Premium
Substituent Effects on the Low‐Lying Singlet and Triplet States of Methylene
Author(s) -
Farrǵs Jaume,
Olivella Santiago,
Solé Albert,
Vilarrasa Jaume
Publication year - 1986
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540070406
Subject(s) - singlet state , chemistry , diradical , carbene , mndo , open shell , methylene , computational chemistry , ab initio , triplet state , substituent , crystallography , stereochemistry , atomic physics , physics , molecule , excited state , catalysis , medicinal chemistry , organic chemistry
The relative energies of the three lower‐lying singlet states (here called S a , S b , and S c for the sake of generality) and the lowest triplet state of CH X and C X 2 carbenes (in which X = Li, BeH, BH 2 , NH 2 , OH, or F) are evaluated by means of the semiempirical MNDO method as well as, for some species, by means of ab initio calculations at the 6‐31G, MP3/6‐31G, and MP3/6‐31G* levels. Calculations for CH(CN) and C(CN) 2 are also reported. In spite of the known MNDO overestimation of the stability of the σ 1 π 1 configurations of methylene, this method turns out to be satisfactory for most carbenes reported here. Emphasis is put on the appearance of the plots of the Δ H f ovalues vs. the carbene bond angles for the different states and on the seldom considered S b states ( 1 B 1 for C 2 v carbenes). A carbene classification is proposed on the basis of the form of these plots. For carbenes with π‐acceptor substituents such as those of “type IA”, open‐shell, diradical configurations are predicted for the lowest singlet states, so that no significant structural differences should be expected between their lowest singlet and triplet states. On the other hand, for carbenes with strong π‐donor substituents, either “type ID” or “IID”, the closed‐shell singlets appear to be the ground states, and the singlet and triplet behaviors should be much more clearly distinguishable.