STRUCTURES OF CNHX(-LESS-THAN-OR-EQUAL-TO-22 AND X-LESS-THAN-OR-EQUAL-TO-5 - EMERGENCE OF PAHS AND EFFECTS OF DANGLING BONDS ON CONFORMATION() MOLECULES FOR N)

Mixed carbon/hydrogen cationic clusters, CnHx+ are generated in a lase r desorption ion source over the size range 0 less than or equal to x less than or equal to 5 and 5 less than or equal to n less than or equ al to 22. These species are mass selected and their conformations dete rmined using ion mobility/ion chromatography methods. The conformation s of pure carbon cationic species have been previously reported as pur e linear chains for n less than or equal to 6, mixed chains and monocy clic rings for 7 less than or equal to n less than or equal to 9, and pure monocyclic rings for n greater than or equal to 10. Addition of a single H atom extends the range of linear species to n = 15, and addi tion of two or more H atoms extends the linear isomer to n greater tha n or equal to 22, primarily at the expense of the fraction of monocycl ic ring present. At n = 15 a new isomer appears for x greater than or equal to 4 and persists to n greater than or equal to 22. Evidence is presented that this isomer is mast likely a benzene ring with a carbon loop attached at ortho positions, providing a starting point for poly cyclic aromatic hydrocarbon (PAH) formation in carbon/hydrogen plasmas . The fraction of linear isomer for CnH2+ is similar to 1.0 up to n = 12 and then smoothly decreases relative to the monocyclic isomer until it is nearly gone at n = 20. This fractional abundance is quantitativ ely matched by C-n(-) anions over this range of n, indicating H atoms act primarily as electron donors tying up dangling bonds. This analogy indicates CnH+ abundances should then reflect C-n abundances, quantit ies that cannot be directly measured experimentally. The CnH+ relative linear isomer abundances fall between C-n(+) and C-n(-) (or CnH2+) ab undances, declining from 1.0 at n = 8 to near 0.0 at n = 16, indicatin g large linear carbon clusters are almost certainly unstable to ring c losure.