Blausäure-N-methylid: Darstellung, spektroskopische Eigenschaften und seine Beziehung zu anderen C2H3N-Isomeren

Synthesis and reactivity of N-heterocyclic carbene (NHC)-supported heavier nitrile ylides

The synthesis and isolation of stable heavier analogues of nitrile ylide as N-heterocyclic carbene (NHC) adducts of phosphasilenyl-tetrylene [(NHC)(TerAr)Si(H)PE14(TerAr)] (E14 = Ge 1, Sn 2; TerAr = 2,6-Mes2C6H3, NHC = IMe4) are reported. The delocalized Si-P-E14 π-conjugation was examined experimentally and computationally. Interestingly, the germanium derivative 1 exhibits a 1,3-dipolar nature, leading to an unprecedented [3 + 2] cycloaddition with benzaldehyde, resulting in unique heterocycles containing four heteroatoms from group 14, 15, and 16. Further exploiting the nucleophilicity of germanium, activation of the P-P bond of P4 was achieved, leading to a [(NHC)(phosphasilenyl germapolyphide)] complex. Moreover, the [3 + 2] cycloaddition and the σ-bond activation by 1 resemble the characteristics of the classic nitrile ylide.

Energetic processing of thioacetamide in cryogenic matrices

There is an ongoing debate on the apparent depletion of sulfur in the interstellar medium (ISM) compared to its universal abundance; therefore, the investigation of sulfurous compounds at low temperatures is of utmost importance. This work aims to study thioacetamide, H3C-C(=S)-NH2, in low-temperature inert Ar and para-H2 matrices by IR spectroscopy. The samples have been exposed to various sources of irradiation, such as Lyman-α or laser UV photons as well as energetic electrons. Using different host materials enabled assessing the matrix's impact on precursor decomposition. The response of the molecule to different types of irradiation has also been evaluated. The existence of three main decomposition channels were deduced: formation of (i) CH3, CH4, and HNCS; (ii) H2S and H2C=C=NH; and (iii) NH3 and H2C=C=S. The H3C-CN and H3C-NC isomers of H2C=C=NH could also be identified. Secondary products such as HNC and HCN were also detected in the quantum solid para-H2 in contrast to the more rigid Ar matrix. The listed decomposition products have been observed in the ISM, with the exception of H2C=C=NH and H3C-NC. The results point to the potential sensitivity of the precursor molecule to energetic radiation in space environments. Finally, the findings of this work will serve as a foundation for future irradiation experiments using the astrochemically more relevant pure thioacetamide ice.

Preparation and Photochemistry of Parent Triplet Vinylarsinidene

Vinyl pnictinidenes are an elusive family of molecules that have been suggested as key intermediates in multiple chemical reactions and commonly display a predisposition toward open-shell electronic ground states (as is evident from quantum chemical computations). However, owing to their expected extremely high reactivity, no vinyl pnictinidene has ever been isolated and characterized spectroscopically. Here, we report the synthesis and spectroscopic characterization of vinylarsinidene, a higher congener of vinylnitrene. As we demonstrate, triplet vinylarsinidene can be prepared through the low-temperature photolysis of diazidovinylarsine at 10 K in an argon matrix. The title compound can also be generated through high-vacuum flash pyrolysis of the diazide at 700 °C and trapped analogously. Triplet vinylarsinidene was characterized by IR and UV/vis spectroscopy and displayed remarkably rich unimolecular photochemistry. Upon selective photoirradiation, it rearranges to vinylidenearsine, 2H-arsirene, triplet ethynylarsinidene or an arsinidene (H-As) acetylene complex. The formation mechanisms of these products were rationalized with DFT and CASPT2 computations.

3-Fluoro-2H-azirine: Generation, Characterization, and Photochemistry

The elusive 3-fluoro-2H-azirine, cyclic NCH2CF, has been generated through the stepwise decomposition of the acryloyl azide CH2CFC(O)N3 in an N2-matrix at 10 K. The characterization of cyclic NCH2CF with matrix-isolation IR spectroscopy is supported by 15N isotope labeling and the calculations with density functional theory (DFT) at the B3LYP/6-311++G(3df,3pd) level of theory. Upon irradiation at 193 nm, cyclic NCH2CF undergoes ring opening by forming the more stable nitrile isomer CH2FCN. In contrast to the photodecomposition reactions, the high-vacuum flash pyrolysis of CH2CFC(O)N3 in the gas phase at 500 °C yields the Curtius rearrangement product CH2CFNCO along with secondary fragmentation to the atmospherically relevant fluorocarbonyl radical (FCO) and cyanomethyl radical (CH2CN). Calculations on the potential energy profile for the decomposition reactions of CH2CFC(O)N3 demonstrate that the excessive energy, arising from the highly exothermic Curtius rearrangement of the azide, plays a key role in driving further dissociation reactions of CH2CFNCO by overcoming the formidable barriers (>50 kcal mol-1) under the pyrolysis conditions.

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.

Formation and interconversion of CCN and CNC radicals resulting from the radiation-induced decomposition of acetonitrile in solid noble gas matrices

Acetonitrile and the species resulting from its dehydrogenation play an important role in the radiation-induced evolution of organic matter in the space environment. In this work, we report on FTIR spectroscopic studies of the degradation of isolated CH3CN and CD3CN molecules induced by prolonged X-ray irradiation in solid noble gas matrices at 5 K. The principal products observed at high conversion degree of the parent acetonitrile molecules (70-90%) are CCN and CNC radicals, which result from prompt or two-step dehydrogenation of the corresponding precursors, H2CN and CH2NC radicals, respectively. CHCN and CHNC were also found as products of dehydrogenation at high absorbed doses, whereas the fragmentation products (CH3, CN, HCN, and HNC) were detected only in minor amounts over the whole dose range studied. CCN and CNC are produced in nearly equal amounts at high absorbed doses. Selective isomerization of CCN to CNC was observed under the illumination with visible light (460-470 nm), while subsequent action of the UV light (254 nm) induced reverse transformation leading to a photostationary state with the relative population of CNC/CCN being ca. 0.7. The astrochemical implications of the obtained results are discussed in connection with the recent discovery of CCN in extraterrestrial objects.

Flash Vacuum Pyrolysis: Techniques and Reactions

Flash vacuum pyrolysis (FVP) had its beginnings in the 1940s and 1950s, mainly through mass spectrometric detection of pyrolytically formed free radicals. In the 1960s many organic chemists started performing FVP experiments with the purpose of isolating new and interesting compounds and understanding pyrolysis processes. Meanwhile, many different types of apparatus and techniques have been developed, and it is the purpose of this review to present the most important methods as well as a survey of typical reactions and observations that can be achieved with the various techniques. This includes preparative FVP, chemical trapping reactions, matrix isolation, and low temperature spectroscopy of reactive intermediates and unstable molecules, the use of online mass, photoelectron, microwave, and millimeterwave spectroscopies, gas-phase laser pyrolysis, pulsed pyrolysis with supersonic jet expansion, very low pressure pyrolysis for kinetic investigations, solution-spray and falling-solid FVP for involatile compounds, and pyrolysis over solid supports and reagents. Moreover, the combination of FVP with matrix isolation and photochemistry is a powerful tool for investigations of reaction mechanism.

Infrared spectra of CH2=M(H)NC, CH3-MNC, and eta2-M(NC)-CH3 produced by reactions of laser-ablated group 5 metal atoms with acetonitrile

Methylidene isocyanides, methyl isocyanides, and eta(2)-nitrile-pi-complexes are observed in the matrix IR spectra from reactions of Group 5 metals with acetonitrile isotopomers. The primary isocyanide products with no trace of cyanide complexes are consistent with the reaction path proposed in the analogous Zr study. The major products (CH(2)=Ta(H)NC, CH(3)-NbNC, eta(2)-Nb(NC)-CH(3), and eta(2)-V(NC)-CH(3)) after codeposition and reaction of metal with CH(3)CN clearly show the increasing preference for the higher oxidation-state complex on going down the group column, and the subsequent photochemistry provides further information for molecular rearrangements. The Group 5 metal methylidene isocyanides exhibit more agostic distortion than the Zr counterparts and are comparable to the previously studied Group 5 metal methylidene hydrides and halides. The computed structures and observed frequencies indicate that the effects of metal conjugation (C=Ta-N=C:) are minor.

Reaction Path of UV Photolysis of Matrix Isolated Acetyl Cyanide: Formation and Identification of Ketenes, Zwitterion, and Keteneimine Intermediates

Irradiations at lambda > 180 nm and lambda > 230 nm of CH3COCN (CD3COCN) trapped in an argon matrix at 10 K have been performed and monitored by Fourier transform infrared spectroscopy. For both wavelengths, the first photoreaction product is acetyl isocyanide. At lambda > 180 nm acetonitrile and methyl isocyanide are obtained as final products. They are formed by photolysis of acetyl isocyanide and acetyl cyanide, respectively. Unstable intermediates, such as ketene:HCN and ketene:HNC complexes, H2CNCH zwitterion, and H2C2NH keteneimine, not observed in the gas phase, are trapped and identified at our experimental temperature. The complexes have an L-shaped structure with a hydrogen bond between the oxygen atom of ketene and the hydrogen atom of HCN or HNC. A pathway process is proposed and compared with the ones determined in the ground state by ab initio calculations.

2-Quinolyl- and 1-isoquinolylnitrenes: ring expansion and ring opening in heteroarylnitrenes

Argon matrix photolysis of tetrazolo[1,5-a]quinoline 8 and tetrazolo[5,1-a]isoquinoline 7 causes nitrogen elimination and ring expansion to 1,3-diazabenzo[d]cyclohepta-1,2,4,6-tetraene 13. The photolysis of tetrazolo[5,1-a]isoquinoline 7 also causes ring opening to o-cyanophenylketenimine 22. Mechanisms of ring opening of heteroarylnitrenes are discussed.

Mass spectrometric studies of elusive molecules that contain an N(+)-X- bond

This review will be concerned with the gas phase chemistry of 1,2- and 1,3-dipolar systems that contain a carbon-nitrogen bond. Although most of these compounds are stable molecules under normal conditions, certain congeners are reactive species that cannot be prepared using conventional procedures. The isolation and observation of these elusive compounds therefore require appropriate experimental conditions such as those provided by the gas phase of a mass spectrometer. In these experiments, the radical cations, corresponding to the molecule under study, must be prepared via indirect procedures, including dissociative electron ionization, on-line flash-vacuum pyrolysis-mass spectrometry, or ion-molecule reactions. Their characterization is mainly based on collisional activation and ion-molecule reactions. The formation of the corresponding highly reactive neutrals is attempted by neutralization-reionization mass spectrometry. This review presents more than one hundred different molecules together with their methods of preparation and the experiment used to identify them.

Photochemistry of Fluorinated Phenyl Nitrenes: Matrix Isolation of Fluorinated Azirines

2,6-Difluorophenylnitrene and pentafluorophenylnitrene were generated in solid argon at 10 K by irradiation of the corresponding phenyl azides and characterized by IR and UV spectroscopy. Selective irradiation with lambda = 444 nm results in the formation of the corresponding azirines, while ketene imines are not produced. On lambda = 366 nm irradiation the azirines rearrange back to the nitrenes. The assignment of the azirines is confirmed by ab-initio calculations.