Ground State Of CH Research Articles

Atropselective synthesis of N-aryl pyridones via dynamic kinetic resolution enabled by non-covalent interactions.

The dynamic kinetic resolution of C-N atropisomeric pyridones was achieved via asymmetric phase-transfer catalysis, exploiting a rotational barrier-lowering hydrogen bond in the starting materials. X-ray and NMR experiments revealed the presence of a barrier-raising ground state CH⋯π interaction in the product, supported by DFT calculations. A co-crystal of the quinidine-derived phase-transfer catalyst and substrate reveals key substrate-catalyst non-covalent interactions.

Imaging the elusive C–C bond dissociation channel of photoexcited ethyl radical

The C–C bond dissociation channel of the ethyl (CH–CH) radical is investigated at 197.4, 199.98 and 201 nm by velocity map imaging and resonance enhanced multiphoton ionisation (REMPI) detection of the produced CH fragments. On the light of high-level ab initio calculations performed in this work, up to four dissociation pathways are identified leading, respectively, to ground state CH radicals in correlation with CH fragments in the first four electronic states. The major pathway is associated with threshold dissociation over an exit barrier located at the point of a conical intersection between the initially populated 3 state and a dissociative valence state. As discussed in the text, the concomitance of both features opens the narrow window in which C–C bond dissociation occurs for ethyl and, presumably, for other similar alkyl radicals.

Spin-tensor multireference first-order polarization propagator approximation in comparison with several multireference methods on the example of low-lying electronic states of the CH+ ion

We propose a spin-tensor (ST) version of the multireference first-order polarization propagator (PP) approximation (MRFOPPA) designed to produce precision-balanced approximations (PBA) of electronic transition energies, ΔE, to low-lying electronic states of selected multiplicity in small molecules in a wide range of their nuclear geometry. Comparing the STMRFOPPA ΔE estimates with those obtained by the full configuration interaction (FCI) method and several multireference approximations on the example of transitions from the ground state of CH+ to its 25 excited states in the aug-cc-pVTZ basis set prompts the following conclusions: (i) STMRFOPPA provides PBA to the corresponding FCI results for the energies of transitions to the noted electronic states and the associated vibronic states and (ii) the accuracy of STMRFOPPA ΔE estimates is higher than that of MRCIS and close to those of MRCISD and XMCQDPT2 that operate in spaces of higher dimensions. Additionally, Supplementary Material to the paper contains the FCI(frozen core)/aug-cc-pVTZ level data for potential energy curves of the 26 electronic states of CH+ that can be useful for future research groups.

Reaction mechanism and product branching ratios of the CH + C3H4 reactions: a theoretical study.

Two ground-state CH radical reactions with the C3H4 isomers allene and methylacetylene occurring along the C4H5 potential energy surface (PES) were studied to probe the reaction mechanisms and final product distributions. The calculations were performed using a CCSD(T)-F12//B2PLYPD3 PES in combination with the 1-D chemical master equation. The reaction between the CH radical and allene was found to lead to exclusive "funneling" of the energized C4H5 intermediates into linear C4H5 configurations before reaching the exit channels, regardless of the specific nature of the initial bimolecular reactive encounter. In the case of the CH radical reaction with methylacetylene, energized C4H5 three-membered ring structures underwent H loss in significant amounts resulting in the production of a cyclic C4H4 methylenecyclopropene product, in accordance with experiments. The theoretical product distribution at room temperature for methylacetylene + CH was ∼35% methylenecyclopropene, ∼36% vinylacetylene, and ∼28% 1,2,3-butatriene, which is in agreement with the available experimental data. The distribution for allene + CH was ∼93% vinylacetylene, ∼4% 1,2,3-butatriene and ∼3% acetylene + vinyl, which overestimates the experimental yield of vinylacetylene and underestimates that of 1,2,3-butatriene by ∼10%. The possible reasons for this slight quantitative deviation of the theoretical results obtained within statistical treatment from the experiment are discussed.

Methane dissociation on Ni(111): A fifteen-dimensional potential energy surface using neural network method.

In the present work, we develop a highly accurate, fifteen-dimensional potential energy surface (PES) of CH4 interacting on a rigid flat Ni(111) surface with the methodology of neural network (NN) fit to a database consisted of about 194 208 ab initio density functional theory (DFT) energy points. Some careful tests of the accuracy of the fitting PES are given through the descriptions of the fitting quality, vibrational spectrum of CH4 in vacuum, transition state (TS) geometries as well as the activation barriers. Using a 25-60-60-1 NN structure, we obtain one of the best PESs with the least root mean square errors: 10.11 meV for the entrance region and 17.00 meV for the interaction and product regions. Our PES can reproduce the DFT results very well in particular for the important TS structures. Furthermore, we present the sticking probability S0 of ground state CH4 at the experimental surface temperature using some sudden approximations by Jackson's group. An in-depth explanation is given for the underestimated sticking probability.

Structural evolution of the methane cation in subfemtosecond photodynamics.

An ab initio quantum dynamics study has been performed to explore the structural rearrangement of ground state CH4 (+) in subfemtosecond resolved photodynamics. The method utilizes time-dependent wave-packet propagation on the X˜(2)T2 electronic manifold of the title cation in full dimensionality, including nonadiabatic coupling of the three electronic sheets. Good agreement is obtained with recent experiments [Baker et al., Science 312, 424 (2006)] which use high-order harmonic generation to probe the attosecond proton dynamics. The novel results provide direct theoretical support of the observations while unravelling the underlying details. With the geometrical changes obtained by calculating the expectation values of the nuclear coordinates as a function of time, the structural evolution is predicted to begin through activation of the totally symmetric a1 and doubly degenerate e modes. While the former retains the original Td symmetry of the cation, the Jahn-Teller active e mode conducts it to a D2d structure. At ∼1.85 fs, the intermediate D2d structure is further predicted to rearrange to local C2v minimum geometry via Jahn-Teller active bending vibrations of t2 symmetry.

Method for traveling-wave deceleration of buffer-gas beams of CH

Cryogenic buffer-gas beams are a promising method for producing bright sources of cold molecular radicals for cold collision and chemical reaction experiments. In order to use these beams in studies of reactions with controlled collision energies, or in trapping experiments, one needs a method of controlling the forward velocity of the beam. A Stark decelerator can be an effective tool for controlling the mean speed of molecules produced by supersonic jets, but efficient deceleration of buffer-gas beams presents new challenges due to longer pulse lengths. Traveling-wave decelerators are uniquely suited to meet these challenges because of their ability to confine molecules in three dimensions during deceleration and the versatility afforded by the analog control of the electrodes. We have created ground state CH($X^2\Pi$) radicals in a cryogenic buffer-gas cell with the potential to produce a cold molecular beam of $10^{11}$ mol./pulse. We present a general protocol for Stark deceleration of beams with large position and velocity spreads for use with a traveling-wave decelerator. Our method involves confining molecules transversely with a hexapole for an optimized distance before deceleration. This rotates the phase-space distribution of the molecular packet so that the packet is matched to the time varying phase-space acceptance of the decelerator. We demonstrate with simulations that this method can decelerate a significant fraction of the molecules in successive wells of a traveling-wave decelerator to produce energy-tuned beams for cold and controlled molecule experiments.

Microwave spectroscopy of [formula omitted]-doublet transitions in the ground state of CH

The Λ-doublet transitions in CH at 3.3 and 0.7GHz are unusually sensitive to variations in the fine-structure constant and the electron-to-proton mass ratio. We describe methods used to measure the frequencies of these transitions with Hz-level accuracy. We produce a pulsed supersonic beam of cold CH by photodissociation of CHBr3, and we measure the microwave transition frequencies as the molecules propagate through a parallel-plate transmission line resonator. We use the molecules to map out the amplitude and phase of the standing wave field inside the transmission line. We investigate velocity-dependent frequency shifts, showing that they can be strongly suppressed through careful timing of the microwave pulses. We measure the Zeeman and Stark effects of the microwave transitions, and reduce systematic shifts due to magnetic and electric fields to below 1Hz. We also investigate other sources of systematic uncertainty in the experiment.

Photoelectron imaging of CH−

The photoelectron images of CH− have been recorded using detachment wavelengths of 532 and 355nm. The spectra show two features arising from photodetachment of the 3Σ− ground state of CH− to the ground, X2Π, and first excited, a4Σ−, states of CH. The electron affinity of CH is measured to be 1.26±0.02eV and the a4Σ− binding energy is 1.98±0.02eV. Photoelectron angular distributions depend on the detachment energy and are characteristic of electron ejection from two different, orthogonal, orbitals in CH−.

Photoelectron spectroscopic study of the E ⊗ e Jahn-Teller effect in the presence of a tunable spin-orbit interaction. III. Two-state excitonic model accounting for observed trends in the $\tilde{\rm X}^{\,2}{\rm {E}}$X̃2E ground state of $\rm {CH}_3\rm {X}^+\,(\rm {X}=\rm {F,\,Cl,\,Br,\,I})$ CH 3X+(X=F, Cl , Br ,I) and $\rm {CH}_3\rm {Y}\,(\rm {Y}=\rm {O,\,S})$ CH 3Y(Y=O,S)

Open-shell molecules in doubly degenerate (2)E electronic states are subject to the E ⊗ e Jahn-Teller effect and spin-orbit interactions. The rotational structure of the ground vibrational level of the X(+) (2)E ground state of CH(3)F(+) has been observed by high-resolution photoelectron spectroscopy. In contrast to what is observed in other members of the isoelectronic families CH(3)X(+) (X=Cl, Br, I) and CH(3)Y (Y=O, S), the spin-orbit interaction does not lead to a splitting of the ground state of CH(3)F(+). Observed trends in the spectra of the X (2)E ground states of these molecules are summarized. Whereas certain trends, such as the reduction of the observable effects of the Jahn-Teller interactions and the increase of the spin-orbit splitting with increasing nuclear charge of X and Y are easily understood, other trends are more difficult to explain, such as the much reduced spin-orbit splitting in CH(3)F(+) compared to CH(3)O. A simple two-state excitonic model is used to account for the trends observed within the series of the methyl-halide radical cations and also the similarities and differences between CH(3)F(+) and the isoelectronic CH(3)O radical. Within this model, the electron hole in the (2)E ground states of CH(3)X(+) and CH(3)Y is described in terms of contributions from the halogenic (or chalcogenic) p(x, y) orbitals and the pyramidal-methylic (e) orbitals. This model enables a global, semi-quantitative description of the combined effects of the Jahn-Teller and spin-orbit interactions in these molecules and also a simple interpretation of the spin-orbit-coupling reduction factor ζ(e).

Vibrationally Resolved Photofragment Translational Spectroscopy of CH3I from 277 to 304 nm with Increasing Effect of the Hot Band

The photodissociation dynamics of CH(3)I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH(3)I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH(3) fragment. There are some extra peaks showing the existence of the hot-band states of CH(3)I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH(3) fragment is determined. The fraction σ of photofragments produced from the hot-band CH(3)I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I* channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel . E(int)/E(avl) of photofragments from ground-state CH(3)I remains at about 0.03 in the I* channel for all four wavelengths, but E(int)/E(avl) decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel . From the ground-state CH(3)I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P(cc) from the hot-band CH(3)I is lower than that from the ground-state CH(3)I. The potential energy at the curve-crossing point is determined to be 32,740 cm(-1).

I2 molecular elimination in single-photon dissociation of CH2I2 at 248 nm by using cavity ring-down absorption spectroscopy

Following single-photon dissociation of CH(2)I(2) at 248 nm, I(2) molecular elimination is detected by using cavity ring-down absorption spectroscopy. The technique comprises two laser beams propagating in a perpendicular configuration, in which a tunable laser beam along the axis of the ring-down cell probes the I(2) fragment in the B (3)Π(ou)(+) - X (1)Σ(g)(+) transition. The nascent vibrational populations for v = 0, 1, and 2 levels are obtained with a population ratio of 1:(0.65 ± 0.10):(0.30 ± 0.05), corresponding to a Boltzmann-like vibrational temperature of 544 ± 73 K. The quantum yield of the ground state I(2) elimination reaction is determined to be 0.0040 ± 0.0025. With the aid of ab initio potential energy calculations, the pathway of molecular elimination is proposed on the energetic ground state CH(2)I(2) via internal conversion, followed by asynchronous three-center dissociation. A positive temperature effect supports the proposed mechanism.

Gas‐Phase Photodissociation of CH3CHBrCOCl at 248 nm: Detection of Molecular Fragments by Time‐Resolved FT‐IR Spectroscopy

By employing time-resolved Fourier transform infrared emission spectroscopy, the fragments HCl (v=1-3), HBr (v=1), and CO (v=1-3) are detected in one-photon dissociation of 2-bromopropionyl chloride (CH(3)CHBrCOCl) at 248 nm. Ar gas is added to induce internal conversion and to enhance the fragment yields. The time-resolved high-resolution spectra of HCl and CO were analyzed to determine the rovibrational energy deposition of 10.0±0.2 and 7.4±0.6 kcal mol(-1), respectively, while the rotational energy in HBr is evaluated to be 0.9±0.1 kcal mol(-1). The branching ratio of HCl(v>0)/HBr(v>0) is estimated to be 1:0.53. The bond selectivity of halide formation in the photolysis follows the same trend as the halogen atom elimination. The probability of HCl contribution from a hot Cl reaction with the precursor is negligible according to the measurements of HCl amount by adding an active reagent, Br(2), in the system. The HCl elimination channel under Ar addition is verified to be slower by two orders of magnitude than the Cl elimination channel. With the aid of ab initio calculations, the observed fragments are dissociated from the hot ground state CH(3)CHBrCOCl. A two-body dissociation channel is favored leading to either HCl+CH(3)CBrCO or HBr+CH(2)CHCOCl, in which the CH(3)CBrCO moiety may further undergo secondary dissociation to release CO.

Evaluation of potential of electric field produced by molecule using symmetrical one-range addition theorems for Coulomb-Yukawa like correlated interaction potentials of integer and noninteger indices

Using symmetrical one-range addition theorems the series expansion formulae in terms of multicenter charge density expansion coefficients for noninteger n Slater type orbitals (STO), parameters of Coulomb-Yukawa like correlated interaction potentials (CIP) of noninteger indices and linear combination coefficients of molecular orbitals are established for the potential of electrostatic field produced by the charges of molecule. The final results are useful for the study of interaction between atomic-molecular systems containing any number of closed and open shells when the Hartree–Fock–Roothaan (HFR) approximation and the explicitly correlated methods based upon the use of STO as basis functions and Coulomb–Yukawa like CIP are employed. As an example of application, the calculations have been performed for the Coulomb interaction potential produced by the ground state of CH 2 molecule \({(1a_1^2 2a_1^2 1b_1^2 3a_1^1 1b_1^1,^3B_1 )}\).

Two-Dimensional (2+n) REMPI of CH3Br: Photodissociation Channels via Rydberg States

(2+n) resonance enhanced multiphoton ionization (REMPI) spectra of CH(3)Br for the masses H(+), CH(m)(+), (i)Br(+), H(i)Br(+), and CH(m)(i)Br(+) (m = 0-3; i = 79, 81) have been recorded in the 66 000-81 000 cm(-1) resonance energy range. Signals due to resonance transitions from the zero vibrational energy level of the ground state CH(3)Br to a number of Rydberg states [Ω(c)]nl;ω (Ω(c) = 3/2, 1/2; ω = 0, 2; l = 1(p), 2(d)) and various vibrational states were identified. C((3)P) and C*((1)D) atom and HBr intermediate production, detected by (2+1) REMPI, most probably is due to photodissociation of CH(3)Br via two-photon excitations to Rydberg states followed by an unusual breaking of four bonds and formation of two bonds to give the fragments H(2) + C/C* + HBr prior to ionization. This observation is supported by REMPI observations as well as potential energy surface (PES) ab initio calculations. Bromine atom production by photodissociation channels via two-photon excitation to Rydberg states is identified by detecting bromine atom (2+1) REMPI.

Accurate Double Many-Body Expansion Potential Energy Surface for the Lowest Singlet State of Methylene

A single-sheeted double many-body expansion potential energy surface is reported for ground-state CH(2) by fitting accurate ab initio energies that have been semiempirically corrected by the double many-body-expansion scaled-external-correlation method. The energies of about 2500 geometries have been calculated using the multireference configuration interaction method, with the single diffusely corrected aug-cc-pVQZ basis set of Dunning and the full valence complete active space wave function as reference. The topographical features of the novel global potential energy surface are examined and compared with other potential energy surfaces.

The spectroscopic characterization of the methoxy radical. I. Rotationally resolved à A21–X̃ E2 electronic spectra of CH3O

New experimental data have been obtained for the methoxy radical by observing at high-resolution laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) transitions between the X (2)E and lowest excited A (2)A(1) state. The SEP transitions were from the A state (pumped from the X (2)E(3/2) spin-orbit component) to the X (2)E(1/2) spin-orbit component. These data for the first time directly connect with high precision the spin-orbit components of the X (2)E ground state of CH(3)O. Surprisingly these new SEP observations are inconsistent with predictions of the X state structure based on long-standing analyses primarily based on the microwave spectra of ground state CH(3)O. It is found that all the experimental data can be understood consistently when the previously accepted value of the spin-orbit coupling constant is adjusted and the reflection parity assignments in the X state are reversed. The latter action changes the sign of a number of reflection-parity-dependent parameters in the X state. The ramifications of the changes and the physical interpretation of the resulting parameters are discussed in some detail.

On the ultraviolet photofragmentation of CH3Xe+

The multireference spin-orbit configuration interaction method is employed to calculate potential energy surfaces for the ground and low-lying excited states of the CH(3)Xe(+) cation as functions of the Xe-C bond length and the Xe-C-H angle. It is shown that the X (1)A(1) ground state of CH(3)Xe(+) is well bound (D(e)=1.78 eV) and dissociates to the CH(3)(+)(X (1)A(1)(')) + Xe((1)S) limit. In contrast, all lowest excited states of CH(3)Xe(+) are repulsive in the Franck-Condon region and converge to the strongly spin-split CH(3)(X (2)A(")) + Xe(+)((2)P(3/2,1/2)) asymptotes. Transition dipole moments for the low-lying valence states are computed at the X (1)A(1) equilibrium geometry. It is shown that the first absorption continuum (A band) of CH(3)Xe(+) is dominated by the parallel (3)Q(0(+))(A(1)) <-- X (1)A(1) transition, which leads to the CH(3) + Xe(+)((2)P(3/2)) dissociation products. The perpendicular transitions to the (1)Q(E), (3)Q(1)(E), and (3)A(1)(E) states are found to be significantly weaker. The CH(3)Xe(+) photodissociation process in its A band is analyzed on the basis of the computed data and compared with the photodissociation of the isovalent RgH(+) (Rg = Ar,Kr,Xe), HI, and CH(3)I systems.

A Multireference Coupled-Cluster Potential Energy Surface of Diazomethane, CH2N2

The intrinsically multireference dissociation of the C-N bond in ground-state diazomethane (CH(2)N(2)) at different angles has been studied with the multireference Brillouin-Wigner coupled-cluster singles and doubles (MRBWCCSD) method. The morphology of the calculated potential energy surface (PES) in C(s)() symmetry is similar to a multireference perturbational (CASPT3) PES. The MRBWCCSD/cc-pVTZ H(2)C-N(2) dissociation energy with respect to the asymptotic CH(2)(ã(1)A(1)) + N(2)(X(1)Sigma(g)(+)) products is D(e) = 35.9 kcal/mol, or a zero-point corrected D(0) = 21.4 kcal/mol with respect to the ground-state CH(2)(X(3)B(1)) + N(2)(X(1)Sigma(g)(+)) fragments.

Theoretical Investigation of Organo-Noble Gas Compounds, HC(Ng)n+, n = 1, 2; Ng = He, Ne, Ar, Kr, and Xe. Evidence for Potentially Isolable HCArn+, HCKrn+, and HCXen+ Species

We present a high-level theoretical study of the interaction of the ground-state CH+ cation with the noble gas, Ng = He to Xe, atoms. Considerably strong C−Ng bonds were detected in the ground-state singlet HC(Ng)+ and HC(Ng)2+ species when Ng = Ar, Kr, and Xe. Bonding occurs in a Lewis acid−base fashion via dative bonds of HC+ ← :Ng type, resembling the bonding nature of the recently isolated N5+ cation. Both closed-shell HC(Ng)+ and HC(Ng)2+ cations are likely to be isolable under appropriate experimental conditions.