Transformation of 2D group-III selenides to ultra-thin nitrides: enabling epitaxy on amorphous substrates

Appraisal of individual hydrogen bond strengths and cooperativity in ammonia clusters via a molecular tailoring approach

In this work, we propose and test a method, based on the molecular tailoring approach (MTA), for the evaluation of individual hydrogen bond (HB) energies in ammonia (NH₃)_n clusters. This methodology was tested, in our earlier work, on water clusters. Liquid ammonia being a universal, non-aqueous ionizing solvent, such information of individual HB strength is indispensable in many studies. The estimated HB energies by an MTA-based method, in (NH₃)_n for n = 3-8, were calculated to be in the range of 0.65 to 5.54 kcal mol^-1 with the cooperativity contribution falling between -0.54 and 1.88 kcal mol^-1 both calculated at the MP2(full)/aug-cc-pVTZ level of theory. It is seen that the strong HBs in (NH₃)_n clusters were additionally strengthened by the large contribution of HB cooperativity. The accuracy of these estimated HB energies was validated by approximately estimating the molecular energy of a given cluster by adding the sum of HB energies to the sum of monomer energies. This approximately estimated molecular energy of a given cluster was found to be in excellent agreement with the actual calculated values. The negligibly small difference (less than 5.6 kcal mol^-1) in these two values suggests that the estimated individual HB energies in ammonia clusters are quite reliable. Furthermore, these estimated HB energies by MTA are in excellent qualitative agreement with the other indirect measures of HB strength, such as HB bond distances and angles, N-H stretching frequency and the electron density values at the (3,-1) bond critical points.

Understanding the Hydrogen-Bonded Clusters of Ammonia (NH3) n (n = 3-6): Insights from the Electronic Structure Theory

Although it is well known that hydrogen bonds commonly exist in ammonia clusters and play an important role, there are still many challenges in understanding the electronic structure properties of hydrogen bonds. In this paper, the geometric and electronic structure properties of cyclic ammonia clusters are investigated by using first-principles density functional theory (DFT) and the Møller-Plesset perturbation theory (MP2). The calculation results show that the pentamer and hexamer have deviated from the perfect plane, while the trimer and tetramer present planarization that has been confirmed by infrared (IR) spectra. The electronic structure analysis further shows that the covalent properties play a non-negligible role in hydrogen bonding. The results also indicate that the electronic structure facilitates structure planarization. Our work not only provides insight into the role and nature of hydrogen bonds in ammonia clusters but also provides a theoretical basis for frontier science in fields such as atmospheric haze and biomolecular functions.

Intramolecular Reactions in Ionized Ammonia Clusters: A Direct Ab Initio Molecular Dynamics Study

Ammonia cluster cations are a chemical species that has recently attracted considerable research attention as an ion-molecule reaction species in the planetary atmosphere, surface reaction species in materials chemistry, and super-alkali species. Reactions of the radical cation of an ammonia cluster, [(NH₃)_n]⁺ (n = 2-6), following the ionization of the parent neutral cluster, were investigated using direct ab initio molecular dynamics to elucidate the reactions of the ammonia cluster cation under astrochemical conditions. The calculations showed that two competing reaction channels-proton transfer (PT) channel and complex formation channel-operate after the ionization of neutral clusters. In the PT channel, a proton of NH₃⁺ was transferred to a neighboring ammonia molecule. The PT channel was found in all clusters (n = 2-6). Reaction via the PT channel became faster with increasing cluster size and saturated around n = 5-6. In the complex formation channel, a face-to-face complex having a H₃N-NH₃⁺ structure (with a N-N bond) was formed. This channel was found only in larger clusters (n = 5-6). Time scales of PT and complex formation channels were calculated to be 20-30 and 40-50 fs, respectively. The reaction mechanism was discussed based on the results of theoretical calculations.