Electronic Spectroscopy of an Isolated Halocarbocation: The Iodomethyl Cation CH2I+ and Its Deuterated Isotopomers

Abstract

Building upon our recent observation of the gas-phase electronic spectrum of the iodomethyl cation (CH2I+), we report an extensive study of the electronic spectroscopy of CH2I+ and its deuterated isotopomers CHDI+ and CD2I+ using a combination of fluorescence excitation and single vibronic level (SVL) emission spectroscopies. The spectra were measured in the gas phase under jet-cooled conditions using a pulsed discharge source. Fluorescence excitation spectra reveal a dominant progression in nu3 (C-I stretch), the frequency of which is markedly smaller in the upper state. Rotational analysis shows that, while the A constant is similar in the two states, the excited state has significantly smaller B and C constants. These results indicate a lengthening of the C-I bond upon electronic excitation, consistent with calculations which show that this transition is analogous to the well-known pi-pi* transition in the isoelectronic substituted formaldehydes. SVL emission spectra show progressions involving four of the six vibrational modes; only the C-H(D) stretching modes remain unobserved. The vibrational parameters determined from a Dunham expansion fit of the ground state vibrational term energies are in excellent agreement with the predictions of density functional theory (DFT) calculations. A normal-mode analysis was completed to derive a harmonic force field for the ground state, where resonance delocalization of the positive charge leads to partial double bond character, H2C+-I <--> H2C=I+, giving rise to a C-I stretching frequency significantly larger than that of the iodomethyl radical.