Internally contracted multireference configuration interaction (icMRCI) calculations of the ground state (X 3 R 2 ), the first excited state (a 1 D) as well as the second excited state (b 1 R 1 ) have been performed for a series of halogenated nitrenes NXs (X 5 Cl, Br, and I). Accurate spectroscopic constants of these lowest three electronic states of each NX were obtained in this work using MRCI methods with aug-cc-pVXZ (X 5 T, Q, 5) basis sets and complete basis set (CBS) limit. In addition, various corrections, including the Davidson correction, scalar relativistic effect, corevalence correlation, and spin-orbit coupling effect, have been studied in calculating spectroscopic constants, especially for heavy-atom nitrenes. Comparisons have been made with previous computational and experimental results where available. The icMRCI 1 Q calculations presented in this work provide a comprehensive series of results at a consistent high level of theory for all of the halogenated nitrenes.
K E Y W O R D Score-valence correlation, icMRCI1Q, nitrenes, scalar relativistic effect, spectroscopic constants, spin-orbit coupling effect 1 | I N TR ODU C TI ON Halogenated nitrenes NXs (X 5 F, Cl, Br, I) are sextet, neutral, and highly reactive molecular species where the nitrogen atom shows a monovalent nature. Derived from the photochemical process of the halogen azides XN 3 !N 2 1 NX (X 5 F, Cl, Br, I), [1][2][3] the radical NXs play an important photochemical role in the upper atmosphere and interstellar space because of their relatively high contents of the constituent elements in the space. [4,5] Halogenated nitrenes are also expected to be a possible candidate for the application in chemical energy storage sources since there exists two metastable excited states, a 1 D and b 1 R 1 of NXs. [6][7][8] Particularly, the a 1 D state of NF or NCl could be used for pumping the 2 P 1/2 -2 P 3/2 transition of atomic iodine in a successful demonstration of an efficient chemical laser. [9][10][11][12][13] Accurate spectroscopic information about these low-lying electronic states is essential to understand the dynamics and kinetics of chemical processes involving halogenated nitrenes.As analogs to halogenated carbenes, the two unpaired electrons of NX can couple either as a singlet or triplet state very close in energy. Being iso-valent with O 2 , all halogenated nitrenes studied to date have a triplet ground state of X 3 R 2 . The two low-lying excited singlet states, a 1 D and b 1 R 1 , resulting from the same electron configuration (p 4 p* 2 ), could exhibit quite different reactivity comparing to the triplet ground electronic state.The spectroscopy of these low-lying states, the singlet-triplet energy splitting, as well as their reactivity, has been the subject of a myriad of investigations in the literature. [14] The spectroscopic study of NXs could date back to more than 70 years ago when the emission spectrum of NBr (later assigned to the b 1 R 1 -X 3 R 2 transition [15] ) was first observed by Elliott. [16] After that, the stru...
