8 have been observed in a Knudsen effusion mass spectrometric investigation of the gas phase above the Y--Ir--Au--graphite system. The enthalpies ΔH/sup circle-open/0 of the reactions Y(g)+n C(graph) = YC/sub n/(g)(n = 2--8) were evaluated by the third law method; the second law method was employed for the gaseous molecules with up to six carbon atoms. The selected values of the reaction enthalpies ΔH/sup circle-open/0 were combined with ancillary literature data to yield the atomization enthalpies ΔH/sup circle-open//sub a//sub ,0/ and the standard enthalpies of formation ΔH/sup circle-open//sub f//sub ,298.15/ in kJ mol-1 of the gaseous yttrium carbides. The values of ΔH/sup circle-open/0 and ΔH/sup circle-open//sub a//sub ,0/ (in kJ mol-1) are 197 +- 5 and 1225 +- 8 for YC2; 377 +- 10 and 1757 +- 12 for YC3; 361 +- 8 and 2484 +- 10 for YC4; 499 +- 10 and 3057 +- 15 for YC5; 536 +- 15 and 3731 +- 20 for YC6; 684 +- 35 and 4294 +- 35 for YC7, and 693 +- 35 and 4727 +- 35 for YC8. The values of ΔH/sup circle-open//sub f//sub ,298.15/ (in kJ mol-1) are 623 +- 8, 804 +- 12, 790 +- 10, 924 +- 15, 966 +- 20, 1105 +- 35, and 1124 +- 35 for YC/sub n/ (n = 2--8), respectively