An ab initio investigation of the ground and low-lying singlet and triplet electronic states of XNO2 and XONO (X = Cl, Br, and I)

Abstract

A systematic ab initio treatment of the nitryl halides (XNO2) and the cis- and trans- conformers of the halide nitrites (XONO), where X = Cl, Br, and I, have been carried out using highly correlated methods with sequences of correlation consistent basis sets. Equilibrium geometries and harmonic frequencies have been accurately calculated in all cases at the explicitly correlated CCSD(T)-F12b level of theory, including the effects of core-valence correlation for the former. Where experimental values are available for the equilibrium structures (ClNO2 and BrNO2), the present calculations are in excellent agreement; however, the X-O distances are slightly too long by about 0.01 Å due to missing multireference effects. Accurate predictions for the iodine species are made for the first time. The vertical electronic excitation spectra have been calculated using equation-of-motion coupled cluster methods for the low-lying singlet states and multireference configuration interaction for both singlet and triplet states. The latter also included the effects of spin-orbit coupling to provide oscillator strengths for the ground state singlet to excited triplet transitions. While for ClNO2 the transitions to excited singlet states all occur at wavelengths shorter than 310 nm, there is one longer wavelength singlet transition in BrNO2 and two in the case of INO2. The long wavelength tail in the XNO2 species is predicted to be dominated by transitions to triplet states. In addition to red-shifting from X = Cl to I, the triplet transitions also increase in oscillator strength, becoming comparable to many of the singlet transitions in the case of INO2. Hence in particular, the latter species should be very photolabile. Similar trends are observed and reported for the halogen nitrites, many of which for the first time.

A theoretical study of the microhydration processes of iodine nitrogen oxides

“…The optimized structures of the five isolated molecules (INO, INO 2 , cis ‐IONO, trans ‐IONO, and IONO 2 ) are shown in Figure while their geometric parameters and corresponding vibrational frequencies are listed in the Supporting Information (Tables S1 and S2) together with the available literature data . The values of the geometric parameters are close to those previously obtained by Bailleux and coworkers for INO at the CCSD(T)‐F12b/cc‐pVQZ‐F12 level of theory, Papayannis and Kosmas for INO 2 , cis ‐IONO, trans ‐IONO, and IONO 2 at the B3LYP/LANL2DZ2df and MP2/LANL2DZ2df levels of theory, Peterson and Francisco for INO 2 , cis ‐IONO, trans ‐IONO at the CCSD(T)‐F12b/cc‐pVQZ‐F12 level of theory, Allan and Plane for IONO 2 at the B3LYP/6‐311 + G(2d,p) level of theory, and Kaltsoyannis and Plane at the B3LYP/aug‐cc‐pVTZ(+all electron basis for iodine atom) level of theory.…”

“…There are only three experimental works for nitrosyl iodide (INO) and nitryl iodide (INO 2 ) using either Ar matrix or gas phase studies . The experimental values of the I‐N stretch (468 cm −1 ) and the out‐of‐plane bend (650 cm −1 ) for INO 2 are not consistent with the theoretical values (see reference for detailed discussion).…”

The X40×10 Halogen Bonding Benchmark Revisited: Surprising Importance of (n–1)d Subvalence Correlation

“…Thus, it would seem that the correlation contribution of ( n –1)­d, and perhaps ( n –1)­spd, electrons needs to be appropriately treated for an accurate benchmark. For calculations on bromine- and iodine-containing systems, core–valence corrections have been applied by several authors. − That being said, it has recently been reported that contributions to bond energies from ( n –1)­sp correlation are typically an order of magnitude smaller than those from ( n –1)­d correlation …”

“…For calculations on bromine-and iodine-containing systems, core-valence corrections have been applied by several authors. [97][98][99][100] That being said, it has recently been reported that contributions to bond energies from (n-1)sp correlation are typically an order of magnitude smaller than those from (n-1)d correlation. 85 For the purpose of examining core-valence correlation contributions, we have chosen four complexes, namely (03) CH 4 …Br 2 , (04) CH 4 …I 2 , (35) HBr…CH 3 OH, and (36) HI…CH 3 OH as test systems; we then performed CCSD(T)/haVTZ,awCVTZ(Br,I) half-CP calculations for (i) valence electrons only, (ii) valence + (n-1)d electrons, and (iii) valence + (n-1)d + (n-1)sp electrons.…”