Infrared multiple photon dissociation (IRMPD) spectroscopy of the proton-bound dimer of 1-methylcytosine in the gas phase

The vibrational spectrum of a hemiprotonated pair of the nucleobase cytosine has been recorded in the 300−1810 cm−1 domain using action spectroscopy on electrosprayed positive ions with a free-electron laser. The singly charged, gaseous cation has the same structure as that reported from crystallography, inferred from a strong absorption seen at 1761 cm−1, identified as the carbonyl stretch of the protonated ring mixed with in-plane bending of the bridging H+ based on a 29 cm−1 shift to lower frequency when all five of the exchangeable hydrogens are replaced by deuterium. IRMPD expels the neutral from the proton-bound dimer, leaving behind a protonated monomer. Whereas the bridging H+ in the dimer ion lies between the nitrogens of the two rings, IRMPD of the protonated monomer ion resulting from the dimer shows its H+ preferably situated on the carbonyl oxygen, indicating that the proton moves from one basic atom to another when the dimer ion dissociates.