Characterization of the X̃ 1A′ state of isocyanic acid

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Abstract
Characteristics of the ground electronic state of HNCO have been investigated theoretically in a series of eight ab initio analyses involving qualitative features of the electronic structure, the barrier to linearity, the NH(3Σ−)+CO fragmentation energy, the H–NCO bond dissociation energy, heats of formation of isomers of HNCO, fundamental vibrational frequencies and anharmonic force fields, the rovibrational spectrum of DNCO, and the precise Re structure of isocyanic acid. Sundry state‐of‐the‐art electronic structure methods were employed in the study, including restricted and unrestricted Hartree–Fock (RHF and UHF), complete‐active‐space self‐consistent‐field (CASSCF), configuration interaction singles and doubles (CISD), Mo/ller–Plesset perturbation theory through fourth and occasionally fifth order (MP2–MP5), coupled‐cluster singles and doubles (CCSD), and CCSD augmented by a perturbative contribution from connected triple excitations [CCSD(T)]. The one‐particle basis sets ranged in quality from (9s5p1d/4s2p1d) to (13s8p3d2f/6s5p3d2f ) on the heavy atoms and from (4s1p/2s1p) to (6s2p1d/4s2p1d) on hydrogen. Several revisions of thermochemical data are proposed, in particular, a larger barrier to linearity of 5.7(3) kcal mol−1, an enhanced bond energy of 85.4(10) kcal mol−1 for D0(NH–CO), and more reliable relative energies for the isomers of HNCO, viz., γe(HOCN)=25.5(10), γe(HCNO)=70(2), and γe(HONC)=84.5(15) kcal mol−1. In addition, the experimental value D0(H–NCO)=113.0(2) kcal mol−1 is confirmed. These results lead to several new proposals for heats of formation (ΔH°f,0, kcal mol−1): HNCO(−26.1), HOCN(−0.7), HCNO(+43.0), HONC (+57.6), and NCO(+35.3). A complete quartic force field has been constructed for HNCO by combining RHF third‐ and fourth‐derivative predictions with CCSD quadratic force constants subjected to the scaled quantum mechanical (SQM) optimization scheme. This force field yields a set of ωi and χij vibrational constants which gives the following fundamental frequencies (with total anharmonicities in parentheses): ν1=3534(−186), ν2=2268(−45), ν3=1330(−9), ν4=778(−50), ν5=576(+9), and ν6=657(+21) cm−1, thus reproducing the observed band origins to 4 cm−1 or less. For DNCO the theoretical force field reveals misassignments of the low‐frequency bending vibrations and predicts ν4(a’)=727, ν5(a’)=458, and ν6(a‘)=633 cm−1. Finally, the theoretical vibration–rotation interaction constants (αi) for five isotopic species of HNCO have been used in conjunction with empirical rotational constants and the Kraitchman equations to determine re(N–H)=1.0030(20) Å, re(N–C)=1.2145(6) Å, re(C–O)=1.1634(4) Å, θe(H–N–C)=123.34(20)°, and θe(N–C–O)=172.22(20)°.