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meso-1,2-Diphenyl-1,2-ethanediol (1) as a hydrogen-bond donor and azanaphthalenes as hydrogen-bond acceptors form cocrystals, mostly in the ratio of 1:2. The X-ray structures provide detailed information on the binding motifs that consist of strong O−H···N hydrogen bonds and C−H···π interactions between C−H bonds of the heterocycles and the phenyl rings of meso-1,2-diphenyl-1,2-ethanediol. These motifs are found whenever a nitrogen atom is in the 2-position of the naphthalene skeleton (isoquinoline (3), quinazoline (2), and phthalazine (4)). Nitrogen atoms in the 1,3- and 2,3-positions, 1,3-diazanaphthalene (quinazoline) (2) and 2,3-diazanaphthalene (phthalazine) (4), alter the crystal lattice only in the case of phthalazine, due to additional hydrogen-bond- and π-stacking interactions. If a nitrogen atom is present in 1-position of naphthalene, 1,5-diazanaphthalene (naphthyridine) (5), the bonding motif consists of O−H···N hydrogen bridges between the nitrogen atoms in the 1- and 5-positions and the O−H groups of 1 leading to infinite strings of alternating naphthyridine and diol molecules. C−H···π interactions are found in this case between C−H groups of the adjacent six-membered “pyridyl” ring and the phenyl group attached to the carbon atom which is not involved in the formation of O−H···N hydrogen bonds as in the other three cases. This motif enables the incorporation of a second naphthyridine molecule without leading to additional hydrogen bonds, but with two C−H···π interactions. Attempts to generate cocrystals between 1 and 1-azanaphthalene (quinoline) as well as a tri-cocrystal between 1, 5, and naphthalene which should be of interest in this context have failed so far. Further attempts at cocrystallization of diazanaphthalenes with 1 also failed, which demonstrates the fine-tuned balance of cooperative and anticooperative effects for secondary intermolecular interactions.

1:1 cocrystals of oxalic and fumaric acid with 1,4-diazanaphthalene (quinoxaline) and 2,3-diazanaphthalene (phthalazine) were prepared. 1,5-Diazanaphthalene (naphthyridine) formed a 1:1 cocrystal only with oxalic acid. X-ray structural analyses are reported. The influence of the position of the two nitrogen atoms in these diazanaphthalenes on the crystal packing is analyzed. The gross crystal structure is governed by strong OH···N hydrogen bonds. The fine-tuning of the packing of the cocrystals of quinoxaline with oxalic and fumaric acid occurs via CH···O hydrogen bonds. The stabilization by the CH···O hydrogen bonds leads to a planar sheetlike structure in the cocrystal with fumaric acid, in contrast to the cocrystal with oxalic acid where the optimization of the CH···O hydrogen bonds results in a nonplanar bent structure. The geometrical arrangement of the nitrogen atoms in 1- and 5-positions in naphthyridine allows a planar sheetlike structure in the cocrystal with oxalic acid. In the case of phthalazine complete proton transfer from one carboxy group of oxalic and fumaric acid to a nitrogen atom of the heterocycle, generating ionic interactions is observed. The hydrogen atom of the second carboxy group of the acids is involved in a OH···O&dbd;C hydrogen bond.

Hydrogen carbonate (bicarbonate, HCO3^-) has been proposed to accelerate the decomposition of N2O3 because N-nitrosation of morpholine via a nitric oxide/oxygen mixture (^•NO/O2) was inhibited by the addition of HCO3^- at pH 8.9 [Caulfield, J. L., Singh, S. P., Wishnok, J. S., Deen, W. M., and Tannenbaum, S. R. (1996) J. Biol. Chem. 271, 25859−25863]. In the study presented here, it is shown that carbon dioxide (CO2) is responsible for this kind of protective effect because of formation of amine carbamates, whereas an inhibitory function of HCO3^- is excluded. N-Nitrosation of morpholine (1−10 mM) at pH 7.4−7.5 by the ^•NO-donor compounds PAPA NONOate and MAMA NONOate (0.5 mM each) was not affected by the presence of large amounts of HCO3^- (up to 100 mM) in aerated aqueous solution. Similar results were obtained by replacing the ^•NO-donor compounds with authentic ^•NO (900 μM). In agreement with data from the study cited above, ^•NO/O2-mediated formation of N-nitrosomorpholine (NO-Mor) was indeed inhibited by about 45% in the presence of 50 mM HCO3^- at pH 8.9. However, 500 MHz ¹³C NMR analysis with ¹³C-enriched bicarbonate revealed that significant amounts of morpholine carbamate are formed from reaction of equilibrated CO2 with morpholine (1−100 mM) at pH 8.9, but only to a minor extent at pH 7.5. The protective effect of morpholine carbamate formation is explained by a significantly reduced charge density at nitrogen. This view is supported by the results of density functional theory/natural population analysis, i.e., quantumchemical calculations for morpholine and morpholine carbamate. In agreement with its lower pKa, another secondary amine, piperazine, had already produced significant amounts of piperazine carbamate at pH 7.4 as shown by ¹³C NMR spectrometry. Consequently, and in contrast to morpholine, N-nitrosation of piperazine (2 mM) by both ^•NO/O2 (PAPA NONOate, 0.5 mM) and the ^•NO/O2^-•-releasing compound SIN-1 (1 mM) was inhibited by about 66% in the presence of 200 mM HCO3^-.

RHF/3-21G ab initio, density functional theory (B3LYP/3-21G), and single-point calculations using an effective core potential (B3LYP/6-31G*-ECP(S)//3-21G) are performed for different mechanisms of polymerization of acrylic acid and methyl acrylate by dicyclopentadienyl−zirconocene enolate complexes. Polymerization is considered involving a cationic zirconocene complex and a neutral zirconocene complex, respectively. In addition, a mechanism proposed by Collins is treated which involves a neutral and a cationic zirconocene complex in the CC bond-forming step at the same time. Catalytic cycles can be devised in all cases which suggest that all three mechanisms may be realizable under suitable conditions. However, the “bimetallic” mechanism proposed by Collins shows the lowest energy of activation for the propagation step. It is shown that bridging of the cyclopentadienyl rings should be important not only with respect to reactivity but also with respect to the possible catalytic cycle. Methyl methacrylate is not included in the treatment; i.e., the problem of tacticity in these polymerizations is not addressed.