Anthony F. Hegarty

An ab initio study of the electronic spectrum of dichlorocarbene CCl2

Abstract The lower-lying electronic states of dichlorocarbene have been studied by ab initio methods at different levels of accuracy. For the 1B1 ← 1A1 transition, the calculated transition energy (1.95 eV) is in good agreement with the gas-phase value (2.10 eV) of Predmore, Murray and Harmony. The discrepancy with the previous CI study by Ha. Gremlich and Buhler is commented on. The vibrational frequencies of CCl2 in both 1A1 and 1B1 states were calculated and compared with experiment. The first four triplet and four cationic states have also been examined.

Elimination–addition mechanism for the hydrolysis of carbamates. Trapping of an isocyanate intermediate by an o-amino-group

The hydrolytic conversion of aryl carbamates to amines is base catalysed and evidence is presented that the process involves an E1cB elimination with the formation of isocyanate intermediates. Substituents in the leaving group (O-aryl ring) have a larger effect (ρ=+3·17) on the observed rate of hydrolysis than those in the N-aryl ring (ρ=+0·64) where the effects of a substituent are compensatory. Using N-(p-nitrophenyl)carbamates, it was possible to measure both the acidity of the carbamate (Ka1) and the rate of reaction of the carbamate anion (k2); k2 was sensitive to substituent effects (ρ=+2·90) for aryl carbamates indicating a high degree of acyl–oxygen bond cleavage in the transition state. A change in mechanism from E1cB to BAC2 attack by hydroxide ion was noted for poorer leaving groups. The pH profile for the conversion of p-nitrophenyl N-(o-aminophenyl)carbamates to o-phenyleneurea was interpreted to show that the o-amino-group traps an isocyanate intermediate after the rate determining E1cB elimination. Substituent effects for NN-disubstituted carbamates which hydrolyse via a BAC2 mechanism are also discussed.

Can the Pentazole Anion (N5-) Be Isolated and/or Trapped in Metal Complexes?

Abstract The 1,3-dipolar cycloreversion pathway of the pentazole anion (N 5 − ) to the azide anion (N 3 − ) plus dinitrogen (N 2 ) has been investigated using ab initio methods. At the MP4SDQ/6–31 + G* level of theory plus zero-point energy contributions, the pentazole anion is predicted to lie at 31 kcal mol −1 above the N 3 − + N 2 system but the energy barrier for decomposition is 22 kcal mol −1 . This indicates that the pentazole anion could be isolated in an inert matrix at low temperature. Comparison between extended Huckel calculations on the (N 5 )M(CO) 3 and (C 5 H 5 )M(CO) 3 complexes (with M = Fe 2+ , Mn + and Cr) suggests that the N 5 − complexes would be formed if the fragments could be brought together. Predicted vibrational frequencies of the N 5 − anion are also reported.

Ab initio study of the hydration of carbon dioxide: Additional comments based on refined calculations

Abstract Ab initio calculations on the formation of carbonic acid from the hydration of carbon dioxide with water dimer are re-examined. Fully optimized geometries of the three stationary points (minima and transition state) with the 3-21G basis set are reported. They possess non-planar structures. The inclusion of polarization (with the 6-31G* basis) and electron correlation (via Moller-Plesset perturbation theory to second through to fourth-order using the 6-31G basis) tends to enlarge the energy barrier (∼35–40 kcal mol−1) for the double hydrogen transfer. This suggests that the neutral hydrolysis of CO2 could require more water molecules (an oligomer) in an autocatalytic process rather than a dimer.

Stereospecific formation of amidines by 1,1-addition of amines to isocyanides

Abstract Addition of secondary amines to isonitriles in the presence of AgCl at low temperature gives isolable but thermodynamically unstable Z-amidines; only the more stable 6E undergoes ring expansion to the imidazoline 7 .

Structure and properties of phosphaketene (H?P?C?O): phosphorus versus oxygen protonation?

Phosphaketenes carrying bulky substituents to limit dimerization have recently been reported and for comparison the simple model phosphaketene (1) was investigated using ab initio methods. It has an E-bent structure with a CP bond length of 1.728A; and a CPH bond angle of 90.6°(using the 4-31 G basis set). This is rationalized in terms of stabilizing interactions between the PH and CO fragments so that the C–P bond is essentially a dative single bond enforced by π-back-donation. Both P and O centres carry an overall negative charge; of five possible structures of protonated HPCO considered, phosphorus protonation is unambiguously preferred and the perpendicular structure (11) calculated to be the most stable. Inclusion of polarization functions and correlation energies favours phosphorus protonation further. Also reported are the vibrational frequencies, dissociation energies of the protonated and neutral phosphaketene, and the predicted reactivity in both cycloadditions and additions of HX; comparison is made with reported experimental data where available.

The isoimide–imide rearrangement

When imidoyl chlorides (8) are solvolysed in the pH range 3–13 in aqueous dioxan in the presence of acetate or benzoate ions, the isoimides (10) are formed in situ. These isoimides undergo acid (pH 11.5) catalysed acyl transfer to the solvent, giving the amides (12). But at intermediate pH (6–11) rearrangement to the N-acyl form (11)(the Mumm rearrangement) alone occurs. The specific rate of this O → N acyl group migration is pH independent and shows a low solvent effect (m 0.175). Substituents attached to carbon or nitrogen have the same effect (ρ–0.84). When the migrating group R is varied a non-linear Hammett plot is observed, with ρ+0.60 for electron-withdrawing and +1.65 for electron-donating substituents. The changeover point is a function of the migrating terminus varying from R = Ph (when Y = H) to R =p-tolyl (when Y =p-NO2). Rate-determining O → N acyl transfer is suggested in all cases, but this is preceded by an equilibrium which favours the Z form (10a) when R is electron donating.

Diels–Alder cycloadditions of stabilised 2,3-pyridynes

Abstract Investigation of electron donating substituents at C-4 on 2,3-pyridyne 1 stability has revealed a novel stabilisation of these reactive species by aryloxy and thiophenoxy groups leading to a relatively high yielding [4+2] cycloaddition between 4-( p -methoxyphenoxy)-2,3-pyridyne and furan at low temperature.

Structures and properties of carboimidophosphene (HPCNH) and carbodiphosphene (HPCPH). An ab initio study

Ab initio calculations are reported for carboimidophosphene (iminomethylenephosphine), carbodiphos-phene (1,3-diphospha-allene) and their conjugate acids. Lsomerization is favoured by inversion about the CN bond and by rotation about CP bonds and is negligibly small (4.2 kcal mol–1 at DZP//4-31 G) for HPCNH, and large (42 kcal mol–1) for HPCPH. The electronic structures, harmonic vibrational frequencies, and proton affinities are reported and compared with those of other phospha-cumulenes such as HPCO and H2CCPH. The protonation occurs preferentially at the P atom in HPCNH (1) and the C atom in HPCPH (4). At the MP4SDQ/6-31 G** level, the proton affinities are predicted to be 210 ± 5 for (1) and 193 ± 5 kcal mol–1 for (4)(with zero-point energy corrections). The regiospecificity of the dimerization, (2 + 2) cycloadditions, additions to HX reagents, and reacticns with metals are rationalized or predicted.

An ab initio study of the ground and excited states of HPO

Abstract A systematic theoretical characterization has been performed for the molecular structure, vibrational and rotational constants and electronic propertie