ION THERMOCHEMISTRY OF LOW-VOLATILITY COMPOUNDS IN THE GAS-PHASE .2. INTRINSIC BASICITIES AND HYDROGEN-BONDED DIMERS OF NITROGEN HETEROCYCLICS AND NUCLEIC BASES

The number densities (NB2) of low-volatility samples (B2) in the ion source of a pulsed high-pressure mass spectrometer were determined on the basis of the proton-transfer rates from t-C4H9+to B2. Subsequently, proton-transfer equilibria between B2and reference compounds B1were observed and equilibrium constants K = (IB2H+NB1/IB1H+NB2) were measured. The proton affinities (PA) of aromatic nitrogen heterocyclics decrease with increasing number of nitrogen atoms in the ring (all PA values related to PA(NH3) = 207 kcal mol-1): pyridine (PA = 222.0 kcal mol-1), 1, 2-diazine (218.3), 1, 3-diazine (215.5), 1, 4-diazine (210.8), 1, 3, 5-triazine (203.3). The addition of one or two fused benzene rings each increases the PA by ca. 6 kcal mol-1as seen in the series pyridine (PA = 222.0), quinoline (227.6), and acridine (233.0); 1, 4-diazine (210.8), quinoxaline (217.1), phenazine (224.8); and 1, 2-diazine (218.3), cinnoline (224.3). The effects of the fused rings are due mainly to electrostatic stabilization of the charge. Linear correlation is observed between the IPs and PAs of the di-and triazines. The hydrogen affinities (HA) of the radical ions (B+.) of these compounds are lower by 10-15 kcal mol-1than those of the monoazines; however, the HAs of all the nitrogen heterocyclics fall in the general range of amines and are quite distinct from carbon bases such as polycyclic aromatics. Solvent effects in H20 attenuate the basicities of the aromatic nitrogen bases, compared with NH3, by up to 32 kcal mol-1.Solvent effected attenuation of basicity increases with molecular size of BH+, owing to inefficient ion solvation, and with increasing polarity of the neutral B, owing to efficient solvation of B. PAs of nucleic bases were measured as follows : adenine (224.6), cytosine (224.9), and thymine (210.9). Thermodynamics of association equilibria BH+B⇄ BH+.B were measured as follows(-ΔH° kcal mor1; -ΔS°, cal mol-1K-1); pyridine (23.7, 28), adenine (30.3, 39), thymine (30.1, 37), cytosine (38.3, 37). The enthalpies and entropies of association indicate two hydrogen bonds in (adenine)2H+and (thymine)2H+and three hydrogen bonds in (cytosine)2H+. © 1979, American Chemical Society. All rights reserved.