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PM3 study of the proton affinities of 2-, 3-, and 4-monosubstituted pyridines in the gas phase

✍ Scribed by Miroslaw Szafran; Jacek Koput

Publisher
John Wiley and Sons
Year
1991
Tongue
English
Weight
445 KB
Volume
12
Category
Article
ISSN
0192-8651

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✦ Synopsis

Heats of formation (AHf) and proton affiiities (PA) of 2-, 3-, and 4-monosubstituted pyridines in the gas phase are calculated using the AM1 and PM3 semiempirical methods. The following substituents are considered: NO2, CN, CF,, CHO, F, C1, COCH3, H, CH,, OCH3, SCH,, NH2, and N(CH,),. The results are compared with the experimental data. Both methods reproduce the AH, with comparble accuracy; the rms deviations are 4.1 (AMl) and 4.5 kcal/mol (PM3) for the free bases and 9.5 (AM1) and 9.7 kcal/mol (PM3) for their codugated acids. The PA are systematically underestimated by both methods, but AM1 appears to be clearly better than PM3 for reproducing the experimental values. The rms deviations for AM1 and PM3 are 5.1 and 9.6 kcal/mol, respectively. This is due to a cancellation of systematic errors in the calculated AH, in the AM1 case and to a summation of the errors in the PM3 case. Both methods correctly reproduce conformations of the molecules under consideration.

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