RELATIVE CARBONYL ISOCYANATE CATION [OCNCO](-STAGE (MS(3)) MASS-SPECTROMETRY() AFFINITIES OF PYRIDINES DETERMINED BY THE KINETIC METHOD USING MULTIPLE)

Ion-molecule reactions in a pentaquadrupole mass spectrometer are used to generate cluster ions in which neutral pyridine molecules are boun d to a linear polyatomic cation, [OCNCO](+). The dimeric adduct, viz. Py(1)[OCNCO](+)Py(2) where Py(1) and Py(2) represent substituted pyrid ines, formed upon reaction of mass-selected [OCNCO](+) with a mixture of pyridines, has a loosely bound structure, as suggested by triple st age mass spectrometric (MS(3)) experiments. Dimeric adducts comprised of meta- and/or para-substituted pyridines (unhindered pyridines) disp lay an excellent linear correlation between the fragment ion abundance ratio In[Py(1)[OCNCO](+)/Py(2)[OCNCO](+)] and the proton affinity dif ference of the pyridines. On the assumption that the effective tempera ture of the [OCNCO](+)-bound dimers is similar to that of the correspo nding Cl+-bound dimers, [OCNCO](+) affinities of the substituted pyrid ines 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 [OCNC O](+) cation affinity and the relative proton affinity (PA) is derived as relative [OCNCO](+) affinity (kcal mol(-1)) = 0.96 Delta PA, using the assumed effective temperature of 555 K. Dimers consisting of orth o-substituted pyridines display substantial steric effects between the ortho-substituted alkyl group and the central [OCNCO](+) cation. A se t of gas-phase steric parameters (S-k) is determined and steric effect s are ordered 2-MePy (-139) < 2,5-diMePy (-302) < 2,4-diMePy (-3.15) < 2,3-diMePy (-3.29) < 2,6-diMePy (5.09) < 2,4,6-triMe (-6.13). A great er steric effect is experienced in the [OCNCO](+) system than in the c orresponding 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 tbe carbon atoms. The calculations also reveal that tbe P y[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[OCNC O]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](+).