Substituent effects in carbocations CX⁺, CHX^+•, and CH₂X⁺, and in singlet and triplet carbenes CHX. Proton affinities of singlet carbenes

Abstract

Abinitio molecular orbital calculations with a 6-31G* basis set have been used to optimise structures for CX+, CHX+•, CHX (both singlets and triplets), H2CX+ and H3CX (X = H, CH3, NH2, OH, F, CN, and NC). Single point calculations were then performed on the 6-31G* optimised structures using a configuration interaction method involving all single and double excitations from the valence shell. Stabilisation energies for the carbocations are compared with those already reported for H2CX+. For the carbenes the single-triplet energy gap is examined as a function of substituent and the stabilisation energies of the singlet carbenes are compared with those of the isoelectronic carbocations H2CX+. Proton affinities are reported for the singlet carbenes and are compared with the proton affinities of similarly substituted ethylenes, H2C=CHX.

Diaminocarbenes; Calculation of Barriers to Rotation about Ccarbene−N Bonds, Barriers to Dimerization, Proton Affinities, and 13C NMR Shifts

“…Pliego and DeAlmeida also report MP2/DZ//HF/DZ calculation of the PA for Ph 2 C, 1151 kJ mol -1 , and fluorenylidene, 1140 kJ mol -1 . Two aryl substituents on the carbene center therefore produce almost the same PA as two dimethylamino groups, but two hydroxyl groups lead to a much lower PA. We feel that these trends are interesting but that the causes are by no means obvious, since both carbene and carbocation are clearly stabilized by π-electron donation (the latter to a larger extent), while it might be expected that the carbene could also be stabilized by σ-electron withdrawal.…”

“…Diaminocarbenes such as 1b and 1c , imidazol-2-ylidenes (e.g., 4a ), and related species such as 6 and 7 have been generated by deprotonation, and it is therefore interesting to enquire what the p K a of the corresponding protonated ions is. We have reported the p K a of the conjugate acid of imidazol-2-ylidene 4b in DMSO, and proton affinities (PA) have been calculated for the parent imidazol-2-ylidene 4c and for some simpler carbenes, of which the most relevant was C(OH) 2 . Accordingly we have calculated the PA values for 1a and 1b and 4c .…”

Dimer Cations of Cyanoacetylene: Theoretical Isomers and Their Laboratory Production in the Absence and Presence of C602+. Implications for Interstellar/Circumstellar Chemistry

“…For each of the dissociations investigated, the puckered singlet state is energetically preferred over the planar triplet state by at least 10 kcal mol -1 at both B3LYP/6-31+G(d) and B3LYP/6-311++G(2df,p). Previous theoretical studies have determined that cyano groups can function either as a π-donor or as a π-acceptor depending upon the environment, although when it is adjacent to a cationic center it generally functions as a π-donor . π-Donation from the cyano groups to the four-membered ring increases the π-population and, as the singlet is a 2π-homoaromatic system, this interaction destroys the aromaticity and is destabilizing.…”

Proton affinities and pKa values of tetraalkylhydrazines

“…Bispyrrolidine (8) has the most negative )//°( +-) value observed, while the N,N' pyrazolidines 10 and 13 also have negative values, as does the , '-seven-membered ring compound 12. In contrast to fiveand seven-membered rings causing negative &Di/°(N+-H) values, that of bispiperidine 9 is the most positive, while those of the N,N1 -six-membered ring compounds 11 and 14 are also both positive. The signs of the deviations for the fiveand six-membered ring bicyclic examples 15 and 16 are turned around from those of the monocyclic examples.…”