Conformational properties of cis- and trans-N-Cyclopropylformamide studied by microwave spectroscopy and quantum chemical calculations

Microwave and Quantum Chemical Study of the Hydrazino Group as Proton Donor in Intramolecular Hydrogen Bonding of (2-Fluoroethyl)hydrazine (FCH2CH2NHNH2)

The microwave spectrum of (2-fluoroethyl)hydrazine (FCH2CH2NHNH2) was studied in the 11-123 GHz spectral region to investigate the ability of the hydrazino group to form intramolecular hydrogen bonds acting as a proton donor. This group can participate both in five-member and in six-member internal hydrogen bonds with the fluorine atom. The spectra of four conformers were assigned, and the rotational and centrifugal distortion constants of these rotameric forms were determined. Two of these conformers have five-member intramolecular hydrogen bonds, while the two other forms are without this interaction. The internal hydrogen bonds in the two hydrogen-bonded forms are assumed to be mainly electrostatic in origin because the N-H and C-F bonds are nearly parallel and the associated bond moments are antiparallel. This is the first example of a gas-phase study of a hydrazine where the hydrazino functional group acts as a proton donor in weak intramolecular hydrogen bonds. Extensive quantum chemical calculations at the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CCSD/cc-pVQZ levels of theory accompanied and guided the experimental work. These calculations predict the existence of no less than 18 conformers, spanning a CCSD internal energy range of 15.4 kJ/mol. Intramolecular hydrogen bonds are predicted to be present in seven of these conformers. Three of these forms have six-member hydrogen bonds, while four have five-member hydrogen bonds. The three lowest-energy conformers have five-member internal hydrogen bonds. The spectrum of the conformer with the lowest energy was not assigned because it has a very small dipole moment. The CCSD relative energies of the two hydrogen-bonded rotamers whose spectra were assigned are 1.04 and 1.62 kJ/mol, respectively, whereas the relative energies of the two conformers with assigned spectra and no hydrogen bonds have relative energies of 6.46 and 4.89 kJ/mol.