| Abstract
| - Z-3-Amino-2-propenenitrile, H2NCHCHCN, a compound of astrochemical and astrobiological interest, hasbeen studied by Stark and Fourier transform microwave spectroscopy along with eight of its isotopologues;the synthesis of five of these are reported. The spectra of the ground vibrational state and of three vibrationallyexcited states belonging to the two lowest normal modes were assigned for the parent species, whereas theground states were assigned for the isotopologues. The frequency of the lowest in-plane bending fundamentalvibration was determined to be 152(20) cm-1 and the frequency of the lowest out-of-plane fundamental modewas found to be 176(20) cm-1 by relative intensity measurements. A delicate problem is whether this compoundis planar or slightly nonplanar. It was found that the rotational constants of the nine species cannot be usedto conclude definitely whether the molecule is planar or not. The experimental dipole moment is μa = 16.45(12), μb = 2.86(6), μc = 0 (assumed), and μtot. = 16.70(12) × 10-30 C m [5.01(4) D]. The quadrupole couplingconstants of the two nitrogen nuclei are χaa = −1.4917(21) and χcc = 1.5644(24) MHz for the nitrogen atomof the cyano group and χaa = 1.7262(18) and χcc = −4.0591(17) MHz for the nitrogen atom of the aminogroup. Extensive quantum-chemical calculations have been performed, and the results obtained from thesecalculations have been compared with the experimental values. The equilibrium structures of vinylamine,vinyl cyanide, and Z-3-amino-2-propenenitrile have been calculated. These calculations have established thatthe equilibrium structure of the title compound is definitely nonplanar. However, the MP2/VQZ energydifference between the planar and nonplanar forms is small, only −423 J/mol. Z-Amino-2-propenenitrile andE-3-amino-2-propenenitrile are formed simply by mixing ammonia and cyanoacetylene at room temperature.A plausible reaction path has been modeled. G3 calculations indicate that the enthalpy (298.15 K, 1 atm) ofthe transition state is about 130 kJ/mol higher than the sum of the enthalpies of the reactants ammonia andcyanoacetylene. This energy difference is comparatively high, which indicates that both E- and Z-3-aminopropenenitrile are not likely to be formed in the gas phase in cold interstellar clouds via a collisionbetween ammonia and cyanoacetylene. An alternative reaction between protonated cyanoacetylene (H−C⋮C−C⋮NH+) and ammonia is predicted to have a much lower activation energy than the reaction between theneutral molecules. Although protonated E- and Z-3-aminopropenenitrile in principle may be formed this way,it is more likely that a collision between NH3 and H−C⋮C−C⋮NH+ leads to NH4+and H−C⋮C−C⋮N.
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