Exploring the distinct conformational preferences of allyl ethyl ether and allyl ethyl sulfide using rotational spectroscopy and computational chemistry

The conformational energy landscapes of allyl ethyl ether (AEE) and allyl ethyl sulfide (AES) were investigated using Fourier transform microwave spectroscopy in the frequency range of 5-23 GHz aided by density functional theory B3LYP-D3(BJ)/aug-cc-pVTZ calculations. The latter predicted highly competitive equilibria for both species, including 14 unique conformers of AEE and 12 for the sulfur analog AES within 14 kJ mol-1. The experimental rotational spectrum of AEE was dominated by transitions arising from its three lowest energy conformers, which differ in the arrangement of the allyl side chain, while in AES, transitions due to the two most stable forms, distinct in the orientation of the ethyl group, were observed. Splitting patterns attributed to methyl internal rotation were analyzed for AEE conformers I and II, and the corresponding V3 barriers were determined to be 12.172(55) and 12.373(32) kJ mol-1, respectively. The experimental ground state geometries of both AEE and AES were derived using the observed rotational spectra of the 13C and 34S isotopic species and are highly dependent on the electronic properties of the linking chalcogen (oxygen vs sulfur). The observed structures are consistent with a decrease in hybridization in the bridging atom from oxygen to sulfur. The molecular-level phenomena that drive the conformational preferences are rationalized through natural bond orbital and non-covalent interaction analyses. These show that interactions involving the lone pairs on the chalcogen atom with the organic side chains favor distinct geometries and energy orderings for the conformers of AEE and AES.

Chromone-methanol clusters in the electronic ground and lowest triplet state: a delicate interplay of non-covalent interactions

Chromone offers two energetically almost equivalent docking sites for alcohol molecules, in which the hydroxyl group is hydrogen bonded to one of the free electron pairs of the carbonyl O atom. Here, the delicate balance between these two competing arrangements is studied by combining IR/R2PI and UV/IR/UV spectroscopy in a molecular beam supported by quantum-chemical calculations. Most interestingly, chromone undergoes an efficient intersystem crossing into the triplet manifold upon electronic excitation, so that the studies on aromatic molecule-solvent complexes are for the first time extended to such a cluster in a triplet state. As the lowest triplet state (T₁) is of ground state character, powerful energy decomposition approaches such as symmetry-adapted perturbation theory (SAPT) and local energy decomposition using the domain-based local pair natural orbital coupled-cluster method (DLPNO-CCSD(T)/LED) are applied. From the theoretical analysis we infer for the T₁ state a loss of planarity (puckering) of the 4-pyrone ring of the chromone unit, which considerably affects the interplay between different types of non-covalent interactions at the two possible binding sites.

Excited states and spin–orbit coupling in chalcogen substituted perylene diimides and their radical anions

A novel series of chalcogen bay-substituted perylene diimides show increasing SOC, which was investigated in detail via (time-resolved) optical spectroscopy, (spectro)electrochemistry, EPR spectroscopy and TD-DFT calculations.

A simplistic computational procedure for tunneling splittings caused by proton transfer

In this manuscript, we present an approach for computing tunneling splittings for large amplitude motions. The core of the approach is a solution of an effective one-dimensional Schrödinger equation with an effective mass and an effective potential energy surface composed of electronic and harmonic zero-point vibrational energies of small amplitude motions in the molecule. The method has been shown to work in cases of three model motions: nitrogen inversion in ammonia, single proton transfer in malonaldehyde, and double proton transfer in the formic acid dimer. In the current work, we also investigate the performance of different DFT and post-Hartree–Fock methods for prediction of the proton transfer tunneling splittings, quality of the effective Schrödinger equation parameters upon the isotopic substitution, and possibility of a complete basis set (CBS) extrapolation for the resulting tunneling splittings.

Gas-phase hydration of nopinone: the interplay between theoretical methods and experiments unveils the conformational landscape

The structure of microsolvated nopinone formed in the supersonic jet expansion is investigated in the gas phase. The rotational spectra of nopinone(H₂O)_n (n = 1, 2, 3) were analysed by means of Fourier transform microwave spectroscopy. In the present study, three monohydrates, two dihydrates and two trihydrates were observed and characterized. The observed structures are the lowest energy conformers predicted by quantum chemical calculations. In all the observed hydrates of nopinone, water was found to be linked to the ketone group (C[double bond, length as m-dash]O) with a strong hydrogen bond (O_NOPH_W) and finishing with a dispersive one (O_WH_NOP). The structure of nopinone was found to alter the structure of water dimer and water trimer, which make nopinone be surrounded with a chain of water molecules. A remarkable decrease in the H-bonding length was observed when the number of attached water molecules is increased. Different DFT and ab initio calculations at the equilibrium structure allowed the identification of the observed conformers. Evaluation of the B3LYP-D3 and ωB97X-D results revealed deficiencies in reproducing the structure of one observed monohydrated structure while MP2 and M06-2X reproduce all the three observed structures. A comparison with similar bicyclic ketones highlights how a small change in the bicyclic ring leads to different effects in the microsolvation of biogenic VOCs. This study presents the first step of molecular aggregation to understand the atmospheric formation of aerosols at the molecular scale.

Hydrogen bonding networks and cooperativity effects in the aqueous solvation of trimethylene oxide and sulfide rings by microwave spectroscopy and computational chemistry

The intermolecular interactions responsible for the microsolvation of the highly flexible trimethylene oxide (TMO) and trimethylene sulfide (TMS) rings with one and two water (w) molecules were investigated using rotational spectroscopy (8-22 GHz) and quantum chemical calculations. The observed patterns of transitions are consistent with the most stable geometries of the TMO-w, TMO-(w)₂, and TMS-w complexes at the B2PLYP-D3(BJ)/aug-cc-pVTZ level and were confirmed using spectra of the ¹⁸O isotopologue. Due to its effectively planar backbone, TMO offers one unique binding site for solvation, while water can bind to the puckered TMS ring in either an axial or equatorial site of the heteroatom. In all clusters, the first water molecule binds in the σ_v symmetry plane of the ring monomer and serves as a hydrogen bond donor to the heteroatom. The second water molecule is predicted to form a cooperative hydrogen bonding network between the three moieties. Secondary C-H⋯O interactions are a key stabilizing influence in trimers and also drive the preferred binding site in the TMS clusters with the axial binding site preferred in TMS-w and the equatorial form calculated to be more stable in the dihydrate. Using an energy partition scheme from the symmetry-adapted perturbation theory for the O, S, and Se containing mono- and dihydrates, the intermolecular interactions are revealed to be mainly electrostatic, but the dispersive character of the contacts is enhanced with the increasing size of the ring's heteroatom due to the key role of longer-range secondary interactions.