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The conformational spaces of the diallyl ether (DAE) and diallyl sulfide (DAS) monohydrates were explored using rotational spectroscopy from 6 to 19 GHz. Calculations at the B3LYP-D3(BJ)/aug-cc-pVTZ level suggested significant differences in their conformational behavior, with DAE-w exhibiting 22 unique conformers and DAS-w featuring three stable structures within 6 kJ mol−1. However, only transitions from the lowest energy conformer of each were experimentally observed. Spectral analysis confirmed that binding with water does not alter the conformational preference for the lowest energy structure of the monomers, but it does influence the relative stabilities of all other conformers, particularly in the case of DAE. Non-covalent interaction and quantum theory of atoms in molecules analyses showed that the observed conformer for each complex is stabilized by two intermolecular hydrogen bonds (HBs), where water primarily interacts with the central oxygen or sulfur atom of the diallyl compounds, along with secondary interactions involving the allyl groups. The nature of these interactions was further elucidated using symmetry-adapted perturbation theory, which suggests that the primary HB interaction with S in DAS is weaker and more dispersive in nature compared to the primary HB in DAE. This supports the experimental observation of a tunneling splitting exclusively in the rotational spectrum of DAS-w, as the weaker contact allows water to undergo internal motions within the complex, as shown based on calculated transition state structures for possible tunneling pathways. [ABSTRACT FROM AUTHOR]

The rotational spectrum of the molecular ion HCNH+ is revisited using double-resonance spectroscopy in an ion trap apparatus, with six transitions measured between 74 and 445 GHz. Due to the cryogenic temperature of the trap, the hyperfine splittings caused by the 14N quadrupolar nucleus were resolved for transitions up to J = 4 ← 3, allowing for a refinement of the spectroscopic parameters previously reported, especially the quadrupole coupling constant eQq. [ABSTRACT FROM AUTHOR]

The rotational spectrum of the molecular ion HCNH+ is revisited using double-resonance spectroscopy in an ion trap apparatus, with six transitions measured between 74 and 445 GHz. Due to the cryogenic temperature of the trap, the hyperfine splittings caused by the 14N quadrupolar nucleus were resolved for transitions up to J = 4 ← 3, allowing for a refinement of the spectroscopic parameters previously reported, especially the quadrupole coupling constant eQq. [ABSTRACT FROM AUTHOR]

The rovibrational spectrum of the molecular ion H $ _2 $ 2 CCCH $ ^+ $ + was investigated in a 4 K cryogenic ion trap instrument employing the leak-out spectroscopy method. Transitions within the fundamental $ \nu _1 $ ν 1 (C–H stretch) and the combination band $ \nu _3 $ ν 3 + $ \nu _5 $ ν 5 (C–C stretches) were detected, the search aided by high level quantum chemical calculations. The analysis of the rovibrational measurements enabled us to predict the rotational structure of the ground state. Using a rotational-vibrational double-resonance scheme, 14 pure rotational transitions were measured. This, in turn, led to the radio astronomical detection of H $ _2 $ 2 CCCH $ ^+ $ + in the interstellar medium, as recently reported (Silva et al., Astron. Astrophys. 676, L1, 2023). [ABSTRACT FROM AUTHOR]

The rovibrational spectrum of the molecular ion H $ _2 $ 2 CCCH $ ^+ $ + was investigated in a 4 K cryogenic ion trap instrument employing the leak-out spectroscopy method. Transitions within the fundamental $ \nu _1 $ ν 1 (C–H stretch) and the combination band $ \nu _3 $ ν 3 + $ \nu _5 $ ν 5 (C–C stretches) were detected, the search aided by high level quantum chemical calculations. The analysis of the rovibrational measurements enabled us to predict the rotational structure of the ground state. Using a rotational-vibrational double-resonance scheme, 14 pure rotational transitions were measured. This, in turn, led to the radio astronomical detection of H $ _2 $ 2 CCCH $ ^+ $ + in the interstellar medium, as recently reported (Silva et al., Astron. Astrophys. 676, L1, 2023). [ABSTRACT FROM AUTHOR]

The antisymmetric C–H stretching vibration $ \nu _3 $ ν 3 ( $ {}^2\Pi \leftarrow {}{}^2\Pi $ 2 Π ← 2 Π) of ionised acetylene, $ {\rm C}_2{\rm H}_2^+ $ C 2 H 2 + , has been revisited using a cryogenic 22-pole ion trap machine. Two action spectroscopic techniques, the novel leak-out spectroscopy (LOS) method and the more established laser-induced reactions (LIR) method, are applied and compared. Mass selectivity and cryogenic temperatures down to 4 K enabled the observation of uncontaminated spectra in which the Λ-doubling components of this open-shell molecule are mostly well resolved, leading to a slight refinement of the spectroscopic parameters. [ABSTRACT FROM AUTHOR]

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 σv 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. [ABSTRACT FROM AUTHOR]

The conformational space of diallylamine (DAA) was investigated using rotational spectroscopy from 7 to 19 GHz aided by quantum chemical calculations. Extensive conformational searches using density functional theory B3LYP-D3(BJ) and the ab initio MP2 method with the aug-cc-pVTZ basis set identified a total of 42 minima for DAA within ∼22 kJ mol−1. This reveals a strikingly rich conformational landscape for this secondary amine with two equivalent substituents. Experimentally, transitions belonging to four low energy conformers (I, II, III, and IV) were unequivocally assigned in the rotational spectrum, and their patterns were confirmed by the presence of the hyperfine structure owing to the 14N quadrupolar nucleus. The relative intensities of the observed transitions suggest a conformational energy ordering of I < II < III < IV. Natural bond orbital and non-covalent interaction calculations reveal that the geometric preferences for the observed conformers are governed by an interplay of subtle attractive interactions (including hyperconjugation involving the lone pair at nitrogen) and repulsive effects. [ABSTRACT FROM AUTHOR]

The conformations of allyl isocyanate (CH2=CHCH2N=C=O) were explored in the gas phase by combining theoretical calculations and Fourier transform microwave spectroscopy, including the chirped pulse and Balle-Flygare types. Three conformers (I, II, and III) were predicted using D3(BJ) dispersion-corrected B3LYP and MP2 methods; however, the lowest energy conformer (conf. I) was absent at the standard B3LYP level. The observed microwave spectra are consistent with the presence of both conf. I and III in the supersonic jet, and surprisingly, this is the first report of the global minimum conf. I both experimentally and theoretically. Rotational transitions from the parent species of both conformers as well as their minor isotopologues (13C, 15N, and 18O) in natural abundance were assigned allowing experimental geometries to be derived. For conf. I, in addition to the typical splitting pattern due to the 14N quadrupole nucleus, the transitions show a tunneling splitting which arises from the interconversion motion between its two mirror images. The experimental observation of conf. I and the absence of conf. II in the jet are rationalized using quantum-chemical calculations to explore the importance of electron correlation and in particular, demonstrate the necessity of including dispersion effects in density functional theory calculations even for seemingly small molecules. [ABSTRACT FROM AUTHOR]

Applying a novel action spectroscopic technique in a 4 K cryogenic ion-trap instrument, the molecule c-C3H2D+ has been investigated by high-resolution rovibrational and pure rotational spectroscopy for the first time. In total, 126 rovibrational transitions within the fundamental band of the ν1 symmetric C–H stretch were measured with a band origin centred at 3168.565 cm−1, which were used to predict pure rotational transition frequencies in the ground vibrational state. Based on these predictions, 16 rotational transitions were observed between 90 and 230 GHz by using a double-resonance scheme. These new measurements will enable the first radio-astronomical search for c-C3H2D+. [ABSTRACT FROM AUTHOR]

A comprehensive description on the conformational behaviour of small peptide models is of primary importance to elucidate polypeptide and protein structures and folding pathways. In this paper, an extensive conformational investigation of N-acetyl-N′-methylprolineamide (Ac-Pro-NHMe) in both isolated phase and in solution is reported. The joint use of 1H NMR spectroscopy and DFT calculations allowed to determine the most stable conformers of Ac-Pro-NHMe and to evaluate its conformational preferences in different media. Theoretical calculations at the M06-2X/aug-cc-pVTZ level of theory led to the identification of two and six stable conformers in isolated phase and solution, respectively. These geometries reveal the presence of two isomers, where the –NC(O)Me bond can assume a trans (ϑ = 180°) or cis (ϑ = 0°) orientation in relation to the –(CO)NHMe group of backbone. The theoretical results show that the trans,trans-IId in a down-puckered γ-turn-like structure and ϑ = 180°, is the most populated conformer in isolated phase. In polar solvents, like acetonitrile and DMSO, there is a mixture of both polyproline II and α-helix-like conformers, while in water, the trans,trans-IIu, in an up-puckered polyproline II-like geometry, is the less energetic. Therefore, our results show that conformer populations change upon solvent variation, indicating that the conformational equilibrium of Ac-Pro-NHMe is sensitive to solvent effects. The experimental population values, obtained through the integration of the peaks in the 1H NMR spectrum, confirm the change of conformer populations in different solvents. The results reveal that the theoretical level employed in this study is in good agreement with experimental data. QTAIM and NCI analyses combined with orbital-based NBO calculations reveal that the conformers trans,trans-IId and trans,trans-IIu are stabilized by an intramolecular hydrogen bond (IHB) C=O…HN in isolated phase. However, these geometries are completely destabilized in solution by the weakening of this interaction. Although IHB is essential to the stability of the two conformers in isolated phase, it represents only a secondary factor to the conformational preferences of the compound. The highest stability of trans,trans-IId in isolated phase can be attributed to hyperconjugative interactions. In solution, an interplay of hyperconjugative, steric and electrostatic effects rules the conformational preferences of Ac-Pro-NHMe. [ABSTRACT FROM AUTHOR]

The conformational preferences of proteinogenic glutamic acid esterified (GluOMe) and N-acetylated (AcGluOMe) derivatives have been determined in solution for the first time. Theoretical calculations at the ωB97X-D/aug-cc-pVTZ made possible the assignment of six and eight stable conformers for GluOMe and AcGluOMe, respectively. The conformational equilibrium of the studied compounds was evaluated in different organic solvents using a combination of the integral equation formalism polarizable continuum model (IEF-PCM) and ¹H NMR spectroscopy data. The results showed that the conformational equilibrium of both derivatives change in the presence of solvent. According to the quantum theory of atoms in molecules (QTAIM), non-covalent interactions (NCI), and natural bond orbitals (NBO) analyses, the conformational preferences observed for GluOMe and AcGluOMe are not dictated by the presence of a specific interaction but are due to a combination of hyperconjugative and steric effects. [ABSTRACT FROM AUTHOR]

The strong gauche preference alongwith the F–C–C–N+fragment in 3-fluoropiperidiniumcation and analogues, in the gas phase, is dictated by electrostaticinteractions, which can be both hydrogen bond F···H(N+) and F/N+attraction. In aqueous solution, wheremost biochemical processes take place, electrostatic effects are stronglyattenuated and hyperconjugation is calculated to be at least competitivewith Lewis-type interactions. [ABSTRACT FROM AUTHOR]

Applying a novel action spectroscopic technique in a 4 K cryogenic ion-trap instrument, the molecule c-C 3 H 2 D + has been investigated by high-resolution rovibrational and pure rotational spectroscopy for the first time. In total, 126 rovibrational transitions within the fundamental band of the ν 1 symmetric C-H stretch were measured with a band origin centred at 3168.565 cm -1 , which were used to predict pure rotational transition frequencies in the ground vibrational state. Based on these predictions, 16 rotational transitions were observed between 90 and 230 GHz by using a double-resonance scheme. These new measurements will enable the first radio-astronomical search for c-C 3 D 2 D + .

The conformational landscapes of diallyl ether (DAE) and diallyl sulfide (DAS) were investigated for the first time using rotational spectroscopy from 6-20 GHz supported by quantum mechanical calculations. A significant difference in the conformational distribution of these chalcogen-bridged compounds is predicted by theory at the B3LYP-D3(BJ)/aug-cc-pVTZ level as DAS has only one low energy conformer while DAE has up to 12 energy minima within 5 kJ mol -1 . This was confirmed by rotational spectroscopy as only transitions corresponding to the global minimum of DAS were observed while the spectrum of DAE was much richer and composed of features from the nine lowest energy conformers. To understand the effects that govern the conformational preferences of DAE and DAS, natural bond orbital and non-covalent interaction analyses were done. These show that unique orbital interactions stabilize several conformers of the ether making its conformational landscape more competitive than that of the sulfide. This is consistent with a bonding model involving decreased hybridization of the bridging atom as one moves down the periodic table which is confirmed by the experimental ground state structures of the lowest energy forms of DAE and DAS, derived using spectra of the 13 C and 34 S substituted species in natural abundance.

The understanding of the conformational behavior of amino acids and their derivatives is a challenging task. Here, the conformational analysis of esterified and N -acetylated derivatives of L-methionine and L-cysteine using a combination of 1 H NMR and electronic structure calculations is reported. The geometries and energies of the most stable conformers in isolated phase and taking into account the implicit solvent effects, according to the integral equation formalism polarizable continuum model (IEF-PCM), were obtained at the ωB97X-D/aug-cc-pVTZ level. The conformational preferences of the compounds in solution were also determined from experimental and theoretical 3 J HH coupling constants analysis in different aprotic solvents. The results showed that the conformational stability of the esterified derivatives is not very sensitive to solvent effects, whereas the conformational equilibrium of the N -acetylated derivatives changes in the presence of solvent. According to the natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM) and noncovalent interactions (NCI) methodologies, the conformational preferences for the compounds are not dictated by intramolecular hydrogen bonding, but by a joint contribution of hyperconjugative and steric effects.