Origin of Rotational Barriers of the N−N Bond in Hydrazine:  NBO Analysis

Abstract

Hydrazine passes through two transition states, TS1 (phi = 0 degrees ) and TS2 (phi = 180 degrees ), in the course of internal rotation around its N-N bond. The origin of the corresponding rotational barriers in hydrazine has been extensively studied by experimental and theoretical methods. Here, we used natural bond orbital (NBO) analysis and energy decomposition of rotational barrier energy (DeltaE(barrier)) to understand the origin of the torsional potential energy profile of this molecule. DeltaE(barrier) was dissected into structural (DeltaE(struc)), steric exchange (DeltaE(steric)), and hyperconjugative (DeltaE(deloc)) energy contributions. In both transition states, the major barrier-forming contribution is DeltaE(deloc). The TS2 barrier is lowered by pyramidalization of nitrogen atoms through lowering DeltaE(struc), not by N-N bond lengthening through lowering DeltaE(steric). Higher pyramidality of nitrogen atoms of TS2 than that of TS1 explains well why the N-N bond of TS2 is longer than that of TS1. Finally, the steric repulsion between nitrogen lone pairs does not determine the rotational barrier; nuclear-nuclear Coulombic repulsion between outer H/H atoms in TS1 plays an important role in increasing DeltaE(struc). Taken together, we explain the reason for the different TS1 and TS2 barriers. We show that NBO analysis is a useful tool for understanding structures and potential energy surfaces of compounds containing the N-N bond.

Natural bond orbital analysis in the ONETEP code: Applications to large protein systems

“…The NBO method has been applied in a wide variety of chemical studies, from rationalizing the electronic origin of nonpairwise additivity in co‐operative hydrogen‐bonded clusters,[5] describing electronic donation between ligand and transition metal orbitals,[6] to explaining molecular rotational barriers in terms of the energetics of NBO delocalization. [7–10] More recently, the NBO method has been applied to study biologically relevant systems, examples including charge determination via natural population analysis (NPA) along an enzymatic reaction co‐ordinate,[11] n → π* backbone interactions in proteins,[12–14] hydrogen bonding in nucleic acid base pairs[15] and observation of hydrogen bond co‐operative strengthening in amides and peptides. [16] However, the applicability of first principles methods in determining the ground state properties of biomolecular assemblies is hindered by the scaling of the computational effort with the number of atoms in the system, which is often cubic or greater.…”

Exploring the G3 method in the study of rotational barrier of some simple molecules

“…Some of these molecules are still seen through a classic model of lone pair and bond pair interactions. Therefore, accurate results are necessary [19] and are currently of great interest in the elucidation of the nature of the rotational barrier considering that many analysis of the rotational barriers are based on the decomposition of the electronic energy in orbitals, attractive, repulsive and quantum contributions, and steric effects [20][21][22][23][24].…”

Physical achirality in geometrically chiral rotamers of hydrazine and boranylborane molecules

“…The skew conformer represented in Figure 2, for the H1─N1─N2─H3 dihedral angle ϕ = 90°, corresponds to calculated values for the minimum energy state, 65–68 and it is even more stable than the trans conformer, which may have the least steric repulsion between the NH 2 moieties, allowing for a simple repulsion model 67 . The dihedral ϕ angle H1─N1─N2─H3 is the same as the dihedral angle lp(N)─lp(N), denoting by lp(N) a lone pair of each N atom, see figure 1 of Reference 68.…”

“…The rotational barrier estimated via a natural bond orbital analysis 68 is approximately twice that of ethane 69–71 . Hydrazine passes through two transition states, cis (eclipsed) TS1 at ϕ = 0° and trans (staggered) TS2 at ϕ = 180°, in the course of internal rotation about the N─N bond; the ground state conformation corresponds to ϕ = 91.2° 68 . Most accurate CCSD(T) values are in the range 89.6–90°, with a recommended best estimate of 89.6° 66 …”