CO2 isolated line shapes by classical molecular dynamics simulations: influence of the intermolecular potential and comparison with new measurements.

Room temperature absorption spectra of various transitions of pure CO2 have been measured in a broad pressure range using a tunable diode-laser and a cavity ring-down spectrometer, respectively, in the 1.6 µm and 0.8 µm regions. Their spectral shapes have been calculated by requantized classical molecular dynamics simulations. From the time-dependent auto-correlation function of the molecular dipole, including Doppler and collisional effects, spectral shapes are directly computed without the use of any adjusted parameter. Analysis of the spectra calculated using three different anisotropic intermolecular potentials shows that the shapes of pure CO2 lines, in terms of both the Lorentz widths and non-Voigt effects, slightly depend on the used potential. Comparisons between these ab initio calculations and the measured spectra show satisfactory agreement for all considered transitions (from J = 6 to J = 46). They also show that non-Voigt effects on the shape of CO2 transitions are almost independent of the rotational quantum number of the considered lines.

High-resolution infrared spectroscopy of trans- and cis-H(18)ON(18)O: equilibrium structures of the nitrous acid isomers.

In this paper, we present the first high-resolution spectra and analysis of the nu 4 fundamental bands of fully (18)O-substituted nitrous acid, trans- and cis-H(18)ON(18)O. These bands are not perturbed by neighboring vibrational levels and were used to determine for the first time accurate rotational and centrifugal distortion constants of the ground and nu 4 = 1 states of trans- and cis-H(18)ON(18)O. The ground-state rotational constants were then used, together with the rotational constants of other HONO isotopic species and with rotation-vibration parameters from ab initio calculations, to determine accurate semi-experimental equilibrium structures of trans- and cis-HONO. Our study confirms the results of a recent work by Demaison et al. (J. Phys. Chem. A 2006, 110, 13609-13617) concerning the structure of trans-HONO, whereas the new structure of cis-HONO obtained in this paper is a significant improvement compared with the previous work of Cox et al. (J. Mol. Struct. 1994, 320, 91-106). The recommended parameters for the equilibrium structure of cis-HONO are r(e)(ON) = 1.1816(10) A, r(e)(N-O) = 1.3887(10) A, r(e)(O-H) = 0.9744(7) A, angle(e)(ONO) = 113.18(1) degrees, and angle(e)(HON) = 104.67(4) degrees.