Exotic SiO₂H₂ Isomers: Theory and Experiment Working in Harmony

Replacing carbon with silicon can result in dramatic and unanticipated changes in isomeric stability, as the well-studied CO₂H₂ and the essentially unknown SiO₂H₂ systems illustrate. Guided by coupled-cluster calculations, three SiO₂H₂ isomers have been detected and spectroscopically characterized in a molecular beam discharge source using rotational spectroscopy. The <i>cis</i>,<i>trans</i> conformer of dihydroxysilylene HOSiOH, the ground-state isomer, and the high-energy, metastable dioxasilirane <i>c</i>-H₂SiO₂ are abundantly produced in a dilute SiH₄/O₂ electrical discharge, enabling precise structural determinations of both by a combination of isotopic measurements and calculated vibrational corrections. The isotopic studies also provide insight into their formation route, suggesting that <i>c</i>-H₂SiO₂ is formed promptly in the expansion but that <i>cis</i>,<i>trans</i>-HOSiOH is likely formed by secondary reactions following formation of the most stable dissociation pair, SiO + H₂O. Although less abundant, the rotational spectrum of <i>trans</i>-silanoic acid, the silicon analogue of formic acid, HSi­(O)­OH, has also been observed

Comparative Study of the Mechanical Unfolding Pathways of α- and β‑Peptides

Using molecular simulations, we analyze the unfolding pathways of various peptides. We compare the mechanical unfolding of a β-alanine’s octamer (β-HAla₈) and an α-alanine’s decamer (α-Ala₁₀). Using force-probe molecular-dynamics simulations, to induce unfolding, we show that the 3₁₄-helix formed by β-HAla₈ is mechanically more stable than the α-helix formed by α-Ala₁₀, although both structures are stabilized by six hydrogen bonds. Additionally, computations of the potential of mean force validate this result and show that also the thermal stability of the 3₁₄-helix is higher. It is demonstrated that β-HAla₈ unfolds in a two-step fashion with a stable intermediate. This is contrasted with the known single-step scenario of the unfolding of α-Ala₁₀. Furthermore, we present a study of the chain-length dependence of the mechanical unfolding pathway of the 3₁₄-helix. The calculation of the dynamic strength for oligomers with chain lengths ranging from 6 to 18 monomers shows that the unfolding pathway of helices with an integer and noninteger number of turns has <i>m</i> + 1 and <i>m</i> energy barriers, respectively, with <i>m</i> being the number of complete turns. The additional barrier for helices with an integer number of turns is shown to be related to the breaking of the N-terminus’ hydrogen bond

Determining Factors for the Unfolding Pathway of Peptides, Peptoids, and Peptidic Foldamers

We present a study of the mechanical unfolding pathway of five different oligomers (α-peptide, β-peptide, δ-aromatic-peptides, α/γ-peptides, and β-peptoids), adopting stable helix conformations. Using force-probe molecular dynamics, we identify the determining structural factors for the unfolding pathways and reveal the interplay between the hydrogen bond strength and the backbone rigidity in the stabilization of their helix conformations. On the basis of their behavior, we classify the oligomers in four groups and deduce a set of rules for the prediction of the unfolding pathways of small foldamers

Importance of Triples Contributions to NMR Spin–Spin Coupling Constants Computed at the CC3 and CCSDT Levels

We present the first analytical implementation of CC3 second derivatives using the spin-unrestricted approach. This allows, for the first time, the calculation of nuclear spin–spin coupling constants (SSCC) relevant to NMR spectroscopy at the CC3 level of theory in a fully analytical manner. CC3 results for the SSCCs of a number of small molecules and their fluorine substituted derivatives are compared with the corresponding coupled cluster singles and doubles (CCSD) results obtained using specialized basis sets. For one-bond couplings the change when going from CCSD to CC3 is typically 1–3%, but much higher corrections were found for ¹<i>J</i>_CN in FCN, 15.7%, and ¹<i>J</i>_OF in OF₂, 6.4%. The changes vary significantly in the case of multibond couplings, with differences of up to 10%, and even 13.6% for ³<i>J</i>_FH in fluoroacetylene. Calculations at the coupled cluster singles, doubles, and triples (CCSDT) level indicate that the most important contributions arising from connected triple excitations in the coupled cluster expansion are accounted for at the CC3 level. Thus, we believe that the CC3 method will become the standard approach for the calculation of reference values of nuclear spin–spin coupling constants

Synthesis, Microwave Spectrum, Quantum Chemical Calculations, and Conformational Composition of a Novel Primary Phosphine, Cyclopropylethynylphosphine, (C₃H₅CCPH₂)

The microwave spectrum of cyclopropylethynylphosphine, C₃H₅CCPH₂, has been investigated in the 26–120 GHz spectral region. The spectrum is dominated by very rich and complex <i>a</i>-type <i>R</i>-branch pile-ups. There must be insignificant steric interaction between the phosphino group and the cyclopropyl ring due to the long distance between these two groups. However, the phosphino group does not undergo free or nearly free internal rotation. Instead, the spectra of two distinct conformers were assigned. Both these two forms have <i>C</i>_S symmetry. The symmetry plane bisects the cyclopropyl ring and the phosphino group in both conformers, and the lone electron pair of the phosphino group points in opposite directions in the two rotamers. The energy difference between the two forms was determined to be 1.9(6) kJ/mol. A simple model that takes into consideration the interaction of the lone electron pair of the phosphino group with the π-electrons of the ethynyl group and the Walsh electrons of the cyclopropyl ring is able to give a qualitative explanation of the observation of two conformers and the nonexistence of free rotation of the phosphino group. The MW work was augmented by quantum chemical calculations using second-order Møller–Plesset perturbation and coupled cluster theory with results that are in good agreement with the experiments

Cholesky Decomposition-Based Implementation of Relativistic Two-Component Coupled-Cluster Methods for Medium-Sized Molecules

A Cholesky decomposition (CD)-based implementation of relativistic two-component coupled-cluster (CC) and equation-of-motion CC (EOM-CC) methods using an exact two-component Hamiltonian augmented with atomic-mean-field spin–orbit integrals (the X2CAMF scheme) is reported. The present CD-based implementation of X2CAMF-CC and EOM-CC methods employs atomic-orbital-based algorithms to avoid the construction of two-electron integrals and intermediates involving three and four virtual indices. Our CD-based implementation extends the applicability of X2CAMF-CC and EOM-CC methods to medium-sized molecules with the possibility to correlate around 1000 spinors. Benchmark calculations for uranium-containing small molecules were performed to assess the dependence of the CC results on the Cholesky threshold. A Cholesky threshold of 10–4 is shown to be sufficient to maintain chemical accuracy. Example calculations to illustrate the capability of the CD-based relativistic CC methods are reported for the bond-dissociation energy of the uranium hexafluoride molecule, UF6, with up to quadruple-ζ basis sets, and the lowest excitation energy in the solvated uranyl ion [UO22+(H2O)12]

Cholesky Decomposition-Based Implementation of Relativistic Two-Component Coupled-Cluster Methods for Medium-Sized Molecules

A Cholesky decomposition (CD)-based implementation of relativistic two-component coupled-cluster (CC) and equation-of-motion CC (EOM-CC) methods using an exact two-component Hamiltonian augmented with atomic-mean-field spin–orbit integrals (the X2CAMF scheme) is reported. The present CD-based implementation of X2CAMF-CC and EOM-CC methods employs atomic-orbital-based algorithms to avoid the construction of two-electron integrals and intermediates involving three and four virtual indices. Our CD-based implementation extends the applicability of X2CAMF-CC and EOM-CC methods to medium-sized molecules with the possibility to correlate around 1000 spinors. Benchmark calculations for uranium-containing small molecules were performed to assess the dependence of the CC results on the Cholesky threshold. A Cholesky threshold of 10–4 is shown to be sufficient to maintain chemical accuracy. Example calculations to illustrate the capability of the CD-based relativistic CC methods are reported for the bond-dissociation energy of the uranium hexafluoride molecule, UF6, with up to quadruple-ζ basis sets, and the lowest excitation energy in the solvated uranyl ion [UO22+(H2O)12]

Spectroscopic Detection and Structure of Hydroxidooxidosulfur (HOSO) Radical, An Important Intermediate in the Chemistry of Sulfur-Bearing Compounds

The rotational spectrum of hydroxidooxidosulfur, HOSO, an intermediate of particular interest in the combustion of sulfur-rich fuels, has been determined to high accuracy from gas-phase measurements. Detection of specific isotopic species using isotopically enriched gases suggests that HOSO is formed in our discharge nozzle via the reaction H + SO₂ (+M) → HOSO (+M). A precise experimental <i>r</i>₀ geometry has also been derived from the isotopic analysis; HOSO has a <i>cis</i> configuration, but the subtle structural question of its planarity remains unresolved. From the derived spectroscopic constants, <i>in situ</i> and remote sensing for this fundamental radical can now be undertaken in a variety of environments, including in combustion reactors, the troposphere of Earth, and Io, the innermost Galilean moon of Jupiter

Gas-Phase Spectroscopic Detection and Structural Elucidation of Carbon-Rich Group 14 Binary Clusters: Linear GeC₃Ge

Guided by high-level quantum-chemical calculations at the CCSD­(T) level of theory, the first polyatomic germanium–carbon cluster, linear Ge₂C₃, has been observed at high spectral resolution in the gas phase through its remarkably complex fundamental antisymmetric C–C stretching mode ν₃ located at 1932 cm^–1. The observation of a total of six isotopic species permits the derivation of a highly accurate value for the equilibrium Ge–C bond length. The present study suggests that many more Ge–C species might be detectable in the future using a combination of laser-ablation techniques for production and high-resolution infrared and/or microwave techniques for spectroscopic detection

Ro-vibrational Spectrum of Linear Dialuminum Monoxide (Al₂O) at 10 μm

Dialuminum monoxide, Al2O, has been investigated in the laboratory at mid-IR wavelengths around 10 μm at high spectral resolution. The molecule was produced by laser ablation of an aluminum target with the addition of gaseous nitrous oxide, N2O. Subsequent adiabatic cooling of the gas in a supersonic beam expansion led to rotationally cold spectra. In total, 848 ro-vibrational transitions have been assigned to the fundamental asymmetric stretching mode ν3 and to five of its hot bands, originating from excited levels of the ν1 symmetric stretching mode and the ν2 bending mode. The measurements encompass 11 vibrational energy states (v1 v2l v3). The ro-vibrational transitions show spin statistical line intensity alternation of 7:5, which is caused by two identical aluminum nuclei of spin I = 5/2 at both ends of the centrosymmetric molecule of structure Al–O–Al. The less effective cooling of vibrational states in the supersonic beam expansion allowed measurement of transitions in excited vibrational states at energies of 1000 cm–1 and higher, while rotational levels within vibrational modes exhibited thermal population, with rotational temperatures around Trot = 115 K. Molecular parameters for 11 vibrational states were derived, including rotation and centrifugal distortion constants and l-type doubling constants for the states (v1 v2l v3) = (0 11 0) and (0 11 1) and an l-type resonance between the states (0 20 0) - (0 22 0) and (0 20 1) - (0 22 1). From the experimental results, rotational correction terms and the equilibrium bond length re were derived. The measurements were supported and guided by high-level quantum-chemical calculations that agree well with the derived experimental results