Relative carbonyl isocyanate cation [OCNCO]+ affinities of pyridines determined by the kinetic method using multiple‐stage (MS3) mass spectrometry

Abstract

AbstractIon‐molecule reactions in a pentaquadrupole mass spectrometer are used to generate cluster ions in which neutral pyridine molecules are bound to a linear polyatomic cation, [OCNCO]+. The dimeric adduct, viz. Py1[OCNCO]+Py2 where Py1 and Py2 represent substituted pyridines, formed upon reaction of mass‐selected [OCNCO]+ with a mixture of pyridines, has a loosely bound structure, as suggested by triple stage mass spectrometric (MS3) experiments. Dimeric adducts comprised of meta‐ and/or para‐substituted pyridines (unhindered pyridines) display an excellent linear correlation between the fragment ion abundance ratio ln [Py1[OCNCO]+/Py2[OCNCO]+] and the proton affinity difference of the pyridines. On the assumption that the effective temperature of the [OCNCO]+ ‐bound dimers is similar to that of the corresponding Cl+‐bound dimers, [OCNCO]+ affinities of the substituted pyridines relative to pyridine are estimated to be 3‐MePy 2.3, 4‐MePy 3.2, 3‐EtPy 3.5, 4‐EtPy 4.1, 3,5‐diMePy 4.9 and 3,4‐diMePy 5.6 kcal mol−1 (1 kcal = 4.184 kJ). A linear relationship between the relative [OCNCO]+ cation affinity and the relative proton affinity (PA) is derived as relative [OCNCO]+ affinity (kcal mol−1) = 0.96 ΔPA, using the assumed effective temperature of 555 K. Dimers consisting of ortho‐substituted pyridines display substantial steric effects between the ortho‐substituted alkyl group and the central [OCNCO]+ cation. A set of gas‐phase steric parameters (Sk) is determined and steric effects are ordered 2‐MePy (‐1.39) <2,5‐diMePy (‐3.02) < 2,4‐diMePy (‐3.15) < 2,3‐diMePy ( −3.29) < 2,6‐diMePy (‐5.09) < 2,4,6‐triMe (‐6.13). A greater steric effect is experienced in the [OCNCO]+ system than in the corresponding Cl+ system, owing to the larger central ion in Py[OCNCO]+ Py. Structural and electronic information regarding the bond angles, bond lengths and charge density distributions in [OCNCO]+, Py[OCNCO]+ and Py[OCNCO]Py+ was obtained from ab initio calculations. The calculations show that [OCNCO]+ is linear with high positive charge densities on the carbon atoms. The calculations also reveal that the Py[OCNCO]+ monomer is a planar ion with the nitrogen atom of pyridine bound to a carbon atom of the [OCNCO]+ cation, and that the Py[OCNCO]Py+ dimer is a symmetrical ion in which the nitrogen atoms on the pyridine molecules bind to the carbon atoms of the central cluster ion, [OCNCO]+.