Ab initio and DFT studies on the elimination kinetics 2-substituted ethyl N,N-dimethylcarbamates [(CH3)2NCOOCH2CH2Z, Z=CH2C6H5, C6H5, C(CH3)6-point double bondCH2] in the gas phase

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Abstract

The theoretical studies on the elimination kinetics of 2-substituted ethyl N,N-dimethylcarbamates [(CH3)2NCOOCH2CH2Z, Z=CH2C6H5, C6H5, C(CH3)6-point double bondCH2] in the gas phase were carried out using the ab initio MP2/6-31G and DFT RMPWP91/6-31G(d,p) levels of theory. These carbamates produce N,N-dimethylcarbamic acid and the corresponding substituted olefin in a rate determining step. On the basis of these calculations, the mechanism appears to be concerted, asynchronous, through a six-membered cyclic transition state structure. The acidity of the benzylic and allylic β-hydrogen is believed to be responsible for faster elimination rates.

Introduction

N,N-Dimethylcarbamates with at least a Cβ–H bond at the alkyl side of the ester are known to decompose in the gas phase through a six-membered cyclic transition state type of mechanism similar as described for acetates, carbonates and xanthates [1], [2], [3], [4], [5], [6] (reaction 1)

Plotting a considerable number of 2-substituted ethyl N,N-dimethylcarbamates [7], (CH3)2NCOOCH2CH2Z, against several method of correlations [8], Hancock's Esc, Taft's Es, Charton's ν steric parameters, the inductive σI values, and the Taft–Topsom method, gave random points with no meaning for mechanistic interpretations.

However, the plot of log kZ/kCH3 vs. Taft σ* values [8] originated at [σ*(CH3)=0.00] three approximate straight lines. These results implied that a small alterations in the polarity of the transition state may be due to changes of electronic transmission at the reaction center. Apparently, a simultaneous effect may be operating during the process of elimination. The mechanisms of these elimination reactions were explained according to each slope of the straight lines. However, the point position of Z=phenyl (C6H5) and isopropenyl [CH26-point double bondC(CH3)] substituents falling far above the three lines were believed to enhance the rate of elimination due to the acidity of the benzylic and allylic Cβ–H bond [9].

A theoretical study using a Møller–Plesset MP2/6-31G method of calculation of the gas phase elimination of 2-substituted alkyl ethyl N,N-dimethylcarbamate was recently described [10]. Results from these calculations also favor the concerted asynchronous six-membered cyclic transition state type of mechanism. In this work [10] when plotting the logarithm of the theoretical rate coefficients against Taft σ* values gives a straight line of ρ*=−1.39, r=0.956 at 360 °C [10], while the experimental correlation [8] yields ρ*=−1.94, r=0.976 at 360 °C. Moreover, the experimental log krel vs. theoretical log krel of these 2-substituted ethyl carbamates gave a good straight line (r=0.9919 at 360 °C), suggesting similar mechanism.

According to papers [7], [9], the present was work is addressed at examining the potential energy surface (PES) at the ‘ab initio’ and DFT level of theories for a further understanding of the transition state nature in the gas phase elimination of 2-susbtituted ethyl N,N-dimethylcarbamates, [(CH3)2NCOOCH2CH2Z, Z=CH2C6H5, C6H5, C(CH3)6-point double bondCH2].

Section snippets

Computational methods and models

The characterization of the potential energy surface for the gas phase elimination reaction of 2-substituted ethyl N,N-dimethylcarbamates was carried out by means of electronic structure calculations using Møller-Plesset MP2/6-31G and DFT RMPWP91/6-31G(d,p) methods as implemented in gaussian 98W [11]. Stable configurations of reactants and products as well as the TS structures were characterized by means of normal-mode analysis. Thermodynamic quantities such as zero point vibrational energy

Kinetic and thermodynamic parameters

The elimination kinetics of 2-substituted ethyl N,N-dimethylcarbamates [(CH3)2NCOOCH2CH2Z, Z=CH2C6H5, C6H5, C(CH3)6-point double bondCH2] in the gas phase produces N,N-dimethylcarbamic acid and the corresponding substituted olefin in a slow step. The N,N-dimethylcarbamic acid is unstable and undergoes rapid decarboxylation and therefore it was not possible to determine experimentally. Parameters for decarboxylation of N,N-dimethylcarbamic acid were calculated at the MP2/6-31G level of theory. The calculated rate

Conclusions

The present work provides additional evidence for the concerted mechanism in the thermal decomposition of 2-substituted ethyl N,N-dimethylcarbamates (Z=CH2C6H5, C6H5, C(CH3)6-point double bondCH2) [8]. Theoretical calculations suggest that the reaction proceeds in a discrete polar concerted six-membered cyclic TS mechanism. The β-hydrogen being transferred is located halfway between the carbonyl oxygen and Cβ. Activation parameters are in better agreement with experimental values for RMPWP91/6-31G(d,p) rather

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