New Diagnostic of the Most Populated Conformer of Tetrahydrofuran in the Gas Phase

Efficient Generation of Torsional Energy Profiles by Multifidelity Gaussian Processes for Hindered Rotor Corrections

Accurate thermochemistry computations often require proper treatment of torsional modes. The one-dimensional hindered rotor model has proven to be a computationally efficient solution, given a sufficiently accurate potential energy surface. Methods that provide potential energies at various compromises of uncertainty and computational time demand can be optimally combined within a multifidelity treatment. In this study, we demonstrate how multifidelity modeling leads to (1) smooth interpolation along low-fidelity scan points with uncertainty estimates, (2) inclusion of high-fidelity data that change the energetic order of conformations, and (3) predicting best next-point calculations to extend an initial coarse grid. Our diverse application set comprises molecules, clusters, and transition states of alcohols, ethers, and rings. We discuss limitations for cases in which the low-fidelity computation is highly unreliable. Different features of the potential energy curve affect different quantities. To obtain "optimal" fits, we apply strategies ranging from simple minimization of deviations to developing an acquisition function tailored for statistical thermodynamics. Bayesian prediction of best next calculations can save a substantial amount of computation time for one- and multidimensional hindered rotors.

Deciphering the conformational preference and ionization dynamics of tetrahydrofuran: Insights from advanced spectroscopic techniques

Tetrahydrofuran (THF) has garnered significant attention due to its pivotal role in biological and chemical processes. The diverse array of conformations exhibited by THF profoundly impacts its reactivity and interactions with other molecules. Understanding these conformational preferences is crucial for comprehending its molecular behavior. In this study, we utilize infrared (IR) resonant vacuum ultraviolet photoionization/mass-analyzed threshold ionization (VUV-PI/MATI) mass spectroscopies to capture distinctive vibrational spectra of individual conformers, namely, "twisted" and "bent," within THF. Our conformer-specific vibrational spectra provide valuable insights into the relative populations of these two conformers. The analysis reveals that the twisted (C2) conformer is more stable than the bent (CS) conformer by 17 ± 15 cm-1. By precisely tuning the VUV photon energy to coincide with vibrational excitation via IR absorption, we selectively ionize specific conformers, yielding two-photon IR + VUV-PI/MATI spectra corresponding to the twisted and bent conformers. This investigation conclusively affirms that both the twisted and bent conformers coexist in the neutral state, while only the twisted conformer exists in the cationic state. These findings not only bridge gaps in existing knowledge but also provide profound insights into the behavior of this pivotal molecule in the realms of biology and medicine.

Photodissociation dynamics of tetrahydrofuran at 193 nm

Tetrahydrofuran (THF), a cyclic ether with the chemical formula C4H8O, can be considered the simplest analog of the deoxyribose backbone component of deoxyribonucleic acid (DNA). As such, it provides a useful model for probing the photochemistry of such biomolecular motifs. We present a velocity-map imaging study into the ultraviolet dissociation of THF at a wavelength of 193 nm. Excitation to the S1 state occurs via a 3s ← n transition involving a lone-pair electron on the oxygen atom, and has been shown by other authors to result in rapid ring opening via cleavage of one of the C-O bonds to form a ring-opened C4H8O diradical, followed by C-C bond cleavage over a longer timescale to form either OCH2 + C3H6 products (Channel 1a), HOCH2 + C2H5 products (Channel 1b), or OCH2CH2 + C2H4 products (Channel 2). The C2H4O products formed via Channel 2 are unstable on the timescale of our experiment and dissociate further to form CH3 and CHO. We also observe a number of minor products resulting from H or H2 loss from the primary photofragments. The speed distributions observed for all photofragments are broad, indicating excitation of a range of rotational and vibrational states of the products. The angular distributions of the photofragments show an interesting speed dependence: the slowest products have almost isotropic angular distributions, but the magnitude of the recoil anisotropy increases monotonically with photofragment speed. The fastest products exhibit highly anisotropic angular distributions, with the recoil anisotropy parameter β approaching its limiting value of -1 (-0.75 for Channel 1 and -0.5 for Channel 2). This behaviour is attributed to the range of timescales over which the diradical intermediate dissociates into the observed photofragments. Rapid dissociation leads to fast photofragments which retain the correlation between the transition dipole moment for the S1 ← S0 excitation (which lies perpendicular to the ring) and the photofragment velocities (which lie predominantly in the plane of the ring). Slow dissociation results in a high degree of energy redistribution into internal modes, slower photofragments, and loss of correlation between the photofragment velocities and the transition dipole. The higher barrier associated with dissociation via Channel 2 suggests somewhat longer lifetimes for the diradical intermediate and is consistent with a corresponding reduction in the maximum observed value for β.

Future of computational molecular spectroscopy-from supporting interpretation to leading the innovation

Molecular spectroscopy measures transitions between discrete molecular energies which follow quantum mechanics. Structural information of a molecule is encoded in the spectra, which can be only decoded using quantum mechanics and therefore computational molecular spectroscopy becomes essential. In this review perspective, the role evolution of computational molecular spectroscopy has been discussed with several joint theory and experiment spectroscopic studies in the past decades, which includes rotational (microwave), vibrational and electronic spectroscopy (valence and core) of molecules. With the development in high resolution and computerized synchrotron sourced spectroscopy, spectral measurements and computational molecular spectroscopy need to be integrated for materials development. Contemporary computational molecular spectroscopy is, therefore, more than merely supporting interpretation but leading the innovation. Future development of molecular spectroscopy lies to identify the niche to integrate experimental and computational molecular spectroscopy. It also requires to engineer molecular spectroscopic databases that function according to the universal approaches of computing, such as those in a Turing machine, to be realized in a chemical and/or spectroscopic programable manner (digital twinning research) in the future.

Low-temperature heat capacity and pseudorotation in 2 methyltetrahydrofuran

The heat capacity and phase transitions of 2-methyltetrahydrofuran in the temperature range from 7 to 350 K were measured by adiabatic calorimetry. The smoothed molar thermodynamic functions in the condensed...

Conformational diversity of the THF molecule in N2 matrix by means of FTIR matrix isolation experiment and Car-Parrinello molecular dynamics simulations

Tetrahydrofuran (THF) is a widely used chemical compound, in particular as a solvent in organic and inorganic synthesis. The THF molecule has also an interesting property, namely, undergoes pseudorotation, similar to the case of the cyclopentane. Low energy difference between the envelope (C_s symmetry) and twisted (C₂ symmetry) conformations of the THF molecule leads to the interconversion between the two conformers. We study the influence of the molecular environment (N₂) on the C_s-C₂ equilibrium of tetrahydrofuran in the THF@N₂ system utilizing nitrogen matrix isolation infrared spectroscopy. We observe a different ratio between envelope (C_s) and twisted (C₂) conformations with respect to a change of the temperature. FTIR experimental studies are supported by the results of the static density functional theory calculations and Car-Parrinello molecular dynamics simulations. We focus on the dynamics of the pseudorotation process, in particular, the lifetime of the THF conformations and their mutual rearrangements. On the basis of the THF@N₂ matrix model, with explicit nitrogen molecules, the anharmonic infrared spectra are generated from the Fourier transformation of the dipole moment autocorrelation function.

Probing Intramolecular Interaction of Stereoisomers Using Computational Spectroscopy

Several model stereoisomers such as ferrocene (Fc), methoxyphenol, and furfural conformers are discussed. It was discovered that the Fc IR spectroscopic band(s) below 500cm−1 serve as fingerprints for eclipsed (splitting 17 (471–488)cm−1) and staggered Fc (splitting is ~2 (459–461)cm−1) in the gas phase. It is revealed that in the gas phase the dominance of the eclipsed Fc (D5h) at very low temperatures changes to a mixture of both eclipsed and staggered Fc when the temperature increases. However, in solvents such as CCl4, eclipsed Fc dominates at room temperature (300K) due to the additional solvation energy. Intramolecular interactions of organic model compounds such as methoxyphenols (guaiacol (GUA) and mequinol (MEQ)) and furfural, ionization energies such as the carbon 1s (core C1s), as well as valence binding energy spectra serve this purpose well. Hydrogen bonding alters the C1s binding energies of the methoxy carbon (C(7)) of anti-syn and anti-gauche conformers of GUA to 292.65 and 291.91eV, respectively. The trans and cis MEQ conformers, on the other hand, are nearly energy degenerate, whereas their dipole moments are significantly different: 2.66 Debye for cis and 0.63 Debye for trans-MEQ. Moreover, it is found that rotation around the Cring–OH and the Cring–OCH3 bonds differ in energy barrier height by ~0.50 kcal⋅mol−1. The Dyson orbital momentum profiles of the most different ionic states, 25a′ (0.35eV) and 3a′ (−0.33eV), between cis and trans-MEQ in outer valence space (which is measurable using electron momentum spectroscopy (EMS)), exhibit quantitative differences. Finally, the molecular switch from trans and cis-furfural engages with a small energy difference of 0.74 kcal mol−1, however, at the calculated C(3)(–H⋅⋅⋅O=C) site the C1s binding energy difference is 0.105eV (2.42 kcal mol−1) and the NMR chemical shift of the same carbon site is also significant; 7.58ppm from cis-furfural without hydrogen bonding.

Determination of the cationic conformational structure of tetrahydrothiophene by one-photon MATI spectroscopy and Franck–Condon fitting

This study affords more accurate information regarding the pseudorotational PES describing the conformational interconversion of THT.

Single-walled carbon nanotubes in tetrahydrofuran solution: microsolvation from first-principles calculations

The molecular interactions between the commonly used solvent tetrahydrofuran (THF) and single-walled carbon nanotubes (SWCNT) are studied using density functional theory calculations and Car-Parrinello molecular dynamics simulations. The competitive interplay between THF-THF and THF-SWCNT interactions via C-H⋯O and C-H⋯π hydrogen bonds is analyzed in detail. The binding energies for different global and local energy minima configurations of THF monomers, dimers, trimers, and tetramers on SWCNT(10,0) were determined. The adsorbed species are analyzed in terms of their coordination to the surface via weak hydrogen bonds of the C-H⋯π type and in terms of their ability to form intermolecular C-H⋯O hydrogen bonds, which are responsible for the self-aggregation of THF molecules and a possible dimerization or tetramerization process. A special focus is put on the pseudorotation of the THF molecules at finite temperatures and on the formation of blue-shifting hydrogen bonds.

Potential-Energy Surfaces for Ring-Puckering Motions of Flexible Cyclic Molecules through Cremer-Pople Coordinates: Computation, Analysis, and Fitting

Ring-puckering motion in 12 flexible cyclic molecules is investigated by calculation and analysis of two-dimensional potential-energy surfaces (PESs) using the so-called ring-puckering coordinates proposed by Cremer and Pople. The PESs are calculated by means of density-functional theory using a B2PLYP-D3BJ exchange-correlation functional with a maug-cc-pVTZ basis set, and results are compared to the available experimental and theoretical data. Special care is devoted to the aspect of symmetry in such two-dimensional PESs, which are here reported for the first time also for molecules whose planar form has symmetry lower than D_{5 h} or C_{2 v}. The issue of PES fitting and that of solving the nuclear dynamics using ring-puckering coordinates are also addressed. Analytical formulations of the computed PESs using suitable functional forms with a limited set of parameters are provided.

Theoretical study of (e, 2e) triple differential cross sections of tetrahydrofuran using multicenter distorted-wave method

We present in this work a theoretical study of the triple differential cross sections for electron-impact single ionization of 12a' and 9b orbitals of a tetrahydrofuran molecule. The calculations are performed by the multicenter distorted-wave (MCDW) method in coplanar asymmetric kinematics with an incident electron energy of 250 eV, where previous (e, 2e) experiments and theories are available for comparison. The present calculations reproduce the experimental measurements satisfactorily, indicating that the MCDW method can be easily extended to study biomolecules with sufficiently high calculation efficiency.

Study of Electron Ionization and Fragmentation of Non-hydrated and Hydrated Tetrahydrofuran Clusters

Mass spectroscopic investigations on tetrahydrofuran (THF, C₄H₈O), a common model molecule of the DNA-backbone, have been carried out. We irradiated isolated THF and (hydrated) THF clusters with low energy electrons (electron energy ~70 eV) in order to study electron ionization and ionic fragmentation. For elucidation of fragmentation pathways, deuterated TDF (C₄D₈O) was investigated as well. One major observation is that the cluster environment shows overall a protective behavior on THF. However, also new fragmentation channels open in the cluster. In this context, we were able to solve a discrepancy in the literature about the fragment ion peak at mass 55 u in the electron ionization mass spectrum of THF. We ascribe this ion yield to the fragmentation of ionized THF clusters. Graphical Abstract ᅟ.

Electron Momentum Spectroscopy Investigation of Molecular Conformations of Ethanol Considering Vibrational Effects

The interpretation of experimental electron momentum distributions (EMDs) of ethanol, one of the simplest molecules having conformers, has confused researchers for years. High-level calculations of Dyson orbital EMDs by thermally averaging the gauche and trans conformers as well as molecular dynamical simulations failed to quantitatively reproduce the experiments for some of the outer valence orbitals. In this work, the valence shell electron binding energy spectrum and EMDs of ethanol are revisited by the high-sensitivity electron momentum spectrometer employing symmetric noncoplanar geometry at an incident energy of 1200 eV plus binding energy, together with a detailed analysis of the influence of vibrational motions on the EMDs for the two conformers employing a harmonic analytical quantum mechanical (HAQM) approach by taking into account all of the vibrational modes. The significant discrepancies between theories and experiments in previous works have now been interpreted quantitatively, indicating that the vibrational effect plays a significant role in reproducing the experimental results, not only through the low-frequency OH and CH₃ torsion modes but also through other high-frequency ones. Rational explanation of experimental momentum profiles provides solid evidence that the trans conformer is slightly more stable than the gauche conformer, in accordance with thermodynamic predictions and other experiments. The case of ethanol demonstrates the significance of considering vibrational effects when performing a conformational study on flexible molecules using electron momentum spectroscopy.

Conformational structures of the tetrahydrofuran cation determined using one-photon mass-analyzed threshold ionization spectroscopy

One-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy was used to characterize the essential conformations of tetrahydrofuran (THF) and thus determine the stereochemistry of the furanose ring constituting the backbones of DNA and RNA.

Vibrational Effects on Electron Momentum Distributions of Outer-Valence Orbitals of Oxetane

Vibrational effects on electron momentum distributions (EMDs) of outer-valence orbitals of oxetane are computed with a comprehensive consideration of all vibrational modes. It is found that vibrational motions influence EMDs of all outer-valence orbitals noticeably. The agreement between theoretical and experimental momentum profiles of the first five orbitals is greatly improved when including molecular vibrations in the calculation. In particular, the large turn-up at low momentum in the experimental momentum profile of the 3b1 orbital is well interpreted by vibrational effects, indicating that, besides the low-frequency ring-puckering mode, C-H stretching motion also plays a significant role in affecting EMDs of outer-valence orbitals of oxetane. The case of oxetane exhibits the significance of checking vibrational effects when performing electron momentum spectroscopy measurements.

Ring-puckering effects on electron momentum distributions of valence orbitals of oxetane

The binding energy spectra and electron momentum distributions for the outer-valence molecular orbitals of oxetane have been measured utilizing (e, 2e) electron momentum spectrometer with non-coplanar asymmetric geometry at the impact energy of 2500 eV. The experimental momentum distributions were compared with the density functional theory calculations employing B3LYP hybrid functional with aug-cc-pVTZ basis set. It was found that the calculation at planar geometry (C2v) completely fails to interpret the large "turn-up" at low momentum region in electron momentum distribution of the highest occupied molecular orbital (HOMO) 3b1, while the calculations considering the thermal abundances of planar (C2v) and bent (Cs) conformers or the thermally populated vibrational states of ring-puckering motion have significantly improved the agreement. The results indicate that the ring-puckering motion of oxetane has a strong effect on the electron density distribution of HOMO.

Reaction of chlorine radical with tetrahydrofuran: a theoretical investigation on mechanism and reactivity in gas phase

Reaction of chlorine (Cl) radical with heterocyclic saturated ether, tetrahydrofuran has been studied. The detailed reactivity and mechanism of this reaction is analyzed using hybrid density functional theory (DFT), B3LYP and BB1K methods, and aug-cc-pVTZ basis set. To explore the mechanism of the reaction of tetrahydrofuran with Cl radical, four possible sites of hydrogen atom (H) abstraction pathways in tetrahydrofuran were analyzed. The barrier height and rate constants are calculated for the four H-abstraction channels. The BB1K calculated rate constant for α-axial H-abstraction is comparable with the experimentally determined rate constant. It reflects that α-axial H-abstraction is the main degradation pathway of tetrahydrofuran with Cl radical. DFT-based reactivity descriptors are also calculated and these values describe α-axial H-abstraction as the main reaction channel.

Photoelectron and electron momentum spectroscopy of tetrahydrofuran from a molecular dynamical perspective

The results of experimental studies of the valence electronic structure of tetrahydrofuran employing He I photoelectron spectroscopy as well as Electron Momentum Spectroscopy (EMS) have been reinterpreted on the basis of Molecular Dynamical simulations employing the classical MM3 force field and large-scale quantum mechanical simulations employing Born-Oppenheimer Molecular Dynamics in conjunction with the dispersion corrected ωB97XD exchange-correlation functional. Analysis of the produced atomic trajectories demonstrates the importance of thermal deviations from the lowest energy path for pseudorotation, in the form of considerable variations of the ring-puckering amplitude. These deviations are found to have a significant influence on several outer-valence electron momentum distributions, as well as on the He I photoelectron spectrum.

Potential energy surfaces for all-trans P5H6(+) and P5Me6(+) may be less complicated than anticipated

Burford and co-workers recently (2007) investigated the synthesis, structure, and pseudorotation of a family of cyclotetraphosphinophosphonium ions comprehensively. We now report theoretical computations through the CCSD(T) level to study the conformations of the parent model rings, P5H6(+) and P5Me6(+). The all-trans conformations on the full pseudorotational cycle for the cyclotetraphosphinophosphonium cation (P5H6(+)) and for its methyl-substituted derivative (P5Me6(+)) were located systematically on their potential energy surfaces. The potential energy for the pseudorotational circuit of P5H6(+) is smooth and monotonic with only one minimum ((3)T4) and one maximum ((4)T3), but the analogous P5Me6(+) circuit has two minima. The P5H6(+) and P5Me6(+) potential surfaces appear to be qualitatively very different from that for the well investigated tetrahydrofuran. Since the (3)T4 form of P5H6(+)avoids unfavorable eclipsing PH-PH and lone pair-lone pair interactions, it is the global minimum among all possible P5H6(+) configurations and conformations.