HIGH-LEVEL COMPUTATIONAL STUDY ON THE THERMOCHEMISTRY OF SATURATED AND UNSATURATED 3-MEMBERED AND 4-MEMBERED NITROGEN AND PHOSPHORUS RINGS

The heats of formation and strain energies for saturated and unsaturat ed three- and four-membered nitrogen and phosphorus rings have been ca lculated using G2 theory. G2 heats of formation (Delta H-f 298) of tri aziridine [(NH)(3)], triazirine (N3H), tetrazetidine [(NH)(4)], and te trazetine (N4H2) are 405.0, 453.7, 522.5, and 514.1 kJ mol(-1), respec tively. Tetrazetidine is unstable (121.5 kJ mol(-1) at 298 K) with res pect to its dissociation into two trans-diazene (N2H2) molecules. The dissociation of tetrazetine into molecular nitrogen and trans-diazene is highly exothermic (Delta H-298 = -308.3 kJ mol(-1) calculated using G2 theory). G2 heats of formation (Delta H-f 298) of cyclotriphosphan e [(PH)(3)], cyclotriphosphene (P3H), cyclotetraphosphane [(PH)(4)], a nd cyclotetraphosphene (P4H2) are 80.7, 167.2, 102.7, and 170.7 kJ mol (-1), respectively. Cyclotetraphosphane and cyclotetraphosphene are st abilized by 145.8 and 101.2 kJ mol(-1) relative to their dissociations into two diphosphene molecules or into diphosphene (HP = PH) and diph osphorus (P-2), respectively. The strain energies of triaziridine [(NH )(3)], triazirine (N3H), tetrazetidine [(NH)(4)], and tetrazetine (N4H 2) were calculated to be 115.0, 198.3, 135.8, and 162.0 kJ mol(-1), re spectively (at 298 K). While the strain energies of the nitrogen three -membered rings in triaziridine and triazirine are smaller than the st rain energies of cyclopropane (117.4 kJ mol(-1)) and cyclopropene (232 .2 kJ mol(-1)), the strain energies of the nitrogen four-membered ring s in tetrazetidine and tetrazetine are larger than those of cyclobutan e (110.2 kJ mol(-1)) and cyclobutene (132.0 kJ mol(-1)). In contrast t o higher strain in cyclopropane as compared with cyclobutane, triaziri dine is less strained than tetrazetidine. The strain energies of cyclo triphosphane [(PH)(3), 21.8 kJ mol(-1)], cyclotriphosphene (P3H, 34.6 kJ mol(-1)), cyclotetraphosphane [(PH)(4), 24.1 kJ mol(-1)], and cyclo tetraphosphene (P4H2, 18.5 kJ mol(-1)), calculated at the G2 level are considerably smaller than those of their carbon and nitrogen analog. Cyclotetraphosphene containing the P = P double bond is less strained than cyclotetraphosphane, in sharp contrast to the ratio between the s train energies for the analogous unsaturated and saturated carbon and nitrogen rings. (C) 1997 John Wiley & Sons, Inc.