Cyanide Isomers Research Articles

Structure, dissociation pathways and thermochemistry of some nitrogen-containing astrophysical molecules: a theoretical study

The structure, relative energies, dissociation pathways and thermochemistry of acetonitrile (CH3CN or methyl cyanide), acrylonitrile (CH2CHCN or vinyl cyanide), propionitrile (CH3CH2CN or ethyl cyanide) and their stable isomers have been studied in detail using density functional B3LYP, B3PW91 and ab initio MP2, QCISD and CCSD(T) methods. Several space missions have already confirmed the abundance of these molecules in different interstellar mediums (ISMs). The existence of related radicals, atoms and molecules are also confirmed using various IR-spectrometric techniques and that may be evolved in solar irradiation in the cold dark core TMC-1 or massive star-forming regions such as Sagittarius B2 or Oion-KL. Possible ways of fragmentation of ground state CH3NC, CH2CHNC and CH3CH2NC through different dissociation channels and their link to other isomers have been considered to justify the presence of such observed radicals in the interstellar cloud. Cyanide(-C≡N), iso-cyanide (-N = C) and imine (-C = N-H) isomeric forms have been considered here and out of these, cyanide isomers are more stable species compared to its iso-nitrile and imine form. Favourable product molecules, such as CH2 radical, C2H2, C2H4 and HCN/HNC, are considered in the dissociation channels.

Understanding propyl cyanide and its isomers formation: ab initio study of the spectroscopy and reaction mechanisms.

Recent detection of propyl cyanide (C3H7CN) with both linear and branched structures has stimulated many experimental and theoretical studies. In this theoretical work, we present the spectroscopic properties of the far infrared spectra of these species and we investigate their different paths of formation in the gas phase. Our spectroscopic study concerns the far infrared spectra of iso, anti and gauche propyl cyanide isomers. The equilibrium structures and the potential energy surfaces are calculated using explicitly correlated cluster ab initio methods (CCSD(T)-F12) and a variational procedure designed for non-rigid species and large amplitude motions. Accurate rotational constants, centrifugal distortion constants, potential energy barriers and surfaces are provided. The rovibrational parameters in the ground vibrational states compare very well with experimental data. The low energy vibrational levels correspond to torsional modes. Far infrared energies are calculated up to 500 cm-1 using the variational approach and the vibrational second order theory (VPT2), and a good agreement with previous experimental values is found. We have also investigated the gas phase formation of the different C3H7CN isomers. After several trials of reacting gaseous species, we considered that a possible formation route of the C3H7CN isomers can be from the bimolecular reaction of HCN with propene. At the UMP2(full)/aug-cc-pVTZ level of theory, this reaction involves two steps for each isomer; the first one corresponds to the association of the two radicals while the second one corresponds to H transfer. From highly correlated ab initio calculations by means of CCSD(T)/aug-cc-pVTZ//UMP2(full)/aug-cc-pVTZ, the geometries, energetics and minimum energy paths of the reactions are obtained. Also, the first step's transition state disappears, as the diradical minimum is much less stabilized with UCCSD(T) than by UMP2(full). We employ the zero curvature tunneling and canonical variational (CVT/ZCT) semiclassical method to predict rate constants for propyl cyanide isomers formation in the gas phase. However, due to the presence of a significant barrier, this reaction leads to very small rate constants. Very recently, we probed reaction mechanisms involving radical addition and we found that these reactions are barrierless and highly exothermic leading to expected fast reactive processes for the formation of propyl cyanide products. The kinetics of such diradical reactions is under study.

Laser-Ablated U Atom Reactions with (CN)2 to Form UNC, U(NC)2, and U(NC)4: Matrix Infrared Spectra and Quantum Chemical Calculations.

Laser-ablated U atoms react with (CN)2 in excess argon and neon during codeposition at 4 K to form UNC, U(NC)2, and U(NC)4 as the major uranium-bearing products, which are identified from their matrix infrared spectra using cyanogen substituted with 13C and 15N and from quantum chemical calculations. The 12/13CN and C14/15N isotopic frequency ratios computed for the U(NC)1,2,4 molecules agree better with the observed values than those calculated for the U(CN)1,2,4 isomers. Multiplets using mixed isotopic cyanogens reveal the stoichiometries of these products, and the band positions and quantum chemical calculations confirm the isocyanide bonding arrangements, which are 14 and 51 kJ/mol more stable than the cyanide isomers for UNC and U(NC)2, respectively, and 62 kJ/mol for U(NC)4 in the isolated gas phase at the CCSD(T)/CBS level. The studies further demonstrate that the isocyano nitrogen is a better π donor, so it interacts with U(VI) better than carbon. Although the higher isocyanides are more stable than the corresponding cyanides, U(NC)5 and U(NC)6 were not observed here most likely because unfavorable or endothermic routes are required for their production from U(NC)4. The computed U-NC bond dissociation energies decrease from 581 kJ/mol for 4[UNC] to 168 kJ/mol for 1[U(NC)6 ]. The ionic nature of U(NC)n decreases as the number of isocyano groups increases.

Matrix Infrared Spectra of Manganese and Iron Isocyanide Complexes.

Mono and diisocyanide complexes of manganese and iron were prepared via the reactions of laser-ablated manganese and iron atoms with (CN)2 in an argon matrix. Product identifications were performed based on the characteristic infrared absorptions from isotopically labeled (CN)2 experiments as compared with computed values for both cyanides and isocyanides. Manganese atoms reacted with (CN)2 to produce Mn(NC)2 upon λ > 220 nm irradiation, during which MnNC was formed mainly as a result of the photoinduced decomposition of Mn(NC)2. Similar reaction products FeNC and Fe(NC)2 were formed during the reactions of Fe and (CN)2. All the product molecules together with the unobserved cyanide isomers were predicted to have linear geometries at the B3LYP level of theory. The cyanide complexes of manganese and iron were computed to be more stable than the isocyanide isomers with energy differences between 0.4 and 4 kcal/mol at the CCSD(T) level. Although manganese and iron cyanide molecules are slightly more stable according to the theory, no absorption can be assigned to these isomers in the region above the isocyanides possibly due to their low infrared intensities.

Formation and Characterization of Homoleptic Thorium Isocyanide Complexes.

Homoleptic thorium isocyanide complexes have been prepared via the reactions of laser-ablated thorium atoms and (CN)2 in a cryogenic matrix, and the structures of the products were characterized by infrared spectroscopy and theoretical calculations. Thorium atoms reacted with (CN)2 under UV irradiation to form the oxidative addition product Th(NC)2, which was calculated to have closed-shell singlet ground state with a bent geometry. Further reaction of Th(NC)2 and (CN)2 resulted in the formation of Th(NC)4, a molecule with a tetrahedral geometry. Minor products such as ThNC and Th(NC)3 were produced upon association reactions of CN with Th and Th(NC)2. Homoleptic thorium cyanide isomers Th(CN)x (x = 1-4) are predicted to be less stable than the corresponding isocyanides. The C-N stretches of thorium cyanides were calculated to be between 2170 and 2230 cm-1 at the B3LYP level, more than 120 cm-1 higher than the N-C stretches of isocyanides and with much weaker intensities. No experimental absorptions appeared where Th(CN)x should be observed.

Major Differences Between Mononuclear and Binuclear Manganese Carbonyl Cyanides and Isoelectronic Binary Chromium Carbonyls Arising from Basicity of the Cyanide Nitrogen Atom

The manganese carbonyl cyanides Mn(CO)n(CN) and Mn2(CO)n(CN)2 have been investigated by density functional theory. The lowest energy structure for Mn(CO)5(CN) is found to be the experimentally known C-bonded cyanide. The experimentally unknown N-bonded Mn(CO)5(NC) lies ~60 kJ mol–1 above its cyanide isomer. The Mn(CO)4(CN) isomers are obtained by removal of a CO group in various ways from Mn(CO)5(CN) or Mn(CO)5(NC). Three structures, cyanide Mn(CO)3(CN), isocyanide Mn(CO)3(NC), and Mn(CO)3(η2-CN), are found for the tricarbonyl. All low-energy binuclear Mn2(CO)n(CN)2 structures have two end-to-end bridging CN groups. These two η2-CN bridges can be oriented in the same or opposite directions. The Mn2(CO)7(CN)2 structures of this type can be derived from these Mn2(CO)8(CN)2 structures by removal of a CO group with relatively little change in the remainder of the structure. These low-energy Mn2(CO)n(CN)2 structures (n = 8, 7) are very different from the previously studied isoelectronic Cr2(CO)n+2 structures in which low-energy end-to-end CO bridged structures are not found.

Neutral cyanide complexes of iron: Structure and stability

Neutral iron-cyanide complexes have been studied through computational techniques. Fe[CN] n complexes are shown to prefer the cyanide arrangement. Only in the case of Fe[CN] the isocyanide isomer is found to lie slightly below the cyanide isomer. According to the complexation energies, Fe(CN) 2 seems to be the main target for possible experimental observation in the gas phase. Mixed complexes with cyanide and cyanogen ligands, Fe(CN) x (C 2N 2) y , have also been studied. The balance between the stronger ligand character of CN and the preference for low coordination numbers, makes Fe(CN) 2, Fe(CN)(C 2N 2), Fe(CN) 2(C 2N 2), Fe(CN)(C 2N 2) 2, and Fe(CN) 2(C 2N 2) 2 the most stable complexes for each stoichiometry.

Cyanide complexes of Ti(IV): A computational study

Density functional theory (B3LYP) and coupled-cluster techniques [CCSD(T)] including solvent effects have been used to study the homoleptic and mixed cyanide/isocyanide complexes of Ti(IV), [Ti(CN)(n)](4-n) (n=1-6). The most stable isomer is found to be the isocyanide form except for n=6 where the cyanide isomer is preferred. Calculations accounting for solvent effects show that, irrespective of the solvent employed, the hexacyanocomplex should be formed. We have additionally analyzed the bonding situation in these complexes in order to shed some light on the reasons for the predicted cyano-/isocyano preference. We have found that the more advantageous sigma-bonding capabilities of the cyanide form become increasingly important for larger n eventually favoring the cyanoisomer for n=6. We finally compare the bonding situation in hexacyanotitanate(IV) with that of hexacyanoferrate(II).

A theoretical study of the [FeCN] + system: Cyanide–isocyanide competition and isomerization barrier

A theoretical study of the [FeCN] + system has been carried out. Density functional methods, as well as single- and multi-reference techniques, were employed. Both cyanide and isocyanide isomers are very close in energy, with FeCN + lying about 0.9 kcal mol −1 below FeNC +. A non-linear minimum, with iron formally bonded just to the nitrogen atom according to a topological analysis of the charge density, is also found just 1.3 kcal mol −1 above the cyanide isomer. The estimated energy barrier for the isomerization process is just 5 kcal mol −1, suggesting that rearrangement of [FeCN] + should take place easily.

The low-lying electronic states of nickel cyanide and isocyanide: A theoretical investigation

At different levels of coupled cluster theory optimum structures, energetics, and harmonic vibrational frequencies for several low-lying doublet and quartet electronic states of linear NiCN and NiNC were studied using four contracted Gaussian basis sets, ranging from Ni[6s5p4d2f], CN[4s3p2d] to Ni[8s7p5d3f2g1h], CN[5s4p3d2f1g]. The most reliable predictions were obtained with a relativistic Douglas-Kroll restricted open-shell-based coupled cluster method including singles, doubles, and perturbative triple excitations [DK-R/UCCSD(T)]. This level of theory was used in conjunction with correlation-consistent polarized valence Douglas-Kroll recontracted quadruple-zeta basis sets (cc-pVQZDK). The energetic ordering of the electronic states of NiCN is predicted to be 2delta < 2sigma+ < 2pi < 4delta < 4pi and that of NiNC is 2delta approximately 2sigma+ < 2pi < 4delta < 4pi < 4sigma-. Our theoretical investigation supports the assignment of the ground-state term symbol, the Ni-C stretching frequency, and the bending frequency for the ground electronic state of NiCN by Kingston et al. [J. Mol. Spectrosc. 215, 106 (2002)] and by Sheridan and Ziurys [J. Chem. Phys. 118, 6370 (2003)]. The predicted structure of the 2delta ground state of NiCN, r(e)(Ni-C) = 1.822 angstroms and r(e)(C-N) = 1.167 angstroms, at DK-R/UCCSD(T)/cc-pVQZDK shows excellent agreement with the experimentally determined Ni-C bond length of 1.826 A and less satisfactory agreement for the C-N bond length of 1.153 angstroms [J. Chem. Phys. 118, 6370 (2003)]. It is also concluded that the metal-to-ligand pi back donation is weak or negligible. Additionally, we found that on the 2delta surface the linear cyanide isomer lies lower in energy than the linear isocyanide isomer by 12.2 kcal mol(-1).

A Stable Structurally Characterized Phosphorus‐Bound Isocyanide and Its Thermal and Catalyzed Isomerization to the Corresponding Cyanide

A PNC linkage has been realized by the formation of a stable isocyano azaphosphatranium ion with a facile synthesis. Subsequent thermal or Lewis acid catalyzed isomerization of the isocyanide derivative to its cyanide isomer (see scheme) was carried out and followed by 31P NMR spectroscopy. Both compounds were characterized by X-ray crystallography.

Reactions of Laser-Ablated Mg, Ca, Sr, and Ba Atoms with Hydrogen Cyanide in Excess Argon. Matrix Infrared Spectra and Density Functional Calculations on Novel Isocyanide Products

Laser-ablated alkaline earth metal atoms reacting with hydrogen cyanide in the presence of excess argon produce matrix-isolated linear metal isocyanides, MNC, but no noticeable amount of the corresponding cyanide isomer, MCN. Supplemental experiments with DCN show that, unlike the corresponding beryllium reactions, no products containing hydrogen are formed. Experiments with isotopically enriched H13CN yield MN13C shifted 40 cm-1 from MN12C in the C⋮N stretching frequency, with the isotopic shift virtually independent of the alkaline earth metal. As the mass of the group 2 metal increases, the C⋮N frequency decreases and the absorption becomes broader, the latter feature indicating a decreasingly rigid linear geometry. The lack of MCN indicates that the products form via attack of the energetic metal on the nitrogen atom, instead of through insertion into the C−H bond.

Correlation effects in the isomeric cyanides: HNC↔HCN, LiNC↔gLiCN, and BNC↔gBCN

Correlated values of the isomerization energy and barrier for the HNC→HCN reaction are obtained from many-body perturbation theory, including the effects of quadruple excitations. Extended basis sets of better-than-triple-zeta plus double-polarization quality are used, as well as basis sets including counterpoise and bond-centered orbitals. The best of these basis sets is sufficient to account for 84% of the valence correlation energy of HCN. These studies predict an isomerization energy for the HNC→HCN rearrangement to be −15±2 kcal/mole, in disagreement with a recent experimental value of −10.3±1. kcal/mole. Correlated isomerization energies of LiCN→LiNC and BCN→BNC are obtained in bases of double-zeta plus polarization quality. In all cases, correlation stabilizes the cyanide isomer more than the isocyanide. Trends in the series R–NC⇄RCN for R=H, Li, and B are discussed.