On the structural parameters and vibrational spectra of some XNCS and XSCN (X=H, F, Cl, Br) molecules

Ab initio calculations with full electron correlation by the perturbation method to second order and hybrid density functional theory calculations by the B3LYP method utilizing the 6-31G(d), 6-311+G(d,p), and 6-311+G(2d,2p) basis sets have been carried out for the XNCS and XSCN (X=H, F, Cl, Br) molecules. From these calculations, force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and structural parameters have been determined and compared to the experimental quantities when available. By combining previously reported rotational constants for HNCS, ClSCN and BrSCN with the ab initio MP2/6-311+G(d,p) predicted parameters, adjusted r0 parameters have been obtained. The structural parameters for isothiocyanic acid are: r(H–N)=0.996(5) Å; r(NC)=1.208(3) Å; r(C=S)=1.568(3) Å; ∠HNC=132.0(5)°; ∠NCS=172.9(5)°. These reported angles have much lower uncertainties than those previously reported from the microwave data alone. The ab initio predicted barrier to molecular linearity of HNCS is 917±44 cm−1 which is in excellent agreement with the experimental value of 900±200 cm−1. The r0 parameters for ClSCN are: r(Cl–S)=2.023(5); r(S–C)=1.694(3); r(CN)=1.156(3) Å; ∠ClSC=99.6(5) and ∠SCN=174.4(5)° and for BrSCN are: r(Br–S)=2.197(5); r(S–C)=1.681(3); r(CN)=1.158(3) Å; ∠BrSC=99.5(5) and ∠SCN=174.7(5)°. Structural parameters are also estimated for HSCN and FSCN. The predicted frequencies for the fundamentals of HNCS and DNCS compare favorably to the experimental values but several of their predicted intensities differ significantly from the observed spectra. The frequencies for the reported bending modes of ClSCN and BrSCN do not agree with the predicted values and further experimental studies are needed for providing confident assignments for these fundamentals.