Syntheses of NCN and NC3N from ionic precursors in the gas phase and an unusual rearrangement of neutral NC3N: A joint experimental and theoretical study

Neutral NCN is made in a mass spectrometer by charge stripping of NCN-., wh ile neutral dicyanocarbene NCCCN can be formed by neutralization of either the corresponding anionic and cationic species, NCCCN-. and NCCCN+.. Theore tical calculations at the RCCSD(T)/aug-cc-pVTZ//B3LYP/6-31+G(d) level of th eory indicate that the (3)Sigma (-)(g) State of NCCCN is 18 kcal mol(-1) mo re stable than the (1)A(1) state. While the majority of neutrals formed fro m either NCCCN-. or NCCCN+. correspond to NCCCN, a proportion of the neutra l NCCCN molecules have sufficient excess energy to effect rearrangement, as evidenced by a loss of atomic carbon in the neutralization reionization (N R) spectra of either NCCCN+. and NCCCN-.. C-13 labeling studies indicate th at loss of carbon occurs statistically following or accompanied by scrambli ng of all three carbon atoms. A theoretical study at the B3LYP/6-31+G(d)//B 3LYP/6-31+G(d) level of theory indicates that C loss is a consequence of th e rearrangement sequence NCCCN --> CNCCN --> CNCNC and that C scrambling oc curs within singlet CNCCN via the intermediacy of a four-membered C-2v-symm etrical transition structure.