Why are smaller fragments preferentially lost from radical cations at low energies and larger ones at high energies?: An experimental and theoretical study

Abstract

A long-standing mystery in gas phase ion chemistry is: why are smaller fragments preferentially lost at low energies and larger fragments at higher energies in competing α-cleavages? This is addressed here by studies of dissociations of ethanol, 2-propanol, 2-butanol, 2-methylpropane and 2-butanone radical cations. The onsets and energy dependencies of the reactions were obtained by photoionization mass spectrometry. Stationary point geometries, critical energies and vibrational frequencies were generated by B3LYP/6-31G(d) and B3LYP/6-311G(d,p) theory (DFT). Dependencies of reaction rates on internal energy were calculated by Rice–Ramsperger–Kassel–Marcus (RRKM) theory. Results obtained establish the generality of a previous finding that loss of H is slower than competing losses of polyatomic fragments. This is attributable to substantial lowering of the frequencies of the vibrations that are converted to rotations and translations in the transition states for the latter reactions, but not in transition states for H losses. It is concluded that lower dissociation thresholds produce preferred losses of smaller alkyl fragments at low energies and that looser transition states favor losses of larger fragments at higher energies. DFT results and abundances of parallel alkane eliminations (two step processes also initiated by simple CC bond cleavage) indicate that cleavage of the CC bond to the larger alkyl fragment is most frequent even below the threshold for complete dissociation (energies at which ion-induced dipole alkyl complexes are formed), i.e., at all energies of bond cleavage, even if not reflected in simple cleavage abundances near threshold. Obtaining realistic rates by RRKM theory required increasing the lowest transition state vibrational frequencies (for modes that become rotations and translations) to well above those obtained by DFT for alkyl losses, further evidence that the rates of those reactions are controlled by minimum entropy transition states occurring earlier than the single imaginary frequency transition states found by DFT. In addition, comparison of DFT critical energies, photoionization critical energies and critical energies that give the best RRKM rates support relaxation of the ground states to lower energy species with long CC bonds following ionization. Preferred losses of larger alkanes at low energies, dominant losses of larger alkyl groups at high energies and formation of long CC bonds to the larger alkyl in the ground state ion all appear to have a common origin.