Reversible fixation of carbon dioxide, considered as a cheap and green C1 feedstock, and its facile transformation into valuable raw and fine materials is one of the most exciting challenges and important priorities for the scientific community.[1] The activation of the thermodynamically and kinetically stable CO 2 molecule has been achieved using metal-containing catalysts, while organic bases are also capable of promoting reactions with CO 2 . The R 2 NH/CO 2 system, which is relevant to industrial processes, has attracted much attention since the beginning of the last century. Hindered amidines and guanidines were found to be particularly efficient catalysts in the synthesis of organic carbonates and urethanes from the respective reactions of alcohols and amines with CO 2 , thus avoiding the use of the highly toxic phosgene and its derivatives. [2][3][4][5] These nitrogen bases also proved to be useful for the coupling of CO 2 and epoxides to give cyclic or polymeric carbonates.[6] It has been suggested that these reactions involve a zwitterionic adduct between the base and CO 2 for 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), which are the most commonly used amidine and guanidine species in these transformations. [2,[6][7][8] The existence of such base-CO 2 adducts was also invoked in the reversible fixation of CO 2 by polymers bearing DBU moieties or aromatic nitrogen bases.[9] While these base-CO 2 adducts are expected to be more stable for amidines and guanidines than for amines because they are capable of greater charge delocalization in the former cases, it was proposed from kinetic studies that such a zwitterionic adduct R 2 NH-CO 2 would be a first intermediate in the synthesis of alkyl ammonium alkyl carbamates [R 2 NH 2 ]-[R 2 NCO 2 ] from CO 2 and primary or secondary amines.[10]Formation of such adducts would also occur between CO 2 and the free amino groups of proteins.[11] Despite their ubiquity from biology to materials science, all attempts to isolate and characterize a zwitterionic adduct between CO 2 and a nitrogen base, especially an amidine or guanidine molecule such as DBU and TBD, were unsuccessful but led in some cases to the formation of the corresponding bicarbonate salt [baseH]- [HCO 3 ] owing to the presence of adventitious water. [7,8] Herein we present the synthesis and characterization, including the X-ray crystal structure, of TBD-CO 2 ; we also analyze the geometry and electronic structure of this adduct by DFT and MP2 calculations including solid or solvent effects.First, an off-white powder of the bicarbonate salt [TBDH][HCO 3 ] was readily obtained upon diffusion of CO 2 into a THF or MeCN solution of TBD in the presence of ambient moisture; colorless crystals were formed after the suspension was heated under reflux. The structure is shown in Figure 1 together with selected bond lengths and angles. The cation and anion are associated through two hydrogen bonds between the nitrogen and oxygen atoms (N1···O1 2.746(2) , N2···O2 2.836(2...