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A pK a is estimated for the protonation of dimethoxycarbene based on the relationship pK a = pK R ÀpK H 2 O , where K H 2 O and K R are equilibrium constants for hydration of the carbene and reaction of the dimethoxymethyl carbocation with water to form dimethoxymethanol, respectively. A value of pK a = 15.5 in aqueous solution is derived based on pK R = À5.7 for the carbocation and an estimate of pK H 2 O = À21.2 based on a heat of formation of the carbene DH°f = À38.8 AE 2 kcal mol À1 , calculated at the G3 level in the gas phase. This value for the pK a compares satisfactorily with pK a = 11 inferred from the dependence of experimental rate constants for reaction of the carbene with alcohols and acetic acid upon the pK a of the oxygen acid in acetonitrile, taking account of the difference in solvents for the calculated and measured values. The calculated heat of formation of dimethoxycarbene in the gas phase is compared with an experimental value of À52.6 kcal mol À1 based on photoacoustic measurements of the heat of reaction of the carbene with methanol solvent to form trimethyl orthoformate. Reanalysis of the experimental data using a heat of solution of trimethyl orthoformate in methanol measured in this work suggests that the discrepancy arises in part from a difference in the heat of reaction of the carbene with methanol in solution and in the gas phase and partly (although less certainly) from a relatively high heat of solution of the carbene in methanol. a From DH v = 5.0 0.41t bp and t bp = 90.3°C. 31 b Ref. 32. c Ref. 33.

Alkaneoxodiazonium ions (R-N=N=O) and (R-O-N 2 ) , which are closely analogous to alkanediazonium ions (R-N 2 ) , were detected as reactive intermediates in acid-catalysed reactions of nitrosohydroxylamines and deamination-type reactions. The isomers were compared by high-level MO investigations of the reactions of nitrous oxide with cationic electrophiles H , CH 3 , Me 3 C and PhCH 2 . Strongly bonded species are formed in reactions with the powerfully electrophilic H and CH 3 , and the resulting ions differ considerably in stabilities; less stable species are formed with Me 3 C and PhCH 2 and the complexes so formed have similar stabilities. Two isomers of Ph(N 2 O) , analogous to phenyldiazonium ion, PhN 2 , are calculated to be stable ions with the end-N-bonded isomer being preferred over the O-bonded ion at the MP4(SDQ)/6-311G(d,p)//MP2(fc)/6-31G(d,p) and G3 levels of theory. Both are non-planar with the phenyl ring at 90°to the non-linear N 2 O residue. Syntheses of salts of Ar(N 2 O) are now planned. Scheme 1. Electronic con®gurations of arene-and alkanediazonium ions Scheme 2. Pro®les for heterolysis of alkane-and arenediazonium ions JOURNAL OF PHYSICAL ORGANIC CHEMISTRY