Singlet CH2 Research Articles

Rovibrational Line Lists of Triplet and Singlet Methylene.

The methylene molecule (CH2) is a short-lived radical with lacking data on its spectral line intensities. Although the lifetime of CH2 is extremely short under Earth's conditions, it exists in a free form in interstellar media. CH2 is an important intermediate species in chemical reactions associated with the formation and destruction of complex hydrocarbons. We present the first rovibrational line lists of CH2 in its ground triplet and first excited singlet electronic state. To this end, our previously developed accurate ab initio potential energy surface (PES) was used for the ground electronic triplet state [Egorov et al. J. Comp. Chem. 2024. V. 45. (2). P. 83] while a new PES for the singlet state was constructed in this work using the single-reference coupled cluster approach [CCSD(T)] combined with the extrapolation to the complete basis set (CBS) limit based on the correlation-consistent orbital basis sets with the core-valence electron correlation effects [aug-cc-pCVXZ, X = T, Q, 5, and 6]. In addition, the contributions to the correlation energy from highly excited Slater determinants [CC(n), n = 3-5] were included as well as the scalar relativistic effects and DBOC. The most accurate description of the infrared band origins of singlet CH2 was thus achieved for the energy range where the impact of the nonadiabatic coupling due to the Renner-Teller effect can be neglected. To obtain the probabilities of the rovibrational transitions, new ab initio DMSs were constructed both for the triplet and singlet CH2 using the CCSD(T)/aug-cc-pCVQZ approach. Finally, the absorption spectra of triplet and singlet methylene were predicted from the variationally computed line lists.

Theoretical Study on Formamide (NH2CHO) and Methylidene (CH2) Reactions for the Formation of Interstellar N-Methylformamide

N-methylformamide (NMF), E & Z containing an amide bond, a vital component of peptides, is one of the simplest derivatives of formamides. It is detected towards the hot molecular core of the Sgr B2(N2) molecular cloud with the Atacama Large Millimeter/submillimeter Array (ALMA). It is proposed that the radical-molecule mechanism can be used as a strong contrivance to investigate the formation of NMF in the interstellar medium (ISM) via some interstellar molecules like methylidene CH2(1Σ1 & 3Σ1) and formamide (NH2CHO). The density functional theory (DFT) at B3LYP/6-311G (d,p) has been applied to study the mechanisms. All the possible cases of E- & Z-NMF formation with singlet and triplet CH2 have been investigated. It is found that the formation of E-NMF with CH2(1Σ1) and Z-NMF with CH2(3Σ1) is more feasible and favorable in comparison to E-NMF formation from CH2(3Σ1) and Z-NMF with CH2(1Σ1).

The effect of benzene on the structure of low-pressure premixed H2/CH4/CO-air flames and related NO formation at different equivalence ratios

Coke oven gas (COG) is a valuable by-product of coke-making process, due to its high H2 content and calorific value. The potential exploitation of COG as an energy carrier requires a proper characterization of its kinetic behavior. Moreover, the residual presence of aromatic compounds in COG can potentially affect both its combustion efficiency and pollutant emissions. In this work, the chemical effect of benzene addition (up to 1.12% by vol.) to H2/CH4/CO mixtures, representative of COG, is assessed. Flame structure and speciation are extensively investigated through a combined experimental and modeling study, carried out in low-pressure (7.5 kPa) premixed laminar flames. Different equivalence ratios (0.8, 1.0, 1.2) are considered, with air as oxidant and a constant cold-gas velocity. It is found that NO formation progressively increases with the amount of benzene at all the investigated equivalence ratios, and such increase mostly occurs at the flame front region. Through a detailed kinetic analysis, it is found that benzene addition increases the NO formation in the H2/CH4/CO/C6H6 flames. This is mainly due to the enhancement of the prompt and thermal NO routes. The kinetic study allowed to identify the sources of the interactions between benzene oxidation and prompt NO chemistry, ultimately involving the ketenyl radical (HCCO) and acetylene (C2H2). This results in an increase of singlet CH2 (CH2(S)) and CH2 with consequent modification of CH profile, via HCCO→CH2(S)→CH2→CH and C2H2→CH2→CH routes. As a result, the higher available CH radical enhances NO formation through the established prompt mechanism, coherently with the experimental results.

Acryloylnitrenes: Spectroscopic Characterization, Spin Multiplicities, and Rearrangement to Vinyl Isocyanates.

The simplest acryloylnitrene, CH2═CHC(O)N (1b), and two halogenated derivatives, CH2═CFC(O)N (2b) and CH2═CBrC(O)N (3b), were generated through the 266 nm laser photolysis of the corresponding azide precursors in solid N2-matrices at 15 K. The IR spectroscopic characterization of these new acylnitrenes is supported by 15N-labeling and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level of theory. For the three nitrenes 1b, 2b, and 3b, two conformers exhibiting syn and anti configurations between the C═C and C═O bonds with respect to the C-C bonds have been identified. Consistent with the CBS-QB3 calculated singlet-triplet energy gaps (ΔEST < 0 kcal mol-1), the IR spectral analysis suggests that all these acryloylnitrenes adopt oxazirine-like structures with closed-shell singlet spin multiplicity. Upon subsequent green light (532 nm) irradiation, these acryloylnitrenes rearrange to form vinyl isocyanates CH2═CXNCO (X = H, F, Br), for which the IR spectra have also been obtained. According to the calculations on the α,β-fluorinated acryloylnitrenes at the CBS-QB3 level, their spin multiplicities can be switched from singlet CH2═CFC(O)N (ΔEST = -0.86 kcal mol-1) to triplet CF2═CFC(O)N (ΔEST = +0.61 kcal mol-1), whereas CFH═CFC(O)N is magnetically bistable due to a rather small ΔEST (+0.09 kcal mol-1).

A quantum chemical study on the formation of ethanimine (CH3CHNH) in the interstellar ice

Ethanimine (CH3CHNH) is an important prebiotic molecule since it is a precursor of amino acid $\alpha $ -alanine in Strecker synthesis. Two isomers (E and Z) of ethanimine were detected in the molecular cloud Sagittarius B2 north during GBT-PRIMOS survey. A possible radical-molecule reaction pathway has been proposed for the formation of ethanimine in the interstellar medium (ISM) from some previously detected interstellar molecules like methylene (both triplet CH2 (3B1) and singlet CH2 (1A1)) and methyenimine (CH2NH). The mechanism has been studied in the gas phase and in water ice with the help of density functional theory at B2PLYPD/6-311++G (2d, p) level of theory. It is observed that E-ethanimine forms efficiently in gas phase but ice reactions are favorable only in the hot core of molecular clouds. Same is true for the formation of Z-ethanimine which forms only at the surface of water cluster as the height of entrance barrier for formation of Z-ethanimine is similar to that of E-ethanimine. Isomerization from E to Z form is also studied and found to be forbidden due to large entrance barrier. Out of the two reaction system CH2 (3B1) + CH2NH and CH2 (1A1) + CH2NH, later is more favorable then the former one due to the small entrance barrier. Still, much of the detected abundance of ethanimine comes from the reaction of CH2 (3B1) with CH2NH as since CH2 (1A1) has very low abundance compared to the CH2 (3B1) in ISM. The proposed pathway seems to be a promising candidate for the ethanimine formation in ISM.

Convergence of coupled cluster perturbation theory.

The convergence of a recently proposed coupled cluster (CC) family of perturbation series [J. J. Eriksen et al., J. Chem. Phys. 140, 064108 (2014)], in which the energetic difference between two CC models-a low-level parent and a high-level target model-is expanded in orders of the Møller-Plesset (MP) fluctuation potential, is investigated for four prototypical closed-shell systems (Ne, singlet CH2, distorted HF, and F-) in standard and augmented basis sets. In these investigations, energy corrections of the various series have been calculated to high orders and their convergence radii have been determined by probing for possible front- and back-door intruder states, the existence of which would make the series divergent. In summary, we conclude how it is primarily the choice of the target state, and not the choice of the parent state, which ultimately governs the convergence behavior of a given series. For example, restricting the target state to, say, triple or quadruple excitations might remove intruders present in series which target the full configuration interaction limit, such as the standard MP series. Furthermore, we find that whereas a CC perturbation series might converge within standard correlation consistent basis sets, it may start to diverge whenever these become augmented by diffuse functions, similar to the MP case. However, unlike for the MP case, such potential divergences are not found to invalidate the practical use of the low-order corrections of the CC perturbation series.

Active Thermochemical Tables: dissociation energies of several homonuclear first-row diatomics and related thermochemical values

The current Active Thermochemical Tables (ATcT) results for the bond dissociation energies of the homonuclear diatomics H2, C2, N2, O2, and F2 are reported and discussed. The role and origin of the distributed provenance of ATcT values is analyzed. Ramifications in terms of the enthalpies of formation of H, C, N, O, and F atoms, which are fundamental thermochemical quantities, are presented. In addition, the current ATcT bond dissociation energies and enthalpies of formation of HF, CH, CO, CN, NO, OH, CO2, H2O, and triplet and singlet CH2 are also reported.

Accurate quantum mechanical study of the Renner-Teller effect in the singlet CH2

The Renner-Teller (RT) effect between the two low-lying electronic states of singlet CH(2), ã(1)A(1) and b̃(1)B(1), is studied using the multi-configuration time-dependent Hartree method with complete treatment of the RT terms. The RT terms, which are the matrix elements of the electronic orbital angular momentum operators, are calculated with ab initio methods and fitted to analytical functions. The ro-vibronic energy levels with complete treatment and constant approximation of the RT terms are calculated and compared. The influences of the geometry dependence of the RT terms on the ro-vibronic energy levels are discussed. The differences of the variation trends and influences of the RT terms between CH(2) and NH(2) are explored. In particular, as the molecule bends from linearity, the curve of the RT term (<linear span>ã(1)A(1)|L(z)(2)|ã(1)A(1)(<linear span>) first goes down to reach a minimum and then goes up leading to decreased zero point energy and bending energy levels for the lower state of CH(2) in contrast to the case of NH(2).

Development of approximately spin projected energy derivatives for biradical systems

AbstractIn this article, we summarize a recent progress in our spin‐projected energy derivative method for broken‐symmetry (BS) solutions of singlet biradical systems. Because, the spin projection procedure that is based on the approximate spin projection (AP) method does not involve a spin contamination error, it improves the accuracy of an optimized geometry and a calculated vibrational frequency for the singlet biradical molecule with the low cost of computation. To examine the spin contamination error in the BS energy derivatives and to demonstrate the efficiency of the AP method, we carry out the geometry optimization and the vibrational calculation of singlet CR2 (R = H, F, and CH3) molecules that are the typical singlet biradical systems. The results indicate that the error in the singlet CH2 and C(CH3)2 molecules are serious, however, the error in the singlet CF2 molecule, which has electron withdrawing fluorine atoms is negligible. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Interactions of Tetrahedrane and Tetrasilatetrahedrane with CH2and SiH2: A Computational Study

Ab initio exploration of the interactions of triplet and singlet methylene (CH2) and singlet silylene (SiH2) with both tetrahedrane and tetrasilatetrahedrane has yielded a number of stable optimized species. Structures 5−7 have been found on the triplet C4H4 plus CH2 potential energy surface (PES), structures 8−13 on the singlet C4H4 plus CH2 PES, structures 14−20 on the triplet Si4H4 plus CH2 PES, structures 21−30 on the singlet Si4H4 plus CH2 PES, structures 31−35 on the singlet C4H4 plus SiH2 PES, and structures 36−44 on the singlet Si4H4 plus SiH2 PES. These structures display a wide variety of geometrical features that in some cases are unusual. Characterization of the transition states connecting several of these species leads to a partial understanding of the various potential energy surfaces. The computational results suggest that some aspects of this chemistry might be profitably explored experimentally, particularly for substituted derivatives.

The role of methylene in prompt NO formation

We address the plausibility of singlet methylene (1CH2) in the prompt NO formation mechanism via examination of experimental species profiles and kinetic flame modeling of several low-pressure methane–oxygen–nitrogen flames. Existing kinetic models assuming CH as the only prompt NO precursor greatly underpredict NO formation under very fuel-lean conditions. We have constructed a kinetic pathway initiated by the recombination of singlet CH2 with molecular nitrogen to form diazomethane, CH2NN, early in the flame. Although the majority of the diazomethane is predicted to react with flame radicals to regenerate N2, a small percentage (approximately 10%) is predicted to react via cleavage of the NN bond leading to NO formation. This leads to accurate prediction of the experimental measurements of NO formation in lean, low-pressure flames. Assuming reasonable kinetic parameters for the reactions of CH2, the large underprediction of NO under lean conditions can be rectified by the inclusion of the 1CH2 prompt NO pathway in the kinetic mechanism.

State Mixing and Predissociation in the c̃ ← ã Band System of Singlet Methylene Studied by Optical−Optical Double Resonance

In an attempt to characterize the state interactions near the dissociation energy of singlet methylene, the near ultraviolet band system of singlet methylene has been studied using a laser optical-optical double resonance scheme. Spectra terminating in several, previously unobserved, higher bending levels of the c(1)A1 state have been detected. The highest energy band has simple rotational structure with lifetime broadened lines and is observed near 32300 cm(-1), which is 500 cm(-1) above the current best estimate for the singlet bond dissociation energy to CH((2)Pi) + H((2)S). Two lower energy bands exhibit a proliferation of rotationally-labeled double-resonance lines in the vicinity of the bright c(0,12,0) and c(0,13,0) bending levels, indicating that at least 7 and 9 strongly coupled vibronic states participate in each of these bands, respectively. The additional states may be associated with kinks in the adiabatic c state potential along the asymmetric stretching coordinate associated with interactions among c(1)A', a(1)A', and 3(1)A' states, as described by Ostojic (J. Mol. Spectrosc. 2002, 212, 130). There is no evidence for lifetime broadening below the singlet dissociation energy; hence we conclude that coupling of spectroscopically accessible singlet CH2 levels to the triplet manifold is very small.

Reactions of Thorium Atoms with Polyhalomethanes: Infrared Spectra of the CH2=ThX2, HC÷ThX3, and XC÷ThX3 Molecules

AbstractLaser‐ablated thorium atoms react with methylene fluoride to form singlet CH2=ThF2, with fluoroform to give triplet HC÷ThF3, and with CF4 to produce triplet FC÷ThF3 molecules as the major products trapped in solid argon. Infrared spectroscopy, isotopic substitution, and density functional theoretical calculations confirm the identity of these methylidene and methylidyne complexes. Parallels with the analogous chloromethane and Group 4 metal reaction products are discussed. Structure calculations show that the C=Th bond lengths decrease and the agostic distortion increases from CH2=ThF2 to CH2=ThFCl to CH2=ThCl2 for the methylidene complexes. The triplet‐state HC÷ThF3 and FC÷ThF3 electron‐deficient methylidyne complexes exhibit delocalized ‐bonding as evidenced by spin densities comparable to those calculated for the analogous zirconium complexes. Chlorine substitution for fluorine supports stronger C÷Th bonds. Thus, thorium appears to react as the early transition‐metal atoms with fluoro‐ and chloromethanes. However, there is a substantial contribution from Th 5f orbitals in addition to 6d in the SOMO forming the weak π‐bonds in these electron‐deficient methylidyne complexes.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Chemical Effects of a High CO2 Concentration in Oxy-Fuel Combustion of Methane

The oxidation of methane in an atmospheric-pressure flow reactor has been studied experimentally under highly diluted conditions in N2 and CO2, respectively. The stoichiometry was varied from fuel-lean to fuel-rich, and the temperatures covered the range 1200–1800 K. The results were interpreted in terms of a detailed chemical kinetic mechanism for hydrocarbon oxidation. On the basis of results of the present study, it can be expected that oxy-fuel combustion will lead to strongly increased CO concentrations in the near-burner region. The CO2 present will compete with O2 for atomic hydrogen and lead to formation of CO through the reaction CO2 + H ⇌ CO + OH. Reactions of CO2 with hydrocarbon radicals may also contribute to CO formation. The most important steps are those of singlet and triplet CH2 with CO2, while other radicals such as CH3 and CH are less important for consuming CO2. The high local CO levels may have implications for near-burner corrosion and slagging, but increased problems with CO emissi...

Correlated product distributions from ketene dissociation measured by dc sliced ion imaging

Speed distributions of spectroscopically selected CO photoproducts of 308 nm ketene photodissociation have been measured by dc sliced ion imaging. Structured speed distributions are observed that match the clumps and gaps in the singlet CH2 rotational density of states. The effects of finite time gates in sliced ion imaging are important for the accurate treatment of quasicontinuous velocity distributions extending into the thickly sliced and fully projected regime, and an inversion algorithm has been implemented for the special case of isotropic fragmentation. With accurate velocity calibration and careful treatment of the velocity resolution, the new method allows us to characterize the coincident rotational state distribution of CH2 states as a smoothly varying deviation from an unbiased phase space theory (PST) limit, similar to a linear-surprisal analysis. High-energy rotational states of CH2 are underrepresented compared to PST in coincidence with all detected CO rotational states. There is no evidence for suppression of the fastest channels, as had been reported in two previous studies of this system by other techniques. The relative contributions of ground and first vibrationally excited singlet CH2 states in coincidence with selected rotational states of CO (upsilon=0) are well resolved and in remarkably good agreement with PST, despite large deviations from the PST rotational distributions in the CH2 fragments. At 308 nm, the singlet CH2 (upsilon2=0) and (upsilon2=1) channels are 2350 and 1000 cm(-1) above their respective thresholds. The observed vibrational branching is consistent with saturation at increasing energies of the energy-dependent suppression of rates with respect to the PST limit, attributed to a tightening variational transition state.

Ketene photodissociation in the wavelength range 193–215 nm: The H atom production channel

The speed and angular distributions of H atom products arising in the photodissociation of jet-cooled ketene (CH2CO) molecules following excitation at 193.3, 203.3, 209, and 213.3 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. The observed product energy disposal is interpreted in terms of one photon absorption to the B11 electronically excited state, internal conversion to high lying vibrational levels of the ground state and subsequent unimolecular decay to yield the observed H (+HCCO) products. H atoms resulting from secondary photolysis of H containing primary products (most probably singlet CH2 radicals) are evident in the measured spectra, especially at high photolysis laser pulse energies. The kinetic energy distributions of the primary H+HCCO products span all energetically accessible product internal energies, peaking at ∼1170 cm−1 in the case of parent excitation at 213.3 nm, and rising to ∼1450 cm−1 (when exciting at 193.3 nm). These distributions are reproduced, qualitatively, by the statistical adiabatic product distribution (SAPD) method proposed recently by Cole and Balint-Kurti (J. Chem. Phys., preceding paper). This method is based on the use of a quantum mechanical, J conserving, Rice–Ramsperger–Kassel–Marcus (RRKM) treatment and provides a prediction of the product quantum state distributions and the total kinetic energy release spectra. Accurate, quadratic configuration interaction, intrinsic reaction coordinates have been computed for both the lowest singlet (S0) and triplet (T1) potential energy surfaces of CH2CO. Quantum mechanical SAPD calculations have been performed using both surfaces; the results favor the conclusion that the dissociation occurs on the S0 surface. This conclusion is further supported by comparison of the calculated and previously measured CO product vibrational quantum state distributions arising from photodissociation at 193.3 nm. The variational RRKM method has also been used to compute the branching ratios for forming H+HCCO and CH2+CO products on both the S0 and T1 surfaces. Different aspects of the SAPD model, such as the inclusion of quantum mechanical tunneling, the attractiveness of the long-range interfragment potential and the assumed adiabaticity of the fragmentation, have been varied in order to shed light on the nature of the dissociation process and the possible origins of the differences between the model calculations and the experimental results. It is found that the agreement between the quantum mechanical statistical model predictions and the experimentally observed total kinetic energy release spectra for the H atom dissociation channel can be greatly improved if the contribution of lower fragment relative orbital angular momenta is increased over that required by the use of a purely statistical model. This finding is equivalent to the conclusion that the dissociation is not entirely statistical, but that the dynamics of the break-up process plays some role. In particular the initial geometry of the parent molecule may restrict the body-fixed angles into which the final products can scatter and, through this, may restrict the relative orbital angular momenta to be on average smaller than that predicted by a purely statistical theory.

Theoretical Study on Mechanism of the 3CH2 + N2O Reaction

The complex triplet potential energy surface of the CH2N2O system, including 49 minimum isomers and 114 transition states, is investigated at the B3LYP and QCISD(T) (single-point) levels in order to explore the possible reaction mechanism of the 3CH2 radical with N2O. The most feasible pathway is the head-on attack of 3CH2 at the terminal N-atom of N2O to form cis-H2CNNO (a1) and trans-H2CNNO (a2). Both a1 and a2 can subsequently dissociate to give P1 (H2CN + NO) via the direct N−N bond rupture. Much less competitively, a1 can undergo a 1,4-H shift, leading to the chainlike isomer HCNNOH (k1), followed by the direct N−N bond cleavage to form product P2 (HCN + 3HON) or interconversion between the isomers k1−k8 and subsequent dissociation to P2. Furthermore, the products P1 (H2CN + NO) and P2 (HCN + 3HON) can undergo secondary dissociation to the same product P12 (HCN + NO + H). The formation of CO, however, seems impossible due to rather large barriers. Our results are in part contradictory with the recent time-resolved Fourier transform infrared spectroscopic study that nascent vibrationally excited products CO, NO, and HCN were observed. Since the initial N-attack step from R to a1 needs a considerable barrier of 14.8 kcal/mol, the title reaction may only be significant at high temperatures, as confirmed by the ab initio dynamic calculations on the rate constants. The reactivity discrepancies between the triplet and singlet CH2 with N2O are compared and discussed in terms of their potential energy surface features. Our calculations suggest that future experimental reinvestigations on the product distributions and rate constants of the title reaction at high temperatures are greatly desired.

The Near Ultraviolet Band System of Singlet Methylene

In a classic paper by G. Herzberg and J. W. C. Johns entitled “The Spectrum and Structure of Singlet CH2” (Proc. Roy. Soc. A295, 107–128 (1966)) the analysis of the b̃1B1←ã1A1 red absorption band system of CH2 is discussed in detail for the first time. In addition to that band system the observation of a fragment of a weak near ultraviolet absorption band system is reported. The three observed bands of the system could not be vibrationally assigned or rotationally analyzed but it was pointed out that they probably involve absorption into the second excited singlet state, c̃1A1. We show this supposition to be true here by simulation. In order to simulate the spectrum we have calculated ab initio the c̃–ã and c̃–b̃ transition moment surfaces and used the MORBID and RENNER program systems with previously determined potential energy surfaces for the ã, b̃, and c̃ states in a calculation of the energy levels and wavefunctions. We find that the three bands seen by Herzberg and Johns are part of the c̃←(ã/b̃) system but that all of the bands of the system above about 31 000 cm−1 are missing as a result of c̃ state predissociation. We vibrationally assign the bands but the weakness of the spectrum, and the presence of perturbations, make it impossible for us to analyze the rotational structure fully. Further experimental and theoretical studies are suggested.

State Correlations in the Unimolecular Dissociation of Ketene

Doppler-broadened absorption spectra of selected rotational levels of singlet CH2 fragments from the 308-nm photolysis of ketene have been observed by frequency-modulated transient absorption spectroscopy in a slit jet expansion. Nascent Doppler profiles of CH2 transitions have been analyzed to yield the internal energy distributions of coincident CO fragments at a total energy 2333 cm-1 above the singlet dissociation threshold. The vibrational branching ratio of CO (υ = 1):(υ = 0) is determined in coincidence with selected rotational levels of CH2. The coincident rotational distributions of CO (υ = 0) are also estimated and found to differ from simple statistical predictions, revealing a relative deficiency of those pairs in which both fragments have a low internal energy.

Absorption Spectroscopy of Singlet CH2 near 9500 cm-1

Absorption Spectroscopy of Singlet CH₂ near 9500 cm^-1