The energetics of protonation and hydrogen-bonded complex formation between the base and its conjugated acid (homoconjugation) of pyridine N-oxide and its selected derivatives was investigated by means of Restricted Hartree-Fock (RHF) and Møller-Plesset (MP2) ab initio calculations. It has been found that introducing the d polarization functions (the 6-31G* basis set) is required to reproduce correctly the geometry of the N−O bond. The proton-transfer energy surface in the homoconjugated cations exhibits a double minimum, with 2.4 kcal/mol energy barrier for the pyridine N-oxide homocomplex; this barrier vanishes when the thermodynamic correction is included. The protonation and homoconjugation Gibbs free energies computed in vacuo correlate well with the acid dissociation and cationic homoconjugation constants determined in acetonitrile.

The reaction of dimethyl ether (1) with atomic oxygen generated by photolysis of ozone or N 2 O was examined in low-temperature matrices. The major reaction products are two conformers of methoxymethanol (5). IR absorptions of the products were assigned by isotopic labeling ( 18 O and D) and DFT calculations at the B3LYP/ 6-311++G(d,p) level of theory. The mechanism of the formation of 5, in particular H abstraction from 1 by atomic oxygen (O 3 P and O 1 D), was investigated using UMP, UCCSD(T), and UDFT. In both the H abstraction and the O( 1 D) insertion reaction, the out-of-plane C-H bonds of 1 are preferentially attacked since the inplane C-H bonds are about 10 kcal/mol stronger. In the case of a reaction with O( 3 P), an Arrhenius activation energy of 3.5 kcal/mol is calculated at 298 K, which compares well with an experimental value of 2.85 kcal/mol. In the exit channel of the reaction, a radical-radical complex between CH 3 CH 2 • and •OH (-2.7 kcal/mol relative to separated products) is found. The latter is the starting point for the formation of 5 and helps to rationalize the stereoselectivity of the reaction leading to particular conformations of 5.

The photochemistry of butatriene (2), matrix-isolated in argon at 10 K, was investigated. UV irradiation (248 nm) produces vinylacetylene (1) and the dimer of acetylene and in addition methylenecyclopropene (3) as a major product. The photochemical 2 → 3 rearrangement is reversible, and on visible light irradiation (λ > 420 nm) 3 is transformed back to 2. The formation of 3 from 2 requires a [1,2]-H migration to allenylcarbene (8). According to MP2 and DFT calculations, 8 is a minimum on both the triplet (T) and the singlet (S) potential energy surface (PES). The comparison of measured and calculated infrared spectra suggests that two forms of 8 (either syn and anti form of S-8 or a mixture of S-8 and T-8) can be trapped at low temperatures in the matrix. The rearrangement 2 → 3 proceeds in a stepwise manner with S-8 as intermediate on the S−PES (calculated heat of formation at 298 K, ΔH°f(298) = 131.7 kcal/mol) and T-8 on the T-PES (ΔH°f(298) = 129.0 kcal/mol).

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