Theoretical Study of Unimolecular Rearrangements of Vinylidenes to Acetylenes RITA KAKKAR, MALLIKA PATHAK, PREETI CHADHA Department of Chemistry, University of Delhi, Delhi-110 007, India Received 24 June 2004; accepted 11 August 2004 Published online 23 November 2004 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/qua.20382 ABSTRACT: The rearrangement of vinylidene to acetylene has been studied in detail by the density functional method, using Becke’s three-parameter exchange functional and the gradient-corrected functional of Lee, Yang, and Parr. The rearrangement of the anion, as well as that of fluoro-substituted systems, has also been investigated, in order to determine the effect of fluorine substitution on the activation barrier to the 1,2- hydrogen shift, as well as the relative migratory aptitudes of hydrogen and fluorine. Natural bond orbital analysis is invoked to gain insight into the mechanisms of the rearrangements. Basis size effects are also discussed, particularly in relation to anionic systems. The need to include diffuse functions in geometry optimizations of anionic systems is reinforced by the present calculations. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem 102: 189 –199, 2005 Key words: acetylene; B3LYP; fluorine substitution; isomerization; NBO; vinylidene Introduction T he 1,2 rearrangements of carbenes and ni- trenes are a versatile route to various classes of compounds utilized in preparative organic chemistry. However, there is still some controversy regarding the involvement of free nitrenes and car- benes in such reactions. In the current work, the 1,2 rearrangement of singlet vinylidene to acetylene has been studied. In its simplest form, this reaction may be treated as a simple allowed [ 2 a +  2 s] pericyclic reaction [1]. The reaction is characterized by the ideal orientation of the migrating group for rearrangement into the vacant orbital of the singlet state of the carbene and by the somewhat shorter distance that the group must traverse when com- pared with alkylidenes. Isomerization reactions that involve migrating hydrogen atoms—such as 1,2-hydrogen shifts, keto-enol tautomerization, and carbocation rearrangements— often play a crucial Correspondence to: R. Kakkar; e-mail: rita_kakkar@vsnl.com This article contains supplementary material available via the Internet at http://www.interscience.wiley.com/jpages/0020- 7608/suppmat. Contract grant sponsors: University Grants Commission; Council of Scientific and Industrial Research. International Journal of Quantum Chemistry, Vol 102, 189 –199 (2005) © 2004 Wiley Periodicals, Inc.