Isomerization Of Methyl Isocyanide Research Articles

Unimolecular Reaction of Methyl Isocyanide to Acetonitrile: A High-Level Theoretical Study.

A combination of high-level coupled-cluster calculations and two-dimensional master equation approaches based on semiclassical transition state theory is used to reinvestigate the classic prototype unimolecular isomerization of methyl isocyanide (CH3NC) to acetonitrile (CH3CN). The activation energy, reaction enthalpy, and fundamental vibrational frequencies calculated from first-principles agree well with experimental results. In addition, the calculated thermal rate constants adequately reproduce those of experiment over a large range of temperature and pressure in the falloff region, where experimental results are available, and are generally consistent with statistical chemical kinetics theory (such as Rice-Ramsperger-Kassel-Marcus (RRKM) and transition state theory (TST)).

Adsorption, decomposition and isomerization of methyl isocyanide and acetonitrile on Pd(111)

The adsorption of methyl isocyanide (CH 3NC) and acetonitrile (CH 3CN) has been studied on Pd(111). Methyl isocyanide adsorbs on Pd(111) at 80 K at low coverages with the isocyanide functionality oriented close to parallel to the surface; the surface-bound species reorients to a geometry in which the isocyanide group is perpendicular to the surface as the coverage increases. Desorbing the excess isocyanide recovers the parallel geometry. Heating to above ∼300 K forms a third, thermally stable, rehybridized species with its CN axis close to perpendicular to the surface. This thermally decomposes to desorb HCN and hydrogen at above 450 K. Acetonitrile is much less reactive and adsorbs in an η 2(C,N) configuration where the adsorbate appears to maintain a reasonably linear CCN geometry on the surface and which desorbs with no thermal decomposition at 225 K.

Vibrational overtone activation of the isomerization of methyl isocyanide

The photoisomerization of methyl isocyanide to form acetonitrile induced by excitation into the fourth (∼1 kcal/mol above the activation barrier) and fifth (∼8 kcal/mol above the barrier) C-H stretch vibrational overtones is reported. The ratio of the collisional deactivation rate constant to the unimolecular rate coefficient, k(∈), was determined by a Stern-Volmer analysis plotting the inverse apparent rate constant against the total pressure. The unimolecular rate coefficients increase monotonically with increasing excitation energies across the rotational band contours

Semiempirical SCF‐MO calculations of kinetic isotope effects

AbstractCarbon and deuterium kinetic isotope effects (KIEs) on the isomerization of methyl isocyanide were calculated by using the geometries and force fields obtained with MNDO, AM1 and PM3 Hamiltonians. These theoretical KIE values were compared with the corresponding experimental values. None of the hamiltonians used proved to be superior to any of the others. The best agreement for the carbon isotope effect was obtained by using the PM3 method; however, the deuterium isotope effect obtained by this method gave the poorest fit to the experimental data. The opposite is true when the MNDO Hamiltonian was used.

ChemInform Abstract: Intramolecular Dynamics of the Overtone‐Induced Isomerization of Methyl Isocyanide

Overtone‐induced isomerization of methyl isocyanide to methyl cyanide is studied by using classical trajectories on several potential‐energy surfaces. The several potential‐energy surfaces are variations of a potential which we have developed based on available experimental and ab initio results. The trajectory results for simple potentials which neglect stretch–bend interactions show that overtone excitation of a CH stretch to the v=6 level at total energies (including overtone excitation energy) of 75, 125, and 150 kcal/mol does not enhance the rate of isomerization. However, at an initial total energy of 200 kcal/mol, the isomerization rate is enhanced by as much as a factor of 3 by selective excitation of a CH stretching overtone. However, the mode specificity is sensitive to the potential‐energy surface. When a more realistic potential is used in which the bending force constants are attenuated as a function of the bond lengths or in which nondiagonal quadratic coupling terms are included, the domina...

Intramolecular dynamics of the overtone-induced isomerization of methyl isocyanide

Overtone-induced isomerization of methyl isocyanide to methyl cyanide is studied by using classical trajectories on several potential-energy surfaces. The several potential-energy surfaces are variations of a potential which we have developed based on available experimental and ab initio results. The trajectory results for simple potentials which neglect stretch–bend interactions show that overtone excitation of a CH stretch to the v=6 level at total energies (including overtone excitation energy) of 75, 125, and 150 kcal/mol does not enhance the rate of isomerization. However, at an initial total energy of 200 kcal/mol, the isomerization rate is enhanced by as much as a factor of 3 by selective excitation of a CH stretching overtone. However, the mode specificity is sensitive to the potential-energy surface. When a more realistic potential is used in which the bending force constants are attenuated as a function of the bond lengths or in which nondiagonal quadratic coupling terms are included, the dominant reaction is CH bond dissociation. The rate coefficient for the CH bond dissociation is an order of magnitude greater than the rate of isomerization at 200 kcal/mol. The initial energy flow out of an excited CH stretch is rapid (occurring on a times scale of less than 0.5 ps) and is primarily into the methyl bending modes. The energy that flows into the bending modes does not transfer out over the time period of 5.4 ps that the trajectories were followed. The methyl bending modes act as an energy ‘‘sink’’.

A theoretical analysis of the overtone-induced isomerization of methyl isocyanide

A master-equation formalism is applied to the problem of overtone-induced isomerization of CH3NC to CH3CN. The results are compared to the experiments of Reddy and Berry, who measured the yield of isomerization as a function of pressure after excitation to the fourth and fifth overtones of the CH stretching mode. The master-equation model predicts the yield and the curvature in the yield−1 vs pressure plots observed in the experiments. For the lower overtone (5←0) the results are consistent with a simple strong-collider model. However, even under strong-collider conditions the yield is very sensitive to the parameters in the master equation. For the upper overtone (6←0) the data do not fit a strong collider model and multistep deactivation dominates. We are able to determine from the data the average energy transferred in a collision by assuming a particular form for the energy-transfer function. In addition, the effect of changing the shape of the energy-transfer function is investigated.

Methyl isocyanaide isomerization. Determination of collisional deactivation parameters following carbon-hydrogen overtone excitation

The isomerization of methyl isocyanide (CH/sub 3/NC) to acetonitrile (CH/sub 3/CN) was studied by excitation of the 5nu/sub C-H/ (726.6 nm) and 6nu/sub C-H/ (621.4 nm) overtone states, which lie about 1 and 8 kcal/mol, respectively, above the isomerization barrier. Products yields were measured as a function of pressure and collision partner. A Stern-Volmer plot (yield/sup -1/ vs. pressure) shows that (1) deactivation by collision with pure CH/sub 3/NC is more rapid than with C/sub 3/H/sub 6/, SF/sub 6/, or Ar, (2) the collisional deactivation efficiencies decrease in going from C/sub 3/H/sub 6/ to SF/sub 6/ to Ar, and (3) the single-collision deactivation approximation (strong collider approximation) fails for both the 6nu/sub C-H/ and 5nu/sub C-H/ data. With the use of a master equation solution, assuming an exponential down energy-transfer function, the average energy transferred in a deactivating collision, - /sub down/, is extracted from each data set, as well as the average energy transferred fer collision, - . It is concluded that the isomerization yield depends markedly on the collision partner and on the average energy transferred per collision, - , even though the single-collision deactivation approximation might have been expected to have its greatest validity in this energy regime.

Interdependence of fluence and pressure in the laser-induced isomerization of methyl isocyanide

The infrared laser-induced isomerization of methyl isocyanide has previously been shown to exhibit a marked pressure dependence; large-scale isomerization occurs when threshold conditions have been exceeded. In this work the wavelength and fluence dependence of the pressure threshold have been investigated. It has been determined that threshold conditions reflect a balance between the average level of excitation and the sample pressure. The multiphoton spectrum and the threshold as a function of wavelength have been determined and both reflect the structure of the linear absorption spectrum, indicating that the effective anharmonicity is small. In addition, the earlier conclusion that the threshold is not due to a laser-triggered, thermal explosion is corroborated. 24 references, 6 figures, 2 tables.

Pressure dependence of the laser-initiated isomerization of methyl isocyanide

Determination de la pression au-dessus de laquelle l'isomerisation est preponderante. En dessous de cette valeur le rendement est une fonction exponentielle de la pression. Comparaison des resultats experimentaux et de ceux de la simulation numerique du systeme considere comme une explosion thermique

Fall-off behavior of the separable exponential Kassel model for thermal unimolecular reactions

For thermal unimolecular reactions a fairly realistic model can be constructed by combining the separable exponential model for the collisional activation and deactivation steps with the Kassel model for the reactive steps. The resulting model admits of interesting results which can be expressed analytically. It is argued that the model has a well defined strong collision limit, and an equation for the efficiency factor β̄ for weak colliders is then derived. This factor is a function of the ratio of the rate coefficient κ to its high pressure limiting value κ∞. The model qualitatively, albeit not quantitatively, reproduces the experimental behavior of β̄(κ/κ∞) for methyl isocyanide isomerization in helium at 518 K. Experiments and previous computations both show β̄(κ/κ∞) to decrease at a decreasing rate. The separable exponential-Kassel model gives β̄’s which either increase or decrease with κ/κ∞, but increasing β̄’s occur only for physically unrealistic values of one of the model’s parameters. The model shows that the decreasing rate of decrease of β̄ is related to the small value of the Kassel parameter s for methyl isocyanide isomerization. It predicts that β̄ decreases at an increasing rate for reactions with large s.

Coordination chemistry of metal surfaces and metal complexes

The coordination chemistry of two isomeric molecules, acetonitrile and methyl isocyanide, has been characterized for (A) single crystal nickel surfaces as a function of crystallography and surface composition, (B) nickel catalyst surfaces and (C) molecular mononuclear and polynuclear metal complexes. Conditions have been established for the catalytic isomerization of methyl isocyanide to acetonitrile within these nickel surface and molecular complex regimes. A comparison is made of the acetonitrile and methyl isocyanide chemistry between the surface and molecular complex areas.

Reaction dynamics of state-selected unimolecular reactants. Energy dependence of the rate coefficient for methyl isocyanide isomerization

Intracavity c.w. dye laser techniques have been used to measure visible absorption spectra of gas-phase CH3NC and CH3CN and to photoactivate CH3NC to reactive states with high selectivity. Photoisomerization kinetics of state-selected CH3NC are reported and are used to determine optical absorption cross-sections and the energy dependence of the inherent unimolecular reaction rate coefficient k(Iµ), over an extended energy range(〈Iµ〉= 38–48 kcal mol–1). The preparation, propagation and reaction of highly vibrationally excited CH3NC are considered on the basis of all available spectroscopic and photochemical data. On the whole, there is only marginal evidence for nonstatistical behaviour in the state-selected CH3NC reaction system.

The unimolecular isomerization of methyl isocyanide to methyl cyanide (acetonitrile)

The unimolecular isomerization of CH3NC→[CH3 NC] →CH3CN is studied by many-body (diagrammatic) perturbation theory. Using double-zeta plus polarization (DZP) contracted Gaussian basis sets (63 CGTO’s), correlated calculations are made by evaluating all second-, third-, and fourth-order diagrams that occur due to single, double, and quadruple excitations [SDQ–MBPT(4)]. These diagrams involve ∼107 CI type configurations. The SDQ–MBPT(4) model predicts the enthalpy of isomerization to be −22.7 kcal/mole [exp(−23.7 ±0.14)]. The activation barrier is found to be ∼41 kcal/mole (exp 38.4). The inclusion of quadrupole excitation diagrams has little effect on the enthalpy of isomerization but increases the activation barrier by 1 kcal/mole.

On the unimolecular isomerization of methyl isocyanide after hot tritium substitution

The Bunker formulation of RRKM theory is used to investigate the yields of CH2TCN and CH2TNC after the substitution of tritium for hydrogen on methyl isocyanide. To rationalize experimental data, large amounts of rotational energy are required at the last collision where either the rate coefficients for the forward and reverse reactions are comparable to the mean free time. This would imply either that the substitution reaction produces even larger amounts of rotational excitation than was previously expected (by typically occurring at larger impact parameters) or that rotational energy is not transferred to collision partners as effectively as in collisions near thermal energy.

Methyl Isocyanide as a Key Molecule in Thermal Explosion Theory

Thermal explosion limits have been measured in the temperature range 285–351 °C for methyl isocyanide and at 351 °C for mixtures of methyl isocyanide and methyl cyanide. We suggest that the methyl isocyanide isomerization is potentially the best reaction system against which to test thermal explosion theories, but at the present time certain kinetic, thermochemical, and thermodynamic data are not available with sufficient accuracy.

The methyl isocyanide isomerization: A CNDO/2 study with partitioning of energy

CNDO/2 calculations together with a partitioning of the energy into various one- and two-centre components have been performed on CH3CN, CH3NC, and a large number of nuclear configurations formed by placing the methyl group at various positions with respect to the fixed CN group. A complete potential energy surface was obtained showing that a minimum in energy occurred for a pyramidal CH3 group placed at approximately 90° to the CN axis. The barrier to the reaction CH3NC→CH3CN is calculated as 32.9 kcal, in reasonable agreement with 38.4 kcal, the experimental value of Rabinovitch. A charge separation equivalent to [CH 3 +0.22 ][CN−0.22] is found for the intermediate compared to [CH 3 +0.08 ][CN−0.08] and [CH 3 +0.12 ][NC−0.22] for the cyanide and isocyanide, respectively.

Assessment of Free Radical Catalysis in Thermal Methyl Isocyanide Isomerization. Variation of Inert Gas Efficiency with k/k∞

Experiments have been made to assess the importance of radical chain effects in thermal isomerization of methyl isocyanide. Three bath gases, propylene, propane, and hexafluoroethane were studied at 230 °C over a small region of fall-off. The relative dilution with propane was varied widely. A negative search was made for possible secondary radical products. The values of K∞ accord with earlier results in the pure system. It is believed that radical catalysis is minor. The observed collisional efficiency of the inert gases [Formula: see text] decreases with increase of k/k∞, in accord with previous theoretical predictions.

Energy transfer in thermal methyl isocyanide isomerization. Relative efficiency of mercury atoms

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEnergy transfer in thermal methyl isocyanide isomerization. Relative efficiency of mercury atomsFa-Mei Wang, T. Fujimoto, and B. S. RabinovitchCite this: J. Phys. Chem. 1972, 76, 14, 1935–1937Publication Date (Print):July 1, 1972Publication History Published online1 May 2002Published inissue 1 July 1972https://doi.org/10.1021/j100658a004RIGHTS & PERMISSIONSArticle Views24Altmetric-Citations6LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (349 KB) Get e-Alerts

Theoretical reaction coordinate for the methyl isocyanide isomerization

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTheoretical reaction coordinate for the methyl isocyanide isomerizationDean H. Liskow, Charles F. Bender, and Henry F. Schaefer IIICite this: J. Am. Chem. Soc. 1972, 94, 15, 5178–5182Publication Date (Print):July 1, 1972Publication History Published online1 May 2002Published inissue 1 July 1972https://doi.org/10.1021/ja00770a005RIGHTS & PERMISSIONSArticle Views101Altmetric-Citations36LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (574 KB) Get e-Alerts Get e-Alerts