Understanding the kinetics of spin-forbidden chemical reactions

Many chemical reactions involve a change in spin-state and are formally forbidden. This article summarises a number of previously published applications showing that a form of Transition State Theory (TST) can account for the kinetics of these reactions. New calculations for the emblematic spin-forbidden reaction HC + N(2) are also reported. The observed reactivity is determined by two factors. The first is the critical energy required for reaction to occur, which in spin-forbidden reactions is often defined by the relative energy of the Minimum Energy Crossing Point (MECP) between potential energy surfaces corresponding to the different spin states. The second factor is the probability of hopping from one surface to the other in the vicinity of the crossing region, which is largely defined by the spin-orbit coupling matrix element between the two electronic wavefunctions. The spin-forbidden transition state theory takes both factors into account and gives good results. The shortcomings of the theory, which are largely analogous to those of standard TST, are discussed. Finally, it is shown that in cases where the surface-hopping probability is low, the kinetics of spin-forbidden reactions will be characterised by unusually unfavourable entropies of activation. As a consequence, reactions involving a spin-state change can be expected to compete poorly with spin-allowed reactions at high temperatures (or energies).     

Ab initio study of the spectroscopy of CH3N and CH3CH2N

Complete active space (CAS) calculations with 6-311++g(3df,3pd) basis sets were performed for a large number of electronic states of the nitrate free radical (CH3N/CH3CH2N) and their positive and negative ions. All calculated states are valence states, and their characters are discussed in detail. To investigate the Jahn-Teller effect on the CH3N radical, Cs symmetry was used for both CH3N and CH3CH2N in calculations. The results (CASPT2 adiabatic excitation energies and CASSI oscillator strengths) suggest that the calculated transitions of CH3N at 32172 and 32139 cm(-1) are attributed to the 2(3)A' ' --> 1(3)A' ' and 1(3)A' --> 1(3)A' ', respectively, which is in accordance with the A3E --> X3A2 emission spectrum at T0 = 31 817 cm(-1). The calculated transitions of CH3CH2N at 334 nm are attributed to the 1(3)A' ' --> 2(3)A' ' and 1(3)A' ' --> 1(3)A', respectively, which is in accordance with the UV absorption spectrum of a series of 11 bands beginning at 335 nm. The vertical and adiabatic ionization energies were obtained to compare with the PES data. These results are in agreement with previous experimental data, which is discussed in detail.     

Comparative theoretical study of the dissociation process of the isoelectronic molecules BH3CO,CH2CO,HNCO, CO2 and BH3N2, CH2N2, HN3, N2O

Anab initio study of the electronic structure of several 22-electrons molecules is presented. The equilibrium geometries of their ground state are calculated at the SCF level using the 6–31G basis set and are found to be in good agreement with the experimental geometries. The dissociation process of these molecules leading to the isoelectronic products CO or N₂ on the one hand and BH₃, CH₂, NH and O on the other hand is studied. The least-energy dissociation paths of the ground states determined at the SCF level are compared on the basis of electron density interactions. The dissociation energies corresponding to the two lowest dissociation channels are calculated. In these calculations, the correlation energy is taken into account using a non-variational method developed previously. The calculated values of dissociation energies are in good agreement with the existing experimental values. The results permit to predict values for HNCO, BH₃CO and CH₂N₂ and to confirm the instability of BH₃N₂.Aspirant du Fonds National Belge de la Recherche Scientifique.     

The heats of formation of diazene, hydrazine, N2H3+, N2H5+, N2H, and N2H3 and the Methyl Derivatives CH3NNH, CH3NNCH3, and CH3HNNHCH3

The heats of formation of N(2)H, diazene (cis- and trans-N(2)H(2)), N(2)H(3), and hydrazine (N(2)H(4)), as well as their protonated species (diazenium, N(2)H(3)(+), and hydrazinium, N(2)H(5)(+)), have been calculated by using high level electronic structure theory. Energies were calculated by using coupled cluster theory with a perturbative treatment of the triple excitations (CCSD(T)) and employing augmented correlation consistent basis sets (aug-cc-pVnZ) up to quintuple-zeta, to perform a complete basis set extrapolation for the energy. Geometries were optimized at the CCSD(T) level with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. Core-valence and scalar relativistic corrections were included, as well as scaled zero point energies. We find the following heats of formation (kcal/mol) at 0 (298) K: DeltaH(f)(N(2)H) = 60.8 (60.1); DeltaH(f)(cis-N(2)H(2)) = 54.9 (53.2); DeltaH(f)(trans-N(2)H(2)) = 49.9 (48.1) versus >/=48.8 +/- 0.5 (exptl, 0 K); DeltaH(f)(N(2)H(4)) = 26.6 (23.1) versus 22.8 +/- 0.2 (exptl, 298 K); DeltaH(f)(N(2)H(3)) = 56.2 (53.6); DeltaH(f)(N(2)H(3)(+)) = 231.6 (228.9); and DeltaH(f)(N(2)H(5)(+)) = 187.1 (182.7). In addition, we calculated the heats of formation of CH(3)NH(2), CH(3)NNH, and CH(3)HNNHCH(3) by using isodesmic reactions and at the G3(MP2) level. The calculated results for the hydrogenation reaction RNNR + H(2) --> RHNNHR show that substitution of an organic substituent for H improved the energetics, suggesting that these types of compounds may be possible to use in a chemical hydrogen storage system.     

Absolute rate constants for F + CH3CHO and CH3CO + O2, relative rate study of CH3CO + NO,and the product distribution of the F + CH3CHO reaction

Using a pulse-radiolysis transient UV–VIS absorption system, rate constants for the reactions of F atoms with CH₃CHO (1) and CH₃CO radicals with O₂ (2) and NO (3) at 295 K and 1000 mbar total pressure of SF₆ was determined to be k₁=(1.4±0.2)×10⁻¹⁰, k₂=(4.4±0.7)×10⁻¹², and k₃=(2.4±0.7)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹. By monitoring the formation of CH₃C(O)O₂ radicals (λ>250nm) and NO₂ (λ=400.5nm) following radiolysis of SF₆/CH₃CHO/O₂ and SF₆/CH₃CHO/O₂/NO mixtures, respectively, it was deduced that reaction of F atoms with CH₃CHO gives (65±9)% CH₃CO and (35±9)% HC(O)CH₂ radicals. Finally, the data obtained here suggest that decomposition of HC(O)CH₂O radicals via C C bond scission occurs at a rate of <4.7×10⁵ s⁻¹. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 913–921, 1998     

Secondary kinetics of methanol decomposition: theoretical rate coefficients for 3CH2 + OH, 3CH2 + 3CH2, and 3CH2 + CH3

Direct variable reaction coordinate transition state theory (VRC-TST) rate coefficients are reported for the (3)CH(2) + OH, (3)CH(2) + (3)CH(2), and (3)CH(2) + CH(3) barrierless association reactions. The predicted rate coefficient for the (3)CH(2) + OH reaction (approximately 1.2 x 10(-10) cm(3) molecule(-1) s(-1) for 300-2500 K) is 4-5 times larger than previous estimates, indicating that this reaction may be an important sink for OH in many combustion systems. The predicted rate coefficients for the (3)CH(2) + CH(3) and (3)CH(2) + (3)CH(2) reactions are found to be in good agreement with the range of available experimental measurements. Product branching in the self-reaction of methylene is discussed, and the C(2)H(2) + 2H and C(2)H(2) + H2 products are predicted in a ratio of 4:1. The effect of the present set of rate coefficients on modeling the secondary kinetics of methanol decomposition is briefly considered. Finally, the present set of rate coefficients, along with previous VRC-TST determinations of the rate coefficients for the self-reactions of CH(3) and OH and for the CH(3) + OH reaction, are used to test the geometric mean rule for the CH(3), (3)CH(2), and OH fragments. The geometric mean rule is found to predict the cross-combination rate coefficients for the (3)CH(2) + OH and (3)CH(2) + CH(3) reactions to better than 20%, with a larger (up to 50%) error for the CH(3) + OH reaction.     

Spectroscopic calculations for the (CH3)3N · BF3 and (CH3)3N · BCL3 complexes

Mean amplitudes of vibration and perpendicular amplitude correction coefficients are calculated for (CH₃)₃N · BF₃ and (CH₃)₃N · BCl₃ complexes using different versions of force fields originating from spectroscopic studies with somewhat different assignments. The results are discussed in conjunction with the electron diffraction results and bonding features.     

Effect of structure on the rate and mechanism of thermolysis of some 4-substituted 3-methylfuroxans

Russian Journal of General Chemistry -     

Hydrogendiazid Synthese und Kristallstruktur von PPh4[N3HN3]

Hydrogen Diazide – Synthesis and Crystal Structure of PPh₄[N₃HN₃] PPh₄[N₃HN₃] has been prepared from PPh₄N₃ and Me₃SiN₃ by the reaction with water or ethanol forming colourless nonexplosive crystal needles, which were characterized by IR spectroscopy and by a crystal structure determination. Space group C2/c, Z = 12, lattice dimensions at –70 °C: a = 3782.4(3), b = 727.8(2), c = 2512.4(2) pm, β = 110.13(1)°, R = 0.0841. The [N₃HN₃]^– ion is characterized by an asymmetric N–H…N hydrogen bridge with a NN distance of 272(1) pm.     

NMR investigations on Stannabicycloundecanes of the type RSn(CH2CH2CH2)3N

The ¹H, ¹³C, and ¹¹⁹Sn NMR data of seven stannabicycloundecanes of the type RSn(CH₂CH₂CH₂)₃N (1, R = Cl; 2 , R = Br; 3 , R = I; 4 , R = OH; 5 , R = SPh; 6 , R = Me; 7 , R = Sn(CH₂CH₂CH₂)₃N) are reported. From ¹H NMR coalescence data at low temperature the free activation enthalpies for the racemisation of the bicyclo[3.3.3]skeleton were estimated to be 37 ± 1 kJ/mol. They are independent of the substituent R. However, it decreases when the tin atom is replaced by silicon for R = Me.     

Solvent effects on the rate of thermolysis of lauroyl peroxide

Thermolysis of lauroyl peroxide in various organic solvents was studied. It was shown that the primary homolytic dissociation of the peroxide group is accompanied by secondary reactions of chain-induced decomposition. The reaction medium affects the rate of both the primary homolytic dissociation and secondary induced decomposition processes. Correlation equations between the rate constants of the reactions in study and the physicochemical parameters of the solvents were proposed.     

Diazabutadienes as ligands: I. Compounds of aluminium: coordination of diazabutadienes to Al(CH3)3 and subsequent intramolecular insertion and rearrangement reactions leading to (CH3)2AIR—N—CH(CH3)—C(R')=N—R(R'= H,CH3) and (CH3)2AIR—N—CH2—C(CH3)=N—R

Reaction of R—N=CH—CH=N—R with [(CH₃)₃Al]₂ affords the coordination product (CH₃)₃AlRN=CH—CH=NR (A) for R = 2,6-(CH₃)₂C₆H₃ and 2,4,6(CH₃)₃C₆H₂. For R = 4 ClC₆H₄, 4-CH₃C₆H₄ and 4-CH₃OC₆H₄, insertion takes place, giving the complexes (CH₃)₂A$\text{lRN—CH(CH}{}_3\text{)—CH=N}$—R (B), in which Al is part of a five-membered chelate ring. Depending on the temperature both the addition and insertion products rearrange intramolecularly to the complexes (CH₃)₂-A$\text{lR—N—CH}{}_2\text{—C(CH}{}_3\text{)=N}$—R (C), in which Al is also part of a five-membered chelate ring. Reactions of the asymmetric (CH₃)₂HC—N=CH—C(CH₃)=N—CH-(CH₃)₂ with [Al(CH₃)₃]₂ also leads to an insertion product, (CH₃)₂A$\text{lRN-—CH(CH}{}_3\text{)—C(CH}{}_3\text{)=N}$—R (B') (R = (CH₃)₂CH), but there is no subsequent rearrangement in this case.A mechanism involving hydrogen migration is tentatively proposed to account for the observed isomerization, which increases in rate in the order:R = (CH₃)₃C>2,4,6-(CH₃)₃C₆H₂> 2,6-(CH₃)2C₆H₃ (A → C)andR = 4-CH₃OC₆H₄>4-CH₃C₆H₄>4-ClC₆H₄ (B → C)Hydrolysis of isomer C gives the unknown imino amines R—NH—CH₂-C(CH₃)=N—R in quantitative yield.     

Theoretical study and rate constants calculation for the reactions of SiH3 radical with SiH3CH3 and SiH2(CH3)2

The multiple‐channel reactions SiH₃ + SiH₃CH₃ → products and SiH₃ + SiH₂(CH₃)₂ → products are investigated by direct dynamics method. The minimum energy path (MEP) is calculated at the MP2/6‐31+G(d,p) level, and energetic information is further refined by the MC‐QCISD method. The rate constants for individual reaction channels are calculated by the improved canonical variational transition state theory (ICVT) with small‐curvature tunneling (SCT) correction over the temperature range of 200–2400 K. The theoretical three‐parameter expression k₁(T) = 2.39 × 10⁻²³T^4.01exp(−2768.72/T) and k₂(T) = 9.67 × 10⁻²⁷T^4.92exp(−2165.15/T) (in unit of cm³ molecule⁻¹ s⁻¹) are given. Our calculations indicate that hydrogen abstraction channel from SiH group is the major channel because of the smaller barrier height among eight channels considered. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Zur solvensfreien Darstellung von Tetramethylammoniumsalzen: Synthese und Charakterisierung von [N(CH3)4]2[C2O4], [N(CH3)4][CO3(CH3)], [N(CH3)4][NO2], [N(CH3)4][CO2H] und [N(CH3)4][O2C(CH2)2CO2(CH3)]

Solvent-free Synthesis of Tetramethylammonium Salts: Synthesis and Characterization of [N(CH₃)₄]₂[C₂O₄], [N(CH₃)₄][CO₃CH₃], [N(CH₃)₄][NO₂], [N(CH₃)₄][CO₂H], and [N(CH₃)₄][O₂C(CH₂)₂CO₂CH₃] A general procedure to synthesize tetramethylammonium salts is presented. Several tetramethylammonium salts were prepared in a crystalline state by solvent-free reaction of trimethylamine and different methyl compounds at mild conditions: [N(CH₃)₄]₂[C₂O₄] (cubic; a = 1 114.8(3) pm), [N(CH₃)₄][CO₃CH₃] (P2₁/n; a = 813.64(3), b = 953.36(3), c = 1 131.3(4) pm, β = 90.03(1)°), [N(CH₃)₄][NO₂] (Pmmn; a = 821.2(4), b = 746.5(3), c = 551.5(2) pm), [N(CH₃)₄][CO₂H] (Pmmn; a = 792.8(7), b = 791.7(3), c = 563.3(4) pm) and [N(CH₃)₄][O₂C(CH₂)₂CO₂CH₃] (P2₁; a = 731.1(2), b = 826.4(3), c = 1 025.2(3) pm, β = 110.1(1)°). The tetramethylammonium salts were characterized by IR-spectroscopy and X-ray diffraction. The crystal structures of the methylcarbonate and the nitrite are described.     

Towards an Understanding of Halide Interactions with the Carbonyl-Containing Molecule CH3CHO

The anion photoelectron spectra of Cl⁻⋅⋅⋅CD₃CDO, Cl⁻⋅⋅⋅(CD₃CDO)₂, Br⁻⋅⋅⋅CH₃CHO, and I⁻⋅⋅⋅CH₃CHO are presented with electron stabilisation energies of 0.55, 0.93, 0.48, and 0.40 eV, respectively. Optimised geometries of the singly solvated species featured the halide appended to the CH₃CHO molecule in-line with the electropositive portion of the C=O bond and having binding energies between 45 and 52 kJ mol⁻¹. The doubly solvated Cl⁻⋅⋅⋅(CH₃CHO)₂ species features asymmetric solvation upon the addition of a second CH₃CHO molecule. Theoretical detachment energies were found to be in excellent agreement with experiment, with comparisons drawn between other halide complexes with simple carbonyl molecules.