Disentangling the Conformational Space and Structural Preferences of Tetrahydrofurfuryl Alcohol Using Rotational Spectroscopy and Computational Chemistry.

The influence of the hydroxymethyl (CH2OH) group on the tetrahydrofuran (THF) ring structure was investigated by disentangling the gas phase conformational landscape of the sugar analogue tetrahydrofurfuryl alcohol (THFA). By combining rotational spectroscopy (6-20 GHz) and quantum chemical calculations, transitions corresponding to two stable conformers of THFA and their 13C isotopologues were observed and assigned in the rotational spectrum. The positions of the C atoms were precisely determined to unambiguously distinguish between nearly isoenergetic pairs of conformers that differ in their ring configurations: envelope (E) versus twist (T). The rotational spectrum confirms that the E ring geometry is favoured when the CH2OH fragment lies gauche (-) to the THF backbone (OCCO ~-60°) whereas the T form is more stable for the gauche (+) alignment of the substituent (OCCO ~+60°). The observed spectral intensities suggest that conformational relaxation of the THF geometry (E↔T) to the more stable form readily occurs within the pairs of g- and g+ conformers which is consistent with the low barriers (1.5-1.7 kJ mol-1) for conversion determined via transition state calculations. Insights into the intramolecular hydrogen bonding and other weak interactions stabilizing the lowest energy structures of THFA were derived and rationalized using non-covalent interaction analyses.

High-Resolution Spectroscopy and Structure of Heavy Carbon Subchalcogenides: Tricarbon Selenide, C3Se.

Linear tricarbon selenide, C3Se, has been studied spectroscopically for the first time using a combination of high-resolution infrared and microwave techniques. Probing laser ablation products from carbon-selenium targets in a free jet expansion with He, initial spectroscopic detection was accomplished in the infrared at a wavelength of 5 µm in search of the ν1 vibrational fundamental. Along with the band of the most abundant isotopic species C380Se found centered at about 2039 cm-1, the corresponding bands of the C382Se, C378Se, C376Se, and C377Se isotopologues were also detected. Pure rotational spectra of the five C3Se isotopologues in the 8-18 GHz frequency range were observed in a supersonic jet expansion using chirped-pulse microwave spectroscopy of the discharge products of selenophene, c-C4H4Se. Spectroscopic analyses were guided by results from high-level quantum-chemical calculations carried out at the coupled-cluster level of theory using large correlation-consistent basis sets. Using the experimentally derived ground-state rotational constants of five isotopologues and calculated zero-point vibrational corrections, an accurate semiexperimental equilibrium carbon-selenium bond length is derived.

Exploring the conformational landscape, hydrogen bonding, and internal dynamics in the diallyl ether and diallyl sulfide monohydrates.

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.

Hyperfine-resolved rotational spectroscopy of HCNH.

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.

High-resolution rovibrational and rotational spectroscopy of the singly deuterated cyclopropenyl cation, c-C3H2D.

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+.

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.

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

The conformational landscape of the monohydrated complex of N-allylmethylamine (AMA-w) was investigated for the first time using rotational spectroscopy from 8-20 GHz and quantum chemistry calculations. From a total of nine possible energy minima within 10 kJ mol-1, transitions for the two most stable conformers of AMA-w were detected, and assigned aided by DFT and ab initio MP2 predictions. The observed rotational transitions displayed characteristic hyperfine splittings due to the presence of the 14N quadrupolar nucleus. Quantum theory of atoms in molecules (QTAIM), non-covalent interaction (NCI) and natural bond orbital (NBO) analyses showed that the observed conformers of AMA-w are stabilized by two intermolecular interactions consisting of a dominant NH-O and a secondary C-HO hydrogen bond (HB) in which the water molecule acts simultaneously as a HB donor and acceptor. The HBs formed with water do not change the relative energy ordering of the most stable conformers of AMA but do affect the stability of higher energy conformations by disrupting the intramolecular forces responsible for their geometries. By comparing the intermolecular interaction energies with those of the monohydrates of the simplest primary (methylamine, MA), secondary (dimethylamine, DMA) and tertiary (trimethylamine, TMA) amines using symmetry-adapted perturbation theory (SAPT) calculations, we find that AMA forms the strongest bound complex with water. This is rationalized through the identification of subtle differences in stabilizing and destabilizing contributions across the amine-w series of complexes.

Characterization of Large-Amplitude Motions and Hydrogen Bonding Interactions in the Thiophene-Water Complex by Rotational Spectroscopy.

The rotational fingerprint of the thiophene-water complex was investigated for the first time using Fourier transform microwave spectroscopy (7-20 GHz) aided by quantum mechanical calculations. Transitions for a single species were observed, and the rotational constants for the parent and 18O isotopomers are consistent with a geometry that is highly averaged over a barrierless large-amplitude motion of water that interconverts two equivalent forms corresponding to the global minimum (B2PLYP-D3(BJ)/def2-TZVP). In this effective geometry, the water lies above the thiophene ring close to its σv plane of symmetry. The observed transitions are split by a second water-centered tunneling motion that exchanges its two protons by internal rotation about its C2 axis with a calculated barrier of ∼2.7 kJ mol-1 (B2PLYP-D3(BJ)/def2-TZVP). Based on quantum theory of atoms in molecules, noncovalent interaction, and symmetry-adapted perturbation theory analyses, the observed geometry enables two intermolecular interactions (O-H···π and O-H···S) whose electrostatic and dispersive contributions favor formation of the thiophene-water complex.

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 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.

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.

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

The highly variable conformational landscape of N-allylmethylamine (AMA) was investigated using Fourier transform microwave spectroscopy aided by high-level theoretical calculations to understand the energy relationship governing the interconversion between nine stable conformers. Spectroscopically, transitions belonging to four low energy conformers were identified and their hyperfine patterns owing to the 14 N quadrupolar nucleus were unambiguously resolved. The rotational spectrum of the global minimum geometry, conformer I, shows an additional splitting associated with a tunneling motion through an energy barrier interconnecting its enantiomeric forms. A two-step tunneling trajectory is proposed by finding transition state structures corresponding to the allyl torsion and NH inversion. Natural bond orbital and non-covalent interaction analyses reveal that an interplay between steric and hyperconjugative effects rules the conformational preferences of AMA.

Conformers of Allyl Isothiocyanate: A Combined Microwave Spectroscopy and Computational Study.

The pure rotational spectrum of allyl isothiocyanate (CH2=CHCH2-NCS) was collected from 4 to 26 GHz using Fourier transform microwave (FTMW) spectroscopy. Its analysis revealed the presence of two conformers that arise due to variation in the CCCN and CCNC dihedral angles. The observed spectrum is consistent with the accompanying potential energy surfaces derived using quantum chemical calculations at the B3LYP-D3(BJ) and MP2 levels of theory. Together, this experimental and theoretical study unequivocally identifies a new conformer (I) as the global minimum geometry. The spectral assignment of this new conformer is verified by the observation of transitions consistent with its 34S, 13C, and 15N isotopologues and with the characteristic 14N quadrupole hyperfine patterns. For conformer I, the substitution (rs) and effective ground state (r0) structures were derived and reveal contributions from a large amplitude motion in the CCNC angle. The remaining geometric parameters compare well with the equilibrium structure (re) from B3LYP-D3(BJ)/cc-pVQZ calculations. The derived CNC bond angle of 152.6(3)° for conformer I of allyl-NCS is found to be ∼15° larger than that of allyl-NCO (137.5(4)°), which is in line with a change in the hybridization at nitrogen from an orbital with more ∼sp character in allyl-NCS to ∼sp1.5 in allyl-NCO as determined via natural bond orbital analyses.

Rotational Spectra and Structures of Phenyl Isocyanate and Phenyl Isothiocyanate.

The pure rotational spectra of phenyl isocyanate (PhNCO) and phenyl isothiocyanate (PhNCS) were investigated using Fourier transform microwave spectroscopy in the range from 4 to 26 GHz. For each molecule, rotational transitions due to the parent species and nine minor isotopologues including seven 13C, one 15N, and one 18O/34S have been observed in natural abundance. The rm(1) geometries were derived from the resulting sets of rotational constants and are consistent with the equilibrium structures (re) from ab initio calculations performed at the MP2/aug-cc-pVTZ level. NBO and Townes-Dailey analyses were conducted to better understand the electronic structure and geometry of each compound. In the case of PhNCS, the nitrogen atom displays more sp-like character resulting in shorter C-N bonds and a larger CNC angle relative to those of PhNCO.

Internal Motions and Sulfur Hydrogen Bonding in Methyl 3-Mercaptopropionate.

The effect of sulfur hydrogen bonding on the conformational equilibrium of methyl 3-mercaptopropionate was investigated using microwave spectroscopy in a supersonic jet expansion. The two most stable conformers (I and II) were assigned in the rotational spectra, and complex splitting patterns owing to the methyl internal rotation and SH tunneling motion were resolved and analyzed in detail. For both conformers, the experimental torsional barriers for the methyl top are similar and about 5.1 kJ mol-1, revealing that their geometrical differences do not affect the methyl internal rotation. The experimentally derived rotational and centrifugal distortion constants, along with the methyl internal rotation barriers, are discussed and compared with results from density functional theory and ab initio calculations. Quantum theory of atoms in molecules, noncovalent interactions, and natural bond orbital analyses show that the global minimum geometry (I), which has the thiol hydrogen oriented toward the carbonyl of the ester, is stabilized by an SH···O=C hydrogen bond. The presence of a hydrogen bond is confirmed by the derivation of an accurate experimental geometry that reveals a hydrogen bond distance and S-H-O angle of 2.515(4) Å and 117.4(1)°, respectively. These results are key benchmarks to expand the current knowledge of sulfur hydrogen bonds and the relationship between internal motions and conformational preferences in esters.

Dispersion-driven conformational preference in the gas phase: Microwave spectroscopic and theoretical study of allyl isocyanate.

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.

Revealing the Conformational Preferences of Proteinogenic Glutamic Acid Derivatives in Solution by 1H NMR Spectroscopy and Theoretical Calculations.

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.

Conformational study of L-methionine and L-cysteine derivatives through quantum chemical calculations and 3JHH coupling constant analyses.

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 1H 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 3JHH 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.

Gauche preference of ß-fluoroalkyl ammonium salts.

The strong gauche preference along with the F-C-C-N(+) fragment in 3-fluoropiperidinium cation and analogues, in the gas phase, is dictated by electrostatic interactions, which can be both hydrogen bond F···H(N(+)) and F/N(+) attraction. In aqueous solution, where most biochemical processes take place, electrostatic effects are strongly attenuated and hyperconjugation is calculated to be at least competitive with Lewis-type interactions.