Nitrogen inversion, molecular mechanics

Semi-empirical models are markedly inferior to all other models dealt with (except the SYBYL molecular mechanics model) for barrier calculations. Major trends in rotation barriers are often not reproduced, for example, the nearly uniform decrement in rotation barrier from ethane to methylamine to methanol. None of the semi-empirical models is better than the others in this regard. One the other hand, AMI is clearly superior to MNDO and PM3 in accounting for nitrogen inversion barriers. All in all, semi-empirical models are not recommended for barrier calculations. 

Already with Jantti s 3PM a timesaving of the stepwise measurement of nitrogen isotherms up to 70 percent was possible 2PM is even faster. For both methods we show that the gain of the shortening of the measurement time is accompanied by a drawback, i.e. the increase of measurement inaccuracy, which is shown to be inversely proportional to the square of the measurement time. In case the molecular adsorption mechanism is not a given factor before the start of the measurement, the application of the time saving measurement will involve extra uncertainties. 

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