High-level computational study on the thermochemistry of saturated and unsaturated three- and four-membered nitrogen and phosphorus rings

Abstract

The heats of formation and strain energies for saturated and unsaturated three- and four-membered nitrogen and phosphorus rings have been calculated using G2 theory. G2 heats of formation (ΔHf298) of triaziridine [(NH)3], triazirine (N3H), tetrazetidine [(NH)4], and tetrazetine (N4H2) are 405.0, 453.7, 522.5, and 514.1 kJ mol−1, respectively. Tetrazetidine is unstable (121.5 kJ mol−1 at 298 K) with respect to its dissociation into two trans-diazene (N2H2) molecules. The dissociation of tetrazetine into molecular nitrogen and trans-diazene is highly exothermic (ΔH298 = −308.3 kJ mol−1 calculated using G2 theory). G2 heats of formation (ΔHf298) of cyclotriphosphane [(PH)3], cyclotriphosphene (P3H), cyclotetraphosphane [(PH)4], and cyclotetraphosphene (P4H2) are 80.7, 167.2, 102.7, and 170.7 kJ mol−1, respectively. Cyclotetraphosphane and cyclotetraphosphene are stabilized by 145.8 and 101.2 kJ mol−1 relative to their dissociations into two diphosphene molecules or into diphosphene (HP(DOUBLE BOND)PH) and diphosphorus (P2), respectively. The strain energies of triaziridine [(NH)3], triazirine (N3H), tetrazetidine [(NH)4], and tetrazetine (N4H2) were calculated to be 115.0, 198.3, 135.8, and 162.0 kJ mol−1, respectively (at 298 K). While the strain energies of the nitrogen three-membered rings in triaziridine and triazirine are smaller than the strain energies of cyclopropane (117.4 kJ mol−1) and cyclopropene (232.2 kJ mol−1), the strain energies of the nitrogen four-membered rings in tetrazetidine and tetrazetine are larger than those of cyclobutane (110.2 kJ mol−1) and cyclobutene (132.0 kJ mol−1). In contrast to higher strain in cyclopropane as compared with cyclobutane, triaziridine is less strained than tetrazetidine. The strain energies of cyclotriphosphane [(PH)3, 21.8 kJ mol−1], cyclotriphosphene (P3H, 34.6 kJ mol−1), cyclotetraphosphane [(PH)4, 24.1 kJ mol−1], and cyclotetraphosphene (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(DOUBLE BOND)P double bond is less strained than cyclotetraphosphane, in sharp contrast to the ratio between the strain energies for the analogous unsaturated and saturated carbon and nitrogen rings. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 373–384, 1997