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 H2C3 H+ 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 66 7(50) and 1629(50) cm(-1) and imply a reduction of symmetry from C-2v in th e 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 cyclopropen yl cation at 3130.4(1.0) cm(-1). Ab initio calculations on the excited stat es of the two isomer cations support the assignment and explain why the ele ctronic transition could not be observed for the cyclic species; it lies be low 200 nm. The A (2)A"<--(XB1)-B-2 and B(2)A'<--(XB1)-B-2 absorptions of t he neutral propargyl radical have also been observed with origin bands at 3 51.9(2) and 343.0(2) nm, respectively. These results provide the basis for the study of these astrophysically interesting C3H3+ species in the gas pha se. (C) 2001 American Institute of Physics.