Cryogenic Ion Spectroscopy of Protonated and Sodiated Methyladenine Derivatives.

Ultraviolet photodissociation (UVPD) spectra of protonated 9-methyladenine (H9MA), protonated 7-methyl adenine (H7MA), protonated 3-methyladenine (H3MA), and sodiated 7-methyladenine (Na7MA) near the origin bands of the S-S transition were obtained using cryogenic ion spectroscopy. The UV-UV hole burning, infrared (IR) ion-dip, and IR-UV double resonance spectra showed that all the ions were present as single isomers in a cryogenic ion trap. The UVPD spectrum of H9MA exhibited only a broad absorption band, whereas the spectra of H7MA, H3MA, and Na7MA displayed moderately or well-resolved vibronic bands. Potential energy profiles were computed to understand the reason for the different bandwidths of the vibronic bands in the spectra. The broadening of the bands was correlated with the slopes between the Franck-Condon point and the conical intersection between the S and S states in the potential energy profiles, thus reflecting the deactivation rates in the S state.