Abstract Fragmentations of cyclobutoxychlorocarbene (13, k frag= 7.1× 10 5 s− 1) and cyclopropylmethoxychlorocarbene (14, k frag= 7.6× 10 5 s− 1) in MeCN proceed to tight and distinct [R+ OC Cl−] ion pairs, which collapse to different distributions of cyclopropylcarbinyl, cyclobutyl and allylcarbinyl chlorides. B3LYP/6–31G* calculations support these conclusions, affording computed fragmentation activation energies of 6.4 (13) and 3.0 (14) ...