Infrared overtone spectroscopy and vibrational analysis of a Fermi resonance in nitric acid: Experiment and theory

Abstract

High resolution infrared spectra of nitric acid have been recorded in the first OH overtone region under jet-cooled conditions using a sequential IR-UV excitation method. Vibrational bands observed at 6933.39(3), 6938.75(4), and 6951.985(3) cm(-1) (origins) with relative intensities of 0.42(1), 0.38(1), and 0.20(1) are attributed to strongly mixed states involved in a Fermi resonance. A vibrational deperturbation analysis suggests that the optically bright OH overtone stretch (2nu1) at 6939.2(1) cm(-1) is coupled directly to the nu1 + 2nu2 state at 6946.4(1) cm(-1) and indirectly to the 3nu2 + nu3 + nu7 state at 6938.5(1) cm(-1). Both the identity of the zero-order states and the indirect coupling scheme are deduced from complementary CCSD(T) calculations in conjunction with second-order vibrational perturbation theory. The deperturbation analysis also yields the experimental coupling between 2nu1 and nu1 + 2nu2 of -6.9(1) cm(-1), and that between the two dark states of +5.0(1) cm(-1). The calculated vibrational energies and couplings are in near quantitative agreement with experimentally derived values except for a predicted twofold stronger coupling of 2nu1 to nu1 + 2nu2. Weaker coupling of the strongly mixed states to a dense background of vibrational states via intramolecular vibrational energy redistribution is evident from the experimental linewidths of 0.08 and 0.25 cm(-1) for the higher energy and two overlapping lower energy bands, respectively. A comprehensive rotational analysis of the higher energy band yields spectroscopic parameters and the direction of the OH overtone transition dipole moment.

The empirical equilibrium structure of diacetylene

“…frequency calculations, while the Dunning basis sets (at least when using the valence sets) show some deficiencies [31][32][33][34][35].…”

Spectroscopic identification and stability of the intermediate in the OH + HONO 2 reaction

“…The fact that the 3260 cm Ϫ1 feature (as well as its two subcomponents) is much broader than the OH radical stretch feature is consistent with strong anharmonic coupling between two (or possibly more) vibrational states. We carried out a vibrational deperturbation analysis (19) in an effort to characterize the energies and couplings of zero-order states that might give rise to the two components of the 3260 cm Ϫ1 feature. A two-state deperturbation analysis indicates that the experimentally observed mixed states would arise from coupling between zero-order states at 3257.6(7) and 3261.2(7) cm Ϫ1 .…”

The equilibrium OH bond length

“…This is in agreement with a rather long OH bond r(OH) ¼ 0.968 Å , see Section 6. On the other hand, the OH stretching vibration, leading to strong overtone bands, has been extensively studied because overtone excitation (by photolysis) of the OH chromophore leads to the dissociation of HNO 3 in the stratosphere [68][69][70][71]. A Birge-Sponer plot of the data 1 v oh 6 leads to!…”