High-resolution infrared spectrum of the v ₁ band of η ⁵-C ₅H ₅NiNO

Gas-phase rotational constants and distortion constants have been determined for the v ₁ (v = 1) excited vibrational state of cyclopentadienylnickel nitrosyl (C ₅H ₅NiNO) using a high-resolution Fourier transform spectrometer system at Kitt Peak, Arizona. The rotationally resolved lines have been measured for the C-H symmetric stretch vibration (v ₁ = 3110 cm ^-1). In the present analysis, over 150 lines have been assigned and fitted using a rigid-rotor Hamiltonian with centrifugal distortion. The vibrational band center, excited-state rotational constants, and distortion constants derived from the measured spectrum for this prolate symmetric-top molecule are v _o = 3110.4129(4) cm ^-1, A′ = 0.14328(8) cm ^-1, B′ = C′ = 0.041285(1) cm ^-1, D _J′ = 0.078(1) kHz, D _JK′ = 2.23(4) kHz, and D _K′ = -2.63(2) kHz, respectively. Several different combination differences, with a common upper state, were calculated for different K stacks for the observed spectra, and the consistency of the lower state rotational constants obtained provided further support for the current assignment. The ground-state rotational constant (B″) derived from this combination differences analysis agrees with the previously obtained Fourier transform microwave value to within 0.15%. However, ground-state rotational constants, A″ and B″, have been fixed in the present analysis to avoid correlation effects and to get more accurate results. The new measured parameters are compared with the previously obtained results from Fourier transform microwave and infrared spectroscopy measurements. The C-H vibration stretching frequency and rotational constants were calculated using density functional theory calculations, and these were quite helpful in resolving ambiguities in the fitting procedure and for initial assignments of measured lines.