Detection of 1 H -Triphosphirene ( c -HP 3 ) and 2-Triphosphenylidene (HP 3 ): The Isovalent Counterparts of 1 H -Triazirine ( c -HN 3 ) and Hydrazoic Acid (HN 3 ).

The hitherto elusive 1 H -triphosphirene ( c -HP 3 ) and 2-triphosphenylidene (HP 3 ) molecules were prepared in low-temperature matrices and detected isomer selectively through photoionization coupled with reflectron time-of-flight mass spectrometry (PI-ReTOF-MS). Our results reveal a thermodynamically preferred cyclic isomer ( c -HP 3 ) compared to the acyclic structure (HP 3 ) in contrast to the isovalent HN 3 system favoring hydrazoic acid (HN 3 ) compared to 1 H -triazirine ( c -HN 3 ). Theoretical computations suggest a ring strain energy of 1 H -triphosphirene ( c -HP 3 ) of only 35 kJ mol -1 , which is significantly lower than the tetrahedral phosphorus molecule (P 4 ) of 74 kJ mol -1 . This work provides a fundamental benchmark to understand the electronic structure and chemical bonding of cyclic molecules and offers an unconventional approach to preparing highly strained, still elusive molecules such as 1 H -triazirine and tetrahedral tetranitrogen (N 4 ) in the near future involving progressive nonequilibrium chemistries.