INTRODUCTION
In a previous work, (1) we reported that the lowest energy channel for thermal decomposition for hexahydroquinazolin-4(1H)-ones compounds (1-4), is the heterocyclic ring opening.
The reaction leads to the formation of isothiocyanates or isocyanates through a cyclic transition state from the thiol or enol tautomer (1-4 XH).
The oxygenated compounds are more stable than the sulfurated ones, this is proved through their decomposition temperatures. The type of ring fusion plays an important role in the stability of the compounds.
We found a relationship between the structural and conformational properties and their thermal reactivities.
The main tools used were experimental infrared spectra and quantum chemical calculations.
RESULTS AND DISCUSSION
There are 90 normal modes of vibration , all infrared active. We adopted a unique scaling factor equal to 0.9648 for the IR spectra.
Figure 2 (A and B) shows each tautomer´s spectrum, its sum and the comparation with the experiment for both cis and trans sulfurated compounds.
The signals at 2724 cm-1and at 2734 cm-1 (A 1exp and B2exp respectively) corresponds to the thiol form presents in a very low quantity.
A factor is obtained from the rate aNH/aSH for scaling the theoretical spectra to compare with the experimentals.
Gas and solid phase calculated populations of each tautomer were obtained.
Figure 3 shows the apropriateness of the selected scaling factors.
