Photoelectron spectroscopy of HCCN− and HCNC− reveals the quasilinear triplet carbenes, HCCN and HCNC

Bimolecular reactions of CH2CN2+ with Ar, N2 and CO: reactivity and dynamics

The reactivity, energetics and dynamics of bimolecular reactions between CH₂CN²⁺ and three neutral species (Ar, N₂ and CO) have been studied using a position sensitive coincidence methodology at centre-of-mass collision energies of 4.3-5.0 eV. This is the first study of bimolecular reactions involving CH₂CN²⁺, a species relevant to the ionospheres of planets and satellites, including Titan. All of the collision systems investigated display two collision-induced dissociation (CID) channels, resulting in the formation of C⁺ + CH₂N⁺ and H⁺ + HC₂N⁺. Evidence for channels involving further dissociation of the CID product HC₂N⁺, forming H + CCN⁺, were detected in the N₂ and CO systems. Proton-transfer from the dication to the neutral species occurs in all three of the systems via a direct mechanism. Additionally, there are product channels resulting from single electron transfer following collisions of CH₂CN²⁺ with both N₂ and CO, but interestingly no electron transfer following collisions with Ar. Electronic structure calculations of the lowest energy electronic states of CH₂CN²⁺ reveal six local geometric minima: both doublet and quartet spin states for cyclic, linear (CH₂CN), and linear isocyanide (CH₂NC) molecular geometries. The lowest energy electronic state was determined to be the doublet state of the cyclic dication. The ready generation of C⁺ ions by collision-induced dissociation suggests that the cyclic or linear isocyanide dication geometries are present in the [CH₂CN]²⁺ beam.

Weak covalent interactions and anionic charge-sharing polymerisation in cluster environments

We discuss the formation of weak covalent bonds leading to anionic charge-sharing dimerisation or polymerisation in microscopic cluster environments. The covalent bonding between cluster building blocks is described in terms of coherent charge sharing, conceptualised using a coupled-monomers molecular-orbital model. The model assumes first-order separability of the inter- and intra-monomer bonding structures. Combined with a Hückel-style formalism adapted to weak covalent and solvation interactions, it offers insight into the competition between the two types of forces and illuminates the properties of the inter-monomer orbitals responsible for charge-sharing dimerisation and polymerisation. Under typical conditions, the cumulative effect of solvation obstructs the polymerisation, limiting the size of covalently bound core anions.

Relatives of cyanomethylene: replacement of the divalent carbon by B-, N+, Al-, Si, P+, Ga-, Ge, and As. Phys Chem Chem Phys 2019;21:26438-26452. [PMID: 31774089 DOI: 10.1039/c9cp05777c]

The lowest lying singlet and triplet states of HBCN-, HCCN, HNCN+, HAlCN-, HSiCN, HPCN+, HGaCN-, HGeCN, and HAsCN+ were studied using the CCSDT(Q)/CBS//CCSD(T)/aug-cc-pVQZ level of theory. Periodic trends in geometries, singlet-triplet gaps, and barriers to linearity were established and analyzed. The first row increasingly favors the triplet state, with a singlet-triplet gap (ΔEST = Esinglet - Etriplet) of 3.5 kcal mol-1, 11.9 kcal mol-1, and 22.6 kcal mol-1, respectively, for HBCN-, HCCN, and HNCN+. The second row increasing favors the singlet state, with singlet-triplet gaps of -20.4 kcal mol-1 (HAlCN-), -26.6 kcal mol-1 (HSiCN), and -26.8 kcal mol-1 (HPCN+). The third row also favors the singlet state, with singlet-triplet gaps of -26.8 kcal mol-1 (HGaCN-), -33.5 kcal mol-1 (HGeCN), and -33.1 kcal mol-1 (HAsCN+). The HXCN species have larger absolute singlet-triplet energy gaps compared to their parent species XH2 except for the case of X = N+. The effect of the substitution of hydrogen with a cyano group was analyzed with isodesmic bond separation analysis and NBO.

Infrared Spectra of CH3CN→M, M-η2-(NC)-CH3, CH3-MNC Prepared by Reactions of Laser-Ablated Fe, Ru, and Pt Atoms with Acetonitrile in Excess Argon

Reactions of laser-ablated Fe, Ru, and Pt atoms with acetonitrile have been carried out in excess argon, and the products identified in the matrix spectra. CH₃CN→Fe and Fe-η²-(NC)-CH₃ observed in the original deposition spectra converted to CH₃-FeNC on uv irradiation. CH₃CN→Ru, the only product detected in the Ru system, dissociated on uv irradiation, but was partly reproduced on subsequent visible irradiation and annealing. Similar behavior was found for CH₃CN→Pt. The major products (CH₃-FeNC, CH₃CN→Ru, and CH₃CN→Pt) are the most stable constituents in the previously proposed reaction path for Group 4, 5, 6, and 7 metal atoms and acetonitrile, parallel with the previous results. The Group 8 metal π-coordination products are weakly bound complexes due to limited back-donation to the π*-orbitals of CH₃CN. Calculations show that the Fe insertion product has a much less bent structure than the Ru analogue, in line with its higher s-character from the first row transition-metal to the C-Fe bond, and the group 8 metal methylidenes are not agostically distorted. The Pt to N bond in CH₃CN→Pt is the strongest of all the metals we have investigated owing in large part to its higher electron affinity, which prevents nitrogen lone pair density from entering the pi* orbitals of the C-N group.

Stripping off hydrogens in imidazole triggered by the attachment of a single electron

Imidazole [C₃H₄N₂] is ubiquitous in nature as an important biological building block of amino acids, purine nucleobases or antibiotics. In the present study, dissociative electron attachment to imidazole shows low energy shape resonances at 1.52 and 2.29 eV leading to the most abundant dehydrogenated anion [imidazole - H]^- through dehydrogenation at the N1 position. All the other anions formed exhibit core excited resonances observed dominantly at similar electron energies of ∼7 and 11 eV, suggesting an initial formation through two temporary negative ion states. Among these anions, multiple dehydrogenation reactions are observed resulting in the loss of 2 up to 4 hydrogens, thus, leading to a complete dehydrogenation of the imidazole molecule, an interesting prototype of complex unimolecular decay induced by the attachment of a single electron. Additionally, the quantum chemical calculations reveal that these multiple dehydrogenation reactions are responsible for the remarkable one electron-induced gas-phase chemistry leading to the opening of the ring. The formation of the observed anions is likely driven by the high positive electron affinity of cyanocarbon molecules supported by quantum chemical calculations. The formation of H^- showed additional resonance at about 5 eV and dipolar dissociation above ∼14 eV.

Spectroscopy and Photochemistry of Triplet 1,3-Dimethylpropynylidene (MeC3Me)

Photolysis (λ > 472 nm) of 2-diazo-3-pentyne (11) affords triplet 1,3-dimethylpropynylidene (MeC3Me, (3)3), which was characterized spectroscopically in cryogenic matrices. The infrared, electronic absorption, and electron paramagnetic resonance spectra of MeC3Me ((3)3) are compared with those of the parent system (HC3H) to ascertain the effect of alkyl substituents on delocalized carbon chains of this type. Quantum chemical calculations (CCSD(T)/ANO1) predict an unsymmetrical equilibrium structure for triplet MeC3Me ((3)3), but they also reveal a very shallow potential energy surface. The experimental IR spectrum of triplet MeC3Me ((3)3) is best interpreted in terms of a quasilinear, axially symmetric structure. EPR spectra yield zero-field splitting parameters that are typical for triplet carbenes with axial symmetry (|D/hc| = 0.63 cm(-1), |E/hc| = ∼ 0 cm(-1)), while theoretical analysis suggests that the methyl substituents confer significant spin polarization to the carbon chain. Upon irradiation into the near-UV electronic absorption (λmax 350 nm), MeC3Me ((3)3) undergoes 1,2-hydrogen migration to yield pent-1-en-3-yne (4), a photochemical reaction that is typical of carbenes bearing a methyl substituent. This facile process apparently precludes photoisomerization to other interesting C5H6 isomers, in contrast to the rich photochemistry of the parent C3H2 system.

Reaction mechanisms and kinetics of the iminovinylidene radical with NO: ab initio study

The nitric oxide (NO) is a notorious compound for polluting environment. Recent year, removing nitric oxide from the atmosphere becomes a focus of the investigation. In our work, we study the iminovinylidene (HNCC) radical reacted with NO molecule. The mechanism and kinetic for reaction of the HNCC radical with the NO molecule is investigated via considering the possible channels of the N and O atoms of NO attacking the N and C atoms of the HNCC based on the high level ab initio molecular orbital calculations in conjunction with variational TST and RRKM calculations. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level and their single-point energies are refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) method. The calculated potential energy surfaces indicated that energetically the most favorable channel for the HNCC + NO reaction was predicted to be the formation of HNC+CNO (P8) product via the addition reaction of the C atom of HNCC radical and the N atom of NO with the head to head orientation. To rationalize the scenario of the calculated results, we also employ the Fukui functions and HSAB theory to seek for a possible explanation. In addition, the reaction rate constants were calculated using VariFlex code, and the results show that the total rate coefficient, ktotal, at Ar pressure 760 Torr can be represented with an equation: ktotal = 6.433 × 10(-11) T (0.100) exp(0.275 kcal mol(-1)/RT) at T = 298-3000 K, in units of cm(3) molecule(-1) s(-1).

Electronic spectroscopy of the HCCN radical

The Ã(3)A"-X̃(3)Σ(-) electronic transition of the HCCN∕DCCN radical was observed by laser-induced fluorescence spectroscopy. Rotationally resolved excitation spectra were observed for HCCN and DCCN, and effective molecular constants of the upper state were determined. The observed intensities of the rotational lines were anomalous, probably due to a level-dependent non-radiative decay process in the excited state. Fluorescence depletion spectroscopy was applied in order to observe non-fluorescent vibronic levels. A dispersed fluorescence spectrum was also observed to determine the vibrational level structure in the ground electronic state. The observed vibrational structures in the fluorescence depletion and dispersed fluorescence spectra were tentatively assigned based on the results of ab initio calculations.

Dicyanogermylenes: a tale of isomers and interconversions

A systematic investigation is carried out using the B3LYP, BLYP, and BHLYP functionals and MP2 level of theory to characterize the low-lying electronic singlet and triplet GeC2N2 isomers. The basis sets used are of double-ζ plus polarization quality with additional s- and p-type diffuse functions, DZP++. Three bent isomers Ge(CN)2, CNGeCN, and Ge(NC)2 are located on the singlet and triplet potential energy surfaces. In visualizing the reaction pathways for the singlet isomerization of the bent isomers, two three-membered [Ge, C, N] cyclic systems, with exocyclic -C-C≡N and -C-N≡C bonding, appear on the energy surface. Four types of electron affinities reported are: the adiabatic electron affinity, the zero-point vibrationally corrected electron affinity, the vertical electron affinity, and the vertical detachment energy of the anion. The ionization energies and singlet-triplet gaps for all isomers are also reported. The energetic ordering (kcal mol(-1)) (B3LYP) with zero-point vibrational energy corrections for the singlet ground state isomers follows: Ge(CN)2 (global minimum) < CNGeCN (2.3) < Ge(NC)2 (3.3) < Cyc_exo_CCN (15.3) < Cyc_exo_CNC (30.6). All the bent and cyclic isomers are found to be below the dissociation limit to Ge ((3)P) + C2N2 ((1)Σg). The rate constants for all interconversions are evaluated using transition state theory.

Electron transfer-induced fragmentation of thymine and uracil in atom-molecule collisions

Ion-pair formation has been studied in hyperthermal (30-100 eV) neutral potassium collisions with gas phase thymine (C(5)H(6)N(2)O(2)) and uracil (C(4)H(4)N(2)O(2)). Negative ions formed by electron transfer from the alkali atom to the target molecule were analysed by time-of-flight (TOF) mass spectrometry. The most abundant product anions are assigned to CNO(-) and (U-H)(-)/(T-H)(-) and the associated electron transfer mechanisms are discussed. Special emphasis is given to the enhancement of ring breaking pathways in the present experiments, notably CNO(-) formation, compared with free electron attachment measurements.

Photoelectron imaging of NCCCN(-): The triplet ground state and the singlet-triplet splitting of dicyanocarbene

The photoelectron spectra of NCCCN(-) have been measured at 355 and 266 nm by means of photoelectron imaging. The spectra show two distinct features, corresponding to the ground and first excited states of dycianocarbene. With support from theoretical calculations using the spin-flip coupled-cluster methods, the ground electronic state of HCCCN is assigned as a triplet state, while the first excited state is a closed-shell singlet. The photoelectron band corresponding to the triplet is broad and congested, indicating a large geometry change between the anion and neutral. A single sharp feature of the singlet band suggests that the geometry of the excited neutral is similar to that of the anion. In agreement with these observations, theoretical calculations show that the neutral triplet state is either linear or quasilinear (X (3)B(1) or (3)Sigma(g) (-)), while the closed-shell singlet (a (1)A(1)) geometry is strongly bent, similar to the anion structure. The adiabatic electron binding energy of the closed-shell singlet is measured to be 3.72+/-0.02 eV. The best estimate of the origin of the triplet band gives an experimental upper bound of the adiabatic electron affinity of NCCCN, EA</=3.25+/-0.05 eV, while the Franck-Condon modeling yields an estimate of EA(NCCCN)=3.20+/-0.05 eV. From these results, the singlet-triplet splitting is estimated to be DeltaE(ST)(X (3)B(1)/(3)Sigma(g) (-)-a (1)A(1))=0.52+/-0.05 eV (12.0+/-1.2 kcal/mol).

Fourier transform microwave spectroscopy of the isocyanomethyl radical, CH(2)NC

The pure rotational spectrum of the isocyanomethyl radical, CH(2)NC, was measured for the first time by using a Fourier transform microwave spectrometer. The molecule was produced by a discharge of isocyanomethane, CH(3)NC, diluted in Ar or Ne. The spectral lines due to the N=1-0 and 2-1 transitions were recorded near 22 and 44 GHz, respectively. The observed spectrum showed a complicated fine and hyperfine structure because of the same order of interaction energies. Among the 39 spectral lines detected and assigned, the transitions with K(a)=1 show no hyperfine splitting due to the hydrogen nuclei, suggesting planarity for the molecule. Molecular constants such as rotational and spin-rotational parameters including centrifugal effects and hyperfine coupling constants due to both the nitrogen and the hydrogen nuclei were accurately determined. The structure and the astronomical implications of the molecule are discussed.

A restricted-open-shell complete-basis-set model chemistry

A restricted-open-shell model chemistry based on the complete basis set-quadratic Becke3 (CBS-QB3) model is formulated and denoted ROCBS-QB3. As the name implies, this method uses spin-restricted wave functions, both for the direct calculations of the various components of the electronic energy and for extrapolating the correlation energy to the complete-basis-set limit. These modifications eliminate the need for empirical corrections that are incorporated in standard CBS-QB3 to compensate for spin contamination when spin-unrestricted wave functions are used. We employ an initial test set of 19 severely spin-contaminated species including doublet radicals and both singlet and triplet biradicals. The mean absolute deviation (MAD) from experiment for the new ROCBS-QB3 model (3.6+/-1.5 kJ mol(-1)) is slightly smaller than that of the standard unrestricted CBS-QB3 version (4.8+/-1.5 kJ mol(-1)) and substantially smaller than the MAD for the unrestricted CBS-QB3 before inclusion of the spin correction (16.1+/-1.5 kJ mol(-1)). However, when applied to calculate the heats of formation at 298 K for the moderately spin-contaminated radicals in the G2/97 test set, ROCBS-QB3 does not perform quite as well as the standard unrestricted CBS-QB3, with a MAD from experiment of 3.8+/-1.6 kJ mol(-1) (compared with 2.9+/-1.6 kJ mol(-1) for standard CBS-QB3). ROCBS-QB3 performs marginally better than standard CBS-QB3 for the G2/97 set of ionization energies with a MAD of 4.1+/-0.1 kJ mol(-1) (compared with 4.4+/-0.1 kJ mol(-1)) and electron affinities with a MAD of 3.9+/-0.2 kJ mol(-1) (compared with 4.3+/-0.2 kJ mol(-1)), but the differences in MAD values are comparable to the experimental uncertainties. Our overall conclusion is that ROCBS-QB3 eliminates the spin correction in standard CBS-QB3 with no loss in accuracy.

Six-Dimensional Potential Energy Surface and Rovibrational Energies of the HCCN Radical in the Ground Electronic State

We report large-scale quantum mechanical calculations for the HCCN radical in its ground electronic state. A six-dimensional potential energy surface based on MR-ACPF/cc-pVQZ ab initio energy points is developed and adjusted to reproduce experimental findings for and nu1 of HCCN. Rovibrational energy levels of HCCN and DCCN are computed for total rotational angular momentum J = 0-4 by making use of combined (functional + point wise) coordinate representations together with contraction schemes resulting from several diagonalization/truncation steps. The classical barrier to linearity is determined to be 287 cm(-1). Spectroscopic parameters are calculated for low lying states and compared with available experimental data. Energy patterns attributed to the nu4 bending mode and to the quasilinear nu5 bending mode are identified. It has been also found that nu2 and nu3 + (nu4(1),nu5(1))(0,0) are coupled in HCCN, while the mixing between nu3 and (2nu4(0), 2nu5(0))(0,0) is seen in DCCN.