State Quartic Centrifugal Distortion Constants Research Articles

The high-resolution infrared spectrum of thiophene between 600 and 1200cm−1: A spectroscopic and theoretical study of the fundamental bands ν6, ν7, ν13, and the c-Coriolis interacting dyad ν5, ν19

Abstract The Fourier transform infrared spectrum of gaseous thiophene, C4H4S, has been recorded in the 600–1200 cm−1 spectral region with a resolution of ca. 0.0030 cm−1. Five fundamental bands ν13 (B1, 712.1 cm−1), ν7 (A1; 840.0 cm−1), ν6 (A1; 1036.4 cm−1), ν5 (A1; 1081.5 cm−1) and ν19 (B2; 1084.0 cm−1) have been analysed by the standard Watson model (A-reduction). Ground state rotational and quartic centrifugal distortion constants have been obtained from a simultaneous fit of ground state combination differences from four of these bands and previous microwave transitions. Upper state spectroscopic constants have been obtained for all five bands from single band fits using the Watson model. A strong c-Coriolis resonance perturbs the close lying ν5 and ν19 bands. We have analysed this dyad system by a model including first and second order Coriolis resonance using the theoretically predicted Coriolis coupling constant ς 19 , 5 c . From this analysis we locate the previously unobserved ν19 band at 1083.969 cm−1. The rotational constants, ground state quartic centrifugal distortion constants, anharmonic frequencies, and vibration–rotational constants (α-constants) predicted by quantum chemical calculations using a cc-pVTZ basis with B3LYP methodology, are compared with the present experimental data, where there is generally good agreement. A complete set of anharmonic frequencies and α-constants for all fundamental levels of the molecule is given.

High-resolution infrared and theoretical study of four fundamental bands of gaseous 1,3,4-oxadiazole between 800 and 1600 cm −1

The Fourier transform infrared spectrum of gaseous 1,3,4-oxadiazole, C 2H 2N 2O, has been recorded in the 800–1600 cm −1 wavenumber region with a resolution around 0.0030 cm −1. The four fundamental bands ν 9(B 1; 852.5 cm −1), ν 14(B 2; 1078.5 cm −1), ν 4(A 1; 1092.6 cm −1), and ν 2(A 1; 1534.9 cm −1) are analyzed by the standard Watson model. Ground state rotational and quartic centrifugal distortion constants are obtained from a simultaneous fit of ground state combination differences from three of these bands and previous microwave transitions. Upper state spectroscopic constants are obtained for all four bands from single band fits using the Watson model. The ν 4 and ν 14 bands form a c-Coriolis interacting dyad, and the two bands are analyzed simultaneously by a model including first and second order Coriolis resonance using the ab initio predicted Coriolis coupling constant ς 14 , 4 c . An extended local resonance in ν 2 is explained as higher order b-Coriolis type resonance with ν 6 + ν 10, which is further perturbed globally by the ν 15 + ν 10 level. A fit of selected low- J transitions to a triad model including ν 2(A 1), ν 6 + ν 10(B 1), and ν 15 + ν 10(A 2) using an ab initio calculated Coriolis coupling constant ς 15 , 6 c is performed. The rotational constants, ground state quartic centrifugal distortion constants, anharmonic frequencies, and vibration–rotational constants ( α-constants) predicted by quantum chemical calculations using a cc-pVTZ and TZ2P basis with B3LYP methodology, are compared with the present experimental data, where there is generally good agreement. A complete set of anharmonic frequencies and α-constants for all fundamental levels of the molecule is given.

Observation and Rovibrational Analysis of the Intermolecular NH3 Libration Band of H3N−HCN

The high-resolution far-infrared absorption spectrum of the gaseous molecular complex H(3)N-HCN is recorded by means of static gas-phase Fourier transform far-infrared spectroscopy at 247 K, using a synchrotron radiation source. The spectrum contains distinct rotational structures which are assigned to the intermolecular NH(3) libration band nu9(1) (nu(B)) of the pyramidal H(3)N-HCN complex. A rovibrational analysis based on a standard semirigid symmetric top molecule model yields the band origin of 260.03(10) cm(-1), together with values for the upper state rotational constant B' and the upper state quartic centrifugal distortion constants D'(J) and D'(JK). The values for the upper state spectroscopic constants indicate that the hydrogen bond in the H(3)N-HCN complex is destabilized by 5% and elongates by 0.010 A upon excitation of a quantum of libration of the hydrogen bond acceptor molecule.