Electronic and infrared spectra of H2C3H+ and cyclic C3H3+ in neon matrices

The A (1)A' <--X (1)A(1) electronic transition of the propargyl cation H2C3H+ With the origin band at 267.8(2) nm has been identified in a neon matrix at 5 K. The frequencies of the two modes excited in the upper state are 667(50) and 1629(50) cm(-1) and imply a reduction of symmetry from C-2v in the ground state to C-s in the excited state. The most intense IR mode of the propargyl cation is observed at 2079.9(1.0) cm(-1) and for the cyclopropenyl cation at 3130.4(1.0) cm(-1). Ab initio calculations on the excited states of the two isomer cations support the assignment and explain why the electronic transition could not be observed for the cyclic species; it lies below 200 nm. The A (2)A<--(XB1)-B-2 and B(2)A'<--(XB1)-B-2 absorptions of the neutral propargyl radical have also been observed with origin bands at 351.9(2) and 343.0(2) nm, respectively. These results provide the basis for the study of these astrophysically interesting C3H3+ species in the gas phase. (C) 2001 American Institute of Physics.