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In order to seek the molecular basis of higher insecticidal activity of the carbamates with two methyl groups, m-xylyl-N-methylcarbamate(MXNMC) than the corresponding unsubstituted phenyl N-methylcarbamate(PNMC), these two derivatives have been studied by molecular orbital(MO) theoretically using extended Hu◎ckel theory(EHT), and analysis of regression and linear free energy relationship(LFER). The most stable stereo structure(Z,Z) shows that the phenyl group occupies vertical(θ=90°) position on the plane of the N-methylcarbamyl group. Regression analysis shows that especially good correlation exists between the pI_(50) values and the calculated MO quantities when the hydrogen atomic charge of metaposition and of m-methyl groups, and LUMO energy are taken as variables. The LFER analysis on the carbamylation indicates that field(F) effect(60%) is slightly larger than resonance(R) effect(40%) in PNMC(F$gt;R), whereas, in case of MXNMC, R effect(98.6%) is much larger than F effect(1.4%)(R≫F). From the basis on the findings, the enhancement of insecticidal activity of MXNMC may be the result of hyperconjugation by m-methyl groups.
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