Microwave and Quantum Chemical Study of Intramolecular Hydrogen Bonding in 2-Propynylhydrazine (HC CCH2NHNH2)

International audienceThe microwave spectrum, of 2-propynylhydrazine (HC CCH2NHNH2) was investigated in the 23-124 GHz spectral interval. The spectra of two conformers denoted I and II were assigned. I is the lower-energy form, and relative intensity measurements yielded an internal energy difference of 3.0(4) kJ/mol between I and II. The spectra of the ground and five vibrationally excited states were assigned for I, whereas only the spectrum of the ground vibrational state was assigned for II. Both I and II are each stabilized simultaneously,by two intramolecular hydrogen bonds. The first of these hydrogen bonds is formed between the hydrogen atom of the -NH- part of the hydrazino group, and the second internal hydrogen bond is formed between One of the hydrogen atoms. of the -NH2 part. The pi-electrons of the triple bond is thus shared by these two hydrogen atoms. The shortest contact between a hydrogen atom of the hydrazino group and. the pi-electrons of the ethynyl group is, found in lower-energy conformer I. The conformational properties of 2-propynylhydrazine were explored by MP2/cc-pVTZ and CCSD/cc-pVQZ calculations. The CCSD method predicts that seven rotameric forms exist for this compound. Five of these rotamers are stabilized by internal hydrogen bonding. The simultaneous sharing of the pi-electrons Of the triple bond by two hydrogen atoms occurs only in Conformers I and II, which are predicted to be the two forms with the lowest energies, with I 2.52 kJ/mol lower hi energy than II. The effective rotational constants of the ground vibrational states of I and II were predicted by a combination of MP2 and CCSD calculations, whereas centrifugal distortion constants and vibration-rotation Constants were calculated by the MP2 method. The theoretical spectroscopic constants are compared with,the experimental counterparts. It is concluded that more refined calculations are necessary to obtain complete agreement