Isocyanide Isomers Research Articles

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.

Importance of the Hydrogen Isocyanide Isomer in Modeling Hydrogen Cyanide Oxidation in Combustion

Hydrogen isocyanide (HNC) has been proposed as an important intermediate in oxidation of hydrogen cyanide (HCN) in combustion, but details of its chemistry are still in discussion. At higher temperatures, HCN and HNC equilibrate rapidly, and being more reactive than HCN, HNC offers a fast alternative route of oxidation for cyanides. However, in previous modeling, it has been required to omit the HNC subset partly or fully in the reaction mechanisms to obtain satisfactory predictions. In the present work, we re-examine the chemistry of HNC and its role in combustion nitrogen chemistry. The HNC + O2 reaction is studied by ab initio methods and is shown to have a high barrier. Consequently, the omission of this reaction in recent modeling studies is justified. With the present knowledge of the HNC chemistry, including an accurate value of the heat of formation for HNC and improved rate constants for HNC + O2 and HNC + OH, it is possible to reconcile the modeling issues and provide a satisfactory prediction o...

Laboratory characterization and astrophysical detection of vibrationally excited states of vinyl cyanide in Orion-KL

New laboratory data of CH$_2$CHCN (vinyl cyanide) in its ground and vibrationally excited states at the microwave to THz domain allow searching for these excited state transitions in the Orion-KL line survey. Frequency-modulated spectrometers combined into a single broadband 50-1900 GHz spectrum provided measurements of CH$_2$CHCN covering a spectral range of 18-1893 GHz, whose assignments was confirmed by Stark modulation spectra in the 18-40 GHz region and by ab-initio anharmonic force field calculations. For analyzing the emission lines of CH$_2$CHCN species detected in Orion-KL we used the excitation and radiative transfer code (MADEX) at LTE conditions. The rotational transitions of the ground state of this molecule emerge from four cloud components of hot core nature which trace the physical and chemical conditions of high mass star forming regions in the Orion-KL Nebula. The total column density of CH$_2$CHCN in the ground state is (3.0$\pm$0.9)x10$^{15}$ cm$^{-2}$. We report on the first interstellar detection of transitions in the v10=1/(v11=1,v15=1) dyad in space, and in the v11=2 and v11=3 states in Orion-KL. The lowest energy vibrationally excited states of vinyl cyanide such as v11=1 (at 328.5 K), v15=1 (at 478.6 K), v11=2 (at 657.8 K), the v10=1/(v11=1,v15=1) dyad (at 806.4/809.9 K), and v11=3 (at 987.9 K) are populated under warm and dense conditions, so they probe the hottest parts of the Orion-KL source. Column density and rotational and vibrational temperatures for CH$_2$CHCN in their ground and excited states, as well as for the isotopologues, have been constrained by means of a sample of more than 1000 lines in this survey. Moreover, we present the detection of methyl isocyanide (CH$_3$NC) for the first time in Orion-KL and a tentative detection of vinyl isocyanide (CH$_2$CHNC) and give column density ratios between the cyanide and isocyanide isomers.

Conformational preferences of RCH2CH2CN (R = CH3, F, Cl) cyanides and their corresponding isocyanides

The structure and relative stability of the different conformers of RCH2CH2CN (R = CH3, F, Cl) cyanides and their corresponding isocyanides have been investigated through the use of high-level ab initio G4 theory as well as B3LYP/aug-cc-pVQZ and M06/aug-cc-pVQZ density functional theory calculations. This theoretical survey ratifies that the gauche conformer of butyronitrile is slightly more stable than the anti one, so that in the gas phase and at room temperature this compound should exist as a mixture of 57 % of the former and 43 % of the latter. Similar stability trends are predicted for the corresponding isocyanide isomer. Conversely, when the terminal methyl group of butyronitrile (or its isocyanide isomer) is replaced by F or Cl, the stability trends are reversed and the anti conformer becomes slightly more stable than the gauche one. These changes in relative stabilities could be traced through an analysis of the reduced density gradient which shows the existence of a stabilizing interaction between the terminal methyl group and the cyano (or isocyano) group in butyronitrile (or its isocyanide isomer), which becomes repulsive when this methyl group is replaced by F or Cl.

Can isocyanogen azide exist?

Ever since the first attempt at its synthesis in 1912, cyanogen azide (NCN3) has continued to attract the attention of chemists in diverse fields such as photochemistry, organic chemistry, and energetic materials. However, to our great surprise, its isocyanide isomer, CNN3, has not been studied experimentally or theoretically. In this paper, for the first time, we report a theoretical spectroscopic and stability study of CNN3 employing the CCSD and CCSD(T) methods with the aug-cc-pVTZ basis set. In light of its potential multi-reference character, the isomers and transition state structures on the reaction pathway were re-optimized using the multi-configurational CASSCF method in conjunction with the cc-pVTZ basis set. It is shown that CNN3 should be intrinsically stable both with respect to isomerization and fragmentation. The structural and spectroscopic properties obtained in this study should provide useful information for the future laboratory identification of CNN3. For comparison, we also calculated the aug-cc-pVTZ-CCSD(T) and cc-pVTZ-CASSCF properties of NCN3.

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.

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.

Cyanides and Isocyanides of First-Row Transition Metals: Molecular Structure, Bonding, and Isomerization Barriers

Cyanides and isocyanides of first-row transition metal M(CN) (M=Sc-Zn) are investigated with quantum chemistry techniques, providing predictions for their molecular properties. A careful analysis of the competition between cyanide and isocyanide isomers along the transition series has been carried out. In agreement with the experimental observations, late transition metals (Co-Zn) clearly prefer a cyanide arrangement. On the other hand, early transition metals (Sc-Fe), with the only exception of the Cr(CN) system, favor the isocyanide isomer. The theoretical calculations predict the following unknown isocyanides, ScNC(3Delta), TiNC(4Phi), VNC(5Delta), and MnNC(7Sigma+), and agree with the experimental observation of FeNC(6Delta) and the CrCN(6Sigma+) cyanide. First-row transition metal cyanides and isocyanides are predicted to have relatively large dissociation energies with values within the range 80-101 kcal mol(-1), except Zn(CN), which has a dissociation energy around 50-55 kcal mol(-1), and low isomerization barriers. A detailed analysis of the bonding has been carried out employing the topological analysis of the charge density and an energy decomposition analysis. The role of the covalent and electrostatic contributions to the metal-ligand bonding, as well as the importance of pi bonding, are discussed.

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).

Interstellar Isomers: The Importance of Bonding Energy Differences

We present strong detections of methyl cyanide (CH3CN), vinyl cyanide (CH2CHCN), ethyl cyanide (CH3CH2CN) and cyanodiacetylene (HC4CN) molecules with the Green Bank Telescope (GBT) toward the Sgr B2(N) molecular cloud. Attempts to detect the corresponding isocyanide isomers were only successful in the case of methyl isocyanide (CH3NC) for its J(sub K) = 1(sub 0) - 0(sub 0) transition, which is the first interstellar report of this line. To determine the spatial distribution of CH3NC, we used archival Berkeley-Illinois-Maryland Association (BIMA) array data for the J(sub K) = 1(sub 0) - 0(sub 0) transitions but no emission was detected. From ab initio calculations, the bonding energy difference between the cyanide and isocyanide molecules is greater than 8500 per centimeter (greater than 12,000 K). Thus, cyanides are the more stable isomers and would likely be formed more preferentially over their isocyanide counterparts. That we detect CH3NC emission with a single antenna (Gaussian beamsize(omega(sub B))=1723 arcsec(sup 2)) but not with an interferometer (omega(sub b)=192 arcsec(sup 2)), strongly suggests that CH3NC has a widespread spatial distribution toward the Sgr B2(N) region. Other investigators have shown that CH3CN is present both in the LMH hot core of Sgr B2(N) and in the surrounding medium, while we have shown that CH3NC appears to be deficient in the LMH hot core. Thus, largescale, non-thermal processes in the surrounding medium may account for the conversion of CH3CN to CH3NC while the LMH hot core, which is dominated by thermal processes, does not produce a significant amount of CH3NC. Ice analog experiments by other investigators have shown that radiation bombardment of CH3CN can produce CH3NC, thus supporting our observations. We conclude that isomers separated by such large bonding energy differences are distributed in different interstellar environments, making the evaluation of column density ratios between such isomers irrelevant unless it can be independently shown that these species are co-spatial.

Silylcyanides and silylisocyanides: a comparative theoretical study

Structural geometrical parameters, rotational constants, dipole moments and vibrational frequencies of the series of silylcyanides SiR 3CN (R=F, H, CH 3) and their isocyanide isomers have been calculated at different levels of approximation using DFT methods with SVWN, Xα, BP86, B3LYP, B3PW91 functionals and 6-311G** basis set and compared with experimental data. The kinetic behavior of the cyano–isocyano isomerization and the nature of the transition states have also been investigated. A comparison with the analogous carbon compounds is also reported.

Do branched structures exist for cyanide-containing magnesium compounds? Computational studies on a range of mixed-ligand compounds XMg(CN) (X = F, Cl, OH, SH, NH2, CH3, CN)

Cyanide compounds of the alkali metals and alkaline earths are commonly found to possess “branched” or π-complex structures in which the metal atom is almost equidistant from both atoms of the CN moiety. Here we present an investigation of the potential energy surfaces for various compounds of the form XMg(CN), using the Gaussian-2 (G2) procedure. Our results suggest that magnesium, at least, is not so prone to π-complex formation with the cyanide ligand as has previously been implied, since the presence of the π complex upon the potential energy surface is strongly dependent upon the level of theory employed in geometry optimizations. We find also that, according to G2 theory, the preference of magnesium for isocyanide (rather than cyanide) formation is small but consistent, with XMgNC isomers having calculated heats of formation between 2 and 5 kJ mol−1 below their XMgCN counterparts. The barriers to interconversion of cyanide and isocyanide isomers are also calculated to be comparatively small, typically ∼25 kJ mol−1. In contrast, calculations for protonated species FMg(CN)H+ and Mg(CN)2H+ have determined that the π complexes in these species are indeed stable against CN-ligand reorientation. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 76: 626–642, 2000

Structural Trends in the Monocyanides of the Second-Row Metal Ions Na+, Mgm+ (m = 1, 2), and Aln+ (n = 1−3)

The G2 (Gaussian-2) theoretical procedure has been used to determine equilibrium geometries, M−(CN) and Mj+−(CN-) bond energies, total energies, and isomerization pathways of the cyanide and isocyanide isomers of the species Na+(CN-), Mgm+(CN-) [m = 1, 2], and Aln+(CN-) [n = 1−3]. At the MP2(full)/6-31G* level, a “T-shaped” isomer is also found for each of the 24-electron species Na(CN), Mg(CN)+, and Al(CN)2+: however, attempted optimization at the QCISD(full)/6-311G** level suggests that T-Al(CN)2+ is not a true minimum. At the G2 level, the linear MCN and MNC geometries are found to be local minima for all species except NaNC, which constitutes a local maximum for rotation of Na+ about the CN- moiety. A very large discrepancy of 162 kJ mol-1 is noted between the G2 and G2Q bond dissociation energies for Al−NC2+, with the G2Q value appearing the more reliable. The calculations at the G2Q level suggest that AlNC2+ is a thermodynamically stable dication, with substantial covalent character in the Al−N bon...

Half-sandwich aminocarbyne complexes of chromium

Efficient methods are described for the synthesis of half-sandwich chromium aminocarbyne complexes starting from trans-X(CO) 4CrCN iPr 2 ( 1a: X  Cl; 1b: X  Br). Complexes 1a and 1b are obtained from Cr(CO) 6 in two steps. The first step involves a nucleophilic addition of LiN iPr 2 to Cr(CO) 6 to give the carboxamido complex Li[(CO) 5Cr{C(O)N iPr 2}], followed by reaction with the Lewis acids ClC(O)C(O)Cl and BrC(O)C(O)Br, respectively. Thermal decarbonylation of 1a or 1b with γ-picoline (4-methylpyridine) results in the quantitative formation of X(CO) 2(pic) 2CrCN iPr 2 ( 2a: X  Cl; 2b: X  Br). Complex 2b reacts with NaCp and KTp′ (Tp′= hydridotris(3,5-dimethylpyrazol-1-yl)borate) to afford Cp(CO) 2CrCN iPr 2 ( 3) and Tp′(CO) 2CrCN iPr 2 ( 4), respectively. Similarly, when 2b is treated with KCp★ (Cp★  C 5 Me 5), the half-sandwich aminocarbyne complex Cp★(CO) 2CrCN iPr 2 ( 5) is obtained. Thermal decarbonylation of 1b with tBuNC in refluxing CH 2Cl 2 leads exclusively to the cationic aminocarbyne complex [( tBuNC) 4(CO)CrCN iPr 2] Br ( 6). Treatment of 6 with NaCp and KTp′ results in the formation of Cp(CO)( tBuNC)CrCN iPr 2 ( 7) and Tp′(CO) ( tBuNC)CrCN iPr 2 ( 8), respectively. In both reactions a minor product, [( tBuNC) 5CrCN iPr 2]Br, is formed by a carbonyl substitution reaction of 6 with the released tBuNC. Complex 6 reacts with KCp★ to give a mixture of the half-sandwich aminocarbyne complex Cp★(CO)( tBuNC)CrCN iPr 2 ( 9) and the Cr 0 isocyanide isomers cis-and trans- Cr(CO)(CN iPr)(CN tBu) 4 ( 10a,b), the latter probably originating from an electron-transfer or a proton-abstraction reaction of 6 with KCp★. Formation of 10a/b is avoided when a more electron-rich (less acidic) aminocarbyne complex, such as Br(CO)( tBuNC) 3Cr CN iPr 2 ( 11), is treated with KCp★.

Alkylation of tricarbonyliron complexes by trimethylsilyl cyanide: synthetic and kinetic studies

Trimethylsilyl cyanide has been shown to be a synthetically useful reagent for the alkylation of tricarbonyl(η5-cyclohexadienyl)iron(1+) salts, particularly in cases where the reactive terminus of the dienyl system bears an alkyl substituent. The reaction proceeds via the isocyanide isomer, with a rate-limiting pre-equilibration of the reagent. In cases where reactive metal complexes were used, the rate of reaction was proportional to the concentration of Me3SiCN, and was independent of both the nature and concentration of the dienyl salt. In the case of the deactivated 2-methoxy substituted dienyl salt, non-linear rate plots indicated a change in the rate-limiting step.

Energy-conformation studies of HCN, HNC and CN?: A comparison of results from EH-SCC and SCF molecular orbital calculations

We have made an Extended Huckel Self Consistent Charge (EH-SCC) molecular orbital calculation for hydrogen cyanide, hydrogen isocyanide and cyanide ion. The main purpose of this calculation was to compare the EH-SCC and the more accurate SCF MO calculations for HCN in order to evaluate the method we used here for future use. Specifically, we have calculated and compared the following properties of HCN: total energy, binding energy, variation of ground state energy with geometric conformation, ionization potential and dipole moment. In addition, we have extended previous calculations of HCN by also considering its energy variation with bond angle for two excited state configurations and deducing some of the characteristics of its electronic spectra. Finally we have also made an MO calculation of the isocyanide isomer HNC and CN− ion to compare with and add to the known characterization of the H, C, N, system.