Five-Membered Ring Molecules Research Articles

Isomeric effects on the electron impact scattering for selected five membered heterocyclic ring molecules

We present electron scattering cross-sectional data for selected isomers of the five-membered ring molecules, specifically oxazole and isoxazole, thiazole and isothiazole, and imidazole and pyrazole. The ab-initio R-matrix method, incorporating static exchange polarization approximations, is employed for this calculation from energy range 0.1 eV to 20 eV. Three shape resonances are identified and characterized in each system, consisting of two π* and one σ* shape resonances. Notably, thiazole and isothiazole exhibit additional σ* resonance which is which is at a lower energy than other σ* resonance. The calculated resonance positions align well with available experimental as well as theoretical data. We also performed electronic structure calculations to aid in characterization of resonances. Comparison of the scattering cross-sectional data with their respective isomers reveal marginal differences in the magnitude of the elastic cross section particularly below 1 eV. This discrepancy may be attributed to variations in the long-range electronic dipole contribution to the electron-molecule interaction process. Further, to understand dependency of dipole moment on elastic cross section, we performed a fitting procedure for the elastic cross sections at 0.1 eV with square of the dipole moment of all the five membered ring molecules studied here. The fitting formula so obtained was used to estimate the cross section for other three five membered ring molecules: Furan, Thiophene and Pyrrole. Additionally, total ionization cross sections are computed using the binary-encounter-Bethe (BEB) model, demonstrating nearly identical results when compared with their isomers. Our findings offer insights into the influence of structural changes on the electron scattering data, providing guidance for other theoretical investigations.

Detection of Interstellar E-1-cyano-1,3-butadiene in GOTHAM Observations of TMC-1

We report the detection of the lowest-energy conformer of E-1-cyano-1,3-butadiene (E-1-), a linear isomer of pyridine, using the fourth data reduction of the GBT Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) deep spectral survey toward TMC-1 with the 100 m Green Bank Telescope. We perform velocity stacking and matched-filter analyses using Markov chain Monte Carlo simulations and find evidence for the presence of this molecule at the 5.1σ level. We derive a total column density of cm−2, which is predominantly found toward two of the four velocity components we observe toward TMC-1. We use this molecule as a proxy for constraining the gas-phase abundance of the apolar hydrocarbon 1,3-butadiene. Based on the three-phase astrochemical modeling code NAUTILUS and an expanded chemical network, our model underestimates the abundance of cyano-1,3-butadiene by a factor of 19, with a peak column density of 2.34 × 1010 cm−2 for 1,3-butadiene. Compared to the modeling results obtained in previous GOTHAM analyses, the abundance of 1,3-butadiene is increased by about two orders of magnitude. Despite this increase, the modeled abundances of aromatic species do not appear to change and remain underestimated by one to four orders of magnitude. Meanwhile, the abundances of the five-membered ring molecules increase proportionally with 1,3-butadiene by two orders of magnitude. We discuss the implications for bottom-up formation routes to aromatic and polycyclic aromatic molecules.

Anomeric Effect in Five-Membered Ring Molecules: Comparison of Theoretical Computations and Experimental Spectroscopic Results.

As demonstrated in previous spectroscopic studies of 1,3-dioxole [ J. Am. Chem. Soc., 1993, 115, 12132-12136] and 1,3-benzodioxole [ J. Am. Chem. Soc., 1999, 121, 5056-5062], analysis of the ring-puckering potential energy function (PEF) of a "pseudo-four-membered ring" molecule can provide insight into understanding the magnitude of the anomeric effect. In the present study, high-level CCSD/cc-pVTZ and somewhat lower-level MP2/cc-pVTZ ab initio computations have been utilized to calculate the PEFs for 1,3-dioxole and 1,3-benzodioxole and 10 related molecules containing sulfur and selenium atoms and possessing the anomeric effect. The potential energy parameters derived for the PEFs directly provide a comparison of the relative magnitudes of the anomeric effect for molecules possessing OCO, OCS, OCSe, SCS, SCSe, and SeCSe linkages. The torsional potential energies produced by the anomeric effect for these linkages were estimated to range from 5.97 to 1.91 kcal/mol. The ab initio calculations also yielded the structural parameters, barriers to planarity, and ring-puckering angles for each of the 12 molecules studied. Based on the refined structural parameters for 1,3-dioxole and 1,3-benzodioxole, improved PEFs for these molecules were also calculated. The calculations also support the conclusion that the relatively low barrier to planarity of 1,3-benzodioxole results from competitive interactions between its benzene ring and the oxygen atom p orbitals.

Effects of nodal planes on strong-field ionization and high-order-harmonic generation in ring-type molecules

We measure the ellipticity dependence of high-harmonic generation (HHG) from unaligned gas-phase ensembles of the five-membered ring molecules 2,3-dihydrofuran (${\mathrm{C}}_{4}{\mathrm{H}}_{6}\mathrm{O}$), furan (${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{O}$), and thiophene (${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{S}$). As is normally the case, the HHG emission is suppressed for increased ellipticity since the recollision wave packet leading to HHG is steered away from the parent ion for large ellipticity. However, through comparison with computations of the first step in HHG, namely, strong-field ionization (SFI), we show that the observed differences in the ellipticity dependence for the three molecular species reflect the lateral momentum distributions of the continuum electron responsible for HHG, which in turn provides information about the particular orientation between the molecular axis and the laser field that maximizes the SFI probability. Strikingly, and contrary to the conventional wisdom in the strong-field community, we find that for furan and thiophene the most probable orientation for SFI occurs when the electric field of the laser is aligned near a nodal plane of the corresponding ionizing orbital. 2,3-dihydrofuran, on the other hand, follows the standard expectation that the most probable orientation for SFI occurs when the electric field is aligned away from any nodal plane.

Theoretical Design of Doubly Bonded Hypervalent Atoms

Molecules that contain a double bond between hypervalent atoms were designed. They included monocyclic five-membered ring molecules [HCO2(FPn)2]- and bicyclic molecules [HCO2Pn]2 (Pn = Sb, Bi), which contain one and two carboxylato (HCO2)- bridges between the doubly bonded pnictogen atoms, respectively. The bicyclic molecules are kinetically and thermodynamically more stable than the monocyclic molecules.

Comparison of the AIM Delocalization Index and the Mayer and Fuzzy Atom Bond Orders

In this paper the behavior of three well-known electron-sharing indexes, namely, the AIM delocalization index and the Mayer and fuzzy atom bond orders are studied at the Hartree-Fock level. A large number of five-membered ring molecules, containing several types of bonding, constitute the training set chosen for such purpose. A detailed analysis of the results obtained shows that the three indexes studied exhibit strong correlations, especially for homonuclear bonds. The correlation is somewhat poorer but still significant for polar bonds. In this case, the bond orders obtained with the Mayer and fuzzy atom approaches are normally closer to the formally predicted bond orders than those given by the AIM delocalization indexes, which are usually smaller than those expected from chemical intuition. In some particular cases, the use of diffuse functions in the calculation of Mayer bond orders leads to unrealistic results. In particular, noticeable trends are found for C-C bonds, encouraging the substitution of the delocalization index by the cheaper fuzzy atom or even the Mayer bond orders in the calculation of aromaticity indexes based on the delocalization index such as the para-delocalization index and the aromatic fluctuation index.

Photoinduced electron transfer (PET) in organic synthesis. [3 + 2]-type cycloaddition, cyclization and CC bond cleavage reactions

An area of growing interest, in organic photochemistry in particular, involves the use of light to induce an electron transfer from a donor (D) to an acceptor (A) molecule, designated photoinduced electron transfer (PET). Excitation of A or D leads to well-defined changes in their redox properties, i.e. A (D) becomes a stronger acceptor (donor). In general, the feasibility of producing radical ions can be predicted using the well-known Weller equation and moreover, polar solvents and salts effects support their formation. Following a brief introduction, we focus on the synthetic applications of PET, and discuss [3 + 2] cycloadditions, reductive and oxidative cyclizations and CC bond cleavage reactions. The PET ring cleavage of azirines results in the formation of 2-azaallenyl radical cations which may be trapped by dipolarophiles to give heterocyclic compounds. Using this method, we have developed a new route to heterophanes. Porphyrins can also be prepared by this procedure. Further studies aim ot reveal the scope and limitations of these new reactions. Radical cations and radical anions can attack π bonds within the same molecule. Depending on the reaction conditions chosen to generate the radical ions, five-or six-membered ring systems can be obtained. Under PET reductive conditions, radical anions arise which preferentially form five-membered ring molecules (Baldwin rule). However using PET oxidative conditions, radical cations are generated which preferentially form six-membered ring molecules due to their electrophilic character. In addition, we have developed a new reactive CC bond cleavage reaction with annular cyclopropanes and cyclobutanes and have applied this method to a new short synthesis of the hirsutene ring system.

A new proposal for a uniform treatment of internal motions in symmetrical and asymmetrical five-membered ring molecules

A system of curvilinear coordinates for five-membered ring molecules is introduced and discussed. In this system all internal motions of any (symmetric or asymmetric) five-membered ring molecule can be treated as Large Amplitude Motions (LAMs). Specific calculations of two-dimensional (ring-puckering, ring-twisting) potential functions for cyclopentene and silacyclopentene molecules are presented. As an example of an asymmetric five-membered ring molecule 2,3-dihydrofuran is discussed. All these calculations are based on the numerical construction of the Hamiltonian for internal motions. An adiabatic approach is used for separation of LAMs from Small Amplitude Vibrations (SAVs).

Die Kristallstruktur von Imidazol, C3N2H4, bei Zimmertemperatur

The crystal structure of imidazole, C 3 N 2 H 4 , at room temperature has been refined by least-squares methods from three-dimensional film data. N–H···N bonds form chains of molecules along the c axis. The five-membered ring molecules are planar.

Harmonic Force Fields for Some Heterocyclic Five-Membered Ring Molecules.

Harmonic Force Fields for Some Heterocyclic Five-Membered Ring Molecules.

Heterocyclic Five-Membered Ring Molecules. II. Mean Amplitudes of Vibration and Related Quantities.

Heterocyclic Five-Membered Ring Molecules. II. Mean Amplitudes of Vibration and Related Quantities.