Rotational Spectrum of 1,1,1-Trifluoro-2-butanone Using Chirped-Pulse Fourier Transform Microwave Spectroscopy

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.

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.

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.

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.

High throughput chirped pulse Fourier-transform microwave spectroscopy of ethanol and water clusters

Here we discuss the design and performance of a novel high-throughput instrument for Chirped Pulse Fourier-transform Microwave (CP-FTMW) spectroscopy, and demonstrate its efficacy through the identification of the lowest energy conformers of the ethanol trimer and mixed water : ethanol trimers. Rotational constants for these trimers were calculated from observed lines in the spectra from 10 to 14 GHz, and compared to the results of anharmonic ab initio computations. As predicted, all trimers share a cyclic donor-acceptor hydrogen bonding structure, with the ethanol monomer favoring the gauche conformation in the lowest energy structures. The increased speed of data collection and resulting sensitivity opens a new avenue into rotational studies of higher order clusters.

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.

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.

Analysis of the Complex Quadrupole Hyperfine Patterns for Two Chlorine Nuclei in the Rotational Spectrum of 2,5-Dichlorothiophene

The rotational spectrum of 2,5-dichlorothiophene (DCT) was measured for the first time using Fourier transform microwave spectroscopy from 5.5-19 GHz. Dense hyperfine splitting patterns due to the two quadrupolar chlorine nuclei (I = 3/2) were resolved and assigned for the ³⁵Cl-³⁵Cl, ³⁷Cl-³⁵Cl, and ³⁷Cl-³⁷Cl isotopologues and for the two ¹³C and one ³⁴S analogues with two ³⁵Cl atoms, allowing derivation of their respective nuclear quadrupole coupling tensors. The rotational constants obtained from fitting the spectra of the six isotopic species allowed derivation of the experimental geometry of DCT for comparison with the equilibrium structure computed at the MP2/aug-cc-pVTZ level. This revealed that the electron-withdrawing effect of chlorine causes small distortions in the ring geometry relative to thiophene, including a 1.1° increase in the two S-C-C angles and a 0.012 Å increase in the two S-C bond lengths.

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.

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.

Understanding (coupled) large amplitude motions: the interplay of microwave spectroscopy, spectral modeling, and quantum chemistry

A large variety of molecules contain large amplitude motions (LAMs), inter alia internal rotation and inversion tunneling, resulting in tunneling splittings in their rotational spectrum. We will present the modern strategy to study LAMs using a combination of molecular jet Fourier transform microwave spectroscopy, spectral modeling, and quantum chemical calculations to characterize such systems by the analysis of their rotational spectra. This interplay is particularly successful in decoding complex spectra revealing LAMs and providing reference data for fundamental physics, astrochemistry, atmospheric/environmental chemistry and analytics, or fundamental researches in physical chemistry. Addressing experimental key aspects, a brief presentation on the two most popular types of state-of-the-art Fourier transform microwave spectrometer technology, i.e., pulsed supersonic jet expansion–based spectrometers employing narrow-band pulse or broad-band chirp excitation, will be given first. Secondly, the use of quantum chemistry as a supporting tool for rotational spectroscopy will be discussed with emphasis on conformational analysis. Several computer codes for fitting rotational spectra exhibiting fine structure arising from LAMs are discussed with their advantages and drawbacks. Furthermore, a number of examples will provide an overview on the wealth of information that can be drawn from the rotational spectra, leading to new insights into the molecular structure and dynamics. The focus will be on the interpretation of potential barriers and how LAMs can act as sensors within molecules to help us understand the molecular behavior in the laboratory and nature.

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 (r_s) 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 (r_e) 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 ∼sp^1.5 in allyl-NCO as determined via natural bond orbital analyses.

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.

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.

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 r_m⁽¹⁾ geometries were derived from the resulting sets of rotational constants and are consistent with the equilibrium structures (r_e) 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.

A highly flexible molecule: The peculiar case of ethynyl isothiocyanate HCCNCS

The rotational spectrum of the parent isotopic species of HCCNCS, along with those of three ¹³C singly substituted variants and one ³⁴S minor isotopologue, has been observed with high resolution using Fourier transform microwave spectroscopy from 4 to 26 GHz. Based on the observed spectral pattern and assignment of the ¹⁴N nuclear quadrupole hyperfine structure, the geometry of HCCNCS is effectively linear, which is further supported by the observation of ℓ -type doubling for rotational transitions in an excited bending state of the parent HCCNCS. The experimental observations are supported by high level quantum chemical calculations using the coupled-cluster singles and doubles model augmented by a perturbative correction for triple excitations, CCSD(T), level of theory that reveal an extremely shallow potential energy well along the CNC bending angle with a surprising basis set dependence.

Microwave spectroscopic investigation and structural determination of the Ar-difluoropyridine van der Waals complexes

The ground state rotational spectra of the van der Waals dimers of 2,3-, 2,4-, 2,5-, 2,6-, and 3,5-difluoropyridine with argon were investigated in the range of 4 to 26 GHz using both Fourier transform microwave (FTMW) and chirped pulse Fourier transform microwave (cp-FTMW) spectroscopy. The spectroscopic parameters derived from fitting the observed transitions were used to estimate the van der Waals coordinates of the five weakly bound complexes and to quantitatively describe their internal dynamics using a pseudodiatomic model. The experimental structures are in fair agreement with MP2 calculations (6-311++G(2d, 2p)) and have the centers of mass of the two moieties separated by 3.486 to 3.545 Å with the Ar atom lying 2-10° from the c' axis of the ring monomer.