Theoretical study on H 2 Y⋯Ag X (X = F, Cl, Br, I; Y = O, S) complexes: Structures, energies and bonding

FFLUX molecular simulations driven by atomic Gaussian process regression models

Machine learning (ML) force fields are revolutionizing molecular dynamics (MD) simulations as they bypass the computational cost associated with ab initio methods but do not sacrifice accuracy in the process. In this work, the GPyTorch library is used to create Gaussian process regression (GPR) models that are interfaced with the next-generation ML force field FFLUX. These models predict atomic properties of different molecular configurations that appear in a progressing MD simulation. An improved kernel function is utilized to correctly capture the periodicity of the input descriptors. The first FFLUX molecular simulations of ammonia, methanol, and malondialdehyde with the updated kernel are performed. Geometry optimizations with the GPR models result in highly accurate final structures with a maximum root-mean-squared deviation of 0.064 Å and sub-kJ mol-1 total energy predictions. Additionally, the models are tested in 298 K MD simulations with FFLUX to benchmark for robustness. The resulting energy and force predictions throughout the simulation are in excellent agreement with ab initio data for ammonia and methanol but decrease in quality for malondialdehyde due to the increased system complexity. GPR model improvements are discussed, which will ensure the future scalability to larger systems.

Understanding Regium Bonds and their Competition with Hydrogen Bonds in Au2 :HX Complexes

A theoretical study of the regium and hydrogen bonds (RB and HB, respectively) in Au₂ :HX complexes has been carried out by means of CCSD(T) calculations. The theoretical study shows as overall outcome that in all cases the complexes exhibiting RB are more stable that those with HB. The binding energies for RB complexes range between -24 and -180 kJ ⋅ mol^-1, whereas those of the HB complexes are between -6 and -19 kJ ⋅ mol^-1 . DFT-SAPT also indicated that HB complexes are governed by electrostatics, but RB complexes present larger contribution of the induction term to the total attractive forces. ¹⁹⁷ Au chemical shifts have been calculated using the relativistic ZORA Hamiltonian.

An ab initio study on coinage atom-inserted cyanide/isocyanide: XMCN/XMNC (M = coinage atoms; X = halogen)

The coinage atom-inserted cyanide/isocyanide compounds, XMCN and XMNC (X = halogens) formed by the insertion of a coinage atom into the X-C(N) bonds of XCN (or XNC), were investigated by ab initio methods. XMCN was predicted to be more stable than XMNC, which is different from the case of XUCN/XUNC reported previously. Based on the analyses on the ionization dissociation pathways, the M-C (or M-N) bond is more easily broken than the X-M bond. Moreover, the order of the M-C (or M-N) bond energy in XMCN (or XMNC) is XAuCN (XAuNC) > XCuCN (XCuNC) > XAgCN (XAgNC). The shift characters of v C-N in XMCN (or XMNC) with respect to the concerning precursor can be used to identified XMCN and XMNC experimentally. The results of charge decomposition analysis (CDA) and atoms-in-molecule (AIM) illustrate that the X-M and M-C(N) bond behaves as a coordination bond, while the C-N bond is a typical polar covalent bond. The higher thermodynamic stability of XMCN is the result of the -CN group having better coordination ability than the -NC group.

An ab initio study on noble gas inserted halogenated acetylene: HNgCCX (Ng = Kr and Xe; X = halogen)

Although HNgCCX (Ng = Kr and Xe; X = F and Cl) have been identified in cryogenic matrices, similar Br and I analogues have not been prepared so far. In this paper, the nature of HNgCCX (Ng = Kr and Xe; X = F, Cl, Br and I) have been investigated by ab initio methods. The main characteristic absorption peak of HNgCCX is the v_H-Ng, which decreases as X varies from F to I. Moreover, the H-Xe bond is stronger than the H-Kr bond. The v_C≡C and v_C-X exhibit red- and blue-shift characters, respectively, especially the C-X bond is abnormal blue-shift halogen bond. AIM results show that the H-Ng bond is essentially covalent bond and the covalent character of H-Xe bond is underestimated, and the trend of the covalent character is C-Cl > C-Br > C-F > C-I. Although HNgCCX is instable thermodynamically with respect to Ng + HCCX, it is kinetically stable with respect to the two-/three-body channels due to the relatively larger energy barriers. The three-body channels of HNgCCX is the main decomposition channel, and the kinetically stability of HXeCCX is more than its Kr analogues. This study is helpful for the preparation of new HNgCCX in cryogenic matrices.

Theoretical Insights into Halogenated Uranium Cyanide/Isocyanide Compounds

Two kinds of halogenated uranium cyanide/isocyanide compounds, XUCN and XUNC (X = halogen) formed by the insertion of uranium atom into X-C(N) bonds of XCN (or XNC), were investigated by DFT and ab initio methods. Although XNC is less stable thermodynamically than XCN, XUNC is more stable than XUCN and is expected to be prepared and characterized in matrix isolation experiments. The C-N stretching vibration mode (ν_C-N) is the primary fingerprint for the identification of these isomers due to its red-shift character with respect to the relevant precursor. Atoms-in-molecule (AIM) analysis illustrates that both X-U and U-C(N) bonds in XUCN and XUNC show closed-shell interaction character, although partial covalent character contributes to them, and can be denoted as X^-U²⁺(CN)^- and X^-U²⁺(NC)^-, respectively. Charge decomposition analysis (CDA) further reveals that the isocyanide exhibits better donation performance than the cyanide, which should be the root cause of the difference between XUCN and XUNC.

Computational insights into CH3MX (M = Cu, Ag and Au; X = H, F, Cl, Br and I)

The thermodynamically stability of CH₃MX with respect to CH₃X + M is CH₃CuX > CH₃AuX > CH₃AgX. Some stable CH₃MX have not been identified experimentally because their vibrational fingerprints (ν_C−M and v_M−X) are too low to be detected.

Gas phase complexes of H3N⋯CuF and H3N⋯CuI studied by rotational spectroscopy and ab initio calculations: the effect of X (X = F, Cl, Br, I) in OC⋯CuX and H3N⋯CuX

Complexes of H₃N⋯CuF and H₃N⋯CuI have been synthesised in the gas phase and characterized by microwave spectroscopy.