Targeting the Rich Conformational Landscape of N ‐Allylmethylamine Using Rotational Spectroscopy and Quantum Mechanical Calculations

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.

Hyperfine-resolved spectra of HDS together with a global ro-vibrational analysis

Despite their chemical simplicity, the spectroscopic investigation of light hydrides, such as hydrogen sulfide, is challenging due to strong hyperfine interactions and/or anomalous centrifugal-distortion effects. Several hydrides have already been detected in the interstellar medium, and the list includes H2S and some of its isotopologues. Astronomical observation of isotopic species and, in particular, those bearing deuterium is important to gain insights into the evolutionary stage of astronomical objects and to shed light on interstellar chemistry. These observations require a very accurate knowledge of the rotational spectrum, which is so far limited for mono-deuterated hydrogen sulfide, HDS. To fill this gap, high-level quantum-chemical calculations and sub-Doppler measurements have been combined for the investigation of the hyperfine structure of the rotational spectrum in the millimeter- and submillimeter-wave region. In addition to the determination of accurate hyperfine parameters, these new measurements together with the available literature data allowed us to extend the centrifugal analysis using a Watson-type Hamiltonian and a Hamiltonian-independent approach based on the Measured Active Ro-Vibrational Energy Levels (MARVEL) procedure. The present study thus permits to model the rotational spectrum of HDS from the microwave to far-infrared region with great accuracy, thereby accounting for the effect of the electric and magnetic interactions due to the deuterium and hydrogen nuclei.

CO2 Aggregation on Monoethanolamine: Observations from Rotational Spectroscopy

The initial stages of the gas-phase nucleation between CO₂ and monoethanolamine were investigated via broadband rotational spectroscopy with the aid of extensive theoretical structure sampling. Sub-nanometer-scale aggregation patterns of monoethanolamine-(CO₂ )_n , n=1-4, were identified. An interesting competition between the monoethanolamine intramolecular hydrogen bond and the intermolecular interactions between monoethanolamine and CO₂ upon cluster growth was discovered, revealing an intriguing CO₂ binding priority to the hydroxyl group over the amine group. These findings are in sharp contrast to the general results for aqueous solutions. In the quinary complex, a cap-like CO₂ tetramer was observed cooperatively surrounding the monoethanolamine. As the cluster approaches the critical size of new particle formation, the contribution of CO₂ self-assembly to the overall stability increases.

Dramatic differences in the conformational equilibria of chalcogen-bridged compounds: the case of diallyl ether versus diallyl sulfide

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 ¹³C and ³⁴S substituted species in natural abundance.

Structures and hydrogen bonding of 1,7-dioxaspiro[5.5]undecane and its hydrates

The conformations of 1,7DSU and its stepwise solvation by up to 5 water molecules were explored using supersonic-jet Fourier transform microwave spectroscopy with the supplement of theoretical calculations. Experimentally, the rotational spectra of the most stable structures of the monomer, monohydrate and dihydrate were observed and assigned. The characteristics of the stability and intermolecular interaction topologies of the 1,7DSU monomer and its hydrated clusters were obtained by CREST conformational sampling followed by B3LYP-D3(BJ)/def2-TZVP geometrical optimizations and MP2/aug-cc-pVTZ single-point energy calculations. The first water molecule links to the 1,7DSU monomer through an O_wH⋯O hydrogen bond. The water molecules tend to aggregate with each other and form cyclic structures for the n = 2-5 clusters. The interactions between water and the 1,7DSU monomer as well as those between water and water were revealed. The analyses of non-covalent interactions and the natural bond orbital suggest that the O_wH⋯O_1,7DSU, O_wH⋯O_w, and CH⋯O_w hydrogen bonds play a prominent role in structural stability.

Conformational preferences of diallylamine: A rotational spectroscopic and theoretical study

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.

Conformational sampling and large amplitude motion of methyl valerate

The microwave spectrum of the fruit ester methyl valerate was recorded using two molecular jet Fourier transform spectrometers covering the frequency range from 2 to 40 GHz. Quantum chemical calculations yielded 11 minima for the anti ester configuration, among them two were identified in the experimental spectrum. The methyl group in the methoxy moiety undergoes internal rotation, leading to torsional splittings of all rotational transitions into doublets. The barrier to internal rotation of the methoxy methyl group was deduced to be 417.724(70) cm-1 and 418.059(27) cm-1 for the C1 and the Cs conformer, respectively, essentially the same values as those in methyl alkanoates with shorter alkyl chains, which are methyl acetate, methyl propionate and methyl butyrate. Geometry parameters such as the rotational and centrifugal distortion constants could be determined with very high accuracy. Optimisations at different levels of theory were performed for a comparison with the experimental results. The MP2/6-311++G(d,p) level of theory failed to calculate reliable rotational constants to guide the assigment of the C1 conformer, while the MP2/cc-pVDZ level fully succeeded.

Exploring the non-covalent interactions behind the formation of amine–water complexes: the case of N-allylmethylamine monohydrate

Rotational spectroscopy and quantum chemical studies reveal the effects of hydrogen bonding with water on the conformer equilibrium of N-allylmethylamine.