Multispectrum analysis of the ν4 band of CH3CN: Positions, intensities, self- and N2-broadening, and pressure-induced shifts

A multispectrum nonlinear least-squares fitting technique was applied to measure accurate zero-pressure line center positions, Lorentz self- and nitrogen (N2)-broadened half-width coefficients, and self- and N2-pressure-induced shift coefficients for over 700 transitions in the parallel ν4 band of CH3CN near 920 cm−1. Fifteen high-resolution (0.0016 cm−1) laboratory absorption spectra of pure and N2-broadened CH3CN recorded at room temperature using the Bruker IFS 125HR Fourier transform spectrometer located at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington, USA, were analyzed simultaneously assuming standard Voigt line shapes. Short spectral intervals containing manifolds of transitions from the same value of J were fitted together. In all, high-precision line parameters were obtained for P(44)-P(3) and R(0)-R(46) manifolds. As part of the analysis, quantum assignments were extended, and the total internal partition function sum was calculated for four isotopologs: 12CH312CN, 13CH312CN, 12CH313CN, and 13CH313CN. Measurements of N2 broadening, self-broadening, N2-shift, and self-shift coefficients for transitions with J up to 48 and K up to 12 were measured for the first time in the mid-infrared. Self-broadened half-width coefficients were found to be very large (up to ∼2 cm−1 atm−1 at 296 K). Ratios of self-broadened half-width coefficients to N2-broadened half-width coefficients show a compact distribution with rotational quantum number in both the P and R branches that range from ∼4.5 to 14 with maxima near ∣m∣=24, where m=−J″, J″, and J″+1 for P, Q, and R lines, respectively. Pressure-induced shifts for N2 are small (few exceed ±0.006 cm−1 atm−1 at 294 K) and are both positive and negative. In contrast, self-shift coefficients are large (maxima of about ±0.08 cm−1 atm−1 at 294 K) and are both positive and negative as a function of rotational quantum numbers. The present measured half-widths and pressure shifts in ν4 were compared with corresponding measurements of rotational transitions.