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Addition of linear methanol associates at the C=N and C=O bonds of methyl isocyanate was studied in terms of the B3LYP/6-311++G(df,p) hybrid quantum-chemical method. The addition at the C=N bond is more favorable than the reaction at the carbonyl group. All reactions involve late asymmetric cyclic transition states. The activity of the reacting system increases in parallel with the degree of methanol association. Isomerization of methyl hydrogen methylcarbonimidate into carbamate is catalyzed by methanol associates. Thermal decomposition of carbamates with formation of isocyanates can occur in autocatalytic mode. * For communication II, see [1].We previously performed quantum-chemical studies on noncatalytic reactions of phenyl isocyanate with linear methanol associates [1, 2]. The obtained results allowed us to conclude that the addition of alcohols at the C=N bond of isocyanates is considerably more favorable than the reaction at the carbonyl group. The reactions involved late concerted asymmetric transition states in which the degree of formation of new C-O bond was higher than the degree of formation of new N-H bond. Alcohol associates were more reactive than monomeric species, which was attributed to enhanced electron-donating properties of alcohol associates. Reactions of isocyanates with alcohol associates were both kinetically and thermodynamically preferable as compared to monomeric species. Reactions of isocyanates with alcohol associates were found to be thermodynamically preferable not only in the gas phase but also in the liquid phase [3].Reactions of alcohols with aliphatic isocyanates are also important from the viewpoint of large-scale applications [4,5]. Therefore, it seemed reasonable to elucidate whether the relations revealed previously by quantum-chemical methods for reactions of aromatic isocyanates with alcohols could be extended to analogous transformations with participation of aliphatic isocyanates. In the present article we report on the results of our quantum-chemical study on the mechanisms of noncatalytic reactions of methyl isocyanate with linear methanol associates (monomer, dimer, and trimer).Quantum-chemical calculations were performed at the B3LYP/6-311++G(df,p) level of theory using Gaussian 03 software package [6,7]. The geometric parameters of the reacting species were preliminary optimized in terms of the density functional theory using Priroda 6 software [8] with L11 basis set (an analog of cc-pCVDZ [9]). Transition states on the potential energy surface were identified by the first negative vibration frequency in the Hessian matrix and were checked by descend toward both sides using IRC (internal reaction coordinate) procedure followed by optimization of geometric parameters of pre-and postreaction complexes. The structures of the most favorable conformers were determined by energy minimization as described in [2].Noncatalytic paths of the reaction of methyl isocyanate with linear methanol associates. We examined reaction paths involving addition of linear methanol associa...

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