Ab initio HF/6-31G* calculations have been performed for the addition mecha
nism of carbenoid CH2CILi with formaldehyde in tetrahydrofuran. An early co
mplex of formaldehyde with CH2CILi is first formed with quite exothermic ef
fect. Only a little activation energy of 14.6 kJ/mol is needed for the comp
lex developing into the product through a transition state with coplanar bi
cyclic structure. In this process, the eletrophilic attack of carbonyl carb
on of formaldehyde is more active than the nucleophilic attack of carbon of
carbenoid. The exothermal effect of this addition process is up to 216.5 k
J/mol.