Abstract
Semiempirical methods of quantum chemistry (CNDOj 2, INDO, MINDOj2) have been tested as a source of molecular constants needed for calculations of equilibrium constants of gas-phase chemical reactions. Equilibrium constants have been expressed in terms of partition functions, and ideal gas, rigid rotator, and harmonic oscillator approximations have been adopted. The set investigated consists of twelve molecules (NH 3 , HzCO, CH4 , CH 3 0H, CH3 NHz, CH3 CN, CHzCHz, CZH4 0, CZH 4 NH, CH3 CH 3 , C3 H 6 , CH3 CHzCH 3 ). It has been found that at temperatures below 2000 K with systems related to the ones investigated the error in the logarithm of the calculated equilibrium constant is smaller than 0'5. The MINDO! 2 method has failed with all compounds containing a-double-bonded oxygen or pyramidal nitrogen. Special attention has been paid to the position of the rigid rotator, harmonic oscillator approximation in modern-day numerical quantum chemistry, and to its validity for calculating equilibrium and rate constants.
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