Vibrational spectra and force constants of symmetric tops: Rovibrational analysis of 2 ν6 and 2 ν6 - ν6 of H 3Si 35Cl and H 3Si 37Cl, and a new method for the determination of A0 and DK0

The gas-phase ir spectra of monoisotopic H ₃Si ³⁵Cl [35] and H ₃Si ³⁷Cl [37] in the 2 ν₆ region near 1300 cm ^-1 have been studied with a resolution of 0.05 cm ^-1. A total of 467 and 1206 lines have been assigned for [35] and [37], respectively, and analyzed by a least-squares procedure, σ( J, K) ∼ 6 × 10 ^-3cm ^-1, to yield three parameters for the 2 ν₆⁰ and six for the 2 ν₆^±2 states for both isotopomers. In the ν₆ region {32}/{47} and {127}/{150} lines have been assigned to the hot bands 2 ν₆⁰ - ν₆^±1 and 2 ν₆^±2 - ν₆^±1 of [35] and [37], respectively. While a hot band 2 ν_t⁰ - ν_t^±1 does not provide any information above that available from ν_t^±1 and 2 ν_t⁰, a hot band 2 ν_t^±2 - ν_t^±1 may supply extra data concerning the ground state. A new method for the evaluation of A₀ and D_K⁰ from combination differences of ν_t^±1, 2 ν_t^±2, and 2 ν_t^±2 - ν_t^±1 of a symmetric top with C_{3 v} symmetry is presented. Application to [35] and [37] yielded A₀ of 2.8447(5) and 2.8437(3) cm ^-1, and D_K⁰ of 2.12(20) and 2.38(9) × 10 ^-5 cm ^-1, respectively.